1    

UNIVERSITÁ  DEGLI  STUDI  DI  MILANO-­‐BICOCCA  

Scuola  di  Scienze  

Corso  di  Laurea  Magistrale  in  Fisica    

 

 

 

Ion  Irradiation  of  Nanocrystalline  Graphene  

on  Quartz  and  Sapphire  

 

Relatore:  Prof.  Alexander  ZAITSEV  

Correlatore:  Prof.  Alberto  PALEARI  

 

Tesi  di  Laurea  di:  

Maria  EDERA    

Matr.  N.  769503  

 

Anno  Accademico  2014/2015  

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I want to thank

Professor Zaitsev for his guidance

For walking me down the path of science

Showing me its beauty

And making me discover a new way to look at it.

Professor Paleari

For his patience and paternal guidance

For believing in me

And sustaining me.

And The One who gives me everything

For His love beyond belief.

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Index Chapter 1 Introduction ........................................................................................... 5

Chapter 2 Theory .................................................................................................... 9

2.1 Graphene ........................................................................................................ 9

2.1.1 Electronic properties ............................................................................ 11

2.1.2 Optical properties ................................................................................ 13

2.1.3 Other Properties ................................................................................... 14

2.1.4 Methods of Synthesis ........................................................................... 15

2.2 CVD .............................................................................................................. 16

2.2.1 Process of growth ................................................................................. 16

2.2.2 Types of CVD ....................................................................................... 17

2.2.3 Pros and Cons ....................................................................................... 18

2.3 Ion irradiation, implantation and sputtering ........................................... 18

2.3.1 Implantation ......................................................................................... 19

2.3.2 Implantation profile ............................................................................. 19

2.3.3 Damage .................................................................................................. 22

2.3.4 Annealing .............................................................................................. 23

2.3.5 Sputtering ............................................................................................. 24

2.3.6 Pros and Cons ....................................................................................... 29

Chapter 3 Equipment ........................................................................................... 31

3.1 Furnace ........................................................................................................ 31

3.2 Vacuum System ........................................................................................... 32

3.3 FIB ................................................................................................................ 34

3.4 Low Pressure Plasma System ..................................................................... 39

3.5 Conductance measuring system ................................................................. 41

3.6 Microscopes ................................................................................................. 42

Chapter 4 Growth of nanocrystalline graphene ................................................. 44

4.1 Experimental procedure for growth ......................................................... 44

4.2 Results .......................................................................................................... 46

4.2.1 Temperature ......................................................................................... 46

4.2.2 Pressure ................................................................................................. 48

4.2.3 Time ....................................................................................................... 50

4.2.4 Pulsed growth ....................................................................................... 52

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Chapter 5 Plasma Treatments ............................................................................. 54

5.1 Procedure ..................................................................................................... 54

5.2 Results .......................................................................................................... 55

5.2.1 Argon treatments ................................................................................. 55

5.2.2 Oxygen treatments ............................................................................... 56

5.2.3 Nitrogen treatments ............................................................................. 57

5.2.4 Hydrogen treatments ........................................................................... 57

5.2.5 Krypton treatments .............................................................................. 58

Chapter 6 FIB Irradiation .................................................................................... 60

6.1 Experimental procedure ............................................................................. 60

6.1.1 Calculation of the dose ......................................................................... 60

6.1.2 Preparation and irradiation of the sample ........................................ 61

6.2 Results and discussion ................................................................................ 65

6.2.1 Conductance after irradiation and after annealing .......................... 65

6.2.2 Results in adhesion after irradiation and annealing ......................... 76

6.2.3 Results in Nucleation improvement of Graphene on Ion-Irradiated Substrates ....................................................................................................... 80

Chapter 7 Conclusion ........................................................................................... 84

Bibliography .......................................................................................................... 87

   

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Chapter 1 Introduction

Graphene, a new prospective electronic nanomaterial, has attracted much

attention as an alternative to traditional semiconductors in a number of

applications [1-7]. Graphene has many advantageous physical properties

making it unique among other electronic materials. One of them is the

combination of a fair electrical conductance and very high optical

transmittance in a broad spectral range. This makes graphene a material of

choice for optoelectronics and photovoltaics where high optical

transmittance is required, but the moderate electrical conductance is not an

issue. Another one is the combination of the 2D atomic structure and a very

high mobility of charge carriers. This makes conductance of graphene very

responsive to the presence of adsorbates on its surface and hence makes

graphene an extremely sensitive electronic chemical sensor [8]. Another

advantage of graphene is that it is a true nanoelectronic material. Unlike

semiconductors with considerable energy bandgap between the valence and

conduction bands, graphene is a semimetal with zero bandgap energy.

Therefore, the unipolar conductivity – the key property of an electronic

material - can be induced in graphene without impurity doping but

controllably by a low external bias. In contrast, in conventional

semiconductors, the unipolar conductivity can be practically achieved only

via impurity doping. The impurity doping is one of the major limitations of

non-zero bandgap semiconductors, which limits the down-scaling of

electronic devices based on the principle of bipolar junction. For instance,

silicon field-effect transistor (FET) ceases to work as a practical device at a

size of 10 nm.

Graphene reveals its highest electronic parameters only when in a form of

isolated single crystal layer. However, direct growth of perfect single crystal

graphene on surface of standard electronic materials (silicon and silicon

oxide) remains an unmet challenge. By now, commercially viable

technologies are those based on two-step procedure of CVD growth of

graphene on copper followed by transfer of the grown graphene onto the

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working substrate. The step of the transfer is one of the major limiting

factors of these technologies. The recently reported direct CVD growth of

graphene on germanium could be a promising alternative [9]. Yet it is not

applicable for direct deposition on silicon or SiO2.

A technologically simple and inexpensive alternative to the high quality

single crystal graphene is nanocrystalline graphene [10, 11]. Nanocrystalline

graphene can be grown by CVD deposition from gas precursor directly on

almost any material over large area [12-13]. Although the electronic

properties of nanocrystalline graphene are inferior to those of single crystal

graphene (low charge carrier mobility in the range 10 to 100 cm2/Vs and

rather high growth temperature over 800°C), nanocrystalline graphene

possesses reasonably high electrical conductance (between 10-4 and 10-5

siemens), high optical transmittance (about 95%) and high chemical

sensitivity (even non-structured nanocrystalline graphene can detect the

presence of NO2 molecules at a concentration of a few tens of ppb).

In the first part of this work, the CVD growth is studied, identifying the

relevant parameters, and finding the values which provide the best quality

nanocrystalline graphene layers.

The CVD growth is studied, in particular its dependence on temperature,

pressure, and time. The knowledge of the dependence on these parameters

allowed us to better understand the growth mechanism, and to interpret the

results of the whole investigation.

When adopting a new material in electronics, along with the technology for

its growth, it is equally important to develop a technology for its structuring

and patterning. Graphene, as a 2D material, is very suitable for planar

patterning, e.g. using lithography.

The second part of the work presents the analysis of the effects of plasma

pre-treatments of the substrate on graphene growth to investigate the

feasibility of patterned growth of nanocrystalline graphene. One part of the

sample was exposed to the plasma treatment, while the other was covered

with a photoresist. After the treatment, graphene growth was performed and

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measurements of conductance were made in order to see the contrast

between the treated and pristine area, if any.

Unfortunately, after pre-treatments in Argon, Oxygen, Nitrogen, Hydrogen

and Krypton plasmas, no relevant effect of growth suppression or

enhancement has been observed.

A specific related feature of graphene, which can be used for its patterning,

is a very high sensitivity of conductance to radiation damage. It has been

shown in a number of publications [14-20], that practically any irradiation

(even with electrons of energy as low as 10 keV) damages graphene and

reduces its conductance. Upon achieving a critical concentration of defects

of 1%, the irradiated graphene becomes actually insulating [21-24].

Although the electron irradiation has been shown to change structural and

electronic properties of graphene [25-31], its efficiency is limited. The ion

irradiation is a more effective technique in many ways. First, the ion

irradiation is much more efficient in the energy transfer from the energetic

ions to the graphene atoms and, consequently, in defect production. Second,

due to the processes of the secondary irradiation by the recoil atoms

generated in graphene and in the underlying substrate, the spectrum of the

structural defects produced by ion irradiation is much broader than that

produced by electrons. Third, the ion irradiation offers unique opportunity

for impurity doping (ion implantation). This impurity doping can be

achieved both by incorporation of atoms from the primary ion beam and via

backscattered atoms from the substrate. Fourth, the ion irradiation is the

most precise technique of deterministic 2D and 3D modification of

materials at a level down to a few nanometers and the method of addressing

single elements in nanostructures. This is the object of the third part of this

work, in which ion irradiation were performed via FIB on nanocrystalline

graphene layers, grown on quartz and sapphire.

The first effect we observed and analyzed is the reduction of conductance

due to irradiation with Ga+ ions via FIB.

This effect offers an opportunity of achieving sharp contrast conductor-

insulator without physical removal of the graphene layer. If the ion

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irradiation is performed with focused ion beam (FIB), the transition between

the conductive and non-conductive areas can be made as sharp as 1 nm.

This opportunity is attracting attention as a way towards the development of

a technology of maskless, resist-free patterning and as an approach to

fabricate novel graphene-based electronic nanodevices, like polarity-

reversible FETs [32] with on-off current ratio by far exceeding that of

conventional graphene FETs [33]. The development of a resist-free

technology of patterning could also resolve the issue of resist residues on

the surface of graphene, which considerably affect its transport properties

[34-36].

Another useful effect of ion irradiation we observed in this work is the

improvement of adhesion of thin films to the underlying substrates. The

effect of the irradiation-enhanced adhesion has been studied for decades and

is well known in material science, chemistry, biology, and medicine [37,

38]. Although the reliable adhesion of graphene to the working substrate is

an important technological property, the influence of irradiation and other

treatments with energetic particles (e.g. plasma) on adhesion of graphene

has not been studied systematically yet [15, 39].

Along with irradiation-induced suppression of conductivity and irradiation-

induced adhesion, the irradiation-induced enhancement of nucleation of

graphene on substrate is another effect we analyzed in the present work.

This effect may be used for patterning, even though its study is at the very

beginning. The only relevant publication we could find is [40], in which the

enhanced growth of graphene on SiC was demonstrated using FIB

irradiation with Si+ ions.

Below we show that all the effects of ion irradiation on graphene just

mentioned can be used for patterning nanocrystalline graphene as well.

Besides, we have found two new effects, specifically the restoration of

conductance after high temperature annealing and the evaporation at high

temperatures, which can also be used for patterning.

 

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Chapter 2 Theory

2.1 Graphene

Nobel prize in physics 2010, devoted to graphene, has collected a lot of

attention in the world of scientific research since its discovery. The

possibility of research, discoveries and applications is becoming broader

every year.

The reason for this attention is due to its very particular, some say

miraculous, properties. It is the thinnest known material in the universe and

the strongest ever measured. Its charge carriers exhibit giant intrinsic

mobility, have zero effective mass, and can travel for micrometers without

scattering at room temperature. Graphene can sustain current densities six

orders of magnitude higher than that of copper, shows record thermal

conductivity and stiffness, is impermeable to gases, and reconciles such

conflicting qualities as brittleness and ductility [1].

It is also a very interesting material, since it is actually not so new. In fact,

graphene has been hidden behind the pencil trace since it was invented in

1656 in England. Ludwig Wittgenstein once commented that ‘the aspects of

things that are most important to us are hidden because of their simplicity

and familiarity’ [7].

Indeed, what we call graphene is simply a monoatomic layer of the much

more known material, graphite. It is the basic structural element of other

carbon allotropes, not only graphite but also carbon

nanotubes and fullerenes.

Picture  1  Carbon  allotropes:  graphite,  fullerene  and  carbon  nanotubes  

10    

It is a monolayer, honeycomb lattice of carbon atoms.

Each atom has four bonds, one σ bond with each of its three neighbors and

one π-bond that is oriented in the z-direction (out of the plane). The atoms

are about 1.42 Å apart.

One can visualize the π orbital as a pair of symmetric lobes oriented along

the z-axis and centered on the nucleus. Each atom has one of these π-bonds,

which are then hybridized together to form what are referred to as the π-

band and π*-bands. These bands are responsible for most of the peculiar

electronic properties of graphene.

The strong σ bounds instead are responsible for the amazing strength of this

material.

The hexagonal lattice of graphene can be regarded as two interleaving

triangular lattice (Picture 2). The atoms A and B in the picture, are actually

exactly the same, but the honeycomb structure is not a good basis to form a

Bravais lattice. With the identification of A and B atoms instead it is easy to

have a basis that can give a Bravais lattice.

