Photovoltaic IV-VI on silicon infrared devices for thermal imaging applications

Abstract

ABSTRACT Narrow gap IV-VI (e.g. Pb1SnSe and PbTe) layers grown epitaxially on Si(111)-substrates by MBE exhibit high qualitydespite the large lattice and thermal expansion mismatch. A CaF2 buffer layer is employed for compatibility. Due to easyglide of misfit dislocations in the IV-VI layers, thermal strains relax even at cryogenic temperatures and after many tern-perature cyclings. This is partly due to the NaC1-structure of the TV-VT materials and at variance to the zinkblende-typesemiconductors. In addition, the high permittivities of the IV-VIs effectively shield the electric fields from charged defects.This makes the materials rather forgiving, higher quality devices are obtained from lower quality material, again at varianceto Hg1CdTe or InSb and related compounds.We describe ways to further improve device performance by lowering the dislocation densities in the lattice mismatchedlayers. This is achieved by temperature rampings, which drive out the threading dislocations from the active parts of thesensors. Presently, densities of 1 x 106 cm2 in layers of a few micrometer thickness are obtained. These densities are suf-ficiently low in order not to dominate the leakage currents in real devices even at 77K.Photovoltaic p-n or Schottky-barrier sensor arrays are delineated by using photolithography. At low temperatures, theultimate sensitivities are presently limited by defects, mainly dislocations. At higher temperatures, the ultimate theoreticalsensitivity was obtained with Schottky barrier devices, this despite the large mismatch and only 3 urn thickness of the layers.Due to the rather low temperatures used during the MBE and delineation (below 450 °C), sensor arrays are obtained bypostprocessing even on active Si-substrates.Keywords: infrared sensors, Pb1SnSe, PbTe, narrow gap semiconductors, thermal imaging, dislocation glide