Novel device functionalities enabled by substitutional doping against native propensity in 2D semiconductors

Summary form only given. Over the last few decades, advances in electronics have fundamentally changed the way we use energy in our everyday life. While the electronic devices have become faster, smarter and compact, the energy consumption for per bit operation have not reduced significantly. With the motivation to reduce energy consumption in electronic systems, we study layered transition metal dichalcogenides (TMDs), which have attracted tremendous attention in recent years as the key semiconducting component for novel energy-efficient electrical, optoelectronic and spintronic devices. Many of such devices require bipolar conduction and tunable carrier density. However, due to omnipresent native defects and band offset, normally only a single type of doping is stable for a specific TMD. In this work, we experimentally demonstrate stable, degenerate p-type conduction in molybdenum disulfide (MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) substitutionally doped with niobium (Nb), which is against the native n-type behavior of not intentionally doped MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . This chemical doping leads to a degenerate hole density exceeding 3 × 1019 cm-3 as verified by Hall-effect measurements. X-ray absorption spectra support the substitutional nature of the p-type doping. When a vertically stacked p-n junction was formed by overlaying the Nb-doped MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> flake onto unintentionally n-doped pristine MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , it can accommodate high current densities across the homojunction, and offer superior tuneability of the junction current. Interestingly, the degree of current rectification is tuneable by modulating the density of free carriers in the bottom n-type layer with electrostatic fields from a back gate. This unique, tuneable rectification of junction current is fully reversible with respect to variation of gate bias, indicating the operation stability and endurability attributed to the stable substitutional doping and atomically flat interfaces. While our study demonstrates stable p-type doping in MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , it not only reveals an effective way to tailor electrical properties of two-dimensional semiconductors far beyond native doping propensity, but also sheds light on many-body interactions in the two-dimensional limit.