A Photocapture Test of DX-Center Models
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A PHOTOCAPTURE TEST OF DX-CENTER MODELS HAROLD P. HJALMARSON, S. R. KURTZ, AND T. M. BRENNAN Sandia National Laboratories, Albuquerque, New Mexico 87185. ABSTRACT The DX-center model is widely used to explain data for the persistent photoconductivity (PPC) effect. An analysis of the DX-center model suggests a new experiment to test its correctness. In this experiment, photons near the threshold energy of the photoionization cross-section for the DX-center induce transitions from the partially occupied conduction band to empty DXcenters. This mechanism, which we call photocapture, competes with the usual photoionization which empties the DX-centers. The photocapture crosssection is estimated and an experimental attempt is made to detect photocapture. The significance of the null result is discussed. INTRODUCTION The persistent photoconductivity (PPC) effect limits the usefulness of field effect transistors made from GaAlAs [1,2]. In this effect, photoconductivity at low temperatures remains after the light source is removed. The effect is caused by a deep level (called the DX-center) which has a barrier that prevents recapture of photoionized electrons [3,4]. However, the photoionization threshold E . is much greater than thermal depth Et of the DX-center (typically, EPt= 0.1 eV and E 1 eV). I These 1 •t t features are consistent with a phenomenological model Xn which lattice relaxation converts a conduction band resonant state to a deep level [4]. The energy difference between the thermal and optical depths represents lattice relaxation energy. An unusual feature of this class of models is that this lattice relaxation energy greatly exceeds the thermal energy; thus these models are called large lattice relaxation (LLR) models. In this paper, we theoretically and experimentally investigate another unusual feature of this model which we call photocapture (PC). In photocapture, photon absorption causes a transition of free electrons in the conduction band to the resonant state which then becomes a bound state by undergoing large lattice relaxation. This mechanism is the reverse of the photoionization (PI) mechanism which creates the free electrons by emptying the DX-centers. In this paper, the photoionization and photocapture mechanisms are discussed, the cross-sections for both are estimated, and an experimental attempt to detect photocapture is also described. The null results suggest that more refined attempts should be made to measure this feature of the LLR class of models. Several microscopic models have been proposed to explain the PPC effect. In the original DX-center model, a pair consisting of a donor (D) and an unknown impurity (X) underwent large lattice relaxation; the unknown defect was suggested to be a vacancy [4]. However, this model was disputed by an experiment on GaAs:Si in which pressure was used to induce the PPC effect [5]. These experiments suggesting that Si alone causes the DX-center have inspired new microscopic models. In one type of model, the substitutional donor itself undergoes large la
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