Lead Chalcogenide on Silicon Infrared Focal Plane Arrays for Thermal Imaging(Review Paper)

Narrow gap IV-VI [lead chalcogenides like Pbl-xSnxSe and PbTe] layers grown epitaxially on silicon( III) substrates by molecular beam epitaxy exhibit high quality despite the large lattice and thermal expansion mismatch. A CaF2 buffer layer is employed for compatibility. Due to easy glide of misfit dislocations in the IV- VI layers, thei1rtal strains relax even at cryogenic. temperatures and after many temperature cyclings. The high permittivities of the IV- VI layers effectively shield the electric fields from charged defects. Higher quality devices are obtained from lower quality material, at variance to narrow gap 11- VI and 111- V compounds. Material characterisation and sensor array properties have been reviewed. Schottky barrier or p-n+ sensor arrays have been delineated using standard photolithography. At low temperatures, the sensitivities are limited by defects, mainly dislocations, and the device performance is predicted by the dislocation density. At higher temperatures, the ultimate theoretical sensitivity is obtained with Schottky barrier devices despite large mismatch and with only 3 µm thickness of the layers. First chara'cterisations of a 96 x 128 array on a silicon substrate containing the read-out circuits show that the concept is functional and gives high yield.