Eindhoven University of Technology

MASTER

The electronic structure of an organo-metallic interfacea test case for ultra-violet photoemission spectroscopy

Berghmans, B.

Award date:2006

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TuI e technische universiteit eind hoven

The electronic structure of an organo-metallic interface:

a test case for ultra-violet photoemission spectroscopy.

Bart Berghmans Eindhoven, January 2006

Report of a graduation project carried out at the Eindhoven University of Technology at the Departement of Applied Physics in the group Physics of NanoStructures from November 2004 to January 2006.

Supervisors: Prof. Dr. B. Koopmans Dr. J.T. Kohlhepp

Abstract

Ultra-violet emission spectroscopy (UPS) is powerful technique for measuring the occupied electronic states of surface. In this report we present the proceedings of the instaU of a UPS excitation source; this includes the working principle and some important considerations about the practical functioning of the apparatus.

Afterwards, we will focus on the production and characterization of the interface between ferromagnetic metals and organic semiconductors. For this procedure was developed for measuring sub-monolayer thicknesses of the organic semiconductor on a metallic surface. This procedure was used to measure the gradual build-up of the interface. Using these samples of the partially constructed interface it was possible to trace the electronic prop­erties up until the interface is fully developed.

We will present the specific case of a Co/ Alq3 interface. It is shown that the energy levels show a very sharp adjustment close to the metallic surface. This sharp energy level alignment has some major implications on the electron injection barrier and the performance of a possible device.

Contents

1 Outline 1

2 Photoemission spectroscopy 3 2.1 Introduetion . 3 2.2 Some theoretica! aspects of photoemission spectroscopy 4

2.2.1 The photo-ionization process . 4 2.2.2 Numerical solutions of the electronk system 6 2.2.3 Final state effects . 9

2.3 Photo electron speetrometry . 10

2.3.1 Electron analyzer 10 2.3.2 Interpretation of a UPS spectrum 12 2.3.3 Secondary electrans . 14

2.4 Excitation sourees . 17 2.4.1 X-rays 18 2.4.2 Gas discharge VUV souree 19 2.4.3 Voltage biasing of the sample 24

3 Organic semiconductors 27 3.1 Introduetion . 27 3.2 Conjugated polymers 28

3.3 Organic semiconductors for spintronie applications . 30 3.4 Metal-semiconductor interfaces .. 31

3.5 Schottky barriers for organic semiconductors 34

3.5.1 Chemisorption . 35 3.5.2 Physisorption 37

4 Growth monitoring by differential ellipsometry 40 4.1 Introduetion . 40

4.2 Ellipsometry fundamentals . 41

4.3 Ellipsometry for thickness monitoring . 42

4.4 Sub-monolayer thickness measurements 45

5 Interfaces between an organic semiconductor an a ferromagnetic mate-rial 47 5.1 Introduetion . . . . 4 7 5.2 Sample preparation 47

5.3 Core level spectroscopy and chemica! interaction . 48 5.4 Binding energy shifts 52 5.5 Valenee band spectroscopy and vacuum level shift 53 5.6 Growth properties . 57 5.7 Energy alignment across the interface 59 5.8 Condusion . 61

6 Condusion and outlook 63

i i

Chapter 1

Outline

This thesis is the refiection of the graduation work performed at the Eindhoven University of Technology (TU/e) in the group Physics of Nano Structures (FNA). The main objec­tive of the graduation project was to install a new excitation souree for photoemission spectroscopy (PES). As will be further discussed in chapter 2, photoemission spectroscopy involves measuring the kinetic energy of an electron that was released after excitation with high energy electro-magnetic radiation. From the kinetic energy of the ejected electrons, the binding energy can be derived. The two most common radiation sourees are X-ray and ultra-violet radiation. An X-ray souree and the electron detection equipment was already presentand operational within the FNA group. The addition of a gas discharge UV souree enabled a study of the occupied electronic states near the Fermi level at a high resolution. A commercial gas discharge UV souree was successfully installed during the part of the graduation work. Same of the teehuical difficulties we encountered are also documented in this thesis.

In the second part of the graduation work we focused on an application for the upgraded equipment. In light of a new focus within the research group, an organic semiconductor was investigated. These organic semiconductors have been extensively stuclied for use in optoelectronic devices and are starting to be commercialized in this domain. One of the next applications could be the use in spintronie devices. This new class of electranies uses the spin of an electron, rather than its charge, as an information carrier. It is believed that organic semiconductors have a relatively long spin-diffusion length. Electrans thus remain relatively long in their original spin state as they are transported through the organic semiconductor. Therefore they make a good candidate for use in these new devices.

In this work we focussed on a single species of organic semiconductor, namely Alq3 .

Chapter 3 gives more insight on this materiaL Also an attempt was made to calculate the electronic structure of this molecule. Alq3 is known as a small molecule organic semi­conductor which means that layered structures of it can be applied by Organic Molecular Beam Epitaxy (OMBE). By using OMBE we were able to produce very clean samples as the entire stack of layers were produced and measuring without breaking the ultra-high vacuum.

In chapter 4 we discuss differential ellipsometry, a methad for measuring the thickness of the organic layer while it is being deposited. After a calibration, which is also given in

1

this chapter, we were able to determine the nominal thickness of sub-monolayer of Alq3

with a very high accuracy.

Because the thickness measurement occurs in real time with the deposition we can control the fin al thickness. This enabled us to produce samples of ferromagnetic metals (Co in our case) covered with a varying thickness of Alq3 , thus gradually building up the metal­semiconductor interface.

From the work aimed at electraluminescent devices it is known that the interface between organic semiconductors and the metallic electrades plays a crucial role in the device performance. An atomie contact between an organic semiconductor and a metal will cause the vacuum levels of the two materials to align, much like a Schottky contact with anorganic semiconductors. However the physical processes at work are more complex than in the anorganic case. In chapter 5 we will present the results of a study on the interface of a ferromagnetic metal ( the electrode of a spintronie device) and an organic semiconductor.

2

Chapter 2

Photoemission spectroscopy

Photoemission spectroscopy is a very powerful technique for an­alyzing the electronic structure of a material. In this chapter we will present the basic formalism and will give the reader an insight on how to read a photoemission spectrum. First the basic physical processes of a photoemission event will be discussed, after which the link of speetral data and binding energies will be made. Also some test measurements of the newly installed ultra-violet radiation souree are presented.

2.1 Intrad uction

For long it has been known that electrans could be released from metals by means of electro-magnetic radiation. The most intriguing part of this phenomenon was that the amount of electrans per unit of time that is released, the photocurrent, does not depend on the frequency of the radiation, but only on the intensity. In as early as 1905 a revolutionary explanation was given by consiclering light as a flux of energy packets, or photons [1]. By this, the new field of quanturn physics was born. The Einstein Photoelectric Law is essentially an energy balance between the maximal electron kinetic energy EK, the ionization potential I, and the photon energy hv.

EK= hv- I, (2.1)

3

In the past 100 years, many advances in science and technology have evolved this ground braking discovery into a widely accepted characterization technique. Photoelectron spectroscopy involves measuring the kinetic energy of electrans that were released by monochromatic radiation (hv is constant) to obtain the binding energy EB spectrum of a specimen (see figure 2.1).

(2.2)

Some remarks must be made for this simplified representation. It is assumed that the electron carries all the kinetic energy after ionization. This violates the conservation of momentum, but the error is negligible sirree the mass of the electron is many orders of magnitude smaller than that of the ion. Secondly, it doesn't take into account the occurrence of a photon colliding with an excited ion, or two photons exciting an atom simultaneously. These ef­fects are extremely rare compared to single photon effects, and can also be neglected.

e

VACUUM

Figure 2.1: Basic rep­resentation of the pho­toemission process.

Modern photoemission spectroscopy is commonly divided by the excitation energy that is used. When the energy ofthe photons is more than typically 100eV it is called X-ray Pho­toemission Spectroscopy (XPS). For lesser energies the term Ultra-violet Photoemission Spectroscopy (UPS) is used. During the graduation workan ultra-violet (UV) souree was installed in a cluster of interconnected ultra-high vacuum (UHV) equipment for structur­ing and analyzing thin-film layered structures (see figure 2.2).

The addition of the UV souree to the vacuum cluster made it possible to study the electronic structure of surfaces near the Fermi levels with much more resolution as was previously the case.

The whole process of exciting an ion and decaying to the ground state by emission of an electron is more complex as the simple photoelectric law might suggest. Therefore it is useful to have a more in-depth look on the photo-ionization process.

2.2 Same theoretica! aspects of photoemission spectroscopy

2.2.1 The photo-ionization process

Assume a system of N electrans of which the initial state is given by the wave function wi(N) with an energy Ei(N). The first step in the photo ionization process involves the absorption of a photon hv, whereafter the N electron wave function is in an excited state \[Jf (N, k), or

(2.3)

4

Figure 2.2: The gas discharge UV souree as it was installed at the TU/e. The individual components that are marked will be further discussed in this work.

5

The index k indicates the mode of excitation, combining all electronic, vibrational and rotational modes of the electron system. By the conservation laws, the energy of the final state Ef(N,k) must be given by

(2.4)

When the excitation energy is sufficiently large, the excited electron becomes very loosely coupled to the remaining ( N - 1) electron system. In that case, the final state can be separated in an ion in the kth excited state iiJ f ( N- 1, k) and a free electron <Px. The state of the free electron is given by spatial wave function <P and a spinor X· Again, energy conservation dictates

(2.5)

where EK is the kinetic energy of the photoelectron. Let 's define the ionization energy h as the energy needed to excite an electron system into an ionized state iiJ f ( N - 1, k). It can be directly linked to the kinetic energy of the photo electron.

h = Ef(N -1,k)- Ei(N)

~ h = hv- EK

(2.6)

What is left to investigate is the relation between the ionization energy h and the binding energy EB ( k) of the kth energy level. It will be shown that in good approximation both energies can be identified. To do this , we will examine the origins of the energy levels of the N electrons system.

2.2.2 Numerical solutions of the electronic system

In order to calculate the energy levels of the N electron system, the time independent Schrödinger equation has to be solved for the total wave function iiJ(N).

'HiJ! = Eif! (2.7)

With 'H a Hamiltonian that is suited for the system. To solve this equation numerically, the problem is usually set in the basis spatial one-electron spinorbitals </Ji(r)xi(O') that combine a spatial vector <Pi (r) and a spin vector X i ( 0').

Because we are looking at an electron system, it has to obey Fermi statistics. To im­pinge the antisymmetry condition associated with this, the wave function iiJ has to be represented by a linear combination of spinorbitals such that

iiJ(<Pl(r)xl(O'), ... , <Pi(r)xi(O'), <Pi(r)xj(O'), ... , <PN(r)xN(O')) = -iiJ(<P1(r)x1(0') , .. . , <Pi(r)xj(O') , <Pi(r)xi(O'), ... , <PN(r)xN(O')) , (2.8)

6

where the index of the spatial and spin coordinates indicate a specific electron. The antisymmetry condition is automatically satisfied when the wave function is given by the determinant of the Slater matrix S

with

1 '11 = 11\TïJSJ,

vN! (2.9)

(2.10)

Note that the Pauli principle is automatically satisfied, because when two spinmbitals are equal there are two identical columns in the Slater matrix, and the determinant will vanish.

What is left is to find the spinmbitals that best describe the system. To find these the condition that the total energy of the system in steady stateE =< 'l'J'HJ'l' > will always be in a minimum can be used. The basic idea is to find the set molecular mbital functions qy with the lowest energy E. In every step of the variational process the molecular orbital wave functions are updated and used in the new Hamiltonian. In chapter 3 the molecular orbitals fora complex molecule (Alq3 ) are calculated by using a computer program based on MOPAC software1

.

When the wave function is well approximated by a linear combination of spinorbitals, the expectation value of the energy Ei of a spinor bi tal i can be calculated by the Fock operator :F [2]. We will use the shorthand notation ra,{3 = Jra,r{3J for the average distance between particles a and (3 (i,j for electrons, l for nuclei). Z1 is the atomie number for nucleus l.

[-!!!_'V;-~ ez Zz] cPi(ri) 2m ~ riz

l=l

+ [t, j r:; <l>j ( r; )<I>; (r; )dr;] </>;( r;)

-ó( a,, a;) t, [t, j r:; </>j ( r; )</>;( r; )dr;] </>; ( r,)

Ei (ePi ( ri)Xi ( O"i)) (2.11)

There are three contributions in this equation. Between the first square brackets the kinetic energy of the electrans and the potential energy due to the Coulomb interaction with the P nuclei with an atomie number Z1 is accounted for. The second term gives the Coulomb repulsion between the individual electrons. And the last term is for the exchange interaction between electrans of opposite spin, hence the Kronecker b in front.

1. MOP AC is a program for theoretica! studies of molecules. It perfarms semi-empirica! molecular orbital calculations to determine molecular properties, such as geometry and electronic structure. While older versions are freely available, the latest version is sold by Fujitsu.

7

As the distance between two nuclei will not significantly change for the timescales of the photoemission process, the Hamiltonian of the inter-nuclear interaction can be considered as a constant. It will not contribute to an energy change during ionization and therefore can be omitted. The only energy contributions that can be resolved in photoemission spectroscopy are the interactions of the electrans as they interact with other electrans and the static energy landscape formed by the nuclei.

