Applications of Surface Science最新文献

A new method is developed to determine the tunneling transmission probability through a thin oxide layer by making use of the switching characteristic of a metal-tunnel oxide-semiconductor thyristor. It is found that the tunneling transmission probability is very sensitive to oxide thickness and is dependent on the voltage drop across the oxide layer.

A modified DLTS technique is proposed for the direct measurement of capture cross-section of MOS surface states. The nature of temperature and energy dependence σ_n is inferred from data analysis. Temperature dependence of σ_n is shown to be consistent with the observed DLTS line shapes.

The technique of surface acoustic wave (SAW) has been applied to doped amorphous GeH films to gain information on the nature of defects. The attenuation of SAW of 300 MHz and 1.5 GHz has been measured in sputtered films of a-GeH doped with P or B at temperatures between 0.5 and 475 K. Sound velocity measurements are also made. A strong absorption maximum at low temperature is observed in a-Ge(H,P) and a-Ge(H,B) films. Distinct effects of annealing on the attenuation and velocity of SAW are observed. These impurity induced features are discussed in the light of acoustoelectric interaction. It is interesting to observe that a-Ge(H,B) film seems to have tunneling states characteristic of glasses.

The adsorption and reactions of methanol, methyl formate and formaldehyde on clean and oxygen-covered copper (110) surfaces has been studied with EELS, UPS and temperature programmed desorption (TPD). We report spectroscopic data for (a) the condensed multilayers at 90 K, (b) the surface monolayers and (c) the reaction intermediates formate (HCOO) and methoxy (CH₃O) generated by reaction with surface atomic oxygen. On the clean surface methanol and methyl formate bond via the oxygen and carbonyl lone pair orbitals, whereas formaldehyde always polymerizes to surface paraformaldehyde above 120 K. Atomic oxygen generates methoxy by reaction with methanol and formate by reaction with formaldehyde as reported previously. Methyl formate undergoes nucleophilic attack to form both methoxy and formate in the presence of atomic oxygen, in a similar fashion to the reaction on Ag (110). Whereas the oxidation reactions of these molecules on copper catalysts can be explained with reference to the observed intermediates, no evidence is found for the dimerization of formaldehyde or the dehydrogenation of methanol to form methyl formate: reactions which are quite facile under higher pressure conditions.

CdTe thin films formed by compound evaporation onto glass substrates, held at between 40 and 460°C, exhibited abrupt changes of physical properties at ca. 270°C. The resistivity changed from 0.1 ω m to 100 kω m, and a marked increase in crystallinity and absorption edge sharpness was noted. These changes resulted from a decrease in the free tellurium content of the films at ca. 270°C.

The chemical deposition method has been employed in the preparation of alloy semiconductor thin films. Thin films of Cd_1−xHg_xS and Cd_1−xPb_xS alloys were deposited onto titanium substrates from thiourea solutions. A monotonic decrease in the bandgaps of the semiconductor alloys was obtained as the Hg and Pb ratio was increased. Bandgap values of 1.8 and 1.6eV were measured for electrodes with a Hg: Cd ratio and a Pb: Cd ratio of 0.18 respectively, considerably lower than the bandgap of pure CdS.

LIMA (Laser-induced Ion Mass Analysis) is a new technique capable of compositional analysis of thin films and surface regions. Under UHV conditions a focused laser beam evaporates and ionizes a microvolume of specimen material from which a mass spectrum is obtained. LIMA has been used to examine a range of thin film materials with applications in electronic devices. The neutral photon probe avoids charging problems, and low conductivity materials are examined without prior metallization. Analyses of insulating silicon oxides, nitrides, and oxynitrides confirm estimates of composition from infrared measurements. However, the hydrogen content of hydrogenated amorphous silicon (a-Si:H) found by LIMA shows no correlation with values given by infrared absorption analysis. Explanations are proposed and discussed.

Peculiarities of the photoelectron yield in conditions of X-ray grazing-incidence diffraction are discussed. The high sensitivity of photoelectron emission angular dependence on structural perfection of the crystal subsurface layers is demonstrated. A method for evaluation of the escape depths of photoelectrons with different energies is proposed.

Self-consistent Hartree-Fock-Slater molecular cluster calculations for the chemisorption of carbon monoxide on a Rh(110) surface both at on-top and at on-fourfold “pocket” sites are presented. The calculations are performed using Rh₅CO clusters, with the carbon-oxygen distance equal to the free molecular value. The bond energy on the on-top site is found to be about two times larger than on the on-fourfold site and the optimized d_Rh-C is equal to 1.80 and 1.85 Å for the on-top and the on-fourfold sites respectively. In the total DOS and difference curve for the on-top site, the 4g̃s, 1g̃p + 5g̃s, and CO 2π-induced peaks are located 10.2, 7.2, and 3.0 eV below E_F, in good agreement with the UPS results. The CO 2π-induced peak is stronger for the on-fourfold site, and the CO 2π orbital will gain more electrons (0.83 compared with 0.55 for the on-top site) if in some way CO is chemisorbed on this site.

Gold/silicon binary mixtures are prepared by depositing thin gold films onto Si(111) single crystals and subsequently annealing at various temperatures. The structure of the films is studied in situ with the help of an ultra-high-vacuum X-ray diffraction chamber. After deposition at room temperature the gold films are homogeneous with a plane-parallel morphology. After annealing at 630 K the Au 111 peak suddenly disappears in the X-ray spectrum indicating that a eutectic melt has been formed. Cooling leads to a regeneration of the peak at 590 K. This temperature is distinctly smaller than the eutectic temperature reported in the literature (643 K). Obviously small gold islands are formed which are embedded into the silicon surface.