Junyan Jiang

Epitaxial growth of vertically aligned ZnO nanowires for bidirectional direct-current driven light-emitting diodes applications

We presented a comparative investigation on the morphological and structural properties of the produced ZnO nanowires (NWs) on c-Al2O3 substrates under different preparation conditions. The effects of a low-temperature nucleation layer, reaction pressure and light irradiation on the alignment, diameter and growth rate of ZnO NWs were evaluated in detail. It was found that the low-temperature nucleation layer acting as a supporting layer favored the formation of a transition layer featuring initially a rough surface and a high density of grain boundaries, guiding and facilitating the subsequent homoepitaxy of ZnO NWs grown on the top of developed (0001) facets of the new activation sites. The crystallinity and vertical alignment of epitaxial ZnO NWs were further optimized to achieve the best value, as the second growth stage was conducted with a higher reaction pressure and light irradiation, such that a relatively low full width at half-maximum (701 arcsec) of the rocking curve could be obtained. The thermodynamical growth kinetics of the produced ZnO NWs via the two-step growth process was also investigated by monitoring the morphology evolution at different growth stages. Coaxial n-ZnO/MgO/p-NiO-core/shell NWs heterostructured light-emitting diodes (LEDs) were then fabricated and characterized, and a unique tunability of the electroluminescence (EL) spectra depending on the bias polarities was observed. We studied in detail the bicolor EL characteristic of the bidirectional direct-current driven LEDs and tentatively proposed carrier tunneling and hole generation models to explain such interesting features.

Improved ultraviolet emission performance from polarization-engineered n-ZnO/p-GaN heterojunction diode

O-polar ZnO films were grown on N-polar p-GaN/sapphire substrates by photo-assisted metal-organic chemical vapor deposition, and further heterojunction light-emitting diodes based O-polar n-ZnO/N-polar p-GaN were proposed and fabricated. It is experimentally demonstrated that the interface polarization of O-polar n-ZnO/N-polar p-GaN heterojunction can shift the location of the depletion region from the interface deep into the ZnO side. When a forward bias is applied to the proposed diode, a strong and high-purity ultraviolet emission located at 385 nm can be observed. Compared with conventional Zn-polar n-ZnO/Ga-polar p-GaN heterostructure diode, the ultraviolet emission intensity of the proposed heterojunction diode is greatly enhanced due to the presence of polarization-induced inversion layer at the ZnO side of the heterojunction interface. This work provides an innovative path for the design and development of ZnO-based ultraviolet diode.

Excellent optical quality versus strong grain boundary effect in a double-layer ZnO structure

ZnO samples with a double-layer structure and top nanorod arrays on the bottom film layer were grown by metal-organic chemical vapor deposition at a temperature range from 340 to 400 °C. The ZnO nanorods show excellent optical quality and no obvious defect related emission can be detected below 40 K except for I6 line and the surface bound exciton emission. The free exciton emission and its phonon replicas dominate the near band edge emission between 40 and 295 K. For the film layer, the temperature-dependent Hall measurements showed that the conduction region is degenerate. In the conduction region, the carrier mobility is mainly limited by the grain boundary effect, which can be weakened by thermal annealing. The conduction mechanism in this region before and after annealing can be fitted by a uniform and a non-uniform conduction model, respectively. The results indicate that grain boundary effects strongly limit the mobility and consume large amounts of carriers by the trap states. Furthermore, we propose a qualitative model to explain the expansion of the conduction regions by annealing. It reveals a mechanism for the improvement of electrical properties of polycrystalline thin films by annealing treatments.

Exciton localization and ultralow onset ultraviolet emission in ZnO nanopencils-based heterojunction diodes

n-GaN/i-ZnO/p-GaN double heterojunction diodes were constructed by vertically binding p-GaN wafer on the tip of ZnO nanopencil arrays grown on n-GaN/sapphire substrates. An increased quantum confinement in the tip of ZnO nanopencils has been verified by photoluminescence measurements combined with quantitative analyses. Under forward bias, a sharp ultraviolet emission at ~375 nm due to localized excitons recombination can be observed in ZnO. The electroluminescence mechanism of the studied diode is tentatively elucidated using a simplified quantum confinement model. Additionally, the improved performance of the studied diode featuring an ultralow emission onset, a good operation stability and an enhanced ultraviolet emission shows the potential of our approach. This work provides a new route for the design and development of ZnO-based excitonic optoelectronic devices.

n-ZnO/p-GaN heterojunction light-emitting diodes featuring a buried polarization-induced tunneling junction

n-ZnO/p-GaN heterojunction light-emitting diodes with a p-GaN/Al0.1Ga0.9N/n+-GaN polarization-induced tunneling junction (PITJ) were fabricated by metal-organic chemical vapor deposition. An intense and sharp ultraviolet emission centered at ∼396 nm was observed under forward bias. Compared with the n-ZnO/p-GaN reference diode without PITJ, the light intensity of the proposed diode is increased by ∼1.4-folds due to the improved current spreading. More importantly, the studied diode operates continuously for eight hours with the decay of only ∼3.5% under 20 mA, suggesting a remarkable operating stability. The results demonstrate the feasibility of using PITJ as hole injection layer for high-performance ZnO-based light-emitting devices.

Polarization-induced enhancement of hole injection efficiency in n-ZnO/p-graded Al x Ga1− x N heterojunction diodes

Vertically aligned O-polar ZnO nanowall networks were prepared on N-polar p-graded Al x Ga1− x N/sapphire substrates by metal–organic chemical vapor deposition. Further, heterojunction light-emitting diodes based on O-polar n-ZnO/N-polar p-graded Al x Ga1− x N were fabricated. A strong and narrow ultraviolet emission at 388 nm, originating from ZnO, was observed under forward bias. The hole injection efficiency of the proposed diode was significantly enhanced owing to the existence of polarization-induced two-dimensional hole gas at the n-ZnO/p-graded Al x Ga1− x N heterointerface, thereby yielding an enhanced light output power. This work provides an alternative path towards the realization of high-performance ZnO-based ultraviolet diodes.