Low Pressure Chemical Vapor Deposition of 1T-TaS₂ Films on Sapphire for Controllable Charge Density Wave Phase Transitions

Metallic two-dimensional (2D) transition metal dichalcogenides such as TaS₂ can host charge density waves (CDWs), periodic modulations of electron density that arise in systems with strong electron-phonon coupling. CDWs can break crystal symmetry, mediate metal-insulator phase transitions and coexist with superconductivity. Furthermore, transitions between CDW phases can be tuned via strain, electric fields, etc. which can potentially be exploited for devices. Crystal disorder can inhibit CDWs, consequently, studies are typically carried out utilizing single crystal flakes exfoliated from bulk TMD crystals. Techniques to deposit large area, high crystal quality TaS₂ films are therefore needed to advance the field toward practical applications.<br/><br/>In this study, we report the synthesis and characterization of <i>1T</i>-TaS₂ films on c-plane sapphire using low pressure chemical vapor deposition (LPCVD). The growths were carried out at 100 Torr in a horizontal hot wall CVD reactor using TaCl₅ and sulfur powder as sources in a 5% H₂/Ar carrier gas mixture. The sulfur powder was placed upstream of the substrate in the quartz tube and was maintained at 150 °C. The TaCl₅powder was contained within a stainless steel bubbler external to the reactor that was heated to 80 °C and maintained at a pressure of 110 Torr. The TaCl₅ was transported to the reactor by passing the H₂/Ar carrier gas at a flow rate of 100 sccm through the bubbler and was introduced downstream of the sulfur boat in the reactor via a separate quartz injector tube. The effects of reactor temperature, gas flow rates and reactor pressure on the uniformity, deposition rate and properties of the films were investigated. Temperatures greater than ~800 °C and low reactor pressures (100 Torr) were required to decompose the TaCl₅ via reaction with H₂ and minimize pre-reactions upstream of the substrate. AFM, Raman spectroscopy and X-ray photoelectron spectroscopy were used to characterize the films after growth. The Raman spectrum of films grown at 900 °C and 100 Torr and cooled rapidly after growth exhibited Raman modes at ~90, ~240 and ~380 cm^-1, consistent with the <i>1T</i> phase of TaS₂. Variable temperature Raman measurements are underway to investigate CDW phase transitions in the films.