NaCl-Assisted Low-Temperature Growth of Few-Layer WSe₂ by Pulsed Laser Deposition

WSe₂, one of the transition metal dichalcogenide (TMD) materials, has been intensively interested owing to its fascinating properties, such as layer-dependent bandgap (~1.7 eV for monolayer and ~1.2 eV for bulk). Harnessing of layer-dependence could enable to tune not only the bandgap but also electrical and optical properties, making TMDs attractive. Therefore, properly controlling the number of layers of TMDs is beneficial for the design and optimization of novel devices.<br/>Recently, many studies have been reported to control the number of layers via chemical vapor deposition (CVD), which enables high-quality and large-area film growth. However, CVD essentially accompanies long-term or high-temperature processes to achieve fine control on layer count. These process conditions limit the metallization of device contacts and interconnections for the fabrication of electronic and optoelectronic devices. In addition, fabricated TMD films by metal-organic chemical vapor deposition (MOCVD) may contain unexpected carbon impurities due to residual organic precursors.<br/>On the other hand, TMDs thin films could be fabricated at relatively low growth temperature and short deposition time via pulsed laser deposition (PLD). The limited source transfer by PLD is advantageous for precise control of the number of layers. It is also easy for the fabrication of heterojunction with other materials through in situ deposition without the additional transfer process. However, it remains a major challenge to synthesize high-quality stoichiometric thin films composed of micron-sized grains by PLD.<br/>Our group successfully synthesized the high-quality WSe₂ thin films at low temperature via home-made PLD system combined with selenium gas flow system. Using our PLD system, selenium vacancies in WSe₂ thin films have been successfully compensated by control of the flow rate of selenium gas. Further, we obtained increased grain size of WSe₂ films by reducing the concentration of water and suppressing the unnecessary nucleation via the NaCl-assisted deposition process. By carrying out Raman spectral analysis and electrical measurements, we have confirmed our WSe₂ thin films show improved properties compared to those deposited in conventional PLD methods.<br/>We expect that our study will pave the way for the development of efficient TMDs synthesis methods and be applied for the fabrication of next-generation electronic devices through follow-up studies.