Ionization and capture kinetics of DX centres in AlGaAs and GaSb: approach for a negative-U defect

Abstract

In this paper the consequences of an idea in which DX centres in semiconductors form a negative-U system for the approach to commonly used DLTS and photoionization measurements are discussed. For the negative-U model of DX centres to be valid the carrier exchange between the DX states and the conduction band must occur via an intermediate one-electron D0 state. In such a system the neutral D0 state must be thermodynamically unstable, but obviously should play a role in all carrier capture and emission processes (D- from or to D0+e- from or to D++2e-). The experimental evidence for the existence of such an intermediate state is presented. It is based upon the detailed observations of the temperature evolution of the photoionization transients of the DX centres in AlxGa1-xAS:Te as well as the capacitance transients of the thermal emission process from DX centres in GaSb:S. Rate equations for photoionization and isothermal DLTS experiments on a defect forming a negative-U system are presented.