Charge state and diffusivity of muonium in n-type GaAs.

The presence of Si donors in GaAs exerts a strong influence on the charge state and diffusion of muonium at the tetrahedral interstitial site (${\mathrm{Mu}}_{\mathit{T}}^{0}$), while it has relatively weak effect on the bond-center muonium (${\mathrm{Mu}}_{\mathrm{BC}}^{0}$). In metallic GaAs:Si, the highly mobile ${\mathrm{Mu}}_{\mathit{T}}^{0}$ center observed in nonmetallic GaAs is replaced by a charged species (probably ${\mathrm{Mu}}^{\mathrm{\ensuremath{-}}}$) which shows a diffusion rate at least ${10}^{\mathrm{\ensuremath{-}}2}$ times smaller than the ${\mathrm{Mu}}_{\mathit{T}}^{0}$ center. The ${\mathrm{Mu}}_{\mathrm{BC}}^{0}$ center is metastable and its transition to ${\mathrm{Mu}}_{\mathit{T}}^{0}$ or ${\mathrm{Mu}}^{\mathrm{\ensuremath{-}}}$ centers (depending on doping concentration) is seen in the 50--150-K range. The current results also suggest that the ${\mathrm{Mu}}_{\mathit{T}}^{0}$ state undergoes fast spin-exchange interaction with conductive carriers prior to the transition ${\mathrm{Mu}}_{\mathit{T}}^{0}$\ensuremath{\rightarrow}${\mathrm{Mu}}^{\mathrm{\ensuremath{-}}}$.