Optically detected magnetic resonance of group-IV and group-VI impurities in AlAs and AlxGa1-xAs with x >= 0.35.

Abstract

Optically detected magnetic-resonance (ODMR) experiments have been performed on n-doped epitaxial layers of AlAs and ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As with x\ensuremath{\ge}0.35 grown on (001) GaAs substrates. The ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As layers were doped during growth or via implantation with Si and Sn impurities from group IV and S, Se, and Te impurities from group VI. The studies were carried out with the as-grown layers on the parent GaAs substrates, removed from the substrates, and attached to substrates with larger lattice constants at low temperatures. Symmetry information was obtained from angular-rotation studies with the magnetic field rotated in the (11\ifmmode\bar\else\textasciimacron\fi{}0) and (001) crystal planes. Also, uniaxial stress along the [11\ifmmode\bar\else\textasciimacron\fi{}0] and [100] directions has been combined with ODMR to further probe the symmetry of the donor states.The magnetic resonance was detected mainly on deep (1.0--1.8 \ensuremath{\mu}m) radiative-recombination processes. The donor state in Si-doped AlAs can be described by the usual hydrogenic effective-mass theory for substitutional donors on the group-III site associated with the X-point conduction-band minima. The g-value anisotropy and splitting observed from the rotation studies in the (11\ifmmode\bar\else\textasciimacron\fi{}0) and (001) planes, respectively, can be understood using an independent-valley model. The Si-donor g values in AlAs are the following: ${\mathit{g}}_{\mathrm{\ensuremath{\perp}}}$=1.976\ifmmode\pm\else\textpm\fi{}0.001 and ${\mathit{g}}_{\mathrm{?}}$=1.917\ifmmode\pm\else\textpm\fi{}0.001 with respect to the long axes of the X-valley ellipsoid. The results obtained for the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As layers doped with S, Se, and Te, particularly for samples with x\ensuremath{\ge}0.6, can be described by the hydrogenic effective-mass theory modified by a finite valley-orbit (i.e., central cell) interaction that mixes the states derived from the ${\mathit{X}}_{\mathit{x}}$,${\mathit{X}}_{\mathit{y}}$, and ${\mathit{X}}_{\mathit{z}}$ valleys to form an ${\mathit{A}}_{1}$ ground state, as predicted by Morgan. Analyses of these results within the virtual-crystal approximation yield valley-orbit splitting energies (i.e., chemical shifts) of \ensuremath{\sim}16--20 meV for these group-VI donors in ${\mathrm{Al}}_{0.6}$${\mathrm{Ga}}_{0.4}$As.The nature of the donor states in the Si-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructures with x1 is more complicated. The monotonic decrease in both the g-value anisotropy and splitting with decreasing Al mole fraction and the increase in the linewidth of the donor resonances from 7 mT for AlAs:Si to 14 mT for ${\mathrm{Al}}_{0.4}$${\mathrm{Ga}}_{0.6}$As:Si indicate a breakdown of the independent-valley model employed to describe the symmetry of the donor ground state in Si-doped AlAs. Various mechanisms that can potentially influence the properties of the donor ground state in Si-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As with x1, such as a finite spin-valley interaction, L-X (or \ensuremath{\Gamma}-X) interband mixing, and alloy disorder, are discussed. The results for the Sn-doped AlAs and ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs heterostructures provide evidence that the optically active states revealed in these studies are much deeper compared to the Si donor states.