electronic devices based on cdte nanowires
DESCRIPTION
dissertation thesisTRANSCRIPT
University of Bucharest
Faculty of Physics Dissertation thesis
Electronic and optoelectronic devices
based on CdTe nanowire arrays
Student: Camelia-Florina FLORICA
2011
Advisors: Prof. Univ. Dr. Ştefan ANTOHE
CS I Dr. Ionuţ ENCULESCU
UNIVERSITY OF BUCHAREST
Faculty of Physics
Dissertation thesis
Electronic and optoelectronic devices
based on CdTe nanowire arrays
Student: Advisors:
Camelia - Florina FLORICA Prof. Univ. Dr. Ştefan ANTOHE
CS I Dr. Ionuţ ENCULESCU
Măgurele - Bucharest, June 2011
Acknowledgements
This thesis was carried out in the laboratory Research and Development Centre for Materials
and Electronic & Optoelectronic Devices (MDEO) within the Faculty of Physics of the
University of Bucharest and in the Multifunctional Materials and Structures laboratory of the
National Institute of Materials Physics.
This work, and with it, its author, have enjoyed a lot of encouragement and support from many
sides. I would like to thank particularly Prof. Dr. Stefan ANTOHE for supervising my training as
well as for the very good guidance during the time spent in his laboratory.
I would like also to express my gratitude to CS I Dr. Ionuţ ENCULESCU for providing the
necessary equipment and time for preparing the nanowire arrays, as well as for all the pieces of
advices which led to constantly improving my work.
Special thoughts and many thanks to Assoc. Prof. Lucian ION, Dr. Elena MATEI and PhD
students Adrian RADU and Sorina IFTIMIE, as well as to all the people in the laboratories for
the pleasant environment and for their help in practical problems.
Contents
1. Introduction ................................................................................ Error! Bookmark not defined.
1.1. Why studying electronic and optoelectronic devices? ........ Error! Bookmark not defined.
1.1.1. State of the art in photovoltaic cells ............................. Error! Bookmark not defined.
1.1.2. State of the art in sensors .............................................. Error! Bookmark not defined.
1.2. Theoretical background ....................................................... Error! Bookmark not defined.
1.2.1. Electrical behaviour of the solar cells ........................... Error! Bookmark not defined.
1.2.2. Single nanowires transistors for sensors ....................... Error! Bookmark not defined.
2. CdTe nanowire arrays for photovoltaic cells ............................. Error! Bookmark not defined.
2.1. Preparation of the CdTe nanowires ..................................... Error! Bookmark not defined.
2.1.1. Electrochemical deposition of CdTe nanowires using Au as the working electrode
................................................................................................ Error! Bookmark not defined.
2.1.2. Electrochemical deposition of CdTe nanowires using Cu as the working electrode
................................................................................................ Error! Bookmark not defined.
2.2. Preparation of the hybrid inorganic/organic photovoltaic cells based on CdTe nanowires
and ZnPc organic dye ................................................................. Error! Bookmark not defined.
2.2.1. Technological steps for producing the hybrid photovoltaic devices . Error! Bookmark
not defined.
2.2.2. Optical, photoelectrical and electrical measurements and results ..... Error! Bookmark
not defined.
2.3. Preparation of the inorganic photovoltaic cells based on CdTe nanowires and CdS thin
film ............................................................................................. Error! Bookmark not defined.
2.3.1. Technological steps for producing the inorganic photovoltaic structures ............ Error!
Bookmark not defined.8
2.3.2. Optical, photoelectrical and electrical measurements and results ..... Error! Bookmark
not defined.0
3. Ni/CdTe/Ni single nanowires properties ................................. Error! Bookmark not defined.2
3.1. Preparation of the Ni/CdTe nanowires ................................ Error! Bookmark not defined.
3.2. Preparation of the Ni/CdTe/Ni nanowires ......................... Error! Bookmark not defined.6
3.3. Aligning nanowires ........................................................... Error! Bookmark not defined.7
3.4. Contacting single nanowires with FIB/SEM ....................... Error! Bookmark not defined.
3.4.1. FIB-SEM Description ................................................. Error! Bookmark not defined.8
3.4.2. Pt contacts .................................................................................................................... 39
3.4.3. Contacting single nanowires ....................................... Error! Bookmark not defined.0
4. Conclusions and further work .................................................. Error! Bookmark not defined.4
5. References ................................................................................ Error! Bookmark not defined.5
1. Introduction
1.1. Why studying electronic and optoelectronic devices?
The arrays of semiconductor nanowires are a new class of materials which lead to larger number
of levels of functionality in building devices for electronics and optoelectronics applications.
These nanoscale devices are enabling a route of development of new technologies in key areas,
such as, communications and information processing, sensors and sensing and renewable energy.
Regarding all these applications it is nowadays usual to use them and to ask for the best results
from it. In order to do this it is obvious the fact that we have to improve them constantly,
therefore to study their properties and find the best compromise between the cost of fabrication
and the best performances.
In this thesis there are reported two types of devices with applications in the renewable energy
sector, photovoltaic cells, and sensors and sensing, single nanowire biofuntionalised transistors.
1.1.1. State of the art in photovoltaic cells
Fossil fuels which took around 400 million years to form and to be stacked away underground
are used by the people for energetic purposes in the last centuries. Besides the depletion of the
fossil fuel noticed in the last decades which is going to become deeper and deeper there is
another extremely important problem which has to be taken into consideration carbon dioxide
emissions and increased global warming.
Most politicians and policymakers agree that a massive redirection of energy policy is essential if
Planet Earth is to survive the 21st century in reasonable shape[1]. This is not simply a matter of
fuel reserves. It has become clear that, even if those reserves were unlimited, we could not
continue to burn them with impunity. The resulting carbon dioxide emissions and increased
global warming would almost certainly lead to a major environmental crisis. So the danger is
now seen as a double - edged sword: on the one side, fossil fuel depletion; on the other, the
increasing inability of the natural world to absorb emissions caused by burning what fuel
remains.
For the good of Planet Earth and future generations we have to invest heavily in renewable
technologies – including solar, wind and wave power – that produce electrical energy free of
carbon emissions.
Sun’s radiation beamed at us day by day, year by year, and century by century, its income can be
used or ignored as we wish. The challenge for the future is to harness such renewable energy
effectively, designing and creating efficient and hopefully inspiring machines to serve
humankind without disabling the planet.
Since the reduction of carbon emissions is a principal advantage of PV, wind, and wave
technologies, we should recognise that this benefit is also proclaimed by supporters of nuclear
power. It is true that all offer electricity generation without substantial carbon emissions, but in
almost every other respect they are poles apart. The renewables offer the prospect of widespread,
relatively small - scale electricity generation, but nuclear must, by its very nature, continue the
practice of building huge centralised power stations. PV, wind, and wave need no fuel and
produce no waste in operation; the nuclear industry is beset by problems of radioactive waste
disposal. On the whole renewable technologies pose no serious problems of safety or
susceptibility to terrorist attack – advantages which nuclear power can hardly claim. And finally
there is the issue of nuclear proliferation and the difficulty of isolating civil nuclear power from
nuclear weapons production.
