EL2 Related Anomalous Splitting in the Photoreflectance Response of Semi-Insulating GaAs
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EL2 RELATED ANOMALOUS SPLITTING IN THE PHOTOREFLECTANCE RESPONSE OF SEMI-INSULATING GaAs
C. Durbin, J. Estrera and R. Glosser Center for Applied Optics and Physics Program, University of Texas at Dallas, Richardson, TX 75083
Walter Duncan Central Research Laboratory, Texas Instruments Incorporated, Dallas,
TX 75265 R.L. Henry, P. Nordquist, N. Bottka and D.K. Gaskill, Naval Research Laboratory, Washington, DC 20375
ABSTRACT Anomalous splitting has been observed in the photoreflectance (PR) response of SI:GaAs in the vicinity of the exciton at 78 K. Recent photolUminescence (PL) measurements suggest the splitting is correlated with the EL2 content of the samples. Separation between the two peaks in PR measurements range from about 2 to 4 meV. A striking effect is that each peak is maximized by a different phase setting of the lock-in. The splitting is sample dependent and is also affected by several other factors including surface conditions, temperature, pump beam intensity and modulation frequency.
Photoreflectance
(PR) is an important
characterization
technique for the study of
semiconductors [1,2]. In this paper we describe splitting near the exciton for semi-insulating (SI) GaAs samples at low temperatures (80 K to 120 K). At 80 K, the high energy peak appears at an energy of 1.504 eV, the band gap energy. Tile energy separation between the two peaks ranges from 2 to 4 meV. An unusual feature of the splitting is that each peak is characterized by a different phase setting on the lock-in. The variation in phase, which is sample dependent, ranges from about 5 to as much as 140 degrees. The photoreflectance response was found to be temperature, pump beam intensity, and modulation frequency dependent. As any of the above parameters are increased, the amplitude of the low energy peak increases while the amplitude of the high energy peak decreases to the point where it is no longer visible in the PR results. Mat. Res. Soc. Symp. Proc. Vol. 262. 01992 Materials Research Society
290
This does not mean there is no contribution to the PR signal from the high energy transition, but rather, the existence of two distinct structures is no longer apparent. Despite the fact that the high energy peak is no longer visible, the variation in phase remains. In an effort to correlate the PR response to properties of the material, photoluminescence (PL) measurements were made. The PL response in samples which exhibit PR splitting exhibit low energy structure in the vicinity of 0.6 to 0.8 eV which differs significantly from the PL response of non-splitting PR samples, as will be discussed later. A total of nineteen samples obtained from several sources were studied. All of the samples were bulk SI:GaAs, grown by several techniques including liquid encapsulated Czochralski (LEC) and vertical zone melt (VZM). All of the samples were undoped except one In doped and one Cr doped. Measured EL2 concentrations ranged from low 1015 to low 1016 cm"3 . Sample resistivity was in the range of 107 to 10' ohm-cm. Samples were etched in
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