IONIZATION AND CAPTURE KINETICS OF DX CENTERS IN ALGAAS AND GASB - APPROACH FOR A NEGATIVE-U DEFECT

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 photoinization 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- reversible D0 + e- reversible 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.