It would however be unfair to pretend that renewable energy is the perfect answer. For a start
such renewables as PV, wind, and wave are generally diffuse and intermittent. Often, they are
rather unpredictable. And although the ‘fuel’ is free and the waste products are minimal, up -
front investment costs tend to be large. There are certainly major challenges to be faced and
overcome as we move towards a solar future.
Regarding photovoltaics this way of producing renewable energy is arguably the most elegant
and direct.
The International Energy Agency has made a scenario for the way it should be invested in the
energy sector until 2035. From their statistics it is noticed the fact that the installed power
generation capacity should increase from 4 722 GW in 2008 to about 8 600 GW by 2035.
Between 2009 and 2035, total gross capacity additions amount to 5 900 GW, with more than
40% installed by 2020.
Nuclear power and renewable energy additions respectively account for 5% and 41% of the total
between 2009 and 2020. Investment in new plants rises more quickly from 2021 to 2035, as
more capital intensive technologies are deployed and more variable resources exploited creating
a need for additional generating capacity (Figure 1.1).
Figure 1.1: World power-generation capacity additions and investment by type in the New Policies Scenario
Source: International Energy Agency study, World energy outlook
In addition to providing support as defined above, governments are engaged in substantial
continuing efforts in research and development (R&D) to bring the costs of renewable energy
technologies down and to improve their performance. Some of these technologies, such as
hydropower, onshore wind and biomass are mature or almost mature and do not require
significant additional spending on R&D, although R&D is still needed for better wind
forecasting and working variable generation into the power supply system.
Photovoltaics and concentrating solar power, though commercially available, depend for their
widespread diffusion on further supportive policy measures. For constantly improving the
efficiency of generating electrical energy investments have to be made.
Total spending on research and development reached $5.6 billion in 2009 (figure 1.2).
Figure 1.2: Global spending on research and development in renewable energy by technology, 2009
Source: International Energy Agency study, World energy outlook.
As seen in the figure above, more than half was spent on solar energies. The estimated energy
generation until 2035 is presented in figure 1.3 and it implies a huge increase of the investment
department.
Figure 1.3: Electricity generation by fuel and region in the New Policies Scenario
Coming back to the history of the field of interest for this thesis, photovoltaic cells have started
to be developed at a large scale since 1958.
Scientist and innovation Year
Becquerel discovers the photovoltaic effect 1839
Adams and Day notice photovoltaic effect in selenium 1876
Planck claims the quantum nature of light 1900
Wilson proposes Quantum theory of solids 1930
Mott and Schottky develop the theory for diodes 1940
Bardeen, Brattain and Shockley invent the transistor 1949
Charpin, Fuller and Pearson announce 6% efficient silicon solar cell 1954
Reynolds et al. highlight solar cell based on cadmium sulphide 1954
First use of solar cells on an orbiting satellite Vanguard 1 1958
All solar cells require a light absorbing material which is present within the cell structure to
absorb photons and generate free electrons via the photovoltaic effect. The photovoltaic (PV)
effect is the basis of the conversion of light to electricity in photovoltaic, for solar cells.
There are many materials that can be used for making photovoltaic cells, such as: silicon
(amorphous, crystalline), cadmium telluride and cadmium sulphide, organic and polymeric
materials. Also the way these materials are processed can be different from thick films
technologies to thin films, from using electrolyte to making nanostructured devices.
In the case of the silicon technologies it is well known that it has been the dominant one for the
supply of power modules into photovoltaic applications and the likely changes are an increasing
proportion of multi-crystalline silicon and monocrystalline silicon being used for high-efficiency
solar cells while thinner wafers and ribbon silicon technology continue to grow[2-5].
Many researchers have investigated the properties of these kind of cells, reporting about their
optical, electrical and structural properties and have searched continuously a way of making a
growth of the efficiency and lowering the price in the same time[6-10].
Amorphous (uncrystallized) silicon is the most popular thin film technology with cell
efficiencies of 5–7% and double- and triple-junction designs raising it to 8–10%. But it is prone
to degradation. Advances made in amorphous-Si PV technology led to the achievement of an
AM 1.5, 13% stable cell efficiency and set the foundation for the spectrum splitting triple-
junction structure being manufactured by the roll-to-roll continuous deposition process[11].
Crystalline silicon offers an improved efficiency when compared to amorphous silicon while still
using only a small amount of material. The commercially available multi-crystalline silicon solar
cells have efficiency around 14–19%[12-18].
Cadmium telluride (CdTe) and cadmium sulphide (CdS) are good replacement materials for the
Si which is highly expensive[19-21]. Work has to be done continuously for improving the
efficiency of this type of cells.
Organic and polymer cells have problems with the stability/ degradation methods for enhancing
the stability through the choice of better active materials, encapsulation, application of getter
materials and UVfilters are done[22-26]. Polymers have demonstrated that the bulk heterojunction
concept is a viable approach towards developing photovoltaic systems by inexpensive solution-
based fabrication technologies[27].
The most important advantage of this type of cells is the low cost of production.
Thin film technologies led to multilayered structures having the biggest known efficiency till
present of 40.7%[28].
Hybrid photovoltaic cells are representing the latest generation of solar cells that is currently
developed. A nanostructured inorganic material in combination with a polymer thin film is
expected to have higher power conversion efficiency at a lower price.
Nanostructures basis of CdTe, nanowires more precisely, will be used to develop photovoltaic
cells in this project.
1.1.2. State of the art in sensors
Sensors are the devices, which are composed of an active sensing material with a signal
transducer. Sensors can be broadly classified in to two categories as chemical sensors and
biosensors. The biosensors can be defined in terms of sensing aspects, where these sensors can
sense biochemical compounds such as biological proteins, nucleotides and even tissues[29-31].
Within these sensors, the active sensing material on the electrode should act as a catalyst and
catalyze the reaction of the biochemical chemical compounds to obtain the output signals[32, 33].
The combination of these two different ways of classifications has given rise to a new type of
sensors which are called electrochemical biosensors, where the electrochemical methods are
applied for the construction and working of a biosensor[34-36]. The selection and development of
an active material is a challenge. The recent development in the nanotechnology has paved the
way for large number of new materials and devices of desirable properties which have useful
functions for numerous electrochemical sensor and biosensor applications[37-41]. Basically by
creating nanostructure, it is possible to control the fundamental properties of materials even
without changing their chemical composition. In this way the attractive world of low
dimensional systems, together with the current tendencies on the fabrication of functional
nanostructured arrays could play a key role in the new trends of nanotechnology[42-44]. Further,
the nanostructures can be used for both efficient transport of electrons and optical excitation, and
these two factors make them critical to the function and integration of nanoscale devices[45-47]. In
fact, nanosystems are the smallest dimension structures that can be used for efficient transport of
electrons and are thus critical to the function and integration of these nanoscale devices. Because
of their high surface-to-volume ratio and tunable electron transport properties due to quantum
confinement effect, their electrical properties are strongly influenced by minor perturbations.
