Electrical properties of zinc selenide

Abstract

The main purpose of the research reported in this thesis was to investigate the behaviour of zinc selenide crystals doped with group III impurities such as indium and gallium, with the aim of understanding the processess of self-compensation involved which prevent them from being converted to p-type. The crystals which were grown from the vapour phase, were doped by adding the group III impurity directly to the charge in metallic form or as ZnSe:In or ZnSesGa. Measurements of the photoluminescence and cathodoluminescence revealed that the crystals behaved very similarly to copper doped ZnSe, emitting an orange-red band at 295 K and in the yellow-green and orange-red at 85 K. Measurements of the electrical conductivity and Hall coefficient were made on as-grown samples and samples heated in zinc vapour, which had been cut from ten crystal boules containing indium or gallium in concentrations between 5 and 1000 ppm. The measurements showed that the crystals obeyed the Meyer-Neldel rule and that their resistivity increased with the dopant concentration. It was also revealed, from the exponential behaviour and low values of the electron Hall mobilities, that the conduction of the lightly doped samples was via impurity centres at 295 K. The heavily doped samples also showed low values of Hall mobility limited by a combination of polar mode and ionized and neutral impurity scattering. The free electron concentration decreased while the activation energies increased with in - creasing dopant concentration. Activation energies ranged from 0.05 to 0,95 eV. The reduction of the free electron concentration with increasing indium or gallium content in the crystals, is attributed to indium (or gallium) substituting on selenium sites and forming acceptors. When the heavily doped crystals were heated in zinc the indium or gallium is precipitated. The precipitates have been examined in the optical, scanning electron and transmission electron microscopesv and are found tohave particular shapes and to decorate defects such as stacking faults. The mechanism of precipitation is discussed in same detail.