Z. G. Yin

Enhancement of conductivity and transmittance of ZnO films by post hydrogen plasma treatment

We studied the effects of hydrogen plasma treatment on the electrical and optical properties of ZnO films deposited by radio frequency magnetron sputtering. It is found that the ZnO H film is highly transparent with the average transmittance of 92% in the visible range. Both carrier concentration and mobility are increased after hydrogen plasma treatment, correspondingly, the resistivity of the ZnO H films achieves the order of 10(-3) cm. We suggest that the incorporated hydrogen not only passivates most of the defects and/or acceptors present, but also introduces shallow donor states such as the V-O-H complex and the interstitial hydrogen H-i. Moreover, the annealing data indicate that H-i is unstable in ZnO, while the V-O-H complex remains stable on the whole at 400 degrees C, and the latter diffuses out when the annealing temperature increases to 500 degrees C. These results make ZnO H more attractive for future applications as transparent conducting electrodes.

Improved electroluminescence from n-ZnO/AlN/p-GaN heterojunction light-emitting diodes

n-ZnO/p-GaN heterojunction light-emitting diodes with and without a sandwiched AlN layer were fabricated. The electroluminescence (EL) spectrum acquired from the n-ZnO/p-GaN displays broad emission at 650 nm originating from ZnO and weak emission at 440 nm from GaN, whereas the n-ZnO/AlN/p-GaN exhibits strong violet emission at 405 nm from ZnO without GaN emission. The EL intensity is greatly enhanced by inserting a thin AlN intermediate layer and it can be attributed to the suppressed formation of the GaOx interfacial layer and confinement effect rendered by the AlN potential barrier layer.

Effects of hydrogen plasma treatment on the electrical and optical properties of ZnO films: identification of hydrogen donors in ZnO.

Wurtzite ZnO has many potential applications in optoelectronic devices, and the hydrogenated ZnO exhibits excellent photoelectronic properties compared to undoped ZnO; however, the structure of H-related defects is still unclear. In this article, the effects of hydrogen-plasma treatment and subsequent annealing on the electrical and optical properties of ZnO films were investigated by a combination of Hall measurement, Raman scattering, and photoluminescence. It is found that two types of hydrogen-related defects, namely, the interstitial hydrogen located at the bond-centered (H(BC)) and the hydrogen trapped at a O vacancy (H(O)), are responsible for the n-type background conductivity of ZnO films. Besides introducing two hydrogen-related donor states, the incorporated hydrogen passivates defects at grain boundaries. With increasing annealing temperatures, the unstable H(BC) atoms gradually diffuse out of the ZnO films and part of them are converted into H(O), which gives rise to two anomalous Raman peaks at 275 and 510 cm(-1). These results help to clarify the relationship between the hydrogen-related defects in ZnO described in various studies and the free carriers that are produced by the introduction of hydrogen.

Observation of dipole complexes in PbWO4:La3+ single crystals

The frequency dependence of dielectric loss factor (tan δ) in undoped and La-doped PbWO4 crystals has been investigated over a frequency range from 10 Hz to 10 MHz and a temperature range from 40 to 370 °C. A typical dielectric relaxation phenomenon in PbWO4:La3+ has been observed. Experimental results indicate that there exist intrinsic mobile defects in pure PbWO4 crystals and lead vacancies are the predominant mobile defects. In La-doped PbWO4 crystals, La3+ ions may be located at the Pb2+ sites and combine with lead vacancies to form the dipole complexes [2(LaPb3+)•−VPb″], which may be the origin of the dielectric relaxation phenomenon in PbWO4:La3+ crystals.

Localized surface plasmon-enhanced electroluminescence from ZnO-based heterojunction light-emitting diodes

We demonstrate the surface plasmon (SP) enhanced n-ZnO/AlN/p-GaN light-emitting diodes (LEDs) by inserting the Ag nanoparticles (NPs) between the ZnO and AlN layers. The ultraviolet/violet near band edge emission of the device is significantly enhanced while the green defect-related emission is modestly suppressed compared to the LEDs without Ag NPs. The red-shift of electroluminescence (EL) peak and the reduced photoluminescence decay lifetime of ZnO suggest that the improved EL performance of the device with Ag NPs is attributed to the resonant coupling between excitons in ZnO and localized SPs in Ag NPs.