To simplify the notations, the Coulomb and exchange operators :Ij and Kj are usually defined such that

j r~j c/Yj(rj)c/Yj(rj)drjc/Yi(ri)

j r~j c/Yj(rj)c/Yj(ri)drjc/Yj(ri).

with the accompanying matrix elements of these operators are given by

< c/Ji ( ri) I :Ij lc/Ji ( ri) > < c/Ji(ri)IKjlc/Yi(ri) >.

(2.12)

(2.13)

(2.14)

(2.15)

Once the orbital wave functions are well approximated, the energy can be easily found by

N

Ei= E? + L (Jij- 6(0'i, O'j)Kij). j=l

In this expression c? is the one-electron energy for the yth orbital.

(2.16)

(2.17)

The total energy of the N electron system is found by adding the energies of the orbital energies. Note that for the Coulomb and the exchange operator the restrietion j > i is added to avoid counting these contributions twice.

N N N

E = L E? + L L (Jij- 6(0'i, O'j)Kij). (2.18) i=l i=l j>i

The potential energy induced by the nuclei is not included here. As stated before, this is static in the time frame of the photoemission experiment and can therefore be omitted.

With these equations we have constructed a methad to calculate the energy of the molecu­lar orbitals and the total energy of the system, provided that the orbital wave functions c/Ji

8

and spinors ai are known. A common method to find these orbitals is finding a minimum for the total energy E by a variational method.

Using these equations it is possible to find an approximation for the binding energy of an electron. That is, the energy difference for the electron neutral wi(N) and the ionized wf (N- 1, k) state.

2.2.3 Final state effects

When photoelectrons are excited, an ionized state of the atom or molecule is created. In principle the creation of a hole infiuences the electronic states that are to be measured. In some cases it can be assumed that these effects are minimal, so that the ionized state can be described in the same basis of molecular orbitals at the neutral state. We can thus see the ionized state a system where one electron has been removed and the energies of the others are not affected. This approximation is also valid when the relaxation happens well after the photoemission process, such that the exiting photoelectron is not infiuenced. Because in that case we know molecular orbitals of the ionized state, the energy of the ( N - 1) electron system can be calculated as

N

Ef(N -1, k) L (Jij- o(msi, msj)Kij) i# i# (j>i,j#)

N N N

L E? + L L(Jij- o(msi, msj)Kij) ioftk i=l j>i

N

- L(Jij- o(msi, msj)Kij). (2.19) i=l

When the difference with the energy of the N electron system ( equation 2.18) is taken, it is found that this is exactly the energy of the kth orbital.

This approximation is known as Koopmans' theorem, named after TjaHing Charles Koop­mans whowas a joint laureate of the 1975 Nobel prize for economics. Unfortunately this approximation is very crude and only applies to few systems. In many cases the relax­ation effects and the effects and the energy shifts that go with it can be so large that they cannot be neglected.

Often the system will react upon the ejection of an electron well before it has left its close environment. This is especially the case for slow (low energy) electrons. When this is the case, a correction term called the relaxation energy has to be incorporated to account for these effects.

In recapitulating this theoretica! section, we can state that the simple photoelectric law Es= hv- EK is aresult of Koopmans' theorem, where Es is the binding energy of the excited molecular orbital with respect to the vacuum level. This simple expression can be used as a first order approximation, but one has to be conscious about the possible final state effects that can change the apparent binding energy of a photo electron.

9

decelaration element

hv

a

hemispherical

b

Figure 2.3: a) A schematic representation of the electron analyzer. The electrans are excited by high energy photons (hv) and are focussed onto the analyzer entrance slit by an lens. After passing through the analyzer, which selects the electrans with a certain energy, the electrans can be detected by a channeltron. b) A close-up of the channeltron. When an electron enters the funnel an hits the inside wall many secondary electrans are generated. These will be accelerated by the high electric potential that is applied along the channeltron. At the end of the channeltron dedicated electranies can detect these amplified electron pulses.

2.3 Photo electron speetrometry

2.3.1 Electron analyzer

After excitation, photoelectrons are emitted in all directions of the sample. Usually the electrans that are emitted perpendicularly to the sample surface are registered because these have traveled less through the sample and are thus less subjected to scattering.

A number of electron analyzer designs exist, but for the measurements presented in this thesis a hemispherical analyzer was used (see part a in figure 2.3). In this design an electrical field EA can be applied between the two hemispheres. An electron with kinetic energy EK can only pass the analyzer if the force exerted by the field equals the centrifugal force.

eEA = e V= mv2

= 2EK d r r

(2.20)

For a specific analyzer this equation can be simplified by stating that the energy of the electrans being passed equals

(2.21)

10

purpose XPS ( overview) XPS (detail) UPS

input slit (mm) 15 x 6 15 x 6 15 x 3

output slit (mm) 10 x 6 10 x 6 10 x 3

CAE energy ( e V) 50 20 10

Table 2.1: Slit sizes and CAE values for common photoemission spectroscopy measurements.

with H a constant defined by the analyzer and V the applied voltage to generate the electric field. A lens is used to transfer electrans from the sample to an entrance slit. The electron opties in the transfer lens assembly also contain an element that can retard ( or accelerate) electrans with a given energy Edec befare they enter the analyzer.

Recording of an electron energy spectrum, i.e. the energy distribution of particles leaving the specimen, can be performed in two modes, the so-called Constant Analyzer Energy (CAE) and Constant Retard Ratio (CRR) mode. In the CAE mode, the electric field in the analyzer, and thus the analyzer pass energy, is kept constant, and a variable retarding voltage is applied to the deceleration element in the transfer lens. In this way, only elec­trans that leave the target with an energy EK = HV- Edec can pass through the analyzer befare they are detected. By recording the number of detected electrans as a function of the retarding voltage the electron spectrum is obtained. Alternatively, an electron energy distribution may be obtained by variation of the pass energy, keeping the ratio between the pass energy and the retardation voltage constant. This is clone in the CRR mode. Since the spatial divergence of the electron trajectories in the analyzer increases with decreasing pass energy, the energy resolution in the CRR mode is proportional to the detected energy tlE ex: E. The CRR mode is used for Auger spectroscopy, which will not be discussed here. In photoemission spectroscopy, spectra are always reearcled in CAE mode, because in this way the analyzer resolution is constant over the entire spectrum.

In any case the resolution is defined by the acceptance angle and the pass energy. The acceptance angle is set by a slit at the analyzer entrance. The analyzer that was used for the experimental was equipped with multiple slits of different sizes. Bath the slit size and the analyzer pass energy HV have to be optimized for the experimental conditions. Low pass energies and smaller slits will result in a better energy resolution and a lower intensity. For UPS a high resolution electron detection is usually required, therefore a small slit and low pass energy are needed. For XPS, resolution is less crucial because the excitation sourees have a larger speetral width tostart with. Therefore a larger slit and a higher pass energy are used to maximize the signal intensity. Table 2.1 gives the slit sizes and CAE values that were used for the experiments presented in this work. For detailed XPS spectra, when we focussed on small energy range, a smaller CAE value was used to maximize the energy resolution.

A channeltron (shown in panel b of figure 2.3) is used as electron detector. This is a bent tube that is coated with an insulator that produces a high secondary electron count. Over this tube, a potential of about 3 kV is applied. When an electron strikes the funnel shaped entrance of the tube, a number of secondaries are produced and are accelerated into the channeltron. These accelerated secondaries in their turn produce secondaries and so on. Ultimately a large number of secondaries is produced from a single electron. The energy

11

required to produce these secondaries is supplied by the channeltron voltage supply. In this way a pulse is produced that indicates the arrival of an electron in the detector. Ultimately, the pulse is shaped and registered by the acquisition electronics.

2.3.2 Interpretation of a UPS spectrum

A photoemission spectrum gives the rate (in counts per second, CPS) that a photo electron with a certain kinetic energy EK is registered. According to Koopmans' theorem, the kinetic energy value can be linked with the binding energy of this electron befare it left the sample. If an electron is not inelastically scattered between the excitation and detection, the binding energy with respect to the vacuum level E~L must be

(2.22)

Usually it is more interesting to give photoemission spectra with respect to the Fermi level. Throughout this work, this will be the preferred energy reference. The binding energy is then given by

EB =EK- hv- W,

with W the work function of the sample.

(2.23)

For XPS this conversion is calibrated into by the electron analyzer. The calibration of the analyzer that was used for the XPS spectra in this report was verified by the position of the 2pl and 2p',l peaks of a Cu(llü) surface. The measured value of these peaks agree

2 2

with reported values [3].

For UPS it is better practice to calibrate each spectrum manually. Therefore, in a UPS experiment the kinetic energy of the photoelectrons is measured. For metallic surfaces, the electronic states are occupied right up to the Fermi level. For lower binding energies the density of occupied states sharply falls to zero. In photoemission spectroscopy this is referred to as the Fermi edge. Due to resolution restrictions of the setup this sharp fall is widened. The exact point of the Fermi energy is therefore placed where the (negative) slope of the spectrum is minimaL In the following, this point will be taken as the zero binding energy reference. Because semiconductors and insuiators have no occupied states at the Fermi level, it is not possible to measure the Fermi level directly. However it is possible to determine the Fermi level by measuring a metal that is in thermadynamie equilibrium with the semiconductor (eg. the sample holder).

In figure 2.4 the UPS spectrum of a Au(lOO) surface is shown. We will use this example as an illustration on how to read a photoemission spectrum. Also this UPS spectrum served as a test measurement for the newly installed radiation source. The spectrum was taken using He I radiation and with a sample bias of -4 V. These technical notes will become clear in this chapter.

The UPS spectrum is a superposition of electrans coming from discrete energy levels onto a continuous background generated by secondary electrons. Secondary electrans are

12

~ I !

0

UPS (He I) Au(100)

: .. 2 4 6 8

spectrum width IE.-EsEcl

-as measured --- secondary electrens -alter correction

vacuum level

work function

excitation energy h v (21.2 eV) ..:

10 12 14 16 18 20 22 24

Kinetic energy (eV)

26

Figure 2.4: UPS spectrum of the (100) surface of Au with a 5x20 reconstruction.

13

photo-electrons that have lost energy in an inelastic scattering event in the solid. Because an electron can only loose energy in an inelastic callision ( and this can occur several times befare escaping into the vacuum), the density of secondary electrans is higher for small values of kinetic energy. The sharp fall to zero, which is labeled as the Secondary Electron Cutoff (SEC), is due tofact that ari electron needs to have a minimal amount of energy, the work function, in order to escape into the vacuum.

Therefore, onset of the SEC provides an excellent marker for the electrans that have just enough energy to overcome the work function. By conservation of energy, this point is exactly the amount of the excitation energy hv below the vacuum level. When the SEC energy EsEC is given with respect to the Fermi level Ep, the work function W is given by

W = hv- EsEC· (2.24)

The work function is graphically found by subtrading the width of a spectrum lEp-EsEc I of the excitation energy hv, as is also shown in figure 2.4. This feature makes UPS a very qualified technique for determining the surface work function. In chapter 5 this methad was used for measuring the work function shift through the interface of a ferromagnetic metal with an organic semiconductor.

After subtradion of the secondary electrons, the features in the UPS spectrum can be analyzed (see figure 2.5). The peaks labeled A and Bare properties of the bulk Au and are seen for all Au spectra [4]. Peaks Dl to D3 are split off states, and are aresult of breaking the bulk symmetry. Feature S is a surface state and is only seen in the 5x20 reconstruction of the Au(lOO) plane. This reconstruction was verified by LEED. The LEED picture is shown in the insert of the figure.

Although secondary electrans provide an excellent marker for determining the work func­tion in UPS, insome case it is better to subtract these using a proper model. The secondary electron intensity will ( slightly) shift any energy peaks to a higher binding (lower kin et ie) energy. More importantly, when one wants to campare the intensity of a peak, the sec­ondary electron background is more important. In the following section we will present a procedure for calculating the secondary electron contribution in a spectrum.

2.3.3 Secondary electrans

Secondary electrans are electrans that were subjected to an inelastic scattering event. The average distance between two such events in a solid is given by the Inelastic Mean Free Path (IMFP). The mean free path of the electrans in the energy regime of photoemission spectroscopy is plotted in figure 2.6. The dashed curve shows a calculation of the mean free path independent of the material and the points are measured data from many elemental solids. The data points scatter more or less around the calculation. Therefore the curve is aften called the universa] scattering curve. The reason for this universality is that the inelastic scattering of electrans in this energy range mostly involves excitations of conduction electrons, which have more or less the same density in all these elements. At lower energies other scattering mechanisms will be important, like the scattering with phonons.

14

11 12 13 14 15 16 17 18 19 20

Kinetic energy (eV)

Figure 2.5: UPS spectrum of a the (100) surface of Au with a 5x20 reconstruction. The original spectrum ( thin solide line) has been correct for the secondary electron contri bution ( dashed line). The corrected spectrum is given by the thick solid line. The inset shows the LEED picture that confirms the 5x20 reconstruction.

15

200

• Al + Hg

100 • o Ag "' Mo \ • Au ~ Ni

\ \ t:J. Be 0 p

.g 0 \ TC o Se \

50 • \ C> Fe +Si .-< ~ \

.r: \theory ~Ge vW 'lij \ a. 0 \

Q) \

~ 20 0. \ • ... T • \ .""'"" c: • \ T, ." \ o• Q) ' , .LS

E 10 0 ' 2b , 0' ~/:; ~ , ,

' y,O"' ' ' ' 0 i. 0 __ , ' ...

