Multivalley Electron Conduction at the Indirect-Direct Crossover Point in Highly Tensile-Strained Germanium

As forward-looking electron devices increasingly adopt high-mobility low-band-gap materials, such as germanium (Ge), questions remain regarding the feasibility of strain engineering in low-band-gap systems. Particularly, the Ge L-T valley separation (∼150 meV) can be overcome by introducing a high degree of tensile strain (ε ≥ 1.5%). It is therefore essential to understand the nature of highly strained Ge transport, wherein multivalley electron conduction becomes a possibility. Here, we report on the competitiveness between L- and T-valley transport in highly tensile-strained (ε ∼ 1.6%) Ge/In_0.24Ga_0.76As heterostructures. Temperature-dependent magnetotransport analysis reveals two contributing carrier populations, identified as lower- and higher-mobility L- and T-valley electrons (in Ge), using temperature-dependent Boltzmann transport modeling. Coupling this interpretation with electron-cyclotron-resonance studies, the effective mass (m*) of the higher-mobility T-valley electrons is probed, revealing m* = (0.049 ± 0.007)m_e. Moreover, a comparison of empirical and theoretical m* indicates that these electrons reside primarily in the first-two quantum sublevels of the Ge T valley. Consequently, our results provide an insight into the strain-dependent carrier dynamics of Ge, offering alternative pathways toward efficacious strain engineering.