Spectroscopic optical second-harmonic generation from semiconductor interfaces

Optical Second-Harmonic Generation (SHG) from semiconductor interfaces has been widely studied, but it is only recently, with the advent of commercial, tuneable, pulsed laser sources that the spectroscopic aspect of SHG has begun to be more widely exploited. Here, results from porous Si and Si(100)-Sb are reported, which illustrate the potential of spectroscopic SHG as a probe of semiconductor interfaces.Paper presented at the 129th Heraeus Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Investigation of self-organizing thiol films by optical second harmonic generation and X-ray photoelectron spectroscopy

The adsorption of self-organizing thiol films on polycrystalline gold substrates was investigated in situ by optical second harmonic generation. Growth of the films could be detected at submonolayer coverage. For a comparison with classical surface analytical methods the thiol coverage was measured ex situ by X-ray photoelectron spectroscopy. We find that formation of ordered thiol films up to monolayer coverage can be described by Langmuir adsorption kinetics.     

Electronic properties of free and supported metal clusters studied by photoelectron spectroscopy

The electronic properties of free and supported metal clusters are studied by photoelectron spectroscopy. Experimental as well as theoretical results clearly demonstrate a dramatic dependence of the level structure on the cluster size. By this an interesting way might be opened to modify the electronic, optical and chemical properties of surfaces.     

Sum-frequency generation at electrochemical interfaces: Cyanide vibrations on Pt(111) and Pt(110)

We use optical sum-frequency generation to investigate the stretching vibrations of cyanide (CN^–) molecules chemisorbed from aqueous electrolytes on single-crystalline Pt(111)- and Pt(110)-electrode surfaces. For clean and well-ordered Pt(111) electrodes, a single vibrational band between 2080 and 2150 cm^–1 with a nonlinear frequency dependence on the potential is observed and assigned to the CN stretching vibration of chemisorbed cyanide. A second band between 2145 and 2150 cm^–1 with very weak potential dependence appears on a surface which was subjected to oxidation-reduction cycles and is attributed to cyanide associated with a microscopically disordered surface. This assignment is supported by preliminary results for a Pt(110) single-crystal electrode. On a well-ordered (110) surface a single and potential-dependent cyanide vibration between 2070 and 2112 cm^–1 is observed. After oxidation of the cyanide and readsorption, this band is replaced by a higher frequency band at 2144 cm^–1 which is essentially not potential-dependent. Occasionally, additional vibrational bands at lower frequencies not reported in corresponding IR studies are observed on Pt(111).Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Model of laser-induced chemical etching of silicon in chlorine atmosphere

A mathematical model for the chemical etching of silicon in a chlorine atmosphere induced by laser irradiation is described. The model takes into account: dissociation of molecules having absorbed radiant energy into chlorine atoms and their diffusion onto the substrate surface, generation of photocarriers in the silicon substrate, kinetics of chlorine atom chemisorption on the silicon surface, chemical reaction of chemisorbed chlorine atoms with silicon atoms, and desorption of reaction products. The obtained results are compared with experimental data.     

Interaction of atomic hydrogen with the graphite single-crystal surface

We have studied the chemisorption of atomic hydrogen on the basal plane of natural graphite single crystals. LEED and angle-resolved photoemission were used to characterize the clean surface. The adsorption of H saturates at rather low exposures, accompanied by a decrease of the work function by =(100±20) meV. The photoemission spectra indicate a clear carbon-hydrogen interaction, leading to shifts of substrate bands by up to about 200 meV. No detectable etching of the surface occurs at room temperature, in agreement with earlier work. Our results are qualitatively consistent with theoretical considerations about a strong H(1s)-C(2p _z) chemical bond.     

The application of photoemission microscopy to the study of heterogeneous reactions over metal single crystal surfaces

The development of UHV instrumentation for photoemission electron microscopy (PEEM) has opened up the possibility of investigating in situ the morphology of metal single crystal surfaces during simple catalytical reactions in the pressure range up to 10^–4 mbar. We have studied the Pt(100) surface during the transition from the high to low rate branches of the CO oxidation reaction. Steps, step bunches and grain boundaries have been identified. They play an important role in the decomposition of CO and the deposition of carbon and probably regulate the reactivity of the Pt surface.     

ESCA studies of Ni-Cr alloys

ESCA examination on Ni-Cr alloys has shown that a thin passive film was formed after 24 h immersion in 0.1 M NaCl. The film contained only chromium oxide in the form of Cr₂O₃. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate of the alloys. Both Ni-10 wt. % Cr and Ni-20 wt. % Cr alloys showed a slightly higher corrosion rate than the Ni-40 wt. % Cr alloy.The present ESCA study of the Ni-Cr system is part of our programme which involves an examination of the four binary alloy systems Fe-Si, Cr-Co, Ni-Cr, and Mo-Ni [1]. The aim is to correlate the structure and composition of the passive films formed in 0.1 M NaCl to the corrosion behaviour in the same solution.     

Elementary processes at semiconductor/electrolyte interfaces: perspectives and limits of electron spectroscopy

Semiconductor device properties based on electrolyte contacts or modified by electrochemical reactions are dominated by the electronic structure of the interface. Electron spectroscopy as e.g. photoemission is the most appropriate surface science techniques to investigate elementary processes at semiconductor/electrolyte interfaces. For such investigations a specific experimental set-up (SoLiAS) has been built-up which allows performing model experiments as well as surface analysis after emersion under different experimental conditions. The experimental approach is presented by a number of experiments performed during the last years with GaAs as substrate material. Model experiments by adsorption and coadsorption of electrolyte species give information on fundamental aspects of semiconductor/electrolyte interactions. Emersion experiments give information on a final composition and the related electronic structure of electrodes after electrochemical reactions. The use of frozen electrolytes will help to bridge the gap between these two approaches. With the combination of the experimental procedures one may expect a detailed analysis of electrolyte (modified) interfaces covering chemical composition, electronic structure of surfaces/interfaces as well as surface/interface potentials.     

