Long Yan

The growth optimization and mechanism of N-polar GaN films with an in situ porous SiNx interlayer

In this paper, N-polar GaN films were grown on vicinal C-face SiC substrates by metal–organic chemical vapor deposition. In situ porous SiNx interlayers with different deposition times of 0 s to 120 s were adopted in the growth processes of N-polar GaN films. We find that the crystalline quality, such as surface morphology and structural characteristics, of N-polar GaN films can be controlled by changing the deposition time of the SiNx interlayers. The N-polar GaN film with a SiNx deposition time of 60 s exhibits the best crystallinity. Based on the observed porous feature of the SiNx interlayer from the phase image obtained using an atomic force microscope, we propose a model to illustrate the epitaxial growth mechanism of N-polar GaN films grown on the top of the porous SiNx interlayer. In this model, we reveal that the SiNx deposition time determines the SiNx coverage and the nucleated island density of overgrown GaN, which markedly affects the structural characteristics of the N-polar GaN films. This model can also be used to analyze the influence of SiNx deposition times on the stress states in N-polar GaN films. Additionally, the threading dislocation propagation behaviors were observed experimentally from the cross-sectional transmission electron microscopy image of N-polar GaN with a SiNx interlayer, which demonstrates the reasonability of this model. It is reasonably believed that this work provide a valuable information to obtain high-quality GaN films that can be used to the fabrication of N-polar GaN-based optoelectronic devices.

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.

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 hole doping in N-polar III-nitride LED grown by metalorganic chemical vapor deposition

Polarization-induced doping has been shown to be effective for wide-bandgap III-nitrides. In this work, we demonstrated a significantly enhanced hole concentration via linearly grading an N-polar AlxGa1-xN (x = 0–0.3) layer grown by metal-organic chemical vapor deposition. The hole concentration increased by ∼17 times compared to that of N-polar p-GaN at 300 K. The fitting results of temperature-dependent hole concentration indicated that the holes in the graded p-AlGaN layer comprised both polarization-induced and thermally activated ones. By optimizing the growth conditions, the hole concentration was further increased to 9.0 × 1017 cm−3 in the graded AlGaN layer. The N-polar blue-violet light-emitting device with the graded p-AlGaN shows stronger electroluminescence than the one with the conventional p-GaN. The study indicates the potential of the polarization doping technique in high-performance N-polar light-emitting devices.Polarization-induced doping has been shown to be effective for wide-bandgap III-nitrides. In this work, we demonstrated a significantly enhanced hole concentration via linearly grading an N-polar AlxGa1-xN (x = 0–0.3) layer grown by metal-organic chemical vapor deposition. The hole concentration increased by ∼17 times compared to that of N-polar p-GaN at 300 K. The fitting results of temperature-dependent hole concentration indicated that the holes in the graded p-AlGaN layer comprised both polarization-induced and thermally activated ones. By optimizing the growth conditions, the hole concentration was further increased to 9.0 × 1017 cm−3 in the graded AlGaN layer. The N-polar blue-violet light-emitting device with the graded p-AlGaN shows stronger electroluminescence than the one with the conventional p-GaN. The study indicates the potential of the polarization doping technique in high-performance N-polar light-emitting devices.

Significantly improved surface morphology of N-polar GaN film grown on SiC substrate by the optimization of V/III ratio

In this paper, N-polar GaN films with different V/III ratios were grown on vicinal C-face SiC substrates by metalorganic chemical vapor deposition. During the growth of N-polar GaN film, the V/III ratio was controlled by adjusting the molar flow rate of ammonia while keeping the trimethylgallium flow rate unchanged. The influence of the V/III ratio on the surface morphology of N-polar GaN film has been studied. We find that the surface root mean square roughness of N-polar GaN film over an area of 20 × 20 μm2 can be reduced from 8.13 to 2.78 nm by optimization of the V/III ratio. Then, using the same growth conditions, N-polar InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) were grown on the rough and the smooth N-polar GaN templates, respectively. Compared with the LED grown on the rough N-polar GaN template, dramatically improved interface sharpness and luminescence uniformity of the InGaN/GaN MQWs are achieved for the LED grown on the smooth N-polar GaN template.

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.