RESEARCH ON NEW HIGH-TEMPERATURE SEMICONDUCTING MATERIALS

摘要

Three years of research on the preparation, purification, crystal growth, and measurement of the fundamental properties of CdS, CdSe, ZnTe, and mixed crystals of CdS•CdSe are summarized. The basic requirements of high-temperature semiconductor devices for operations to 500℃ were closely examined and led to the choice of the above materials for further investigation. A detailed account, with liberal references to previous quarterly reports, is presented on the following topics:(l) purification of elemental zinc, cadmium and tellurium by zone refining and vacuum distillation using spectrograph and radioisotopic techniques for evaluation; (2) preparation of compounds by high-temperature reaction of the component elements; (3) growth of 10- to 100-gram single crystals by vapor-phase deposition and recrystallization; (4) mechanism of crystal growth in the Reynolds-Czyzak process, including analog studies of soap-bubble and iodine-crystal models; (5) preparation of test specimens, covering crystal orientation, wire and band sawing, lapping, polishing, etching, and electroding; (6) measurement of basic properties. Measurements taken on a variety of doped and undoped crystals include; (l) X-ray crystallographic data; (2) thermal expansion data from 25 to 200℃;(3) optical absorption (energy gap) data, including the variation of △e with composition of mixed CdS-CdSe crystals; and (4) data on the temperature dependence of Hall effect and conductivity from 77°K to 800 °K. The room-temperature energy gaps are typical values of interest and for CdS, CdSe, ZnTe and ZnSe are 2.38, 1.70, 2.21 and 2.43 eV, respectively. △e Of mixed crystals are nonlinear with composition (△E of Cd(S5Se5) = 1.91 ev). Room temperature carrier mobilities in doped crystals (p= 10-2 to 1 ohm cm) are 350,550, and 100 cm2 /volt sec., respectively, for CdS(n-type),CdSe(n-type), and ZnTe (p-type). At high temperatures Wp=∝T-3/2 for In-doped CdS, Wn=∝T-1 for Ga-doped CdSe, and Wp=∝T-3/2 for Cu-doped ZnTe. Low temperature electron mobility in Ga-doped CdSe = 1000 cm /volt-sec. with n = 1017 cm .Thermoelectric power of Ga-doped CdSe =250WV/OK. Data on photoconductivity, piezoelectricity, cleavage, twinning, impurity gradients, and other miscellaneous effects are also presented.