p-Doping of 2D materials by Phosphorus Ion Implantation

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are considered as next-generation materials to replace silicon. TMDCs consist of atomically thin crystal layers with dangling-bond-free surfaces that exhibit high mobility, which is advantageous over silicon that exhibits decreased mobility at nanoscale thickness. However, efficient doping processes for TMDCs are yet to be fully established. Various doping processes such as electrostatic doping, charge transfer doping, and incorporation of impurity atoms during growth have been investigated but these methods are typically challenged by the lack of precise control over the doping location, concentration, or depth. Ion implantation is a doping method that is widely used in conventional silicon-based processes and has become essential for fabricating advanced devices and integrated circuits since it offers precise control over the doping concentration and depth and is compatible with existing semiconductor fabrication processes.<br/>In this study, phosphorus ion implantation was conducted to enable the p-type doping of WSe₂, which intrinsically exhibits ambipolar transport characteristics which can be effectively tuned to either n- or p-type. WSe₂ flakes were mechanically exfoliated from a bulk WSe₂ crystal and transferred onto a sapphire substrate. 150 nm SiO₂ layer was deposited on the WSe₂ flakes on the sapphire substrate using high-density plasma chemical vapor deposition process. The SiO₂ layer was deposited to minimize defects generated by high energy ion collision and to control penetration depth of phosphorus ions in WSe₂. The optimal implantation energy to effectively introduce phosphorus dopants into the WSe₂ flakes was determined using the Monte Carlo simulation. After the ion implantation process, the samples underwent rapid thermal annealing (RTA) under an Ar gas environment to heal the possible damages induced by the ion implantation process and to ensure the stable incorporation of the dopants into the crystal lattices of the WSe₂ flakes. The RTA process was performed under temperatures ranging from 600 °C to 900 °C with 100 °C increments to investigate the effect of the annealing temperature in the activation of the phosphorous dopant in WSe₂. The effectiveness of the phosphorus ion implantation and thermal annealing process on the p-doping of WSe₂ was investigated through structural and electrical analyses. The change of crystallinity of the WSe₂ lattice structure was analyzed systematically before and after ion implantation and thermal annealing using Raman spectroscopy. Transmission electron microscopy imaging technique was also utilized before and after thermal annealing to verify the restoration of the lattice structure. Electronic devices based on the p-doped and intrinsic WSe₂ crystals were fabricated and their current-voltage characteristics were investigated to analyze the change in the transport characteristics and carrier concentration using a semiconductor parameter analyzer. The detailed process and results will be given during the presentation.<br/>This study demonstrates the effective p-type doping of WSe₂ through phosphorous ion implantation and thermal annealing, illustrating the efficacy of the widely used ion implantation technique for the TMDCs and diversifying their doping methods. This work lays the groundwork for enabling precise doping in future nanoelectronics based on TMDCs.