Transport and optical properties of low-resistivity CdSe.

Abstract

Transport and optical properties of n-type CdSe crystals together with microanalysis of residual impurities from two different sources (A and B) were investigated and compared in detail. It was found that the electronic properties of the crystals are governed by two kinds of shallow donors. At low temperatures (T150 K) a shallow level 10 meV is dominating the transport properties, while at Tg150 K a 22-meV level is the predominant one for crystal A. In addition, crystal B exhibits higher density of free carriers at all temperatures and the activation energy of its only shallow donor level is 10 meV. Laue patterns (x-ray measurements) showed that crystal B has a mosaic structure. Low-temperature photoluminescence spectra were measured for two crystals of A source, both low-resistivity n type (one undoped, the other In doped), and for crystal B. Two kinds of donors were identified in accordance with the transport measurements: one having an energy of about 10 meV, the other with energy of about 20 meV. The high-temperature photoluminescence of crystal B consisted of two peaks. They could be associated with emission from the A and B gaps of Wurzite-type crystals. Room-temperature photoresponse measurements showed that the energy gap of crystal B is higher by 6 meV than that of crystal A, which was attributed to the strain in this crystal. The origin of the two shallow donor levels seen in the transport and photoluminescence measurements is discussed in light of the elemental microanalysis and the existing literature. We suggest that In is responsible for the 10-meV level, while the deeper level (20 meV) is attributed to Na.