ESCA studies of Fe-Ti alloys

Fe-Ti alloys containing 5 to 47% Ti have been studied by ESCA. The alloys were exposed to 0.1 M NaCl for 24 h under open-circuit potential (OCP) during which passive films were formed. The passive film consisted of FeO and TiO₂ in the inner layer while Fe₂O₃, water, and hydroxyl groups were present in the outermost monolayers, irrespective of composition. The thickness of the passive layer was reduced from 4.4 nm to 1.0 nm with increasing Ti content. The amount of iron oxide in the passive layer also decreased with increasing Ti. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate of the alloys. Alloys with 5–28% Ti showed a relatively high corrosion rate but that with 47 wt.% Ti had a much lower corrosion rates.     

Chemisorption of halogens on silver a critical comparison of results from photqemission and thermal desorption spectroscopy

Conflicting thermal desorption (TDS) results from submonolayer coverages of atomic Cl, Br or I on Ag(100), Ag(110) or Ag(111) stimulated us to check for the possible influence of different sample preparation procedures. We present experimental evidence, that a combined approach using TDS and angle-resolved ultraviolet photoemission may help to distinguish between better and worse surface preparation. We present detailed new results for the Cl/Ag(110) system. From our observations we conclude that all three halogens listed above give one and only one TDS peak in the submonolayer regime and show very similar adsorption/desorption-characteristics. This fact removes the apparent contradictions reported in the literature and demonstrates that the halogen-Ag bond is not very sensitive to the surface crystallography.     

Interaction of atomic hydrogen with the graphite single-crystal surface

The reaction of atomic hydrogen (or atomic deuterium) with highly orientated pyrolytic graphite surfaces has been studied by means of thermal desorption spectroscopy. In some cases atomic deuterium instead of atomic hydrogen, was used solely to assign the desorbed masses unambiguously to the different hydrocarbons. The desorption of D₂ and fourteen hydrocarbons was observed. D₂ desorbed at higher temperatures than the CH-(CD) compounds, the desorption spectra of the hydrocarbons contained two peaks. The dependence of the desorption spectra of several hydrocarbons on the heating rate, the atomic hydrogen exposure and the composition of the desorption products was investigated in detail. The kinetic parameters of the desorption process were determined for CH, C₂H₂, and CD₄. The spectra showed that there must be a first order desorption process for all the hydrocarbons, the values for the activation energy and the frequency factor were the same within experimental errors. The results were discussed by means of a simple model.     

Oxidation and thermal annealing effects on native and ion-irradiated PbTe films grown by molecular beam deposition

Investigations are reported into the effect of low-pressure oxygen exposure and thermal annealing on the carrier transport properties of native and 350 eV Ar⁺ bombarded PbTe films. The electrical measurements were madein situ on MBD-grown PbTe films without breaking vacuum. On native surfaces, oxidation was initially sustained by diffusion of a donor species from the film bulk to the surface, where reaction with oxygen occured. This diffusion process was apparently inhibited on ion irradiated films and direct doping of the film surface effected a gradual reduction in the ion-induced electron accumulation resident at the film surface. The native properties and behavioural characteristics of the films could be recovered by thermal annealing of the ion-irradiated and/or oxidized films at 300–350 °C.     

Studies of buried interfaces by optical second-harmonic generation

Optical Second-Harmonic Generation (SHG) has been used to study the technologically very important buried semiconductor-metal and magnetic multilayer interfaces. For the case of GaAs-Au, the SHG intensity is shown to depend on the applied bias and Schottky barrier height, and is strongly affected by the sweepout of the carriers generated by the fs excitation pulses. For the M/Co/M multilayers, with M=Cu or Au, the SHG signals appear to depend strongly on the magnetization and can be shown to be interface specific.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Phase shifts in a Nd3+ fibre amplifier

Phase shifts due to optical pumping have been measured in a Nd³⁺-doped fibre amplifier. The measurements were made with a two-fibre Mach-Zehnder interferometer. It is shown that phase shifts are mainly due to fibre heating. The results are compared with heat transport theory.     

Quantum-chemistry simulations of second-harmonic and sum-frequency generation of organic layers

By using quantum-chemistry approaches, the second-order nonlinear optical responses of molecules and two-dimensional molecular arrays containing the p-nitroaniline chromophore are evaluated in order to highlight key features of the simulation of the second-harmonic and sum-frequency generation spectra of organic layers. For the electronic component, which dominates the second-harmonic generation response and which constitutes the weakly frequency-dependent background contribution to the vibrational sum-frequency generation phenomenon, the time-dependent Hartree–Fock scheme based on the semi-empirical AM1 parameterization is suitable for predicting the microscopic responses as well as for accounting for the surrounding effects within a simple multiplicative scheme. For the vibrational resonant part of the sum-frequency generation response, ab initio density functional theory approaches turn out to be necessary for locating the resonances and estimating their intensities. Received: 16 October 2001 / Published online: 29 May 2002     

Field-emission flicker noise from potassium adsorbed on W(111) bounded and unbounded surface diffusion by spectral analysis

The spectral density functionsW(f) of the field-emission flicker noise of potassium adsorbed on the tungsten (111) region were determined. The analysis in terms of the Gesley-Swanson or the Timm and Van der Ziel model gives surface diffusion energies between 0.54 and 0.31 eV for (average) coverages from 0.15 to 0.38 and diffusion prefactors between 3×10^–1 and 10^–4 cm²/s. The fitting procedures are successful for both bounded and unbounded diffusion though in opposite order of temperature, as expected.On leave from Wrocaw University