 Picture  3  Unit  cell  in  the  cristalline  lattice  and  first  Brillouin  zone  

Picture  2  Graphene’s  Triangular  sublattice  and  graphene’s  bonds,  sigma  and  pi

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2.1.1 Electronic properties

Carbon is a chemical element of the group IV of the periodic table. It has

atomic number 6 and an electronic configuration 1s22s22p2. It is a non

metallic element and has four valence electrons, since the ones in orbital 1s

do not form bonds. Two are in the 2s subshell and two in the 2p subshell

when it is in the ground state.

Hybrid orbitals sp are formed when carbon is forming bonds with other

carbon atoms.

It also promotes one of its 2s electrons into its empty 2p orbital.

Three different types of sp orbitals can form, sp, sp2 and sp3 depending on

the number of s and p orbitals involved.

Carbon atoms with sp2 and sp3 hybrid orbitals are able to form three and

four bonds with neighboring carbon atoms, respectively, which form the

bases of graphene and diamond [6].

Looking at graphene band structure, the linear dispersion between energy

and momentum is evident (Picture 4). Because of this property, graphene

exhibits electronic properties for a two-dimensional (2D) gas of charged

particles described by the relativistic Dirac equation, rather than the non-

relativistic Schrodinger equation with an effective mass, and so the carriers

behave like particles with zero mass and an effective ‘speed of light’ of

around 106 m s–1 [5].

 Picture  4  Graphene's  band  structure,  taken  from  [7]  

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The electronic structure of single layer graphene (SLG) can be described

using a tight-binding Hamiltonian. Because the bonding and anti-bonding σ-

bands are well separated in energy (>10 eV at the Brillouin zone centre Г),

they can be neglected in semi-empirical calculations, retaining only the two

remaining π-bands. The electronic wavefunctions from different atoms on

the hexagonal lattice overlap. However, any such overlap between the pz(π)

and the s or px and py orbitals is strictly zero by symmetry. Consequently,

the pz electrons, which form the π-bonds, can be treated independently from

the other valence electrons. Within this π-band approximation it is easy to

describe the electronic spectrum of the total Hamiltonian and to obtain the

dispersion relations

E(kx, ky) restricted to first nearest-neighbour interactions only:

!±(!!,!!) = ±!! !+ ! !"# !!!!!

!"# !!!!+ ! !"# !!!

!

! (1)

where a = √3 acc (with acc = 1.42 A being the carbon–carbon distance) and

γ0 is the transfer integral between first-neighbour π-orbitals (typical values

for γ0 are 2.9–3.1 eV). The k = (kx, ky) vectors in the first Brillouin zone

constitute the ensemble of available electronic momenta.

With one pz electron per atom in the π–π* model (the three other s, px, py

electrons fill the low-lying σ-band), the (–) band (negative energy branch) in

equation (1) is fully occupied, whereas the (+) branch is totally empty.

These occupied and unoccupied bands touch at the K points. The Fermi

level EF is the zero-energy reference, and the Fermi surface is defined by K

and K′.

Expanding equation (1) at K(K′) yields the linear π- and π*-bands for Dirac

fermions:

!± к = ±ħ!! к (2)

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where κ = k – K and νF is the electronic group velocity, which is given by νF

= √3 γ0a/(2ħ) ≈ 106 m s–1.

The linear dispersion given by equation (2) is the solution to the following

effective Hamiltonian at the K(K′) point H = �±ħνF (σ • κ), where κ = –i�

and σ are the pseudo-spin Pauli matrices operating in the space of the

electron amplitude on the A–B sublattices of graphene [5].

2.1.2 Optical properties

Graphene has shown also amazing optical properties, which combined with

its mechanical one make it a promising material for optoelectronic devices.

The linear dispersion of the Dirac electrons makes broadband applications

possible.

The optical image contrast can be used to identify graphene on top of a

Si/SiO2 substrate. This is possible because of interference between the

different layers, and with SiO2 acting as a spacer. The effect depends on the

number of layers.

Starting from the Fresnel equations in the thin-film limit is possible to

derive the transmittance of a freestanding SLG (single layer graphene) :

! = 1+ 0.5!" !! ≈ 1−  !"   ≈ 97.7% (3)

With α ≈ 1/137 the fine-structure constant. The high transmittance of

graphene is remarkable, as much as its low reflectivity, since it only reflects

<0.1% of the incident light in the visible region. Many layers stuck together

have a higher reflectivity, around 2% per layer. Because of this, we can take

the optical absorption of graphene layers to be proportional to the number of

layers.

We can take each layer absorbing A ≈ 1 – T ≈ πα ≈ 2.3% over the visible

spectrum.

14    

While in a few-layer graphene (FLG) sample, each sheet can be seen as a

2D electron gas with little perturbation from the adjacent layers, making it

optically equivalent to a superposition of almost non-interacting SLG.

The absorption spectrum of SLG is quite flat from 300 to 2,500 nm with a

peak in the ultraviolet region (~270 nm), due to the exciton-shifted van

Hove singularity in the graphene density of states [5].

One of the biggest issues for graphene researchers is the fact that graphene

has no “band gap,” meaning that its conductive ability can’t be switched “on

and off” like that of silicon [3].

Chemical doping of graphene is one way to get around this problem,

together with producing bilayared graphene with a different symmetry,

changing the lattice orientation of the two layers.

One more option is to grow carbon nanotubes along different orientations of

the lattice, which brings as well to different bandgaps depending on the

chosen direction of growth.

2.1.3 Other Properties

Many works investigated other properties of graphene, beside the electronic

and optical ones, with equally surprising results, as very well reported in a

review by A.K. Geim [1].

The first ones to mention are graphene mechanical and thermal properties.

Graphene has a breaking strength of ~40 N/m, reaching the theoretical limit.

It has a room-temperature thermal conductivity of ~5000 Wm–1 K–1 and a

Young modulus of ~1.0 TPa.

Neither the melting temperature nor the order of the phase transition is

known.

The one-atom-thick foil of graphene showed also to be impermeable to

gases, including helium, opening the possibility to use it also for bio-

chemical applications as a filter. It also showed a high sensitivity to its

chemical environment, which suggests one more possible application as a

chemical sensor, for lab-on-chip technology.

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Only a few of the other possible applications of graphene are: touch screens,

photovoltaic solar cells, graphene-ink for ink jet printing, gas diffusion

barrier, strain sensor, photo detectors, light emitting devices, ultrafast

tunable wavelength lasers, and many more keep being discovered and

studied by researchers.

With such good properties, one would expect an exorbitant price for such a

material.

Instead, one of graphene most exciting features is its cost.

Every few months, researchers develop new, cheaper methods of mass-

producing graphene and experts predict prices to eventually reach as low as

$7 per pound for the material . The thinnest, strongest material in the

universe may be closer to commercial applications than initially imagined

[3].

2.1.4 Methods of Synthesis

We mention here only some of the possible methods for the production of

graphene, and will go in depth only of the one that was used in this work.

The first method used to produce graphene, the one that the winners of

Nobel Prize in 2010 for its discover used, is mechanical exfoliation. It

consists basically in peeling off of a bulk of graphite the thinnest possible

layers, with adhesive tape. Other techniques are micromechanical cleavage,

carbon segregation from metal or SiC, deposition from Hydrocarbon gas on

metal, liquid phase exfoliation (graphite is put in a solution and exfoliation

happens applying ultrasounds to the solution) and chemical synthesis on

substrate , otherwise called CVD, chemical vapor deposition.

This is the technique that was used in this work.

 

 

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2.2 CVD

2.2.1 Process of growth

Reactant gases are introduced in the chamber, and chemical reactions occur

on wafer surface leading to the deposition of a solid film.

The process starts introducing reactive gases into the chamber.

The gases are activated or decomposed by a source of energy which can

either be heat or plasma. The gas is adsorbed by the substrate surface. The

reaction takes place on the surface, leading to the deposition of the film on

top of the surface substrate.

The chemical reaction is exactly what makes the growth happen, exploiting

the energy provided to lead the molecules available to be deposited to

arrange on top of the sample surface forming a film.

The reaction can also produce byproducts, which are transported away from

the substrate together with the exhaust gases.

CVD is a very widely used technique, deposited films ranging from metals

to semiconductors to insulators. It is one of the premier techniques for

epitaxial growth of thin layer structures (semiconductors, oxides,

superconductors).

It also has a wide application for devices such as Lasers, LEDs, solar cells,

photo-detectors, HBTs, and FETs.

In each CVD procedure, a specific chemical reaction has to be studied, and

different parameters are involved. Not any chemical reaction on any type of

substrate will lead to an efficient deposition.

The reaction that was used for this deposition is thermal dissociation of

methane, and consequent deposition of nanocystalline graphene on quartz

and sapphire.

The details of the chemical reaction and the features of the resulting

graphene films were characterized in a previous work in the same

laboratory. The next section briefly summarizes the main results.

17    

The data collected suggest that these films are nano-polycrystalline with the

crystallite size varying from 10 to 30 nm. The thinnest of these films have

thickness in the range of 1 nm, they are as transparent as 1–2 layer graphene

and their conductance is about 5x10-5 siemens. The charge carriers in these

films are holes and the hole mobility is a few tens of cm2/Vs [11].

2.2.2 Types of CVD

Chemical vapor deposition uses a precursor of the material to be deposited,

which can be from gas, volatile liquid or solid.

When CVD is performed from solid the target of material to deposit is

sputtered through a plasma.

The type of CVD that we used instead uses a gaseous precursor, which is

decomposed via heating.

The reaction can either happen in atmospheric pressure or at low pressure.

The atmospheric pressure one is cheaper, easier to realize and faster, but

produces less quality films with a worse coverage of the surface.

The low pressure CVD (gas pressure of a few torr) is slower, and requires a

good control of the temperature in the chamber, but films turn out of a much

better quality and a good coverage of the substrate is achieved.

In some types of CVD, the material can be dissociated via plasma or

photochemical CVD.

As mentioned above, the source of energy that allows the reaction to happen

in our case is heat. In most cases, as in ours, the heat was produced by a hot

filament, within or around the chamber.

It is possible to have the walls of the chamber heated up at the same

temperature of the substrate (hot wall reactors) or have the walls kept at a

lower temperature than the substrate (cold wall reactors).

In this work the CVD performed was in low pressure and with hot walls

reactor.

In our furnace a hot filament was running through the walls heating up all

the different areas uniformly.

18    

The main pro of cold wall reactors is that deposition does not happen from

the walls of the chamber, which are not heated up, but only from the

precursor material.

This problem was avoided in our chamber since each part of it is graphite-

made, in this way any contamination from the walls of the chamber is

avoided. The pro of the hot wall reactor is that no gradient of temperature is

present, avoiding inconvenient convections in the chamber which can lead

to non uniform films.

2.2.3 Pros and Cons

Here we list briefly the pros and cons of this technique of growth.

Main advantages are the following ones: i) high growth rates are possible,

with a good reproducibility; ii) with this technique it is possible to deposit

materials which are normally hard to evaporate, through sputtering

performed with a plasma; iii) CVD allows to grow epitaxial films, and in

this case is also termed as “vapor phase epitaxy (VPE)”. For instance one

can grow also organic films with MOCVD (metal-organic CVD) which is

also called OMVPE (organo-metallic VPE); iv) it gives generally better film

quality, and a more conformal step coverage.

A few drawbacks have to be noticed: i) high process temperatures are

required; ii) it is a complex process, sometimes requiring toxic and

corrosive gasses; iii) exactly because of such gasses, which can be used as

catalysts, the films grown may not be pure (hydrogen incorporation…) [41].

2.3 Ion irradiation, implantation and sputtering

The basic process consists in the introduction of atoms in a solid phase,

through the ionization of a certain atomic species and its acceleration with

enough energy to cause the penetration in the solid.

Depending on the energy of the beam and dose of the atomic species the

damages produced in the solid are of very different entity.

19    

This is because the ions will penetrate at different depths, depending on the

energy of the beam, and will cause collisions of different entity.

It is a very convenient method overall because it allows to control the

number of ions, the position and depth of the implantation.

2.3.1 Implantation

The implanter used in this work was a FIB, Focused Ion Beam, through

which the ion implantation was performed. FIB is described in detail in the

next chapter.

A very important parameter to characterize the implantation is the dose. It

expresses the number of incident ions in a unitary surface, and it indicates

the concentration of ions introduced.

Its unit can be expressed in atoms/µm2. Sometimes the level of doping can

also be expressed in number of impurities/volume (atoms/µm3) or in

Coulombs/µm2.

! = !"#$!!! ~

!"#$%!"! ~ !"

!!! ~!!"#$×!!×!

(4)

With the assumption that all ions within the beam have the same electrical

charge, the number of ions is proportional to the beam current integrated

over the time of exposure to the ion beam.

