Extrinsic self-trapping and negativeUin semiconductors: A metastable center in InP

A fascinating electron-irradiation-induced defect (the $M$ center) in $n$-type InP has been reported recently. Two entirely different capacitance transient spectra can be obtained for this center in the same sample (a $p-n$ junction diode) depending upon its bias condition during the cool down to the initial measurement temperature (~ 30 K). We present a conceptually simple configuration-coordinate model that explains the unusual properties of this center. In our model the metastable center can exist in either of two configurations, one which displays a very large lattice relaxation, and another ordinary (no anomalously large lattice relaxation) configuration. Photoionization rates have been measured for the defect levels of the two different defect configurations as a function of photon energy. These data are used to confirm the qualitative properties of our model and to determine its parameters. Classification of the $M$-center behavior within the general context of selftrapped configurations is proposed.