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In capacity measurements, a strong frequencydependent capacity has been observed at higher potentials which was discussed as a contribution of surface states.8In photocurrent spectra, a potential-dependent onset energy similar to the one described for thin films on zirconium is observed at low potentials. This is another indication that cathodic photocurrents are important at low potentials. The experimental results for passive iron, however, are not yet as detailed as for the other systems, allowing for no quantitative interpretation. ConclusionsDetailed results of photocurrent measurements on passive films revealed the presence of cathodic photocurrents at potentials above the flatband potential. The potentials, U0, at which the photocurrent changes sign are found to be wavelength dependent which indicates that U0 is not the flatband potential. It is also found that UQ is usually more positive than Uvalues obtained from capacity measurements. Passive films are generally highly disordered or amorphous. This means that a large number of localized states are present in what is the bandgap of the corresponding crystalline material. For a photoexcitation process that involves localized states, the removal of the photoexcited charge carrier from the localized state can be rate determining.1 2The model presented here takes into account reverse tunneling, i.e., tunneling processes against the field, which can result in cathodic photocurrents. The model calculations generate results that are very similar to experimental results and which are able to explain experimental results described above.

It is believed that atomic-scale surface steps cause defects in single-crystal films grown heteroepitaxially on SiC substrates. A method is described whereby surface steps can be grown out of existence on arrays of device-size mesas on commercial “on-axis” SiC wafers. Step-free mesas with dimensions up to 200 μm square have been produced on 4H-SiC wafers and up to 50 μm square on a 6H-SiC wafer. A limiting factor in scaling up the size and yield of the step-free mesas is the density of screw dislocations in the SiC wafers. The fundamental significance of this work is that it demonstrates that two-dimensional nucleation of SiC can be suppressed while carrying out step-flow growth on (0001)SiC. The application of this method should enable the realization of improved heteroepitaxially-grown SiC and GaN device structures.

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In situ transmission x-ray absorption spectroscopy was used to investigate changes in the structure of Ni(OH)2 as it was cycled as a nonsintered nickel oxide electrode in concentrated alkali. Results were obtained for dry as-prepared electrodes and electrodes that were charged or discharged. Changes were observed in the (0001) plane of the Ni(OH)2 as a function of cycling. In the charged material there was a contraction in the Ni-O and Ni-Ni distances. The EXAFS results and the XANES spectra strongly indicate that the Ni-O coordination is symmetrical in Ni(OH)2, whereas the coordination is asymmetric in the charged material.The nickel oxide electrode, also known as the nickel or nickel hydroxide electrode (NOE), is used in batteries, fuel cells, and electrolyzers. Other possible uses are in electrochromic and switching devices. Because of its many applications it has been the subject of numerous investigations and reviews (1-4). In spite of this, the electrode processes remain an enigma. Much of this confusion is due to the amorphous nature of the reactants and products. Extended x-ray absorption fine structure (EXAFS) technique can be used to study the average local structure of systems that do not possess long range order. A previous paper has presented preliminary results of an in situ EXAFS study of a nickel oxide electrode (5). This work has shown that the EXAFS spectroscopy can be used to study electro- chemical systems under working conditions (in situ).The present paper is a more extensive report of the previous work which includes analysis of higher shells and x-ray absorption near edge structure (XANES). ExperimentalElectrode fabrication.--In order to obtain high quality x-ray absorption data, samples of good uniformity and an absorption step (h~x) of -1.5 at the nickel K absorption edge are required (6). From the data interpretation point of view, the data analysis procedure is considerably simplified if only a single compound of the element of interest is present in the sample. For this reason, nonsintered electrodes were fabricated in which nickel was present only in the hydroxide form. Details of the electrode fabrication process are given elsewhere (7,8). The electrodes consisted of ~-Ni(OH)2, cobalt hydroxide, graphite powder, chopped vitreous carbon fibers, and a polyvinylidene fluoride plastic binder. The electrodes were circular and were 25.4 mm in diam and 0.25 mm thick. The electrode composition was 69.1 mg Ni(OH)2, 5.4 mg Co(OH)2, 32.4 mg graphite, ll,6mg carbon fibers, and 12.4mg polyvinylidene fluoride. The theoretical electrode capacity was 20 mAh. Excellent utilization was obtained with these electrodes.Electrochemical cell--The electrochemical cell has been described in detail elsewhere (5, 7). The cell body consisted of two machined acrylic plastic blocks (76 • 76 x 19 ram) with a hole (40 x 10 ram) machined at the center of each block. Acrylic windows (76 x 76 x 0.75 mm) were glued to one face of each block. The cell was clamped between each window. The cell consisted of the ...

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