Typically the implanted ions are O2, N2, B, P, As, Sb at a typical density

between 1015-1017 atoms/cm2. These species though are very rarely used in

FIB systems, and more common for other implantation systems. The most

common specie used in FIB is Gallium.

2.3.2 Implantation profile

One very important and useful function to calculate is the implantation

profile.

Normally the crystal is considered to be isotropic and that no channeling is

happening. We will see later how this is not the case in our work, but this

20    

function is still very important to estimate and still gives a good

approximation of the phenomenon studied.

The phenomenon of channeling is observed in ions moving in certain

directions in a crystalline material, where there are long range open spaces

through which the ions can travel without significant scattering. The

presence of these open spaces depends on the symmetry and crystalline

structure of the crystal.

During implantation, the high-energy-ion penetrating in the substrate

undergoes a series of stochastic collisions which deviate its trajectory, force

it to decelerate and in the end stop it.

 Picture  5  Monte  Carlo  calculation  of  128  ion  trajectories  for  50  keV  boron  implanted  into  Silicon,  

taken  from  [41]

During the implantation the ion can produce two types of collisions: ion-

electron collision and ion-nucleus collision.

In the ion-nucleus collisions the interaction is dominated by the Coulombian

repulsion between the ion and the nucleus of the crystalline specimen. This

type of collisions causes abrupt deviations and decelerates the ions. These

collisions are the main reason for the displacement of the nuclei of the

21    

crystal, thus the main cause of disorder and irradiation damage in the

crystal.

The collisions between ions and electrons only slow down the ions and

make them deflect slightly. The interaction is between the ions and the

electrons around the nuclei and the free conduction electrons on the

substrate. This type of interaction does not produce any crystal damage.

We introduce now some parameters to calculate the implantation profile.

SN and Se are the nuclear stopping power and the electronic stopping power.

The nuclear stopping power depends on the atomic mass and atomic number

of the incident ions and the atoms of the substrate.

!! ∝  !!!!

!!! !!!!

! !

!!!!!!!!!

(5)

Where Zi and Mi are the atomic number and the atomic mass of the incident

ion (i=1) and of the substrate atom (i=2).

The electronic stopping power depends on the square root of the ion energy:

!! ∝ ! (6)

The implantation profile N(x) is Gaussian, because of the randomness of the

collisions.

! !! = !!!∆!!

(7)

Rp is the average ionic penetration. The Gaussian has a standard deviation of

∆Rp.

Q is the quantity of ions introduced over a unitary area of the substrate.

The following equation is useful to calculate the average total distance that

the ions can travel in the bulk, before completely losing their energy and

stopping:

22    

! = !"!! = !"

!! ! !!! !!!! (8)

An example of the Gaussian profile and its dependence on the beam energy

is shown in the graph:

 Picture  6  Boron  implanted  atom  distributions,  comparing  measured  data  points  with  Gaussian  

fitted  distributions,  taken  from  [41]

The depth profiling of a Gaussian is centered at a certain distance from the

surface, typically fractions of micrometers inside the sample.

2.3.3 Damage

The ionic damage is provoked by ions over a certain dose and energy, as it

was also observed in this work.

The damage is caused by the collisions of ions that cause a displacement of

the atoms from their positions of equilibrium in the lattice.

High energy impacts produce a limited damage in the first superficial layers:

the thermalization of the incident ions happens mainly when the ion energy

is low and consequently the damage is localized close to the maximum of

the implantation N(x).

Regarding the recoil of the atoms, the number of substrate atoms pushed

back by the impacts grows with growing energy of the incident ions.

23    

Usually, damage caused by ion implantation includes:

1) formation of crystal defects such as Frenkel defects, vacancies, di-

vacancies, higher-order vacancies, and interstitials;

2) creation of local amorphous regions included in the crystalline structure;

3) formation of continuous amorphous layers as the localized amorphous

regions grow and overlap.

Damage types 1 and 2 are categorized together as 'primary crystalline

damage'.

Normally the irradiation is followed by an annealing in order to eliminate

the damage and activate the dopant making it diffuse, if needed.

2.3.4 Annealing

Annealing is a process where the wafer is heated to repair the damage of the

lattice.

It has also the effect to activate the ions implanted as dopants, if they can

interact with the substrate and create new energy levels, and has also the

function of inducing dopant diffusion into the implanted target.

This second function of annealing was not applicable in our work, since

Ga+ ions were implanted on graphene, and these two materials do not

interact, Gallium is an inert material in graphene, electronically and

optically. This is why we focused more on the first function of annealing,

which is restoration of the broken bonds and of the pristine atomic structure

of graphene.

Primary crystalline damage annealing basically consists of:

1) recombination of vacancies and self-interstitials in the low temperature

range (up to 500 °C);

2) formation of dislocations at 500-600 °C which can capture impurity

atoms;

3) dissolution of these dislocations at 900-1000 °C.

24    

Annealing of the continuous amorphous layers that extend to the surface has

been shown to occur by solid-phase epitaxy between 500-600 °C. Under this

phenomenon, the crystalline substrate beneath the amorphous layers initiates

the recrystallization of the amorphous layers, with the regrowth proceeding

towards the substrate surface. Factors affecting the recrystallization rate

include crystal orientation and the implanted impurities. Amorphous layers

that do not extend to the surface anneal differently, with the solid-phase

epitaxy occurring at both amorphous-crystal interfaces and the regrowth

interfaces meeting below the surface.

One more phenomenon that for sure cannot be restored through annealing is

the sputtering of the surface.

2.3.5 Sputtering

The picture below shows the main events that occur during ion irradiation of

a substrate (Picture 7).

The primary beam consists of Gallium ions, that are implanted in the target

generating defects and creating disorder in the crystalline structure.

When the energy of the incident ion is high enough it can happen that it hits

an atom of the specimen so hard to make it exit from the bulk.

When this phenomenon happens it leads to the corrosion of the bulk,

sputtering away considerable quantity of the material exposed to the beam.

25    

 Picture  7  Example  of  the  events  occurring  during  ion  implantation,  taken  from  [42]

 

Sputtering is a process whereby atoms are ejected from a solid target

material due to bombardment of the target by energetic particles. It only

happens when the kinetic energy of the incoming particles is much higher

than conventional thermal energies (� 1 eV). This process can lead, during

prolonged ion or plasma bombardment of a material, to significant

corrosion, and can thus be harmful. On the other hand, it is commonly

utilized for thin-film deposition, etching and analytical techniques.

 Picture  8  Example  of  sputtering  of  the  target  by  ion  beam,  taken  from  [42]

A good parameter in the study of sputtering is the etching rate.

26    

Three different “Etching Rate” definitions are encountered in FIB literature:

!"#$%&  !"#  !"#  ~  !!!

!"

!"#$%&  !"#  !"#$  !"  !"#$  ~  !!!

!

!"#$ℎ  !"#  !"#$  !"  !"#$  ~  !"!

The sputtering yield is given by ! = !"#$%"&  !"#$%!"#$%&"'  !"#$

= !"!!

(9).

It depends also on the angle of incidence.

Depth – Dose Relationship:

As a first-approximation depth of FIB etching is directly proportional to the

“Dose” of ion beam exposure. This approximation holds very well if:

- Actual depth is less then x3 of the narrowest dimension of the milled

area;

- Etching is done at 90° incident angle;

- Etching is done without any reactive gas

The phenomenon of channeling was mentioned in the previous paragraph. It

normally is omitted when calculating the implantation profile, but it actually

is relevant in our case.

The effects of channeling are well presented in Picture 9, which gives an

example of how changing simply the orientation of the same crystalline

structure the sputtering yield can change by far.

27    

 Picture  9  Dependece  of  Yield  on  angles  of  orientation,  effect  of  channeling,  taken  from  [42]

In our work in particular, channeling has a role since graphene on our

samples is of nanocrystalline type.

This means that the graphene layers consist mainly of crystalline grains.

Because of this, graphene layers do not have in general the same crystalline

orientation, and in this sense each and every grain has a different

orientation.

Because of this, it is impossible to evaluate in which direction the ion

channeling can happen, in order to avoid it, since in every crystal grain it

will be different.

On the other hand this fact can be convenient, since in order to have a full

channeling all the crystals should be oriented in the same way along the ion

trajectory, which is concretely almost impossible for the presence of the

grains.

Picture  10  Effect  of  channeling  on  different  crystal  grains  taken  from  [42]

28    

Another effect that was mentioned in the previous paragraph, as a

consequence of ion irradiation at high energy is complete amorphization of

parts of the sample.

The secondary electron yield and the atoms sputtering yield depend as well

on the orientation of the sample and on the energy of the beam, which

influences the implantation profile, and thus the area of the sample that is

interested in the ion irradiation.

This is well shown in Picture 12.

 Picture   12   Example   of   dependence   of   secondary   electron   yield   and   sputtering   yield   on   the  orientation  of  the  crystal,  taken  from  [42]

One more effect that can happen during the irradiation is the sample

charging.

Picture  11  Example  of  amorphization  of  the  sample  with  FIB  irradiating  the  sample  from  the  right,  taken  from  [42]  

29    

The impact of ions produces charged particles. Some of them leave the

sample, while some of them do not have enough energy to exit and are

trapped in the sample, charging it and generating a current. This follows

Kirchoff’s Law, which applied for FIB is:

!!"#$ + !!"#$%&'()  !"!#$%&'( −  !!"#$%&'()  !"#$ =   !!"#$%& > 0 (10)

Due to impossibility of balancing current of the primary ion beam by

currents of secondary charged particles,, any un-grounded sample subjected

to FIB irradiation will accumulate positive charge. Under continuous ion

beam irradiation, the charge will keep accumulate until electric breakdown

creates a discharge path.

It is possible to solve this problem by charge neutralization, with an electron

gun dispensing electrons to compensate the positive charge of the

backscattered ions, or with biases placed around the chamber (negative bias

where positive ions will be sputtered back and bias applied to the sample).

Sometimes it is possible with certain instruments (with FIB for example) to

perform gas-assisted etching when the beam itself is not able to provide the

energy and depth necessary for the requested etching characterization of the

sample. This was not the case in our work.

2.3.6 Pros and Cons

Some of the advantages of this technique are the possibility to introduce any

type of impurity/ions in different substrates and to control accurately the

quantity of doping introduced together with control of the speed of the ions

and the depth of penetration.

It is possible to set the thickness of the irradiated layer and independently to

set also the implantation dose.

It gives a good uniformity and reproducibility of the operation.

30    

This procedure allows us to set and control the implantation profile setting

the energy of the beam, and also to use photoresists in order to shield the

atoms in parts of the sample.

The operation can be performed at room temperature.

The disadvantages consist in the fact that it is hard to control the quality of

the implantation. The procedure causes damage of the crystalline structure

which has to be recovered with annealing, and it is not always possible to

fully recover.

When the irradiation is performed with masks it is possible that some ions

will pass through the mask and be implanted. What also needs to be

considered is the fact that the equipment is complicated and expensive.

“At present disorder at the atomic level produced by ion implantation in

Silicon is not understood in detail. Electron diffraction studies agree with

the interpretation above that ion implantation drives the silicon lattice

toward an amorphous-like structure, however the details of this transition

are difficult to ascertain.”

This is taken from the book Silicon VLSI Technology by Plummer, Deal

and Griffin, [41], which studies in detail the case of Silicon, but its

conclusions can be reasonably extended also to other materials, at least in

their general assertion.

31    

Chapter 3 Equipment 3.1 Furnace

The main parts of the furnace are the heater, the sample holder, the

electrodes, the external vacuum chamber, the cooling system and the inlets

for gas and the pressure gauge.

The interior of the furnace is made of graphite (heater, sample holder and

electrodes), in order to avoid contaminations by other materials during CVD

growth. The external vacuum chamber is made of stainless steel.

The furnace is connected to a vacuum system, which keeps it in the pressure

range of a few millibars.

From a gas cylinder the precursor gases for the growth are injected in the

chamber, through a pipe connected to it, controlled by a valve.

Picture  13  Graphite  and  stainless  steel  furnace

32    

For this type of growth the gas is methane, but also hydrogen or other gases

can be introduced in the chamber to enhance or suppress the growth, or

clean the chamber.

The heating system is connected and powered by the Sorensen DC power

supply DCR 20-125.

Through applying a certain potential difference (from 0 to 2,5 V with an

error of 0,25 V) to the heater (a graphite filament) it produces a current

(measured in Ampere and displayed on the same Power Supply) with an

error of about 2,5 A. It is possible to control the voltage with coarse and fine

adjusters.

The chamber is also connected to a pressure gauge that allows to control the

pressure in the chamber.

