SHALLOW DONORS AS A GAUGE OF THE FLUCTUATING ELECTRIC-FIELDS IN A SEMICONDUCTOR

The magnetospectroscopy of shallow donor states is used to study the f luctuating electric fields in semiinsulating GaAs. This material conta ins a large number (similar to 10(16) cm(-3)) of ionized impurities, a nd (at 4.2K) no free electrons to screen them. Since the Fermi level i n semiinsulating GaAs is pinned at the midgap intrinsic donor EL2, sha llow donor states at low temperature are not populated. A constant ill umination with near band gap light was used to achieve a dynamic, stea dy state population of the donor states. Far infrared magnetophotocond uctivity and magnetoabsorption due to the shallow donors was then meas ured, and found to be a sensitive gauge of the fluctuating electric fi elds in the sample. Transitions to metastable, autoionizing states wer e observed, as well as transitions to stable excited states. It was fo und that the fluctuating electric field greatly increases the probabil ity of ionization of resonant excited states. The linewidth of the tra nsitions to stable states proved to be an effective toot for observing changes in the distribution of the local electric fields caused by po pulating different charge states of the deep level EL2.