5 + '~---!'-V'-- •

o ... 6 o

32 5 10 20 50 100 200 500 1000 2000

electron kinetic energy (eV)

Figure 2.6: The mean free pathof the electrans in solid. The dots are measurements and the dashed curve is a model calculation. [5]

The mean free path curve has a broad (note the log-log scale) minimum around a kinetic energy of about 70 eV. There it is less than 10 A. Therefore if an electron is observed in this kinetic energy range without having suffered an inelastic scattering event it must have originated from the first few layers.

A common method for eliminating the secondary electron background is the Taugaard algorithm. In this method secondary electrans are modeled to be coming from higher energy free electrans that scatter according to the universal scattering curve.

Suppose an electron intensity j(E') is registered for the measured energy E'. A part of these electrans are not coming from the supposed energy state, but have lost some energy in an inelastic scattering event and were actually generated with a higher kinetic energy (which corresponds toa lower binding energy). The fraction has been found to be [6]

(2 .25)

for electrans that had an original binding energy E , where C = 1643 eV2 and B1 is a fitting paramater. To know the intensity of scattered electrans that end up with an energy E' , the scattering fraction 2.25 has to be integrated for all values starting from E . After subtraction of the background, the true photoemission intensity F(E) is found.

+oo

F(E) = j(E)- B J (E'- E) j(E')dE' 1 [C + (E'- E)2] 2

E

(2 .26)

The background in figure 2.5 was calculated using this algorithm. The Taugaard method

16

:i' ~ ~ ëii

18 16

XPS (Al Ka)

$ UPS (He I) c

0 2 4

14 12

6 8

Binding energy (eV)

10 8 6 4 2 0 -2

10 12 14 16 18 20 22

Kinetic energy (eV)

Figure 2. 7: A comparison of the valenee band photoemission spectrum of a 5x20 reconstructed Au(lOO) surface. The upper spectrum was excited with AlK a X-rays, the lower spectrum with He I UV light.

was also used for some XPS spectra in chapter 5. After a correction for the secondary electrans is was possible to campare the intensity of speetral features for different samples. The Taugaard methad is not always as effective for UPS, but it has been proven to work for many XPS examples [7].

2.4 Excitation sourees

In photoemission spectroscopy different types of radiation can be used to excite the elec­tron levels. Most common radiation sourees are X-ray anodes and gas discharge lamps. Also synchrotron radiation can be used, but will not be discussed as it wasnotpart of the experimental work in this thesis. Depending on the type of radiation photoemission spec­troscopy is classified as XPS (X-rays) or UPS (UV radiation). Figure 2.7 demonstrates the difference between the two excitation sources. Both spectra are measured from the 5x20 reconstructed surface of Au. The top spectrum was excited using X-rays (AlKa) and the bottorn spectrum with UV light (He I). These nomendatmes will become clear in this section.

17

-c;; c;; c:: 0

..c '-' '-"' C>.

"' ë ;;, 0

<..)

105 ,.............----~. -. -· Klz· KL 3

I

i1o 2 ~ K KL CKL4 l

' <11, 2 l

~-

I j

~ ~ r <13 +o r f

1 103f- (15 (16 -;

•' !3

11 < r .. •'

,.;"·,,;~,. a.~ a,o ,., Re+ .....

' . Q13 Q\4 o/' i· Rhv_,,_ ·.. . ........ 1 '

.·: ... ~~.,. ./. 'N:,J',:.,\ - .. ~.·.:·:-:.·-. ' 1 >~ 10" 1

( I

10 1~----~~~----~~~~_J--~~ -IQ 0 20 40-

6. E, rel at ive energy CeVl

>,

~ c "' .S ~

+-' 0

-c;; c:t::

Figure 2.8: The K X-ray emission spectrum of Mg. The peaks 0:1, o:2, ... , (3 indicate the various transitions to the K = 1s shell. The dots represent the measured values, the dashed line is the average background and the solid line is the fit of the net spectrum. Reproduced from [9].

The valenee band of the Au surface is measured in both spectra. The spectra are aligned such that the Fermi levels coincide. The top scale gives the binding energy and the bottorn scale gives the kinetic energy of the electrans in the UPS experiment. The difference in resolution immediately becomes clear. The He I radiation has a much smaller line width than AlK o:. The other large difference is the amount of secondary electrons. For valenee band electrans that are excited with X-rays, the kinetic energy is larger than 1 keV, and they have a small probability toscatter inelastically. For UPS however, the kinetic energy of the photo electrans is much smaller (in the order of 10 e V), and therefore the IMFP is much smaller (in the order of 10 Á[8]).

UPS is used for making high resolution spectra of the occupied states near the Fermi level; the valenee band. XPS has less resolution but is able to probe the deeper lying core electron states. These core electrans levels are specific for each chemical element. XPS can therefore also be used to characterize the chemical content of a surface. Sametimes the term Electron Spectroscopy for Chemical Analysis (ESCA) is used instead of XPS.

2.4.1 X-rays

The standard X-ray souree consistsof a heated-filament cathode from which electrans are accelerated to an anode of a specific materiaL The energy of these electrans is typically around 15keV.Holes are formed in the anode material by the electron bombardement that are filled by transitions from higher lying levels under emission of a photon. Accompanied with this is a continuous background of Bremsstrahlung, which comes from electrans that are decelerated by the positively charged nucleus. The energy difference is then

18

L1 = 280 L2

Figure 2.9: The technica! drawing of the Omicron HIS-13 noble gas discharge source.

compensated by the emission of a photon. By optimizing the souree design and anode material, an intense, monochromatic X-ray beam can be achieved. For XPS studies Al and Mg are mostly used as an anode materiaL These materials have an X-ray spectrum (see figure 2.8) that is dominated by a very intense degenerate K a 1 - K a 2 doublet resulting from transitions from the 2p::I and 2pl to the ls shell. This is aften called Ka radiation. The X-ray energies for the aforementioned anode materials are

MgKa : 1253.6 eV

AlK a : 1486.6 eV.

Bath materials are used in the dual-anode X-ray souree that was used for the experiments. The Mg anode was preferred as it has a smaller line width than Al ( ,6.EFWHM = 0. 7 e V and 0.85 eV respectively).

2.4.2 Gas discharge VUV souree

The UV light for UPS is produced by a discharge through a noble gas. This radiation is typically well above lüeV, which is sufReient to overcome the work function and excite a considerable part of the valenee band. The experimental difficulty lies in the fact that above the LiF cutoff of 12 eV there doesn't exist a material for optical windows that is capable of transmitting this type of radiation. Just very thin metal foils or organic compounds can transmit some percents of the original intensity and almast all gasses are opaque because of their high photoabsorption cross section. Therefore this band of electromagnetic radiation is aften called Vacuum Ultra Violet (VUV). This means that the discharge chamber, where the VUV light is generated, has to have an open conneetion to the experimental chamber, where UHV conditions are required.

The VUV souree that was installed (model HIS-13 by Omicron NanoTechnology GmbH) is given in figure 2.9. The light is generated by a high voltage gas discharge between the anode and a cathode that is formed by the body of the discharge chamber. The gas discharge needs a pressure in the millibar regime when operating. In order to overcome this huge

19

VUV souree energy (eV) rel. intensity (%) wavelength ( nm) satellite shift ( e V) H Lyman a 10.20 100 121.57 0 H Lyman j3 12.09 10 102.57 2.67 He I a 21.22 100 58.43 0 He I j3 23.09 1.2 ... 1.8 53.70 1.87 He I 1 23.74 0.5 52.22 2.52 He 11 a 40.81 100 30.38 0 He 11 j3 48.37 <10 25.63 7.56 He 111 51.02 n.a. 24.30 10.2 Ar I 11.62 100 106.70 0

11.83 50 104.80 0.21 Ar 11 13.30 30 93.22 0

13.48 15 91.84 0.18

Table 2.2: Photon energy and intensity of the atomie spectra for gasses that can be used in a gas discharge radiation source, data provided by Omicron NanoTechnology GmbH.

1.0 1.0 [O) (bi

' He II

~ I He I öii I >- ... c: I t " i!. I 111

" " He I c: .!: \ 2! " \ 0.5 ...

0.5 .!: ... He II !l! I ...

\ " += .. .9 \ >

:;: OI \ 0

"' \ 'ai ... " \

"' /

\ / /

' ... ' " ' \

0 s 10 0 100 200 Gas pressure (mbarl Discharge current <mAl

Figure 2.10: The intensity of the He I and He II as a function of gas pressure and discharge current. Not that the curves in are not up to scale.

pressure difference with the UHV (typically 10-10 mbar) a differential pumping method is required. Along the light capillary, which is the open conneetion between the discharge and experimental chamber, differential pumpingacross three transverse bores is possible. Note that in figure 2.9 only two differential pumping stages are drawn. An additional pumping stage enables a lower operational pressure in the experimental chamber. In each stage the pressure is gradually reduced. The first differential pumping stage is performed by a high-eapacity roughing pump that will reduce the pressure to less than w-l mbar. The second and third differential pumping stage are performed by two turbo molecular pumps that further reduce the pressure.

The pressure inside the discharge chamber can be evaluated by measuring the pressure at the first differential stage, because there is a one-to-one correspondence between both. This has the benefit that there are less components connected directly to the discharge chamger and therefore reduces the possibility of contaminating the noble gas.

20

Hel

18 16 14 12 10 8 6 4 2 0

Binding energy (eV)

Figure 2.11: A comparison of photoemission spectra excited with He I and He II of the same sample.

As can be seen from table 2.2, the Omicron HIS-13 gas discharge souree can be used to produce photons with different energies depending on the atomie species an the ionization state. For example when He is used, as is the case for the experimental work in this thesis, two ionization states for the He atom can occur: neutral (He I) and singly ionized (He II). The ionization state can be tuned by the pressure and the discharge current, as displayed in figure 2.10. For a large He I contribution a moderate pressure and current should be used, while for He II the pressure should be as low as possible and the current at the maximal value. It should be noted that the scale of the plots in both graphs in figure 2.10 are not equal. The contribution of He II will always be several orders of magnitude less than that of He I, even in He II-optimized conditions.

In figure 2.11 the He I and He II spectrafora sample of 8 Á Alq3 (see chapter 3) on top of a Co layer are compared. Although the position of the different peaks are the same, there are some striking differences. Most and fore all the intensity of the He II is much less than that of the He I spectrum, resulting in a poorer sensitivity. However the He II spectra has much less background because these photoelectrons have a higher kinetic energy and therefore a lower scattering rate (see section 2.3.3).

21

Both ionization states also have multiple satellite peaks, be-He 1 a cause many transitions between energy levels are possible,

provided that the selection rule !:lL = 1 is obeyed. Further more, only transitions to the 1s state will produce photons in the UV range. Therefore in neutral He atoms (where there are two electrans) both electrans must have different spin numbers, so only singlet state transitions of He I can pro­duce spectrallines that can be used for UPS. For He II there is only electron, so a transition to the ground state is always allowed provided the conservation of angular momenturn is

He 1 P respected. --o

4 ·2

Binding energy (eV)

In figure 2.13 the ground state and first excited states for He I (singlet states only) and He II are shown. Also the two most probable transitions to the ground state are given. The photon energies in table 2.2 can be directly deduced from this graph.

Figure 2.12: Satellite peak near the Fermi level of poly­crystalline Co due to the He I (3 line. In some cases the satellite peaks can be resolved. For example

in figure 2.12 where the He I spectrum of polycrystalline Co is shown. In this measurement two satellite spectra are superimposed. The spectrum excited with He I {3 is shifted by 1.87 eV and the intensity is only 2% of that of He II o:. In this example the satellite peak could be resolved because Co has a very high density of states at the Fermi level. Usually the satellite peaks are to low in intensity to be well resolved. However one must always be aware not to mistakenly identify these with true energy states.

Experimental experience has shown that the gas purity is crudal for a good photon intensity. Especially for He II radiation, any impurity in the gas inlet will have a dramatic effect on the radiation intensity. For this reason a cold trap was designed and added at the gas inlet. This is basically a tubular heat exchanger that can be submersed in liquid nitrogen. The He gas passes through the tubes before it enters the discharge chamber of the UV source. In this way any water or oil impurities are condensed out of the gas stream. Even with these modifications it is not straightforward to obtain a high intensity t of He II radiation.

22

Total momenturn J

0 2 1/2 3/2 5/2

4~ 4~ 4~

25 50 3d

1s3d

20 40

>. 15 30 e' Q) c: Q)

c: ,g .l!l ë3 ><

UJ 10 20

5 10

0 0

He I singlet states He 11 states

Figure 2.13: The energy levels for the singlet states of He I and He II. The most probable transitions to the ground state are indicated by the arrows.

23

~ ëii c: Q)

ë

Hel (21.2 eV) UPS polycrystalline Co

molybdenum sample holder

18 16 14 12 10 8 6 4 2 0

Binding energy (eV)

Figure 2.15: Polycrystalline Co measured on different sample holders. Due to the poor con­ductivity it is not possible to measure a full spectrum with stainless steel sample holders.

2.4.3 Voltage biasing of the sample

When electrons are ejected from a sample, positively charged ions remain. For solid state samples, an electrical conneetion with the spectrometer is made to compensate for the lost negative charges. Without this the sam­ple would slowly charge, generating an addi­tional electric field that the photoelectrons have to counter in order to reach the detec­tor. These charging effects are almost never uniform, such that substantial distortions of the spectrum will occur. The result is a gradual shift to a higher binding energy of the entire spectrum and a lossof resolution.

