Formation of Radiative Binding States for the Pairs Between Acceptors in Heavily Acceptor-Doped Gaas
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FORMATION OF RADIATIVE BINDING STATES FOR THE HEAVILY ACCEPTOR-DOPED GaAs.
PAIRS
BETWEEN ACCEPTORS
IN
NOBUKAZU OHNISHI*, NUNOSUKE MAKITA, HAJIME SHIBATA, ABOUBAI(--ER.C.BEYE,AKIMASA YAMADA and MASAHIKO MORI. Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi,305 Japan *Institute of Fundamental Analysis, 3-24-3 Yoyogi, Shibuya-ku,151 Japan ABSTRACT We carried out the photoluminescence measurements at low temperature for Be-doped GaAs. Samples were grown by molecular beam epitaxy and Be concentration was varied by controlling the cell temperature of Be. Besides the well-defined emission, [g-g], which is observed for the acceptor con16 3 8 3 centration [A] from - 5x10 cm- to - lxl01 cm- and which shows steep red shift with increasing [A], we observed two unusual emissions temporarily denoted by [g-g]a and [g-g]f . These emissions were commonly observed just below bound exciton emissions for heavily-doped region of [A]>1x1018 cm- 3 . [g-g]a indicated a strong blue shift with increasing [A] and its energy can become larger than the direct band-gap energy of undoped GaAs at 2K when [A] exceeds 5x1019 cm- 3 . On the other hand, [g-g]fl was observed in the vicinity of the band-to-acceptor emission and showed red shift with increasing [A]. We here proposed a model that these two emissions are attributed to the binding states of acceptor-acceptor pair formed by the overlapping of the wave functions in the ground state of an isolated acceptor. This situation is principally different from that of [g-g] in which the formation of acceptor-acceptor pair was ascribed to the overlapping of the wave functions of 2P excited state. It was revealed that the theoretical estimation made through the simplified calculation agreed qualitatively well with the dependence of [g-g]fl on [A]. However this calculation presented unsatisfactory agreement with the features of [g-g]a and it was suggested that another physical conjecture such as a formation of GaAs-Be alloy should be proposed. INTRODUCTION Systematic optical investigation of impurity doped GaAs for wide range of impurity concentration is necessary to fabricate the versatile high quality optoelectronic devices. Recently we carried out the investigation of acceptor-doped GaAs for various species of acceptor (Mg,C,Zn,Be and Ge)[1-6] for wide range of acceptor concentration, [A] by the photoluminescence (PL) at low temperature. It was shown that some acceptor related emissions were masked by the unintentionally incorporated donor impurities and that these emissions can be observed only when the residual impurity concentration is extremely small. At [A] greater than Ix1017 cm- 3 , an emission denoted by [g-g] appears just below bound exciton emissions, B.E.. This emission shows a strong energy shift towards the lower energy side with increasing [A]. This red shift seems to be locked at a critical [A] ,which is more distinctly observed for heavy acceptor impurity (Cd and Zn) incorporated GaAs by means of ion-implantation (optical self-compensation effect)[4]. [g-g] is suggested to
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