Two different pressure gauges were used during the experiment: Omega

DPG 3500B-2000MBARA [pressure range 0-1000 mbar] and Adixen ACC-

2009 [pressure range 5E-9 to 1000 mbar]. Temperature was measured with

a temperature controller and wire thermocouple.

The display with which readings of vacuum and temperature were made had

four digits for the temperature, and an error of 1°C and three digits for

pressure with an error of 10-2 mbars. The thermocouples and gauge errors

were both lower.

3.2 Vacuum System

The vacuum system consists of two pumps, that work in series.

The first one is a rotary vacuum pump, Two stage Edwards E2M2.

The second one which works at a higher vacuum regime is a

Turbomolecular pump, CFF 450 Turbo Alcatel Adixen ACC 2009.

Rotative pumps are mechanical pumps which work in idrodinamic regime,

able to produce 1.3x10-2 - 1.3x10-3 mbar vacuum, starting from atmospheric

pressure.

33    

They basically consist of a rotor mounted on the same ax of the rotor of an

electronic motor.

The two rotors are at the center of a cylindrical chamber, in which the gas is

injected.

Two or four palettes are mounted on the rotor that firmly adhere to the

internal side of the chamber.

They have the function to “wipe” the volume of the chamber and compress

the air and push it toward the exhaust valve of the pump, that vents it out of

the chamber. All the parts are immersed in silicon oil that has the function

of lubricant and sealant between the rotor and the chamber stopping the

back diffusion of the pressurized gas toward the aspiration area.

The second pump used is a turbomolecular pump. It is a versatile type of

pump, since it can generate different degrees of vacuum, from intermediate

(~10-4 mbar) up to ultra-high vacuum (~10-10 mbar).

The physical principle it uses is furnishing moment to the gas molecules,

through rapidly spinning blades. The molecules in this way are pushed out

of the chamber, lowering the pressure of the gas.

What furnishes the momentum to the molecules is the repeated collisions

with the blades, that increases the speed of the molecules that are directed

from the inlet of the pump towards the exhaust in order to create or maintain

vacuum.

The gas molecules pass through different stages, all with the same working

principles, mounted in series. Once the gas passed from the first stage it is

directed toward the next one in which an higher vacuum is obtained by the

different size, shape and orientation of the blades. This process is continued,

finally leading the molecules of gas outwards through the exhaust.

Performance of a Turbo Molecular Pump (TMP) is strongly related to the

frequency of the rotor. As rpm increases, the rotor blades deflect more.

Turbomolecular pumps must operate at very high speeds, and the friction

heat buildup imposes design limitations. At atmospheric pressure, the mean

free path of air is about 70 nm. A turbomolecular pump can work only if

34    

those molecules hit by the moving blades reach the stationary blades before

colliding with other molecules on their way. To achieve that, the gap

between moving blades and stationary blades must be close to or less than

the mean free path.

3.3 FIB

FIB stands for focused ion beam. The basic mechanism of this machine is

very simple.

It has an ion source, from which ions are extracted, focused through the

main column by lenses into a concentrated beam, to hit a target.

In the beam the energy of the ions, the sharpness of the beam and density of

ions can be calibrated, together with many more features.

The beam is directed toward the sample on which the ions are meant to

impact.

The area of the sample irradiated the beam can be changed by deflecting the

beam by octopole electrodes present in the column. Size of the area of

Picture  14  Elements  of  primary  column  in  an  ion  beam,  taken  from  [42]  

35    

irradiation can be set as well by changing amplitude of deflection.

Magnitude of ion beam current is defined by changing excitation of the

condenser lens and measured at the blanking aperture.

A detector using the secondary electrons produced by ion beam interaction

with the sample allows to obtain images of the area where the beam is

rastering on the sample.

In the images are shown the main components of the ion column (Picture

14), and the block diagram of the whole system (Picture 15).

The sample, together with the sample holder and the whole system is kept in

a high vacuum (10-7 torr).

Different vacuum pumps and controllers are involved. In particular the

specimen holder can be pulled out in order to introduce the sample, and

inserted back in. Sample exchange is the main reason for different chambers

to exist, at a different vacuum levels. Multiple-chamber configuration

allows to keep the beam and the rest of the system under high vacuum while

the sample is introduced into the chamber, from atmospheric pressure.

Picture  15  Block  diagram  of  Micrion  2500  FIB,  taken  from  manual  

36    

The system of vacuum pumps and gauges, the ion beam column, the

detector and the imaging system are controlled by a computer, from which

is possible to set all the different parameters and

control the irradiation.

The source of Gallium Ions is a liquid metal.

The metal is placed on top of a tungsten needle.

The needle is heated by a tungsten wire coil that

runs around the needle. In order to heat up the

coil a current runs through it, and this current can

be easily regulated.

The heat melts the metal drop on the needle, and

the ions are attracted from the source by high-

strength electric field of the extractor electrode. Since the metal has to be

melted normally a material with a low melting point is chosen.

In order to extract the ions a potential difference is applied between the

electrodes, one of them is the needle itself and the other is a metal plate

toward which the ions are attracted. This second electrode is called the

extractor electrode. Potential applied between the needle and extractor

generates en electric field that influences the shape of the drop of metal on

the needle. In the picture is depicted the difference of the shape of the liquid

Ga surface at the tip of the needle depending on the applied field (picture

17).

 Picture  16  Gallium  ion  source,  taken  from  [42]  

37    

 

Picture  17  Dependence  of  shape  of  liquid  Ga  on  the  applied  field,  taken  form  [42]  

Through all the column of the FIB there are electrostatic lenses and

blanking plates with the function of focusing and directing the beam of ions.

The parameters that can be set for the beam are many, in particular to

calibrate the number of ions one can change both the final aperture and

potential of the condenser lens to define the ion beam current. The aperture

changes the size of the beam and potential of the condenser lens changes the

current the density of ions in the beam. Two beams with same aperture can

have very different number of ions, depending on the potential applied to

the condenser lens. Both the beam current and the final aperture can be set

from the computer that controls the FIB system.

The octopole has the function of correcting astigmatism by shaping the ion

beam. Shape correction is achieved by applying same polarity potentials to

the opposing plates of the octopole, thus either compressing with positive

potential or expanding it with negative potential. Such shape correction

removes ellipticity from the ion beam, thus correcting astigmatism.

The deflector plates have the role to change the direction of the beam, and

use as well an electric field of different polarity applied to opposite plates of

38    

octopole, thus pushing ion beam away from the positive plate and attracting

it to the negative.

The sample can also be tilted, so that implantation of ions can happen also at

an angle different from the normal.

As mentioned, the whole system is kept in vacuum. The specimen is

introduced in a secondary chamber, placed on its holder and the pushed

forward, under the beam. From here, once the sample gets on the stage, it

can be moved in all directions and tilted.

The computer controls the position of the stage, so that it’s possible to

calibrate exactly its movements. In this way orientation on the sample is

possible also without using the function of imaging, available on Micrion

2500 FIB only through primary ions. The impact of ions during the

observation of the sample can damage the surface, so normally when

possible it was avoided during our experiment.    

In most FIBs though a second column of electrons is present, specifically

with the function of imaging (Picture 18). The system of imaging is based

on secondary electrons (it is so even when the imaging happens through

bombardment of the sample by ions, but only the electrons are detected

from the detector and used to depict the image). The principle is the same

one used in SEM microscopes.

Picture  18  SEM  layout  and  function,  taken  from  [42]  

39    

Also a system to neutralize current in order to avoid damaging of the sample

when it’s overcharged is present in the FIB system. There are two main

ways to neutralize the current that runs through the sample. One of them is

through an electron gun that injects electrons directly on the sample (since

as explained in chapter 2 the current on the sample is always positive, as a

consequence of Kirchoff’s law), or through applying a bias to the sample

[42].

3.4 Low Pressure Plasma System

The low pressure plasma system that was used in order to treat with plasma

the samples, is produced by Diener electronic. It is the PICO plasma-

surface-technology model, with vacuum pump trivac D2,5E.

This machine can be used for different purposes, like cleaning of surfaces,

activation of surfaces, etching of surfaces and deposition of surfaces-

plasmapolymerization. [Manual of the instrument]

As will be explained more in depth in chapter 5, in this work the plasma

treatments were performed to see if the growth of graphene could be

enhanced or suppressed through such treatments, in order to characterize the

surfaces, isolating the not treated areas covering them with a photoresist.

A plasma can be created by heating a gas or subjecting it to a strong

electromagnetic field applied with a laser or microwave generator. This

decreases or increases the number of electrons, creating ions, and is

accompanied by the dissociation of molecular bonds, if present.

There are several means for plasma

generation, however, one principle is

common to all of them: there must be

energy input to produce and sustain

it. For this case, plasma is generated

when an electrical current is Picture  19  Artificial  plasma  produced  in  air  by  a  Jacob's  Ladder

40    

applied across a dielectric gas or fluid (an electrically non-

conducting material) as can be seen in the image to the left, which shows

a discharge tube as a simple example (DC used for simplicity).

The potential difference and subsequent electric field pull the bound

electrons (negative) toward the anode (positive electrode) while

the cathode (negative electrode) pulls the nucleus. As the voltage increases,

the current stresses the material (by electric polarization) beyond

its dielectric limit (termed strength) into a stage of electrical breakdown,

marked by an electric spark, where the material transforms from being

an insulator into a conductor (as it becomes increasingly ionized). The

underlying process is the Townsend avalanche, where collisions between

electrons and neutral gas atoms create more ions and electrons. The first

impact of an electron on an atom results in one ion and two electrons.

Therefore, the number of charged particles increases rapidly (in the

millions) only "after about 20 successive sets of collisions", mainly due to a

small mean free path (average distance travelled between collisions).

This is also the way in which the plasma is produced by the generator used

in this work.

The chamber contains two metal plates, about 4 inches wide and 16 inches

long.

They are kept in a medium vacuum (between 0,1 to 10 mbars) and the

sample is placed on the underneath one, on top of an holder.

It is possible to connect the machine to two different gas cylinder.

Once the sample is placed inside the vacuum rotary pump is turned on and

once a vacuum between 0,2 and 0,4 mbar is reached the gas is let flow

inside, and the power is turned on, in order to create an electric field

between the plates that generates the plasma.

41    

3.5 Conductance measuring system

The conductance measuring system consists of a working table (Went

Worth Labs), with a stage in the center where the samples are placed in

order to perform electrical measurements on them (Picture 20)

Supports for the two tungsten needles are installed on the table.

The needles can be moved in all directions, be tilted, raised and lowered.

The stage as well can be set at different heights. This system gives a full

mobility and allows to take measurements in a wide area of the stage and

with different needles distances.

 

Picture  20  Working  table,  with  needles  and  needle  holders  and  microscope  

On top of the stage is placed an optical Bausch&Lomb Microscope, which

allows a better view of the sample and of the contacts with the needles.

42    

The electrical conductance was measured using Keithley 2300

Semiconductor characterization system analyzer placing the tungsten

needles on the sample surface with identical force.

The distance between the needles tips touching graphene was about 20

microns. Most of the measurements were performed at a voltage of 1 V.

Through the computer is possible to set the voltage, start the measurements

and read on the monitor the level of conductance on the sample.

3.6 Microscopes

Microscope Olympus SZ30 was used for three dimensional observation of

the sample in order to verify the cleanness of the surface before deposition

and for quick and three dimensional observation of the samples in general. It

is a stereo microscope.

The stereo or stereoscopic or dissecting microscope is an optical

microscope variant designed for low magnification observation of a sample,

typically using light reflected from the surface of an object rather than

transmitted through it. The instrument uses two separated optical paths with

two objectives and eyepieces to provide slightly different viewing angles to

the left and right eyes. This arrangement produces a three-

dimensional visualization of the sample being examined.

The model Olympus SZ30 is developed primarily for mounting onto

bonders and probers, offers excellent cost performance. The combination of

a long working distance (110 mm) and large zoom range of 0.9X to 4X

means the user can expect superb optical performance and maneuverability.

It is provided with Auxiliary Objectives 0.3x, 0 4x, 0 5x, 0.75x. 1.5x, 2x.

[Manual of the instrument]

Optical microscope Nikon Eclipse LV100- Marrell is an Advanced

polarized light microscope that works under both diascopic and episcopic

illumination. This more precise instrument was used to make more detailed

43    

observation of the samples, in particular to identify the irradiated areas of

graphene. The lenses are 5X/0,15A 10X/0,30A 50X/0,80A 100X/0,90A,

all installed together on the microscope.

Nikon's Eclipse polarizing microscopes are renowned for their ability to

produce brighter, clearer, and higher contrast images. The LV100 POL,

available in diascopic and episcopic microscope illumination types,

continues this tradition and offers a completely reengineered base for even

easier operation. It also features an exclusive high-intensity halogen light

source, which delivers brighter images, lower power consumption and less

heat generation, thereby reducing the chance of heat-induced focus drift.

