Solid state effects in the Auger spectrum of zinc and oxidised zinc

Measurements of the zinc L_2,3M_4,5M_4,5 Auger spectra are reported. The line shapes in solid zinc are similar to those in zinc vapour but the Auger energies have increased by about 15 eV and the line breadths have broadened from 0.5 eV to 1.0 eV fwhm. The ratio of the L₂:L₃ groups differ from the vapour suggesting that L₂L₃M_4,5 Coster-Kronig transitions occur in the solid but not in the vapour. Changes in the spectra with oxidation have been observed. The Auger lines broaden on oxidation and a line breadth of 3.2 eV fwhm gives the best fit to the spectrum of almost fully oxidised zinc. The oxide L₃M_4,5M_4,5¹G₄ peak progressively shifts from 2.6 eV to 4.2 eV below the metal peak as the oxide thickness increases, the latter value being close to the measured shift in crystalline zinc oxide. Similar energy variation is reported for solid Argon condensed onto clean silver and the shifts are explained in terms of variation in “extra electron relaxation” with film thickness.     

High energy (~ 107 eV) Si peak in Ar excited auger emission from silicon and silicides

The Ar⁺ excited electron emission from Si and Si-Ni compounds (from NiSi₂ to Ni₃Si) was studied, with emphasis on the high energy peak in the Si (L₂₃-related) spectrum. This peak is associated with the decay of Si atoms having two 2p holes (Si²*); therefore it originates in asymmetric (Ar-Si) collisions only. It has been investigated to determine the occurrence of these collisions with respect to the symmetric (Si-Si) ones and their relative weight.The threshold energy for the Si^2* related Auger emission was found to be about 2.9 keV in Si and silicides, significantly lower than previously reported. Above this threshold, the relative weight of the asymmetric collisions increases with ion energy and depends on the target stoichiometry, being greater in metal-rich silicides. However in the investigated ion energy range (1 to 5 keV) the total Auger yield was found to be mainly related to the symmetric collisions.We also investigated the dependence of the high energy Si peak on the excitation and acceptance geometry. The results indicate that asymmetric collisions are mainly “surface events”, resulting in the ejection of an anisotropic flux of energetic Si atoms.     

SiO2 surface defect centers studied by AES

A theoretical model is proposed on how a Si dangling bond associated with an oxygen vacancy on a SiO₂ surface (E_s′ center) should be observed by Auger electron spectroscopy (AES). The Auger electron distribution N_A(E) for the L₂₃VV transition band is calculated for a stoichiometric SiO₂ surface, and for a SiO_x surface containing Si-(e^?O₃) coordinations. The latter is characterized by an additional L₂₃VD transition band, where D is the energy level of the unpaired electron e^?. The theoretical N_A(E) spectra are compared with experimental N(E) spectra for a pristine, and for an electron radiation damaged quartz surface. Agreement with the theoretical model is obtained if D is assumed to lie ≈2 eV below the conduction band edge. This result shows that AES is uniquely useful in revealing the absolute energy level of localized, occupied surface defect states. As the L₂₃VD transition band (main peak at 86 eV) cannot unambiguously be distinguished from a SiSi₄ coordination L₂₃VV spectrum (main peak at 88 eV), supporting evidence is presented as to why we exclude a SiSi₄ coordination for our particular experimental example. Application of the Si-(e^?O₃) model to the interpretation of SiO₂Si interface Auger spectra is also discussed.     

Electron states in α-quartz (SiO2)

Valence electron states of α-quartz SiO₂ are calculated self-consistently using the pseudopotential method. Excellent agreement is found with photoemission and u.v.-absorption data. X-ray emission spectra are calculated in an OPW scheme and compared to experiments. While the Si- and O-K spectra agree well with experiment, the Si L_{2, 3} spectrum shows substantial differences. An explanation is offered based on the formation of amorphous Si in SiO₂ during electron irradiation.     

