Epitaxial films, heterostructures and composites of A, B and M phases of VO2

Abstract

Transition metal oxides exhibit various polymorphic structures, among which many are neither stable in ambient conditions nor can be easily synthesized. Integration of these metastable phases on Si substrates promises novel device functionalities. Prime among them is metal insulator transition based functionality using transition metal oxides such as VO2(M). VO2 exhibits two other layered polymorphs which are promising materials to study strong electronic correlations resulting from structure [VO2(A)] or their use as electrode materials for batteries [VO2(B)]. However, growing single crystal thin films of these novel metastable phases have remained a challenge. I demonstrate for the first time that high quality single phase films of VO2(A, B, and M) can be grown on Si substrate by controlling the vanadium arrival rate (laser frequency) and oxidation of the V atoms. Single phase monoclinic VO2(M), tetragonal VO2(A) and monoclinic VO2(B) thin films were grown on (100) SrTiO3 (STO) and (100) STO (28 nm) buffered Si substrates using PLD. A phase diagram has been developed (oxygen pressure versus laser frequency) for various phases of VO2. A detailed structural analysis, coupling X-ray diffraction and transmission electron microscopy, revealed a [011]VO2(M)||[100]STO, [110]VO2(A)||[100]STO, [001]VO2(B)||[100]STO epitaxial relationship and the presence of 90? oriented domains for VO2(A) and VO2(B) thin films respectively. The transport measurement showed that B is semi-metallic, A is insulating while M is semiconducting which was corroborated by the HAXPES measurements. Furthermore, the presence of the V-V dimers (present in all phases with varying amounts) probed by Raman and infrared spectroscopic measurements in the three polymorphs underscores the importance of dimerization that strongly influences the electronic properties of VO2. Considering the R/M system, orbital band diagram and relative position of different bands for the VO2(A) and VO2(B) with respect to VO2(M) are proposed. In order to corroborate our model a deep study on the behavior of these two polymorphs grown on STO and STO-Si substrate, in term of structural behavior as well as electronic transport behavior is performed. I present a detailed study on composite films of VO2(A) and VO2(B) phases and show that these composite films exhibits a metal insulator transition similar to the VO2(M/R) phase transition. However, extensive TEM and temperature dependent XRD studies reveal that the film is mainly comprised of VO2(A) and VO2(B) phases and very little of M phase. The A phase is under compressive stress while the B phase is under tensile stress and we believe this stress leads to the dimer induced metal insulator transition in this system presumably triggered by the small amount of M phase present. This raises the question ?Is a structural phase transition necessary for the metal to insulator transition (MIT) in VO2(M)?? I report the study on a coherently coupled interfaces of ZnO/VO2(M) in a heterostructure form to study the effect of strain exerted due to the structural phase transition of VO2(M) on the over-layer. This strain induced defects in the over layer (ZnO) was monitored by measuring the photoluminescence from ZnO which exhibited a temperature dependent hysteresis similar to the hysteresis in transport exhibited by the VO2 layer below. Considering the strong potential application in devices of the two polymorphs VO2(A and B), I report on the electronic properties of the junctions formed in VO2(A)/ Nb-SrTiO3 and VO2(B)/ Nb-SrTiO3. Both the junctions showed rectifying behavior while temperature dependent I-V and 1/C2-V behaviors confirmed that for VO2(B)/ Nb-SrTiO3 rectified junction, the surface electronic structure of VO2(B) is distinct from that of the interface of the film to substrate and does not undergo the transition seen in bulk.