The Effects of High Pressure on the Photovoltage Spectrum of Cadmium-Sulfide and Cadmium-Selenide Schottky Diodes.
The effects of high pressure to 10 kilobars on the photovoltage spectrum of CdS and CdSe Schottky diodes has been investigated. These diodes exhibit a photovoltage at photon energies less than the energy gap of the semiconductor. This photovoltage is due to excitation of electrons from interface states to the conduction band of the semiconductor. A model where the interaction of the diode contact metal with the semiconductor surface gives rise to a sub-band of interface energy levels is used to explain the low energy response. Excitations from the sub-band also give rise to a recombination mechanism which causes a decrease in the photovoltage at photon energies slightly less than the semiconductor band gap energy. The recombination level is defined as the energy at which the recombination of photogenerated electrons and holes begins to dominate the photovoltage spectrum. We find that with the CdS Schottky diodes both the sub-band and the recombination level have pressure coefficients equal to that of the CdS conduction band edge (10 meV/kbar). CdS diodes which have been previously heated to 300(DEGREES)C show the sub-band and the recombination level to have pressure coefficients equal to the valence band edge (5 meV/kbar). This difference in pressure coefficients correlates to the extent of metal diffusion into the CdS as determined by Auger spectroscopy. CdS MIS diodes using Al(,2)O(,3) as an insulating layer between the CdS surface and the contact metal show the sub-band to have the same pressure coefficient as the CdS conduction band edge (10 meV/kbar). The recombination level in these diodes, however, has the same pressure coefficient as the CdS valence band edge (5 meV/kbar). Auger measurements on these diodes reveal a complicated interfacial layer which is a mixture of metal, Al(,2)O(,3) and CdS rather than a layered structure. The atmospheric pressure spectra of all of these CdS devices are surprisingly similar. Pressure is essential for determining differences in the interfacial sub-band in these diodes. The atmospheric pressure photovoltage spectrum of CdS Schottky diodes is similar to that found for CdS Schottky diodes. With the CdSe diodes we find that the sub-band and the recombination level both have a pressure coefficient equal to that of the CdSe valence band edge (2 meV/kbar). In this respect the CdSe diodes more nearly resemble the heated CdS diodes rather than the unheated CdS Schottky diodes.
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- Physics: Condensed Matter