Dependence of Semiconductor to Metal Transition of VO2(011)/NiO{100}/MgO{100}/TiN{100}/Si{100} Heterostructures on Thin Film Epitaxy and Nature of Strain

We have studied semiconductor to metal transition (SMT) characteristics of VO2(011) thin films integrated epitaxially with Si(100) through NiO{001}/MgO{001}/TiN{001} buffer layers and correlated with the details of epitaxy and nature of residual stresses and strains across the VO2/NiO interface. Thin film epitaxy at both room and elevated temperatures is studied in detail by electron microscopy and in situ high-temperature X-ray diffraction techniques. The epitaxial relationship across the interface between monoclinic VO2 and NiO is determined to be (011)(VO2)||{100}(NiO) and [01 (1) over bar](VO2)||[001](NiO) at room temperature. The epitaxial alignment at the temperature of growth where tetragonal VO2 is stable is determined as: (110)(VO2)||{100}(NiO) and [001](VO2)||[100](NiO). A cube-on-cube crystallographic alignment is established across the NiO{100}/MgO{100}/TiN{100}/Si{100} interfaces. The misfit strains across the VO2/NiO interface at the growth temperature are calculated and the mechanism of strain relaxation is discussed. The out-of-plane orientation is found to be relaxed in both monoclinic and tetragonal states of VO2. It is shown that a compressive strain of 31.65% along the [001] direction of tetragonal VO2 is fully relaxed via matching of multiple domains. However, a small tensile misfit strain of about 5% along [1 (1) over bar >0] direction cannot relax and remains in the lattice. This tensile residual strain leads to a compressive strain along [001] axis which, in turn, results in an SMT temperature slightly lower than that of freestanding strain-free VO2. SMT characteristics of VO2(011) epilayers are assessed where an amplitude of near five orders of magnitude, and a hysteresis of less than 3.6 C-degrees are obtained. This study introduces VO2/NiO thin film heterostructure integrated with silicon as a promising candidate for multifunctional devices with novel characteristics where a combination of sensing, manipulation, and response functions is needed.