Only some of the key features of this instrument are: the diascopic

(transmitted) light source, lead and arsenic free objective lenses, reverse

type nosepiece, Bertrand Lens incorporated into design of intermediate tube,

reduced focus drift and many more [43].

 

44    

Chapter 4 Growth of nanocrystalline graphene 4.1 Experimental procedure for growth

The method used to grow graphene layers on quartz substrate is Chemical

Vapor deposition, CVD.

The substrates are single crystal quartz plates of size 4 mm×4 mm×0.5 mm

with the large surfaces cut perpendicular to the z-axis and polished to a

roughness of Ra < 1 nm (commercial product of MTI company).

The quartz is cleaned in acetone in ultrasonic bath for five minutes and,

when needed, wiped with cotton soaked in acetone to mechanically remove

dust.

After cleaning it, the sample is placed on the sample holder. As mentioned

before, both the sample holder and the furnace are all-graphite, in order to

avoid contaminations during the growth.

The furnace is opened and the holder is placed inside of it. The furnace is

closed and the pump (Rotary Two stage Edwards E2M2) is turned on, and

keeps working until the furnace reaches a vacuum of at least 10-2 mbar.

Pressure was measured with two gauges: Omega DPG 3500B-2000MBARA

and Adixen ACC-2009.

Methane is then inflated in the chamber and let flow through it for about 30

seconds, to make sure that no other gas except methane is present in the

chamber during the growth.

Then, the methane in the chamber is pumped out, down to the pressure

chosen for the growth (normally between 1 and 5 mbar). Through the

heating system (Sorensen DC power supply DCR 20-125) the temperature is

raised, up to the value chosen for the growth (normally between 900°C and

1200°C). In these conditions of pressure and temperature, methane

decomposition occurs and nanocrystalline graphene layers start to grow on

top of the substrate.

The temperature is kept constant during the growth. To stop the growth the

heating system is quickly turned off and the chamber cools down rapidly.

45    

When the temperature is low enough (around 200°C), air is inflated in the

chamber and the sample can be taken out and measured. To prevent the

damage of rubber parts of the vacuum system, these are constantly cooled

down by a water cooling system.

Electrical conductance of graphene films was measured at room temperature

using a Keithley 2300 analyzer and tungsten needles placed on the sample

surface with identical force. Since we had to perform hundreds of

measurements in different places of many samples, we practically could not

use a four probe method with deposition of metal electrodes. Instead, a

simple two probe method was used. The distance between the needle tips

touching the graphene surface was about 20 microns.

The apparatus is connected to a computer in which the KYTE software

allows us to set a bias between the needles and control the current passing

through them when they touch a conductive layer. Most of the

measurements were performed at a voltage of 1 V. This voltage was low

enough to avoid non-linear effects in the current flow. The level of

sensitivity of the measurements was in the range 0.01 to 0.1 pA. The quality

of the electrical contacts between the tungsten tips and the graphene layer

was not always good and that could cause considerable fluctuations of the

current. After having performed many measurements on different samples at

different voltages and at different conditions of placing probes on sample

surface, we estimated the experimental error of the measurements as an

order of magnitude. However, despite this seemingly high experimental

error, the discussion of the electrical measurements is not compromised

since the changes in conductance occurred over many orders of magnitude.

46    

4.2 Results

In order to better understand the growth process and which parameters play

a role in it, many growths were performed in different conditions.

The parameters that we focused on in particular are temperature, pressure

and time of growth. They most likely influence the growth of the films in

their quality, thickness and conductance. Other factors that can influence the

growth are the substrate surface roughness, substrate chemical composition

and surface termination. In each growth was possible to choose the

temperature of growth, the pressure of the methane in the chamber and the

time of growth, which determine the methane decomposition and the

deposition of graphene. Our study was performed to see if and how these

parameters influence the growth of graphene.

Growths were performed keeping two of the three parameters fixed and

changing the third one in order to observe the influence of one parameter at

a time.

In this way was also possible to find the conditions in which the growth is

more effective. In fact, judging by the transparency of the layers and the

conductance, it was possible to deduce when a higher quality graphene was

grown, and so record which were the best conditions of temperature,

pressure and time to effect the growth.

4.2.1 Temperature

The first parameter to be studied was the temperature.

In this case the growths were all performed with pressure in the chamber of

5mbar for 15 minutes.

The lowest temperature at which the growth was performed is 900°C. We

considered that a lower temperature did not make sense because the

standard temperature for methane decomposition is around 1200°C on

average. The highest temperature of growth was 1300°C. We did not

47    

performed growths at higher temperatures since the threshold was already

found and 1300°C is the average temperature of quartz degradation.

As mentioned before the way to detect a layer of graphene is by measuring

its conductance. The lowest values found are around 10-14S, which is

comparable with the sensitivity of the instrument. When the conductance

reaches a value of 10-6 S or over, it is the clear sign of the presence of

graphene layers.

The conductance was measured on both the top and the bottom side, since

from previous studies we had evidence that graphene was growing also on

the bottom side of the sample.

Between 900°C and 1300°C, a sharp step is observed. For the top side, the

conductance is in the range of 10-14 S in the growths at 900°C, 1000°C and

1050°C.

When the growth was performed at 1075°C, a conductance of almost

6x10-6 S was observed. From this temperature on, the conductance was

between 6x10-6 S up to 7,9x10-5 S.

On the bottom side of the sample, the conductance was around 10-14 S up to

1175°C, at which the conductance resulted to be 2,3x10-5 S.

These results show how the dependence of graphene’s growth on

temperature follows the trend of a sharp threshold. Under a certain

temperature no conductance is present. This could either be the sign of the

complete absence of graphene on the substrate, or islands of graphene could

be present, which are not connected yet and do not result in a high

conductance of the sample.

Over a certain temperature, conductance suddenly increases, revealing the

formation of the first complete layer of graphene. From that temperature on,

the conductance remains stable, slowly growing linearly with the increasing

of the temperature.

The same trend is observed also in the case of the bottom side of the sample,

but with a higher temperature threshold. This is probably because this side

of the sample is less exposed to the methane flow. In fact it is in contact

48    

with another piece of quartz or sapphire, to keep it clean and prevent the

direct contact with the graphite support.

In both cases the temperature dependence of the growth has the same trend.

There is a certain temperature at which the growth of graphene starts. This

is probably linked to the quantity of methane decomposed, and consequently

the number of C atoms available for the formation of graphene.

 

Picture  21  Temperature  dependence  of  growth  

4.2.2 Pressure

In order to study how pressure can influence the growth of graphene on

quartz, a series of growths were performed at constant temperature of

1200°C for 15 minutes, at different pressure values, as measured by the

gauge into the chamber, after it was filled with methane, and then pumped

out to the chosen pressure value. The pressure therefore is most likely the

value which indicates the quantity of methane present in the chamber. In

fact, pressure is not given exclusively by the presence of methane, because

little leaks are always possible as well as presence of other gases influencing

the pressure. However, it is a good parameter to indicate quite accurately the

relative quantity of methane from growth to growth.

49    

Picture 22 registers, also in the case of pressure dependence, a sharp step

behavior. In this case, the top and bottom side of the sample show the same

trend of conductance.

The growths performed at 0,5 mbar and 0,75 mbar did not show any

presence of graphene deposited. From 2 mbar up, instead, the conductance

was found to be in the range of 10-5-10-4 S. The sudden appearance of

conductance is associated with the presence of a layer of graphene. It is

possible to notice how the increase of the pressure, and so the quantity of

graphene, does not determine a major change in the conductance. We can

affirm that it stays in the same range from 2 to 10 mbar.

This is probably because once there is enough Methane, and consequently

Carbon in the chamber to have a continuous layer of graphene, an increase

in the quantity of Carbon does not increase the conductance of the layer,

since most likely simply more layers are formed, one on top of the other, all

more or less of the same quality and having the same conductance.

 

Picture  22  Pressure  dependence  of  growth  

50    

4.2.3 Time

The threshold as a function of the time was studied at three different

pressures: 3 mbar, 4 mbar and 5 mbar. In the case of 5 mbar, the threshold is

between 2 and 3 minutes for the top side and between 3 and 5 for the bottom

side. When the growth was performed with 4 mbar pressure, the observed

time threshold is between 2 and 3 minutes for both the top and the bottom

side. For the growth at 3 mbar the threshold in time is between 3 and 5

minutes for both sides. The temperature was kept stable at 1200°C in all

three cases.

In the time dependence measurements, quartz degradation is observed after

prolonged treatment at high temperatures. The degradation occurs in

particular on the bottom of the sample, which is likely at higher temperature

because of the contact with the graphite support. This phenomenon is

accompanied by the lack of detectable conductance in the samples,

specifically in the growths at 5 mbar for 30 minutes, at 4 mbar for 60

minutes and at 3 mbar after fifteen minutes.

5mbar:

 

Picture  23  Time  dependence  of  growht,  pressure  5  mbar  

51    

4mbar:

 

Picture  24  Time  dependence  of  growth,  4  mbar  

 

3mbar:

 

Picture  25  Time  dependence  of  growth,  3  mbar  

52    

4.2.4 Pulsed growth

Pulsed growths were also performed, in order to find out at which

temperature graphene starts to grow instantaneously.

The method consisted of a very quick growth, with growth time of about

one second.

Once the sample is placed inside the furnace, starting from room

temperature, the temperature is raised very quickly, increasing the voltage

applied to the heating wire by DC Sorensen Power Supply .

In about ten seconds, the temperature reached the chosen value (between

1200°C and 1600°C) and then voltage was promptly turned down. The

estimated time of reaction to turn off the voltage regulator and reaction of

the machine is estimated to be around one second, time during which the

growth occurs.

This procedure was used to study at which temperature graphene can grow

instantaneously, in order to better control the growth of nanocrystalline

graphene in our furnace.

Different measurements were performed in this way, from 1200°C to

1600°C, finding the growth threshold at 1225°C. Growths were performed

at pressure of 5 mbar.

Picture  26  Pulsed  growth  

53    

We observed in all cases that there are thresholds of temperature, pressure

and time at which continuous conductive film starts to form. The threshold

value of each parameter depends on the other parameters as well as on the

surface roughness, the surface termination and its atomic structure (presence

of defects). Therefore, the growth temperature, the methane pressure and the

growth time could be chosen so that a continuous conductive film formed

only on the surfaces of particular physical and chemical parameters.

The carbon films grown for this research were homogeneous, transparent

and possessed conductance at a level around 10-4 siemens. As shown in

[11], these films can be identified as a few layer nanocrystalline graphene of

a thickness of 1 nm.

54    

Chapter 5 Plasma Treatments We carried out a thorough analysis of the effects of plasma treatments on

the substrate to study the possibility of changing the growth of graphene

selectively, either promoting or suppressing it.

If any effect like this was to be found, the specific gas would be used in the

following FIB treatments.

In the FIB Micrion 2500 it is possible to inject a gas together with the Ion

Beam, during the process of ion sputtering. The presence of a gas that could

promote or suppress the growth could be helpful to nanostructure the

graphene film.

Treatments were performed with Argon, Oxygen, Nitrogen, Hydrogen and

Krypton plasmas.

5.1 Procedure

The quartz sample was cleaned in ultrasonic bath for five minutes.

A photoresist was used in order to cover half of the sample. The drop of

photoresist was dried putting the sample on a heated plate for 15-20 min.

When the drop was dry the sample was placed into the Diener electronic

Plasma system PICO.

The chamber was pumped out until reaching a pressure of about 0,2-0,3

mbar. Then the gas was inflated into the chamber up to the chosen pressure

(between 0,15 and 1,75 mbar).

The plasma is ignited turning on the generator, at the power of 85 Watt. The

sample is kept into the chamber for about five minutes.

After the treatment air is let flow into the chamber.

The sample is extracted and the photoresist is removed with an ultrasonic

bath in acetone for five minutes.

Then the growth is performed on the sample as described above. The

temperature was always 1200°C, pressure 5 mbar for 15 minutes.

55    

After the growth the sample is extracted and the conductance is measured in

the two different areas.

The presence of the photoresist covered half of the sample, thus it is

possible to compare the growth in the part which was treated with plasma

and the part that was covered, and is intact.

