Effect of point defects and dislocations on electrical and optical properties of III-V semiconductors
Raman scattering, cathodoluminescence (CL), transmission electron microscopy (TEM) as well as positron annihilation technique (PAT) have been applied to investigate the effect of point defects, such as dopants, gallium vacancies, and vacancy-related complexes, and dislocations on electrical and optical properties of III?V compound semiconductors, GaAs and GaN by concentrating on the interactions of point defects with dislocations. A so-called diffusion-drift-aggregation (DDA) model has been developed to describe the microscopic kinetic processes of point defects due to the interactions between them and dislocations. Computer simulations based on the DDA model have been carried out to reveal that the conventional Cottrell atmosphere cannot always correctly describe the aggregation of point defects at the dislocation and it is the formation of arsenic precipitates at the dislocation that results in the spatially extended increase in the free-electron concentration from the matrix to the dislocation in n-type GaAs:Si and GaAs:S, as indicated by Raman scattering. The arsenic precipitation at the dislocation in GaAs is found to be kinetically and energetically favorable by using the DDA model and molecular dynamics (MD) simulations. The aggregation of point defects at the dislocation in n-type GaAs is elucidated to depend on annealing time, temperature, arsenic pressure, and the doping level. The spatial variation of the luminescence from the matrix to the dislocation has been explained by considering the aggregation of point defects around the dislocation. The difference of the luminescence bands from the low doping level to the high doping level has been analyzed and the defects responsible for them have been identified. The energy levels of the corresponding defects in band gap have been determined. The gallium-vacancy-related complexes have been deduced to be responsible for the yellow luminescence band at 2.2 eV in n-type GaN. Their aggregation at the dislocation is revealed to result in the decrease in the free-electron concentration from the matrix to the dislocation by Raman scattering and cathodoluminescence.
Keywords: Raman, cathodoluminescence; transmission electron microscopy; positron annihilation; point defects; complexes; vacancies; dislocations; gallium arsenide; gallium nitride; diffusion; simulation