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 Ca F2 on Si(111) substrates is relieved by the glide of dislocations in t he principal [110]{100} glide system. The strain in the BaF2(111) and CaF2(111) buffer layers is relaxed at room temperature regardless of w hether 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 o f relieving the strain induced by the thermal-expansion mismatch even at 77 K, and after many temperature cycles between room temperature an d 77 K. Even after 1400 such cycles, plastic relaxation still occurs o n 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 therm al cycles, no new thermal-strain-relieving dislocations are created, b ut the existing ones move back and forth on the same atomic glide plan es with each cycle.