Dengkui Wang

Ultraviolet Electroluminescence from ZnS@ZnO Core-Shell Nanowires/p-GaN Introduced by Exciton Localization.

We investigate the electroluminescence (EL) from light emitting diodes (LEDs) of ZnO nanowires/p-GaN structure and ZnS@ZnO core-shell nanowires/p-GaN structure. With the increase of forward bias, the emission peak of ZnO nanowires/p-GaN structure heterojunction shows a blue-shift, while the ZnS@ZnO core-shell nanowires/p-GaN structure demonstrates a changing EL emission; the ultraviolet (UV) emission at 378 nm can be observed. This discrepancy is related to the localized states introduced by ZnS particles, which results in a different carrier recombination process near the interfaces of the heterojunction. The localized states capture the carriers in ZnO nanowires and convert them to localized excitons under high forward bias. A strong UV emission due to localized excitons can be observed. Our results indicated that utilizing localized excitons should be a new route toward ZnO-based ultraviolet LEDs with high efficiency.

Investigation of Localized States in GaAsSb Epilayers Grown by Molecular Beam Epitaxy

We report the carrier dynamics in GaAsSb ternary alloy grown by molecular beam epitaxy through comprehensive spectroscopic characterization over a wide temperature range. A detailed analysis of the experimental data reveals a complex carrier relaxation process involving both localized and delocalized states. At low temperature, the localized degree shows linear relationship with the increase of Sb component. The existence of localized states is also confirmed by the temperature dependence of peak position and band width of the emission. At temperature higher than 60 K, emissions related to localized states are quenched while the band to band transition dominates the whole spectrum. This study indicates that the localized states are related to the Sb component in the GaAsSb alloy, while it leads to the poor crystal quality of the material, and the application of GaAsSb alloy would be limited by this deterioration.

Effect of rapid thermal annealing on the optical properties of GaAsSb alloys

GaAsSb ternary alloys are fundamental components of advanced electronic and optoelectronic devices in the future. The presence of localized states could greatly affect the optical properties in GaAsSb alloy, which depend on the fluctuation of alloy composition. In order to optimize the optical properties, GaAsSb alloys were treated by rapid thermal annealing (RTA) at different temperatures, and the optical behaviors of the annealed samples were investigated in detail. During RTA, a significant reduction of the localized states was observed by photoluminescence (PL) spectral analysis. Furthermore, the RTA process also altered the distribution of the components of the GaAsSb alloy, which caused a slight red-shift of the maximum PL peak at 150 K. The relationship between the localized states and the temperature of the RTA process was also investigated. The process involving the conversion of localized carriers to free carriers was proposed. Under the suitable RTA conditions, the Sb component was homogenized and the depth of carrier localization was decreased.

Ordered and Disordered Phases in Mo 1−x W x S 2 Monolayer

With special quasirandom structure approach and cluster expansion method combined with first-principle calculations, we explore the structure and electronic properties of monolayer Mo1−xWxS2 alloy with disordered phase and ordered phase. The phase transition from ordered phase to disordered phase is found to happen at 41 K and 43 K for x = 1/3 and x = 2/3, respectively. The band edge of VBM is just related with the composition x, while the band edge of CBM is sensitive to the degree of order, besides the concentration of W. Near the CBM band edge, there are two bands with the Mo-character and W-character, respectively. It is found that in disordered phase the Mo-character band is mixed with the W-character band, while the opposite happens in ordered phase. This result leads to that the splitting of two bands near CBM in ordered phase is larger than in disordered phase and gives rise to the smaller band gap in ordered phase compared to the disordered phase. The electron effective mass in ordered phase is smaller than in disordered phase, while the heavy hole effective mass in ordered phase is larger than that in disordered phase.

Surface sulfurization of ZnO/ZnS core shell nanowires and shell layers dependent optical properties

With the unique properties and superior performance, ZnO/ZnS core shell nanostructures have been applied in many photoelectric devices. Varied growth methods can synthesize ZnO/ZnS core shell samples, which exhibit different crystal and optical properties, and have the positive or negative effect on performance of related devices. To investigate the growth and properties, ZnO/ZnS core shell nanowires were grown through surface sulfurization. X-ray diffraction and scanning electron microscopy measurements indicated surface sulfurization degree can affect crystal quality. In addition, photoluminescence results exhibited that ZnO/ZnS heterojunction emission properties were dependent on shell layers quality. By increasing the amount of surface powder, it would improve shell layers’ crystal quality. And with optimal surface sulfurization degree, the vacancies and interstitial atoms defects can be restrained, which may improve photo-generated carriers separation efficiency.

The optical property improvement of GaAs via surface passivation and carriers restriction of MgO film

ABSTRACT Surface passivation is an effective way to eliminate the surface states and to improve the optical properties of GaAs. Moreover, carriers restriction of wide bandgap materials also is a path to enhance photoluminescence. In this paper, Be-doped GaAs epilayer was growth on GaAs substrate by molecular beam epitaxy. The MgO films were deposited by atom layer deposition. It was used as not only passivation layer to eliminate surface states, but also barrier layer to restrict carriers. The carriers restriction was confirmed by photoluminescence and current-voltage characteristics of metal-insulator-semiconductor structure. As the result, the photoluminescence intensity of Be-doped GaAs with 5 nm MgO was 2.5 times higher than the bare Be-doped GaAs.

Facile synthesis and formation mechanism of uniform antimony nanotubes

One-dimensional tubular antimony (Sb) nanomaterials were successfully synthesized via a facile template-free solvothermal reduction process. These as-synthesized Sb nanotubes exhibited a middle-hollow, multi-walled, open-ended structures with the average size of lengths and diameters for 370×80nm. Also, they show the rhombohedral phase and good crystallinity in nature. Based on the crystal structures and morphologies evolution of Sb nanomaterials by the temperature-dependent reactions, the formation mechanism of Sb nanotubes has been proposed to be that the nanosized clusters nucleated first and then self-assembly aggregated into thin nanosheets and finally rolled into the nanotubes through some driving forces, namely, a rolling-up mechanism.

Band alignment at a MgO/GaSb heterointerface using x-ray photoelectron spectroscopy measurements

The valence band offset (ΔE V) of a MgO/GaSb heterostructure was determined using x-ray photoelectron spectroscopy measurements. A ΔE V value of 2.84 ± 0.10 eV was calculated by using Ga 3d3/2 and Mg 2p1/2 binding energies as references. Taking the empirical band gaps of 7.83 eV and 0.73 eV for MgO and GaSb thin films into consideration, respectively, we obtained the type-I band alignment of a MgO/GaSb heterostructure with a conduction band offset (ΔE c) of 4.26 ± 0.10 eV, suggesting a nested interface band alignment.