ELECTRON SPECTROSCOPY OF SURFACES

Transcription

1 ELECTRON SPECTROSCOPY OF SURFACES Characterization of Solid Surfaces and Thin Films by Photoelectron and Auger Electron Spectroscopy F-Praktikum in den Masterstudiengängen Physik Versuch Nr. 35 Lehrstuhl E20, Raum 229 Betreuer: Francesco Allegretti 1 Introduction Electrons are well suited probes for the investigation of the electronic and geometric properties of clean and adsorbate-covered surfaces. Consequently, the use of electrons is the basis of a plethora of experimental techniques commonly employed in surface science. This experiment focuses on two techniques, Photoelectron Spectroscopy (PES) and the photon induced Auger Electron Spectroscopy (AES), which both entail the detection of ejected electrons and are well-suited for investigation of the electronic properties of atoms, molecules and solids and for elemental and chemical analysis of surfaces. One of the main reasons for the widespread use of electrons as probes in surface science is their short inelastic mean free path (λ e ) in solids (Fig. 1). This quantity describes the average distance that an electron can travel through the material without suffering energy losses by inelastic scattering; it therefore provides an indication of the electron escape depth. For electrons with kinetic energy in the range between 10 and 1000 ev, λ e can vary between 5 and 30 Å depending on the material and thus amounts to only a few atomic layers. This ensures that in a typical experiment based on electron emission only electrons emitted from a narrow region at the solid-vacuum interface can reach the detector without energy loss. As a result, the PES and AES methods are highly surface sensitive. During the experiment, the students will become familiar with the fundamental principles, the basic operation and methodology of PES and AES and will get an insight into the underlying physics. The potential of the techniques and the range of information that can be gained will be illustrated by considering a prototypical application, the condensation of a magnesium film onto a tungsten surface and its subsequent oxidation. It will also be shown that these nondestructive techniques are of relevance for a vast range of systems not only in condensed matter physics, chemistry, and materials science, but also in applied physics and even biophysics. The technical basis of the experiment encompasses: basics of ultra-high vacuum (UHV) technology generation of X-ray radiation analysis in energy and detection of electrons sample preparation: deposition of a Mg film by evaporation in UHV and subsequent oxidation by molecular oxygen

2 Figure 1: Inelastic mean free path of electrons as a function of kinetic energy for various elements. computer control of the measurements; acquisition of a comprehensive data set Gaining insight into the analysis and interpretation of the data and drafting a short scientific report (in German or English) are further achievements of this FOPRA experiment. 2 Photoelectron Spectroscopy 2.1 Basic principles PES is based on the photoelectric effect, which was first observed experimentally by Hertz and Hallwachs in and explained theoretically by Einstein in 1905 [1]. Although the effect has been well known for a long time, it took more than six decades of experimental developments before Siegbahn and coworkers succeeded in establishing PES as a standard analytical tool in atomic, molecular, and solid state physics. The basic process is illustrated in Fig. 2a: an incident photon of energy ω is absorbed by an electron system (e.g., by a solid surface), promoting an electron from an occupied electronic level into an unoccupied state. If this unoccupied state lies above the vacuum level, the electron can escape into the vacuum and be detected by an energy sensitive analyzer. The energy balance is the following: ω + E i (sys) = E kin + E f (sys) (1) where E i (sys) and E f (sys) are the total energies of the system (atom, molecule, cluster, or solid) before and after photon absorption, respectively, and E kin is the kinetic energy of the emitted photoelectron. In a simplified one electron picture (i.e., neglecting the energy of mutual electron-electron interactions) the difference E f (sys) E i (sys) can be simply regarded as the binding energy of the ejected electron: E b = E f (sys) E i (sys) = ω E kin (2) For a solid surface (Fig. 2b), binding energies are conventionally measured with respect to the Fermi level rather than to the vacuum level, and the previous relationship is written in the form: E b = ω E kin Φ w (3) where Φ w represents the work function of the material. 2

3 photon hω θ p z θ e e - photoelectron in vacuum final state Φ w E kin vacuum level Fermi level y E b hω ω φ p sample φ e initial state x (a) (b) Figure 2: Schematic view of the photoemission process: (a) a photon of energy ω impinges on a solid surface, and after absorption of the photon an electron is ejected in a given direction (θ e, φ e ) with respect to the reference system; (b) corresponding energy balance within the framework of the single particle (or one electron) picture. The work function is the minimum energy required to remove an electron from the solid; it can be seen as the energy barrier that electrons have to overcome in order to escape from the surface into the vacuum. For a given photon energy, recording the number of photoelectrons as a function of their kinetic energy yields a spectrum of distinct lines, which reflects - in the first approximation - the energy distribution of occupied orbitals of the electron system under investigation. Since the binding energies of the occupied orbitals and their sequence are different for (and therefore characteristic of) each chemical element, photoemission spectra can serve as fingerprints of the respective elements (Fig. 3). This renders PES a very useful analytical tool, making it possible - for instance - to determine the composition of an unknown sample and to assess the degree of cleanliness of solid surfaces. Depending on the photon energy we discriminate core level PES (electrons excited from inner shells E b 40 ev) and valence electron PES (electrons excited from outer shells 0 E b 40 ev). The latter is named UPS (Ultraviolet Photoelectron Spectroscopy) because UV photons from gas discharge sources or UV synchrotron beamlines are commonly used. PES with soft X-rays ( ω below 2-3 kev), which can excite core level electrons, is labeled XPS (X-ray Photoelectron Spectroscopy). Typical light sources are X-ray tubes with or without an additional monochromator, and soft X-ray synchrotron beamlines. In the present experiment, XPS measurements will be performed Figure 3: Photoemission spectra of the 1s core levels of the second row elements (taken from Ref. [2]). 3