This phenomenon is also observed when iso­lating or semiconducting materials are mea­sured. Figure 2.14 gives the UPS measure­

~ ëii c Q)

ë

4 6

He I (21.2 eV) A/q

3(1000A)

8 10 12

-1' -·-·- 3' ...... 5'

14 16

Kinetic energy (eV)

18

Figure 2.14: The effects of charging on a thick organic semiconductive layer. UPS spec­tra taken 1', 3' and 5' after focussing of the VUV souree on a spot.

ments of a 0.1 f.-lm layer of Alq3 . As the VUV souree is focused on a certain spot, this will immediately start to charge. The Fermi levels drifts to lower kinetic energy values and the intensity drops.

24

hv

sample e·~ spectrometer

+ kinetic energy

T EK'

i EK

1 vacuum level

1 i <!>speet

hv <Ps

1 1 Fermi level

i Ea

excitation level l

Figure 2.16: The energy level diagram of the measurement setup.

During the experimental work the importance of the effects of charging became clear when newly fabricated stainless steel sample holders were used for measuring UPS spectra. In figure 2.15 the spectra of a polycrystalline Co layer are shown when a stainless steel and a Mo sample holder. The intensity is very low, and there is no clear secondary electron cutoff. The only feature that is well resolved is the sharp Fermi edge. Probably there is a charge build up somewhere between the Co and the grounding of the detector. When the first photoelectrons are excited, a positive charge remains that is not compensated due to the poor grounding. This charge build up will generate an electric field from the sample to the grounded areas that can act as an electric lens. The result is that ejected electrans will deftect from their original position. Especially electrans with a low kinetic energy are susceptible to this. Electrous with a higher kinetic energy are less inftuenced. This might be the reason that the XPS spectra do not seem to suffer from this effect and for UPS only the high binding energy region ( near the SEC) is affected.

It is remarkable that stainless steel has such poor conductance, even more so when taken into account that extremely small currents in the order of 10 nA are needed to compensate for the potential that is created by the ejected electrons. Most likely a small oxide layer is formed at the steel surface, isolating it from the rest of the equipment.

When a sample is electrically connected to the setup a thermadynamie equilibrium will be established between the sample and the detector. The detector is also grounded and can act as charge reservoir, so the Fermi level will be the same for every sample that is measured. However, the vacuum levels do not need to be equal. With the use of figure 2.16 the position of the vacuum levels can be easily deduced.

25

Suppose that the sample is also at ground level (so in the schematic VB = 0). Because both the sample and the detector are at ground potential, the kinetic energy of the electrans will not change between the sample and the detector. When exiting the sample and entering the detector, the electrans will feel the effects of the respective work functions Ws and Wspect in different directions. The net potential to which the electrans are subjected is given by Ws- Wspect· The kinetic energy of the electrans as measured by the detector E~ and the kinetic energy of the electrans at the surface of the sample EK are thus related by

(2.27)

To compensate for the effects of the detector work function, an negative bias voltage VB is usually applied to the sample. Otherwise it could occur that low kinetic energy electrans are not able to reach the detector, making it impossible to determine the secondary electron cut-off. This voltage will generate an electric field between the sample and the detector, accelerating the electrans when they are in the vacuum. This way the electrans in the detector will have a energy of

(2.28)

The additional voltage is generally only applied for UPS because fram the position of the SEC it is possible to accurately define the work function of the sample. XPS gives a quite broad SEC, making it of less interest. Also the bias voltage would require to recalibrate the binding energy for every voltage ( or at least take it into account). A common voltage to use in UPS is -5 V.

26

Chapter 3

Organic semiconductors

Ever since the VanSlyke group were able to obtain electrolumi­nescence from organic semiconductors {10}, many advances have been made in this new field of material science. As the fi.rst com­merciallight emitting devices based on organic semiconductors are starting to emerge, new applications for these materials are being researched. It is though that organic semiconductors are very suit­able for use in spintronie applications, which is an ultimate goal for this project. As with all electronic devices, interfaces between different materials play a large role in the device performance. This chapter is a short introduetion to organic semiconductors and will focus on the interface with metals.

3.1 Introduetion

In all electronk devices the working principle is based on efficiently cantrolling charge transport through the device. Traditionally this is achieved by using extremely uniform crystals of semiconducting materials. This high order of symmetry leads to a very broad delocalization of the electronic states near the Fermi level and a braaderring of the levels into energy bands. By introducing minute amounts of dopants into this lattice, charge carrier states are introduced into the conduction or valenee band enabling electron or hole currents through the device.

The large delocalization of the carrier states is essential for traditional electronics, sirree it enables the charge carriers to move from one dopant site to the next. Part of the difficulty in the fabrication of semiconductor devices is exactly this high order of delocalization, which requires semiconductor crystals without defects.

For organic semiconductors the situation is much different. Within the molecule there is a strong covalent bond as in the case of inorganic semiconductors, but between the individual molecules there is only a weak van der Waals interaction. The charge transport in organic semiconductors is much different within a molecule as in between molecules.

When discussing charge transport the distinction between polymers and oligomers be­comes important. Polymers are very large molecules that consist of several repeating

27

ground state excited state hybridized state

Figure 3.1: The formation of sp2 hybridization. Two 2p electrans of the excited state combine with the remaining 2s electron to form three sp2 states.

groups (mers). In the solid state polymers usually form disordered phases with many de­fects that can act as traps for charges. For electronic applications oligomers are frequently used. These small molecule organic materials can form crystal structures with a very small interaction between the molecules, which results in small energy bands near the Fermi level.

This work will focus on the current injection from a ferromagnetic material into a organic semiconductor by means of two prototypical materials. We will discuss our choice of organic material: tris(8-hydroxyquinoline) (Alq3 ) in the following sections of this chapter. But first a general introduetion to conjugated polymers and their electronic properties is g1ven.

3.2 Conjugated polymers

Carbon has four electrans in its outer shells: two in the 2s and two in the 2p orbital. When it is chemically bond it can show different type of hybridization. In semiconducting organic materials these are always sp2 hybridized orbitals. This hybridization occurs when a carbon atom is attached to three groups. For sp2 hybridization to take place first one 2s electron has to be excited to a 2p state. With four unpaired electrans four boncis have to be created. Two of these 2p states will mix up with the unpaired 2s state to form three sp2 hybridization. The remaining 2p state remains unchanged (see figure 3.1).

28

One of the finest examples of an sp2 hy­bridization is found in benzene (C6H6). The C atoms are positioned in a ring shaped structure with one H atom attached in the plane of the ring. The remarkable feature is that all C atoms in this molecule are identi­cal. As seen above every C has four unpaired electron. It will form one CJ bond by the in­teraction of an sp2 with Hls. The other two sp2 electrans bind with other sp2 electrans of the two neighboring C atoms.

This leaves one 2p electron remairring on

Figure 3.2: The Pz statesof the C atoms in a benzene ring form delocalized 1r orbitals paral­lel to the atomie plane.

each C atom. These will join in a delocalized 1r state that is much weaker than the CJ

bonds. These electrans are completely delocalized around two rings above and below the molecule (see figure 3.2). Because each atom gives only one electron to the 1r bond, one could imagine that inside the ring there is an alteration of single and double bonds.

:---~a+

f 1t + \ ' ....... f--···-······\·-·····+

p T'······i.. ..AL .• .\.······ P, Z I '"'"''""'""lf""•"' '" "' I

sp2+ + ~ 7t- ~ + +sp2 \ ! \ ; ~ : \ ;

~-tt-W (J-

The benzene ring is a prime example of a conjugated polymer, where an alteration of single and double bonds exist in a sequence of bonded atoms. The CJ bonds pro­vide the structural frameworkof the molecule, while the delocalized 1r electrans determine the electronic prop­erties of the molecule.

Upon formation the hybridized states will consist of a bonding (-) part and an anti-bonding ( +) part. Wh en the energy diagram (figure 3.3) is drawn forthese levels, we see that the Highest Occupied Molecular Orbital

Figure 3.3.: !he. energy splitting (HOMO) is given by 1r- and the Lowest Unoccupied by the hybndizatiOn. Molecular Orbital (LUMO) by 1r+ . These energy levels can be compared to the valenee and conduction band in traditional solid state physics with a typical bandgap of 2eV. With organic molecules it is possible to tune the HOMO and LUMO in order to obtain the desired properties for a molecular electronic device.

29

25.0

- 22.5 ~ Q) 0

~ ~

20.0

17.5

~ p~

40

30

~ 20 ~

~ 10

0

~--~--~--~--~--~--~--~--~~-10

-1,500 -750 0 750 1,500 Magnetic field (Oe)

Figure 3.4: A typical GMR loop of a spin-valve device with an organic spacer. The blue (red) curve denotes GMR measurements made while increasing (decreasing) H . The anti-parallel (AP) and parallel (P) configurations of the FM magnetization orientations are shown in the insets at low and high H, respectively. The electrical resistance of the device is higher when the magnetization directions in FMl and FM2 films are paralleltoeach other.

3.3 Organic semiconductors forspintronie applications

One of the most simple spintronie devices is the spin valve. A typical device consists of 2 ferromagnetic electrades (FM) and an organic semiconductor spaeer (see figure 3.5). When a positive bias is applied, a spin polarized current flows from FM1 , through the organic spacer, to FM2 . An in-plane magnetic field H is swept to change to the direction of the two FM electrades separately. By sweeping the magnetic field, the magnetization of the bottorn and top electrades can aligned parallel or anti-parallel. Both contigura­tions have a different electrical resistance. This effect is known as Giant Magneto Resistance (GMR) and is of used in magnetic field sensors.

-H

Figure 3.5: Schematic representa­tion of a typical organic magneto­resistance device, taken from [11] .

30

The use of organic semiconductors as a spaeer layer has received some attention [11] as they are relatively cost­effective to produce, and are thought to have a long spin ditfusion length because of the weak spin orbit interaction in C atoms.

The prototype molecule Alq3 was chosen because it has good electron transporting properties and from its use in organic LED's , it is a well known materiaL It can be de­posited in UHV by using Organic Molecular Beam Epitaxy (OMBE), which allows making very clean surfaces. This is necessary when prohing with a surface sensitive technique such as photoemission spectroscopy. For the electrode ma­terial Co was used because it has a high spin polarization,

Figure 3.6: Structural representation of the Alq3

molecule.

it is possible to plasma sputter and the analysis is more straightforward than some ferro­magnetic alloys. Alq3 is an oligomer that is made up of quinoline rings . A quinoline ring is double benzene ring with one N atom incorporated on a C position ( this are also the basis of the yellow food colouring E104). Three of these quinoline molecules are tied to a central Al atom by the N atom and an 0 atom that acts a bridge with a C atom (see figure 3.6 for the structural representation).

From other organic semiconductor devices it is known that the interface between the semiconductive layer and the contact electrades is very important for a good device per­formance. Also for new spintronie devices this interface should have a large impact on the functioning of the device.

A computer simulation was performed to visualize the electronic orbitals of the HOMO and LUMO states. These calculation were performed on an Intel PC using Ghemical, an open souree software package for chemica! calculations [12] . The atomie coordinates of the Alq3 molecule [13] were directly loaded into the program. It was found that the three highest occupied states (HOMO, HOM0-1, and HOM0-2) are quasi degenerate. The same happens with the three lowest unoccupied states (LUMO, LUM0+1, LUM0+2). When the molecular orbitals associated with these energy levels are visualized in the molecule, it can be seen that both for the HOMO's as for the LUMO's the degeneracy originates from almost identical molecular orbitals in the three individual quinoline rings . In figure 3.7 the first three HOMO's and LUMO's are plotted into the molecular frame of Alq3 .

Interestingly, the electronk states near the Fermi level are defined by the quinoline rings. It is this part of the molecule that defines the electronk transport properties.

3.4 Metal-semiconductor interfaces

When an organic semiconductor is brought into atomie contact with a metal the energy levels across the interface have to align. lf thermadynamie equilibrium across the interface is to be assumed, the Fermi energy Ep must be constant throughout the device. This will infiuence the position of the vacuum leveland may introduce a harrier for electron or hole

31

HOMO

Figure 3. 7: Computer simulation of the first three HOMO and LUMO levels. These levels are quasi- degenerate because of the high symmetry for the three quinoline ligands and are drawn on the same picture.

32

injection. The height of these energy barriers is important factor in the device performance, because for a efficient spintronie device a good electron injection is required.

For traditional (anorganic) semiconductors the Schottky-Mott model [14] can describe the metal-semiconductor interface very well, but for the case of organic semiconductors there is still a some debate about the physical processes at work. It has been shown that different organic semiconducting materials show a multitude of different behaviors at metallic interfaces. In chapter 5 we will present the result of a photoemission spectroscopy study of the interface between Co and Alq3 .

For the analysis the organic semiconductor will be compared with an n type anorganic semiconductor, as it bears the most resemblance with an electron transporter such as Alq3 .

First a review of the Schottky-Mott model will be given. Typically metal-semicondutor interfaces are explained [14] by a Gedanken experiment where the distance between the two componentsis decreased until atomie contact is reached. In general, the work functions of the metal WM and the semiconductor Wsc are not equal. Therefore an electric field will exist between the two surfaces and as a consequence there will be an accumulation of surface opposite charges on the metal and semiconductor.

