Theoretical Insights to Niobium-Doped Monolayer MoS2–Gold Contact

We report a first principles study of the electronic properties for a contact formed between Nb-doped monolayer MoS₂ and gold for different doping concentrations. We first focus on the shift of energy levels in band structure and the density of states with respect to the Fermi level for a geometrically optimized 5 × 5 MoS₂ supercell for both pristine and Nb-doped structures. The doping is achieved by substituting Mo atoms with Nb atoms at random positions. It is observed that for an experimentally reported sheet hole doping concentration of (ρ_2D) 1.8 × 10¹⁴ cm^-2, the pristine MoS₂ converts to degenerate p-type semiconductor. Next, we interface this supercell with six layers of (111) cleaved surface of gold to investigate the contact nature of MoS₂-Au system. By careful examination of projected band structure, projected density of states, effective potential and charge density difference, we demonstrate that the Schottky barrier nature observed for pure MoS₂-Au contact can be converted from n-type to p-type by efficient Nb doping.

I. Introduction

Covalently bonded monolayer MoS₂ is a potential channel material for future ultrathin body MOSFET [1]. MoS₂ crystals are shown to have n-type, nature mostly in experimentally reported transistors [2]–[6]. However, for the successful realization of CMOS circuitry, high-performance p-channel MoS₂ transistor is equally required. It is challenging to find a suitable atom for MoS₂ crystal which is competent enough to donate holes and be thermodynamically stable at the same time. Using the first principles analysis, niobium emerged as a possible choice among all the p-type dopants for Mo [7]. A p-channel transistor making use of p-type MoS₂ crystals fabricated using chemical vapor transport [8] and chemical vapor deposition [9] is attained successfully. The p-type nanoparticles made of Nb-doped MoS₂ are also synthesized [10]. Since the nature of MoS₂–metal contacts significantly affects the functioning of transistor, it is of utmost importance to probe into the theoretical insights of the electronic properties of Nb-doped MoS₂–metal contact apart from the experiments [8].