Jin-Ping Ao

1 mW AlInGaN-based ultraviolet light-emitting diode with an emission wavelength of 348 nm grown on sapphire substrate

By introducing the AlInGaN/AlGaN quaternary system as an active region, we fabricated an UV light-emitting diode (LED) with an emission wavelength of 348 nm. The optical power is 1 mW at an injection current of 50 mA under a bare-chip geometry, which is the highest report among UV–LEDs with an emission wavelength of around 350 nm grown on sapphire substrate. It means that the optical power of such LEDs is high enough to be used in practical application. In contrast to it, a similar UV–LED based on GaN/AlGaN system as an active region has been also grown, whose optical power is less than that of the AlInGaN/AlGaN-based UV–LED by one order of magnitude. The temperature-dependent photoluminescence study indicates that there exists a strong exciton-localization effect in the AlInGaN/AlGaN material system, while there is no distinguished exciton-localization effect in the GaN/AlGaN material system. Therefore, the high performance of the AlInGaN/AlGaN-based UV–LED can be attributed to the enhanced exciton-local...

Copper gate AlGaN/GaN HEMT with low gate leakage current

Copper (Cu) gate AlGaN/GaN high electron mobility transistors (HEMTs) with low gate leakage current were demonstrated. For comparison, nickel/gold (Ni/Au) gate devices were also fabricated with the same process conditions except the gate metals. Comparable extrinsic transconductance was obtained for the two kinds of devices. At gate voltage of -15 V, typical gate leakage currents are found to be as low as 3.5/spl times/10/sup -8/ A for a Cu-gate device with gate length of 2 /spl mu/m and width of 50 /spl mu/m, which is much lower than that of Ni/Au-gate device. No adhesion problem occurred during these experiments. Gate resistance of Cu-gate is found to be about 60% as that of NiAu. The Schottky barrier height of Cu on n-GaN is 0.18 eV higher than that of Ni/Au obtained from Schottky diode experiments. No Cu diffusion was found at the Cu and AlGaN interface by secondary ion mass spectrometry determination. These results indicate that copper is a promising candidate as gate metallization for high-performance power AlGaN/GaN HEMT.

Fabrication of high performance of AlGaN/GaN-based UV light-emitting diodes

Abstract A high-performance AlGaN/GaN-based ultraviolet (UV) light-emitting diode (LED) is successfully fabricated on sapphire substrate by metal-organic-chemical-vapor-deposition technique. Generally, a p–n junction is grown on a thick GaN layer on sapphire substrate, which results in a strong internal-absorption effect. Simultaneously, a thick AlGaN cladding layer on the GaN layer also easily produces crack. In order to avoid the internal absorption, a thick AlGaN layer is immediately introduced on a thin low-temperature GaN buffer (LT GaN buffer) instead of a thick GaN layer, which successfully avoids crack formation. However, an enhanced lattice-mismatch of AlGaN/LT GaN buffer/sapphire compared with that of GaN/sapphire might result in an enhanced dislocation density, which leads to the degraded performance of UV-LED. An AlGaN/GaN supperlattice that is applied in UV-LED instead of the thick AlGaN layer strongly decreases the dislocation density, confirmed by transmission electron microscope. Furthermore, this AlGaN/GaN supperlattice successfully avoids crack formation. Consequently, the optical power of UV-LED is greatly increased. Based on the above results, we successfully fabricate a crack-free UV-LED with a high performance.

Monolithic Blue LED Series Arrays for High‐Voltage AC Operation

Design and fabrication of monolithic blue LED series arrays that can be operated under high ac voltage are described. Several LEDs, such as 3, 7, and 20, are connected in series and in parallel to meet ac operation. The chip size of a single device is 150 μm x 120 μm and the total size is 1.1 mm x 1 mm for a 40 (20 + 20) LED array. Deep dry etching was performed as device isolation. Two-layer interconnection and air bridge are utilized to connect the devices in an array. The monolithic series array exhibit the expected operation function under dc and ac bias. The output power and forward voltage are almost proportional to LED numbers connected in series. On-wafer measurement shows that the output power is 40 mW for 40 (20 + 20) LED array under ac 72 V.

High-Performance 348 nm AlGaN/GaN-Based Ultraviolet-Light-Emitting Diode with a SiN Buffer Layer.

