SI DELTA-DOPING IN GAAS - INVESTIGATION OF THE DEGREE OF CONFINEMENT AND THE EFFECTS OF POSTGROWTH ANNEALING

A stack of 60 delta-planes, each containing a Si areal concentration of 3.4 x 10(14) cm-2 (approximately 0.5 monolayers, ML), grown in GaAs by molecular beam epitaxy at 400-degrees-C has been examined before and after post-growth annealing by high-resolution x-ray diffractometry, transmission electron microscopy, secondar-ion mass spectrometry and infrared absorption localized vibrational mode (LVM) spectroscopy. These techniques provided complementary information concerning the concentration, spatial distribution and site occupancy of the Si atoms. It was found for the as-grown samples that the Si was located on Ga lattice sites (Si(Ga)) and confined to layers no more than 2 ML in thickness. Annealing at 600-degrees-C resulted in spreading of the delta-layers, with some Si remaining on the original planes, and the remainder diffusing away, resulting in a Si concentration of 2.1 x 10(19) cm-3 between the delta-planes. LVM spectroscopy indicated that the diffused Si atoms were present as Si(Ga) donors, Si(As) acceptors, Si(Ga)-Si(As) pairs and Si-X complexes. After the samples were annealed at 800-degrees-C or 950-degrees-C only a uniform Si concentration of 3 x 10(19) cm-3 was detected, although there was excess Si in the surface region where dislocation loops were observed. It is concluded that there is a maximum silicon concentration in solution that is in equilibrium with the delta-layers at 600-degrees-C and that the Si in the delta-layers is present as dimers or larger 2D clusters.