Hualong Wu

High hole concentration in p-type AlGaN by indium-surfactant-assisted Mg-delta doping

High hole concentration was achieved in Mg-doped AlxGa1−xN (x ∼ 0.4) by using indium-surfactant-assisted delta doping method. A maximum carrier concentration of 4.75 × 1018 cm−3 was obtained, which is three times higher than that of the conventionally delta-doped sample. Sheet resistivity as low as 2.46 × 104 Ω/sq was realized, benefiting from the high hole concentration (p). Analysis results show that the Mg incorporation is effectively enhanced, while the compensation ratio and acceptor activation energy (EA) are significantly reduced by using In surfactant. It was also found that the In surfactant may induce stronger valence-band modulation, contributing to the decrease of EA and the increase of p.

Enhanced Mg Doping Efficiency in P-Type GaN by Indium-Surfactant-Assisted Delta Doping Method

An indium-surfactant-assisted delta doping method is reported to enhance the hole concentration and doping efficiency of Mg-doped p-type GaN grown by metal organic chemical vapor deposition. The hole concentration is increased to 1.5×1018 cm-3 by using this method, which is 92% higher than that of conventional delta doping. This higher carrier concentration leads to an improved doping efficiency of 12%. Secondary ion mass spectroscopy reveals that the Mg incorporation is increased by the In surfactant. Photoluminescence analysis suggests that the nitrogen vacancies may be suppressed by the induced indium. Temperature-dependent Hall measurements indicate that the Mg ionization energy and compensation ratio are reduced by the In surfactant.

All AlGaN epitaxial structure solar-blind avalanche photodiodes with high efficiency and high gain

Solar-blind avalanche photodiodes were fabricated with an all AlGaN-based epitaxial structure on sapphire by metal–organic chemical vapor deposition. The devices demonstrate a maximum responsivity of 114.1 mA/W at 278 nm and zero bias, corresponding to an external quantum efficiency (EQE) of 52.7%. The EQE improves to 64.8% under a bias of −10 V. Avalanche gain higher than 2 × 104 was obtained at a bias of −140 V. The high performance is attributed to the all AlGaN-based p–i–n structure comprised of undoped and Si-doped n-type Al0.4Ga0.6N on a high quality AlN layer and highly conductive p-type AlGaN grown with In-surfactant-assisted Mg-delta doping.

Improved carrier injection and efficiency droop in InGaN/GaN light-emitting diodes with step-stage multiple-quantum-well structure and hole-blocking barriers

Hole transport control and carrier injection improvement have been demonstrated in the InGaN/GaN light-emitting diodes (LEDs) with step-stage multiple-quantum-well (MQW) structure and Si-doped hole-blocking barriers. Single-wavelength emission was obtained under electrical pumping in these LEDs by utilizing hole-blocking effect. The light emission around 450 nm showed a substantial increase compared with the reference sample with single or step-stage indium-content MQWs. The droop behavior and wavelength stability were also improved significantly. These improvements were attributed to the enhanced carrier injection to the active region due to the alleviation of the quantum-confined Stark effect and the effective hole-blocking effect of the Si-doped barriers.

Observation of Electroluminescence From Quantum Wells Far From p-GaN Layer in Nitride-Based Light-Emitting Diodes

We report the observation of electroluminescence from the first to fourth quantum wells (QWs) from the p -GaN layer in InGaN/GaN multiple-QW light-emitting diodes (LEDs) with various indium contents (4%-16%) in each QW. The investigated LED sample showed a lower turn-on voltage and ideality factor as well as a reduction of etching pit density compared with the reference sample. Also, the X-ray reciprocal space maps revealed a partial strain relaxation in the active region. The enhanced hole injection efficiency was attributed to the weakening of strain-induced polarization field in the QWs and the good crystalline quality.

Demonstration of solar-blind AlxGa1−xN-based heterojunction phototransistors

Al0.4Ga0.6N/Al0.65Ga0.35N heterojunction phototransistors have been fabricated from the epi-structure grown by low-pressure metal organic chemical vapor deposition on c-plane sapphire substrates. P-type conductivity of the AlGaN base layer was realized by using indium surfactant-assisted Mg-delta doping method. Regrowth technique was used to suppress the Mg memory effect on the n-type emitter. The fabricated devices with a 150-μm-diameter active area exhibited a bandpass spectral response between 235 and 285 nm. Dark current was measured to be less than 10 pA for bias voltages below 2.0 V. A high optical gain of 1.9 × 103 was obtained at 6 V bias.

Effect of Si Doping Level in n-Cladding Layer on the Performance of InGaN-Based Light-Emitting Diodes

In this study, we have systematically investigated the effect of Si doping level in the n-cladding layer on the performance of InGaN/GaN-based light-emitting diodes (LEDs). Detailed structural, optical and electrical properties of the sample with different Si doping level were studied. Based on these investigations, it was found that with a low level of Si doping (0.738 nmol/min, ~ 5.4×1017 cm3), the subsequent multiple-quantum-well (MQW) structure showed superior crystalline quality with low-density threading dislocations (TDs); however, the operation voltage of the LED chips became unbearable. On the other hand, the highly-doped (14.40 nmol/min, ~ 1.1×1019 cm3) samples showed a substantially lower operation voltage, but the MQW quality deteriorated with much higher TD density. Finally, the effect of inserting a 100-nm-thick undoped spacer GaN layer between the n-cladding layer and active layers was also investigated. The electrical and emission properties both deteriorated while the crystalline quality improved for LEDs with this structure, proving the importance of electron injection on the performance of LEDs.

Performance improvement of InGaN/GaN multiple quantum well visible-light photodiodes by optimizing TEGa flow*

The performance of an InGaN/GaN multiple quantum well (MQW) based visible-light Schottky photodiode (PD) is improved by optimizing the source flow of TEGa during InGaN QW growth. The samples with five-pair InGaN/GaN MQWs are grown on sapphire substrates by metal organic chemical vapor deposition. From the fabricated Schottky-barrier PDs, it is found that the smaller the TEGa flow, the lower the reverse-bias leakage is. The photocurrent can also be enhanced by depositing the InGaN QWs with using lower TEGa flow. A high responsivity of 1.94 A/W is obtained at 470 nm and −3-V bias in the PD grown with optimized TEGa flow. Analysis results show that the lower TEGa flow used for depositing InGaN may lead to superior crystalline quality with improved InGaN/GaN interface, and less structural defects related non-radiative recombination centers formed in the MQWs.