Electron Localization by a Metastable Donor Level in $n-\mathrm{GaAs}$: A New Mechanism Limiting the Free-Carrier Density

We observe in Si-doped GaAs, by capacitance transient spectroscopy, electronic occupation of a highly localized state of the donor-related $\mathrm{DX}$ center. The emission and capture kinetics are those of a metastable state which lies above the conduction-band edge. The state is so spatially localized that its emission kinetics are not measurably perturbed by neighboring Si atoms (donors or acceptors) at an average distance ≅3.5 nm. Occupation of this state is therefore a previously unsuspected mechanism which can limit the free-carrier density in very heavily doped $n-\mathrm{GaAs}$.