Hong Quang Le

Synthesis and optical properties of well aligned ZnO nanorods on GaN by hydrothermal synthesis

Uniformly distributed ZnO nanorods with diameter 80–120 nm and length 2 µm have been grown at low temperatures on GaN by a catalyst-free and inexpensive aqueous solution method. The formation of the ZnO nanorods and the growth parameters are controlled by reactant concentration, temperature and pH. XRD and TEM studies show that the ZnO nanorods are single crystals and are well oriented along the c-axis of the crystal plane. The room-temperature photoluminescence measurements have shown a dominant ultraviolet peak at 3.23 eV with high intensity, which are comparable to those found in high-quality ZnO films. Through the photoluminescence measurement at a low temperature, the ZnO nanorods show a strong near band edge emission with weak deep level emission and also a free-to-bound transition with its phonon replicas. The temperature dependence of these excitonic emission and multiple phonon replicas from 4 to 250 K have been observed and their origins have been identified.

The influence of nitrogen plasma treatment on the lattice vibrational properties of hydrothermally grown ZnO nanorods

In this study, the authors have investigated the optical properties of hydrothermally grown ZnO nanorods subjected to the combination of thermal annealing and nitrogen plasma treatments. In particular, ultraviolet-visible micro-Raman scattering has been used to study the influence of nitrogen incorporation in ZnO nanorods grown on GaN/sapphire templates. The band-edge photoluminescence spectra show significant changes due to nitrogen plasma treatment. In addition, visible Raman spectra show intensity enhancement of the disorder-activated vibrational modes from plasma-treated ZnO nanorods. Multiple longitudinal optical (LO) phonons are observed under ultraviolet resonant Raman excitation from these nanorods. The first-order resonant LO phonon line shape fitting is correlated to the nitrogen-induced lattice disorder.

Optical and Electrical Properties of Ga-Doped ZnO Single Crystalline Films Grown on MgAl2O4(111) by Low Temperature Hydrothermal Synthesis

Single crystalline gallium-doped ZnO films have been hydrothermally grown at 90°C using a ZnO seed layer on MgAl 2 O 4 (111) substrates. High resolution X-ray diffraction showed an epitaxial relationship between the ZnO single crystalline film and the spinel substrate with an out-of-plane orientation of Ga:ZnO〈001〉∥MgAl 2 O 4 〈111〉 and an in-plane orientation of Ga:ZnO[110]∥MgAl 2 O 4 [112] and Ga:ZnO[110]∥MgAl 2 O 4 [110]. The effects of the doping concentration as well as the postannealing treatments on the electrical and optical properties were examined. After thermal treatment, the carrier concentration in the 2.48% Ga-doped ZnO films was 2 orders of magnitude higher at 3.1 × 10 20 cm ―3 with a carrier mobility of 28 cm 2 /V s. The blueshift of the optical bandgap was also observed from the photoluminescence and optical absorption measurements as the doping concentration increased. This can be explained in the framework of the induced stress and Burstein―Moss effects at high carrier concentration.

Gallium and indium co-doping of epitaxial zinc oxide thin films grown in water at 90 °C

Zinc oxide (ZnO) thin films were intentionally co-doped with group III elements (gallium) in order to investigate and understand the effects of co-doping on the morphological, electrical and optical properties of gallium-doped ZnO (GZO) films. The co-doped films were grown on MgAl2O4 spinel substrates using a low-temperature solution-phase method known as hydrothermal synthesis. Gallium with indium co-doped ZnO (GIZO) films displayed a dramatic improvement in surface morphology as compared with the Ga-doped ZnO (GZO) films due to the compensation effect of gallium and indium doping which reduced the lattice strain. The 0.0033M gallium with 3.3 × 10−4M indium co-doped film exhibited an electron concentration of 3.14 × 1020 cm−3 and resistivity of 7.4 × 10−4 Ω cm which were both enhancements of 1.5 times over the GZO film. These films were comparable to the films fabricated by more expensive and complicated vapour-phase methods. The figure of merit for this film was determined to be 1.63 × 10−2 sq/Ω which was very close to the indium tin oxide conducting films currently used commercially. Finally, the GIZO film was hydrothermally grown on a p-GaN film to form an n-ZnO/p-GaN heterojunction light-emitting diode (LED). This LED showed diode I–V characteristics and exhibited strong cool-white light emission which signified the prospect of using GIZO as an effective and low-cost n-type layer in LEDs.

Nanorod assisted lateral epitaxial overgrowth of ZnO films in water at 90 °C

A novel method was used to grow epitaxial ZnO films by employing a nanorod assisted lateral epitaxial overgrowth process at a low growth temperature of 90 °C in water utilizing a continuous circulation reactor. The relatively smooth films had an epitaxial relationship with the sapphire substrate, as confirmed by off-axis φ scans. Films grown for 72 hours had a high carrier concentration of 3.12 × 1018 cm−3 and mobility of 9.75 cm2 V−1 s−1 after thermal treatment at 300 °C. The threading dislocation density of the thickest film was 5 × 108 cm−2, 2 orders of magnitude lower than that of normal solution grown ZnO films.

Electrical Properties and UV Response of Single ZnO Nanorod

Single ZnO nanorods with diameters of 100nm were directly grown on GaN surface by using low-temperature hydrothermal synthesis. Individual nanorods were removed from the substrate and placed between the Au contact pads and the current voltage measurement was proceeded to characterize the electrical properties of the ZnO nanorods. By using thermionic field emission model of Padovani and Stratton , the resistivity, carrier concentration, electron mobility can be extracted with the values of 0.14 Ωcm, 9.2x1016/cm3,33.82cm2V-1s-1, respectively. The single ZnO nanorods also showed high sensitivity to the UV light (325nm). Under the UV illumination, the UV induced current increase nearly 10 times than the dark current. In addition, the nanorods also exhibited a slow UV response due to the effects of the oxygen ions on the surface of the nanorods.