5.2 Results

5.2.1 Argon treatments

1. Argon plasma 0,3 mbar 5 min:

Conductance S Error Top treated with plasma 4,5E-05 ±1E-05 Top covered with photo resist 9,4E-05 ±1E-05 Bottom 3,0E-05 ±1E-05

2. Argon plasma 0,35 mbar 5 min:

Conductance S   Error  Top treated with plasma ∼E-13 ±1E-13  Top covered with photo resist 9,6E-5 ±1E-05 Bottom ∼E-13 ±1E-13   Note: Area covered with photo resist visible with optical microscope

3. Argon plasma 0,35 mbar 5 min:

Conductance S Error Top treated with plasma 9,9E-05 ±1E-05 Top covered with photo resist 9,5E-05 ±1E-05 Bottom 5E-06 ±1E-06

4. Argon plasma 0,35 mbar 5 min:

Conductance S Error Top treated with plasma 2,6E-5 ±1E-05 Top covered with photo resist 1,1E-4 ±1E-04 Bottom 2,4E-5 ±1E-05

56    

5. Argon plasma 0,35 mbar 10 min:

Conductance S Error Top treated with plasma 1,4E-4 ±1E-04 Top covered with photo resist 2,0E-4 ±1E-04 Bottom ∼E-13 ±1E-13  

In the treatment with Argon only once, during the second treatment, was

seen an evident difference in conductance between the area treated with

plasma and the pristine one.

The area treated with plasma is completely non conductive, and the one

covered with photoresist shows a conductance of about 1E-4 S.

In the rest of the treatments no important difference in the conductance

between the treated and not treated area is apparent. Because of this the

episode of the only difference of conductance was not judged as relevant

enough to proceed with plasma treatments with Argon during FIB

irradiations.

5.2.2 Oxygen treatments

1. Oxygen plasma 0,35 mbar 5 min

Conductance S Error Top treated with plasma 1,2E-4 ±1E-04 Top covered with photo resist 9,8E-5 ±1E-05 Bottom 1,5E-5 ±1E-05

2. Oxygen plasma 0,35 mbar 5 min:

Conductance S Error Top treated with plasma 8,4E-5 ±1E-05 Top covered with photo resist 8,1E-5 ±1E-05 Bottom 1,7E-5 ±1E-05

3. Oxygen plasma 0,35 mbar 5 min:

Conductance S Error Top treated with plasma 7,1E-5 ±1E-05 Top covered with photo resist 7,3E-5 ±1E-05 Bottom 8E-6 ±1E-06

57    

In the case of oxygen plasma no relevant difference is evident between the

treated and not treated area. This result was expected since in the surface

(SiO2) a big quantity of oxygen is already present, and the oxygen plasma

does not change significantly the condition of the surface and the growth on

it.

5.2.3 Nitrogen treatments  

1. Nitrogen plasma 0,35 mbar 5 min

Conductance S Error Top treated with plasma 1,0E-4 ±1E-04 Top covered with photo resist 1,0E-4 ±1E-04 Bottom 5,0E-5 ±1E-05

2. Nitrogen plasma 0,35 mbar 5 min:

Conductance S Error Top treated with plasma 2,0E-4 ±1E-04 Top covered with photo resist 1,6E-4 ±1E-04 Bottom 7,0E-5 ±1E-05

3. Nitrogen plasma 0,35 mbar 5 min:

Conductance S Error Top treated with plasma 1,2E-4 ±1E-04 Top covered with photo resist 9,3E-5 ±1E-05 Bottom 3,4E-5 ±1E-05

5.2.4 Hydrogen treatments 1. Hydrogen plasma 0,35 mbar 5 min:

Conductance S Error Top treated with plasma 1,2E-4 ±1E-04 Top covered with photo resist 1,2E-4 ±1E-04 Bottom 8,4E-5 ±1E-05

2. Hydrogen plasma 0,35 mbar 5 min:

Conductance S Error Top treated with plasma 9,1E-5 ±1E-05 Top covered with photo resist 1,2E-4 ±1E-04 Bottom ∼E-13 ±1E-13  

58    

3. Hydrogen plasma 0,2 mbar 5 min:

Conductance S Error Top treated with plasma 2,7E-4 ±1E-04 Top covered with photo resist 3,7E-4 ±1E-04 Bottom 2,0E-5 ±1E-05

4. Hydrogen plasma 1,75 mbar 5 min:

Conductance S Error Top treated with plasma 1,1E-4 ±1E-04 Top covered with photo resist 1,1E-4 ±1E-04 Bottom 3,1E-5 ±1E-05

5. Hydrogen plasma 0,2 mbar 5 min:

Conductance S Error Top treated with plasma 7,2E-5 ±1E-05 Top covered with photo resist 2,3E-4 ±1E-04 Bottom 4,1E-5 ±1E-05

5.2.5 Krypton treatments

1. Krypton plasma 0,35 mbar 5 min:

Conductance S Error Top treated with plasma 2,1E-4 ±1E-04 Top covered with photo resist 3,1E-4 ±1E-04 Bottom 9,2E-5 ±1E-05

2. Krypton plasma 0,35 mbar 10 min:

Conductance S Error Top treated with plasma 2,0E-4 ±1E-04 Top covered with photo resist 2,5E-4 ±1E-04 Bottom 1,1E-4 ±1E-04

3. Krypton plasma 0,2 mbar 5 min:

Conductance S Error Top treated with plasma 4,3E-4 ±1E-04 Top covered with photo resist 4,5E-4 ±1E-04 Bottom 1,2E-4 ±1E-04

59    

4. Krypton plasma 1,5 mbar 5 min:

Conductance S Error Top treated with plasma 9,3E-5 ±1E-05 Top covered with photo resist 1,3E-4 ±1E-04 Bottom 5,4E-5 ±1E-05

 

In the cases of Nitrogen and Hydrogen no difference is apparent in the

conductance of the areas treated with plasma and the ones not treated with

plasmas.

This is true also in the case of Krypton, in which different attempts were

made with different pressures in the treatments (0,2 0,35 and 1,5 mbar).

Plasma treatments were performed in order to see if the interaction of the

substrate with the plasma could promote or suppress the growth of graphene

selectively.

Unfortunately no relevant effect was observed with any of the gases, so we

decided to perform the FIB irradiations with no plasma injected.  

60    

Chapter 6 FIB Irradiation 6.1 Experimental procedure The source in the FIB is liquid Gallium ion source. The graphene films were

irradiated with Gallium ions that collided on the graphene layer, in some

cases only partially damaging it, in other cases completely sputtering the

whole film.

The irradiations were performed at various ion energies and doses.

In order to better understand and study the events happening during the ion

irradiations, TRIM software was used. It is a software the produces

simulations of irradiations, giving predictions about distributions of atoms,

vacancies, number of collisions and sputtered atoms, useful to better

interpret our results.

6.1.1 Calculation of the dose

The dose indicates the concentration of ions introduced, it is defined as the

number of incident ions over a unitary surface.

There are more than one parameter that can be changed in order to have

different doses.

The parameter that more obviously regulates the number of incident ions is

the ion beam current.

Primary ion beam current is the current of ions after main aperture, which is

defined by combination of the main aperture diameter and voltage applied to

the condenser lens. This primary ion beam current is the current of ions

which irradiate the sample. When beam is blanked this primary ion beam

current is measured at the beam blanker. However measurements taken at

the beam blanker are not very accurate because of noise. If a more accurate

measurement is required then the beam is un-blanked and directed into the

ion Faraday cup for accurate measurement. This is the procedure that was

followed in our work.

61    

With a fixed potential, the area of exposure is inversely proportional to the

dose, in fact with the same number of ions if they are spread through a

wider area the resulting concentration of ions per unit area will be smaller.

We kept the area always the same size, 150×150 µm2, and instead changed

the other parameters.

Because of problems focusing precisely the beam and for unpredictable

shakings and movements of the machine the area is never precisely a square,

but it approximates pretty well the area exposed to the beam.

With the constant flow of beams hitting the sample, the third parameter on

which clearly the dose depends is the time of exposure.

The dose of ions irradiating the target is directly proportional to the time of

exposure to the beam.

In the same treatment, energy of the beam, current, and area were kept

constant, and a line of subsequent squares was exposed to the beam for

various amounts of time.

The formula that was used in order to calculate the dose was:

!"#$ =   !∗  !!∗!

(11)

Where I is the Faraday current, t is the time of exposure, Q is the ion

charge 1,6*10-19 C and a is the area of exposure.

Ion irradiation was performed with Ga+ ions using focused ion beam (FIB)

system Micrion 2500. The ion energy varied from 5 to 50 keV at an ion

beam current in the range from 0.6 to 1.3 nA. The doses were in a range

from 1013 to 1016 cm-2 by scanning ion beam.

6.1.2 Preparation and irradiation of the sample The quartz and sapphire samples were cleaned in acetone in ultrasonic bath

for five minutes.

Then the growth of graphene was performed as described in Chapter 4.

62    

Only in the case of samples for the study of FIB-induced nucleation

enhancement, the growth of graphene was performed after the treatment

with FIB.

The samples were placed into the FIB and loaded on the stage.

Through the use of the console and the camera placed into the chamber was

possible to move the stage and place the sample right under the beam.

If necessary, before placing the sample under the beam, further

measurements and checking were made, to maximize the focusing of the

beam and set its energy.

To set correctly the beam current, the stage was moved so that the Faraday

cup is placed right under the beam. In order to ensure homogeneity of

irradiation, the ion beam was defocused to a spot of 2 µm. Vacuum in the

sample chamber during ion irradiation was higher than 2×10-7 mbar. Thus

we believe that no considerable contamination of the sample surface

occurred during irradiation.

Imaging is possible detecting secondary electrons that come from the impact

of primary ions with the surface, even though caution has to be paid not to

damage the sample. In fact, such a function was only used to identify the

sample and the exact zone where to perform the irradiations. Imaging

parameters (focus and contrast) can be set, by moving the beam on specific

copper grids and adjusting the settings.

After positioning the sample under the beam for the first irradiation, the

beam is turned on and the treatment starts. The time of exposure was

calculated from equation (11) consistently with the dose of ions to be

deposited on the sample. After turning off the beam, the sample is moved to

the next square and the beam is turned on again to treat the following area

with the chosen dose.

After FIB irradiation, the sample is extracted and the conductance of the

treated areas is measured with Keithley 2300 analyzer, as explained in detail

in Chapter 4.

63    

In some cases, after the measurements, the sample undergoes a treatment of

annealing, to see if and to which extent the original conductance can be

restored.

Annealing temperature ranged from 400°C to 1500°C.

Annealing treatments were performed in the same all-graphite furnace in

which the samples of nanocrystalline graphene were grown.

The furnace was purged with Ar of ultra-high purity grade. Through this

expedient, and because of the fact that each and every part of the furnace is

high purity graphite made, we can be sure that no noticeable contamination

was present in the chamber during the annealing treatment.

A high vacuum is needed for the annealing process, and it was provided by

a turbomolecular pump, that works in series with a diffusive pump. Once a

vacuum of 10-4 mbar is reached the heater is turned on to increase the

temperature up to a first stay at 600 °C. The temperature is kept constant

around 600°C, in order to allow gases and impurities to be released, until the

chamber pressure reaches 10-4 mbar again. Then, the temperature is

increased to the set temperature of annealing and kept constant for the

chosen time.

After the annealing the system is let cool down and the sample is extracted

to be measured again.

Optical images of the irradiated structures were taken with Nikon Eclipse

LV100 microscope in the regimes of transmitted and reflected light.

Visualization of the irradiated areas was an issue and this should be

addressed separately. It was not problematic to find the irradiated squares

when the optical transmittance contrast between the irradiated and non-

irradiated areas was considerable, like for high dose irradiation.

In particular was found a high optical sensitivity of quartz to irradiation,

which made the irradiated areas easy to recognize.

Ion irradiation, even with relatively low doses, makes quartz permanently

gray and this color change is seen in microscope. In contrast, sapphire is not

so sensitive to ion irradiation. An irradiation test of pristine quartz and

sapphire substrates was performed in the regimes used for the irradiation of

64    

the samples with graphene. It was found that the quartz substrates changed

their color in almost all irradiated areas, whereas the sapphire substrates did

not show any detectable changes in color (Picture 27). Thus, the color

changes observed on the graphene-on-quartz samples could be caused both

by graphene and quartz, while the color changes observed on the graphene-

on-sapphire samples were due to graphene only.

 

Picture  27  Optical  images  in  reflected  light  of  ion-­‐irradiated  areas  on  quartz  (a,  b)  and  sapphire  (c,  d)   substrates   after   irradiation   (a,   c)   and   after   subsequent   annealing   at   1400°C   and   complete  removal  of  graphene  (b,  d).  The  irradiation  dose  for  different  squares  varies  from  1014  to  1016  cm-­‐2.  All  the   irradiated  squares  on  quartz  are  clearly  seen  immediately  after   irradiation  and  after  high  temperature   annealing   and   removal   of   graphene.   On   sapphire,   only   squares   irradiated   at   high  

65    

doses   (left   squares   on   (c))   are   clearly   seen.   The   low   dose   irradiations   shown   on   picture   (c)   are  hardly   seen   if   at   all.   After   high   temperature   annealing   and   complete   removal   of   graphene,   all  irradiated  squares  on  sapphire  are  not  visible.  (e,  f)  Condensation  of  water  on  sapphire  substrate  covered  with  graphene  after  ion  irradiation.  The  pattern  of  the  condensed  water  droplets  allows  recognition  of  all  irradiated  areas.  