The electric field thus enters both the metal and semiconductor layer, but because the large density of states the screening length in metals is typically less than 1 A the field and the charges are restricted to the first atomie layer. However in semiconductors the screening length is typically in the order of 100 A. Therefore the charge in the metal is referred to as a surface charge and in the semiconductor as a space charge.

For the situation as sketched in figure 3.8, the semiconductor will be positively charged because of a depletion of mobile electrons. The difference in potential due to this depletion region is indicated in the figure by VD. The depletion voltage will bend all energy levels equally. In this case, the electron affinity xsc and ionization potential I se of the semiconductor are raised by the same amount of eVD.

The barrier height <I>B,n for electrans to pass from the metal Fermi level into the semiconductor conduction band is given by

<f>B,n = WM- XSC· (3.1)

MET AL SEMICONDUCTOR

This last equation is known as the famous Mott-Schottky Figure 3.8: Development of law. If this rule were valid, the barrier height for a given a Schottky harrier as a func­semiconductor follows the work function directly. In other tion of decreasing the metal­words, a change in metal work function results in the same semiconductor distance 8. quantitative change in barrier height.

Most semiconductors do portray a linear electron barrier behavior , however the slope S = ~~~n is often less than unity as would be for the case for the Schottky-Mott limit . Bardeen attributed these results to electronk interface states that accommodate additional charge.

33

The charge neutrality equation has the be completed with an additional term for this interface charge Qi.

It is assumed that the interface states have a constant density Di across the band gap and a charge neutrality level (CNL) EcNL· The charge density at the interface is then found as

(3.2)

where <I>~ ,n = XiECNL is the barrier height for Qi = 0, so the Fermi level coincides with the CNL. The voltage drop over the interface .6. is found in the energy band diagram as

(3.3)

This voltage is generated by a charge at the interface, such that

(3.4)

By relating these expression to the charge neutrality condition, it can be found that

<I>B,n = S(WM- xs)- (1- S)<I>~,n' (3.5)

with the slope parameter

(3.6)

We can conclude again that the barrier height relates linearly to the metal work function, but the slope is dependent on the density of interface states Di. When this density ap­proaches zero, the Schottky-Mott rule is recovered. In the Bardeen limit, when Di --+ oo the barrier height becomes independent of the metal work function (S = 0). It is said that the Fermi level is pinned at the CNL of the interface states.

3.5 Schottky harriers for organic semiconductors

Organic semiconductors have a similar behavior for metal interfaces: at the interface there exist a region where the energy levels align. For many organic semiconductors it was found that a thin dipale layer exists at the interface.

For Alq3 an interface parameter SA1q3 = 0.23 is found [15], meaning that the Fermi level is pinned to a certain extent near the charge neutrality level. This was obtained by a linear fit of the interface dipale .6. as function of the metal work function for Alq3 deposited on

34

2.5,----------------------,

2.0

> ~ 1.5 Q)

8. '6 ~ 1.0 -ê .l!l .f:

0.5

Mg ... ..

Ag Au Al • liï .. ...

SD = 0.23

0.0 +--.----.--...---,---.--~.------.---,---,-----1 3.0 3.5 4.0 4.5 5.0 5.5

Metal work tunetion (eV)

Figure 3.9: The interface dipale of Alq3 as a function of the work function of the metal onto which it was deposited.

Mg, Al, Ag and Au (see figure 3.9). In the experimental section we will add a data point for Co into this graph (see figure 5.12), which will modify the slope parameter considerably.

The difference with the Schottky-Mott model is that this dipole layer can be created by many effects. These effects are discussed further.

3.5.1 Chemisorption

Chemisorption is the effect of a chemical bond formation upon adsorption onto a surface. In this chemical bond a charge transfer can occur through an ionic or a covalent bond. When the molecules donate or accept charges from the metal, the bond is called ionic. A covalent bond is a hybridization (mixing-up) of the electron wave functions of the adsorbed molecules and the metallic surface. In general the distinction between both bonding mechanisms is rather diffuse. A chemisorbed system will usually have ionic as wel as a covalent character.

In both cases a qualitative analysis can be made by consiclering the chemical potential of the electrans J.L of the two systems: in this case the metal and the organic semiconductor. When the electrans are allowed to interact the chemical potential J.L will try to equalize [16]. A partial electron transfer will occur such that the chemical potential is equal for the two systems.

The chemical potential is defined as the amount of energy needed to add an electron, or

BE J.L= aN· (3.7)

35

~ ·u; c 2 c

speetral width = 16.0 eV

18 16 14 12 10 8 6 4

Binding energy (eV)

2 0 -2

Figure 3.10: The UPS spectrum of a polycrystalline Co surface. By the width of the spectrum the work function can be obtained.

For bare metals the chemical potential is well approximated by the negative work function [17].

(3.8)

The chemical potential for non-conducting solids can be estimated by the average of the ionization potential I P and electron affinity (EA).

IP+EA 1-L~ ----

2 (3.9)

The experimental UPS data confirm the work function value for polycrystalline Co of f-Lco = -5.2eV [18]. This spectrum is shown in figure 3.10.

The ionization potential of bulk Alq3 has been reported in literature and has a value of I P = 5. 7 e V [19][20] and the electron affinity has estimated to be in the range EA = 1-3 eV. Using these values the chemical potential of Alq3 is estimated at /-LAtq3 = ( -3.9 ± 0.5) eV. In a chemical reaction a charge transfer from a system with a hightoa system with a low chemical potential, ie. from the organic semiconductor into the metal is expected.

The charge transfer 6.N between the Co and Alq3 system is not only dependent on the difference in chemical potential, but also on the chemical softness S of the systems.

(3.10)

36

6

4

- 2 > (!) 0 -lL

-2 w ~ -4

>- -6 ~ (!) -8 c w -10

-12

-14 -5

Fermi level

! ;/ eDmet - ---t----- --

/ ... ~erfX) / :

,...·\.~p(x) .. ··············-·-··- ············lt·······' ··-~.,-· .... ··········· ······----· -4 -3 -2 -1 0

Distance x (Á)

2 3

2.5

2.0 g a. >.

1.5 -ëi) c (!)

1.0 Cl c 0 .....

0.5 -u (!)

üJ 0.0

Figure 3.11: DFT calculations within the jellium model by Lang and Kahn [21] give the evolution of the electron density p(x) across the metal/vacuum interface (bottom dotted curve), the effective potential f.Leff( x) (top dotted curve), and the electrastatic potential <P( x) (top solid line). The large circles depiet the atoms on the metal surface (x < 0 indicates the metal bulk); Xim defines the image plane position.

The softness is a measure of the response of the electron system to the change of an external potential, or

(3.11)

For metals this is exactly the density of states near the Fermi level DOS(EF ). For molecules the softness is approximated in a finite-difference approximation by 2/(J P­EA). For Co the Fermi level is located in the 3d band, which has a large DOS. This can also be seen in the UPS spectrum of the polycrystalline Co surface. N ear the Fermi level ( 0 e V binding energy) there is a high intensity peak.

From these considerations, a partial charge transfer at the Co/ Alq3 interface is expected when a chemical bond is formed. Because of the large difference in chemical potential and high DOS near the Fermi level of Co the charge transfer could be relatively large. The direction of transfer will be such that a dipale with the negative side pointing to the metallic surface is generated. Such a dipale willlower the electron injection barrier from the metal into the semiconductor because the dipale lowers the potential energy of an electron passing through the dipole.

3.5.2 Physisorption

As seen before, a chemical bond can introduce a change in the work function of a metal. However, this is not the only effect at work. When a molecule or atom is adsorbed without

37

> 1.4 ~ s 1.2 Rh /

<l • /

Pd /

(!) 1.0 /

• Ti Ni / / .er 0> .// c:

0.8 • CU /

..c: / /

() / • 0.6 / . c: Ag /

Cu Fe / 0 / u 0.4

e/ 7

/ c: / • :::1 Li // 32 0.2 Mg ..... K Na/; 0 _yi •ca s 0.0 /

0 1 2 3 4 5 6 Metal Surface Dipale eDmet (eV)

Figure 3.12: Evolution of the work function change of various metals upon adsorption of a Xe monolayer, with respect to the met al surface dipale [22].

a charge transfer (physisorbed) on a metal, the electrans in the metal will react instanta­neously to the charges in the molecules and completely screen them.

To analyze the electronic structure of metal surfaces, the jellium model can be used (see figure 3.11). In jellium the surface is represented by a step of nucleus density, i.e. the nucleus density drops to zero at the interface. Outside the metal, in the vacuum, the electron density decays exponentially over several Ángströms. At the interface an electric dipale Dmet is created by the deficit of electron density in the metal and the nonzero electron density in the vacuum close to the surface edge. As a result the electron potential cjJ(x) jumps from a negative bulk value toa positive vacuum value. The difference between the bulk and vacuum values of the electronic potential is defined by the interface dipole.

Dmet = c/J(vac)- c/J(bulk) (3.12)

When the Fermi level is taken as the energy scale reference, it is found by the respective definitions that the chemical potential p, and the work function W are related to the electronic potential by

w M

ecjJ(vac)- Ep

ecjJ(bulk)- Ep.

Combining these, the work function for the jellium model is easily found by

W = eDmet- M

(3.13)

(3.14)

(3.15)

It has been shown that the preserree of a physisorbed species near the metal surface leads to reduction of the electronic tail outside the metal, and therefore reducing the surface dipale eDmet and the work function W by the same amount. An archetype example of this

38

effect is that of a Xe atom adsorbed on different metal surfaces [23]. For this well stuclied adsorbent, a work function change ,6.W ~ 0.2eDmet has been observed (see figure 3.12).

For all molecules adsorbed on a metallic surface, a shortening of the electron density tail occurs. This will reduce the metal work function because of a reduced effective potential outside the metal surface. Physisorption will thus always reduce the barrier height for metal to molecule electron injection.

When the molecules have an intrinsic dipole, an alignment of the individual molecules could also lead to an additional dipale layer. The sign and size of the dipale will depend on the net dipale moment. For this to happen a high order of molecular arrangement will have to take place, which is usually only seen after annealing.

39

Chapter 4

Growth monitoring by differential ellipsometry

For studying interfaces by stepwise deposition it is necessary to ob­tain an accurate measurement of the layer thickness. In this chapter we will present a metbod for high accuracy in-situ thickness char­acterization basedon diHerential ellipsometry More specifically the case of Alq3 on Co will be explored, as this will be used in chapter 5.

4.1 Introd uction

For metallic MBE growth a microbalance is frequently used as a thickness monitor. The quartz crystal in the microbalance is set to oscillate at its eigen frequency. As more material is deposited onto it, the frequency of the microbalance will change. If it can be assumed that the same amount of material is deposited onto the microbalance as on the sample, the deposition thickness can be calculated from the frequency change of the quartz oscillator. However for organic materials the sticking coefficient on the different substrates can deviate significantly to that of Alq3 which would on the microbalance from previous measurements. For these materials differential ellipsometry is a more preferred technique for thickness monitoring as it measures the film thickness in a more direct way.

Figure 4.1: The ellipsometry setup for the thickness measurement during growth.

40

The ellipsometry setup used in this workis integrated in the depositing chamber, as shown in figure 4.1. The evaporation cell (not depicted) is mounted in the same chamber, pointing upwards to the sample. In this cell organic materials (in this case Alq3 ) can be resistively heated such that a molecular beam is formed at the cell exit. This molecular beam can aimed at a target sample. To stress the fact that organic molecules are deposited, the term Organic Molecular Beam Epitaxy (OMBE) is generally used.

First, the plane of incidence is defined as the plane perpendicular to the surface of the sample. The polarization state of the light is then defined by two components, one par­allel and one perpendicular to the plane of incidence. These components are called the s (senkrecht, German for perpendicular) and p (parallel) components respectively.

4.2 Ellipsometry fundamentals

Differential ellipsometry makes use of the fact that the reflection coefficient of a sample depends on the thickness of a deposited layer. Experimentally this can be measured by using light that is polarized at an angle a with respect to the s direction. The reflected light is than passed through a photoelastic modulator (PEM) and a second polarizer ( the analyzer). The PEM induces a harmonie phase shift with an amplitude (0 and frequency v on one of the polarization directions, and thereby modulating the light between left and right circularly light. In our case the PEM has an operating frequency of v = 50kHz. The angle of this phase shift at a certain time t is thus given by

( = (o sin(21rvt). ( 4.1)

The signal V, as registered by the photo detector, will be composed out of harmonies with integer frequencies of v. It can be shown [24] using the Jones formalism that the amplitudes of the first three harmonies are given by

VDc A (1- lo((o)) (1- cos(2a)) lr~l2

+ (1 + 10 ((0 )) (1 + cos(2a)) lr~l2

V1v A Jl((o) sin(2a) ~(rs)~(rp); ~(rs)~(rp)

V2v AJ2((0 ) [lr~l2

(cos(2a) -1) + lr~l2

(cos(2a) + 1)], (4.2)

where ri(i = s,p) is the complex reflection index in direction i and In is the Bessel function of the nth kind. The factor A accounts for the conversion from the intensity of light to a registered voltage.

As the layer thickness of the top layer increases, the reflection coefficients will change. Therefore, the layer thickness could be monitored by one of these signals. For an analyses some ellipsometry definitions have to be introduced. The indices of reflection ri (i = s, p) are complex numbers that can be represented by its complex modulus !ril and complex

41

angle fi. In describing ellipsometry data, it is useful to introduce the change in amplitude IJ! and phase ~ of polarization between the incident and refiected waves.