Properties of the SiO2/SiC interface investigated by angle resolved studies of the Si 2p and Si 1s levels and the Si KLL Auger transitions

Angle resolved photoemission studies of the Si 2p and Si 1s core levels and the Si KL_2,3L_2,3 Auger transitions from SiO₂/SiC samples are reported. Most samples investigated were grown in situ on initially clean and well ordered √3×√3 reconstructed 4H-SiC(0 0 0 1) surfaces but some samples were grown ex situ using a standard dry oxidation procedure. The results presented cover samples with total oxide thicknesses from about 5 to 118 Å. The angle resolved data show that two oxidation states only, Si⁺¹ and Si⁺⁴, are required to explain and model recorded Si 2p, Si 1s and Si KLL spectra.The intensity variations observed in the core level components versus electron emission angle are found to be well described by a layer attenuation model for all samples when assuming a sub-oxide (Si₂O) at the interface with a thickness ranging from 2.5 to 4 Å. We conclude that the sub-oxide is located at the interface and that the thickness of this layer does not increase much when the total oxide thickness is increased from about 5 to 118 Å.The SiO₂ chemical shift is found to be larger in the Si 1s level than in the Si 2p level and to depend on the thickness of the oxide layer. The SiO₂ shift is found to be fairly constant for oxides less than about 10 Å thick, to increase by 0.5 eV when increasing the oxide thickness to around 25 Å and then to be fairly constant for thicker oxides. An even more pronounced dependence is observed in the Si KLL transitions where a relative energy shift of 0.9 eV is determined.The relative final state relaxation energy ΔR(2p) is determined from the modified Auger parameter. This yields a value of ΔR(2p)=−1.7 eV and implies, for SiO₂/SiC, a “true” chemical shift in the Si 2p level of only ≈0.4 eV for oxide layers of up to 10 Å thick.     

Influence of the geometric structure on the V L3 near edge X-ray absorption fine structure from vanadium phosphorus oxide catalysts

We present the V L₃ near edge X-ray absorption fine structure (NEXAFS) of a vanadium phosphorus oxide (VPO) catalyst. The spectrum is related to the V3d–O2p hybridised unoccupied states. The overall peak position at the V L₃-absorption edge is determined by the formal oxidation state of the absorbing vanadium atom. Details of the absorption fine structure are influenced by the geometric structure of the compound. Empirically we found a linear relationship between the energy position of several absorption resonances and the V–O bond length of the participating atoms. This allows identification of the contribution of specific V–O bonds to the near edge X-ray absorption fine structure. The bond length/resonance position relationship will be discussed under consideration of relations between geometric structure and NEXAFS features observed in X-ray absorption experiments and theory.     

The energy spectra of secondary ions emitted during ion bombardment

Secondary ion energy spectra have been measured for singly charged ions emitted from targets irradiated with 43 keV A⁺ ions. Targets studied include the 3d transition metals (Sc, Ti, V, Cr, Fe, Ni) Cu and Zn, Zr, Al and Si and the compounds SiO₂, Al₂O₃, NaCl, KCl. Energy spectra were measured in the energy range 1–600 eV. In several cases a peak in the energy spectrum in the region around 200 eV has been found. This is in addition to the usual low energy peaks in the region of 5–10 eV. In many cases the low energy peak was observed to decay steadily with irradiation time or to increase with oxygen pressure. In the case of the cleanest Zn spectrum, only the high energy peak can be detected. The data are discussed in relation to current models of secondary ion emission. We conclude that, in general, elemental metal targets which are clean are characterised by the high energy peak in the secondary ion energy spectrum. The slower ions emitted have been neutralised by electron exchange processes. The low energy peaks in unclean, partially clean, oxide coated or compound targets (NaCl, KCl) arise because the neutralisation of the slower ions is either not as efficient or is not possible. The secondary ion emission model of Blaise and Slodzian could account for the emission of ions from most targets.     

