Z.Z. Zhang

A highly glass-forming alloy with low glass transition temperature

A rare-earth Pr-based bulk metallic glass (BMG) is obtained in the shape of rod up to 5 mm in diameter by die cast. Unlike other rare-earth-based BMGs, it exhibits a distinct glass transition, the low glass transition temperature (Tg=409 K), a large and stable supercooled liquid region, and paramagnetic property. The glass transition as well as its kinetic nature and the fragility parameters of the BMG have been studied. The BMG offers an ideal model to investigate the nature of glass transition as well as the relaxation and nucleation with a large experimentally accessible time and temperature window at low temperatures.

Single-crystalline cubic MgZnO films and their application in deep-ultraviolet optoelectronic devices

By employing a relatively low growth temperature and oxygen-rich conditions, single-crystalline cubic MgZnO films were prepared. A solar-blind deep ultraviolet (DUV) photodetector was finished on the MgZnO film. The maximum responsivity of the photodetector is 396 mA/W at 10 V bias, which is almost three orders of magnitude larger than the highest value ever reported in MgZnO-based solar-blind photodetectors. The dark current density is 1.5×10−11 A/cm2, comparable with the smallest value ever reported in solar-blind photodetectors. The improved performance reveals that the single-crystalline cubic MgZnO films have great potential applications in DUV optoelectronic devices.

High responsivity ultraviolet photodetector realized via a carrier-trapping process

Metal-semiconductor-metal structured ultraviolet (UV) photodetector has been fabricated from zinc oxide films. The responsivity of the photodetector can reach 26 000 A/W at 8 V bias, which is the highest value ever reported for a semiconductor ultraviolet photodetector. The origin of the high responsivity has been attributed to the carrier-trapping process occurred in the metal-semiconductor interface, which has been confirmed by the asymmetric barrier height at the two sides of the metal-semiconductor interdigital electrodes. The results reported in this paper provide a way to high responsivity UV photodetectors, which thus may address a step toward future applications of UV photodetectors.

Effects of nitrogen doping and illumination on lattice constants and conductivity behavior of zinc oxide grown by magnetron sputtering

A yellow-orange nitrogen-doped zinc oxide (ZnO:N) film was deposited on a quartz glass substrate at 510K by reactive radio-frequency magnetron sputtering of a ZnO target with sputtering gas of nitrogen. The lattice constants of the as-grown ZnO:N are much larger than those of undoped ZnO, and decrease with increasing annealing temperature due to escape of the nitrogen from the ZnO:N and decrease of tensile stress, accompanied with color change from yellow-orange to pale yellow. The nitrogen occupies two chemical environments in the ZnO:N based on x-ray photoelectron spectroscopy measurement. One is NO acceptor formed by substitution of N atom for O sublattice, and another is (N2)O double donors produced by substitution of N molecular for O site, which make the lattice constants expanded. The as-grown ZnO:N film shows insulating, but behaves p-type conduction in the dark after annealed at 863K for 1h under 10−3Pa. Unfortunately, the p-type conduction is not stable and reverts to n type soon. However, after...

X-ray photoelectron spectroscopy measurement of n-ZnO/p-NiO heterostructure valence-band offset

Valence-band offset (VBO) of n-ZnO/p-NiO heterojunction has been investigated by x-ray photoelectron spectroscopy. Core levels of Zn 2p and Ni 2p were used to align the VBO of n-ZnO/p-NiO heterojunction. It was found that n-ZnO/p-NiO heterojunction has a type-II band alignment and its VBO is determined to be 2.60±0.20 eV, and conduction-band offset is deduced to be 2.93±0.20 eV. The experimental VBO value is in good agreement with the calculated value based on the electron affinity of ZnO and NiO.

Nitrogen-related recombination mechanisms in p-type ZnO films grown by plasma-assisted molecular beam epitaxy

The recombination mechanisms of nitrogen-related emissions in p-type ZnO films were investigated by photoluminescence (PL) measurements. The enhanced peak at 3.260 eV was confirmed as a donor-acceptor pair (DAP) transition and the emission around 3.310 eV was assigned to the free electron to acceptor (FA) recombination by studying the temperature evolution of DAP and FA luminescence, calculating the energy level of the corresponding nitrogen acceptor, and measuring the decay time of DAP transition. By comparing the PL spectra of the samples with various hole concentrations, it can be found that as the hole concentration increases, the DAP peak significantly dominates the spectra. In addition, the acceptor binding energy is estimated to be about 120 meV from the FA transition, which is in good agreement with the value calculated by a hydrogenic acceptor model.

ZnO light-emitting devices with a lifetime of 6.8 hours

Lithium-nitrogen doped p-type Mg0.25Zn0.75O films have been realized, and p-Mg0.25Zn0.75O/n-ZnO single-heterostructured light-emitting devices (LEDs) have been constructed. Obvious emission at around 392 nm has been observed from the LEDs under the injection of continuous current, which can be attributed to the near-band-edge emission of ZnO. The LED can work continuously for 6.8 h under a continuous current of 20 mA, revealing the good reliability of the LED. The results reported in this letter reveal that reliable ZnO-based LEDs can be realized, thus high-performance ZnO-based LEDs may be promised in the future.

Biaxial stress-dependent optical band gap, crystalline, and electronic structure in wurtzite ZnO: Experimental and ab initio study

The relationship between band gap and biaxial stress in wurtzite ZnO thin films has been investigated by side-inclination x-ray diffraction technique and optical absorbance spectrum as well as ab initio calculation. The experimental result shows that differing from other semiconductor thin films with hexagonal structure, such as GaN, the band gap of ZnO thin films increases with the increase in biaxial tensile stress. For explaining the difference, ab initio calculation is performed to simulate the relationship between band gap and biaxial stress of wurtzite ZnO and GaN. The calculated result indicates that the band gap of ZnO increases under biaxial tensile stress but GaN is opposite, supporting our experimental result. The band offset calculation shows that the conduction-band minimum (CBM) and the valence-band maximum (VBM) of ZnO and GaN offset to low energy under biaxial tensile stress. The VBM offset of ZnO is larger than the CBM, responsible for the increase in band gap. The VBM offset of GaN is sm...

Valence-band offset of epitaxial ZnO∕MgO (111) heterojunction determined by x-ray photoelectron spectroscopy

The valence-band offset of ZnO∕MgO (111) heterojunction has been directly measured by x-ray photoelectron spectroscopy. Excluding the strain effect, the valence-band offset is determined to be 0.87±0.20eV, and the conduction-band offset ΔEC is deduced to be −3.59±0.20eV, indicating that ZnO∕MgO heterojunction has a type-I band alignment. The conduction-band and valence-band offset of MgO∕ZnO is used to interpret the origination of p-type conduction in undoped MgxZn1−xO alloy and deeper acceptor level in undoped and N-doped p-type MgxZn1−xO alloy than in ZnO.

Investigation on the formation mechanism of p-type Li-N dual-doped ZnO

Lithium and nitrogen dual-doped ZnO films [ZnO:(Li, N)] with Li concentrations of 0%–11.2% were grown on sapphire by plasma-assisted molecular beam epitaxy, and a stable p-type ZnO:(Li, N) film was obtained by doping 6.1% of Li. The p-type conductivity of ZnO:(Li, N) is attributed to the formation of the Lii–NO complex, which depresses the compensation of Lii donor for LiZn acceptor and the generation of (N2)O donors. It is demonstrated that the Lii–NO complex can form an impurity band above the valance band maximum, resulting in a decrease in the ionization energy of the acceptor and an improvement in the conductivity and stability of the p-type ZnO:(Li, N).