Sputtering of High Quality Layered MoS2 films

Sammanfattning: We have deposited bulk, monolayer and few-layers as well as large-scale 2D layered MoS2 thin films by pulsed DC magnetron sputtering from an MoS2 target. MoS2 has gained great attention lately, together with other layered Transition Metal Dichalcogenides (TMDCs), for its unique optical and electrical properties with thickness-dependent bandgap. MoS2 also transitions from an indirect to a direct bandgap when thinned down to monolayer. This is intriguing in the fabrication of novel solar cells and photodetectors. Sputter-deposition has the advantage of producing large-scale, high-quality films, which is paramount for layered MoS2 to be applicable on an industrial level. The quality in terms of crystallinity and c⊥-texture of sputtered bulk MoS2 was evaluated as a function of several deposition process parameters: process pressure, substrate temperature and H2S-to-Ar ratio. X-ray Diffraction (XRD) results revealed that the high substrate temperature of 700 °C together with reactive H2S process gas improved the quality regardless of pressure. However, the quality was slightly improved further with increasing pressure up to 50 mTorr. We also found that the quality improved with increasing temperature up to 700 °C using pure Ar as the process gas. Rutherford Backscattering Spectrometry (RBS) analysis showed that with the addition of H2S the stoichiometry of MoSx improved from MoS1.78 using pure Ar to fully stoichiometric MoS2.01 at 40% H2S in the H2S/Ar mixture. Cross-sectional Transmission Electron Microscopy (TEM) imaging revealed the high-quality 2D layered structure of the MoS2 films and a maximum thickness of 5 nm of c⊥-growth MoS2 before the onset of the undesirable c∥-growth. These results provide a solution with respect to the ongoing challenge of obtaining high quality and good stoichiometry of sputtered TMDC films at elevated temperatures. Formation of monolayer and few-layers MoS2 was confirmed by Raman and Photoluminescence (PL) spectroscopy. The peak separation of the E12g and A1g Raman-active modes for MoS2 monolayer was measured to 19.3 cm-1 on SiO2/Si, increases substantially in the transition to bilayer MoS2 and exhibits bulk values from four layers MoS2 and above. This result serves as a good indicator of monolayer as well as few-layers MoS2 formation. The monolayer film exhibits a strong photoluminescence peak at 1.88 eV owing to its direct optical bandgap, as compared to the indirect one of bilayer and thicker films. X-ray Photoelectron Spectroscopy (XPS) spectra of the monolayer MoSx film indicate successful sulfurization of the molybdenum atoms and absence of residual sulfur. XPS also showed ideal stoichiometric MoS2.03 ± 0.03 of the monolayer film. Furthermore, a uniform MoS2 monolayer was successfully grown on a 4" SiO2/Si wafer, demonstrating the large-scale uniformity that can be achieved by sputter-deposition, making it highly applicable on an industrial level.