4 using the characteristic X-ray radiation from an aluminium anode (the Al K α line at ω = ev). 2.2 Spectral features and quantitative aspects of core level PES Binding energy and the chemical shift When comparing the binding energies of core electrons of a distinct atom in different systems (i.e., in different molecules, compounds or solids), one finds that the exact value of the binding energy depends on the chemical environment of the atom and that energy shifts may occur when inequivalent atoms of the same elemental species are present (Fig. 4). These energy shifts are termed chemical shifts. By comparison with data from wellknown standard substances the values of these shifts can be used in many cases for the determination of oxidation states and of the nature of the chemical bond formed by the atoms. This application of XPS, which is denoted by the alternative name ESCA (Electron Spectroscopy for Chemical Analysis), is an important application of core level PES in chemistry, surface physics and materials research. (a) (b) Figure 4: (a) Chemical shifts in isolated molecules (C 1s and N 1s levels); (b) 4f 7/2 core level shift for a W(111) surface. In (b) the surface component at lower binding energy is clearly discerned, whereas the component arising from the first sub-surface layer can only be detected by means of a more sophisticated analysis. This special type of chemical shift originates from the different coordination ( different chemical environment) of surface atoms relative to bulk atoms: it is usually referred to as surface core level shift. Panels (a) and (b) are both taken from Ref. [2]. 4

5 2.2.2 Initial and final state effects in photoemission In the analysis of the energy shifts in core level photoemission one can separate different energetic contributions arising from distinct effects and different physical processes. These effects can be classified depending on the fact that they act, respectively, on the initial state of the system under study or on the final state reached after the photoelectron ejection [3]. Initial state effects Ground state effects: Ground state effects are those contributions to the observed energy shifts that are due to variations of the distribution of valence electrons depending on the chemical environment, i.e. that are due to the electronic part of the potential. Experimentally observed chemical shifts are only partly due to ground state effects. So-called final state effects (see below) can also contribute to chemical shifts, and although their contribution is often assumed not to be dominant, it cannot be neglected a priori. Reference level effects: Shifts of the reference level for the kinetic energy scale may also contribute to experimentally obtained initial state shifts. In order to be able to compare E b values, all photoemission spectra have to be calibrated against identical reference levels. This can be difficult for atoms in different environments. For gas phase measurements E b values are commonly referenced to the vacuum level, while - as noted before - for solid state investigations the Fermi level is the commonly adopted zero point of the binding energy scale. Therefore, if gas phase and solid state data are compared one has to know the energy separation between the vacuum and the Fermi level ( = the work function Φ w, see Fig. 2b). Final state effects Relaxation shift: A very important final state effect is the relaxation of the electron system in response to the creation of a core hole (= the vacancy in the inner shell) upon photoemission. The core hole represents a positive charge acting on the remaining electrons, which therefore rearrange themselves to shield the electron vacancy and to minimize the total energy of the ionized atom. The screening of the core hole typically comprises an intra-atomic and an extra-atomic contribution. The intra-atomic part is due to the relaxation of the electrons (essentially, the outer shell electrons) of the excited atom itself. For isolated atoms this is the only contribution. For atoms bound in molecules or solids the electrons of the neighboring atoms are also polarized by the creation of the core hole, although to a lesser extent, thus giving rise to the extraatomic component of the relaxation. Due to the energy conservation the relaxation of the electron system reduces the apparent binding energy E b measured for the photoelectron. The energy balance before and after photoemission is, in fact: ω + E i (N) = E kin + E f (N 1) = E b = E f (N 1) E i (N) where E i (N) is the energy of the ground state of the atom with N electrons and E f (N 1) the energy of the final state of the ionized atom. If relaxation takes place, the minimization of E f (N 1) leads to a decrease of the expected binding 5

6 energy. Because the screening of the core hole depends on the electronic environment 1, relaxations shifts differ from system to system for a specific core level line. This difference, in combination with the initial state effects (see above), contributes to the chemical shift. Multiplet splitting: Removal of a core electron may create different final state configurations with different spin and orbital angular momentum, and therefore different binding energies in PES (= different lines in the spectrum). In particular, multiplet splitting occurs if the initial state of the photoionized atom contains unpaired electrons. For instance, in the N 1s photoionization of an isolated NO molecule two lines are obtained, which correspond to parallel or anti-parallel orientation of the spin of the residual 1s electron with respect to the unpaired electron in the molecular 2π orbital (triplet and singlet final states, respectively). Another example is given by some transition metal ions, such as Mn 2+ [3]. Mn 2+ has a high spin configuration, S = 5/2, in which all five 3d electrons in the outer shell are unpaired and with parallel spins. If a photoelectron is ejected from the 3s level, the exchange interaction between the spin of the remaining 3s electron and the total 3d spin produces different binding energies for the two possible final states S ± 1 2 = 3, 2. As a consequence, the 3s photemission line is split into a doublet, where the 7 : 5 intensity ratio of the two components stems from the ratio of the respective m S degeneracies. Spin-orbit coupling in the final state: In the photoemission from p, d and f inner subshells the spectral lines are split into two-component peaks (Fig. 5a). This splitting arises from spin-orbit coupling effects in the final state [4, 5]. While in the initial state the inner subshells are completely filled (for example, the configuration of the inner shells in gold is: (1s) 2 (2s) 2 (2p) 6... (4f) ), in the final state one electron has been removed and an unpaired spin is left behind. The spin can be oriented up or down, and if the core hole belongs to an orbital with non-zero orbital angular momentum (l > 0) there will be a coupling between the unpaired spin and the orbital angular momentum: the two states j + = l + 1/2 and j = l 1/2 are not degenerate and a spin-orbit split doublet is observed in the photoelectron spectrum. The intensity ratio of the two spin-orbit components is dictated by the ratio of the respective multiplicities: (2j + + 1)/(2j + 1) = (2l + 2)/2l (Fig. 5b), which determines the relative probability of transition to the two states upon photoionization. Generally, in the photoemission spectrum from a given subshell the core level subpeak with maximum j is detected at lower binding energy. Obviously, no spin-orbit splitting occurs in the photoemission from s core levels. Multi-electron excitations and satellites: The reorganization of the electrons upon creation of the core hole can lead to electronically excited final states. Because of energy conservation this excitation energy is missing in the kinetic energy of the photoelectron: the corresponding satellite lines 1 For example, the screening ensured by electron rearrangement is typically more effective in metals than in ionic solids, thanks to the availability of valence electrons (called conduction electrons) that are free to move inside the material. Accordingly, the magnitude of the binding energy reduction due to relaxation is more pronounced in metals than in ionic materials. 6

