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Narrow-gap IV-VI lead chalcogenide layers grown on Si substrates with the aid of an intermediate CaF,/BaF, buffer relax the thermal-mismatch-induced strain by misfit dislocations which glide on the primary (100) glide planes inclined 54.7' with respect to the (111) surface. Macroscopic strain measurements show near complete mismatch strain relaxation down to cryogenic temperatures even after multiple cycling for binary PbSe, while ternary PbSe,-,Te, (x = 0.4) strains elastically below room temperature. The morphology of the resulting dislocation steps is revealed by scanning tunnelling microscopy for the IV-VIS and atomic force microscopy for the fluoride buffer, and the number of dislocation step lines agrees with the calculated thermal strain relaxation. FZ 1 pm h-l, and substrate temperatures are in the range 300 to 400°C. X-ray rocking curve linewidths are N 180 arcsec for the 2-3 pm thick layers used [2, 51.s337

MBE growth of epitaxial IV-VI lead salt layers on Si (111) substrates and fabrication of photovoltaic infrared devices in the layers is reviewed. IV-Vi on Si IR sensors have potential as a low-cost technique of fabrication of large IR focal plane arrays for both t h e 3-5 pm and 8-12 pm ranges because of t h e easy fabrication procedure and because uniformity problems are much less severe in IV-VIS due to the weaker composition dependence of the bandgap compared with Cd, -,Hg,Te. Sensor arrays are fabricated in 2-4 pn thick PbTe. PbS, -,Se, and Pb, _.Eu,Se for 3 4 p m and in Pb,-,%,Se for 8-12 pm cut-off. An intermediate epitaxial stacked 0.2 pm thick CaF,BaF, bilayer serves for compatibility and helps to overcome the large lattice and thermal expansion mismatch between the Si substrate and the IV-VI layer. Perfectly smooth surfaces with surface defect concentrations down to 103cm-'. and x-ray rocking-curve linewidth of ~1 5 0 arcsec are obtained. Sensor arrays with 66 and 256 elements are described, the latter having been grown on standard Si chips with Ai metallization.

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MBE growth and infrared device fabrication with epitaxial lV-VI layers on Si-substrates is reviewed and some new results are included. Epitaxy is achieved using a stacked BaF2/CaF2 or CaF2 buffer layer. While photolithographic delineation techniques are somewhat difficult with BaF2 (whiôh is soluble in water), reliable wet-etching techniques are easy with the CaF2 buffer. Photovoltaic IV-VI sensors on Si(1 1 1 ) substrates are fabricated with cut-off wavelengths covering the whole atmospheric 3-5 and 8-1 4 im window. They offer the possibility for low cost infrared focal plane arrays with sensitivities similar to MCT, but with much less demanding material processing steps. This is because the structural quality of even heavily lattice mismatched lV-Vl layers suffices to fabricate devices with good sensitivities, and because the bandgap in Pb1SnSe for the 8-12 jim range depends much less on composition x than for the corresponding Hg1CdTe. A 13 mm long linear array with 10.5 jim cutoff wavelength has inhomogeneities in cut-off below 0.1 .tm. Some arrays were grown on prefabricated active Sisubstrates containing the whole read-out circuits. Process temperatures were below 450°C because of the Almetallization. First thermal images using these chips are demonstrated.The induced mechanical strain due to the different thermal expansions relaxes down to cryogenic temperatures even after many temperature cycles. This is due to dislocation glide in the main {100}-glide planes even at low temperatures.