Electron-limiting defect complex in hyperdoped GaAs: TheDDXcenter

The physical properties and chemical trends of defects in GaAs under the hyperdoping situation are investigated and found to be very different from those in the impurity limit. We show that at high dopant concentrations, a defect complex denoted as the $DDX$ center becomes the dominant ``killer'' to limit the electron carrier concentration, whereas in the impurity limit, the electron carrier concentration is usually limited by the well-known $DX$ center. The $DDX$ center shows some opposite chemical trends compared to the $DX$ center. For example, to avoid the $DX$ center, anion site donors are preferred, but to avoid the $DDX$ center, cation site donors are better. Our proposed mechanism is able to explain some puzzling experimental observations.