7 (a) Au 4f 5/2 Au 4f 7/2 (b) Subshell Total angular momentum j components Ratio of the degeneracies (g = 2j +1) Counts s p 1/2-3/2, 1/2 4/2 d f 5/2, 3/2 7/2, 5/2 6/4 8/6 Binding Energy (ev) Figure 5: (a) Spin-orbit coupling leads to a splitting of the 4f photoemission line of gold into a doublet [2]. (b) Table listing spin-orbit split components and intensity ratios for different subshells. The notation n[l-subshell] j is commonly used (e.g., 4f 7/2 ). are therefore shifted to higher binding energy relative to the main core level line 2. Different processes can be identified. Shake-up satellites occur when an additional electron is promoted from an occupied energy level into a bound unoccupied state, whereas shake-off satellites when the additionally excited electron is promoted into a state in the continuum above the vacuum level (Fig. 6). Other possible satellites are related to plasmon excitations. Plasmons are quasi-particles arising from the quantization of plasma oscillation, i.e. of the collective oscillation of the free electron gas density with respect to the positively charged ion cores. Plasmon satellites appear displaced to higher binding energy from the elastic line by amounts n( ω p ) + m( ω s ), where n and m are integers and ω p and ω s are the energy of a bulk and a surface plasmon, respectively. Finally, interband satellites in solids correspond to transitions of additional electrons into unoccupied states above the Fermi level and to the creation of electron-hole pairs. They may show up as discrete lines in some cases, but commonly they give rise to an asymmetric broadening of the high binding energy side of the photoemission peaks. (a) shake-up shake-off (b) shake-up structure CuO Cu 2p 1/2 Cu 2p 3/ binding energy (ev) Figure 6: (a) Schematic of the shake-up and shake-off transitions; (b) Shake-up loss structure in the Cu 2p spectrum of copper (II) oxide (CuO), adapted from Ref. [6]. 2 Additional satellites in PE spectra occur when the photon source is not monochromatic (see 5.2). 7

8 2.2.3 Surface sensitivity and collection geometry As noted in section 1, the surface sensitivity of photoelectron spectroscopy stems from the short inelastic mean free path of electrons. While X-rays interact only weakly with matter and consequently can penetrate deeply into a solid (1000 nm or more at ω 1 kev), electrons with energy in the range ev readily lose energy by inelastic scattering. XPS experiments are mainly concerned with the intensity of emitted photoelectrons that have suffered no energy losses: therefore, the XPS sampling depth refers to a characteristic length over which electrons can travel without undergoing inelastic scattering events. This quantity is the inelastic mean free path λ e plotted in Fig. 1, which is defined as the average distance that an electron of given kinetic energy E kin travels between two successive inelastic collisions [4]. In other terms, the probability for an electron to cover a distance z in the solid without losing energy can be written as an exponential decay: P (z) e z λ e = < z >= zp (z)dz = λ e If an electron experiences energy losses, but still has enough energy to escape from the surface into the vacuum, it will not contribute to the main photoemission line but rather to a background signal on its low kinetic energy side. For the correct determination of peak areas, peak positions and peak widths this background is always substracted 3. If one considers for simplicity the electrons emitted in the direction perpendicular to a solid surface, the previous considerations imply that a small element of thickness dz at a distance z from the surface (z = 0 denotes the surface plane) gives a contribution di to the measured intensity: di e z λ e dz (4) For the total intensity I coming from a thin film of thickness t right underneath the surface this yields (i.e., by integrating Eq. (4) between 0 and t): I = I 0 [1 e t/λ e ] (5) The sampling depth of the XPS experiment is often defined as 3λ e, which means that 95% of the photoemission intensity of the core level line comes from the corresponding region (3λ e thick) below the surface [4]. When detecting photoelectrons emitted at an angle θ e away from the surface normal (see Fig. 2a), one has to consider the effective path z/cosθ e of the electrons in the solid, and Eq. (4) becomes: z di exp[ ] dz (6) λ e cosθ e Hence, in grazing emission geometry the sampling depth of XPS is reduced to 3λ e cosθ e and the surface sensitivity is strongly enhanced (e.g., θ e 80 = cosθ e 0.17) Quantitation From the previous discussion it is clear that an extended X-ray photoelectron spectrum contains peaks that appear superimposed on a smooth background and can be associated with the different elements (except H and He) present in the outer 50 Å or less of the sample. Determination of the peak areas (appropriately corrected to take into account 3 Particularly at low kinetic energies (< 100 ev) the inelastic electron background is large due to the increasing number of secondary electrons emitted in cascade processes. 8

