Electron-paramagnetic-resonance measurements of Si-donor-related levels in ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As

We report measurements of an EPR signal in indirect-gap Si-doped ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As whose intensity increases after illumination at low temperature. The data indicate that this signal comes from a hydrogenic level associated with the X valley of the conduction band. Measurements of the spectral dependence of the enhancement of the EPR signal show that electrons are transferred from the DX level, the lowest energy state of the Si donor, to this higher-lying state. No other signal which might be associated with the DX level was observed. The results are consistent with a large lattice relaxation model of the DX center but do not, at this time, distinguish between positive- and negative-U models.