Electron states in α-quartz (SiO2)

Valence electron states of α-quartz SiO₂ are calculated self-consistently using the pseudopotential method. Excellent agreement is found with photoemission and UV-absorption data. X-ray emission spectra are calculated in an OPW scheme and compared to experiments. While the Si- and O - K spectra agree well with experiment, the Si L_2,3 spectrum shows substantial differences. An explanation is offered base on the formation of amorphous Si in SiO₂ during electron irradiation.     

The origin of narrowing of the Si 2p coincidence photoelectron spectroscopy main line of Si(1 0 0) surface

The Si 2p photoelectron spectroscopy (PES) main line of Si(1 0 0) surface measured in coincidence with the singles (noncoincidence) Si L_2,3-VV Auger-electron spectroscopy (AES) elastic peak is calculated. The agreement with the experiment is good. The present work is the first many-body calculation of the experimental coincidence PES spectrum of solid surface. The narrowing of the coincidence Si 2p PES main line compared to the singles one is due to the mechanism inherent in the coincidence PES. The inherent mechanism is explained by a many-body theory by which photoemission and Auger-electron emission are treated on the same footing.     

Formation of the buffer layer of silicon suboxides SiOx in the Si/SiO2 low-dimensional heterosystem after Si+ ion implantation: Si L2, 3 X-ray emission spectra

Nanosized heterostructures n-Si/SiO₂ with different thicknesses of the oxide film (20, 500 nm) after implantation by Si⁺ ions with energies of 12 and 150 keV have been investigated using Si L _{2, 3} X-ray emission spectroscopy (the Si 3d3s → Si 2p _{1/2, 3/2} electronic transition). The ion-beam modification of the interface has been revealed and studied for the heterostructure with a silicon dioxide thickness of 20 nm. An analysis of the Si L _{2, 3} X-ray emission spectra has demonstrated that the Si⁺ ion implantation leads to the self-ordering of the structure of the initially amorphous SiO₂ film 20 nm thick due to the effect of high doses. A mechanism of ion-beam modification of the insulator-semiconductor interface has been proposed. No substantial transformation of the atomic and electronic structures of the heterostructure with a silicon dioxide thickness of 500 nm has been revealed after the ion implantation.     

Influence of natural aging on photoluminescence from porous silicon

The influence of natural aging on the photoluminescence intensity and the position of a photolu-minescence peak in n-type por-Si (por-Si) is studied. The variation of the phase composition and the relative content of the amorphous and oxide phases of silicon in por-Si during aging is determined by fitting simulated spectra to experimental ultrasoft Si L _2,3 X-ray emission spectra using reference spectra.     

Oxygen chemisorption and initial oxidation of Cr(110)

The initial stages of the interaction of oxygen with a Cr(110) surface have been investigated at 300 K by LEED, AES, electron energy loss spectroscopy (ELS), secondary electron emission spectroscopy (SES) and work-function change measurement (Δφ). In the exposure region up to 2 L, the clean-surface ELS peaks due to interband transition weakened and then disappeared, while the ～5.8 and 10 eV loss peaks attributed to the O 2p → Cr 3d transitions appeared, accompanied with a work-function increase (Δφ = +0.19 eV at2L). In the region 2–6 L the work function decreased to below the original clean-surface value (Δφ_min = ?0.24 eV at6L), and five additional ELS peaks were observed at ～2, 4, 11, 20 and 32 eV: the 2 and 4 eV peaks are ascribed to the ligand-field d → d transitions of a Cr³⁺ ion, the 11 eV peak to the O 2p → Cr 4s transition, the 20 eV peak to the Cr 3d → 4p transition of a Cr³⁺ ion and the 32 eV peak probably to the Cr 3d → 4f transition. A new SES peak at 6.1 eV, being attributed to the final state for t he 11 eV ELS peak, was observed at above 3 L and identified as due to the unfilled Cr 4s state caused by charge transfer from Cr to oxygen sites in this region. In the region 6–15 L the work function increased again (Δφ_max = +0.32 eV at15 L), the 33 and 46 eV Auger peaks due to respectively the M_2,3(Cr)L_2,3(O)L_2,3(O) cross transition and the M_2,3VV transition of the oxide appeared and the 26 eV ELS peak due to the O 2s → Cr 4s transition was also observed. Above 10 L, the ELS spectra were found to be practically the same as that of Cr₂O₃. Finally, above 15 L, the work function decreased slowly (Δφ = +0.13 eV at40L). From these results, the oxygen interaction with a Cr(110) surface can be classified into four different stages: (1) dissociative chemisorption stage up to 2 L, (2) incorporation of O adatoms into the Cr selvedge between 2–6 L, (3) rapid oxidation between 6–15 L leading to the formation of thin Cr₂O₃ film, and (4) slow thickening of Cr₂O₃ above 15 L. The change in the Cr 3p excitation spectrum during oxidation was also investigated. The oxide growth can be interpreted on the basis of a modified coupled current approach of low-temperature oxidation of metals.     

