Semiconductor-metal transition characteristics of VO2 thin films grown on c- and r-sapphire substrates

Abstract

We have made a comparative study of epitaxial growth of VO2 thin films on c-cut (0001) and r-cut (11¯02) sapphire substrates, and the semiconductor to metal transition (SMT) characteristics of these films have been correlated with their structural details. On c-sapphire, VO2 grows epitaxially in (002) orientation. These (002) oriented VO2 films have 60° twin boundaries due to three equivalent in-plane orientations. The epitaxial VO2 films on r-sapphire consisted of two orientations, namely (200) and (2¯11). The coexistence of these two orientations of VO2 has been explained on the basis of similarity of atomic arrangements in (200) and (2¯11) planes. The thermal hysteresis (ΔH), sharpness of the transition (ΔT), and the transition temperature for VO2 films on c-sapphire were found to be 4.8, 8.5, and 72.6 °C, respectively, which were higher than the corresponding values of 3.3, 5.4, and 60.3 °C for films on r-sapphire. The SMT temperature for VO2 films on c-sapphire was close to the bulk value of 68.0 °C. The significant decrease in transition temperature to 60.3 °C for VO2 films on r-sapphire has been attributed to the compressive strain along [002] direction of VO2.

Resistance switching of epitaxial VO2/Al2O3 heterostructure at room temperature induced by organic liquids

“…From this (020) peak, we calculated the out-of-plane lattice constant to be 4.52 Å. This value equals that of bulk VO 2 , 32 indicating the epitaxial strain was fully relaxed. Using the rocking curve of the (020) peak (inset of Fig.…”

Textured metastable VO2 (B) thin films on SrTiO3 substrates with significantly enhanced conductivity

“…11 It is obvious that the peak at 42 can be indexed as VO 2 (M) (002) or (020). 8,10 The interesting lattice matching relation between M-phase VO 2 and sapphire has been previously reported to be the dominating mechanism for the growth of VO 2 (M) on sapphire. 8 However, similar deposition conditions produce completely two different VO 2 phases, suggesting lattice matching between the film and the underlying substrate could be the main driving force for the formation of VO 2 films with different structures.…”

Near bulk semiconductor to metal transition in epitaxial VO2 thin films

“…Since the tensile/compressive strain along VO 2 ͓001͔ leads to a higher/ lower SMT temperature, complete relaxation of strain along c-axis in VO 2 films deposited on epitaxial NiO buffer causes SMT to occur at ϳ341 K which is the SMT temperature for bulk VO 2 single crystals. 6,10,13 Also, for large misfit systems such as VO 2 / NiO ͑planar misfit= 25%͒, the critical thickness is less than 1 to 2 monolayers, so dislocations corresponding to full lattice relaxation are generated at or within 1 monolayer of the interface; and the remainder of the film can be grown virtually strain and misfit dislocation-free leading to better SMT characteristics. 11 In conclusion, we have used epitaxial NiO template to grow fully relaxed VO 2 thin films on c-sapphire substrates.…”