Anomalous Luminescence Properties of GaAs grown by Molecular Beam Epitaxy
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ANOMALOUS LUMINESCENCE PROPERTIES OF GaAs GROWN BY MOLECULAR BEAM EPITAXY I. SZAFRANEK, M.A. PLANO, M.J. MCCOLLUM, S.L. JACKSON, S.A. STOCKMAN, K.Y. CHENG 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 A shallow acceptor-like defect labeled "A" is frequently incorporated in molecular beam epitaxial GaAs. We report here anomalous photoluminescence effects that are induced by this defect. With increasing concentration of the "A" defect: (1) neutral and ionized donor-bound 3 4 exciton peaks disappear almost completely even for donor concentration as high as 7x101 cmand compensation ratio ND/NA-0.3; (2) a new, sharp line emerges at 1.5138 eV, and (3) the relative intensity and line shape of the free exciton transition change dramatically. These observations are discussed in the perspective of previous reports, where similar effects were, in our opinion, misinterpreted. INTRODUCTION GaAs layers grown by molecular beam epitaxy (MBE) often exhibit unique photoluminescence (PL) features associated with unidentified shallow defects. The best known are the series of lines in the 1.504-1.511 eV range due to defect-bound excitons (d,X), first reported by Kiinzel and Ploog1 and denoted hereafter as K-P peaks. Recently, we have found that the g(d,X) peak at the high energy limit of the K-P series originates in an exciton bound to the shallowest known acceptor-like defect in GaAs, labeled "A". 2 The activation energy of this defect is -24.8 meV, about 1.7 meV less than that of CAs acceptors. In the present paper further investigation of the "A" defect is reported. We have characterized a set of MBE grown GaAs samples in which the relative PL intensities of the "A" defect-induced transitions, g(d,X) at 1.5112 eV and the free electron-to-"A" defect A(e,AO) at 1.4946 eV, varied over a wide range, allowing for study of anomalous effects induced by this defect in the exciton recombination luminescence. Our observations are important, because they demonstrate that if some commonly used methods of semiquantitative assessment of semiconductors with PL spectroscopy are used indiscriminately, they may provide an entirely misleading indication of purity and compensation in GaAs grown by MBE. EXPERIMENTAL Nominally undoped, p-type layers were grown in Phi 430P MBE system using solid Ga and As sources. A Phi As cracker with a rhenium baffle was operated at a current of 4 A (no temperature calibration has been available). Approximately 10 .tm thick layers were grown under the As-stabilized (2x4) surface reconstruction conditions on semi-insulating, undoped liquid-encapsulated Czochralski GaAs substrates oriented 20 off the (100) orientation. The samples were characterized electrically with van der Pauw Hall-effect measurements at a magnetic field of 0.66 T. The ohmic contacts on these p-type samples were formed with alloyed In-Zn spheres. Concentrations of electrically active donor and accept
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