A good method for generating any kind of nanostructures should enable simultaneous control of
the dimensions, properties, and morphology. In general, nanostructures are synthesized by
promoting the crystallization of solid-state structures along one direction by various mechanisms
which includes, the use of templates with nanostructure morphologies to direct the formation of
nanostructures; the use of intrinsically anisotropic crystallographic structure of a solid to achieve
nanowires growth; the use of a liquid/solid interface to reduce the symmetry of a seed and the
use of appropriate capping agents to kinetically control the growth rates of various facets of a
seed[48]. Among these, highly successful methods to obtain nanowires and nanowire arrays are by
using the synthesis on templates and self assembly or self-organization processes. The first and
most important process of this strategy is the creation of a desirable surface structure on which
the nanowires growth should take place; for this goal several top-down techniques like molecular
beam epitaxy, electron beam evaporation, phase-shift optical lithography and sputtering have
been used[49-51].
Polycarbonate membranes are used to create nonporous structures which are in turn being used
as templates to grow functional nanowires of different species by mainly self-assembling process
and electroplating techniques. By means of electrodeposition processes, it could be possible to
take control on, ordering degree, i.e., the size of crystalline single domains (up to several square
micrometers); the single one dimensional structure diameter (from 15 to 200 nm) and length
(from tens to thousands of nanometers); the lattice parameter of ordered arrays (between 65 and
500 nm).
1.2. Theoretical background
1.2.1. Electrical behaviour of the solar cells
The photovoltaic effect appears when light interacts with a semiconductor material, if two
conditions are fulfilled:
- the absorbed light should generate particles that can move through the material
(electrons, holes, excitons, polarons) by direct transport inside an energetic band or by
hopping
- there should exist an electric field which have to separate the charge carriers created by
direct photogeneration or by excitons dissociation and sent them to the exterior circuit
At open circuit the structure is polarised with the voltage VOC, and therefore there exist in the
structure a dark current opposite to the current generated by the light.
The shortcircuit current has the density jSC and it is the current generated by the light
(photocurrent). In the presence of the light, through the structure there will be a current of
density j (through a resistance); the resulting power is negative which thermodynamically
corresponds to an energy generator.
In figure 1.4 the current-voltage characteristics of an ideal photovoltaic cell is shown.
Figure 1.4 The current-voltage characteristics of an ideal photovoltaic cell
The power conversion efficiency is defined by:
inc
SCOC
inc
m
P
IVFF
P
P . (1)
Where, Pm is the maximum power generated by the cell, Pinc is the incident power on the sample
and FF is the fill factor (SCOC
mm
IV
IVFF ).
The current voltage characteristic of the photovoltaic cell is described by:
L
Sh
SkT
IRVe
S IR
IRVeII
S
1 (2)
And it is obtained for the real solar cell, equivalent with the circuit from figure 1.4, where RS is
the series resistance given by the semiconductor and the contacts resistance, RSh is the shunt
resistance, which is in parallel with the p-n junction.
Figure 1.4 Equivalent circuits for a real photovoltaic cell
I
VOC
VmV
mI
SCI
0
mP
LI
SR
0R
ShR
dI
I
A detailed study of the photovoltaic effect assumes the knowledge of the mechanism, of its
sources, the generation processes and charge carries recombination, doping effects and likewise
the dependence of the photocurrent of the wavelength and of the intensity of the incident
radiation.
When discussing about heterojunctions we are actually talking about the contact between two
semiconductors which are different because of the type of conduction and of the band gaps, also
they have different effective masses and dielectric constants. In this way at the photovoltaic
effect, besides the electrostatic field there can be the contribution of an effective field of forces.
There is a large variety of heterojunctions that can be analyzed using different models. In the
case of CdTe and ZnPc, or CdTe and CdS the corresponding model is Anderson. This is for a
heterojunction p-n, where the light enters the structure through the n type semiconductor, which
has a wide band gap, with the role of the window. The band diagram is illustrated in figure 1.5.
The p type region where the photovoltaic effect is dominant is called a basis. The limited space
charge regions have the widths l1, l2.
Figure 1.5 Band diagram of the p-n heterojunction
The photons with the energy 21 gg EhE will pass through the n type region (the window)
without being absorbed in this layer getting to the p region where non equilibrium charge carriers
will be created, separated by the electric field from the barrier, leading in this way to the
photovoltaic effect. The window effect of the n type semiconductor gives the generation of the
charges in the barrier determining the lowering of losses because of recombination, while the
diffusion of the charge carriers takes place in the neutral regions until the field region.
1CE
2CE
2VE
1VE
FE
CE
VE
h
1l 2l
0x
d
n)2(
p)1(
In order to calculate the total current through the structure at illumination the current density in
the two regions has to be analysed. In this way, in region (1), where the photovoltaic effect takes
place with a percentage of 60-80% the photocurrent contains the minoritary charge carriers jl1
which are generated in the passing region and the diffusion current, jn1 of the electrons which
pass through the p type region and get to the separation field.
111 nl jjj (3)
The generation rate is calculated at the x distance from the junction plane, for semiconductor 1
being xedgxg 1
11
, where d
AeIdg 2
111
. In this case the electron current density
generated in the barrier layer is:
112
1
12 11
0
111
ld
A
l
dd
Al eeIqdxeeIqj
(4)
Or:
L
l
L
d
Al eeIqj
1
111 (5)
Where 1 the quantum efficiency of generating and d is is the thickness of each semiconductor.
The diffusion current density of the electrons generated in the neutral region of the p type
semiconductor is given by:
1
1
lx
nndx
ndqDj
(6)
The concentration of the excess electrons is obtained solving the continuity equation imposing
the right limit conditions:
0
0
0
1
2
1
22
2
n
ln
L
xg
L
n
dx
nd
n
n
n
(7)
The first condition shows that the electrons generated in the neutral region (1) are all directing to
the edge of the charge space limited region because of the field from the junction and then are
sent to the n type semiconductor (2) such as 01 pnln .
The second condition shows that the neutral region of type p is larger than the electron diffusion
length and therefore the electrons are recombining with the majoritary charge carriers (holes), in
the way to get to the back electrode.
The solution of the equation (7) is:
nL
lx
L
lx
L
l
L
d
n
A
n
n eeeeD
I
LL
LLxn
1
1
1
1
1
21
22
1
1
2
(8)
With this solution the equation (6) becomes:
1
1
2
1
11
L
l
L
d
n
Ann ee
LL
IqLj
(9)
The electronic current density in the basis is:
1
1
2
1
1
1111 1
L
l
n
L
d
Anl eLL
LeIqjjj (10)
The total current generated in the n type semiconductor has contributions from the holes current
generating in the passing region of the semiconductor, jl2 and from the holes diffusion current
generated in the neutral region, jp2 which is sent to the p type semiconductor.
