Density functional theory calculations are performed to explore the electronic and transport properties of monolayer α-GeTe under uniaxial strain. It is found that monolayer α-GeTe has an indirect band gap of 1.75 eV and exhibits worthwhile anisotropy along with high electron mobility. The electron mobilities reach 1974 cm 2 • V −1 • s −1 and 1442 cm 2 • V −1 • s −1 along the zigzag and armchair directions, respectively. When uniaxial strain is applied, our results show an appreciable strain sensitivity of electron mobility. The electron mobility dramatically increases by an order of magnitude around a special strain due to the shifts of conduction band minimum. In addition, we also construct a double gate tunneling field effect transistor (TFET) with a channel of monolayer α-GeTe. The steeper sub-threshold swing and higher ON/OFF ratio are observed by applying tensile strain to the channel. As a result, it indicates that the appropriate strain can significantly improve the performance of α-GeTe TFETs.

GeSe has recently emerged as a photovoltaic absorber material due to its attractive optical and electrical properties as well as earth abundancy and low toxicity. However, the efficiency of GeSe thin-film solar cells (TFSCs) is still low compared to the Shockley-Queisser limit. Point defects are believed to play important roles in the electrical and optical properties of GeSe thin films. Here, we perform first-principles calculations to study the defect characteristics of GeSe. Our results demonstrate that no matter under the Ge-rich or Se-rich condition, the Fermi level is always located near the valence band edge, leading to the p-type conductivity of undoped samples. Under Se-rich condition, the Ge vacancy (VGe) has the lowest formation energy, with a (0/2–) charge-state transition level at 0.22 eV above the valence band edge. The high density (above 1017 cm-3) and shallow level of VGe imply that it is the p-type origin of GeSe. Under Se-rich growth condition, Sei has a low formation energy in the neutral state, but it does not introduce any defect level in the band gap, suggesting that it neither contributes to electrical conductivity nor induces non-radiative recombination. In addition, Gei introduces a deep charge-state transition level, making it a possible recombination center. Therefore, we propose that the Se-rich condition should be adopted to fabricate high-efficiency GeSe solar cells.