The problem with the visualization of the irradiated areas on sapphire could

be overcome by gentle blowing warm air saturated with water vapor onto

the samples. Tiny water droplets condensate immediately on the sample

surface and make the irradiated squares clearly seen (Pictures 27e, 27f).

This effect of the selective water condensation was observed both on

sapphire and quartz and was used for the precise positioning of the

measuring probes over the irradiated areas.  

6.2 Results and discussion 6.2.1 Conductance after irradiation and after annealing Dose dependences of the conductance of irradiated samples are shown in

Picture 28.

28  a  

66    

28  b  

28  c  

67    

28  d  

Picture  28.  Dose  dependence  of  conductance  of  graphene  on  quartz  after  irradiation  with  Ga+  ions  of  energies  40  keV  (a),  20  keV  (b),  5  keV  (c)  and  on  sapphire  after  irradiation  with  50  keV  Ga+  ions  (d).  Annealing  time  for  all  temperatures  but  1500°C  was  5  to  10  min.  The  annealing  at  temperature  1500°C  was  performed  for  1  min.    

The ion irradiation with doses below 3x1014 cm-2 has not affected the

conductance of nanocrystalline graphene both on quartz and sapphire

substrates. However, a dose of 3x1014 cm-2 of Ga+ ions is sufficient to

considerably suppress the conductance of single crystal graphene. An

obvious explanation of this difference is the inherent disorder of

nanocrystalline graphene [11]. This critical dose 3x1014 cm-2 does not

depend much on the ion energy (at least in the range of a few tens of keV)

and can be taken as a phenomenological parameter of the atomic disorder in

nanocrystalline graphene. Based only on this dose we cannot give a

meaningful estimation of the concentration of defects. The reason for that is

that the ion irradiation damage of graphene on substrate is a very complex

process involving the primary damage by fast ions, the damage by recoil

atoms from the substrate, the implantation of atoms from the substrate, the

68    

sputtering as well as the processes of the secondary defect transformations.

We can just say that Ga+ ion irradiation with an energy of a few tens of keV

at doses up to 3x1014 cm-2 does not produce more atomic disorder than it is

present in nanocrystalline graphene CVD-grown on quartz and sapphire.

This conclusion is supported by the similarity of the Raman spectra of

nanocrystalline graphene [10] and single crystal graphene irradiated with

Ga+ ions [19, 44]. Both spectra are very much alike and typical of low

quality graphene (Picture 29). The main features are D-band (at about 1350

cm-1), G-band (at about 1580 cm-1) and 2D band (at about 2700 cm-1). All

these bands are broadened suggesting a considerable disorder of crystal

structure.

Picture  29.  Raman  spectrum  of  nanocrystalline  graphene  on  quartz  (re-­‐plotted  from  [11])  is  compared  with  that  of  single  crystal  graphene  irradiated  with  Ga+  ions  to  a  dose  causing  an  order  of  magnitude  reduction  of  its  conductance  (re-­‐plotted  from  [19]).  

At doses over 3x1014 cm-2, the conductance of irradiated graphene decreases

rapidly with the dose and becomes undetectable over the sensitivity at doses

exceeding 2x1015 cm-2. This irradiation-induced reduction in conductance is

an expected behavior for crystalline semiconductors and semimetals, the

conductance of which is strongly affected by defects working as scattering

centers and charge carriers traps. Another reason for the irradiation-induced

69    

decrease in conductance of graphene is its gradual ion sputtering and

actually its physical removal. Indeed, the dose 2x1015 cm-2 is comparable

with the atomic density of graphene (3.9x1015 cm-2). Thus at this dose a

considerable fraction of graphene atoms experiences direct collisions with

the incident Ga ions and gets knocked out of the graphene layer.

Ion irradiation at doses of complete suppression of conductance can be used

as a method of maskless fabrication of electronic structures on graphene. In

this case, areas with as-deposited graphene are conductive, whereas

irradiated ones are insulating.

In Picture 30, the conductance contrast between an irradiated square and the

surrounding non-irradiated area is shown. Although a high conductance

contrast can be achieved this way, it may disappear after annealing, when

the graphene remaining in the irradiated area restores its conductance. That

is why in this case a sharp difference of doses also should be used, possibly

directly sputtering the region that is desired to be non conductive.

Picture  30.  Change  in  conductance  of  nanocrystalline  graphene  on  quartz  measured  with  probes  passing  across  a  square  as-­‐irradiated  with  a  dose  of  5x1015  cm-­‐2.  The  irradiated  area  is  insulating,  while  the  surrounding  non-­‐irradiated  graphene  exhibits  its  original  conductance  of  10-­‐5  S.  Zero  mark  on  horizontal  axis  is  set  at  one  of  the  edges  of  the  irradiated  square.  

70    

The observed reduction in conductance of graphene after irradiation is not

unique among carbon materials. This effect is also known for bulk graphite.

In order to find out whether the ion dose range causing the reduction of

conductance is specific of graphene only, we performed comparative

measurements on high purity, high density polycrystalline graphite

irradiation in the same regimes. It was found that the current-voltage

characteristics of the irradiated areas on graphite revealed an electric

breakdown behavior (Picture 31). The current-voltage characteristics of this

type are typical for conductors covered with thin insulating layer. Thus this

result suggests formation of an insulating layer on the surface of graphite

after ion irradiation.

Picture   31.   Current-­‐voltage   characteristics   measured   on   the   surface   of   a   polished   plate   of  

polycrystalline   graphite   before   irradiation   (dotted   curve)   and   after   irradiation  with   50   keV  Ga+  

ions   at   doses   1.2x1014   (solid   curve)   and   1016   cm-­‐2   (dashed   curve).   The   characteristics   of   the  

irradiated  graphite  exhibit  current  jump  typical  for  electrical  breakdown.    

At low voltages, current is proportional to voltage and the conductance

remains unusually low till a certain voltage threshold. At this threshold,

current jumps up to a high value (compliance limit of the instrument) and,

71    

with sweeping voltage down, current follows the dependence of the non-

irradiated graphite. This current jump is a typical electrical breakdown

through a thin insulating layer formed on the surface (which is what

determined the first trend of the curve, of low conductance for the voltage

applied). The breakdown starts as an avalanche sustained by injection of

high current and then ends up as a thermal breakdown, when the insulating

layer gets destroyed. The curve (1) in Picture 31 is an example of the

current-voltage characteristics when the thermal breakdown has not

occurred yet and the insulating layer survives. In this case, current returns

back to low values when the voltage is reduced. The curve (2) shows

current-voltage characteristic of full breakdown with the complete

destruction of the insulating layer. This is attested by the new high value of

conductance/voltage.

In this case, the conductance becomes high at any voltage and reaches the

value of non-irradiated graphite.

Conductance after Annealing

After the as-irradiated samples had been measured, they were annealed at

different temperatures up to 1500°C. The data show that the annealing at

400°C did not cause noticeable changes in conductance. After annealing at

temperatures over 600°C, a considerable recovery of conductance was

observed, and, at temperatures from 1000°C to 1300°C, the recovery

reached its maximum. The onset of the restoration of conductance at

temperatures 600°C to 800°C is expected. It is known that vacancies are the

most abundant defects in as-irradiated materials. This was confirmed also by

a TRIM simulation at energy 50keV, which is shown in Picture (32).

72    

Picture  32  TRIM  simulation  with  Ga  ions  irradiation  with  50  keV  energy,  distribution  of  vacancies  

is  displayed  

In graphite, vacancies start to move at a temperature of 700°C [45]. Since

the atomic structure of a few-layer nanocrystalline graphene is closer to that

of bulk polycrystalline graphite than to single crystal single layer graphene,

we can also expect similar temperatures of the activation of defect mobility

in graphite and nanocrystalline graphene.

At temperatures over 1300°C, a reverse process of reduction of conductance

was observed. The most drastic changes occurred at doses in the range from

1x1015 to 4x1015 cm-2. Picture 33 shows the change in conductance of

nanocrystalline graphene on quartz after irradiation at a dose of 1015cm-2

with Ga+ ions of different energy. It is seen that after annealing at 1000°C

and 1200°C the conductance increases over 7 orders of magnitude and

actually restores its original value of 10-5 S. After annealing at temperatures

over 1300°C, the conductance reduces again and completely disappears

after annealing at a temperature of 1500°C.

73    

Picture   33.   Conductance   of   nanocrystalline   graphene   grown   on   quartz   after   irradiation  with   20  and   40   keV   Ga+   ions   at   doses   1.3x1015   cm-­‐2   and   1.4x1015   cm-­‐2   respectively   versus   annealing  temperature.  Although  the  ion  energies  differ  by  a  factor  of  two,  there  is  no  essential  difference  between  the  annealing  dependences.    

Complex behavior of conductance of graphene subjected to ion irradiation

and annealing suggests involvement of several processes, the obvious ones

being the annealing of irradiation-induced defects and the degradation of

graphene at high temperature. We exclude surface contamination as a

possible cause of the observed conductance change for two reasons: first,

we took every precaution to minimize the contamination during the

irradiation and annealing. Second, any surface termination possibly

occurring in the vacuum chamber at residual pressure below 10-5 mbar

cannot change the conductance of graphene for many orders of magnitude.

A cause of graphene degradation at high temperature could be a strong

chemical interaction with the substrate and the sublimation of graphene. In

order to find out which of these mechanisms prevails, a few samples of

graphene on quartz and sapphire were subjected to isothermal annealing at

different temperatures in vacuum. Some of these samples were annealed

74    

with their graphene-carrying surfaces fully open to vacuum. The graphene-

carrying surfaces of the other samples were covered with clean substrates of

the same type and same size (stack of two substrates with graphene film in

between) and so annealed. The result of this annealing test is shown in

Picture 34.

Picture   34   (a)   Change   in   conductance   of   non-­‐irradiated   graphene   film   on   quartz   as   a   result   of  isothermal   annealing   at   1300°C   (3,   4)   and   1500°C   (1,   2);   graphene   film   open   to   vacuum   (1,   3);  graphene   film  between  two  quartz  plates   (2,  4).   (b)  Change   in  conductance  of  graphene   film  on  sapphire   with   annealing:   comparison   of   sublimation   of   as-­‐grown   graphene   and   graphene  irradiated  with  doses  of  partial  sputtering.    

75    

At 1300°C, a slow reduction of conductance of non-covered graphene

occurs for 1 hour of annealing and then conductance disappears abruptly. At

temperature 1500°C, the conductance vanishes in less than 2 minutes. In

contrast, the conductance of the covered graphene reveals no degradation

after annealing at 1300°C and only a 10 times reduction after annealing at

1500°C for 7 minutes. This behavior well supports the idea of sublimation

of graphene. Assuming that the thickness of highly transparent

nanocrystalline graphene film is about 1 nm, the sublimation rate at

temperature 1500°C can be estimated as 1 nm per minute. This is

unexpectedly high value, which is at least three orders of magnitude greater

than the sublimation rate of bulk graphite in vacuum [46-48] and free-

standing multilayer graphene. We do not have any solid explanation of this

discrepancy yet, however the chemical interaction with the substrate and

first of all with its volatile component (oxygen atoms) could be the reason of

this highly stimulated sublimation.

Combination of the two effects, the healing of the radiation damage and the

sublimation, can well explain the changes in conductance during annealing.

At temperatures below 1200°C, sublimation is negligible and the restoration

of conductance occurs due to healing of the damaged graphene lattice.

Full restoration occurs only if the irradiation dose has not exceeded 2x1015

cm-2. For higher doses, from 2x1015 to 6x1015 cm-2, conductance restores

only partially. For doses over 6x1015 cm-2, the irradiated areas do not show

any conductance neither after irradiation, nor after subsequent annealing at

any temperature. This dose 6x1015 cm-2 of the total irreversible destruction

of conductance is most probably the dose of the complete ion-sputtering of

the graphene layer. The dose range from 2x1015 to 6x1015 cm-2 corresponds

to a partial sputtering of the graphene film. TRIM simulation [49] of the

radiation damage of 1 nm thick carbon film on quartz and sapphire predicts

the sputtering yield of carbon atoms by Ga+ ions of energy from 10 to 50

keV to be from 1.6 to 2. Thus the area density of the carbon atoms

sputtered by a dose of 6x1015 cm-2 is expected to be about 1.2x1016 cm-2.

