The DX Center: How Complicated Can a Point Defect Be?

Abstract

ABSTRACTI summarize a large body of experimental and theoretical work, especially in Sidoped GaAs and AlxGal−xAs, which has led to our present understanding of the DX center. There is good evidence that the DX center is just the simple donor, but each donor atom can exist in either of two distinct lattice configurations, each with its own spectrum of bound electronic states. At present the model which appears to best explain this unexpected complexity is that of Chadi and Chang, which predicts a large bond-breaking distortion of the lattice, accompanied by capture of two electrons by the donor. I argue that the best evidence to date for the predicted distortion is provided by observations of alloyperturbations of the DX level. Furthermore, there is now very convincing evidence that the DX level is a two-electron state. I briefly summarize what is known about the bound state spectra of the substitutional and relaxed (DX) configurations, and then discuss the very interesting question of how the donor captures two electrons from the conduction band to theDX level. There is good, although indirect, evidence that an excited one-electron state acts as an intermediate in the thermal emission and capture process. A different excited state appears to 15e involved in photoemission. Although much of this complexity was unforeseen by Chadi and Chang, it nevertheless seems to be consistent with their model. However, I point out a number of issues which still require experimental or theoretical resolution.