DEFECT STUDIES IN GALLIUM ARSENIDE PHOSPHIDE AND CONFIRMATION OF NEGATIVE-U PROPERTY OF SULPHUR RELATED DX CENTER

Abstract

This thesis deals with several crucial topics on defects in gallium arsenide phosphide. First, the role played by Te in GaAs0.6P0.4 was investigated. Second, a novel experiment, which involves the co-doping of Te and S in GaAsP, was designed and implemented to clearly demonstrate the negative U property of S DX center in GaAsP. Finally, the C-V profiling technique used in this experiment is reviewed for the case of ion-implanted samples and samples with high concentration of deep centers. GaAs0.6P0.4 with uniform Te doping was studied using deep level transient spectroscopy (DLTS), capacitance-voltage (C-V) and thermally stimulated capacitance (TSCAP). DLTS scanning reveals two deep levels both with defect densities of at least two orders of magnitude lower than the concentration of Te. Carrier concentrations measured by C-V at both room temperature and liquid nitrogen temperature are identical. TSCAP measurements under different initial conditions produce indistinguishable spectra reflecting the small concentrations of the two deep levels observed in DLTS. The results of these independent measurements agree with each other and lead to the conclusion that Te in GaAs0.6P0.4 is a shallow donor only. The S atoms were introduced into Te doped GaAs0.6P0.4 by ion implantation. The S DX center level and an ion implantation induced defect level, trap SA, are detected in the co-doped sample. The emission and capture behaviours of the S DX center level and trap SA were studied using DLTS. It was found that trap SA has a large enough emission time constant but a very small capture time constant at 77 K. This property implies that trap SA can only capture electrons but not emit them during any single C-V measurement (around 1 minute) at 77 K, and therefore permits the determination of the concentration of S DX centers alone. The analysis of carrier distributions measured by C-V both in the dark and after light illumination show unambiguously that the S DX center in GaAsP traps two electrons in its ground state and thus has a negative U property. In the presence of either a doping gradient or a high concentration of deep centers in semiconductor materials, the C-V profile has to be examined carefully in order to extract meaningful results. Simulated C-V profiles were examined in terms of Debye length. The comparison between the simulated profiles using the parameters of our co-doped sample and the original doping profiles shows that the diffusion current due to the doping gradient has negligible effect on the experimental C-V results. The C-V profile of our co-doped sample was corrected by taking into consideration of the edge-zone effect due to the high concentration of trap SA.