Jiming Bian

Thickness-modulated metal–insulator transition of VO2 film grown on sapphire substrate by MBE

VO2 films with precisely controlled thickness on the nanoscale ranging from 15 to 60xa0nm were grown on single crystal sapphire substrates by molecular beam epitaxy. X-ray diffraction and atomic force microscopy measurements indicated that high quality single phase VO2 films with condense and smooth surface and free of cracks could be achieved only when the film was thicker than 30xa0nm. The temperature-dependent resistance measurement indicated a drastic modification of metal–insulator transition (MIT) properties which was achieved through the variation of film thickness, especially the transition magnitude and curve abruptness. The corresponding mechanism was supposed to be associated with the tensile stress relaxation effect with increasing thickness caused by thermal mismatch within VO2 films, as demonstrated by Raman spectra. Our present finding provides an effective and convenient alternative to modulate the MIT properties of VO2 films.

Controllable end shape modification of ZnO nano-arrays/rods by a simple wet chemical etching technique

The well-aligned ZnO nano-arrays/rods synthesized by a chemical bath deposition method on a highly conductive Si substrate were chemically etched in an ammonia chloride aqueous solution. An obvious end shape modification of ZnO nano-arrays/rods was realized in this report. The hexagonal frustum end of ZnO nano-arrays/rods changed into a pyramid and the diameter of ZnO nano-arrays/rods decreased gradually with the increasing etching time. The evolution mechanism of the wet etching process was discussed based on a proposed evolution model. Photoluminescence measurements indicated that the near band edge emissions of ZnO nano-arrays/rods increased greatly after wet etching. The controllable end shape modification of ZnO nano-arrays/rods on a highly conductive Si substrate by this simple wet etching technique will further explore the application of ZnO in field emission devices and 1D based nano-devices with various end shapes.

Deposition and properties of highly c-oriented of InN films on sapphire substrates with ECR-plasma-enhanced MOCVD

InN films with highly c-axis preferred orientation were deposited on sapphire substrate by low-temperature electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition (ECR-PEMOCVD). Trimethyl indium (TMIn) and N2 were applied as precursors of In and N, respectively. The quality of as-grown InN films were systematically investigated as a function of TMIn fluxes by means of reflection high-energy electron diffraction (RHEED), X-ray diffraction analysis (XRD), and atomic force microscopy (AFM). The results show that the dense and uniform InN films with highly c-axis preferred orientation are successfully achieved on sapphire substrates under optimized TMIn flux of 0.8 ml·min−1. The InN films reported here will provide various opportunities for the development of high efficiency and high-performance semiconductor devices based on InN material.

Growth of high c-orientated crystalline GaN films on amorphous Cu/glass substrates with low-temperature ECR-PEMOCVD

A low temperature growth method based on electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition system (ECR-PEMOCVD) was proposed for the growth of gallium nitride (GaN) films on ordinary soda-lime glass substrates with sputtered Cu as intermediate layer (Cu/glass substrates). The influence of deposition temperature on the properties of the GaN films on Cu/glass substrates was systematically investigated by means of In-situ reflection high energy electron diffraction, X-ray diffraction, atomic force microscopy and photoluminescence spectra. With this method, high c-orientated crystalline GaN films with relatively smooth surface were achieved on amorphous Cu/glass substrate at an extremely low temperature of ~400xa0°C. The successfully growth of crystalline GaN films on amorphous Cu/glass substrates show great potential for significant improvements in the scalability and cost of GaN based devices, since the adverse effects with high temperature process for glass substrates can be effectively suppressed by this technique.

Sunlight-induced resistance changes and their effects on the semiconductor–metal transition behavior of VO2 film

High-quality VO2 films with precisely controlled thicknesses were grown on sapphire substrates by plasma-assisted oxide molecular beam epitaxy (MBE). To evaluate the degradation of semiconductor–metal transition (SMT) behavior of VO2 films under solar radiation, the temperature-driven SMT was investigated by measuring the electrical resistance during heating and cooling processes under solar simulator AM1.5, which provided illumination approximately matching the natural sunlight. The distinct reversible SMTs were observed for all the samples, whereas a remarkably conflicting trend in resistance change for extremely thin and thick samples was observed after exposure to the sunlight soaking system. The corresponding mechanism was proposed based on sunlight-induced resistance changes due to the transformation in the electron correlation and structural symmetry. The results might be especially attractive for some specific applications of VO2 films where solar radiation was inevitable.

Deposition and characteristics of GaN films on Ni metal substrate by ECR-PEMOCVD

Gallium nitride (GaN) films were deposited on Ni metal substrate using electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition system. With this approach, highly c-oriented GaN films with smooth surface were obtained at an extremely low temperature of ~480xa0°C. The trimethyl gallium (TMGa) flux dependent structural, morphological, and optical characteristics of GaN films were investigated by X-ray diffraction analysis, reflection high energy electron diffraction, atomic force microscopy and photoluminescence analysis. The results indicate that it is feasible to deposit GaN films on Ni metal substrate under the proper deposition procedures. The high quality GaN films with high c-axis orientation and strong ultraviolet emission peak are successfully achieved under the optimized TMGa flux of 1.2xa0sccm. The GaN/Ni structure has great potential for the development of high power devices with excellent heat dissipation.

ZnO films on transferable and low thermal resistance graphite substrate grown by ultrasonic spray pyrolysis

ZnO thin films were deposited on graphite substrates by ultrasonic spray pyrolysis method with Zn(CH3COO)2·2H2O aqueous solution as precursor. The crystalline structure, morphology, and optical properties of the as-grown ZnO films were investigated systematically as a function of deposition temperature and growth time. Near-band edge ultraviolet (UV) emission was observed in room temperature photoluminescence spectra for the optimized samples, yet the usually observed defect related deep level emissions were nearly undetectable, indicating that high optical quality ZnO thin films could be achieved via this ultrasonic spray pyrolysis method. Considering the features of transferable and low thermal resistance of the graphite substrates, the achievement will be of special interest for the development of high-power semiconductor devices with sufficient power durability.