Photoenhancement mechanism for oxygen chemisorption on GaAs(110) using visible light

Visible light from an argon ion laser (514.5 nm, 3 W/cm²) is seen to increase oxygen chemisorption on cleaved GaAs(110) surfaces up to a final coverage between one and two monolayers. Using photoemission spectroscopy to measure the oxygen coverage after simultaneous exposure of the surface to oxygen and light, we have determined that oxygen uptake for photoenhanced exposures is independent of sample temperature and doping type. In addition, significantly less enhancement is seen for weakly bound oxidizing molecules (N₂O) relative to the effects with molecular oxygen. These results are explained by a photoenhancement mechanism in which energy is released in a surface recombination event, possibly in the form of nonthermal phonons, causing physisorbed gas molecules to dissociate and thereby overcoming a major rate limiting step of the reaction in the dark. This reaction mechanism is supported by calculations of the surface recombination rates and free carrier densities at the surface which show that only the recombination rate is correlated with enhanced oxygen uptake. Other mechanisms and experimental data are also discussed.     

The process of oxygen chemisorption on the Si(111) surface

The chemisorption of oxygen on the Si(111) surface has been studied by the ASED-MO method. Three steps of the initial oxidation process have been proposed. The first step is molecular oxygen chemisorption. The second step is that of dissociated oxygen chemisorption in which the atomic short bridge site (between the first layer and second layer silicon atoms) can be occupied only after the saturation of the dangling bonds of the surface silicon with oxygen. The third step is the diffusion of atomic oxygen from the short bridge positions into the bulk of silicon to form an SiO₂ film. For molecular chemisorption, both the peroxy vertical and peroxy bridge models are possible although the peroxy vertical model is the more stable. The dissociated atomic oxygen can chemisorb for both the on-top and the short bridge models. Our results can explain, and are consistent with, most experimental results.     

Evolution of Photocurrent during Coadsorption of Cs and O on GaAs （100）

The photocurrent and spectral response characteristics of gallium arsenide （GaAs） are obtained by a multiinformation measurement system, and the evolution of the photocurrent versus the Cs：O flux ratio is investigated. The experimental results show that the photocurrent increases approximately exponentially after the first exposure to Cs until a maximum sensitivity is reached, the detailed evolution process and the ultimate photocurrent are different for different samples. These differences are analysed, and according to the process of coadsorption of Cs and oxygen on GaAs, an equation is presented to explain the increase of photocurrent.     

Evidence for two oxygen chemisorption sites on the Cr(100) surface at room temperature

The interaction of oxygen with the clean Cr (100) surface has been investigated by work function measurements, angle resolved photoemission spectroscopy and low energy electron diffraction. At low coverages (≈0.15 monolayer (ML)) the adsorbed oxygen is characterized by a work function decrease, a feature at 6.8 eV binding energy in UPS and a weak diffuse C(2 × 2) diffraction pattern. This form of oxygen labeled form a is quite stable at temperatures below 500°C. Upon further exposure at room temperature (⪆0.20 ML) a second form of oxygen labeled form b and characterized by a work function increase and peaks at 3.8 and 5.9 eV at normal photo-emission is identified. It is proposed that oxygen in forms a and b is chemisorbed in the fourfold hollow and on-top sites respectively.     

Generation and removal of adatom-induced electronic states on the Cs/GaAs(001) surface

JETP Letters - A nonmonotonic behavior of band bending φ S as a function of cesium coverage ? on the Cs/GaAs(001) surface is observed in the form of several maxima and minima. This...     

Generation and removal of adatom-induced electronic states on the Cs/GaAs(001) surface

A nonmonotonic behavior of band bending φ _S as a function of cesium coverage ? on the Cs/GaAs(001) surface is observed in the form of several maxima and minima. This behavior indicates the formation of the quasi-discrete spectrum of the adatom-induced electronic surface states. The hysteresis of the φ _S (?) dependence under adsorption and subsequent thermodesorption of cesium indicates the metastability of the Cs/GaAs(001) system.     

