Nonequilibrium VLS-grown stable ST12-Ge thin film on Si substrate: a study on strain-induced band engineering

The current work describes a novel method of growing thin films of stable crystalline ST12-Ge, a high pressure polymorph of Ge, on Si substrate by a nonequilibrium VLS technique. The study explores the scheme of band engineering of ST12-Ge by inducing process stress into it as a function of the growth temperature and film thickness. In the present work, ST12-Ge films are grown at 180–250 °C to obtain thicknesses of ~4.5–7.5 nm, which possess extremely good thermal stability up to a temperature of ~350 °C. Micro-Raman study shows the stress induced in such ST12-Ge films to be compressive in nature and vary in the range of ~0.5–7.5 GPa. The measured direct band gap is observed to vary within 0.688 eV to 0.711 eV for such stresses, and four indirect band gaps are obtained to be 0.583 eV, 0.614–0.628 eV, 0.622–0.63 eV and 0.623–0.632 eV, accordingly. The corresponding band structures for unstrained and strained ST12-Ge are calculated by performing DFT simulation, which shows that a compressive stress transforms the fundamental band gap at M-Γ valley from ‘indirect’ to ‘direct’ one. Henceforth, the possible route of strain-induced band engineering in ST12-Ge is explored by analyzing all the transitions in strained and unstrained band structures along with substantiation of the experimental results and theoretical calculations. The investigation shows that unstrained ST12-Ge is a natural n-type semiconductor which transforms into p-type upon incorporation of a compressive stress of ~5 GPa, with the in-plane electron effective mass components at M-Γ band edge to be ~0.09m e . Therefore, such band engineered ST12-Ge exhibits superior mobility along with its thermal stability and compatibility with Si, which can have potential applications to develop high-speed MOS devices for advanced CMOS technology.