Deep Level Luminescence Measurements of MBE CdTe Growth Quality and Processing
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DEEP LEVEL LUMINESCENCE MEASUREMENTS OF MBE CdTe GROWTH QUALITY AND PROCESSING J. L. SHAW*, L. J. BRILLSON**, S. SIVANANTHAN***, and J. P. FAURIE*** *Naval Research Laboratory, Washington D. C. 20375 **Xerox Webster Research Laboratory, Webster, NY 14580 ***Microphysics Laboratory, Physics dept., Univ,_rsity of Illinois at Chicago, Chicago, IL 60680
ABSTRACT We used photoluminescence spectroscopy to monitor CdTe in-situ during surface processing in order to optimize the surface electronic quality. Spectra of optimally-prepared (100) films grown by molecular beam epitaxy reveal low intensity emission from deep states relative to typical bulk-grown material.
INTRODUCTION CdTe is used as a substrate for epitaxial growth of II-VI compounds as well as an active part of various electronic devices [1]. The characteristics of such devices are often sensitive to the surface and interface properties of CdTe as well as the the bulk properties. Photoluminescence (PL) is widely used as a characterization tool for CdTe, and is sensitive to the presence of a variety of mid-gap as well as shallow states, which may be local to the CdTe surface [2,31. The presence of deep states at the surface can increase the recombination rate, alter the surface band bending, and influence the Schottky barrier height, all of which may influence the PL spectrum [2,4]. Thus to ensure reliable comparison of bulk-related features, the surface condition must be controlled. High quality (110) CdTe surfaces may be obtained by cleaving the crystals in ultra high vacuum (UHV) [2-41. However, characterization of epitaxially-grown material or orientations other than (110) require am alternative preparation method. Here we report the use of PL to optimize (100) CdTe surfaces prepared by chemical etching followed by thermal desorption in UHV [5].
EXPERIMENTAL DETAILS We grew CdTe films from CdTe sources on (100), (111), and (211) CdTe substrates (GalTech) in an Riber 2300 MBE machine. In some cases a Te source as well as a CdTe source was used. Substrate preparation involved etching in 0.05% Br:methanol (to remove -0.4 jim/min), rinsing in methanol, blowing dry with N2, and transferring to UHV; all under N2 atmosphere. This process leaves excess Te on the surface but no contamination. Heating to the growth temperature (250-335 C) in UHV desorbs the excess Te. The finished films were stored in air for several weeks, then re-etched in the same way prior to analysis in a second UHV chamber. PL spectra were measured with the films held at 80 K. The focused beam of either a HeCd (442 nm, extinction length 100 nm) or HeNe laser (extinction length 200 nm) provided excitation. Only the blue HeCd laser was used for the spectra reported here. Luminescence in the range 0.41.6 eV was dispersed by flint glass or CaF2 prisms and phase-detected with Ge or InSb photodiodes. The response function of the detection system was deconvolved from the spectra. Auger electron spectra (AES) were mt asured at room temperature with a normal incidence, 2 kV, 0.5 mA, 1 um electron beam
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