LOCAL-ENVIRONMENT DEPENDENCE OF THE DX CENTER IN GAALAS - ALLOY AND SUPERLATTICE STUDIES

We present new experimental data on the capture of electrons onto Si-induced impurity states, which give evidence of the existence of several configurations for the DX centre with a large lattice relaxation. The investigated samples were Si-doped GaAlAs alloys and GaAs/AlAs short-period superlattices, grown by molecular beam epitaxy, with different aluminium contents. Persistent photoconductivity in the samples has been investigated by means of Hall measurements under pressures up to 15 kbar, between 4.2 and 400 K. In GaAlAs alloys several steps have been observed on the thermostimulated capture curves between 90 and 140 K. The mechanism responsible for the presence of these steps is not only related to capture processes, but also results from the equilibrium between capture and emission processes, involving the different states of the DX centre. Our data confirm that the DX centre presents several configurations, each of them with discrete emission and capture barriers, related to the local environment of the Si atom. For the short-period superlattices selectively doped in GaAs layers or in AlAs layers, the thermal annealing curves decrease monotonically, as expected for a single barrier between 90 and 140 K. In uniformly doped GaAs/AlAs superlattices the n(T) curve shows a two-step behaviour, indicating that two barriers are involved in the capture process. These results confirm the existence of several configurations for the DX centres in short-period superlattices. The selective doping of short-period superlattices in AlAs layers made it possible to study the all-Al environment of the Si atom, unobserved until now.