EFFECTS OF THE POPULATION OF LOCALIZED LEVELS ON SHALLOW DONOR TRANSITIONS IN SEMIINSULATING GAAS

The dominant deep donor in semiinsulating GaAs, the EL2 defect, may at low temperatures be transferred into a metastable configuration. Unde r hydrostatic pressure this metastable configuration can capture an ex tra electron. It is thus possible to observe three different charge st ates of the EL2 defect in a sample: EL2(0), EL2(+) and (EL2)(-). Diff erent distributions of electrons among these states in turn lead to di fferent distributions of the fluctuating electric fields in the sample . The far infrared magnetophotoconductivity due to shallow donors was measured in semiinsulating GaAs under hydrostatic pressure. The observ ed transitions between ground and excited states of dynamically popula ted shallow donor states are shown to be extremely sensitive to the lo cal electric fields. The halfwidth of the 1s-2p(+), transition, which was previously found to increase when the EL2 was transferred to the m etastable state,is observed to increase far more when the same experim ent is performed under pressure. The dominant effect is a purely elect ronic one-population of the acceptor-like level of the EL2-and is not connected with lattice distortion accompanying the transition to the m etastable state. The broadening of the linewidth due to EL2 bleaching can therefore also be explained in terms of electrostatics.