Auger electron spectroscopy of silicon surfaces

The measurements of the spectra of Auger electrons of the silicon surfaces performed at the pressure of (2–5)×10^?7 Torr are described. In this pressure range rapid oxidation and carbonization of the uppermost layers take place. The changes of characteristic energies in Auger and loss spectra are related to the change of chemical composition of the surface. The combination of the characteristic loss spectroscopy with Auger electron spectroscopy makes possible the determination of the chemical shifts. The measurements of the chemical shifts of the individual energy levels of the silicon atoms in both the pure and contaminated silicon surfaces, in quartz and Fe∶Si alloy are given. Finally, the possibilities and limitations of the heating for the silicon surface cleaning are examined.     

Mechanisms for oxygen adsorption on the Si(110) surface studied by Auger electron spectroscopy

Oxygen adsorption on the Si(110) surface has been studied by Auger electron spectroscopy. For a clean annealed surface chemisorption occurs, with an initial sticking probability of ～6 × 10^?3. In this case the oxygen o_kll signal saturates and no formation of SiO₂ can be detected from an analysis of the Si L_2,3VV lineshape. With electron impact on the surface during oxygen exposure much larger quantities are adsorbed with the formation of an SiO₂ surface layer. This increased reactivity towards oxygen is due to either a direct effect of the electron beam or to a combined action of the beam with residual CO during oxygen inlet, which creates reactive carbon centers on the surface. Thus in the presence of an electron beam on the surface separate exosures to CO showed adsorption of C and O. For this surface subsequent exposure in the absence of the electron beam resulted in additional oxygen adsorption and formation of SiO₂. No adsorption of CO could be detected without electron impact. The changes in surface chemistry with adsorption are detectable from the Si L_2,3VV Auger spectrum. Assignments can be made of two main features in the spectra, relating to surface and bulk contributions to the density of states in the valence band.     

KrF-excimer-laser-induced native oxide removal from Si (100) surfaces studied by Auger electron spectroscopy

The mechanism of KrF-excimer-laser cleaning of Si(100) surfaces was studied by Auger Electron Spectroscopy (AES) and Low-Energy Electron Diffraction (LEED) spectroscopy. The dependence of the cleaning efficiency on the laser fluence was investigated by using a mildly focused laser beam and carefully measuring the energy density distribution of the laser spot impinging on the sample. These values were compared with the AES spectra measured in different points of the irradiated area and with the morphology observed by optical microscopy. Samples as received from the manufacturer were first investigated. It was found that desorption of weakly bonded organic adsorbates occurs at energy densities as low as 0.3 J/cm², whereas significant oxide removal takes place only at an energy density above 0.8 J/cm², which produces damaged surface morphologies. The experimental findings, in agreement with the temperatures calculated for the laser-induced Si heating, indicated that a large fraction of the oxide film is dissolved in the molten silicon, leading to oxygen concentration below the AES detection limit only when the melted depth was of the order of several hundred nanometers. Atomically clean, damage-free Si(100) surfaces were obtained after irradiation of samples pre-etched for 1 min in a HF: H₂O (5%) solution, which had only a thin SiO _x (x < 2) layer and F, C and O containing adsorbed species. Complete contaminant elimination was achieved in this case with 15 pulses at 0.8 J/cm² without any damaging of the surface.     

Effect of ion bombardment at low energy on (100)InP surfaces,studied by Auger electron spectroscopy

Surface cleaning of (100)InP substrates with an Ar⁺ ion beam of 250–400 eV is analysed by AES and shown as a function of time. The results obtained show the possibility of removing the contamination layer without any significant chemical damage to the InP surface.     

The adsorption of CO on Cu surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) in the energy range of electronic transitions (primary energy 30 < E₀ < 50 eV, resolution ΔE ≈ 0.3 eV) has been used to study the adsorption of CO on polycrystalline surfaces and on the low index faces (100), (110), (111) of Cu at 80 K. Also LEED patterns were investigated and thermal desorption was analyzed by means of the temperature dependence of three losses near 9, 12 and 14 eV characteristic for adsorbed CO. The 12 and 14 eV losses occur on all Cu surfaces in the whole coverage range; they are interpreted in terms of intramolecular transitions of the CO. The 9 eV loss is sensitive to the crystallographic type of Cu surface and to the coverage with CO. The interpretation in terms of d(Cu) → 2π¹(CO) charge transfer transitions allows conclusions concerning the adsorption site geometry. The ELS results are consistent with information obtained from LEED. On the (100) surface CO adsorption enhances the intensity of a bulk electronic transition near 4 eV at E₀ < 50 eV. This effect is interpreted within the framework of dielectric theory for surface scattering on the basis of the Cu electron energy band scheme.     

