Growth and Characterisation of GaAsBi

This thesis reports the optical and structural properties of GaAs(1-x)Bi(x) alloys grown on GaAs by Molecular Beam Epitaxy (MBE). The photoluminescence (PL) of a GaAs(0.97)Bi(0.03) alloy was measured over a wide range of temperatures and excitation powers. The temperature dependence of the PL peak energy indicated significant exciton localization at low temperatures and the band gap varies more weakly with temperature than in GaAs. An analysis of dominant carrier recombination mechanism(s) was also carried out indicating that radiative recombination is dominant at low temperature.

The PL results indicate that dilute fractions of bismuth (Bi) with x < 0.025 improve the material quality of these low temperature growth alloys by reducing the density of gallium (Ga) and/or arsenic related defects. The crystal quality starts to degrade at higher Bi concentration probably due to a significant amount of Bi-related defects, i.e BiGa. However, the room temperature PL intensity continues to increase with Bi content for x up to 0.06 due to the greater band-gap offset between GaAs and GaAs(1-x)Bi(x).

To improve the quality of GaAs(1-x)Bi(x) alloys, annealing and growth studies were carried out. At room temperature, the annealed GaAs(1-x)Bi(x) showed a modest improvement (~ 3 times) in PL while the PL peak wavelength remained relatively unchanged. Also, the optimum annealing temperature is Bi composition dependent; for samples with x < 0.048, the optimum annealing temperature is 700 oC but it reduces to 600 oC for higher compositions.

Two growth parameters were investigated which are growth rate and As4/Bi beam equivalent pressure (BEP) ratio. It was found that growth rate significantly affects Bi incorporation and the accumulation of surface Bi. Decreasing the As4/Bi BEP ratio has been shown to increase Bi concentration but is limited by the formation of Bi double PL peaks.