Dunjun Chen

On the reverse gate leakage current of AlGaN/GaN high electron mobility transistors

In this work, we include the polarization effect within the AlGaN barrier into calculation of the near-surface electrical field ES underneath the Schottky contact metal which determines the field-dependent characteristics of reverse gate leakage current of AlGaN/GaN high electron mobility transistors. High-frequency capacitance-voltage measurement combined with electrostatic analysis is used to estimate ES as a function of reverse bias voltage. The resultant log(I/ES) versus ES curves over a temperature range from 293 to 453 K agree well with the predicted model of Frenkel–Poole (FP) emission of electrons up to the conductive states of threading dislocations. Around zero bias, the reverse polarization-field-induced FP emission current is balanced by a forward defect-assisted tunneling current, both of which follow the same temperature dependent characteristics.

Back-illuminated separate absorption and multiplication AlGaN solar-blind avalanche photodiodes

This letter reports the fabrication and performance of back-illuminated separate absorption and multiplication AlGaN solar-blind avalanche photodiodes. Devices with a 60-μm-diameter active area and a double-mesa structure exhibit a low dark current density of 1.06 × 10−8 A/cm2 at the reverse bias of 20 V and a maximum multiplication gain up to 3000 at the reverse bias of 91 V. The temperature dependence of avalanche voltage shows a large positive temperature coefficient of 0.05 V/K, confirming that avalanche multiplication is the dominant gain mechanism in the photodiodes.

Forward tunneling current in GaN-based blue light-emitting diodes

Forward tunneling current in InGaN/GaN multiquantum-well blue light-emitting diodes grown on sapphire substrate was studied by temperature-variable current-voltage (I-V) measurement. All semilog I-V curves obtained in the temperature range from 100 to 300 K exhibit two successive linearly dependent regions at low forward bias. The corresponding slopes appear to be insensitive to temperature, which indicates a dominant defect-assisted tunneling process. It is found that the tunneling current varies approximately as a function of ∼exp(−βEg+λeV), where β and λ are constants independent of temperature and voltage. The temperature- and voltage-dependence of forward tunneling current are explained by thermally induced band gap shrinkage and bias-induced route change of diagonal tunneling, respectively. The likely tunneling entities involved in the forward tunneling process are also discussed.

Stable response to visible light of InGaN photoelectrodes

The photoelectrochemical properties of InxGa1−xN∕GaN (0⩽x⩽0.20) epitaxial films on sapphire (0001) substrates have been investigated. The flatband potential of InxGa1−xN is shifted to more positive voltages with increasing indium incorporation. In aqueous HBr solution, the turnover number of the In0.20Ga0.80N electrode reaches 847 after 4000s illumination, which suggests that In0.20Ga0.80N has good photostability. Moreover, In0.20Ga0.80N shows highest visible-light response among InxGa1−xN (0⩽x⩽0.20) and the incident photon conversion efficiency is about 9% at 400–430nm in the HBr solution.

Enhanced bias stress stability of a-InGaZnO thin film transistors by inserting an ultra-thin interfacial InGaZnO:N layer

Amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistors (TFTs) having an ultra-thin nitrogenated a-IGZO (a-IGZO:N) layer sandwiched at the channel/gate dielectric interface are fabricated. It is found that the device shows enhanced bias stress stability with significantly reduced threshold voltage drift under positive gate bias stress. Based on x-ray photoelectron spectroscopy measurement, the concentration of oxygen vacancies within the a-IGZO:N layer is suppressed due to the formation of N-Ga bonds. Meanwhile, low frequency noise analysis indicates that the average trap density near the channel/dielectric interface continuously drops as the nitrogen content within the a-IGZO:N layer increases. The improved interface quality upon nitrogen doping agrees with the enhanced bias stress stability of the a-IGZO TFTs.

High-Gain AlGaN Solar-Blind Avalanche Photodiodes

This letter reports high performance AlGaN solar-blind avalanche photodiodes (APDs) with separate absorption and multiplication structure grown by metal-organic chemical vapor deposition on AlN templates. In fabricating APD devices, we applied a photo-electrochemical treatment process after mesa etching to reduce damage induced by etching. After introducing this process, the leakage current of the fabricated devices was reduced obviously and a record-high gain of 1.2×104 at the reverse bias of 84 V was achieved under the measurement condition with the protection current constrained to 10-5 A.

Ultra-Low Dark Current AlGaN-Based Solar-Blind Metal–Semiconductor–Metal Photodetectors for High-Temperature Applications

Solar-blind metal-semiconductor-metal (MSM) photodetectors (PDs) with Ni/Au semi-transparent interdigitated contact electrodes are fabricated on Al0.4Ga0.6N epi-layer grown by metal-organic chemical vapor deposition on sapphire substrate. The PD exhibits ultra-low dark current in fA range at room temperature (RT) under 20-V bias and a corresponding breakdown voltage higher than 300 V. The PD also shows a maximum RT quantum efficiency of 64% at ~ 275 nm under 10-V bias with a solar-blind/ultraviolet (UV) rejection ratio up to four orders of magnitude. Even at a high temperature of 150°C, the dark current of the PD is still in fA range with a reasonable rejection ratio of more than 8000, suggesting its potential applications for high-temperature deep-UV detection. The ultra-low dark current achieved is believed to be related to the high-temperature AlN buffer layer used in the epi-structure as well as the coplanar configuration of the MSM PD itself.

Remarkable enhancement in photocurrent of In0.20Ga0.80N photoanode by using an electrochemical surface treatment

The photocurrent and incident photon conversion efficiency of In0.20Ga0.80N increased about 2 times after a simple electrochemical surface treatment. X-ray photoelectron spectroscopy and photoluminescence analysis suggested that In-rich InGaN region on the surface of the In0.20Ga0.80N electrode was removed by using the electrochemical surface treatment. The enhancement of the photocurrent was attributed to the removal of In-rich InGaN phases caused by indium segregations on the surface of the electrode, which played a major role as surface recombination centers of photo-generated electron-hole pairs.

Efficiency droop behavior of direct current aged GaN-based blue light-emitting diodes

By direct current (dc) stressing, GaN-based blue light-emitting diodes (LEDs) with different density of nonradiative recombination centers in the active region of InGaN/GaN multiple quantum wells were obtained and studied for injection-current-induced efficiency droop. It is found that with increasing stressing time, the overall quantum efficiency of the aged LEDs drops while the peak-efficiency-current shifts toward higher magnitude. At selected injection current levels, the electroluminescence spectra of the aged LEDs show little change in peak position and shape. The shift in peak-efficiency-current, which follows the same trend as the degree of luminescence decay, is explained by a rate-equation model in which the newly created defects by dc stressing enlarge the dominant low-current region of nonradiative recombinations.

Field-dependent carrier trapping induced kink effect in AlGaN/GaN high electron mobility transistors

An anomalous kink effect featuring an abrupt recovery of drain current following current collapse is observed in the room-temperature output characteristics of AlGaN/GaN high electron mobility transistors. The kink is largely caused by trapping electrons from the gate leakage current by deep levels within the AlGaN barrier at high drain bias, resulting in a positive shift in threshold voltage and a reduction in reverse gate leakage current. The release of the trapped electrons is likely due to impact ionization of traps by hot electrons, which starts to play a role at relatively lower drain bias. Both sub-bandgap illumination and temperature rise could reduce the kink.