Si delta -doping in GaAs: investigation of the degree of confinement and the effects of post-growth annealing

Abstract

A stack of 60 delta -planes, each containing a Si areal concentration of 3.4*1014 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, secondary-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 (SiGa) 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*1019 cm-3 between the delta -planes. LVM spectroscopy indicated that the diffused Si atoms were present as SiGa donors, SiAs acceptors, SiGa-SiAs 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*1019 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.