Electroluminescence behavior of ZnO/Si heterojunctions: Energy band alignment and interfacial microstructure

n-ZnO/p-Si heterojunction light-emitting diodes (LEDs) show weak defect-related electroluminescence (EL). In order to analyze the origin of the weak EL, the energy band alignment and interfacial microstructure of ZnO/Si heterojunction are investigated by x-ray photoelectron spectroscopy. The valence band offset (VBO) is determined to be 3.15 +/- 0.15 eV and conduction band offset is -0.90 +/- 0.15 eV, showing a type-II band alignment. The higher VBO means a high potential barrier for holes injected from Si into ZnO, and hence, charge carrier recombination takes place mainly on the Si side rather than the ZnO layer. It is also found that a 2.1 nm thick SiOx interfacial layer is formed at the ZnO/Si interface. The unavoidable SiOx interfacial layer provides to a large number of nonradiative centers at the ZnO/Si interface and gives rise to poor crystallinity in the ZnO films. The weak EL from the n-ZnO/p-Si LEDs can be ascribed to the high ZnO/Si VBO and existence of the SiOx interfacial layer.

Ultraviolet electroluminescence from ordered ZnO nanorod array/p-GaN light emitting diodes

The highly ordered and aligned ZnO nanorod arrays were grown on p-GaN substrates via a facile hydrothermal process assisted by the inverted self-assembled monolayer template, from which the ZnO nanorod/p-GaN heterojunction light emitting diodes (LEDs) were fabricated. The ZnO nanorod-based LEDs exhibit a stronger ultraviolet emission of 390 nm than the ZnO film-based counterpart, which is attributed to the low density of interfacial defects, the improved light extraction efficiency, and carrier injection efficiency through the nano-sized junctions. Furthermore, the LED with the 300 nm ZnO nanorods has a better electroluminescence performance compared with the device with the 500 nm nanorods.

Enhancement of field emission of the ZnO film by the reduced work function and the increased conductivity via hydrogen plasma treatment

The ZnO films deposited by magnetron sputtering were treated by H/O plasma. It is found that the field emission (FE) characteristics of the ZnO film are considerably improved after H-plasma treatment and slightly deteriorated after O-plasma treatment. The improvement of FE characteristics is attributed to the reduced work function and the increased conductivity of the ZnO:H films. Conductive atomic force microscopy was employed to investigate the effect of the plasma treatment on the nanoscale conductivity of ZnO, these findings correlate well with the FE data and facilitate a clearer description of electron emission from the ZnO:H films.

Dielectric, hypersonic, and domain anomalies of (PbMg1/3Nb2/3O3)1-x(PbTiO3)x single crystals

Dielectric permittivities, Brillouin backscattering spectra, polarization-electric field hysteresis loops, and domain structures have been measured as a function of temperature in relaxor-based ferroelectric single crystals (PbMg1/3Nb2/3O3)1−x(PbTiO3)x(PMN-xPT) for x=0.24 and 0.34. For PMN-24%PT, a diffuse phase transition which is associated with a broad frequency-dependent dielectric maximum was observed near 380 K. As the temperature increases, the PMN-24%PT crystal gradually develops cubic regions and is fully converted into the cubic state near 375 K. An extra dielectric anomaly appears at 370 K, possibly due to the percolating polar cluster induced by an external electric field. PMN-34%PT exhibits a nearly normal ferroelectric phase transition near 445 K from the tetragonal to the cubic phase. In addition, a weak diffuse phase transition observed near 280 K may result from partial conversion of rhombohedral phase to tetragonal phase. The dielectric thermal hysteresis confirms that the transitions ne...

Controllable Growth of Highly Ordered ZnO Nanorod Arrays via Inverted Self-Assembled Monolayer Template

This article presents a facile and effective approach to the controllable growth of highly ordered and vertically aligned ZnO nanorod arrays on the GaN substrate via a hydrothermal route by using the TiO(2) ring template deriving from the polystyrene microsphere self-assembled monolayer. The size of TiO(2) ring template can be flexibly tuned from 50 to 400 nm for the 500 nm polystyrene microspheres by varying the time of reactive ion etching and the concentration of TiO(2) sol. As a result, the diameter of the individual ZnO nanorods can be potentially tuned over a wide range. The combination of several characterization techniques has demonstrated that the ordered ZnO nanorods are highly uniform in diameter and height with perfect alignment and are epitaxially grown along [0001] direction. This work provides a novel and accessible route to prepare oriented and aligned ZnO nanorod arrays with high crystalline quality.