Tunable uniaxial vs biaxial in-plane strain using compliant substrates

In this letter, the relaxation of strained rectangular islands on compliant substrates is used to achieve semiconductor thin films with either uniaxial stress or uniaxial strain in the plane of the film over an area of tens of microns. The work is demonstrated using silicon and silicon-germanium alloy single-crystal thin films, with uniaxial strain values approaching 1%. The biaxially strained SiGe or SiGeSi films on borophosphorosilicate glass (BPSG) were fabricated by a wafer bonding and layer transfer process. When the viscosity of BPSG drops at high temperatures for short times, films patterned in a rectangular shape can move laterally to relieve stress only in one in-plane direction. Thus one can tailor the strain from biaxial to uniaxial in the thin films.