222 pl jjj (11)
If the generation rate in semiconductor 2 is: 22
2
2222
L
x
Ad
A eL
IeIxg
, the current density
of the holes is the passing region (d – l2, d) is:
d
ld
L
l
L
d
A
L
x
Al eeIqdxe
L
Iqj
2
2
2
22 1
2
2
22
(12)
The diffusion current density of the holes generated in the neutral region is:
2
2
ldx
ppdx
pdqDj
(13)
The concentration of the excess holes is obtained solving the continuity equation imposing the
right limit conditions:
0
0
0
0
2
2
2
22
2
pSdx
pdD
ldp
L
xg
L
p
dx
pd
x
p
p
p
p
(14)
The first condition shows that if the illumination is not very strong, non equilibrium holes which
get to the edge of the passing region, are trapped by the field from the junction and send to
semiconductor 1 such that 02 npldp and 02 ldp .
The second condition shows that the holes generated in the neutral region of the semiconductor
1 are going towards the illuminated surface, and a total recombination phenomena is happening
at x = 0, where the superficial recombination speed is very high because the electrons
concentration in this region is very high. So region 2 has to be neutral.
With the obtained solution we get the diffusion current for holes and then the current density of
holes in the window layer:
22
2
2
2
2
2
2
2
22
2
2
2
22
1
111
L
d
p
pp
ppL
ld
p
p
A e
D
SLD
SL
L
Le
L
L
LL
LIqj (15)
where:
pp
p
p
pp
p
p
L
ldsh
D
SL
L
ldch
L
ldch
D
SL
L
ldsh
ldS22
22
2,
(16)
The total current density across the heterojunction at illumination is: 21 jjj .
Having the I – V characteristics of type:
L
AV jejj 10 (17)
Which is valid for an ideal heterojunction and from it the open circuit voltage can be determined:
1ln
1
0
21
j
jj
AVOC (18)
Knowing the absorption spectrum , and calculating j1, j2 and VOC, the spectral characteristics
of the heterojunction can be obtained. Because of the « window effect » the region of sensibility
is in a band in the incident photons energies between Eg1 and Eg2.
1.2.2. Single nanowires transistors for sensors
For NW biosensors operated as FETs[52] the sensing mechanism is the field gating effect of
charged molecules on the carrier conduction inside the NW.
The sensitivity of the nanodevices is much better than the micromaterial or bulk materials and it
is given by the reduced dimensionality and he much larger surface/volume ratio.
Therefore it is natural to expect that the highest sensitivity should be achieved when the whole
volume of the nanodevice is gated by surface charges.
The transport mechanism for such a device, when discussing about Si has been calculated last
year by Xuan P. A. Gao et all[53] and it can be clearly seen that the changes in conductance could
be evaluated and measurements on single nanowire field emission transistors based sensors:
}1)/{exp(2 2
0
TkepRedrpreG Bmaterial
R
(19)
where: G is the conductance, R is the nanowire size (radius), material is surface potential shift,
p is the hole density, e is the electron charge and µ is the mobility.
Figure 1.5 The dependence of screening length on the carrier
density. In the high carrier concentration regime NW-FET
works in the linear regime, where the conductance G varies with
gate voltage linearly. In the low carrier concentration regime the
NW-FET works in the depletion (subthreshold) regime where the
G varies with gate voltage exponentially.
Source: Nano Letters 2010 10 (2), 547-552
2. CdTe nanowire arrays for photovoltaic cells
2.1. Preparation of the CdTe nanowires
2.1.1. Electrochemical deposition of CdTe nanowires using Au as the working
electrode
Using template based electrochemical deposition, as a growth method for the CdTe nanowire
arrays, several steps have to be performed in order to get the desired nanostructured material.
Electrochemical deposition, or electrodeposition for short, has been around for a very long time
now and is exceptionally versatile, and valuable applications keep being invented. Why should
serious scientists and technologists still get excited by it? There are several answers. To begin,
electrodeposition is a fascinating phenomenon. That one can put a shiny coating of one metal on
another simply by donating electrons to ions in a solution is remarkable, and studies of the
process at an atomic level continue to yield surprises.
In order to grow nanowires using this method an electrochemical cell with 3 electrodes
configuration is needed as shown in figure 2.1.
The current flow is taking place between the working electrode (cathode) and the
counterelectrode (anode), being controlled with the help of the reference electrode.
Figure 2.1 Electrochemical cell used for growing the nanowires
The working electrode can be a metal, graphite or a semiconductor and its surface must be
constant and measurable.
Primarily, before sputtering the necessary gold working electrode, the template must be
prepared. It consists of polycarbonate foils (Makrofol N, Bayer), that have a thickness of 30 m.
In order to have pores through the membrane, it is irradiated with swift heavy ions (with a
specific energy 11.4 MeV/nucleon) at different fluencies as in figure 2.2. Varying the fluencies
of the ions in the range of 104– 10
9 ions/cm
2 is necessary for having a broad range of density of
the pores in the membranes, for different applications[54].
Figure 2.2 Polycarbonate foil irradiated with swift heavy ions of specific fluencies for a range
of the density of the pores
Once the nuclear tracks have been made in the polycarbonate foils, in order to get to a specific
diameter of the pores, these were subsequently chemically etched with aqueous solutions
containing 5M NaOH and 10% volume methanol at 50C (figure 2.3).
Figure 2.3 Etching of PC foils for having a specific diameter of the pores
A good control of the diameter of the cylindrical pores is controlled by the low etching rate of
200 nm/h.
Once the template is prepared as desired the next step is the deposition of the gold working
electrode, 50 nm thick, on one surface of the foils, by sputtering (figure 2.4).
Figure 2.4 Sputtering of Au thin film
To complete the closing of the pores and to improve the mechanical stability of the template, a
copper layer, 10 m thick, was electrochemically deposited onto the gold film (figure 2.5).
Figure 2.5 Deposition of a Cu thin for mechanical stability
The counterelectrode has a big influence upon electrodeposition. It is usually made by a material
which does not produce new species by electrolysis. For this process platinum counterelectrode
is used.
The working electrode potential is controlled by a potentiostat with the help of a commercial
saturated calomel electrode (SCE) as reference.
Deposition of CdTe nanowires (figure 2.6) was performed in a potentiostatic mode using an
acidic deposition bath (1 M CdSO4, 0.3 mM TeO2 at a pH of 1.6 adjusted with H2SO4). The
reaction mechanism leading to the formation of CdTe nanowires is described by:
HTeO2+ + 3H
++ 4e
- Te +2H2O (20)
Cd2+
+ Te + 2e- CdTe. (21)
Figure 2.6 Growing CdTe nanowires onto Au substrate
The peculiarity of CdTe forming in this case is that the deposition process is Te diffusion
limited. The polarization curves for the deposition process in a membrane containing 108
pores/cm2 of 80 nm diameter at 74C are presented in figure 2.7.
