Optical characterization and device application of the semiconductors ZnSe and ZnS
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We have presented evidence that the blue emission band, which is dominant at room temperature in ZnSe made under a wide variety of conditions, arises from a transition between a free hole and an electron bound to a donor, but not all donors contribute. There is a cutoff energy within the spread of donor levels above which there is negligible contribution to the emission. It is suggested that the cutoff corresponds to a localization edge of the same nature as the mobility edge. The line shape calculation based on a simple model agrees well with the experimental data. The origin of the blue emission seen at room temperature in the electroluminescence is examined to be the same. We have also discussed the injection mechanism of minority carriers in the ZnSe MIS diodes. It has been shown that annealing as-grown MOCVD ZnSe in the temperature range 300-400 °C can lead to large increases in resistivity. The effect is large for annealing in air or selenium and smaller for annealing in vacuum or zinc vapour. The process involved has an activation energy of only 0.26 eV and appears to be caused by a lattice defect acting as an acceptor. The photocapacitance spectra show that the acceptor is likely to be the so-called M-centre in ZnSe. We have shown that the attribution of the M-centre to copper-red centre is by no means conclusive. The possibility is still open that the M-centre is a lattice defect. We have made double light source steady-state photocapacitance measurements on ZnS single crystals. The Schottky diodes were made by evaporating a metal contact onto a chemically cleaned ZnS surface. Levels were found at 0.9 eV and 2.0 eV below the conduction band and 0.8 eV above the valence band in both melt-grown and iodine-transported material. These centres might be due to lattice defects. An additional level at 1.6 eV below the conduction band occurred in the iodine-transported material.
Thesis, PhD Doctor of Philosophy
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