S. Roy Morrison

Anodic properties of n-Si and n-Ge electrodes in HF solution under illumination and in the dark

Abstract In HF solution, the n-Si electrode is found to show unique properties under illumination and in the dark. It shows a quantum efficiency of the photocurrent as high as 400% (current quadrupling effect) when the light intensity is weak, i.e. less than 5×10−3 mW cm−2 in 1 M (M=mol dm−3) HF at pH 4.9. In the dark, an n-Si electrode that has been covered with an oxide layer shows a peak of anodic current a short time after it is immersed in a HF solution. Both these phenomena are attributable to the dissolution of the oxide layer by the HF solution and to electron injection from transient species, special forms of partially oxidized silicon at the Si/HF solution interface. The properties of an n-Ge electrode are also studied and compared with those of an n-Si electrode.

Bulk and Surface Characterization of the Silicon Electrode

Abstract The properties of silicon as an electrode are investigated. Techniques for reproducible measurement of the doping level are described, techniques designed to avoid surface films associated with fluoride ions. A peak in the capacity/voltage curve that appears near the flatband voltage for both n- and p-type silicon is characterized in detail and shown to be associated with interface states between a surface oxide layer and the silicon. The possible chemical origin of the interface states when the electrode is in solution is discussed.

Measurement of surface state energy levels of one-equivalent absorbates on ZnO

Abstract Surface state energy levels associated with selected one-equivalent chemical redox couples deposited on zinc oxide are determined by two experimental methods. The results are compared with an earlier quasi-theoretical estimation based on the chemical redox potential. The experimental methods include, (1) a previously reported method involving a study of the electron injection rate from such surface states as monitored by a surface potential measurement, and (2) a new method based on control of the intergranular contact resistance in a pressed ZnO pellet by nonvolatile surface states. It is found that all three methods of surface state energy determination lead to the same energy level for ions with known and stable ligand shells, such as iron cyanide (energy ~0.1 eV below the conduction band) and iridium hexachloride (energy ~ 0.7 eV below the conduction band). For ions where a stronger adsorbate/surface interaction can be expected, the methods lead to differing energy values, which can be reconciled in terms of such an interaction. The chromium, manganese, and oxygen species (O 2 − ) are in this category. For these species the energy level of the surface state when occupied (surface potential measurement) is found to be about 1.1, 0.8, and 0.9 eV below the conduction band, respectively. The activation energy for electron injection from O − is about 0.4 eV.

Electron capture by ions at the ZnO/solution interface

Abstract The electron reactivity (electron capture rate) of one-equivalent oxidizing agents at the ZnO surface is determined by cathodic electrochemical reduction. Because in this system the Helmholtz potential is insensitive to the oxidizing agent used, the energy level of a particular species with respect to the ZnO conduction band is not considered a variable Thus the electron capture by (reduction of) the species can be interpreted according to surface state capture theory. The energy levels are estimated by analysis of the redox potential of the species. A maximum in electron reactivity is found for energy levels just below the conduction band minimum, the reactivity decreasing rapidly for higher energy levels. There is some indication that the electron reactivity decreases for very low energy levels, in accordance with expectations if the process is controlled by a multi-phonon electron capture model.

Photoanodic properties of an n-type silicon electrode in aqueous solutions containing fluorides

Abstract The n-Si electrode shows two distinct characteristic, photoanodic corrosion properties in solutions with fluoride ion or hydrofluoric acid together with a reducing agent. In the presence of a low concentration of fluoride ion or hydrofluoric acid, the photocurrent is observed to decay more rapidly than in the absence of the fluorides. In the presence of a relatively high concentration of hydrofluoric acid such that the oxide dissolves rapidly, current doubling occurs, suggesting that the photoproduced holes all react to corrode the silicon rather than oxidizing the reducing agent. Models to explain the two-way influence of the fluoride ion in the photocorrosion of silicon are presented.

