The properties of mixed crystals of zinc-cadmium-sulphide(Zn(_x)Cd(_1-x)S)

Abstract

CdS and ZnS are two binary compounds of II-VI semiconductors which form a continuous series of solid solutionsthroughout the whole range of composition (Zn(_x)Cd(_1-x)S). The main purpose of the work reported in this thesis was to characterise some electrical properties of Zn(_x)Cd(_1-x)S crystals grown in the department. Atomic Absorption Spectroscopy (AAS) was used to determine the actual composition of the solid solutions to compare with Energy Dispersive Analysis by X-Ray (EDAX) measurements. A calibration curve of ratio intensity versus composition can then be used to estimate thecomposition the X-Ray of Zn x Cdl -x S diffraction non-destructively. From the x-ray diffraction studies, all Zn(_x)Cd(_1-x)S crystals with x<0.85 were found to have the hexagonal, wurtzite form while ZnS crystals have the cubic, zinc blende phase. Conductivity and Hall coefficient measurements were also carried out. At 300K, conductivities up to about lxl0(^-1) ohm(^-1)cm(^-1) could be achieved for composition up to x=0.45. The Hall mobility was found to decrease drastically at x=0.2. Scattering due to a combination ofpolar optical and piezoelectric processes was thought to Control the carrier mobility for crystals with x<0.2. With x>0.2, scattering due to ionised impurities, space charge or alloy processes is probably more important. The barrier heights of Au-Zn(_x)Cd(_1-x)S (x<0.5) Schottky diodes were calculated from forward I-V characteristics, C-V and photoelectric measurements were also carried out. A quite linear relationship with composition was observed for barrier heights measured by photoelectric method. Barrier heights calculated by the other methods was very much dependent on the nature of the barrier. From steady state photo capacitance studies, a level situated at about l.l-1.2eV above the valence band is the dominant feature for copper doped samples and crystals grown by the Piper-Polich method. This centre was pinned to the valence band. A second deep centre situated at about 1.4-1.5eV below the conduction band was also observed in some samples and was pinned to the conduction band. The existence of this centre is thought to depend very much on the history of the individual sample.