9 instrumental factors) allows, in principle, an estimate of the elemental ratios, such that the composition of the sample can be determined semi-quantitatively. To this purpose, the absolute intensity I of the photoemission line i from an element A is usually expressed as [4] 4 : z I i,a = K J 0 σ i,a ( ω) T (E A ) L i,a (γ) n A (z) exp[ ] dx dy dz (7) λ e (E A ) cosθ e Here, K is an instrumental constant; J 0 is the incident photon flux, which is assumed for simplicity to be independent of the (x, y, z) position inside the sample; σ i,a ( ω) represents the photoionization cross-section of peak i from the element A, related to the probability of creating a photoelectron in the orbital i of A at the photon energy ω; T is the transmission function of the electron analyzer; L i,a (γ) describes the angular dependence of the photoemission process (γ = angle between the incident X-ray beam and the direction of the ejected electrons); n A (z) is the density of atoms A at a distance z below the surface (neglecting lateral variations, i.e. parallel to the surface plane), and λ e (E A ) is the inelastic mean free path of electrons of energy E A in the material. In most cases, calibration of absolute intensities based on Eq. (7) is not possible, since instrumental factors may be difficult to determine. However, measurements of relative intensities are valuable in practical experiments where elemental ratios are important, such as - for instance - for the detection of relative surface coverages of adsorbates and contaminants or for the determination of the stoichiometry of a compound Peak width of core level lines The observed line shape of a given photoemission peak results from the convolution of different contributions. These are related: i) to the equipment used in the experiment (instrumental resolution); and ii) to the photoemission process itself (lifetime of the core hole, presence of satellite features). To i) contribute primarily the linewidth and lineshape of the photon source used for the excitation, and the energy resolution of the electron energy analyzer employed to analyze in energy the distribution of photoelectrons. An essential contribution to ii) is the peak broadening due to the finite lifetime of the core hole; broadening by coupling to the nuclear motion (intra-molecular vibrations including rapidly dissociating antibonding states, and phonons) and due to the presence of shake-up satellites, electron-hole pair excitation and multiple splitting effects may be in many cases non-negligible. The spectral lineshape due to the core hole lifetime τ is Lorentzian [7]: I(E) = I(E c ) Γ 2 (E E c ) 2 + Γ 2 (8) where I(E) is the intensity at the energy E, and E c is the center of the Lorentzian peak (when the spectrum is plotted in the binding energy scale, E c is the binding energy E b of the main line satisfying the energy conservation law with the relaxation shift taken into account). The energy broadening is therefore quantified by the the full-width at halfmaximum (FWHM) of the line, 2Γ, which is called intrinsic or natural linewidth and is related to the lifetime through the following expression [8]: 2Γ = τ = ev sec τ [sec] (9) 4 Photoelectron diffraction effects arising from coherent scattering of the photoelectron wavefield by neighbouring atoms are neglected in formula (7). This is correct, for example, in amorphous or polycrystalline samples and for angle-integrated mesurements. 9

10 2Γ is typically larger for inner shell orbitals, because these can be filled rapidly by electrons from the outer shells. Therefore, for a given element 2Γ tends to increase with increasing binding energy (for example: the natural broadening increases in the order 4f < 4d < 4p < 4s in gold). In a similar way, for a given core level orbital (e.g. 1s) 2Γ increases as the atomic number Z increases, due to the higher valence electron density in heavier atoms. At variance with the Lorentzian broadening produced by the core hole lifetime, instrumental effects and vibrational broadening yield Gaussian lineshapes. In metals electronhole excitations at the Fermi edge can cause additional asymmetric line broadening, with an asymmetric tail on the low kinetic energy side of the photoemission peak. Multiple splitting and shake-up satellites also contribute with asymmetric broadening: depending on their binding energy and the instrumental resolution they may or may not be resolvable from the main photoemission peak. The contributions of intrinsic and instrumental effects to the experimentally observed linewidth can be combined to a first approximation into a quadratic sum [4]: F W HM 2 tot = (2Γ) 2 + F W HM 2 X + F W HM 2 A (10) with the second and third term on the right side due to the X-ray source and analyzer resolution, respectively. 3 Auger Electron Spectroscopy 3.1 Decay of a core hole Photoionization of inner shell levels creates electronically excited states. These excited states can decay via two different de-excitation routes: (a) The hole in the inner shell X is filled by an electron from an outer shell Y, and a photon is emitted. This is the well-known X-ray emission process or fluorescent decay. The energy balance is: ω = E i (X) E f (Y ) (11) where ω is the energy of the fluorescence photon, and E i (X) and E f (Y ) are the total energies of the system with holes in the shells X and Y, respectively. (b) The hole in the inner shell X is filled by an electron from an outer shell Y as in the previous case, but the excess energy is transferred to a third electron from an outer shell Z, which is emitted. The process, called Auger decay, involves three electrons: the electron knocked out in the initial ionization (photoelectron), the electron that decays to fill the inner shell vacancy, and the emitted electron from the outer shell (Auger electron). As a consequence, the final state is a doubly ionized state. Since the energy is conserved during the transition, the kinetic energy of the emitted Auger electron, measured with respect to the Fermi level E F, is: E kin,f = E i (X) E f (Y, Z) (12) Here, E i (X) and E f (Y, Z) represent the total energies of the system with one hole in shell X, and two holes in shells Y and Z, respectively. The non-radiative decay is, of course, only permitted for E kin,f > 0; this condition limits the number of possible X,Y,Z combinations. The branching ratio of processes (a) and (b) is governed by the respective decay crosssections. For the binding energy range below 1.5 kev, as in the present experiment, and especially for the lighter elements, the non-radiative decay by far prevails. 10