(4.3)

The angle of incident polarization a and the PEM amplitude (0 are parameters of the set up that can be adjusted freely. In this case (0 was set at 2.407. The incident polarization is set at a = 1r /4. Together with the definitions of IJ! and ~ this reduces the above expressions to much simpler ones.

A' 10 ((0 ) cos(21J!) + 1

A' Jl((o)(cos(21J!) +cos(~))

A' J2((o)cos(21J!)

(4.4)

(4.5) (4.6)

The factor A' = A 4 c~:J~w) is equal in all equations and is separated for ease of future calculations. Simple arithmetics reveal that IJ! and ~ are found by

IJ! = (4.7)

(4.8)

Note that by use of equations 4.7 and 4.8 IJ! and ~ can be extracted from a measurement of the DC and first two Fourier components of the refiected intensity. In the next section we will present the measurement of IJ! and ~ for Co onto which a layer of Alq3 was deposited using the above results. From these measurements a thickness calibration will be derived.

4.3 Ellipsometry for thickness monitoring

In figure 4.2 IJ! and ~are given with respect to the thickness of Alq3 while it was deposited on a Co substrate. In this procedure the evaporation cell is heated above the evaporation temperature of Alq3 . When the chamber pressure and the evaporation cell temperature have stabilized, the sample is brought into place and the deposition commences. Because of the constant temperature and pressure, a constant growth rate can be assumed. This way the (horizontal) time axis can be transformed to a thickness axis by setting the zero thickness point at the starting point of deposition and the final thickness at the end point. The fin al thickness is found a low angle XRD measurement.

42

0 100 200 300 400 500 600

50

48 ~

e..... 46 ."..

44

35

30

25

20

~ 15

e..... <I 10

5

0

-5

-10 0 100 200 300 400 500 600

Alq3

thickness (À)

Figure 4.2: W and ~ as a function of Alq3 coverage.

1400 I~

1200

1400 ~ 1200 en 1000

1000 en Q) c:

I~

''~* ,,

"''"r" ~ I r

800 ~ 800 (.)

600 ~ ... , .,

400 Q) 600 >-

200 ..!2 ..,

, , 0"

0 <i: 400 l

200

0 -90 -75 -60 -45 -30 -15 0 15 30 45

700

700

90 80

70 60

5 0 4o 'I' n

Figure 4.3: 3D plot of W vs ~ for increasing Alq3 coverage.

43

40

20

u 0 0 > ...._ >

>~

-20 R' = 0.99983

11' 0

( -7.49063 ± 0.01064)

-40 '1 ( 0.05772 ± 0.00003 ) A·'

-60 -200 0 200 400 600 800 1000 1200 1400

Alq3

thickness {À)

Figure 4.4: The normalized lv signal as a function of Alq3 thickness. For low thicknesses there is a linear correspondence. The dotted line is a linear fit of which the parameters are given in the box. ~~ is the offset and r/ is the slope of the fit.

Figure 4.3 gives the same data in a more traditional way as W vs. b.. From these graphs it can be seen that for low ( t < 500 Á) cover a ge W remains constant and b. increases linearly or

4 b. ~ b.o + T/t.

(4.9)

(4.10)

If one wants to compare ellipsometry data from different measurements it is better to compare the ratio of two of these signals, such that the conversion factor A is eliminated. For instance, it appears that the ratio V1v/VDc is well suited for monitoring the thickness of the grown layer. Together with the assumptions presented above, this gives

2]1 ( (o) sin( b.o + T/t)

~ 2Jl((o)(b.o + Tlt)

(b.~ + T/'t). (4.11)

In the last approximation the sine was replaced by its first order Taylor expansion, which is reasonable given the small angles of b. in figure 4.2. With this we have found a simple and reproducible method for measuring the thickness of organic material t that is being

44

g rJJ rJJ Q)

J2 ()

;s

2.5

2.0

1.5

r:::f' 1.0 <(

0.5

0.0

40

30

10

0

0 5 10 15 20 25 30 35 40 45 50 55 60

time (s)

Figure 4.5: An example of a thickness measurement. 2 A is deposited on a layer of Co. The resolution of the thickness characterization is less than 0.1 A

evaporated. The only parameter that remains unsolved is r/, which can be experimentally found by calibrating ellipsometry data with XRD data, as shown in figure 4.4.

As can be seen from figure 4.4, the V1v/VDc ratio is linear with respect to the Alq3

thickness for t < 500 A. The slope of the curve in this domain has been found to be r/ = 0.05772 Á -l. For future measurements this result can be used to accurately define the thickness of a layer while it is being deposited.

4.4 Sub-monolayer thickness measurements

An example of thickness monitoring is presented in figure 4.5. The scale on the right hand side gives the ratio V1v/VDc and has been corrected with an offset such that the beginning of the measurement is at zero. Now these axis can be transformed to yield the deposition thickness using the factor r/ as obtained by the calibration measurement in the previous section.

The spikes at 17 s and 38 s result from the shutter blocking the laser beam as it moved away from and back to the evaporation cell. Incidentally they serve as good marker for the startand end of the deposition. When the deposition starts, there is a delay before the ellipsometry signal starts to change. This is probably due to a difference in the sticking coefficient for the clean and a partially covered surface. At appears the Alq3 needs some nucleation sites for a good growth on Co. After this first delay there is a linear increase in layer thickness, indicating a constant growth rate. After the second peak, which marks the end of deposition, the ellipsometry signal remains constant as the shutter prevents material exiting the evaporation cell.

45

The technique as described above is an excellent method for measuring and monitoring the deposition of extremely thin layers of organic materiaL The growth ra te ( or a variation therein) is not important in the thickness measurement. The only relevant factor is the change in the ellipsometry signal before and after the deposition.

As can beseen from the example in figure 4.5, a thickness of only 2 A can be measured with a very high accuracy ( rv 0.2 A). This is even much astonishing when one considers that one monolayer of Alq3 is already 12 A thick [25]. An additional feature of this technique is that it is possible to pinpoint exactly when a certain layer thickness is achieved.

In the next chapter we will present a study of the interface of Alq3 on Co. A large number of samples were prepared by evaporating Alq3 on a clean Co surface. The thickness of the top Alq3 layer was varied, such that an interface is gradually built up. Differential ellipsometry enables the high resolution thickness characterization without breaking the vacuum.

46

Chapter 5

Interfaces between an organic semicon cl uctor an a ferromagnetic material

We have performed an extensive photoemission spectroscopy study on the Co/ Alq3 interface. This way more information is gathered about the physical and chemical processes at work and the energy level diagram of the interface is constructed, which is important for a current injection from the metal in the semiconductor. The interface was built up by deposition of different thicknesses of Alq3

on a Co surface. For every thickness UPS and XPS spectra were taken.

5.1 Introduetion

When an organic semiconductor is brought into atomie contact with a metal the energy levels across the interface have to align. lf thermadynamie equilibrium across the interface is to be assumed, the Fermi energy EF must be constant throughout the device. This will infl.uence the position of the vacuum level and may introduce a harrier for electron or hole injection. The height of these energy harriers is an important factor in the device performance, because for a efficient spintronie device for a good electron injection a hole harrier is required as the electron mobility of Alq3 is about 100 times larger than its hole mobility [26] .

Similar experiments have been performed for low work function metals in light of use in organic LED's, for instanee [27] and [28]. To our knowledge , the electronk properties of the interface of Alq3 with ferromagnetic metals has nat been studied.

5.2 Sample preparation

For making the samples described in this chapter the following procedure was used. SiOx substrates where ultrasonically cleaned in ammonia, acetone, and ethanol. After­warcis they were rinsed in isopropanol vapor. Consequently they were mounted on OMBE daughter plates using a conductive adhesive (aka. silverpaint). The sample holders for the

47

conductive ..... N~>:s\ adhesive

Figure 5.1: The sample plates used for the PES experiments. The small daughter plate slides into the large mother plate. Conductive paste and a W wire ensure a good electrical conneetion from the top layer to the detector.

OMBE have a different shape as those for the other growth and measurement setups in the UHV cluster. Therefore a special adapter ( the mother plate) is needed outside the organic chamber (see figure 5.1). To prevent charging of the isolating SiOx substrates, a droplet of silverpaint is placed on the edge of the substrate to form a conductive bridge between the daughter plate and the Co layer. For the same purpose a W wire is mounted on the mother plate under which the daughter plate can be slid.

The ferromagnetic contact was prepared using argon DC sputtering. First a 50 A layer of tantalum (Ta) was deposited as a buffer layer, on which 100 A of Co was sputtered. For every experiment a new clean sample was prepared in this way under identical conditions to minimize any risk of contamination of the surface.

The organic layer of Alq3 was deposited using organic molecular beam epitaxy (OMBE) and the nominal thickness was monitored using differential ellipsometry, as described in chapter 4. The evaporation cell temperature was set at 185 oe and the pressure in the evaporation chamber never rose above 10-8 mbar.

A completed sample is then transported in a pressure below 10-8 mbar to the XPS/UPS chamber where the photoemission experiments are performed. The pressure in this cham­ber is always well below w-9 mbar.

5.3 Core level spectroscopy and chemical interaction

For studying complex structures such as oligomers, XPS can be a valuable tool to accom­pany UPS data. By using X-rays as an excitation souree it is possible to probe the core levels of electrons. Because for every element these core levels have specific binding ener­gies it is possible to identify the different species that are present on a surface. Moreover, when an element chemically bonds with a neighboring element the core levels of both elements will perturb and the binding energies will show a charaderistic deviation from the unbound state [29].

This feature makes XPS a suitable technique to trace specific elements and the state wherein they occur. To analyze the Co/ Alq3 interface, a MgK a X-ray souree with an

48

element peak area height atoms in Alq3 binding energy ( e V) c ls 0.25 0.25 27 283.4 N ls 0.42 0.42 3 401.6 0 ls 0.66 0.66 3 532.0 Al 2p 0.185 0.18 1 73.1 Co 2p 3.8 2.5 - 778.6

Table 5.1: Peak area and height (relative to Fls) for largest peak of the elements present in Alq3 and Co as taken from [30]. Also the amount of atoms per molecule Alq3 and the binding energy [31] in the unbound state are given.

excitation energy of 1253.6 eV was used. For each sample a low resolution overview spec­trum (as in figure 5.2) was taken to identify the elements that are present on the surface. As expected, all spectra show clear evidence of the Co substrate, and the 0, N and C atoms of Alq3. The Ols peak is more difficult to distinguish as it is superpositioned on the Auger spectrum of the Co underlayer. It is not possible to see a clear peak of Al (around 78 e V) in these spectra.

A quantitative analysis of the elements on the surface is also possible if one takes into account that the XPS sensitivity differs for each peak. The sensitivity factors for the energy levels that were investigated in this work are given in table 5.1.

For example if one takes ratio of the heights of the Cls to Nls peak in figure 5.2, the experimental value of 5.0 is more or less in agreement with the predicted value.

fels = 27 · 0.25 = 5.36 fNls 3 · 0.42

(5.1)

From this table it is also clear why the Al2p peak is not visible in the overview. Firstly, there is only one Al atom per Alq3 molecule. Secondly the sensitivity of the Al2p level is much less than that of the other elements in the molecule. The sensitivity of the Hls levels is very small. Therefore it is not possible to visualize these with XPS.

For all the samples some selected regions of binding energy were measured at higher resolution to see possible changes in the chemical state. No change in chemical state was seen for the Co2p, Ols or Nls peaks. The binding energy of the latter two agree with literature values of bulk Alq3. The Co2p peak matches that of metallic Co. The intensity of the Al2p was insufReient to draw conclusions on the chemical state of Al.

The Cls spectra show a significant change when the thickness of the Alq3 top layer increases. Some representative examples are given in figure 5.3. After subtradion of the background, all (13) spectra were simultaneously fitted with a 4-peak Gaussian fit. Three of these peaks have fixed energy positions of those that have been identified as [32] a C that bonds with two other C's and H (C-H), bridged C bonded with three other C's (C-C) and C bonded with two other C's and an 0 or N (C-X). For samples with a high coverage of Alq3, these three sufRee to fit the measured data. This is a good indication that Alq3 is deposited in the OMBE process.

49

Co2p

-:::::l ro ........ >. ..... (i) c Q) ..... c

1000 900 800

520

C1s N1s

700 600 500 400 300

Binding Energy (eV)

MgKaXPS Alq

3 (8 A) I Co

200 100 0

Figure 5.2: An overview XPS spectrum of a Co surface covered with a sub-monolayer of Alq3. The Co, 0, N and C peaks can be clearly resolved.

50

t

-:::::1 ro -~ ëi) c 2 c

289 288

C-X C-H C-C

287 286 285 284

Binding energy (eV)

C-Co

28A

8 A

0.8 A

0.2 A

<0.1 A

283 282 281

Figure 5.3: XPS of the Cls peaks for different thicknesses of Alq3 as deposited on a Co layer. After subtraction of the background, all spectra were fitted with a four-peak Gaussian fit (indicated by the fulllines).

51

0.25 ... ...

-"' (IJ

0.20 !• (]) 0. Ul ... ü 0.15

0 ... c 0 0.10 n ......

