Kasvatusparametrien vaikutus GaAsBi-rakenteiden ominaisuuksiin

In this thesis the properties of GaAsBi structures are investigated with respect to their growth parameters in molecular beam epitaxy. The GaAsBi alloy is a novel III-V semiconductor material with many beneficial material properties, including large band gap reduction in relation to change in Bi concentration and lattice constant, which make it a promising candidate for a wide range of applications in optoelectronics. However, the progress of GaAsBi research has been hindered by challenges in its growth. Due to the weak reactivity of the Ga-Bi system, unconventional growth conditions, such as low growth temperatures and stoichiometric As/Ga flux ratios, are required for efficient Bi incorporation. Furthermore, small changes in these growth conditions induce large changes in Bi incorporation and material properties, to an extent that the poor accuracy in As flux control limits the control of material properties and reproducibility. This work aims to resolve this issue by using an unconventional growth method where the substrate rotation is stopped. Due to the inherent spatial non-uniformity of the molecular fluxes, the stationary growth method transforms the uncertainty in As flux into spatial uncertainty of the As/Ga gradient, which can be determined accurately by ex-situ methods. Simultaneously, as the growth parameter distributions over the substrate can be determined by growth of calibration samples, the GaAsBi material properties can be examined as a function of growth conditions. This work focuses on the growth of GaAsBi bulk layers and single quantum wells with relatively high nominal Bi-fractions of around 5 %. The Bi incorporation, structural properties and optical quality of the GaAsBi structures were found to be extremely sensitive to the growth parameters. GaAsBi grown at a low temperature of ~220 °C showed a pronounced growth window at the stoichiometric and slightly above As/Ga flux ratio range with efficient Bi incorporation, good crystal quality and smooth surfaces. At higher growth temperatures, the growth window was pinned down to only the stoichiometric range and the overall Bi incorporation was reduced. Additionally, growth at a high temperature of ~370 °C enabled efficient photoluminescence at the growth window, due to reduction in low temperature growth related defects. Interestingly, spontaneous changes in the Bi incorporation were observed at specific growth regimes, indicating fundamental growth mode changes