Impact of impurities on the electrical conduction of anisotropic two-dimensional materials

Anisotropic two-dimensional materials possess intrinsic angle-dependent physical properties that originate from their low crystal symmetry. Yet, how these properties are affected by external impurities or structural defects in the material is still wholly unclear. Here, we address this question by investigating the electrical transport in the anisotropic layered model system germanium arsenide. First, we show that the ratio of conductivities along the armchair and zigzag crystallographic directions exhibits an intriguing dependence with respect to both temperature and carrier density. Then, by using a conceptually simple model, we demonstrate that this unexpected behavior is directly related to the presence of impurity-induced localized states in the band gap that introduce isotropic hopping conduction. The presence of this conduction mechanism in addition to the intrinsic band conduction significantly influences the anisotropic electrical properties of the material, especially at room temperature, i.e., at application-relevant conditions.