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

Abstract

Low-noise infrared (IR) absorption measurements of localized vibrational modes (LVM's) showed that ${\mathrm{Si}}_{\mathrm{As}}$ acceptors, ${\mathrm{Si}}_{\mathrm{Ga}}$-${\mathrm{Si}}_{\mathrm{As}}$ pairs, and a deep trap Si-X (${\mathit{V}}_{\mathrm{Ga}}$-${\mathrm{Si}}_{\mathrm{As}}$-${\mathrm{As}}_{\mathrm{Ga}}$), as well as isolated ${\mathrm{Si}}_{\mathrm{Ga}}$ donors, were present in silicon \ensuremath{\delta}-doping superlattices in (001) GaAs grown under an As flux by molecular-beam epitaxy (MBE) at 400 \ifmmode^\circ\else\textdegree\fi{}C for areal concentrations (per layer) 0.05 ML\ensuremath{\leqslant} [Si${]}_{\mathit{A}}$\ensuremath{\leqslant}0.5 ML. These observations supersede previous data, and agree with recent Raman-scattering measurements. For [Si${]}_{\mathit{A}}$\ensuremath{\geqslant}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${]}_{\mathit{A}}$\ensuremath{\geqslant}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 \ensuremath{\delta}-doping superlattices that all have [Si${]}_{\mathit{A}}$=0.01 ML (per layer) showed only the ${\mathrm{Si}}_{\mathrm{Ga}}$ LVM as the interlayer spacing was reduced to 5 ML when the volume carrier concentration n approached \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. For interlayer spacings of 2 and 1 ML, compensating complexes ${\mathrm{Si}}_{\mathrm{As}}$, ${\mathrm{Si}}_{\mathrm{Ga}}$-${\mathrm{Si}}_{\mathrm{As}}$, and Si-X were present, and n tended to zero. Compensating complexes were also present in homogeneously doped MBE GaAs grown at 350 \ifmmode^\circ\else\textdegree\fi{}C, but n remained at a value of 2\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ as [Si] was increased to 1.3\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. N never exceeded 2\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ in any sample. The formation of ${\mathit{V}}_{\mathrm{Ga}}$, ${\mathrm{As}}_{\mathrm{Ga}}$, etc. is attributed to diffusion jumps of Si atoms originally located on Ga lattice sites. The formation of the ``Si-like'' structure in \ensuremath{\delta} layers must result from the aggregation of such displaced atoms. We speculate that these processes are facilitated by the initial displacements of ${\mathrm{Si}}_{\mathrm{Ga}}$ donors to DX locations. \textcopyright{} 1996 The American Physical Society.