Electron transport in Ge(Sn)metal-oxide-semiconductor field-effecttransistors at cryogenic temperatures

Germanium-tin (GeSn) alloys are promising for electronic and photonic applications due to their direct-bandgap characteristics. However, the transition from indirect- to direct-bandgap in Ge(Sn) has not been widely investigated using electronic approaches. In this Letter, electron transport in Ge(Sn) n-type metal–oxide–semiconductor field-effect transistors (n-MOSFETs) is characterized at temperatures of 300 K to 4 K. Distinct temperature dependence of mobility across all devices with varying Sn fractions is observed, which is attributed to the dual-valley electron transport in the indirect L- and direct Γ-valley. An anomalous mobility upturn between 100 and 20 K is observed exclusively in the Ge0.895Sn0.105 n-MOSFET due to electron condensation to the direct Γ-valley, suggesting that the Ge0.895Sn0.105 alloy is a direct-bandgap material.