Liang-Wen Ji

Ultraviolet photodetectors based on selectively grown ZnO nanorod arrays

Metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors with ZnO nanorods (NRs) have been fabricated and characterized in this investigation. The NR arrays were selectively grown on the gap of interdigitated electrodes by chemical solution method through a photolithography process. Compared to a traditional ZnO MSM photodetector with no NRs, the fabricated NR UV photodetector showed much higher photoresponsity. As a result, it can be attributed to high surface-to-volume ratio of ZnO NRs and such a high photoresponse could strongly depend on oxygen adsorption/desorption process in the presence of trap states at the NR surface.

Buckling characterization of vertical ZnO nanowires using nanoindentation

Nanomechanical characterization of vertical well-aligned single-crystal ZnO nanowires on ZnO:Ga/glass templates was performed by nanoindentation technique. The buckling loads were found to be 1465 and 215μN for the ZnO nanowires of 100 and 30nm diameters, respectively. Furthermore, the buckling energies for the ZnO nanowires of 100 and 30nm diameters were 3.62×10−10 and 3.69×10−11J, respectively. Based on the Euler buckling model, Young’s modulus of the individual ZnO nanowire has been derived from two possible modes in this work.

Growth of nanoscale InGaN self-assembled quantum dots

It has been demonstrated that we can use interrupted growth mode in metalorganic chemical vapor deposition (MOCVD) to fabricate nanoscale InGaN self-assembed quantum dots (QDs). With a 12-s growth interruption, we successfully formed InGaN QDs with a typical lateral size of 25 nm and an average height of 4.1 nm. The QDs density is about 2 x 10(10) cm(-2). In contrast, much larger InGaN QDs were obtained without growth interruption. Compared with samples prepared without growth interrupt, a much larger photoluminescence (PL) intensity and a large 67meV PL blue shift was observed from samples prepared with growth interrupt. These results suggest such a growth interrupt method is potentially useful in nitride-based optoelectronic devices grown by MOCVD

Nanoscale mechanical characteristics of vertical ZnO nanowires grown on ZnO:Ga/glass templates

The mechanical properties of vertical single-crystal ZnO nanowires on ZnO:Ga/glass templates were characterized by nanoindentation experiments in this work. The results from x-ray diffraction and Raman spectra show good crystal quality for the ZnO nanowires. The buckling loads were found to be 1465 and 215 μN for ZnO nanowires of 100 and 30 nm diameters, respectively. When the fixed‐fixed column mode was used, it was found that the Young’s modulus values of the ZnO nanowires of 100 and 30 nm diameters were 117 and 232 GPa, while the critical buckling strains were 0.62% and 0.35%, respectively. On the other hand, when we employed the fixed‐pinned column mode, it can be seen that the Young’s modulus values were 229 and 454 GPa, while the critical buckling strains were 0.32% and 0.18%, respectively. Buckling behaviour of the ZnO nanowires was significantly predicted by the Euler buckling model in this work.

Characterization of ZnO metal–semiconductor–metal ultraviolet photodiodes with palladium contact electrodes

ZnO metal–semiconductor–metal (MSM) photodiodes with palladium (Pd) contact electrodes were fabricated. It was found that the barrier height at the Pd/ZnO interface was 0.701 eV. With an incident wavelength of 370 nm and 1 V applied bias, it was found that the maximum responsivity of the Pd/ZnO/Pd MSM photodiodes was 0.051 A/W, which corresponds to a quantum efficiency of 11.4%. For a given bandwidth of 100 Hz and 1 V applied bias, we found that the noise equivalent power and the corresponding detectivity D ∗ were 1.13 × 10 −12 W and 6.25 × 10 11 cmHz 0.5 W −1 , respectively.

ZnO Schottky diodes with iridium contact electrodes

Schottky diodes with iridium (Ir) contact electrodes on ZnO films were fabricated and characterized in this work. The Schottky barrier height between Ir and ZnO was determined to be 0.824 ± 0.04%, 0.837 ± 0.04% and 0.924 ± 0.04% eV by the thermionic emission model, the Norde model and capacitance–voltage measurement, respectively. It was also found that the ideality factor of the fabricated ZnO-based Schottky diode was 1.68.

Buckling instabilities in GaN nanotubes under uniaxial compression

We report experimental observations of shell buckling instabilities in free-standing, vertically aligned GaN nanotubes subjected to uniaxial compression. Highly uniform arrays of the GaN nanotubes standing on a GaN template were fabricated and subjected to uniaxial compression using a nanoindenter. The buckling load was found to be of the order of 150 microN for the GaN nanotubes with an outer radius of 40 nm, an inner radius of 20 nm, and heights of 500 and 300 nm. Good agreement was found between the experimental observations, the stress-strain relation equation study findings and the predictions from the cylindrical shell buckling theory.

InGaN/GaN blue light-emitting diodes with self-assembled quantum dots

InGaN/GaN blue light-emitting diodes (LEDs) with multiple quantum dot (MQD) active layers were successfully fabricated by using an interrupted growth method in metal-organic chemical vapour deposition (MOCVD). We have successfully formed nanoscale QDs embedded in quantum wells with a typical 3 nm height and 10 nm lateral dimension. It was found that a huge 68.4 meV blue shift in electroluminescence (EL) peak position as the injection current is increased from 3 to 50 mA for the MQD LED. The large EL blue shift reveals that deep localization of exitons (or carriers) originates from QDs will strengthen the band-filling effect as the injection current increases.

A novel method to realize InGaN self-assembled quantum dots by metalorganic chemical vapor deposition

An in situ bending-plate method is used to measure intrinsic stress during the growth process of nanocrystalline-diamond film. Nanometer-sized diamond film is prepared by continuous H+ ion bombardment under different energies induced by applying a negative-bias voltage at the substrate relative to the grounded-vacuum chamber using the microwave-plasma, chemical-vapor-deposition (MWPCVD) method. The effects of substrate-bias voltage, temperature, total pressure, and CH4 concentration on intrinsic stress during the film-growth process are investigated. The results indicate that high bias voltage and high substrate temperature are beneficial when synthesizing smooth nanocrystalline-diamond film. This film usually relates to high intrinsic compressive stress. This stress can be attributed to the high secondary-nucleation rate and grain-boundary density. The evolution of intrinsic stress presents a complicated process at different pressures and CH4 concentrations. The compressive stress of the film can be controlled by modifying the grain size at various deposition parameters

Physical Behavior of Nanoporous Anodic Alumina Using Nanoindentation and Microhardness Tests

In this paper, the mechanical response and deformation behavior of anodic aluminum oxide (AAO) were investigated using experimental nanoindentation and Vickers hardness tests. The results showed the contact angle for the nanoporous AAO specimen was 105° and the specimen exhibited hydrophobic behavior. The hardness and the fracture strength of AAO were discussed and a three-dimensional finite element model (FEM) was also conducted to understand the nanoindentation-induced mechanism.