11 3.2 Important aspects of AES In most cases, core ionization and core hole decay can be treated as independent (twostep process). Besides via photoionization, the primary core hole can be also created by inelastic electron scattering. One can thus distinguish between XAES (X-ray induced Auger Electron Spectroscopy) and EAES (Electron induced Auger Electron Spectroscopy), depending on whether the primary hole has been created by photoionization or by electron scattering. Excitation by electrons is very popular in conventional laboratories, because electron sources are much cheaper than photon sources and the experimental set-up can be relatively simple [9]. Moreover, electron guns can have very small beam spots enabling element-specific Auger electron microscopy. Photon induced core ionization, on the other hand, has the advantage that the concomitant rate of valence excitations is lower than in the EAES case. Valence excitations are the main sources of irradiation damage, therefore XAES is a less destructive tool. Because of this, XAES is an important technique for investigation of radiation sensitive layers on surfaces. As in the PES case, discrete, element-specific lines associated with Auger transitions appears in AES spectra (see Fig. 7 for a survey of the different processes in photoexcited electron spectra). Similarly to PES, therefore, Auger spectra can be used for elemental analysis. Moreover, chemical specificity is ensured by the occurrence of chemical shifts caused by shifts in the inner shell energy levels due to changes in the bonding. It should also be noted that the emitted Auger electrons carry a well-defined kinetic energy, which directly reflects the sequence of energy levels involved in the Auger transition and does not depend on the energy of the photons/electrons used to create the primary hole in the inner shell. At variance with core level peaks in PES, which are characterized by a constant binding energy ( the kinetic energy increases linearly with the photon energy), Auger lines are better identified by their distinctive kinetic energy and AES spectra are therefore commonly plotted on the kinetic energy scale. The nomenclature of Auger transitions follows the electron shells contributing to the Figure 7: Features of photoexcited electron spectra. From right (high kinetic energy) to left (low kinetic energy): d) photoionization of valence band levels; c) Auger transitions; b) core level excitation; a) inelastic loss processes. 11

12 Figure 8: Explanation of different types of Auger decay processes based on the energy level diagrams: a) KLL decay; b) LM M decay; c) Coster-Kronig transition; d) Auger decay involving valence band levels of a solid. Figure taken from Ref. [10]. decay. An Auger line labeled XY Z corresponds to a primary hole in the X shell that decays into final holes in the Y and the Z shells (see Fig. 8). Commonly, the X-ray nomenclature is used for identification of the holes: the K shell denotes 1s electrons, while L 1, L 2, L 3 indicates 2s, 2p 1/2, and 2p 3/2 electrons, respectively, etc. The decay crosssection is maximum if one of the two final holes has the same principal quantum number (i.e., is from the same shell) as the primary hole. These XXZ processes are called Coster- Kronig transitions. For super Coster-Kronig decays all participating holes have identical principal quantum numbers (e.g., N 4 N 6 N 6 and N 5 N 7 N 7 for tungsten). When the atom in which the Auger process takes place is bound in a solid, extended electronic bands can be involved in the decay and the label V is used. For example, a L 3 V V transition occurs when an electron from the valence band de-excites into the L 3 core hole, with the excess energy being transferred to another valence band electron. Because strong redistribution of valence electrons may occurr upon formation of new chemical bonds, valence band Auger transitions supply chemical information. Similarly to PES, satellite lines occur in AES. Multiplet features are even richer for AES in comparison to PES because of the doubly ionized final state, i.e. there are at least two holes which interact with each other. Depending on the atomic number Z, L S (for low Z) or j j coupling (for large Z) prevails. Possible final states of KLL transitions are presented in Fig. 9. Decay satellites, such as shake-up or shake-off processes triggered upon decay, can render the situation even more complicated. Characteristic energy losses due to excitation of plasmons and asymmetric peak broadening caused by interband transitions are also frequently observed in solids. For all these reasons, Auger lines appear - in general - broader than typical core level photoemission peaks, and they may also exhibit a more complex shape. Calculation of the kinetic energies of Auger electrons according to Eq. (12) can be difficult for electron systems larger than isolated atoms. Hence, approximations are required. One possibility is to take the binding energy for the energy level X of the primary hole, remove from it the binding energy of the decay electron from shell Y in order to quantify the de-excitation energy, and finally subtract the binding energy of the level Z from which the Auger electron is ejected [11]. Inserting an additional term U h h that accounts for the 12

13 Figure 9: Relative KLL Auger energies from theory (lines) and experiments (points) as a function of the atomic number Z. Light elements (left side): L S coupling, heavy elements (right side): j j coupling; intermediate coupling case in between. The doubly ionized electronic configurations are indicated in brackets, preceded by the spectroscopic notation ( 1 S 0, 1 P 1, etc.). See Ref. [9]. interaction of the two holes in the final state, one can write: 5 E kin,f = E P ES b (X) Eb P ES (Y ) Eb P ES (Z) U h h (13) By neglecting U h h, one obtains the approximate kinetic energy range of the Auger transition. U h h can be determined experimentally by comparison with the measured E kin,f, thus providing important information about the interaction energy of the two holes. 4 Adsorption on surfaces The term adsorption indicates the process of trapping and binding of foreign species (atoms or molecules) onto a surface by condensation from the gas phase. The energy released in this process is called energy of adsorption U ads or also binding energy (not to be confused with the binding energy in PES). The term substrate refers to the solid surface onto which adsorption occurs, while adsorbate is used to indicate the species that are adsorbed onto the substrate. Adsorption can be associative (if a particle or molecule is adsorbed as a single unit) or dissociative (if the particle or molecule is dissociated upon adsorption), and moreover adsorption processes may be reversible or irreversible. The reverse process, the removal of adsorbed species from the surface, is called desorption. Desorption may be stimulated by heat (thermal desorption), by electrons (electron stimulated desorption), or by photons or ions (photon or ion stimulated desorption). 4.1 Types of adsorption Physisorption: Physical adsorption (= physisorption) is governed by weak Van der Waals interactions with the surface. The adsorption energy U ads is small, typically < 30 kj/mol or 0.3 ev/particle. Formation of multilayers (condensates) is possible at sufficiently low temperatures. Example: He on noble metal surfaces at cryogenic temperatures. 5 To express E sample kin,vacuum the work function has to be substracted from E kin,f in Eq. (13). 13

