Defect Engineering and Doping of 2D Transition Metal Dichalcogenides: A First-Principles Study

Abstract

The foundation of the electronic industry is based on n-type and p-type semiconductor materials, which is obtained by the ubiquitously present point defects. The presence of defects affects the electronic and optoelectronic properties in a semiconductor material ranging from providing free carriers to acting as undesirable nonradiative carrier trap centres. According to Moore’s law, the dimensions of field-effect transistors (FETs) continue to decrease, however, it cannot be scaled down infinitely. The limit of the channel length of the Si-based FET device is predicted to be 5 nanometers under which short channel effects (SCE) occur. Therefore, modern electronics need the channel materials to be ultra-thin, and hence low-dimensional materials like two dimensional (2D) materials are required. Recently, 2D transition metal dichalcogenides are found to be a promising candidate due to their excellent nano (opto)-electronics applications. Among the TMDs, monolayer (ML) MoS2 and WS2 are front runners and are found to be the promising candidates, due to their direct band gap (∼ 1.9 eV of MoS2, ∼ 2.1 eV of WS2) and good electrical mobility (200 cm2V−1s−1 for MoS2, 486 cm2V−1s−1 for WS2) at room temperature. MoS2 (WS2) based field-effect transistors (FETs) exhibit a high current on/off ratio of ∼ 108 (∼ 107). Moreover, these materials also show high optical absorption, which makes them promising for optoelectronic applications. However, the origin of their unintentional n-type conductivity is completely unknown. On the other hand, a cation vacancy in MoS2 and WS2 acts as deep acceptor defect, which makes the device development even more challenging. Tuning these deep defect levels to shallow remains an open challenge and must be overcome to develop the electronic devices based on these materials. Layered TMDs have attracted a great deal of attention in nano-electronics with remarkable features such as layer-dependent tunable band-gap and layer-dependent electrical conductivity. ML MoS2 and WS2 are found to be direct band gap semiconductors in contrast to their bulk phase that has an indirect band gap. ML MoS2 and WS2 based FET device exhibit good electrical mobility compared to its bulk phase, which show very low mobility in the range of ∼ 0.3−0.5 cm2V−1s−1. The origin for this drastic change in mobility from single-layer to bulk is not entirely clear and still is an open question. In emerging stretchable electronics, a major challenge is the choice of stretchable channel semiconducting material, which is reasonable for high-performance FETs on a polymer substrate. In this regard, ML MoS2 display excellent mechanical stretchability with high strength and found to be a promising stretchable active channel material for a transistor. However, the theoretical or experimental investigation of stretchability of MoS2 is completely unexplored and hence need the analysis. Therefore, to enhance the diverse functionality of these 2D materials, a deep understanding of defect physics is required. In this regard, we have studied the defect engineering and doping strategy of these layered materials using density functional theory (DFT) based calculations.