... (IJ

.)::

0 ü 0.05 ü

0.00

0 5 10 15 20 25 30

Alq3

nominal thickness (A)

Figure 5.4: The fraction of C-Co intensity within the total Cls range as a function of the top layer thickness.

For low coverage, a fourth peak at 283.4 eV is clearly present. This in an undocumented feature of the Cls spectrum of Alq3 , and will be postulated as C atoms of the quinoline structure that bond with Co atoms at the surface. In figure 5.3 the relative contribution of the C-Co peak to the total Cls spectrum is plotted as a function of the Alq3 coverage. The bottorn graph in this figure shows a clear contribution of Alq3 in the Cls energy range. This was probably due to some contamination somewhere in the vacuum system. From the intensity of the peak it could be deduced that the amount of Alq3 on this sample is less than 0.1 Á.

The points in figure 5.3 give the contribution of the C-Co peak in the total Cls spectrum. It is seen that after deposition of more than 10 A of Alq3 onto the Co layer, most of the C-Co intensity is lost. The behavior of the C-Co decay is caused by two effects: the relative t amount of reacted Alq3 will decrease as more Alq3 is deposited, and the reacted Alq3 layer will be buried at the interface making it harder to probe. A more detailed discussion is given further in this chapter.

5.4 Binding energy shifts

From the Cls spectra it is found that at the Co-Alq3 interface there are C atoms of which the ls spectra is shifted to a higher binding energy. Some attempts have been made to correlate Cls binding energies to atomie charges [33], but face some difficulties as there are many factors that have to be considered. In a first approximation this change in the binding energy can be attributed to a change in the electrastatic potential due to a charge

52

transfer from or to the specific atom. In XPS, this is known a Siegbahn shift, named after one of the pioneers of XPS.

However, in this case the binding energy shift cannot be attributed to a partial charge transfer from the metal to the organic semiconductor molecules. If only for this chemica! effect, a decrease in the binding energy of the Cls energy levels should be caused by a higher electron density near the atomie site. A partial charge transfer in this direction will generate a dipale with the positive side pointing to the metallic surface. This would contradiet the prediction made in §3.5.1. This would also mean that the electron injection banier from metal to organic would be higher instead of lowered [34]. Such a dipale would heighten the work function. This would not concur with the work function measurements in the next that were found with UPS.

However the apparent contradiction can be explained. The C atoms that are chemically bonded with the Co surface have a distance to the Co atoms of typically 2 A, much smaller then physisorbed species would have. The C atoms that are bonded with the Co surface will appear at a lower binding energy because of metal screening effect [35]. Therefore the C atoms that are very close to the Co surface will appear to have a lower binding energy due to the image charge in the metal that was induced by the core hole in the C atom. The C atoms that are not chemically bonded with the Co atoms are further away from the metal surface and in a much lesser extent subjected to such an image charge. The effect of the image charge on the chemisorbed atoms is large enough to undo the change to a higher binding it would have if the electron density on the chemisorbed species decreases.

5.5 Valenee band spectroscopy and vacuum level shift

Figure 5.5 shows the valenee band spectra of Alq3 taken with He I radiation (21.22 eV) as it was gradually deposited on a Co surface. The spectra were shifted in such a way that the Fermi edge is at zero binding energy. As can be seen, even for sub-monolayer coverage the UPS spectra show a significant deviation from the uncovered ( 0 A) spectrum. For example: 4 A nominal coverage of Alq3 relates to about 30% of surface coverage if flat growth is assumed. Still the molecular orbitals for Alq3 are well resolved. This indicates the surface sensitivity of the technique.

We can also roughly verify the correctness of the thickness measurements. One monolayer of Alq3 is expected to be in the order of 10 - 15 A thick. Because the IMFP of the low­energy electrans generated by the VUV souree is less than one monolayer it is expected that the Fermi edge of Co disappears once the surface has a close layer of Alq3 . From the spectra it can be seen that this occurs between 9 and 28 A.

The change in work function (see figure 5.6) can be derived from the secondary electron cut-off (SEC) positions in the UPS spectra for Co surfaces that are covered with Alq3 . It can be seen that work function drops by an amount of 1.8 e V for an Alq3 thickness of only 10 A. An exponential fit gives a typical decay lengthof 2.54 A. This is much less than the typical depletion regions that are encountered for non-organic semiconductors. Because

53

He I UPS (21.22 eV) Co I Alq

3 (variable thickness)

E

lil :!: c:

45A :::J

..ei Cl) .._ 28A Cl)

~ Q) c:

~ 9A .I<: (.)

ïii :ë c: -Q)

"' - 0'" c: <( Cl c: ïii C1l Q) .._ (.) c:

18 16 14 12 10 8 6 4 2 0 -2

Binding energy (eV)

Figure 5.5: The valenee band spectra for an increasing Alq3 top layer.

54

5.2

5.0

4.8

> ~ 4.6 c: 0

:;= 4.4 u c: .a 4.2 ~ .._ 0 3: 4.0 (!) u -ê 3.8 :I rJl

3.6

3.4

0.0

•

0

W = A*exp(-Ut) + W0

R2 = 0.95572

W0

( 3.4 ± 0.1 ) eV

A ( 1.5 ± 0.2 ) eV ( 2.5 ± o.a )A

10 20 30

Alq3

nomina! thickness t (A)

40 50

Figure 5.6: The work function change for different thicknesses of the Alq3 top layer.

55

0 * feature A

-·

.... feature C 1 ·----- ------- ·-------·------------·····-··--···- · ·-····-······· • feature D

• feature E .... feature F

·-2 -····----··-------- ---- --- -- ------- -- --------- --- --- --·----

3 * * * 4

> 5 ~

-----• -------· ------·------·---------- --·-----·----------- ---·-----·-----··------ -·-.... ....

>-6 Cl ....

Q) c Q)

Cl 7 c '5 c

..... ... • ...... ..• .. äl 8

9 --· -ïï ·-----------------------------------------------------------------------------.......... • .... . .. .. ..• ..

.. ··•···· .. 10

11 ....... ·············· · -·· ·- ---- -- -- --- --·--- -- ·- -·-···· ·· ·· · ···-···· · -· --- ---- -·-···- ·· ··--·· -· -·· · ....

.... .... .... 12 ·- --··- --- ----- ----- ---··-------------·· -·· ---- --- --------- -··--·--··---·------ ·---- ----- ---

0 10 20 30 40 50

Alq3

nominal thickness (A)

Figure 5. 7: Shift of the valenee band features as defined in figure 5.5.

56

the extremely short range over which the work function change occurs is so small, this is called an interface dipale layer.

The features A toF in figure 5.5 portray a similar shift, although with a smaller magnitude. The binding energies of these features are given in figure 5. 7. Note that one feature represents several quasi-degenerate energy levels. For instanee feature A is the combination of the HOMO levels of the three ligands in the Alq3 molecule. These are the energy levels that are associated with the molecular orbitals represented in the HOMO part of figure 3.7.

This rigid shift is very characteristic for an interface dipole, as it acts as a builcl-in potential that raises all energy levels equally. Still there is a discrepancy between the magnitude of the vacuum level shift and that of the molecular orbitals. One explanation can be that some relaxation effects have to be taken into account. In principle the occupied energy levels shift equally with the vacuum level, but as first monolayer completes more and more molecules become surrounded by other Alq3 molecules. This means that after a photoemission event a targeted molecule has more chance to relax via a neighboring molecule, which is less polarizable than the metal surface. The relaxation energy will therefore be less for layers with a higher Alq3 coverage.

Another explanation could be that the work function of the interface is changed by inter­face states. However this is less likely as they would show up in the valenee band spectrum between the Fermi energy and the vacuum level.

5.6 Growth properties

Because photoemission is a very surface sensitive technique it is possible to gather infor­mation about the growth of the first layer(s) of Alq3 . Let's assume that Alq3 grows in a uniform way. Also assume that the Co atoms remain in their original lattice positions when the Alq3 is deposited. This means that the reacted Alq3 will be confined in a zone at the interface thickness tR. Take "' as the fraction of C atoms that react with the Co surface. It is evident that up until the reacted layer is completed, the fractional amount of reacted C atoms T/C-Co stays constant at

(5.2)

For thicker layers, the reacted layer will be buried under a layer of bulk Alq3 (see figure 5.8). The incident X-rays are capable of penetrating the whole sample, but the exiting electrans are scatteredon a distance given by the inelastic mean free path À. Cls electrans that are excited with MgK a radiation have a kinetic energy of lke V. For these electrans typical values of À are in the order of 20 Á.

57

Figure 5.10: The fraction of the C-Co in the Cls spectrum if the growth is layer-by-layer and if all Alq3 molecules contribute equally to the C-Co peak.

The Lambert-Beer law states that when an beam of electrans has to pass through a layer of thickness t, the intensity will be reduced by a factor e-t/>- . To know the intensity of photoelectrons coming from the reacted layer IR , the electron emission per unit of surface area I 0 has to be integrated over the layer thickness.

l f 1

t

IR = J e-3: "'Iodz

t-tn

"'IoÀ ( e!f - 1) e-*

Figure 5.8: Schematic represen­( 5. 3) tation of the samples under in­

vestigation. The thickness of the Alq3layer was varied from 0 to 28

(5.4) A.

The running variabie z indicates the vertical position in the layer as measured from the top surface. Sim­ilarly, the intensity of the total Alq3 layer is given by

(5 .5)

The fraction of reacted carbon peaks in the whole C1s spectrum is thus given by

!..11 e >- - 1 TJC-Co(t::; tR) = K, t •

ex -1 (5.6)

Alq3 islands

Figure 5.9: Island growth of Alq3 on a smooth metallic surface.

58

If the assumptions on the growth process are correct, the relative intensity of the C-Co peak should be similar to that of figure 5.6. As stated before, this layer does not seem to portray a layer-by-layer growth.

It is often seen that they form islands (as in figure 5.9) when they are deposited on another materiaL For analysis purposes, let's define the fraction of Alq3 surface coverage a and the average pilar height tav· Note that the nominal thickness t as measured with differential ellipsometry is actually volume measurement and should be interpreted as the thickness of this volume if is was evenly spread over the surface, thus

t = (]' tav· (5.7)

The apparent shortening of the constant plateau in the C-Co ratio could thus be caused by a preferential growth on other Alq3 molecules as to on the Co surface. By consequence we can interpret the surface coverage fraction a as a sealing parameter for the physical height. However for the C-Co behavior as seen in figure 5.3, this would mean that Alq3

would grow on very high, isolated islands of a large number (a-1 ) of monolayers of Alq3 .

This would also mean that it would take the same multiple ( a- 1) of nominal thickness to cover the full Co surface with Alq3 . From the UPS spectra it is found the that d state peak of Co vanishes between 9 and 28 A.

It is more likely that the sudden drop of the C-Co ratio is because the chemical state of the Alq3 molecules at the surface also depends on the local environment. It could well be that isolated Alq3 molecules have a higher probability to chemically bond with the surface as molecules that are in the close presence other Alq3 molecules. In the sub-monolayer regime, the probability for a molecule that lands on the surface to find other molecules goes linear with the nominal thickness. The effect on the C-Co ratio on the Alq3 thickness would be an exponentially decaying function, as seen from the data.

5. 7 Energy alignment across the interface

Regardless of the true origin of the vacuum level alignment, the data from the UPS measurements can be used to construct the energy level diagram for the Co/ Alq3 interface (figure 5.11). The position of the HOMO level is the lower binding energy edge of feature A as measured with UPS. The LUMO cannot be measured directly with UPS as it is un unoccupied state. The LUMO was estimated by assuming a constant band gap E9 .

As seen in figure 5.6, the vacuum level shows a very rapid transition of 1.8eV in less than 10 A, which is less than one monolayer [25]. Physically this shift must be much shorter because the Alq3 molecules at the interface only partially react with the Co surface, as there is always a sizeable contribution of non-reacted Alq3 in the Cls XPS spectra. It is therefore more realistic to estimate the width of the dipole interface 8 in fractions of molecular size. From computer simulations [36] it has been shown that the Alq3 molecule can change shape under the infiuence of metallic adsorbance. This would shorten the

59

VACUUM l ................ 'ltl I

qJa,jj··:········= .... ......,1f---.t-+---"" HOM 0

hv

SEC

Co

I I IE-~

I I I

CD:hv

I I I I I I I I Co:Alq3

SEC

Alq3

Figure 5.11: Vacuum level alignment for the Co/Alq3 interface.

interface width even more. Therefore the dipale width could well be in the order of the tunneling probability decay, which is typically 1 A [37] for hydracarbon spacers.

When tunneling thraugh the dipale layer can be assumed, the injection barriers for elec­trans and holescan be estimated by the energy difference of the Fermi leveland the LUMO and HOMO respectively. The position of the HOMO upon completion of the dipale layer is measured to be 2.1 eV below the Fermi level, which gives the barrier for injection holes from Co into Alq3 ( 1> B,p). The electron injection barrier is found by subtradion of the band gap Eg.

1>B,n =Eg -1>B,p = 2.5 eV- 2.1 eV = 0.4eV (5.8)

The value used for the band gap Eg [38] is the transport gap for Alq3 . This is takes into account the effects of a the polarization clouds the are generated by the electrans and holes. The transport gap is considerably lower than the difference between the HOMO and LUMO.

When we return to the interface results for bottorn electrades withother materials (figure 3.9), we see that Co has a somewhat different behavior when it is interfaced with Alq3 .