Ethylene and acetylene adsorption on Cu(111) and Pt(111) studied by Auger spectroscopy

High resolution, electron impact excited, carbon Auger spectra of ethylene and acetylene adsorbed on Cu(111) and Pt(111) are compared. The spectra of ethylene on the two metals provide the first example of the sensitivity of AES to the nature of metal-adsorbate bonding for molecular adsorbates. The acetylene spectra are identical on the two metals. The changes in the carbon Auger spectra resulting from thermal decomposition of the two adsorbates on Pt(111) are discussed in the context of results from electron energy loss spectroscopy.     

Studies of acetylene and oxygen adsorption on silicon surfaces by low energy electron loss spectroscopy

Acetylene and oxygen adsorption on disordered and ordered silicon crystal surfaces of (111) orientation were studied by low-energy electron loss spectroscopy (ELS). There are marked structure-dependent differences in the ELS spectra in the presence of these adsorbates. A model for the acetylene molecule orientation on the silicon (111) ordered surface is proposed to explain the experimental results. ELS data suggest that the electrons in the silicon-oxygen chemisorption bond are more tightly bound on the disordered than on the silicon (111) surface with a (7 X 7) surface structure.     

The effect of adsorbed atomic hydrogen on the growth of ultrathin silicon films on Ge(100) studied by positron-annihilation-induced Auger electron spectroscopy (PAES)

The effects of adsorbed atomic hydrogen on the stability of silicon films grown on a Ge(100) substrate were studied by using positron-annihilation-induced Auger electron spectroscopy (PAES) and electron-induced Auger electron spectroscopy (EAES). PAES is almost exclusively sensitive to the topmost atomic layer due to the trapping of positrons in an image potential well just outside the surface before annihilation. This surface specificity was exploited in the study of film stability and interfacial mixing during the growth of silicon on Ge(100). The PAES results show that the prior adsorption of hydrogen prevented the segregation of germanium on top of the deposited silicon, and that the hydrogen adsorption was useful in growing a thermally stable structure.     

Auger electron spectroscopy of a hydrogenated amorphous silicon

Thin layers of a hydrogenated amorphous silicon were studied by means of the Auger electron spectroscopy (AES). It was found that the spectra of the a-Si : H samples exhibit a large peak at 34 eV which was ascribed to the L₁L₂₃V Coster-Kronig transition and that the intensity of the L₂₃VV transition was lowered, due to hydrogenation. The explanation of this feature is given on the basis of the electronic structure and the transition probabilities changes in silicon, due to hydrogenation. The results on the a-Si : H layer were compared with measurement of the a-Si layer and the influence of an electron and an ion bombardment, an elevated temperature and an exposure to oxygen on both layers was studied.The author would like to expres hiss thanks to Dr. J. Zemek for supplying the a-Si and a-Si : H layers, to Dr. J. Drahokoupil and Dr. J. imnek for stimulating discussions and to Dr. V. Cháb for helpful discussions and for his help with measurements.     

Auger electron spectroscopy and electron energy loss spectroscopy studies on carbonization of Si(100) and (111) surfaces with ethylene

The reactions of Si(100) and Si(111) surfaces at 700 °C (973 K) with ethylene (C₂H₄) at a pressure of 1.3×10⁻⁴ Pa for various periods of time were studied by using Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS). For a C₂H₄ exposure level, the amount of C on the (111) surface was larger than that on the (100) surface. The formation of β-SiC grain was deduced by comparing the C_KLL spectra from the sample subjected to various C₂H₄ exposure levels, and from β-SiC crystal.     

The adsorption of fluor-carbon complexes on GaAs(110) studied by electron energy loss spectroscopy

The room temperature adsorption of CF₃COOH, CH₃COOH and CO on cleaved GaAs(110) surfaces has been studied by vibrational electron energy loss spectroscopy (HRELS), second derivative electron energy spectroscopy (ELS) and electron diffraction (LEED). CO does not adsorb on the GaAs surfaces in measurable quantities. Acetic acid CH₃COOH is dissociatively adsorbed as an acetate bonded to Ga surface atoms with the split-off hydrogen on As surface atoms. The fluorated acid CF₃COOH decomposes via an acetate intermediate CF₃COO into active CF₃ groups which adsorb on Ga surface atoms. The split-off hydrogen sticks to surface As atoms while the generated CO₂ desorbs. The adsorption models are consistent with the LEED c(2×2) superstructure observed after saturated adsorption of both acids.     

