Thermal-mismatch-strain relaxation in epitaxial CaF2, BaF2/CaF2, and PbSe/BaF2/CaF2 layers on Si(111) after many temperature cycles.

Abstract

The thermal mismatch strain in stacks containing PbSe, ${\mathrm{BaF}}_{2}$, and/or ${\mathrm{CaF}}_{2}$ on Si(111) substrates is relieved by the glide of dislocations in the principal 〈110〉{100} glide system. The strain in the ${\mathrm{BaF}}_{2}$(111) and ${\mathrm{CaF}}_{2}$(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 ${\mathrm{Pb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sn}}_{\mathit{x}}$Se 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 arc 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.