Facile Phase Control of Multivalent Vanadium Oxide Thin Films (V₂O₅ and VO₂) by Atomic Layer Deposition and Postdeposition Annealing

Abstract

Atomic layer deposition was adopted to deposit VO thin films using vanadyl tri-isopropoxide {VO[O(CH)], VTIP} and water (HO) at 135 °C. The self-limiting and purge-time-dependent growth behaviors were studied by ex situ ellipsometry to determine the saturated growth conditions for atomic-layer-deposited VO. The as-deposited films were found to be amorphous. The structural, chemical, and optical properties of the crystalline thin films with controlled phase formation were investigated after postdeposition annealing at various atmospheres and temperatures. Reducing and oxidizing atmospheres enabled the formation of pure VO and VO phases, respectively. The possible band structures of the crystalline VO and VO thin films were established. Furthermore, an electrochemical response and a voltage-induced insulator-to-metal transition in the vertical metal-vanadium oxide-metal device structure were observed for VO and VO films, respectively.

Phase identification of vanadium oxide thin films prepared by atomic layer deposition using X-ray absorption spectroscopy

“…The dominance of VO 2 (B) appears inconsistent with a recent report, 29 in which XRD data showed the existence of crystalline VO 2 (M), and its insulator-metal transition, when voltage was applied. However, XAS reects the averaged local structure regardless of the crystallinity of each microstate, whereas XRD only shows the structure the crystallites.…”

“…This is probably due to the signicant structural disorders in the asdeposited VO x , which is supported by the lack of prominent peaks in the XRD data. 29 Additionally, the intensities of the rst peaks in the O K-edge region, which is related to the V 3d (t 2g ) states, are weaker than those for the O 2 -annealed (Fig. 2b) or FGannealed sample (Fig.…”

“…This manifests the possibility of tuning the composition of ALD VO x via the PDA process. 29 To scrutinize the chemistry and local structures of the VO x , the spectra taken at two different temperatures (RT and 87 C) are displayed in Fig. 2 and 3.…”

“…More details of the growth process are described elsewhere. 29 The as-deposited lms were then annealed at 500 C for 1 h in an FG (95% N 2 + 5% H 2 ) or O 2 atmosphere in a box furnace (Hantech, C-A14P, Korea).…”

“…4,26,28 In an earlier work we reported the transformation of ALDgrown amorphous VO x lms into crystalline VO 2 and V 2 O 5 by annealing in forming gas (FG; 95% N 2 + 5% H 2 ) and O 2 , respectively, and their electrochemical and metal-insulator transition properties were investigated. 29 However, a detailed investigation of the local atomic structure was not undertaken to evaluate the formation of vanadium oxides with phase purity and appropriate stoichiometry. Structural characterizations of the ALD VO x lms in previous reports used conventional X-ray diffraction (XRD).…”