14 Chemisorption: Chemical adsorption (= chemisorption) involves the formation of a chemical bond between the adsorbate and the surface, e.g. transfer of electrons and/or formation of covalent bonds (hybrid molecular orbitals). In most cases the adsorption energy is much larger than 30 kj/mol, 100 kj/mol or 1 ev/particle. Chemisorption is often dissociative and may be irreversible; activation barriers are possible, and chemisorbates form typically only a single layer (= direct contact of surface atoms and the chemisorbate is required). Moreover, specific binding sites may be favoured because of the highly corrugated surface potential. Examples: CO and oxygen chemisorption on transition metals at room temperature. 4.2 Adsorption order First order adsorption: One particle from the gas phase yields one particle of the adsorbate, i.e., the impinging particles are molecules that do not dissociate upon adsorption. The reverse process is first order desorption. Examples: CO/nickel (CO chemisorbs on Ni surfaces molecularly with U ads 120 kj/mole); Ar/Ruthenium (Ar physisorbs on Ru with U ads 10 kj/mol). Second order adsorption: A molecule dissociates upon adsorption. One particle in the gas phase yields two particles on the surface. The reverse process is associative desorption or second order desorption. Example: H 2 /Nickel (H 2 dissociates into two H atoms on Ni, U ads 90 kj/mole). 4.3 Nomenclature Other concepts that are common in adsorption studies in the realm of surface science: Surface (relative) coverage Θ: Θ N ads N sub with N ads = number of adsorbate particles per unit area; N sub = number of particles per unit area in the substrate surface layer(s). Θ is typically expressed in monolayers or as a fraction of a monolayer (ML), 1 ML corresponding to an adsorbate surface density equal to the surface density of substrate atoms. Collision rate ν: it is defined as the number of particles hitting a surface per unit time and unit area. The Hertz-Knudsen equation following from the gas kinetic theory states that: p ν = [particles s 1 m 2 ] 2πMkB T with P = pressure [N m 2 = Pa]; M = particle mass [Kg]; T = temperature [K]; k B = Boltzmann constant. Sticking coefficient s: s = dn ads dt / dncoll dt 0 s 1 probability that a particle colliding with the surface is adsorbed and stays on it. Exposure: measure of the amount of gas that a surface has been exposed to; more specifically, the product of the gas pressure and the time of exposure. Normal unit is the Langmuir, where 1L = 10 6 Torr s (1 Torr = Pa = mbar). 14

15 5 Experimental set-up The experimental set-up, schematically depicted in Fig. 10, consists of an ultra-high vacuum (UHV) chamber on which the PES apparatus and the equipment for sample insertion, preparation and manipulation are mounted. 5.1 Vacuum system The UHV chamber is maintained at a base pressure lower than mbar ( Pa). The pressure is measured by an ion gauge (hot filament ionization gauge) mounted inside the chamber. Pressure in the UHV regime is required in order to permit electron detection (avoiding scattering of the photoelectrons between the sample surface and the detector) and to ensure that atomically clean surfaces can be prepared and maintained free of contaminants during the experiment. The chamber is pumped by an ion getter pump directly mounted inside the chamber. Auxiliary pumping by a titanium sublimation pump is possible. In addition, an external turbomolecular pump (with attached a backing roughing pump) can be connected to the UHV chamber through a gate valve. This turbomolecular pump is also used to pump the load-lock system adjacent to the main chamber. The UHV regime can only be reached after baking out the chamber for about 24 hours (or longer) at temperatures exceeding C. This procedure allows to eliminate residual molecules (predominantly water) adsorbed on the stainless steel walls of the chamber, which otherwise would be released at a low rate thus increasing the pressure. A brief introduction to the UHV technology, instrumentation and related protocols can be found in Refs. [12, 13]. hemispherical electron energy analyzer (HSA) detector UHV chamber pressure gauge X-ray source e - holder leak valve line O 2 W sample Mg evaporator gate valve load lock system titanium sublimation pump turbomolecular pump + roughing pump ion pump Figure 10: Scheme of the UHV chamber with the PES apparatus and ancillary equipment. 15