When our result is added in the set to analyze the interface dipale dependance on the metal work function, we get figure 5.12. The fitted lines are for the original data ( dotted line) and the data with Co added (dashed line). The slope parameter increases from 0.23 to 0.51, and the fit is less reliable.

60

2.5,...---------------------.

2.0

> ~ 1.5 Q)

0 a. '6 ~ 1.0 -ê Q)

'E

0.5

Mg

SD = 0.51

Ag Au Al __ .. --·• lf

------·· SD = 0.23

Co

•

0.0 +---.---,---,---.-------,.-----,-----.----r--.---1 3.0 3.5 4.0 4.5 5.0 5.5

Metal work tunetion (eV)

Figure 5.12: The interface dipole of Alq3 as a function of the work function of the metal onto which it was deposited. The data point for Co is the outcome of this study. The dotted line is the fit as in figure 3.9, the dashed line is a fit which includes the Co data. After [15]

This is not unexpected as the physical processes of the energy alignment differ for the different metals. For example for Au it is known that no chemical reaction occurs [39] and that the interface dipole is generated by physisorption. The work function of Au is more or less the same as that of Co. We can therefore assume that the additional interface dipale contribution for the case of Co is mainly because of a charge transfer by a chemical bond.

It would be very interesting to investigate the interface dipale for Alq3 when deposited on other transition metals such as Fe of Ni. For these metals a similar chemical effect as with Co could occur. This could help clarify which mechanisms play a role in the energy level alignment for this organic semiconductor. Up until now the interfaces with different metal have been treated on a case-by-case basis. It could be useful to assign classes of materials that have behaviors when interfaced with metals.

5.8 Condusion

In this chapter we have presented the results of a study of the Co/ Alq3 interface. The interface was build up by deposition of different thicknesses of the organic material on a clean metallic electrode. By using X-ray and ultra violet photoemission spectroscopy, the electronk properties at the interface were examined. It was found that a chemical reaction takes place at between the Co atoms of the surface and C atoms of the Alq3 .

Via this chemical bond a charge transfer is responsible for a very large interface dipale of

61

1.8 eV. The effect of this interface dipale is that the electron injection from the Co into the Alq3 is facilitated and the hole injection from the Alq3 into the Co is more difficult.

62

Chapter 6

Conclusion and outlook

In this thesis we have tried present experimental data that were gathered by using a UPS souree that was installed as part of the graduation work. We were successfully in making this a reliable tool for high resolution valenee band photoemission spectroscopy. Still the intensity of the souree is not up to the manufacturers specifications. It is the author's believe that the gas purity in the discharge lamp is the key to solve this problem. From the experience with the device, it was seen that the intensity of the radiation ( especially that of He II) can be increased after a bake out of the gas inlet system including the cold trap. When the performance of the system is to be increased, special attention has to go to a method getting even lower impurity levels of the He gas.

Despite of the less than optimal UV souree performance it was still possible to obtain high quality spectra of the interface of yet unstudied system. It was found that a very large interface dipole is formed when Alq3 is evaporated onto a Co surface. It was shown that the origin of the large dipole is partly a chemical reaction between the C atoms of the Alq3 and the Co atoms at the surface. It would be interesting to verify this with numerical simulations, which can be performed using Density Functional Theory, but were outside the scope of this project.

The electron and hole injection could become very important parameters for organic magneto resistance devices. Very recently [40][41] a new effect called Organic MAgneto Resistance (OMAR) was proposed. This effect has yet to be fully explained, but it is thought to be an intrinsic effect in organic semiconductors. However it was found that the OMAR effect is dependent on the electron and hole injection barrier of the organo-metallic interface. It might be necessary to engineer these interfaces such that these barriers have a large magneto resistance or a small operating voltage.

Therefore it would be very interesting repeat these experiments with Fe or Ni. Both metals are also very reactive and ferromagnetic, as is Co. Because research on metal interfaces with Alq3 has been focussed on low work function materials for use in organic LED's, the electronk structure of these interfaces are not documented. Interfaces with these metals, as well as magnetic alloys, could become important for future use in spintronie devices.

The same samples could be stuclied with Inverse Photo Emission Spectroscopy (IPES). In IPES, the unoccupied states of a material are measured by injecting electrans and measuring the spectrum of the light that is emitted. Because did not have access to IPES

63

facilities, the unoccupied states could not be measured. However they were estimated by assuming a constant gap between the HOMO and LUMO. This is common practice in photoemission interface studies and have shown to produce reliable results.

It might also be valuable to do an AFM analysis on the growth of the first monolayer of Alq3 on a Co surface. The UPS measurements suggest that the first monolayer grows rather uniform, but harder evidence of this is required to make this claim. If the first layer is indeed uniform, the rapid decay of the C-Co fraction in the XPS spectra have to be attributed to a change in the chemica! reactivity of Alq3 depending on the presence of other molecules on the surface.

64

Dankwoord

Dit verslag is voor mij een kroon op mijn studie Technische Natuurkunde. Zonder de hulp en steun van mijn familie en vrienden zou dit niet mogelijk geweest kunnen zijn. In het bijzonder zou ik mijn vriendin Katrien willen bedanken voor haar steun en de heerlijke chocolade-banaan muffins, mijn ouders voor hun steun en mijn Bommavoorde logies in de laatste maanden aan de TU/ e.

Verder wil Bert Koopmans en Jürgen Kohlhepp bedanken voor hun uitstekende begelei­ding tijdens mijn afstudeerwerk. Ook wil ik Gerrie Baselmans in het bijzonder bedanken voor zijn goede raad op technisch vlak en daarbuiten. Ik ben zeer dankbaar voor de in­spirerende tijd die ik heb mogen meemaken tijdens mijn afstudeerjaar bij de vakgroep FNA. Vandaar dat ik alle groepsleden hiervoor wil bedanken.

Last, but not least: alle mensen met wie ik tijdens mijn studentenjaren in Eindhoven in contact ben gekomen. In het bijzonder de mensen van het dispuut Barabas en mijn oud­huisgenoten. Jullie hebben ervoor gezorgd dat Eindhoven mijn tweede thuis is geworden.

Bedankt.

65

Bibliography

[1] A. Einstein. Über einen die Erzeugung und Verwandlung des 1ichtes betreffenden heuristischen Gesichtspunkt. Annalen der Physik, 17(132), 1905.

[2] C.S. Fadley. Basicconceptsof X-ray photoelectron spectroscopy, volume 2, chapter 1. Academie Press, 1978.

[3] A.F. Carley, P.R. Davies, and R.V. Jones. An stm and xps study of the chemisorption of methyl mercaptan at a cu(110) surface. Surf. Sci., 490:1585-1591, 2001.

[4] H.-G. Zimmer and A Goldmann. Surface-atom valenee-band photoemission from Au(111) and Au(100)-(5x20). Surface Science, 176:115-124, 1986.

[5] A. Zangwill. Physics at surfaces. Cambridge University Press, Cambridge, 1988. [6] S. Tourgaard. Practical algorithm for background subtraction. Surface Sience,

216:343-360, 1989. [7] S. Tougaard. Practical algorithm for background subtraction. Surface Science,

216:343-360, 1989. [8] R. Bindi, H. 1antéri, and P. Rostaing. Application de l'équation de Boltzmann á

l'émission électronique secondaire, de réflexion, du cuivre et de l'or. J. Phys. D: Appl. Phys., 13:461-470, 1980.

[9] M.O. Krause and J.G. Ferreira. K X-ray emission spectra of Mg and al. J. Phys. B: At. Mol. Phys., 8:2007-2014, 1975.

[10] C.W. Tang, S.A. VanSlyke, and Chen C.H. Electroluminescence of doped organic thin films. Joumal of Applied Physics, 65:3610-3616, 1989.

[11] Z.H. Xiong, Di Wu, Z. Valy Vardeny, and Jing Shi. Giant magnetoresistance in organic spin-valves. Nature, 427:821-824, 2004.

[12] T et al. Hassinen. Ghemical 1.02, a software package distributed under the gnu licence via http: I /www. uku. fi;-thassine/ghemical/.

[13] M. Cölle. Evidence for the facial isomer in the blue luminescent phase of tris(8-hydroxyquinoline)aluminum(iii), an online artiele at http://www.fkf.mpg.de/xray/html/body_isomers.html.

[14] S.M. Sze. Physics of Semiconductor Devices. Whiley, New York, 1981. [15] C. Shen, A. Kahn, and I. Hill. Organic Molecular Interfaces: Investigations of elec­

tronic structure, chemistry, and carrier injection properties; in Conjugated polymer and molecular interfaces : science and technology., chapter 11, pages 351-500. Marcel Dekker, Inc., 2002.

[16] W.J. Mortier, S.K. Ghosh, and S. Shankar. Electronegativity equalization method for the calculation of atomie charges in molecules. J. Am. Chem. Soc., 108:4315-4320, 1986.

66

[17] J.F. Janak. Proof that EJEjEJni =Ei in density-functional theory. Physical Review B, 18(12):7165-7168, 1978.

[18] D.R. Lide, editor. CRC Handbook Chemistry and Physics. CRC Press, 85th edition edition, 2004-2005.

[19] l.G. Hill, A. Rajagopal, A. Kahn, and Y. Hu. Molecular level alignment at organic semiconductor-metal interfaces,. Appl. Phys. Lett., 73:662-664, 1998.

[20] S.T. Lee, Y.M. Wang, X.Y. Hou, and C.W. Tang. Interfacial electronic structures in an organic light-emitting diode. Appl. Phys. Lett., 74:670, 1999.

[21] N.D. Lang. Theory of the Inhamogeneaus Electron Gas, page 309. Plenum Press, New York, 1983.

[22] V. Crispin, X. andGeskin, A. Crispin, J. Cornil, R. Lazzaroni, W. Salaneck, and J.-L. Brédas. Characterization of the interface dipale at organic/ metal interfaces. J. Am. Chem. Soc., 124:8131-8141.

[23] Y.C. Chen, J.E. Cunningham, and C.P. Flynn. Dependenee of rare-gas-adsorbates dipale moment on substrate work function. Phys. Rev. B, 30:7317-7319, 1984.

[24] M.V. Tiba. Organo-metallic structures forspintronie applications. PhD thesis, Eind­hoven University of Technology, 2005.

[25] M. Brinkmann, G. Gadret, M. Muccini, C. Taliani, N. Masciocchi, and A. Sironi. Correlation between molecular packing and optical properties in different crystalline polymorphs and amorphous thin films of mer-tris(8-hydroxyquinoline)aluminum(III). J. Am. Chem. Soc, 122:5147-5157, 2000.

[26] R.G. Kepler and et al. Appl. Phys. Lett., 66:3618-3620, 1995. [27] L. Yan and Y. Gao. Interfaces in organic semiconductor devices. Thin Solid Films,

417:101-106, 2002. [28] l.G. Hill, A.J. Mäkinen, and Z.H. Kafafi. Initial stages of metal/organic semiconduc­

tor interface formation. J. Appl. Phys., 88:889-895, 2000. [29] T.L. Barr. Modern ESCA: the principles and practice of X-ray photoelectron spec­

troscopy. [30] D. Briggs and M.P. Seah. Practical Surface Analysis by Auger and X-ray Spec­

troscopy. John Wiley & Sons, 1983. [31] J.A. Bearden and A.F. Burr. Reevaluation of X-ray atomie energy levels. Rev. Mod.

Phys., 39:125-142, 1967. [32] T.-W. Pi, T.C. Yu, C.-P. Ouyang, J.-F. Wen, and H.L. Hsu. Electronic structure

of tris( 8-hydroxyquinolato) aluminum at room temperature and during annealing. Phys. Rev. B, 71:205310, 2005.

[33] V. Patil, S. Oke, and M. Sastry. On the application of a modified sanderson formal­ism to atomie charge-Cis binding energy correlation in some aromatic molecule. J. Electron Spectrosc. Relat. Phenom., 85:249-256, 1997.

[34] X. Crispin. Interface dipale at organic/metal interfaces and organic solar cells. Solar Energy Materials & Solar Cells, 83:147-168, 2004.

[35] E. Kratscheck and W. Kohn. Nonlocal screening in metal surfaces. Phys. Rev. Let., 57:862-865, 1986.

[36] A. Curioni and W. Andreoni. Computer simulations for organic light-emitting diodes. IBM J. Res. & Dev., 45:101-113, 2002.

67

[37] H. Oevering, M.N. Paddon-Row, M. Heppener, A.M. Oliver, E. Cotsaris, J.W. Ver­hoeven, and N.S. Rush. Long-range photoinduced trough-bond electron transfer adn radiatvie recombination via rigid nonconjugated bridges: Distance and solvent de­pendence. J. Am. Chem. Soc., 109:3258-3269, 1987.

[38] S.T. Lee, X. Hou, M.G. Mason, and C.W. Tang. Energy level alignment at AlQ/metal interfaces. Appl. Phys. Lett, 72:1593-1595, 1998.

[39] A. Turak, D. Grozea, X.D. Feng, Z.H. Lu, H. Aziz, and A.M. Hor. Metal/ AlQ3

interface structures. Appl. Phys. Let., 81:766-768, 2002. [40] T.L. Francis and et al. New J. Phys., 6:185, 2004. [41] Ö. Mermer, G. Veeraraghavan, T.L. Francis, and M. Wohlgenannt. Large magne­

toresistance at room-temperature in small-molecular-weight organic semiconductor sandwich devices. Solid State Communications, 134:631-636, 2005.

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