Photoluminescence of ZnSe Epilayers on GaAs Under Hydrostatic Pressure
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PHOTOLUMINESCENCE OF ZnSe EPILAYERS ON GaAs UNDER HYDROSTATIC PRESSURE
JUDAH A. TUCHMAN, ZHIFENG SUI, IRVING P. HERMAN, R. L. GUNSHOR,* L. A. KOLODZIEJSKI,** D. A. CAMMACK,*** and M. SHONE*** Department of Applied Physics, Columbia University, New York, NY 10027 *School of Electrical Engineering, Purdue University, West Lafayette, IN 47907 **Department of Electrical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 ***North American Philips Corporation, Briarcliff Manor, NY 10510
ABSTRACT
The near bandgap photoluminescence of ZnSe epilayers grown on GaAs substrates is measured for pressures up to -25 kbar using a diamond anvil cell at T = 9 K. The bandgap changes with pressure, dE/dp, for pseudomorphic and nonpseudomorphic films are obtained, and are compared with results for bulk crystalline ZnSe.
INTRODUCTION
Commensurate growth of ZnSe on GaAs has been achieved for thicknesses up to -0.15 pm [1-3]. Elastic strain due to the lattice mismatch (0.25% at room temperature) is accommodated until the critical thickness is obtained. For larger thicknesses misfit dislocations lower the total energy of the system and incommensurate growth ensues. Previous studies have demonstrated the relation of this strain to epilayer thickness, with its concomitant effect on the density of misfit dislocations [2,4]. Moreover, photoluminescence (PL) studies have demonstrated the effect of this strain on band-edge emission [4-6]. The presence of biaxial strain in very thin films is also expected to alter the effect of applied hydrostatic pressure (p) on the band-edge photoluminescence relative to its effect on the PL from bulk material. Previous reports have determined dE/dp, the change in bandgap energy with pressure, to be in the range 6.0 - 7.5 meV/kbar for bulk ZnSe at room temperature down to 77 K [7-9]. The current study determines dE/dp for ZnSe epilayers of contrasting thickness, grown by MBE on GaAs, and compares these values with the bulk crystalline value, which is also measured here. All measurements are made at 9 K.
Mat. Res. Soc. Symp. Proc. Vol. 161.
i1990 Materials
Research Society
472
EXPERIMENTAL PROCEDURE Photoluminescence measurements were made on 0.1 and 2.1 jim-thick ZnSe epilayers grown on GaAs by MBE and on bulk ZnSe grown by zone melting. The "thin" epilayer was commensurately grown, while the "thick" layer was thicker than the critical thickness for commensurate growth. Experiments were conducted at a temperature of T = 9 K and at pressures up to -25 kbar. The GaAs substrate was initially -300 gim thick, and by mechanical polishing. was thinned down to -50 mim High pressure measurements were made in a gasketed Merrill-Bassett diamond anvil cell (DAC) [10,11], contained within a closed cycle refrigerator (9 K). The ZnSe sample was loaded with ruby chips in a liquid argon bath within the DAC to obtain nearhydrostatic pressure conditions at low temperature [12]. ZnSe and ruby photoluminescence were excited using the 4067 A line from a krypton ion laser, and were detected using a 0.8
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