The Epitaxial Growth And Property Research Of RuO₂and VO₂Thin Films

Transition metal oxides with rutile structure (CrO2、TiO2、VO2、RuO2、SnO2and so on) are the hotspot of new functional materials research, because of the special physical properties. Ruthenium dioxide (RuO2) with a tetragonal rutile structure (a=b=0.4490nm, c=0.3106nm), is an attractive electrode oxide because of low room-temperature resistivity(-35μΩm), high work-function (～5.0eV), good thermal stability and high chemical corrosion resistance. It has been widely applied in thick film resistors, ferroelectric random access memory devices, dynamic random access memory (DRAM) devices, organic thin film transistors (OTFT) and so on. Moreover, besides using as a conducting layer, RuCO2is deposited as a rutile seed layer to promote the epitaxial growth of some isostructural rutile transition metal oxide films (such as rutile TiO2), which are hard to grow normally. VO2exhibits a fast and first-order metal-insulator phase transformation at a critical temperature of～68℃, accompanied by dramatic changes in crystal structure, optical and electrical properties. And it is found that, except the thermal method, the metal-insulator transition (MIT) can be triggered by a voltage or electric field and optical perturbation. Moreover, the transition temperature can be modified, even to a room temperature, due to strain in the films by choosing special substrate. With these excellent properties, VO2thin films have wide range of applications, such as two-terminal electronic switches, three-terminal (gated) electronic switch devices, electronic oscillators, micro-bolometer, smart window, laser protection, optical switches, optical storage and so on. This paper is mainly about the epitaxial growing of high quality VO2and RuO2thin films on rutile MgF2substrate and the small lattice mismatching Al2O3(1-102) substrate, and the research on structure and properties.There have been a lot of reports on conductive RuO2thin films deposited on LaAlO3、Si、MgO、YSZ and so on. Because of the big lattice mismatch between the RuO2film and the substrates, the films have polycrystalline nature or are single orientation but existing several variants in plane, which is not beneficial for epitaxial rowth of other films. Here, using the rutile MgF2and Al2O3(1-102) substrates which have good lattice match with RuO2, high quality single-crystal RuO2thin films have been epitaxially deposited by PLD. The films are highly conductive, with a room-temperature resistivity of38μΩ·cm and45μΩ·cm for MgF2(110) substrates and Al2O3(1-102) substrate respectively. And the epitaxial details have been studied by High resolution x-ray diffraction (HRXRD), High resolution transmission electron microscopy(HRTEM).Owing to the small lattice mismatch, VO2thin films have been commonly grown on rutile TiO2single crystal substrates in research. Especially, the transition temperature can reduce to a room-temperature of300K, by epitaxial strain in thin films, for ultrathin VO2films on TiO2(001) substrate. But the commercial rutile TiO2single crystal substrates arc expensive that are limited for research and application. Here, we epitaxially grow high quality VO2films on another rutile structure substrate of MgF2which are cheap and universally applied as components in optical elements. The epitaxial relationship is confirmed by X-ray diffraction. The structure from monoclinic phase to tetragonal phase by rising the temperature is presented though the Raman spectrum. The VO2films(thickness～110nm) on MgF2(001) and MgF2(110) substrates have a wonderful transition property, with the transition temperature below the value of bulk single-crystal (340K) and the change of resistance near104magnitude.We also deposited VO2hin films on Al2O3(1-102) substrate under a optimized conditions. For the VO2films on Al2O(1-102) substrate, the transition temperature were Thot≈332.6K, Tcool≈328.3K, and the change of resistance up104magnitude.In order to investigate the electrically triggered MIT (E-MIT) characteristics, we fabricate VO2/RuO2/MgF2(110) and VO2/RuO2/Al2O3(1-102) heterostructures to research the phase transition and resistance changing properties out the films plane. We found the transition temperature are smaller than those directly deposited on substrates without RuO2buffered layer. For the capacitor device of Au/VO2/RuO2/Al2O3, an abrupt current jumping is observed, induced by a static electric field. Conclusion, using the PLD method, we successfully fabricated highly conductive RuO2thin films and VO2thin films with good transition property on MgF2and R-cut sapphire substrates. The epitaxial growing relationship of rutile oxides with isostructure, including their properties, were systematically researched. These were beneficial for the exploring of application devices based on rutile oxides.