Strain-free, ultra-high purity ZnSe layers grown by molecular beam epitaxy
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Strain-free, ultra-high purity ZnSe layers grown by molecular beam epitaxy R. M. Park, C. M. Rouleau, and M. B. Troffer Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611
T. Koyama and T. Yodo Tsukuba Research Laboratory, Nippon Sheet Glass Co. Ltd., 5-4 Tokodai, Tsukuba City, Ibaraki Pref. 300-26, Japan (Received 19 October 1989; accepted 21 November 1989) ZnSe layers have been grown by molecular beam epitaxy on high-purity, high-quality ZnSe wafers [(100) oriented] cut from ingots grown by the iodine vapor transport method. Photoluminescence (PL) analysis indicates the homoepitaxial ZnSe material to be of ultra-high purity as well as being strain-free relative to ZnSe/(100) GaAs layers which exhibit in-plane biaxial tension. The 10 K PL spectra recorded from homoepitaxial layers exhibit unsplit free- and donor-bound exciton transitions of comparable intensity together with a strong peak at 2.7768 eV believed to be the so-called Iv transition in relaxed ZnSe. The ultra-high purity nature of the homoepitaxial layers is attributed to the high purity of the substrate material in addition to the use of high purity Zn and Se source materials in this work.
A photoluminescence (PL) characterization of ultrahigh purity ZnSe heteroepitaxial layers was recently reported by Shahzad et a/.,1 the layers being grown by molecular beam epitaxy (MBE) on GaAs using 6 nines Zn and Se obtained from Osaka Asahi Metal Manufacturing Co., Ltd. Shahzad et al. suggest that the presence of strong free-exciton related peaks in the low temperature PL spectra recorded from ZnSe layers together with a significantly strong peak, termed Iv at 2.7738 eV, is indicative of ultra-high purity ZnSe material. A thermal expansion coefficient mismatch between ZnSe and GaAs, however, results in ZnSe heteroepitaxial layers thicker than —0.8 ^tm experiencing inplane biaxial tension.2 Such tensile strain is evidenced by the observation of exciton (both free and bound) peak splitting in low temperature PL spectra.1'2 Splitting of exciton transitions results from light and heavyhole valence band splitting at k = 0 under conditions of biaxial tension. Consequently, although high purity ZnSe layers can be grown by MBE on GaAs, the heteroepitaxial approach is nonideal since the ZnSe layers remain elastically stressed at room temperature. Attempts to grow high purity, strain-free homoepitaxial ZnSe layers, however, have been hampered by the nonavailability of high purity, high quality ZnSe bulk wafers. Park et al.3 and Menda et al.4 recently reported homoepitaxial growth by MBE of ZnSe using ZnSe substrate material [(100) oriented] from EaglePicher Industries, Inc. In both cases, however, low temJ. Mater. Res., Vol. 5, No. 3, Mar 1990
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perature PL analysis indicated the layers to contain a significant concentration of impurities, as evidenced by the presence of strong donor-bound exciton peaks as well as donor-to-acceptor pair transition peaks. Park et al.3 sho
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