2, University of California, Berkeley, Santa Barbara, California, United States
Vanadium dioxide (VO2) undergoes a first order structural phase transformation at a critical temperature (Tc) of 340 K. The structure converts from a low temperature insulating (monoclinic) phase to a high temperature metallic (tetragonal) phase, along with significant changes in the electronic, optical, and thermal properties. Through the transition the electrical resistance has been known to change over 4 orders of magnitude over a narrow temperature range. The phase transition is highly dependent upon both the process parameters and the nature of the film substrate interface. Using pulsed laser deposition (PLD) we were able to optimize the deposition parameters by isolating 3 unique polymorphs of VO2, namely the, monoclinic M1 phase, triclinic T phase, and the metastable tetragonal A phase as determined by XRD. By depositing on 3 common substrates; thermally oxidized SiO2 on Si, p-type Si <100>, and c-plane sapphire (0001) we demonstrate the ability to fine tune the metal-insulator transition (MIT). HRTEM was used to determine the growth behavior of the thin films, and to determine, if any, the orientation relationship between film and substrate. An orientation relationship of <001>(010)VO2 || <100>(0001)Al2O3 was established for VO2 grown on sapphire, while a native oxide on the Si substrate, revealed by HRTEM, and the amorphous nature of thermal SiO2 prevented growth of a preferential orientation. AFM analysis of the samples revealed the island like growth behaviour of VO2 on Si and SiO2, and layer-by-layer mode on sapphire indicating significantly different stress minimization techniques were present. Interfacial strain can lead to significant increase (tensile strain) or decrease (compressive strain) in the absolute position of the MIT, while grain size will affect the onset and severity of the transition. Deposition temperature, which controlled the degree of grain growth, resulted in modulation of the MIT with respect to the grain size. The MIT observed for our sapphire system was larger and over a narrower temperature range than both the Si and SiO2 systems. The ability to controllably deposit and manipulate the MIT of VO2 thin films is of paramount importance for integration into device fabrication and application.