Electrical contact properties of 2D metal-semiconductor heterojunctions composed of different phases of NbS2 and GeS2

Metal-semiconductor heterojunction (MSJ) is the basis for developing novel devices. Here, we consider different two-dimensional van der Waals MSJs consisting of different-phase metals H- and T-NbS2 and semiconductor GeS2, and conduct an in-depth study of their structural stabilities, electronic and electrical contact properties, with an emphasis on exploring the dependence of the electrical contact properties of the MSJs on the different phases of metals. Calculation results of their binding energy, phonon spectra, AIMD simulations, and mechanical properties show that both heterojunctions are highly stable, which implies that it is possible to prepare them experimentally and feasible to use them for designing electronic devices. The intrinsic H-NbS2/GeS2 and T-NbS2/GeS2 heterojunctions form p-type Schottky contacts and quasi-n-type Ohmic contacts, respectively. It is also found that their Schottky barrier heights (SBHs) and electrical contact types can be effectively modulated by an applied electric field and biaxial strain. For example, for the H-NbS2/GeS2 heterojunction, Ohmic contact can be achieved regardless of applying a positive/negative electric field or planar biaxial compression, while for the T-NbS2/GeS2 heterojunction, Ohmic contact can be achieved only at a very low negative electric field. The planar biaxial stretching can achieve quasi-Ohmic contact. In other words, when the semiconductor GeS2 monolayer is used as the channel material of the field effect transistor and contacts different metal NbS2 monolayers to form the MSJ, the interfacial Schottky barriers are distinctly different, and each of them has its own advantages in different situations (intrinsic or physically regulated). Therefore, this study is of great significance for understanding the physical mechanism of the electrical contact behaviors for H(T)-NbS2/GeS2 heterojunction, especially for providing the theoretical reference for selecting suitable metal electrodes for the development of high-performance electronic devices.