Fundamental issues of device-relevant low temperature GaAs and related materials properties

In the past few years, a flurry of activity has been devoted to the studies and device applications of non-stoichiometric GaAs grown by molecular beam epitaxy at the extremely low temperature (LT) of 250°C or below using, in essence, identical growth conditions to those of 'normal', high temperature (> 500°C) grown materials. These materials are highly non-stoichiometric, containing a massive amount of excess As in the lattice which totally dominates the electro-optical characteristics. This essentially defect-controlled material, in both its as-grown state and after annealing, leads thereafter to a number of device concepts and applications ranging from buffer layers to fast photoconductors. In this study, a detailed investigation of the growth conditions, including growth dynamics, has established that the non-stoichiometry of LT GaAs is not an intrinsic property but a perfectly controllable one. We report here on new phenomena associated with the growth of GaAs and related compounds at < 250°C and present data on highly electrically and optically active material demonstrating electro-optical qualities comparable to those grown at high temperatures, as evidenced by photoluminescence (PL) of quantum wells and sheet densities and mobilities in HEMT structures. It is therefore surmised that non-stoichiometry in low-temperature-grown GaAs can be overcome, leading to the growth of stoichiometric low temperature (SLT) materials possessing properties similar to those of conventional, high-temperature-grown layers.