Molecular cluster theory of CO chemisorption on a nickel(100) surface

Self-consistent Hartree-Fock-Slater molecular cluster models for the chemisorption of carbon monoxide on a (100) transition metal surface are presented. Energy levels and charge distribution for the CO: Ni₅ cluster in C_4v symmetry are obtained, and the variation of binding energies with height of the CO molecule above the surface of nickel is studied in detail. Comparison is made with experimental binding energy spectra and with the multiple-scattering results of Batra and Bagus. The redistribution in energy of free-atom valence levels is studied by means of local-densities-of-states diagrams.     

Theoretical study of oxygen chemisorption on zinc surface by cluster models

Theoretical calculations by DV-Xα-MO have been performed for clusters representing a Zn(0001) surface with oxygen. The calculated DOS is in good agreement with the experimental UPS spectra found by Briggs or by Abbati et al. A large amount of electron flow from Zn atoms into the oxygen atom occurs and this causes (1) a remarkable upward shift of the oxygen 2p level and (2) formation of an unoccupied level near the highest occupied level. The bonds between the oxygen 2p and the zinc 4s or 4p orbitals have a strong bonding character, and the contribution of the zinc 3d orbital to the Zn-O bond is not important.     

Time-dependent chemisorption on a semiconductor surface

The microscopic aspects of interfacial growth are only crudely accessible to experiment. We have therefore developed a technique for computer-experimental studies of atoms and molecules deposited on solid surfaces, in which forces are determined from the electronic structure rather than atomic potentials. Here we report simulations of Cl, O, Se, N, As, Si, Ge, Al, Ga, Zn, Hg, Cu, and Au on the (1 1 0) surface of GaAs. The results exhibit non-trivial variety in both the dynamics and final bonding sites.     

Theoretical study of oxygen chemisorption on (111) and (100) silicon surfaces

The chemisorption of atomic oxygen on (111) and (100) silicon surfaces has been studied by the MNDO method using a cluster approach. The results show that, for both surfaces, chemisorption occurs preferentially on bridge positions, but chemisorption on top positions can play a significant role especially for the (111) surface.     

NEA光电阴极的(Cs,O)激活工艺研究

在自行研制的负电子亲和势光电阴极性能评估实验系统上,首次利用动态光谱响应技术和变角X射线光电子能谱(XPS)表面分析技术研究了GaAs光电阴极的(Cs,O)激活工艺.获得了首次导Cs、(Cs,O)导入以及(Cs,O)循环的优化激活条件.XPS分析给出GaAs(Cs,O)的最佳激活层厚度为0.82 nm,首次导Cs达到峰值光电发射时的Cs覆盖率为0.71个单层.在优化激活条件下,可以在国产反射式GaAs上获得1025 μA/lm的积分灵敏度.     

Chlorine chemisorption and surface chloride formation on Au(111)

The adsorption-desorption properties of the gold(111)-chlorine system have been investigated. Thermal desorption experiments following chlorine adsorption at 298 K indicated two desorption processes: the high temperature peak (ΔH = 217 kJmol^?1) showed desorption of equal numbers of molecukr and atomic chlorine species, while the lower temperature peak (ΔH = 140 kJmol^?1) was due to the desorption of Cl₂ only. Chlorine adsorption led to a maximum work function change of +0.5 V and electron-stimulated desorption proceeded with constant cross-section (1.4 × 10¹⁸ cm²), until all chlorine had been removed from the surface These observations are consistent with the immediate formation of a surface chloride (AuCl₃) during chlorine adsorption on Au(111) at 298 K without the intervention of an initial adsorbed overlayer.     

The chemisorption of nitrogen on the (001) surface of ruthenium

High resolution electron energy loss spectroscopy (EELS), thermal desorption mass spectrometry (TDMS) and low energy electron diffraction (LEED) have been used to investigate the molecular chemisorption of N₂ on Ru(001) at 75 K and 95 K. Adsorption at 95 K produces a single chemisorbed state, and, at saturation, a (√3x√3) R30° LEED pattern is observed. Adsorption at 75 K produces an additional chemisorbed state of lower binding energy, and the probability of adsorption increases by a factor of two from its zero coverage value when the second chemisorbed state begins to populate. EEL spectra recorded for all coverages at 75 K show only two dipolar modes — ν(RuN₂) at 280–300 cm^?1 and ν(NN) at 2200–2250 cm^?1 — indicating adsorption at on-top sites with the axis of the molecular standing perpendicular to the surface. The intensities of these loss features increase and ν(NN) decreases with increasing surface coverage of both chemisorbed states.