Properties of IV-VI Narrow Gap Semiconductors on Fluoride Covered Silicon
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PROPERTIES OF IV-VI NARROW GAP SEMICONDUCTORS ON FLUORIDE COVERED SILICON
H. Zogg*, C. Maissen', S. Blunier', J. Masek, V. Meyer'- R.E. Pixley'
AFIF (Arbeitsgemeinschaft fur industrielle Forschung) at Swiss Federal Institute of Technology, ETH-Honggerberg, CH-8093 ZOrich, Switzerland "Physik-Institut, University of Zurich, CH-8001 ZOrich ABSTRACT
We present new results on structural and electronic properties of epitaxial lead chalcogenides on Si-substrates. A stacked MBE-grown CaF2 BaF 2 buffer of 200 nm thickness serves to overcome the large lattice- and thermal expansion mismatches. Lead chalcogenide layers are grown by MBE or HWE with thicknesses of a few Jim. The X-ray rocking curve widths of these layers are below 200 arc sec. They are as low as curve widths of similar layers grown on bulk BaF2, or GaAs layers of comparable thicknesses on Si. The mechanical strain-state of the layers was determined with x-ray measurements and RBS channelling angular scans. Strain is below 4.10-4 at room temperature, indicating a near complete relaxation of the thermal and lattice misfit induced strains. The quality of the layers is sufficient to integrate whole photovoltaic IR-sensor arrays. We have fabricated linear arrays with cut-off wavelengths of 12 gIm by using Pb.1 ,SnSe, 5 gm with PbTe, and 3 - 4 gm with PbS and Pb,.,EuSe. The structures withstand repeated cooling to the 80K operation temperature. INTRODUCTION
IV-VI narrow gap lead chalcogenides with the cubic rocksalt structure are employed as infrared sensors [1] and laser diodes [2]. While polycrystalline photoconductive sensors were developed over 40 years ago for a few binary materials, single crystal photovoltaic sensors can be fabricated with ternary materials. Their cut-off wavelengths can be chemically tuned from below 3 gm up to above 15 lam by using different chemical compositions (see table 1). Optimal carrier concentrations for photovoltaic sensors lie in the 1017 cm- range, which is easily obtained by slight deviations from exact stoichiometry. Contrary to narrow gap MCT (Hg,.Cd.Te), the band gap of the binary end compositions of lead chalcogenides corresponds to cut-off wavelengths which lie already in the infrared range. This leads to a much smaller dependence of the cut-off on the chemical composition x, i.e. uniformity problems are much less severe. For example, to maintain the same spread in cut-off wavelength of an array with 12.4 g±m cut-off (0.1 eV bandgap), the required compositional homogeneity is 5 times smaller for Pb,. Sn.Se than for Hg,.1 Cd.Te. In addition, MBE of IV-VI materials is simple, in routine use for laser applications [2], and it does not involve difficult to handle Hg. While epitaxy of IV-VI materials on alkali halide substrates has always led to considerable mosaicity, high quality epitaxial films result when deposited on BaF 2(111) surfaces, and photovoltaic IR-devices can be fabricated in such films [1]. The reasons why epitaxy works so well on BaF 2 despite an up to 5% lattice mismatch are not well understood. Although th
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