A 348 nm ultraviolet-light-emitting diode (UV-LED) based on an AlGaN/GaN single quantum well (SQW) with a high optical power is reported. In this structure, a thin SiN buffer is introduced before the growth of a conventional low-temperature GaN buffer layer. Such a buffer layer can dramatically reduce the density of threading dislocation as we have previously reported. Since the optical performance of UV-LED is generally known to be sensitive to the density of threading dislocations, unlike the InGaN/GaN- based blue LED, our UV-LED has a higher optical power than that of a similar structure but without a SiN buffer layer. Since our new buffer technology is much easier than the so-called epitaxial lateral overgrowth (ELO) or pendeo-epitaxy method, it is highly recommended for use in the fabrication of GaN-based optical devices, particularly AlGaN/GaN-based UV-LED.

Normally-off GaN recessed-gate MOSFET fabricated by selective area growth technique

In this letter, a normally-off GaN recessed-gate MOSFET is demonstrated using a nonplasma gate recess technique, in which the access region with AlGaN/GaN heterostructure was selectively grown on a semi-insulating GaN/Si template to naturally form a recessed gate. The normally-off recessed-gate Al2O3/GaN MOSFET presents a high threshold voltage of 3.5 V and a maximum drain current density of 550 mA/mm (at a positive gate bias of 12 V). A maximum field-effect mobility of 170 cm2 V−1 s−1 and a large on/off current ratio of more than 107 was obtained, which indicates the high quality of the Al2O3/GaN interface.

Fabrication of High-Output-Power AlGaN/GaN-Based UV-Light-Emitting Diode Using a Ga Droplet Layer

We report a new method of increasing the output power of an ultraviolet light-emitting diode (UV-LED) based on an AlGaN/GaN single quantum well (SQW). In this method, a thin Ga droplet layer is intentionally grown before the growth of an AlGaN/GaN SQW active layer. The Ga droplet layer causes a spatial and compositional fluctuation on the SQW active layer, which induces exciton localization in the potential minima. The LEDs fabricated with the Ga droplet layer show an emission peak of 353 nm and a higher optical output power than those of the same structure but without the Ga droplet layer.

Effect of strain relaxation and exciton localization on performance of 350-nm AlInGaN quaternary light-emitting diodes

The optical and structural properties of AlInGaN quaternary single and multiple quantum-well structures have been investigated by means of photoluminescence and x-ray diffraction. This comparative study of single quantum-well (SQW) and multiple quantum-well (MQW) structures was carried out in terms of the exciton localization effect and the strain relaxation. A detailed analysis indicated that 13% strain relaxation occurs in the MQW compared to the SQW, which is assumed to be fully strained. Furthermore, the AlInGaN SQW structure showed a stronger localization effect than the MQW. Both these effects result in enhanced emission efficiency for the SQW structure, indicating that it is better suited as the active region for ultraviolet light-emitting diodes (UV-LEDs). Finally, the UV-LEDs with an emission wavelength of about 350nm based on such SQW and MQW active regions were grown. The output power of the SQW UV-LEDs is around 2.3 times higher than that of MQW UV-LEDs.

NiO/GaN heterojunction diode deposited through magnetron reactive sputtering

NiO films were prepared with different O2/Ar reactive sputtering gas ratios. The morphology, crystalline structure, and optical properties of the as-deposited films are dependent on sputtering gas ratios. The NiO/GaN heterojunction diode was fabricated with NiO film obtained at medium O content (25%). Compared with the Ni/GaN Schottky diode, the NiO/GaN heterojunction diode shows a relatively higher turn-on voltage and lower reverse leakage current. The temperature-dependent current–voltage characteristics demonstrate that the thermionic emission dominated the reverse leakage current of the NiO/GaN diode.

Evaluation of a Gate-First Process for AlGaN/GaN Heterostructure Field-Effect Transistors

In this study, we evaluated the annealing temperature and time-dependent electrical properties of AlGaN/GaN heterostructure field-effect transistors (HFETs) utilizing TiN/W/Au as the gate electrode. With the annealing temperature increasing from 750 to 900 °C for the annealing time of 1 min, the sheet resistance of TiN/W/Au films increased gradually while that of the ohmic contact was minimum (0.66 Ω mm) at 800 °C. From the current–voltage characteristics of the Schottky diode and HFETs, it is demonstrated that annealing at 800 °C showed the lowest on-resistance and highest maximum drain current. By prolonging the annealing from 0.5 to 10 min at 800 °C, good device performance was achieved when the annealing time was 1 and 3 min, while the device performance degraded showing an increased gate leakage current and gate resistance with increasing annealing time. These results demonstrated that the TiN/W/Au gate, which can withstand 800 °C annealing for a short time, is suitable for application in the gate-first process for AlGaN/GaN HFETs.