-700 -600 -500 -400 -300 -200 -100 0 100-0.30
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
sweep1
sweep2
sweep3
Cu
rre
nt
(mA
)
Potential vs. SCE (mV)
Figure 2.7 Polarisation curves for CdTe nanowires deposition (sweep rate of 5 mV/s)
The peak at -500 mV corresponds to the potential where CdTe is deposited. A shift towards
more positive potential can be observed at the second and third sweep. This is most probably due
to the fact that the deposition is further favorite because of the presence of Te atoms in the
substrate. Also depositions at other voltages has been performed (-550mV, -600mV) for finding
the most stoichiometric compound.
In order to measure the properties of the nanowires, the PC foil has been removed by dissolving
it in chloroform (figure 2.8).
Figure 2.8 Exposed nanowires after the removal of PC
Structural and compositional analysis of the CdTe wires array show a stoichiometric
compound (figure 2.9) containing 50.5% Cd and 49.5% Te in the case of the sample grown at -
550 mV with a zinc-blend cubic crystalline structure (figure 2.10).
Figure 2.9 EDX spectrum of CdTe wires array grown at -550 mV, 50.5% Cd and 49.5% Te
Figure 2.10 X-ray diffraction pattern of the CdTe wires
By SEM micrograph the CdTe nanowire array can be seen, being 108/cm
2 dense (figure
2.11). In this way an image about how the future structure will look like, is formed.
0 10 20 30 40 50 60 70
0
50
100
150
200
250
300
Cu
(2
00
)
CdT
e(3
11)
Cu (111)
CdT
e(2
20)
CdT
e(1
11)
I(a
.u.)
2
Figure 2.11 SEM micrograph showing the CdTe wire array deposited on Au after dissolving the PC membrane
The Au electrode is not completely covered as it can be seen and for further processing of the
samples this should be taken into account.
The free standing nanowires are fit for being used in photovoltaic application.
2.1.2. Electrochemical deposition of CdTe nanowires using Cu as the working
electrode
In this case the procedure is similar with the one presented in sub-chapter 2.1.1. After preparing
as shown the polycarbonate foil, a copper film is sputtered on its surface (figure 2.8).
Figure 2.12 Sputtering of Cu thin film
In the same conditions as before the CdTe nanowires are grown (figure 2.9) having a polarization
curve for a membrane with the density of the pores of 109/cm
2 shown in figure 2.10.
Figure 2.13 Growing CdTe nanowires on Cu substrate
The peak around -0.5 V corresponds to the potential where CdTe is deposited, without being too
different from the deposition of Au electrode (figure 2.14).
a)
b)
Figure 2.14 a) Polarisation curves for CdTe nanowires deposition (sweep rate of 5 mV/s)
b) Deposition curves of CdTe at -550 mV
0.0 3.0x103
6.0x103
9.0x103
1.2x104
0.0
1.0x10-4
2.0x10-4
3.0x10-4
4.0x10-4
0.0 3.0x103
6.0x103
9.0x103
1.2x104
-0.60
-0.58
-0.56
-0.54
-0.52
-0.50
Cu
rre
nt (A
)
Elapsed Time (s)
Po
ten
tia
l (V
)
Elapsed Time (s)
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1
-1.8
-1.5
-1.2
-0.9
-0.6
-0.3
0.0
sweep2
sweep3
sweep1
Cu
rre
nt (m
A)
Potential (V)
Figure 2.15 Exposed CdTe nanowires on Cu substrate after the removal of the PC membrane
Reflexion spectrum of the grown nanowires were measured using a Perkin Elmer Lambda 45
UV/VIS Spectrophotometer (figure 2.15).
Figure 2.15 Reflexion spectra of the CdTe nanowires grown at -550 mV
The theory which makes possible to use reflexion spectrum was proposed by Kubelka
and Munk [55].
Figure 2.16 Kubelka-Munk function versus the photon energy for determining the energy bandgap of the CdTe
nanowires
600 700 800 900 1000
4
6
8
10
R (
%)
[nm]
1.4 1.6 1.8 2.00
4
8
12
16
Eg=1.48 eV
Photon energy (1240/) [eV]
F(R
)2
From the graphic in figure 2.16 the energy bandgap of the nanowires was determined, having a
value of 1.48 eV.
The compositional analysis of the CdTe nanowire array shows an almost stoichiometric
compound (figure 2.17) containing 47% Cd and 53% Te in the case of the sample grown at -550
mV.
Figure 2.17 EDX spectrum of CdTe wires array grown at -550 mV, 47% Cd and 53% Te
In the SEM image (figure 2.18) can be noticed a more dense CdTe nanowire array
(109pores/cm
2) with free standing nanowires having properties that make them suitable for
photovoltaic applications.
Figure 2.18 SEM image of the CdTe nanowire array deposited on Cu film after dissolving the PC membrane
2.2. Preparation of the hybrid inorganic/organic photovoltaic cells based
on CdTe nanowires and ZnPc organic dye
Using the CdTe nanowires grown on Au electrode described in sub-chapter 2.1.1 two different
photovoltaic devices were designed and measured. The photoelectrical response of the second
design which was best suited and will be used for other cells as well will be compared below, in
sub-chapter 2.2.2, with the one of the first design which had problems with collecting the charges
at the electrodes.
The difference between the two designs is the addition of a CdTe thin film of about 300 nm
which is covering the Au electrode completely and it is decreasing the charge recombination rate
at this electrode (published in [56]).
2.2.1. Technological steps for producing the hybrid photovoltaic devices
To continue the processing of the solar cell a 300 nm CdTe thin film was deposited by thermal
vacuum evaporation at a temperature of 300oC and on top of it a 400 nm ZnPc organic dye, with
the help of the same technique (figure 2.19).
a)
b)
Figure 2.19 a) Schematic representation of the of the photovoltaic device after deposition of CdTe thin film
b) Schematic representation of the of the photovoltaic device after deposition of ZnPc thin film
The SEM image of the structure at this moment of the preparation shows that the nanowires are
still standing and have a larger diameter (figure 2.20). Also at the basis of the nanowires the Au
electrode is completely covered.
Figure 2.20 SEM image after covering CdTe wires with CdTe thin film (300 nm) and ZnPc (400 nm)
As seen the morphology of the nanowires is changed. In order to finalize the structures a
transparent electrode was coated by pulsed electron deposition (schematic view in figure 2.21).
The superficial resistivity for the 300 nm ZnO thin film was 88Ω/cm2.
Figure 2.21 Supposed structure of the photovoltaic device
In the SEM image the different morphology can be noticed (figure 2.22) after this final step.
Ag contacts had been made for being able to measure the properties of the cell (one on ZnO, the
other on Au).
