Reassessment of Acceptor Passivation Models in p-Type Hydrogenated GaAs
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REASSESSMENT OF ACCEPTOR PASSIVATION MODELS IN p-TYPE HYDROGENATED GaAs I. SZAFRANEK AND G.E. STILLMAN Center for Compound Semiconductor Microelectronics, Materials Research Laboratory and Coordinated Science Laboratory, University of Illinois at Urbana-Champaign, IL 61801 ABSTRACT The existing microscopic models of acceptor passivation in p-type hydrogenated GaAs are reviewed in light of new experimental results concerning the relative thermodynamic stability of the passivating complexes. In particular, the present model for neutralization of Group II acceptors, Be, Mg and Zn, on Ga sites is shown to be inadequate to account for the observed trends, which imply existence of a strong interaction between the hydrogen and acceptor. It is proposed that a direct acceptor-hydrogen bond is formed due to attractive Coulomb interaction between the ionized species. The relative stability of the pair complex can be then explained based on electronegativity of the acceptor species. Passivation at intermediate pair separations up to about twice the Bohr radius of the nearest acceptor, is also discussed. INTRODUCTION It is a well established fact that the electrical and optical activity of shallow substitutional acceptors in p-type GaAs can be neutralized by exposure to hydrogen plasma.'. 2 By analogy2 with silicon it is assumed that interstitial hydrogen atoms form a deep donor level in GaAs. Thus, in p-type material, in the extrinsic temperature regime compensation takes place, and this effect is sufficient to explain the observed reduction in carrier concentration upon hydrogenation. However, in order to account for phenomena such as mobility enhancement 3 or changes in photoluminescence (PL) 4 and infrared local vibrational mode (IR LVM) 5 -8 spectra after hydrogenation, the concept of passivation is used to imply formation of neutral acceptorhydrogen complexes (Acc-H). 9 In GaAs two types of acceptors are distinguished: Group II elements Be, Mg, Zn and Cd on Ga sites, and Group IV elements C, Si and Ge on As7sites. Accordingly, models have been proposed for the two possible passivating configurations.1, The underlying mechanism, invariant with the acceptor type, is assumed to be Coulomb-field enhanced diffusion of protons toward the ionized acceptor sites. Any proposal for the microscopic structure of the passivating complexes should be consistent with the experimentally established relative extent and thermodynamic stability of passivation of different acceptor species. However, no such data has been available so far. Recently, we have reported the first results on this aspect of hydrogenation,1 0 based on4PL investigation of passivation efficiency and susceptibility to light-induced reactivation (LIR) of different acceptors in high-purity p-type GaAs. The observed stability trends do not fully comply with predictions of the existing model for passivation of Group [Il]Ga acceptors.1 In this paper the experimental results are reviewed and the discrepancies with the model predictions analyzed. A modified passivation mechani
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