16 5.2 X-ray source A conventional X-ray laboratory source provides characteristic Al Kα radiation. The source consists of a cathode filament, which emits electrons upon heating by direct current (typical emission current: ma), and an anode, onto which the electrons are accelerated by applying a high voltage (typically, kv). Water cooling is required during continuous duty operation of the X-ray source, in order to prevent melting of the anode target. Here, the anode is an Al film - some 10 µm thick - supported on a copper rod. During operation, the electrons from the cathode collide with the atoms of the Al target, causing vacancies in inner electron shells. The subsequent de-excitation by fluorescence decay (see 3.1) produces X-ray radiation of characteristic energy. energy L L L p 2p 2s 3/2 1/2 Kα K α 1 2 Al Kα 1,2 K 1s ħω = ev FWHM = 0.78 ev (a) (b) Figure 11: Characteristic radiation from an Al anode source: (a) schematic energy level diagram explaining the production of the individual Kα 1 and Kα 2 lines; (b) emission spectrum showing the convolution of the Kα 1 and Kα 2 lines into a composite Kα 1,2 line centered at ev with 0.78 ev FWHM. Adapted from Ref. [12]. As illustrated in Fig. 11, the Kα 1 and Kα 2 lines are generated, corresponding to L 3 K and L 2 K transitions, respectively, where L 3, L 2 and K denote the 2p 3/2, 2p 1/2 and 1s levels. The L 2,3 subshell is split due to spin-orbit coupling, such that the relative intensity of the two lines reflects the L 3 to L 2 population ratio of 2 : 1. As a consequence, the Kα 1,2 line is in principle a doublet with an energy splitting of 0.43 ev. However, since each line has a natural width of 0.47 ev caused by lifetime broadening, the doublet is not resolved and the resulting composite line has a linewidth of 0.78 ev (FWHM) and is centered at ω = ev. In addition, a Kα 3,4 satellite is also produced, attributed to emission from doubly ionized atoms [12]. This satellite line is centered at about 1496 ev and its intensity is 10% of that of the main Kα 1,2 emission. Additional emission lines can arise from contamination (e.g., the O Kα line centered at ω = ev if the anode material is partly oxidized) or other modifications of the anode (e.g., when the Al film becomes too thin, the Cu L α line from the copper rod beneath may be excited, ω = ev). 5.3 Electron energy analyzer An electrostatic 180 hemispherical analyzer (HSA) is used to analyze in energy the electrons emitted from the sample. This electrostatic device consists of two metallic hemispheres, concentrically arranged as in Fig.12, and enables to disperse the electrons as a function of their kinetic energy similarly to a prism that disperses light depending on its wavelength. For this purpose, a potential difference V is applied between the inner and outer hemispheres, so that the trajectory of the incoming electrons is bent into a curve 6. 6 To prevent perturbation of the electron trajectories by stray magnetic fields, the analyzer region is shielded by a µ-metal shield inside the vacuum vessel. 16

17 V(R ) = - V 2 2 electron optics V(R ) = - V 1 1 detector V = V(R ) - V(R ) 1 2 Figure 12: Schematic of a concentric hemispherical analyzer. Both hemispheres are at negative potential, with V (R 2 ) < V (R 1 ) ( V 2 > V 1 ). Adapted from Ref. [14]. An electrostatic lens between the sample and the entrance slit S of the analyzer focuses the photoelectrons from the sample onto S. At a given V, ideally only the electrons with a well-defined kinetic energy E p are able to complete their path along the median trajectory of radius R 0 = (R 1 +R 2 )/2 (R 1 and R 2 radii of the inner and outer hemisphere) and emerge at the exit slit F. E p is called pass energy and its typical values range between 5 and 50 ev for XPS measurements. An important concept is that of energy resolution of the analyzer. This is a measure of the ability of the analyzer to distinguish and separate ( resolve) peaks that differ in energy by only small amounts. As discussed in 2.2, the energy resolution of the analyzer contributes to the overall FWHM of a core level peak (Eq. (10)). For the HSA, the energy resolution can be expressed by the formula [12, 13]: E/E p = W S + W F 4R 0 + α2 2 (14) in which E is the FWHM, W S and W F are the widths of the entrance and exit slit, respectively, and α is the angular spread of the electron beam. The analyzer is operated at constant pass energy to yield a constant absolute resolution E. The lower the pass energy, the better the energy resolution. In order to enable scans over arbitrary kinetic energy ranges, the electrons are retarded (or accelerated) to the kinetic energy E p by a potential difference R inside the lens system situated between the sample and the entrance slit. A channel-type electron multiplier (channeltron) is located behind the exit slit of the analyzer, where the emerging electrons are counted. Therefore, by scanning R a spectrum of the photoelectron intensity as a function of the kinetic energy is recorded, the measured kinetic energy being simply R+E p. When R = 0, the Fermi levels of sample and analyzer are aligned (in practice, both objects are grounded), whereas they are separated by R when the retarding voltage is applied. However, due to the different values of the work function, the vacuum levels of sample and analyzer do not coincide even for R = 0. From the energy level diagram illustrated in Fig. 13 it then follows: E b = ω (E p Φ analyzer R) = ω E analyzer kin, vacuum Φ analyzer (15) 17

18 e - E p E kin,f hω Φ analyzer E vacuum,analyzer E vacuum,sample Φ w R E F,analzyer E F,sample E b Core Levels E kin,f analyzer E kin,vacuum sample E kin,vacuum E p = R + + = R + = R + E p E p Φ analyzer (measured) + (Φ - Φ ) analyzer w Sample Analyzer Figure 13: Photoelectron detection by an electron energy analyzer: schematic diagram of relevant energy levels and voltages. Eq. (15) shows that the work function of the analyzer, rather than the work function of the sample, needs to be taken into account. Φ analyzer enters the equation because in the experiment the kinetic energy scale is referenced to the vacuum level of the analyzer. To keep Φ analyzer constant, all electrodes are covered with a chemically inert material (gold or graphite; in the present case graphite). 5.4 Samples Three samples will be investigated during the experiment. A tungsten (W) ribbon will be studied initially. It can be heated up to 2500 K by direct current flow in order to remove contamination from the surface. The W ribbon is mounted on a four-degree-of-freedom manipulator enabling translational motion along three orthogonal directions and rotation around the axis of the manipulator. In this way, the polar angle θ e defined in Fig. 2a can be changed, thus varying - when needed - the surface sensitivity of the PES measurements. Further, the W ribbon will serve as substrate for the growth of a Mg film that will be deposited in-situ under UHV conditions. The subsequent oxidation of magnesium will provide a clear illustration of the concept of chemical shift; moreover, measurements with different surface sensitivity will provide structural information on the depth of the oxidation process. Finally, a third sample to be analyzed will be introduced through the custom-made load-lock system, which allows fast insertion of ex-situ prepared samples without breaking the vacuum of the main chamber. An UHV compatible sample chosen by the students will be inserted into the chamber and characterized by PES. Possible examples: a metal substrate covered by an organic sulphur-containing self-assembled monolayer (SAM); a coin; a (cheap!) piece of jewellery; the crown cork of a bottle; a fragment taken from a broken DVD, CD or hard disk; a piece of special glass, etc. 18

