Semiconductor-to-metal transition in atomic layer deposition (ALD) of VO2 films using VCl4 and water

• Published in: Applied physics letters Vol. 118; no. 26
• Main Authors: Ganesan, Jeya Prakash, Dev, Durjoy, Krishnaprasad, Adithi, Feit, Corbin, Moser, Daniel, Kanjolia, Ravindra K., Roy, Tania, Banerjee, Parag
• Format: Journal Article
• Language: English
• Published: Melville American Institute of Physics 28-06-2021
• Subjects: Applied physics; Atomic layer epitaxy; Chlorine; Electrical measurement; Ellipsometry; Gold; High resistance; Low resistance; Oxygen consumption; Photoelectrons; Refractivity; Temperature; Temperature dependence; Titanium; Vanadium dioxide
• ISSN: 0003-6951; 1077-3118
• DOI: 10.1063/5.0053566

The semiconductor-to-metal transition of vanadium dioxide (VO2) films is studied using temperature-dependent Raman, optical, and electrical measurements. The VO2 films are deposited via an atomic layer deposition (ALD) process using alternate pulses of vanadium tetrachloride and H2O at 350 °C. A growth rate of 0.021 nm/cycle and a thickness of 33 nm of VO2 are obtained for all films studied. The phase of the film is determined using x-ray diffraction. The as-deposited films are amorphous and are transformed to the monoclinic phase with a post-deposition, forming gas anneal at temperatures ≥ 500 °C for 60 min. The purity of the films is determined using x-ray photoelectron spectroscopy and no evidence of residual chlorine is detected. The temperature-dependent Raman Ag mode of the monoclinic VO2 phase is observed to monotonically decrease from 25 °C to 78 °C; where no evidence of the Ag peak is observed in the film beyond 68 °C. The refractive index and extinction coefficient extracted from temperature-dependent ellipsometry confirm that, beyond 68 °C, free carriers are generated in the film. Electrical measurements performed on a fabricated p++Si/VO2/Ti/Au device show a semiconductor-to-metal transition behavior with a high resistance of 14701 ± 2284 Ω at 62 °C and a low resistance of 1064.1 ± 143 Ω at 67 °C. This work demonstrates that a halide-based ALD process provides a clean and robust approach to synthesizing high-quality VO2 films.