H. F. Lui

Luminescent and structural properties of ZnO nanorods prepared under different conditions

The morphology and optical properties of ZnO nanostructures prepared by thermal evaporation of Zn under different conditions was investigated. ZnO nanostructures prepared in air, dry and humid argon flow, and dry and humid nitrogen flow were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and photoluminescence. Tetrapod nanorods were obtained for fabrication in air, while for fabrication in argon or nitrogen flow nanowires and tetrapod nanorods were obtained. Growth of nanowires from the end of the tetrapod nanorod was observed. Influence of the preparation conditions on the structure and the room-temperature photoluminescence is discussed.

Magnetic properties of Mn doped ZnO tetrapod structures

ZnO tetrapod nanostructures were prepared by evaporating Zn metal under humid argon flow. After the fabrication, Mn diffusion doping was performed at two different temperatures (600 and 800 °C). The samples were characterized by scanning electron microscopy, transmission electron microscopy, x-ray fluorescence, x-ray diffraction (XRD), superconducting quantum interference device magnetometer, and photoluminescence. Diffusion doping resulted in the increase of the size of tetrapods, but no new peaks were found in XRD spectrum. Mn doped ZnO tetrapod structures were found to be ferromagnetic with Curie temperature ∼50 K, and showed large coercive field (∼3500 Oe for 800 °C sample, ∼5500 Oe for 600 °C sample).

GaN/ZnO nanorod light emitting diodes with different emission spectra

Light emitting diodes (LEDs) consisting of p-GaN epitaxial films and n-ZnO nanorods have been fabricated and characterized. The rectifying behavior and emission spectra were strongly dependent on the electronic properties of both GaN film and ZnO nanorods. Light emission under both forward and reverse bias was obtained in all cases, and emission spectra could be changed by annealing the ZnO nanorods. The emission spectra could be further tuned by using a GaN LED epiwafer as a substrate. Both forward and backward diode behavior has been observed and the emission spectra were significantly affected by both the properties of the GaN substrate and the annealing conditions for the ZnO nanorods.

ZnO nanorod/GaN light-emitting diodes: The origin of yellow and violet emission bands under reverse and forward bias

ZnO nanorods have been prepared by electrodeposition under identical conditions on various p-GaN-based thin film structures. The devices exhibited lighting up under both forward and reverse biases, but the turn-on voltage and the emission color were strongly dependent on the p-GaN-based structure used. The origin of different luminescence peaks under forward and reverse bias has been studied by comparing the devices with and without ZnO and by photoluminescence and cathodoluminescence spectroscopy. We found that both yellow-orange emission under reverse bias and violet emission under forward bias, which are commonly attributed to ZnO, actually originate from the p-GaN substrate and/or surface/interface defects. While the absolute brightness of devices without InGaN multiple quantum wells was low, high brightness with luminance exceeding 10 000 cd/m2 and tunable emission (from orange at 2.1 V to blue at 2.7 V, with nearly white emission with Commission internationale de l’eclairage (CIE) coordinates (0.30,...

The influence of the ZnO seed layer on the ZnO Nanorod/GaN LEDs

We have studied the influence of the seed layer on the growth of ZnO nanorods on GaN by vapor deposition and the performance of the p-GaN/ZnO nanorod light emitting diodes (LEDs). The seed layer had a significant influence on the orientation and density of the ZnO nanorods as well as on the current-voltage curves of the devices, while optical properties exhibited a weaker dependence on the seed layer. A uniform and bright yellow electroluminescence was observed in all the devices, while the photoluminescence spectra exhibited a prominent UV emission and a weak green emission.

Nitrogen doped-ZnO/n-GaN heterojunctions

Nitrogen-doped ZnO nanorods were prepared by electrodeposition using two different Zn precursors (zinc nitrate and zinc acetate), while all other growth conditions (dopant precursor, concentration, growth temperature, and bias) were identical. We have shown that the precursor used affects the properties of the ZnO nanorods, and that the presence of rectifying properties in n-GaN/N:ZnO heterojunctions is strongly related to the use of nitrate precursor for ZnO growth. The difference in the properties of ZnO obtained from two precursors is attributed to the differences in native defect and impurity concentrations, which could affect the electronic properties of the samples.

Characterizations of bismuth telluride/gallium nitride heterojunction photovoltaic detector for MWIR detection under room temperature

Uncooled infrared detectors have aroused keen interest in narrow bandgap semiconductor research. We propose to use Bismuth Telluride (Bi2Te3) bandgap energy of about 0.15 eV for middle wavelength IR (MWIR) detection. In this work, a heterojunction structure of Bi2Te3/GaN/Al2O3 was fabricated to form a photovoltaic detector. The structural properties of Bi2Te3 thin films were characterized by high resolution x-ray diffraction spectroscopy. Crystallinity of Bi2Te3 thin films was found to be strongly dependent on deposition temperatures. The spectral current responsivity for this heterojunction in NIR-to-MWIR region was evaluated under back-side illumination. The devices were also characterized by studying their low-frequency noise spectra measurement. The experimental results suggest that, the noise level of the devices is correlated to the crystallinity of the Bi2Te3 thin films.

Growth of high quality Cu 2 ZnSnS 4 thin films on GaN by co-evaporation

We report on the growth of high quality Cu2ZnSnS4 (CZTS) thin films on GaN-on-sapphire (GOS) substrates by thermal co-evaporation. Structural characterization was performed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The results are compared to films grown by thermal co-evaporation on glass and those synthesized by the sulfurization of stacked precursors on glass. Our results show that single phase epitaxial quality CZTS films with improved crystallinity can be grown on GOS substrates. The optical and electrical properties such as optical transmission, carrier concentration and Hall mobilities of the films are also reported.

Characteristics of MBE-Grown GaN Detectors on Double Buffer Layers Under High-Power Ultraviolet Optical Irradiation

In this paper, present experimental investigations on the radiation hardness of GaN-based Schottky diode photodetectors. High-power ultraviolet (UV) radiation obtained from a Xenon lamp is used as the light source for the optical-stressing experiment. Two types of devices are being investigated. One has a double-buffer-layer structure that consists of a conventional high-temperature AlN buffer layer and an intermediate temperature buffer layer (type I), and the control device was fabricated with only a conventional AlN buffer layer (type II). Detailed current-voltage, capacitance-voltage, flicker noise, and responsivity measurements performed on the detectors show that the degradations of the devices arose from the defects present at the Schottky junctions due to the exposure of the devices to the high-power UV radiation. Both types of devices exhibit degradation in their optoelectronic properties. However, type-I devices, in general, exhibit gradual and slow degradation, whereas type-II devices exhibit catastrophic breakdowns in the device characteristics. The experimental data indicate significant improvement in the radiation hardness for type-I devices

ZnO nanorods for light emitting diode applications

We investigated the influence of the growth method, growth conditions, and post-growth treatments on the ZnO nanorod properties and the performance of heterojunction light emitting diodes (LEDs) based on ZnO nanorods. Due to small lattice mismatch between GaN and ZnO, we will mainly consider p-GaN/n-ZnO nanorod heterojunctions. The influence of p-GaN substrate and the influence of growth method and growth conditions used for ZnO nanorods on the LED performance will be discussed.