The preparation and properties of large single crystals of cadmium sulphide

Abstract

The photoconductive and luminescent properties of cadmium sulphide have been of interest, and have only been studied extensively since 1947 when Frerichs reported a convenient method of preparing small single crystals. The properties of the material are summarised in Chapter two of this thesis, following an outline of the essential theory of electrical conduction in semiconductors. Conventional methods of single crystal preparation such as growth from the melt of from solution are not practicable with cadmium sulphide and most measurements have been made on small plates or rods grown by sublimation in a flow of carrier gas. However the preparation of large single crystal samples is desirable for certain measurements, e.g. Hall effect measurements, and essential if recent applications such as gamma-ray detectors or ultrasonic amplifiers are to be pursued. Various methods of growing large single crystals by sublimation have been reported, and these are reviewed in chapter 4. However crystal produced by similar methods can show marked differences in their properties. The resistivities of crystals as grown can be as high as 10(^10) ohm-cm., or as low as 1 ohm-cm, with wide variations in the carrier mobility. Close control of the properties of the material produced is essential for any application of for any detailed study of the material itself. The work which forms the basis of this thesis therefore consists of an investigation into the feasibility of growing large crystals by several methods. The effect on the resultant properties is observed when parameters such as the growth temperature or the partial pressures of the components are altered. The suitability of various starting material is also discussed. The conclusions from the present work are that departures from stoichiometry in the growth system are responsible for much of the difficulty in preparing large single crystals of good quality at low temperatures (~1050 C). Further, at the present time the properties of the material are controlled by departures from stoichiometry rather than by contamination from chemical impurities. The final section of the thesis indicates which of the reported methods is most suitable for small-scale production of large single crystals of high resistivity cadmium sulphide. It also described a growth method embodying the conclusions drawn from the present work as a basis for further study.