Figure 2.22 SEM image of CdTe nanowires covered with CdTe film, ZnPc and ZnO (300 nm)
In order to perform the optical, photoelectrical and electrical measurements on this device
contacting the electrodes is a crucial step in order to avoid having a shortcircuit (figure 2.23).
Figure 2.23 Schematic view of the sample with electrical contacts
2.2.2. Optical, photoelectrical and electrical measurements and results
The spectral dependence of the external quantum efficiency (EQE) of two photovoltaic
structures, CdTe wires/ZnPc is shown in figure 2.24, respectively CdTe wires/CdTe (300
nm)/ZnPc at ambient temperature (25°C), in figure 2.25.
200 400 600 800 1000
0.0
0.2
0.4
0.6
0.8
1.0
No
rmali
zed
Ab
sorb
an
ce
No
rmali
zed
EQ
E
Wavelength (nm)
0.0
0.2
0.4
0.6
0.8
1.0
Figure 2.24 EQE spectrum of an Au/CdTe (nws)/ZnPc/ZnO photovoltaic structure and the corresponding
absorption spectra of ZnPc
Figure 2.25 EQE spectrum of an Au/CdTe (w)/CdTe/ZnPc/ZnO photovoltaic structure. For comparison purposes,
absorption spectra of ZnPc and CdTe films deposited on optical glass in the same conditions are also given (in red
line, respectively in green line)
In the case of the structure without the thin CdTe film deposited the external quantum efficiency
follows the features in the absorption spectra of ZnPc in the investigated spectral region. Those
features correspond to Q absorption bands of ZnPc (in the range from 500 nm to 800 nm),
500 600 700 800 900 1000 1100
0.0
0.2
0.4
0.6
0.8
1.0
No
rmali
zed
Ab
sorb
an
ce
No
rmali
zed
EQ
E
Wavelength (nm)
0.0
0.2
0.4
0.6
0.8
1.0
associated to electronic excitations. Under illumination with higher energy photons (low
wavelength) EQE increases abruptly, due to B (0, 0) and B (0, 1) electronic excitations (Soret B
band, extending below 400 nm). The shoulder at 900 nm is probably due to the onset of light
absorption in the CdTe wires. The maximum value of EQE is 0.8%.
At the second sample, the external quantum efficiency of the Au/wire array CdTe/CdTe (300
nm)/ZnPc/ZnO structures was significantly increased on the measured range of wavelengths,
with respect to that of Au/wire arrays CdTe/ZnPc/ZnO structures and, as it seen in figure 2.25, it
follows the features in the absorption spectra of ZnPc, but also the absorption spectra of the
CdTe thin film in the investigated spectral region.
For all the samples the electrical characteristics were not good because of the shortcircuit which
appeared while measuring.
2.3. Preparation of the inorganic photovoltaic cells based on CdTe
nanowires and CdS thin film
The main idea for developing these structures was to improve even more the performances of the
cells presented in the previous sub-chapter. Trying to densify the array of CdTe nanowires, using
this time 109 pores/cm
2 and growing it on Cu electrode as described in sub-chapter 2.1.2 it is
expected to increase the efficiency at a lower cost.
By searching the best option and observing the number of trials for CdTe/CdS devices[57], for this
type of cells, using a nanostructured CdTe and a thin film of CdS an important difference can be
seen when comparing to the other structure responses.
The photoelectrical response of the cell having the second design (with a thin film of CdTe
deposited onto the nanowires) which was best suited has been measured.
2.3.1. Technological steps for producing the inorganic photovoltaic structures
In figure 2.26 is schematically shown the next step in processing the photovoltaic structures by
adding a 88 nm CdTe thin film with the help of thermal vacuum evaporation technique in the
following conditions: temperature of the substrate was 90oC, temperature of the source was
450oC, having a deposition rate of 0.5 nm/sec.
Figure 2.26 Schematic view of the design of the sample after the CdTe thin film deposition
In the actual processing the design was the same as the one proposed, the nanowires were
standing after the deposition of the thin film and their thickness increased due to it as seen in
figure 2.27.
Figure 2.27 SEM image of the CdTe nanowire array covered with 300 nm CdTe thin film
The morphology of the nanowires has been changed also after the next step, the deposition of the
CdTe layer by thermal vacuum evaporation. For making the deposition the temperature of the
substrate was 90oC, temperature of the source was 600
oC, having a deposition rate of 1.6 nm/sec.
The thickness of the resulting CdS layer was about 327 nm.
The SEM image (figure 2.29) shows the nanowires with a bigger diameter, having the same
shape as the projected design (figure 2.28).
Figure 2.26 Schematic view of the design of the sample after the CdTe and CdS thin films deposition
The shape of the nanowires is followed by the thin films, assuring a large number of interfaces
between CdTe and CdS.
Also the recombination rate is significantly lowered by covering the Cu electrode with the CdTe
thin film before the deposition of the CdS thin layer.
Figure 2.29 SEM image of the CdTe nanowire array covered with CdTe and CdS thin films
After processing the final step, the deposition of the top transparent electrode by magnetron
sputtering technique was made. The deposition was performed at a current of 30 mA, when the
Ar gas flow was of about 3 sccm at a pressure of 3 mtorr.
The sample has been contacted (figure 2.30) for further measurements.
Figure 2.30 Schematic view of the design of the sample after processing with electrical contacts
2.3.2. Optical, photoelectrical and electrical measurements and results
When measuring the photocurrent at each wavelength in the domain where both semiconductors
have absorption peaks, the external quantum efficiency can be calculated.
When illustrating the EQE spectrum (figure 2.31) it can be noticed that it follows the absorption
spectra of the CdTe and CdS, overlapping it in some parts. The maximum value of EQE is
1.13%.
Figure 2.31 EQE spectrum of an Cu/CdTe n(ws)/CdTe/CdS/ITO photovoltaic structure. For comparison purposes,
absorption spectra of CdS and CdTe films deposited on optical glass in the same conditions are also given (in red
line, respectively in green line
For this type of structures the electrical characteristics have been possible to measure. The
current voltage characteristics in the dark, at room temperature and current voltage characteristic
in AM 1.5 illumination conditions are shown in figure 2.32.
The supralinear behavior of the characteristics and the power generator part shows that indeed
the structure acts as a solar cell having small power conversion efficiency (figure 2.32).
Figure 2.32 I-V characteristics in the dark (black) and under AM 1.5 illumination of the
Cu/CdTe(nws)/CdTe(300nm)/CdS(400nm)/ITO cell
-1.0 -0.5 0.0 0.5 1.0-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Cu
rre
nt (
A)
Voltage (V)
500 600 700 800 9000.0
0.2
0.4
0.6
0.8
1.0
1.2
No
rma
lize
d E
QE
(nm)
No
rma
lize
d O
ptica
l A
bso
rptio
n
EQE Cu/CdTe(nws)/CdTe(100nm)/CdS/ITO
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Absorption CdS
Absorption CdTe
When enlarging the fourth quadrant and calculating the power generated by the cell (figure 2.33)
a primary idea of the performances of the cell is putted in evidence.