Auger electron spectroscopy and leed studies of adsorption isotherms: Xenon on (0001) graphite

The passivation mechanism and related natures of silicon surfaces with a very thin natural oxide film baked at low temperature and low pressure (≤ 600°C, 1–5 × 10^?6Torr) was studied principally by the measurement of ESR absorption. Two sorts of resonance lines, which are called the broad and the narrow line hereafter, were observed in the dark by vacuum baking after introducing air. Paramagnetic centers responsible for the broad line have a one-to-one correspondence to such surface states at the silicon-silicon oxide interface that have an electrically amphoteric nature. The narrow line with an intense g-anisotropy originated from trivalent silicons. These ESR lines interact very sensitively with atmospheric gases such as water and oxygen. In addition, light illumination induced two ESR lines different from those observed in the dark. The electron trapped at the surface state forms an intrinsic layer at the surface of n-type silicon. It has been confirmed by present ESR experiments and surface conductance measurements that the passivation effects of this surface to various atmospheric ambients such as water vapor results from the existence of an intrinsic layer at the surface of n-type silicon.     

Surface segregation in Pt-Au alloys studied using Auger electron spectroscopy

Temperature dependent surface segregation studies using Auger electron spectroscopy have been performed on three different Pt-Au alloys, containing 2, 5 and 90 wt% Au. By utilizing Auger transitions of different kinetic energies and model segregation profiles, an estimate of the in-depth variation in composition was made. Strong surface segregation of Au was observed in the three alloys.     

Scandium and lutetium surfaces studied by reflection electron energy-loss spectroscopy

The reflection electron energy-loss spectra of the (1 0 0) and (0 0 1) surfaces of Sc single crystals and the (0 0 1) surface of a Lu single crystal have been studied with primary energies in the range 50–2000 eV. Scandium is congeneric with lutetium and the loss spectra of the two elements are very similar in both the collective excitations and the interband transitions. Strong excitations observed at around 41 eV are attributed to 3p → 3d and 5p → 5d transitions in Sc and Lu, respectively. The loss data of Sc fit the characteristic energy-loss data of the other elements of the first group of transition metals. Oxygen adsorption and nitrogen adsorption on the (1 0 0) surface of Sc influence the loss spectra. The observed differences are correlated with density-of-states calculations for Sc, ScO and ScN.     

Oxygen exposure of Sm,Gd and Tb studied by Auger electron spectroscopy

The present paper treats the characteristics of the Auger transitions out of the valence band of the lanthanides Sm, Gd and Tb engaging in a chemical compound. These facts have been observed by AES on the basis of a slow oxidation process for these elements. The appearing Auger transitions have been identified as far as possible and compared with studies of density of states. The changes in the Auger spectra and the oxidation rate have been discussed.     

Silicon diffusion through thin tungsten films on silicon,studied with Auger spectroscopy

Silicon out-diffusion through ? 3000 Å tungsten films deposited on silicon by r.f. sputtering was studied using Auger spectroscopy. Silicon first diffuses to the tungsten film surface by grain boundary diffusion and surface migration. The out-diffusion kinetics were most strongly dependent on the thickness of the silicon dioxide layer between tungsten and silicon, and this (native) oxide thickness varied with substrate doping. The out-diffusion rate was independent of tungsten film thickness at 540 Å and 2400 Å. For substrates from which the native oxide was removed by backsputtering just prior to tungsten deposition, no Si out-diffusion to the W film surface was observed until almost the entire film had converted to WSi².     

Auger electron spectroscopy - a local probe for solid surfaces

The Auger electron transition in solids is discussed under the aspect of a local excitation due to the strongly localized primary hole in an inner atomic core level. In first approximation the solid is represented by a cluster model, consisting of the excited atom and its neighbors. Using this simple model it is possible to describe the Auger electron energies, intensities and line shapes of transitions in solids in a satisfactory way. Only for the angular dependent Auger emission, characteristic long-range crystalline order has to be taken into account. It is the aim of this introductory review to point out that Auger spectra bear more information about the solid surface and particularly on its chemical bonds as has yet been exploited by surface spectroscopists.     

Electron beam interactions with CO on W {100} studied by Auger electron spectroscopy

The interaction of 2500 eV electrons with carbon monoxide chemisorbed on tungsten {100} was investigated by rapid-scan Auger electron spectroscopy. When no α state was present the O and C signals from the β state of CO were invariant during electron bombardment, giving an upper limit estimate for the electron stimulated desorption cross section, Q_β of 2 × 10^?21 cm². With the crystal at room temperature and saturated with CO, however, electron-beam induced accumulation of carbon was observed and characterised, the rate of the process being independent of CO pressure at pressures above 2 × 10^?8 Torr. At 450 K the rate was found to be pressure dependent up to at least 6 × 10^?7 Torr. A model is proposed for the accumulation process, which is based on electron beam dissociation of α₂-CO to form adsorbed carbon and gaseous O and the creation of new sites for further α₂-CO adsorption; it is in quantitative agreement with the results and yields a cross section for ESD of α₂-CO (Q_α2 = 1.55×10^?18cm²) in close agreement with direct measurements.