Thickness-dependent phase transformation in nanoindented germanium thin films

Abstract

We investigate the mechanical response of 50–600 nm epitaxial Ge filmson a Si substrate using nanoindentation with a nominally spherical(R≈4.3 µm) diamond tip. The inelastic deformation mechanism is found to depend critically on thefilm thickness. Sub-100 nm Ge films deform by pressure-induced phase transformation,whereas thicker films deform only by shear-induced dislocation slip and twinning.Nanoindentation fracture response is similarly dependent on film thickness. Elastic stressmodelling shows that differing stress modes vary in their spatial distribution, andconsequently the film thickness governs the stress state in the film, in conjunction with theradius of the nanoindenter tip. This opens the prospect of tailoring the contact response ofGe and related materials in thin film form by varying film thickness and indenterradius.