Figure 2.33 I-V characteristics under AM 1.5 illumination and the generated power of the
Cu/CdTe(nws)/CdTe(300nm)/CdS(400nm)/ITO cell
Taking into account the experimental determined parameters of the cell from the table below the
fill factor (FF = 27%) and the power conversion efficiency (η = 0.0064%) can be calculated.
The performances of the measured cells are not great, but an important step has been made
starting with the possibility of having electrical results.
Once this important step has been made a way of continuously improving the performances at
least at this price if not possible lower is being developed.
3. Ni/CdTe/Ni single nanowires properties
In order to make the multisegmented nanowires, first a Ni/CdTe junction has to be prepared to
find the most suitable conditions for having a stoichiometric compound of CdTe grown on Ni
(figure 3.1).
Figure 3.1 Schematic view of the metal- semiconductor junction
Isc Uoc Imax Umax Illuminated area
-2.318 x10-5
A 0.26 V -1.143 x10-5
A 0.14 V 0.2512 cm2
0.0 0.1 0.2 0.3-4.0x10
-5
-3.0x10-5
-2.0x10-5
-1.0x10-5
0.0
I (A
)
U (V)
P (
W)
-1.6x10-6
-1.2x10-6
-8.0x10-7
-4.0x10-7
0.0
3.1. Preparation of the Ni/CdTe nanowires
Preparing the electrodeposition of the nanowire with metal – semiconductor junctions was made in the same way as for the above preparation of the CdTe nanowires. First the metal was electrodeposited in the pores of the membranes at a tension of -1000mV. Deposition of nickel was performed from a Watts bath containing besides the “classical” components PVP as pore wetting additive. By adding PVP, the pore filling efficiency reaches 80%[58]. The deposition temperature was 50°±1° C. After, the deposition bath was changed and CdTe was grown from its specific above described bath. More trials were made for finding the best deposition potential. Bellow, SEM images and EDX spectrums of different processed samples at some potentials are shown. For CdTe grown at -450mV there was a Te excess in the composition shown in the EDX spectrum (figure
3.2). Ni was grown for 4 minutes and Cd Te for 4 hours.
a) c)
Figure 3.2 a) Scanning electron microscopy image of an
array of Ni – CdTe multisegment( secondary electrons
detector image) b)Scanning electron microscopy image of an
array of Ni – CdTe multisegment wires (left back scattered
detector image/ right secondary electron detector image) c)
EDX spectrum showing 44.3%Cd and 55.7% Te
b)
For CdTe grown at -500mV there was a Te excess in the composition shown in the EDX spectrum (figure
3.3). Ni was grown for 4 minutes and Cd Te for 2 hours.
a) b)
Figure 3.3 a) Scanning electron microscopy image of
an array of Ni – CdTe multisegment( secondary
electrons detector image) b)Scanning electron
microscopy image of an array of Ni – CdTe
multisegment wires (left back scattered detector
image/ right secondary electron detector image) c)
EDX spectrum showing 44.7%Cd and 55.3% Te c)
For CdTe grown at -550mV there was an almost stoichiometric compound taking into account the EDX
errors (figure 3.4). Ni was grown for 4 minutes and Cd Te for 3 hours.
a)
c)
Figure 3.4 a) Scanning electron microscopy image of an array of
Ni – CdTe multisegment( secondary electrons detector image)
b)Scanning electron microscopy image of an array of Ni – CdTe
multisegment wires (left back scattered detector image/ right
secondary electron detector image) c) EDX spectrum showing
48%Cd and 52% Te
b)
For CdTe grown at -700mV there was a Cd excess in the composition shown in the EDX spectrum (figure
3.5). Ni was grown for 4 minutes and Cd Te for 2 hours.
a) c)
Figure 3.5 a) Scanning electron microscopy image of an array of
Ni – CdTe multisegment( secondary electrons detector image)
b)Scanning electron microscopy image of an array of Ni – CdTe
multisegment wires (left back scattered detector image/ right
secondary electron detector image) c) EDX spectrum showing
56%Cd and 44% Te
b)
Two graphics (figure 3.6) were made in order to see the percentages of both Cd and Te at
different growth voltages.
Figure 3.6 Percentages of Cd and Te at different growth voltages
The right growing conditions can be now chosen for making the Ni/CdTe/Ni nanowires in order
to further try to contact them using a FIB/SEM.
The variation of the current and potential in time during electrodeposition can be seen in figure
3.7. It can be noticed that the deposition current for CdTe is higher than the one for the metal.
-700 -650 -600 -550 -500 -450
44
46
48
50
52
54
56
Cd
(%
)
U (mV)
-700 -650 -600 -550 -500 -450
44
46
48
50
52
54
56
Te
(%
)
U (mV)
Figure 3.7 Variation of the current and potential in time during electrodeposition of Ni and CdTe
3.2. Preparation of the Ni/CdTe/Ni nanowires
Knowing that is possible to grow Ni/ZnO/Ni nanowires[59], in the established conditions above,
another part of the nanowires was electrodeposited, again a Ni segment. Schematic view can be
visualized in figure 3.8.
Figure 3.8 Schematic view of the grown multisegments
Figure 3.8 Variation of the current and potential in time during electrodeposition of Ni/CdTe/Ni
0 65 130-0.005
-0.004
-0.003
-0.002
-0.001
0.000
0 6000 12000
Cu
rre
nt (A
)
Elapsed time (s)
Ni
CdTe
0 65 130
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0 6000 12000
Pot
entia
l (V
)
Ni
Elapsed time (s)
CdTe
0 65 130-0.005
-0.004
-0.003
-0.002
-0.001
0.000
0 6000 12000 0 600 1200 1800
Cu
rre
nt (A
)
Elapsed time (s)
Ni part 1 Ni part 2
CdTe
0 65 130
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0 6000 12000 0 600 1200 1800
Po
ten
tia
l (V
)
Ni part 1
Elapsed time (s)
CdTe
Ni part 2
Choosing a deposition voltage of -550 mV for CdTe the electrodeposition was made in the same
conditions as the previous samples.
After removing the template with chloroform SEM images of the nanowires were taken and also
a mapping of one of the multisegment nanowires was done (figure 3.9).
Figure 3.9 a) Scanning electron microscopy image of an array of Ni – CdTe multisegment (left back scattered
detector image/ right secondary electron detector image) b)Selecting an area of the image for mapping c) EDX
mapping showing Ni with blue, Cd with red and Te with green
These free standing nanowires can be used for making devices based on single nanowires with
the help of the FIB/SEM. One of the reasons for choosing Ni as the metal from the junction is
that it can be easily aligned using a magnetic field.
Once the copper is chemically etched, the nanowires are spread in a solution. From that solution
they can be arranged parallel on the substrate as it will be shown bellow.
