Vanadium oxide (VO2) exhibits a very interesting semiconductor to metal transition as the crystal structure changes from tetragonal or rutile to monoclinic upon cooling close to 68°C. The characteristics of this transition are very interesting scientifically and are of immense technological importance due to a variety of sensor- and memory-type applications. We have processed high-quality films of VO2 by pulsed laser deposition, which were grown epitaxially on (0001) sapphire substrate via domain matching epitaxy, involving matching of integral multiples of lattice planes between the film of monoclinic structure and the sapphire substrate. These films exhibit a sharp transition near 68°C, large amplitude, and very small hysteresis, similar to bulk single crystal of VO2. The sharpness and amplitude of the transition and the hysteresis upon heating and cooling are found to be a strong function of crystal structure and microstructure (grain size, characteristics of grain boundaries, and defect content). Here, we propose a model to establish microstructure-property correlations in crystalline and amorphous phases of VO2, which can be used to rationalize our experimental observations as well as those available in the literature. Based upon this model, we predict specific microstructures leading to properties needed for various sensor- and memory-type devices.

In this paper, we report on the metal-semiconductor transition behavior observed in transparent and conducting ZnO:Ga films grown by pulsed-laser deposition. The electrical resistivity measurements were carried out on ZnO films with varying Ga concentration in the temperature range of 14to300K. The electrical properties were correlated with film structure, and detailed structural characterization was performed using x-ray diffraction, transmission electron microscopy, and x-ray photoelectron spectroscopy. The room-temperature resistivity of these films was found to decrease with Ga concentration up to 5% Ga, and then increase. The lowest value of resistivity (1.4×10−4Ωcm) was found at 5% Ga. Temperature dependent resistivity measurements showed a metal-semiconductor transition, which is rationalized by localization of degenerate electrons. A linear variation of conductivity with T below the transition temperature suggests that the degenerate electrons are in a weak-localization regime. It was also found that the transition temperature is dependent on the Ga concentration and is related to the increase in disorder induced by dopant addition. The results of this research help to understand the additional effects of dopant addition on transport characteristics of transparent conducting oxides (TCOs) and are critical to further improvement and optimization of TCO properties.

This letter reports the metallic conductivity in Ga:ZnO system at room temperature and a metal-semiconductor transition (MST) behavior at low temperatures. Zn0.95Ga0.05O films, deposited by pulsed laser deposition in the pressure range of ∼10−2Torr of oxygen, were found to be crystalline and exhibited degeneracy at room temperature with the electrical resistivity close to 1.4×10−4Ωcm and transmittance >80% in the visible region. Temperature dependent resistivity measurements of these highly conducting and transparent films also showed, for the first time, a MST at ∼170K. Mechanisms responsible for these observations are discussed in the terms of dopant addition and its effect on ionization efficiency of oxygen vacancies.

We report room temperature ferromagnetism in single crystal Ce1_ _ A .CoA .0 2__< 5 (xssO.OS) films deposited on a LaAlC^OOl) substrate. Films were grown by a pulsed laser deposition technique and were thoroughly characterized using x-ray diffraction, high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy and scanning transmission electron microscopy-Z contrast, x-ray photoelectron spectroscopy, optical transmission spectroscopy, and magnetic measurements. These films are transparent in the visible regime and exhibit a very high Curie temperature ~740-875 K with a giant magnetic moment. Our results indicate that the ferromagnetic property is intrinsic to the C e02 system and is not a result of any secondary magnetic phase or cluster formation.

We report the growth of epitaxial molybdenum oxide (MoOx,2&lt;x&lt;2.75) films on c plane of sapphire substrate using pulsed laser deposition in oxygen environment. The structure was characterized using x-ray diffraction, high resolution transmission electron microscopy and x-ray photoelectron spectroscopy (XPS). Electrical resistivity and optical properties were investigated using four-point-probe resistivity measurements and spectroscopy techniques, respectively. It was found that the film had a monoclinic structure based on MoO2 phase and showed an unusual combination of high conductivity and high transmittance in the visible region after annealing. The unusual combination of these properties was realized by systematically controlling the relative fraction of different oxidation states of molybdenum, namely Mo4+, Mo5+, and Mo6+ in the monoclinic phase. For a film 60nm thick and annealed at 250°C for 1h, the ratio of Mo6+∕(Mo4++Mo5+) was determined to be ∼2.9∕1 using XPS, and a typical value of transmittance was ∼65% and resistivity close to 1×10−3Ωcm. These results demonstrate growth of epitaxial MoOx films with tunable electrical and optical properties. Further optimization of these properties is expected to result in applications related to display panels, solar cells, chromogenic (photochromic, electrochromic, gasochromic) devices, and transparent conducting oxides. Our ability to grow epitaxial MoOx films can further aid their integration with optoelectronic and photonic devices.