19 5.5 Magnesium evaporator The evaporator consists of a small high-melting-point crucible containing Mg grains. The crucible is heated by passing current through a conducting wire coiled around it. Prior to evaporation, the Mg grains need to be outgassed repeatedly, in order to ensure that a clean Mg film is deposited on the tungsten ribbon during the experiment. 5.6 Gas handling line A gas line filled with molecular oxygen is connected to the main chamber through a highprecision leak valve. In this way O 2 can be admitted into the chamber in a controllable fashion during the experiment. 6 Concepts to be understood before the experiment Fundamental principles of PES and AES: the photoemission process, Auger transitions. Core level lines, valence band excitation. Binding energy. Work function. Satellite lines. Spin-orbit splitting. Chemical shifts. Surface sensitivity of PES and AES. Importance of the electron detection geometry: normal emission vs. grazing emission. Generation of characteristic X-rays by fluorescent decay. Working principle of the Al anode X-ray source. Hemispherical electron energy analyzer. Pass energy. Retarding voltage. Instrumental resolution. Ultra-high vacuum. Adsorption of foreign species. Dissociative adsorption. 7 Instruction for the experimental session The goal of the experiment is to characterize by core level PES the tungsten ribbon mounted inside the chamber, and to study by PES and AES the modifications introduced by the deposition of a Mg film. The effect of the oxidation of the Mg film is also part of the investigation. In addition, insight into the performance of the hemispherical analyzer will be gained, in particular as to the dependence of the core level intensity and energy resolution on the pass energy. In order to stimulate the scientific curiosity of the students, in the final part of the experiment the possibility of studying an additional sample at choice is offered. In this way, the experiment provides an overview on the main information that can be obtained from the two experimental techniques, on some technical aspects and on typical issues faced in adsorption studies and surface science. Proceed according to the following steps: A. Performance of the analyzer and characterization of the W sample 1. Get an introduction to the experiment and to the equipments by your advisor. 2. Clean the W ribbon following the advisor s instructions. After setting the pass energy to E p = 50 ev, record a survey spectrum 7 with 100 ev E kin 1500 ev. In this broad scan you are not interested in the fine details of the core level lines, 7 Unless otherwise stated, photoemission spectra will be taken at an electron emission angle of 45, which corresponds to an incidence angle of 45 of the photon beam relative to the surface normal. 19

20 so you can use a large energy step (1 ev). Try to assign all maxima to known core level and Auger peaks: this may be easier if you plot the spectrum in the binding energy scale, using the appropriate computer software. 3. Zoom-in into the W 4f doublet using an energy step of 0.1 ev and record a sequence of spectra with different values of E p (e.g., 7.5, 10, 15, 20, and 50 ev). In order to have good statistics, at each value of the pass energy select appropriately the integration time per energy point. 4. Zoom-in into the C 1s and O1s core level regions at E p =50 ev. Is the surface completely free of contaminants? 5. At E p = 20 ev record a photoemission spectrum of the W valence band (see Fig. 7). Determine Φ analyzer from the position of the Fermi edge. B. Cleanliness of the Al anode of the X-ray source 6. Using E p = 50 ev, acquire a PES spectrum for 200 ev E kin 530 ev with the X-ray source operated at reduced anode voltage of 1.4 kv and an emission current of 6 ma (consult the advisor to change the operation conditions). Now that the Al Kα emission is suppressed, what is the origin of the spectral features observed? C. Deposition of a Mg metal film 7. The advisor will now deposit a Mg film on the W sample: record a survey spectrum from 50 to 1500 ev (E p = 50 ev, E step = 1 ev). Assign all the features of this spectrum and assess qualitatively the cleanliness and thickness of the film. Can you see any signal from the W substrate? 8. Record the Mg KLL Auger spectrum in the kinetic energy range from 1060 ev to 1200 ev (E p = 20 ev, E step 0.3 ev) for: (1) nearly normal electron exit (θ e = 10 ); (2) nearly grazing electron exit (θ e = 80 ). Keeping in mind the different probing depth of the two geometries, can you conclude that the (electronic) structure of the metallic film is uniform across its thickness? 9. Zoom-in into the Mg 1s region (E p = 10 ev). Write down the kinetic energy of the main line. Record a photoemission spectrum (E p = 10 ev) from the O 1s region. Any feature? D. Oxidation of the Mg film at room temperature 10. After the Mg overlayer has been oxidized by dosing 45 L O 2 at room temperature (P O2 = mbar), repeat step 9. Any changes? Explain the results in the light of what you have learnt about the chemical shift. 11. Repeat step 8. Which conclusions can be drawn based on the different surface sensitivity of the two collection geometries? Is the film uniformly oxidized and structurally homogeneous across its thickness? 20