3.3. Aligning nanowires
Simply by using the magnetic field between two small magnets Ni nanowires can be arranged
parallel with the field lines. One of the reasons for doing this is to avoid conglomerations of
nanowires and to be able to select single nanowires for contacting.
Immediately after putting a drop of the solvent with nanowires on the substrate, it is placed
between the two magnets which are positioned at a specific distance for allowing the nanowires
to align (figure 3.10).
a) b)
c)
Figure 3.10 Scanning electron microscopy image of aligned Ni nanowires
.
Because of the relative smaller time of growing this type of nanowires comparing with
semiconductors they will be used later for single nanowire contacting with the help of a
FIB/SEM.
3.4. Contacting single nanowires with FIB/SEM
3.4.1. FIB-SEM Description
It is a favorable combination of the electron and ion sources and optical columns attached on one
chamber. It extends imaging qualities of the scanning electron microscope with the possibility of
surface modification by a focused ion beam.
It is based on a high resolution SEM column with a Schottky field emission gun.
FIB control is fully integrated in the SEM software and simultaneous SEM imaging with FIB
etching or deposition can be done. The powerful software toolbox DrawBeam for basic shapes
creation with programmable process parameters is helping to etch or make depositions in the
desired geometries[60].
The gas injection system has ideal geometrical configuration with respect to SEM and FIB
columns and 3-axis microstage with automatic nozzles positioning (figure 3.11).
When positioning the point of interest at 55 degrees with respect to the electron gun, meaning at
the FIB- SEM intersection the design can be processed. The equipment in question has also a
nanomanipulator with which the nanowires can be moved for contacting, if necessary.
Figure 3.11 GIS nozzle for Pt deposition
3.4.2. Pt contacts
For the present experiments, a commercial “dual beam” instrument (Lyra microscope from
TESCAN) has been used. It integrates a 30 kV field-emission electron column and a Ga based 30
kV ion column placed forming 55degrees. For Pt deposition, an automatized gas-injection
system (GIS). The deposition details can be found in the table below.
Chemistry Pt deposition
Composition of deposited material (atomic fraction)4
Pt 0.2; Ga 0.1; C 0.7 (IBID)
Precursor material Trimethyl-methylcyclopentadienyl-
platinum – C9H
16Pt
Deposition rate 0.5 μm3
/nC (IBID)
0.008 μm3
/nC (EBID)
Electrical resistivity 200 μΩcm (IBID)
500000 μΩcm (EBID)
Minimum linewidth 50 nm (IBID,
15 nm (EBID)
All the rates and Pt properties can differ from contact to contact depending on the used currents,
pressure, and temperature.
3.4.3. Contacting single nanowires
When using aligned nanowires it is more easily to find one to put contacts on. Usually the
contacts are made in such a way to get to pads made with the help of photolithography or mask
assisted deposited contacts.
With the purpose of finally having a sensor based on a single nanowire transistor, Pt contacts are
deposited at the ends of a Ni nanowire.
A single nanowire is chosen (figure 3.12) and using a current of 15 pA the testing of the
deposition of the contacts begins.
Figure 3.12 Chosen nanowire for contacting
Before the actual deposition begins there are various steps to perform. The image of the
nanowire from figure 3.11 is taken at the FIB-SEM intersection, meaning at 55 degrees tilt with
respect to the e-gun direction. Once the SEM and FIB images are put in order, eliminating the
aberrations and having the beams in the best focus point the design of the contact is drawn with
the help of a program included in the apparatus software.
After setting the exact position and choosing the ion beam deposition mode the actual deposition
can start.
The design is rastered in steps and once one rectangle is filled the deposition starts for the
second. In figure 3.13 the first part of the contacts is deposited. Using relatively low currents the
surface of the nanowire should not be affected.
Figure 3.13 Pt contacts deposition with FIB – SEM with a current of ~15 pA
Because of the use of the SEM while deposition it can be noticed in figure 3.14 that not only the
pattern has Pt but it is also deposited around it.
This is pointing to a more careful use of the instrument, implying a better imaging and an
improved focusing of the images in both SEM and FIB.
Figure 3.14 Contacts depositions with artifacts
Several trials were made in order to find the best parameters for a Pt deposition which would not
affect the nanowire (figure 3.15 and figure 3.16).
Taking into account the tests made it can be concluded that for having good contacts with the
deposition only in the desired design, without touching the nanowire the smallest currents have
to be used. Also it is recommended not to use the SEM mode for viewing the insitu deposition
because it will interfere with the ion beam deposition.
In this microscope also electron beam deposition can be made, but the deposition time is highly
increased with respect to ion beam deposition.
Making a compromise between the waiting time for an electron beam deposition and the quality
it is summarize the fact that making an ion beam deposition with a low current is much more
advantageous.
For the future electrical measurements which are planned, the resistivity of the Pt layer has to be
taken into account, at different deposition rates for different conditions[61].
Figure 3.15 Pt contacts deposited at ~400 pA
Figure 3.16 Pt contacts deposited at ~200 pA
4. Conclusions and further work
Summarizing the presented experimental work it can be stated that there exist an obvious
improvement of the previously developed devices.
Taking into account the growth of the nanowires, the best conditions have been brought up for
having stoichiometric compounds with a cylindrical shape.
The structural properties of the nanowires have shown a well formed stoichiometric compound
with a zinc-blend cubic crystalline structure.
The changes made in the design of the cells got to the reduced rate of recombination at the back
electrode then increasing the holes collection.
For the Au/nanowire array CdTe/CdTe(300 nm)/ZnPc/ZnO cell the EQE follows closely the
features in the absorption spectrum of the organic dye, showing a good charge transfer at the
organic/inorganic interfaces. Also it was increased with about four orders of magnitude, with
respect to that of Au/wire arrays CdTe/ZnPc/ZnO structures.
Unfortunately electrical measurements of these samples were not made but the good
photoelectrical characteristics were pushing forward towards finding a solution for getting the
best possible results.
Once the idea of using CdTe/CdS flourished the possibility of having measurable electrical
results appeared. The quality of the nanowires and the preparation procedures were undoubtedly
the best till the present moment as a result the electrical behavior showed the characteristics of a
photovoltaic cell as predicted.
Nevertheless the efficiency was not high, but using these results as a starting point for further
development of this type of cells is the next logical step.
Further work includes varying the diameter of the nanowires, their length, making thermal
treatments of the nanowires for a better crystalline structure. Also varying the thickness of the
CdS layer and changing the top transparent electrode. All this is expected to lead to an important
increase of the power conversion efficiency.
Also the possibility to have multisegment nanowires has been properly investigated and the
success of preparing it leads to the development of many applications. Junctions made along the
wire with a perfect metal/semiconductor contact aims to less measuring errors of the properties
of this type of wires. When contacting the metallic parts of the wires it is obvious that the contact
metal/metal is much better than metal/semiconductor made by deposition. Functionalized
nanowires of this type can be used with success in fabricating sensors of different types.
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