Growth and photoluminescence characterization of ZnSe layers grown on (100)Ge by molecular beam epitaxy
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(Received 10 April 1986; accepted 25 June 1986) ZnSe layers have been grown on ( 100)-oriented Ge substrates by molecular beam epitaxy for the first time. The optimum in-situ Ge substrate preparation technique was found to consist of argon-ion sputtering at room temperature followed by annealing at -400 "C. This cleaning technique provided Ge surfaces which exhibited ( 2 X 2) surface reconstruction as observed by reflection high-energy electron diffraction and which had 0 and C contamination levels below the detection limit of the Auger electron spectroscopy system. ZnSe layers grown on argon-ion sputtered and annealed ( 100)Ge substrates using a variety of substrate temperatures and Zn to Se beam pressure ratios were characterized by 4.2 K photoluminescence ( P L ) measurements. The highest quality layers as judged by PL analysis were grown in the substrate temperature range, 310-350 "C, with a Zn to Se beam pressure ratio around unity. ZnSe layers grown under these conditions exhibited a dominant donorbound exciton peak at 2.7976 eV having a minimum linewidth of 1.1 meV.
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I. INTRODUCTION ZnSe is rapidly becoming an important material with regard to the fabrication of optoelectronic devices operating in the blue region of the visible spectrum. However, due to the unavailability of high-quality bulk ZnSe crystals, it is necessary to grow the material heteroepitaxially. ZnSe layers have been grown heteroepitaxially on substrate materials such as G ~ A s ' - and ~ Si.4 Another possible candidate substrate material for ZnSe heteroepitaxy is Ge since the lattice mismatch between ZnSe and Ge is only -0.17% and their thermal expansion coefficients (at least at room temperature) are also closely matched ( 7 10-6"C-' ~ for ZnSe and 5.8 x lo-' 'C-' for Ge). In addition, Ge is available in the form of high-quality single crystals at low cost. Moreover, device possibilities exist for ZnSe/Ge heterojunctions including high-speed photodetectors, switching devices, and image convertors (see, for example, Ref. 5). Although some early work on the growth of ZnSe on Ge by vacuum evaporation showed promise, the quality of the ZnSe layers and resultant heterojunctions was poor.' Probable reasons for this might include inadequate in-situ cleaning, poor vacuum conditions, and the use of relatively impure source material. More recently, Werthen et d.'reported the growth by metalorganic chemical vapor deposition of n-type (AIdoped) ZnSe layers on both ( 100)-and ( 111)-oriented Ge substrates. The doped ZnSe layers had carrier concentrations around 5 x 10'' cm ( ntype) while the Ge substrate material had carrier concentrations around l O " ~ m - ~( p type). Strong band gap luminescence was ~
J. Mater. Res. 1 (4), Jul/Aug 1986 http://journals.cambridge.org
reported from the layers grown on ( 100)Ge and nearideal diode behavior was observed from the ZnSe/ ( 100)Ge n-p heterojunctions, indicating the ZnSe/Ge system to have device potential. In this paper the MBE growth of unintentionally doped ZnSe layers on n-type ( 100)Ge substrates is
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