Lattice locations of silicon atoms in delta-doped layers in GaAs at high doping concentrations

Low-noise infrared (IR) absorption measurements of localized vibrational modes (LVM's) showed that SiAs acceptors, SiGA-SiAs pairs, and a deep trap Si-X (VGa-SiAs-AsGa), as well as isolated SiGa donors, were present in silicon delta-doping superlattices in (001) GaAs grown under an As flux by molecular-beam epitaxy (MBE) at 400 deg C for areal concentrations (per layer) 0.05 ML<= (Si)A <= 0.5 ML. These observations supersede previous data, and agree with recent Raman-scattering measurements. For [Si]A<=0.5 ML, the LVM's were not detected by either technique, but Raman measurements revealed a broad line that has been attributed to small two-dimensional Si clusters. For [Si]A <= 0.5 ML, electrical conductivity was lost. These observations led to a reappraisal of simulations of high-resolution x-ray 002 and 004 diffraction profiles. IR and Raman measurements for delta-doping superlattices that all have [Si]A <= 0.01 ML (per layer) showed only the SiGa LVM as the interlayer spacing was red uced to 5 ML when the volume carrier concentration n approached about 2 x 10(exp 19) cm(exp -3). For interlayer spacings of 2 and 1 ML, compensating complexes SiAs, SiGa-SiAs, and Si-X were present, and n tended to zero. Compensating complexes were also present in homogeneously doped MBE GaAs grown at 350 deg C, but n remained at a value of 2 x 10(exp 19) cm(exp -3) as [Si] was increased to 1.3 x 10(exp 20) cm(exp -3). N never exceeded 2 x 10(exp 19) cm(exp -3) in any sample. The formation of VGa, AsGa, etc. is attributed to diffusion jumps of Si atoms originally located on Ga lattice sites. The formation of the "Si-like" structure in delta layers must result from the aggregation of such displaced atoms. We speculate that these processes are facilitated by the initial displacements of SiGa donors to DX locations.