Reversible Semiconducting-to-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe$_{\mathrm{2}}$ and Applications for Devices

Two-dimensional (2D) semiconducting monolayer transition metal dichalcogenides (TMDCs) have stimulated lots of interest because they are direct bandgap materials that have reasonably good mobility values. However, contact between most metals and semiconducting TMDCs like 2H phase WSe$_{\mathrm{2}}$ is highly resistive, thus degrading the performance of field effect transistors (FETs) fabricated with WSe$_{\mathrm{2}}$ as active channel materials. We applied a phase engineering method to chemical vapor deposition (CVD) grown monolayer 2H-WSe$_{\mathrm{2}}$ and demonstrated semiconducting-to-metallic phase transition in atomically thin WSe$_{\mathrm{2}}$. We have also shown that metallic phase WSe$_{\mathrm{2}}$ can be converted back to semiconducting phase, demonstrating the reversibility of this phase transition. In addition, we fabricated FETs based on these CVD-grown WSe$_{\mathrm{2}}$ flakes with phase-engineered metallic 1T-WSe$_{\mathrm{2}}$ as contact regions and intact semiconducting 2H-WSe$_{\mathrm{2}}$ as active channel materials. The device performance is substantially improved with metallic phase source/drain electrodes, showing on/off current ratios of 10$^{\mathrm{7}}$ and mobilities up to 66 cm$^{\mathrm{2}}$/V\textbullet s for monolayer WSe$_{\mathrm{2}}$.