Ultraviolet bleaching and regeneration of ⋅Si≡Si3 centers at the Si/SiO2 interface of thinly oxidized silicon wafers

The ⋅Si≡Si3 defect revealed by electron spin resonance (ESR) at the Si/SiO2 interface has been found to be very sensitive to ultraviolet (UV) radiation at 2537 A. With native oxides or very thin thermal oxides, UV bleached the signal, which then recovered slowly in room temperature air, or rapidly upon water immersion. Details of the bleaching suggest outer‐oxide surface trapping by adsorbed oxygen, rather than bulk oxide traps. In HF‐stripped silicon, an opposite uv effect was observed, i.e., an accelerated growth of ⋅Si≡Si3 centers. The physico‐chemical responses of ⋅Si≡Si3 reflected several stages in the development of nascent oxides on silicon wafers. The uv trapping phenomenon further offers a potential method for determination of the energy levels of the ⋅Si≡Si3 defect, and thus for clarification of its role in interface trapping.

Electron exchange processes in olefin oxidation

Abstract A new experimental technique is described for measurement of the relative Fermi energy of various catalysts used for partial oxidation of olefins and alcohols. The technique is based on a measurement of the conductance of a semiconducting support (TiO 2 ). For a number of catalyst compositions of industrial importance (in terms of selectivity and activity) the bulk Fermi energy is found to have a common value. This recurring value is near the electrochemical potential of electrons on adsorbed oxygen. Catalyst systems studied were the bismuth/molybdenum, copper oxide, vanadium oxide and iron/molybdenum systems. An attempt is made to separate the macroscopic requirements (where the Fermi energy is considered a macroscopic parameter) from the microscopic ones (where local bonding orbitals and acid centers may be considered microscopic parameters). We conclude that for reactions for which the rate-limiting step depends on electron transfer to the catalyst that the Fermi energy should be near or just above the electron exchange level for oxygen. Also the bulk Fermi energy should be stable against small variations in reactant gas compositions. Impurity band pinning of the Fermi energy may account for the excellent stability of bismuth molybdate.

Surface states associated with acid sites on solids

Abstract It is shown that present models of Lewis acidity or basicity and of surface states on ionic solids have substantial overlap, although the former is designed to describe chemical interactions at the surface site, and the latter electron capture. The site requirements for a Lewis acid are compared to the requirements for an acceptor surface state to suggest under what conditions a site should exhibit both strong acidity and a deep surface state: The expected influence of electronic properties of the solid on acid strength, and the influence of adsorbed water, acids or bases on surface state energies are discussed. Experimental measurements are reported where both chemical acidity measurements and electrical surface state measurements are made on a series of Lewis acids. A positive correlation is found. Where there is correlation, it is concluded that chemical interaction measurements should provide a valuable tool to describe the energy distribution of surface states on a semiconductor surface. Also, the use of the surface state models and measurements from semi-conductor physics should help in understanding and classifying acid and basic sites on ionic semiconductors and insulators.

Surface states on a chromia catalyst

Experiments were performed on pressed pellets of microcrystalline chromia, using measurements of adsorption, temperature-programmed desorption, and electrical conductivity. From the electrical conductivity measurements, a donor surface state was identified and calculated to be at an energy of 0.3 eV above the valence band. (Presumably, the bulk valence band is the Cr3+ band of the semiconductor.) It was concluded that this donor surface state is associated with the same coordinatively unsaturated surface Cr3+ ions that have been shown in other work to be the catalytically active centers on the catalyst. Studies of oxygen adsorption, CO adsorption, and CO oxidation are presented. The results are consistent with, but provide new details regarding, the accepted models of oxidation catalysis over chromia.

Mechanism of cathodic processes on the semiconductor zinc oxide

Abstract The cathodic reduction of the aqueous ferricyanide ion was investigated on a single crystal zinc oxide electrode. The experimental results substantiate that this chemical reduction process obeys the model used in semiconductor physics for electron capture by surface states. It is concluded that the rate determining step of the reduction process is the capture of electrons from the conduction band of the ZnO by the sorbed ferricyanide ions. This process was shown to be irreversible, i.e., electrons are not transferref from sorbed ferrocyanide (reduced ferricyanide) to the semiconductor electrode. The capacitance, voltage and current were measured as a function of the concentration. The rate of ferricyanide reduction was measured by the current and was found to be first order in sorbed ferricyanide ion and first order in the electron concentration at the surface. The electron concentration at the surface of the electrode was determined from the capacitance-voltage measurement. The sorption isotherm for ferricyanide was found to be linear in concentration over the range from 7 × 10−5 to 0.7 molar.