Low-Temperature Photoluminescence of Mocvd GaAs Grown Directly on Si
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LOW-TEMPERATURE PHOTOLUMINESCENCE OF MOCVD GaAs GROWN DIRECTLY ON Si 5
B. A. WILSON , CARL E. BONNER', T. D. HARRIS*, M. G. LAMONT*, R. C. MILLER*, S. K. SPUTZ*, S. M. VERNON"*, V. E. HAVEN", R. M. LUMt, AND J. K. KLINGERTt * AT&T Bell Laboratories, Murray Hill, NJ 07974 * Spire Corporation, Bedford, MA 01730
t AT&T Bell Laboratories, Holmdel, NJ 07733
ABSTRACT We present a systematic study of the low-temperature photoluminescence from undoped GaAs layers grown directly on Si substrates by MOCVD. GaAs layers from 100A to 4 pm in thickness were deposited on Si substrates prepared with a variety of doping levels and orientations. The emission from thicker samples is dominated by pairs of lines in the band-edge region. Photoluminescence excitation measurements show that this multiplicity results from two regions in the material with different levels of strain. The stress-induced splitting of the valence band is also studied using excitation spectroscopy. In thinner samples we observe strong emission in the midgap range due to stoichiometric defects. The nature of the defects near the interface depends strongly on the character of the substrate. INTRODUCTION The possibility of growing high quality III-V materials directly on Si substrates has attracted increasing interest over recent years [1-91. This interest stems both from the advantages Si offers as a stronger, cheaper substrate with higher thermal conductivity, as well as from the desire to integrate high speed and optically active III-V devices with standard Si technology. Although a number of groups [5-91 have applied optical techniques to characterize the material quality of heteroepitaxial GaAs layers on Si, to date there has been no extensive examination of the effects on the optical properties resulting from variations in the growth parameters. In this paper we present a systematic study of the effects of substrate character and layer thickness on the photoluminescence (PL) characteristics of GaAs layers deposited by MOCVD on Si. We have also carried out a more detailed examination of the PL and photoluminescence excitation (PLE) spectra in the band-edge region, providing a more definitive identification of the source of the PL bands. SAMPLES AND EXPERIMENTAL TECHNIQUES We have measured the photoluminescence (PL) and photoluminescence excitation (PLE) spectra of a variety of GaAs samples deposited by MOCVD on Si substrates. In order to study the effects of substrate character and layer thickness, one set of samples was prepared at Spire Corporation in a vertical reactor described previously [101 with 6 different thicknesses, each deposited on 4 different substrates. The thinnest samples consisted only of a -100A, nucleation layer deposited at 450'C after the substrates were annealed in H 2 for 10 mins at 1000"C to desorb any surface oxide. For the thicker samples, an additional layer of -250A, 0.2 pm, 0.5 ;pm, 1.5 ptm or 4 prm was grown at 650 C, at a growth rate of 3.6 pm/h. One set of samples was deposited directly on (100) n-type Si. Two other sets wer
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