High resolution Auger electron spectroscopy of metallic copper

Part of the LMM Auger spectrum from metallic copper has been studied in a high resolution X-ray photoelectron spectrometer. Fine structure not earlier reported has been observed. The main L₃M_4,5M_4,5 peak is very narrow, 1.0 eV, although the valence band is involved in the transition. The agreement between experimental and calculated Auger electron energies is very good. Since fine structure is found to be an intrinsic property in Auger spectra the interpretation of “satellite” peaks as due to electron—plasmon interactions should be used with care. The L₃M_4,5M_4,5 peak is very sensitive to the copper surface conditions. Surface oxygen affects the peak in a characteristic way.     

Oxidation of the porous silicon surface under the action of a pulsed ionic beam: XPS and XANES studies

The changes in the electronic structure and phase composition of porous silicon under action of pulsed ionic beams have been studied by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) using synchrotron radiation. The Si 2p and O 1s core photoemission spectra for different photoelectron collection angles, valence band photoemission spectra, and X-ray absorption near-edge fine structure spectrain the region of Si L _2,3 edges of the initial and irradiated samples have been analyzed. It has been found that, as a result of the irradiation, a thin oxide film consisting predominantly of higher oxide SiO₂ is formed on the porous silicon surface, which increases the energy gap of the silicon oxide. Such film exhibits passivation properties preventing the degradation of the composition and properties of porous silicon in contact with the environment.     

Silicon L2,3VV Auger lineshape and oxygen chemisorption study of Pd4Si

The SiL_2,3VV Auger Lineshape for Pd₄Si was measured and found to be in good agreement with the self-fold of the Si partial density of states model calculated by Riley et al. Oxygen chemisorption altered both the Auger lineshape and the HeI photoemission spectrum, especially near the Fermi energy.     

A solution of the core-exciton issue for silicon and germanium

The large discrepancies among the Si L_2,3 core-excitonic shifts measured by different techniques can be explained by the recently discovered surface shifts of the Si 2p level. New, accurate photoemission measurements of both the L_2,3 edge and of the 2p binding energy with equal surface sensitivity have been performed. Our present results and those of previous experiments are consistent with a single value 0.3^-00_+0.15 eV for the Si L_2,3 core excitonic shift. Preliminary results for the 3d core exciton in Ge give a shift of 0.35 ± 0.25 eV.     

Aes study of silicon bonding states during oxidation of Si(111)

During thermal oxidation of Si(111) the fine structure of the Si LVV spectrum is correlated with the ratio of SiO₂ to Si peak heights and with the attenuation of the Si peak. Criteria are given for recognizing the onset of silica formation. The width of the transition region, characterized by a precursor of silica, is evaluated as 6–7 Å. Above this thickness, SiO₂ appears to grow layer by layer. Comparison between low-pressure oxidation of clean Si and Auger sputter profiling of thick oxides suggests that the interface has a similar structure.     

