THERMAL-MISMATCH-STRAIN RELAXATION IN EPITAXIAL CAF2, BAF2/CAF2, AND PBSE/BAF2/CAF2 LAYERS ON SI(111) AFTER MANY TEMPERATURE CYCLES

The thermal mismatch strain in stacks containing PbSe, BaF2, and/or CaF2 on Si(111) substrates is relieved by the glide of dislocations in the principal [110]{100} glide system. The strain in the BaF2(111) and CaF2(111) buffer layers is relaxed at room temperature regardless of whether they form the top layer in the stack or are overgrown by other layers. PbSe (as well as Pb1-xSnxSe and PbTe) top layers are capable of relieving the strain induced by the thermal-expansion mismatch even at 77 K, and after many temperature cycles between room temperature and 77 K. Even after 1400 such cycles, plastic relaxation still occurs on each cycle. The x-ray rocking curves, typically 150-190 are sec wide, do not broaden on cycling. The total cumulative plastic deformation of the layer corresponds to as much as 400%. After the first few thermal cycles, no new thermal-strain-relieving dislocations are created, but the existing ones move back and forth on the same atomic glide planes with each cycle.