Crystallization and semiconductor-metal switching behavior of thin VO2 layers grown by atomic layer deposition

Abstract Crystalline vanadium dioxide (VO 2 ) thin films were prepared by annealing amorphous VO 2 films which were deposited by atomic layer deposition on a SiO 2 substrate. A large influence of the oxygen partial pressure in the annealing ambient was observed by means of in-situ X-ray diffraction. In the range between 1 and 10 Pa of oxygen the interesting VO 2 (R) phase crystallized near 450 °C. Between 2 and 10 Pa of oxygen, metastable VO 2 (B) was observed as an intermediate crystalline phase before it transformed to VO 2 (R). Anneals in inert gas did not show any crystallization, while oxygen partial pressures above 10 Pa resulted in oxidation into the higher oxide phase V 6 O 13 . Film thickness did not have much effect on the crystallization behavior, but thinner films suffered more from agglomeration during the high-temperature crystallization on the SiO 2 substrate. Nevertheless, continuous polycrystalline VO 2 (R) films were obtained with thicknesses down to 11 nm. In the case where VO 2 (R) was formed, the semiconductor–metal transition was observed by three complementary techniques. This transition near 68 °C was characterized by X-ray diffraction, showing the transformation of the crystal structure, by spectroscopic ellipsometry, mapping optical changes, and by sheet resistance measurements, showing resistance changes larger than 2 orders of magnitude between the low-temperature semiconducting state and the high-temperature metallic state.