This number is very close to 2D atomic density of a three-layer, 1 nm thick

76    

graphene (1.15x1016 cm-2). Experimental studies of the ion beam sputtering

of bulk graphite with 5 keV Ar+ ions give a value of sputtering yield of 1.5

[50]. This number is in good agreement with our data. Thus we may

conclude that the total destruction of conductance of graphene after high

dose ion irradiation is the result of the physical sputtering of the graphene

film and that the ion sputtering is one of the major effects causing the

degradation of graphene during ion irradiation.

It is seen in Fig. 28 that the conductance of the ion-irradiated graphene

increases with annealing temperature up to 1200°C and then disappears after

annealing at higher temperatures due to sublimation. The conductance

vanishes first in the areas irradiated with high doses and then all irradiated

areas become non-conductive. This behavior is quite expected as the

sublimation efficiency of materials strongly depends on their structural

quality. The higher sublimation rate of the irradiated graphene is an effect,

which can be used for patterning. Regimes of irradiation and annealing can

be found so as to completely remove the graphene, whereas the non-

irradiated graphene still retains continuity and a considerable conductance.

6.2.2 Results in adhesion after irradiation and annealing

It has been found that even low dose ion irradiation improves adhesion of

graphene grown on quartz and sapphire. The enhancement of adhesion is

especially pronounced for quartz substrates. Our tentative explanation of the

different efficiency of quartz and sapphire is the different chemical

composition and, first of all, the presence of silicon atoms. TRIM simulation

of the Ga ion irradiation of graphene on quartz shows that a considerable

intermixing of C, Si and O atoms occurs at the graphene-quartz interface.

Thus the formation of stable Si-C bonds between graphene and quartz

during ion irradiation, and especially after subsequent annealing, is quite

possible.

The results of the simulations and the distributions of C, O and Si are shown

in the following graphs (Picture 35):

77    

Picture  35  TRIM  simulation  with  Ga  ions  with  50  keV  energy,  distribution  of  C,  O  and  Si  

 

78    

The irradiation enhanced adhesion was found when the samples with the

irradiated graphene were gently rubbed with cotton soaked in acetone

(Picture 36). Usually rubbing with cotton could easily remove as-deposited

graphene from all substrates. In contrast, the ion-irradiated graphene could

not be easily removed this way and, if on quartz, it could not be removed

completely even after vigorous rubbing.

Picture   36.   (a)   As-­‐grown   graphene   of   quartz   substrate.   Considerable   removal   of   graphene   film  occurs   after   single   gentle   stroke   with   cotton   soaked   in   acetone.   (b)   Graphene   on   sapphire  substrate  after  irradiation  and  annealing  at  1200°C.    

Graphene film remains on irradiated square areas after multiple strokes with

cotton soaked in acetone, while it is completely removed from the non-

irradiated areas. Graphene uniformly covers the areas irradiated with doses

over 1014 cm-2 (upper row of squares), whereas it has been partially removed

79    

from the areas irradiated with doses below 1014 cm-2 (bottom row of

squares). For 50 keV Ga+ ions, the irradiation-stimulated adhesion of

graphene occurs at doses over 2x1014 cm-2 and its strength comes to

maximum at doses over 7x1014 cm-2 (Picture 37a). The dose 7x1014 cm-2 is

low enough not to affect much the conductance of nanocrystalline graphene

(Picture 28).

Picture  37.  Conductance  of  graphene  on  sapphire  after  50  keV  Ga+  ion  irradiation,  annealing  at  a  temperature   of   1300°C   and   single   stroke   with   cotton   over   the   sample   surface.   (a)   Dose  dependence   of   the   irradiation-­‐enhanced   adhesion.   At   doses   below   2x1014   cm-­‐2,   the   adhesion  

80    

remains  negligible  and  graphene  is  wiped  away  completely.  In  the  dose  range  from  2x1014  cm-­‐2  to  7x1014   cm-­‐2,   the   adhesion   improves   and   graphene   remains   partially   on   the   irradiated   areas.   At  doses   over   7x1014   cm-­‐2,   complete   retention   of   the   graphene   layer   takes   place.   (b)   Conductance  contrast   between   the   irradiated   and   non-­‐irradiated   areas.   Conductance   is   measured   with   the  probes   scanning  over   two  areas   irradiated  with  doses   2x1014   cm-­‐2   and  2x1015   cm-­‐2.  Graphene   is  completely   removed   from   the   non-­‐irradiated   surface   leaving   it   nonconductive.   The   irradiated  areas  retain  conductance  due  to  remaining  graphene  film.  The  zero  mark  on  horizontal  axis  is  set  at  one  of  the  edges  of  the  irradiated  area.    

The enhancement of adhesion takes place immediately after the irradiation.

Subsequent annealing results in a further improvement of the adhesion.

After the irradiation and annealing, the adhesion of graphene on quartz may

become so strong that the conductive graphene film cannot be removed

from the substrate even by an intense rubbing.

The effect of the irradiation-enhanced adhesion of graphene to the substrates

can be used for the patterning and development of imprint lithography for

graphene. We have found that the application of a sticky tape to the samples

with the irradiated graphene and then its peeling off is a way to remove the

non-irradiated graphene and to leave the irradiated graphene areas in place.

When scanning the measuring probes over the irradiated areas of these

samples, a high conductance contrast between the irradiated areas with

graphene and the surrounding non-irradiated areas has been revealed. This

contrast was similar to that shown in Picture 37b.

6.2.3 Results in Nucleation improvement of Graphene on Ion-Irradiated

Substrates

Ion irradiation can be also used for selective growth of nanocrystalline

graphene on quartz and sapphire. It has been found that the formation of

continuous well conductive graphene on ion-irradiated areas can be

achieved in the temperature-pressure-time regimes at which no conductive

film grows on non-irradiated surface (Picture 38).

81    

Picture  38  Deposition  of  graphene  on  sapphire  substrate  at  temperature  1200°C,  methane  pressure  1  mbar,  growth  time  1  minute.  At  these  parameters,  visible  and  conductive  graphene  film  grows  only  on  the  ion-­‐irradiated  areas.    

Our experiments show that the higher the pressure of methane the broader

the dose range of the stimulated growth. For instance, at a pressure of 0.1

mbar and temperature 1280°C, the growth of graphene is observed on the

areas irradiated with doses from 7x1014 cm-2 to 2x1015 cm-2, whereas at a

pressure of 2 mbar and the same temperature all the areas irradiated with

doses from 2x1014 cm-2 to 6x1015 cm-2 reveal deposition of highly

conductive film (Picture 39). In both cases, the non-irradiated surface

remains insulating.

82    

Picture  39  Deposition  of  nanocrystalline  graphene  on  quartz  ion-­‐irradiated  with  different  doses.  (a)  Conductance  of  the  irradiated  areas  after:  irradiation  and  deposition  at  methane  pressure  of  0,1  mbar  (light  blue  symbols);  cleaning  in  oxygen  plasma  (green  symbols);  plasma  cleaning  and  deposition  at  methane  pressure  of  0.1  mbar  (red  symbols);  plasma  cleaning  and  deposition  at  

methane  pressure  of  2  mbar  (blue  symbols).  (b)  Measuring  conductance  with  probes  moved  over  the  squares  after  irradiation  with  doses  1.1x1015  and  1.8x1015  and  graphene  deposition  at  0.1  mbar  pressure  (  pink  and  purple  symbols);  after  irradiation  with  doses  2x1014  and  5.7x1015  cm-­‐2  

and  graphene  deposition  at  2  mbar  pressure  (orange  and  green  symbols).    

83    

The enhancement of nucleation of graphene on ion-irradiated quartz and

sapphire is a very stable effect. It persists after multiple cleanings in

solvents, high temperature annealing and even after cleaning in oxygen

plasma. This stability suggests that the enhanced nucleation is the result of

permanent ion damage of the substrate and, as such, remains until the ion-

damaged layer is removed to its whole depth.

As to the preferential growth on the areas irradiated with certain doses after

plasma cleaning (oxygen plasma termination), as seen in Picture 39 (a), it is

hard for us to give a solid explanation, since we had no chance to study this

effect in detail.

We suppose it occurs as a manifestation of three concurring processes:

(i) stimulation of growth by low dose ion irradiation (below 2e15 cm-2),

(ii) suppression of growth by high dose irradiation (above 3e15 cm-2) and

(iii) suppression of growth by O-plasma treatment. The latter is a weaker

effect compared with ion irradiation and hence it is observed only for low

doses of irradiation (below 7e14 cm-2). At higher doses, the irradiation

stimulation of growth is stronger than the plasma suppression and thus the

growth happens at doses over 7e15 cm-2. However at high doses (over 2e15

cm-2), the damage of the substrate surface becomes too high and this

suppresses the growth.

84    

Chapter 7 Conclusion Graphene, a very promising true 2D nanoelectronic material, is studied in

many different aspects.

Its zero bandgap and consequent unipolar transport makes it a very

interesting material for micro and nano-technology.

One aspect that we believe has not been explored with appropriate attention

yet is the characterization of its surface.

The aim of this work was to find new ways to characterize nanocrystalline

graphene.

As a first step the growth of this material on top of single crystal quartz and

sapphire was studied in detail, understating the dependence of the growth on

three main parameters, temperature pressure and time, and getting to know

under which conditions the best quality nanocrystalline graphene can be

grown.

The first attempt to characterize graphene, creating a sharp contrast between

a conductive and a non conductive area, was made with plasma pre-

treatments of the substrates.

Oxygen, Argon, Nitrogen, Hydrogen and Krypton plasmas did not produce

relevant results of selective enhancement or suppression of growth.

The main focus of the work was the study of the characterization of

nanocrystalline graphene with Gallium ions via FIB. The irradiations were

performed in a wide range of doses, with energy from 5 to 50 keV.

The results show that using ion irradiation it is possible to control the

electrical conductance of nanocrystalline graphene on quartz and sapphire

over many orders of magnitude. It is shown that nanocrystalline graphene

stands fairly high doses of ion irradiation (up to 3x1014 cm-2 for 5 to 50 keV

Ga+ ions) without degradation in conductance.

At higher doses, nanocrystalline graphene rapidly loses its conductance and

at doses over 2x1015 cm-2 becomes actually insulating.

Annealing in vacuum at temperatures over 600°C restores the conductance

of the ion-irradiated nanocrystalline graphene and, if the irradiation does not

85    

exceed a dose of 3x1015 cm-2, this restoration can be almost complete. This

reverse increase in conductance after annealing occurs via the healing of the

radiation damage.

Ion irradiation at very high doses - approaching 1016 cm-2 - results in the

complete sputtering of a few graphene layers. This is confirmed by the

lowering of conductance, and the impossibility to recover it even after high

temperature annealing. A quick calculation, based on graphene density and

TRIM simulations of irradiation, confirms that at such high doses the

nanocrystalline graphene layers are sputtered by the colliding ions.

This can be a first method of graphene patterning, by irradiating areas of the

sample which give rise to non conductive areas with a sharp contrast with

respect to untreated areas in which highly conductive graphene is preserved.

It has also been found that along with radiation damage and ion beam

sputtering, the high temperature sublimation is an important effect involved

in the reduction of conductance of the ion-irradiated and annealed

nanocrystalline graphene.

It is also shown that ion irradiation improves adhesion of graphene to quartz

and sapphire substrates. As a result, graphene in the non treated areas can be

easily wiped away with cotton and acetone, while in the areas treated with

gallium ions the adhesion of graphene is increased. When the treated areas

undergo the same acetone cleaning, graphene is not wiped away, and

conductance measurements confirm its presence, with a sharp contrast

between pristine and treated areas. This can be an additional method of

patterning graphene.

For 50 keV Ga+ ions, irradiation-enhanced adhesion is observed at doses

over 2x1014 cm-2.

The increased efficiency of the graphene nucleation on these materials

during CVD growth is another effect studied. After observing the

enhancement of adhesion of nanocrystalline graphene as a result of

irradiation, we investigated whether the same effect could be induced by

treatments performed before the growth of graphene.

86    

The results of irradiations in a broad dose range (from 1x1014 up to

5x1015cm-2) demonstrate the occurrence of selective growth on the treated

areas. A deeper study of the effect is desired to better understand the

responsible mechanisms and to optimize the range of irradiation to have the

most effective growth contrast.

The promoted graphene nucleation is observed in a broad dose range.

The above effects can be used for developing methods of patterning of

graphene deposited on insulating substrates and methods of imprint

lithography of graphene. As the modern FIB systems permits ion irradiation

with a sub 10 nm resolution [18, 51, 52], the patterning of graphene at a few

nanometer scale is feasible. It is important that the patterning done in this

way is maskless and resist-free, and, as such, would not require steps of

cleaning of graphene surface from the resist residues.

87    

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