Effects of substrate on surface morphology, crystallinity, and photoluminescence properties of sputtered ZnO nano films

Effects of substrate on crystallinity, surface morphology, and luminescence properties of radio frequency sputtered zinc oxide (ZnO) thin films were investigated. A variety of materials such as Si (100), Si (111), Al₂O₃, quartz, and silicon carbide (SiC) wafers were examined as substrates for deposition of ZnO thin films. The results showed smooth and uniform growth of c-axis orientation films. The thickness of the layers was about 50 nm. The average grain sizes of films were about 10, 13, and 12 nm for Si (111), quartz, and SiC samples, respectively. The deposited film on Al₂O₃ showed the largest grain size, about 500 nm. Grazing incidence x-ray diffraction patterns of the samples revealed that sputtered layers on Al₂O₃ and quartz had better crystallinity with higher peak at (002) orientation compared to Si and SiC substrates. Moreover, the Al₂O₃ sample exhibited a weak peak at position of (100) planes of ZnO too. The photoluminescence spectra of the samples showed a typical luminescence behavior with a broad UV band, including a main peak at around 388 nm and a weak shoulder peak at around 381 nm, corresponding with bound excitonic recombination and free excitonic recombination, respectively. The luminescence peak revealed that the intensity of UV emission is not necessarily dependent on the grain sizes and the micro-structural quality of ZnO films.     

Features of atomic and electronic structure of oxides on porous silicon surface according to XANES data

Based on synchrotron research of the fine structure main parameters of SiL _{2, 3} X-ray absorption edges (X-ray absorption near edge structure (XANES)) in porous silicon on boron-doped Si(100) wafers, the thickness of the surface oxide layer and the degree of distortions of the silicon-oxygen tetrahedron in this layer were estimated. The thickness of the oxide layer formed on the amorphous layer coating nanocrystals of porous silicon exceeds the thickness of the native oxide on the surface of Si(100) : P and Si(100) : B single-crystal (100) silicon wafers by several times. Distortion of the silicon-oxygen tetrahedron, i.e., the basic unit of silicon oxide, is accompanied by Si-O bond stretching and an increase in the angle between Si-O-Si bonds.     

Comparative AES and RHEED study of the formation of Pd silicide on clean and oxide covered Si(100) and (111) surfaces

The formation and epitaxial orientation of Pd silicide on clean and native oxide covered Si(100) and (111) surfaces was studied by Auger electron spectroscopy (AES) and reflection high energy electron diffraction (RHEED). Pd was vapor deposited in UHV on to the substrates up to thicknesses of about 6 nm. On clean Si substrates, ultra-thin Pd deposits reacted to form Pd₂Si already at room temperature, as detected by a characteristic splitting of the Si LVV Auger peak. However, a polycrystalline structure with very small crystallite sizes was indicated by diffuse ring patterns in RHEED. When the initial thickness of the Pd deposit exceeded about 3 nm, the diffraction ring pattern of unreacted metal developed. During annealing of room temperature deposits of Pd, the (100) and (111) substrates behaved differently. Larger crystallites formed on Si(100), but the films remained polycrystalline, though textured. On Si(111), virtually perfect epitaxial re-orientation of the silicide was found. When the substrates were initially covered with native oxide of about 2 nm thickness, silicide formation started at about 200°C, resulting in polycrystalline, but strongly textured Pd₂Si. Upon further annealing at temperatures up to 600°C, an additional phase of epitaxially oriented Pd₂Si developed on Si(111), similar to that on clean Si(100). In all experiments, extended annealing at temperatures above 250°C caused segregation of Si to the surface. This was accompanied by the development of an additional peak in the Auger electron spectra at about 313 eV, which we assign to a plasmon loss of δE = 17 eV in the Si overlayer, being excited by Pd Auger electrons of energy 330 eV.