Lattice locations of silicon atoms in δ-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 {δ}-doping superlattices in (001) GaAs grown under an As flux by molecular-beam epitaxy (MBE) at 400 ^∘{}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 {δ}-doping superlattices that all have [Si]A=0.01 ML (per layer) showed only the SiGa LVM as the interlayer spacing was reduced to 5 ML when the volume carrier concentration n approached {~}2×{}10¹⁹ cm^-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 ^∘{}C, but n remained at a value of 2×{}10¹⁹ cm^-3 as [Si] was increased to 1.3×{}10²⁰ cm^-3. N never exceeded 2×{}10¹⁹ cm^-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 {δ} 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. {} 1996 The American Physical Society.