The growth and characterisation of zinc telluride for use as a nuclear radiation detector
As grown ZnTe is a highly conductive p-type semiconductor. In an attempt to produce highly resistive material suitable for fabrication into nuclear radiation detectors various dopants have been added during the Bridgman growth process to try to compenate the shallow acceptor defects that are responsible for the as grown electrical conductivity. Van der Pauw resistivity measurements an samples from each boule have showed that indium and aluminiun doping offers a consistant way of producing the high resistivity material. In total 12 devices were fabricated from the indium, aluminium and the one boule of high resistivity lead doped material but it was found that only one device, from an aluminium doped boule, was able to detect nuclear radiation. Further studies on this device showed that it was capable of detecting alpha-particles but not gamma-rays and also that it exhibited the normaI polarisation efect. In an attempt to correlate device performance (or lack of it) with the presence of point defects in the material all of the material was subject to studies using electrical, optical and magnetic resonance techniques. The information gathered in these ways has proven, to a great extent, to be inconclusive. TSC studies on indium doped material revealed the presence of two hole trapping levels. One defect with an activation energy of Ev +0.0geV was found to be present in all samples whereas the activation energy of the deeper defect was found to vary from sample to sample. Only one defect, with an activation energy of Ev +0. 13eV, was observed in the aluminium doped material and was suggested as being the (VznAlzn)' A centre. The concentrations of the defects found using this method give an indication why indium doped material did not act as a detector. The presence of the deep Fe+ centre has been observed using EPR. The observation of this signal has been correlated with with near infra-red luminescence common to ZnTe and observed in these samples suggesting the involvement of the Fe+ centre in the luminescence process. The signs of the shallow donor g-factors in CdS in ZnS in ZnSe and in the mixed crystal ZnS₀.₆Se₀.₄ have, for the first time, been measured directly through ODMR experiments which employ circularly polarised microwaves. In all cases the sign was measured to be positive and provides unequivical experimental conformation of the theoretical calculations carried out by Cardonna. ODMR studies on the 695 run bound exciton emission in CdS₀.₉₈Te₀.₂ indicate that this emission does indeed involve the Te pair bound exciton and is the first reported observation of such ODMR signals. These measurements serve to confirm the Te₂ bound exciton assignment to this emission by Goede et al.