BISTABILITY OF THE DX CENTER IN GAAS AND ALXGA1-XAS, AND EXPERIMENTAL TESTS FOR NEGATIVE-U OF THE DX LEVEL

We summarize a large body of experimental and theoretical work, especially in Si-doped GaAs and Al(x)Ga1-xAs, regarding the bistability of the DX center. There is good evidence that the DX center is just the simple donor, and that each donor can exist in either of two distinct lattice configurations, each with its own spectrum of bound electronic states. Generally, the substitutional configuration binds electrons in shallow hydrogenic states, but many observations also indicate a deep (highly localized) state of A1 symmetry. These states are to be distinguished from bound states of a lattice-distorted configuration, the lowest-lying of which is the deep DX level. The occupation of the DX level in thermal equilibrium with the states of the conduction band can be reasonably well modeled by assuming that DX is either a one-electron or a two-electron state, and we discuss the reasons for this ambiguity. However, we then show that such thermal equilibrium results are consistent with thermal capture and emission kinetics only if we assume that DX is a two-electron state. Our results thus support the model of Chadi and Chang in which the distorted configuration is stabilized by capture of two electrons. In other words, the defect exhibits negative effective correlation energy (negative U).