Jian Kai Liou

Improved Light Extraction Efficiency of a High-Power GaN-Based Light-Emitting Diode With a Three-Dimensional-Photonic Crystal (3-D-PhC) Backside Reflector

An interesting approach to improving the light extraction efficiency of high-power GaN-based light-emitting diodes (LEDs) by the use of a 3-D-photonic crystal (3-D-PhC) backside reflector is studied. A 3-D-PhC backside reflector is formed by coating a self-assembled SiO2 nanosphere monolayer between the hybrid reflector and backside of a sapphire substrate. The 3-D-PhC structure is used to enhance light scattering. At 350 mA, as compared with a conventional LED (with a hybrid reflector while only without the 3-D-PhC structure), the studied device exhibits 23.6% enhancement in the light output power without the degradation of electrical properties. Therefore, the performance of high-power GaN-based LEDs could be further improved by using a 3-D-PhC backside reflector.

Effects of the Use of an Aluminum Reflecting and an

A GaN-based light-emitting diode (LED) with an aluminum (Al) reflecting and an SiO2 insulating layers (RILs) deposited on the naturally textured p-GaN surface is fabricated and studied. The use of RIL could enhance the current spreading performance and reduce the photon absorption by the p-pad metal. The textured surface is used to limit the total internal reflection and increase photon scattering. In this paper, effects of the use of an Al RL and/or an SiO2 insulating layer on the performance of GaN-based LEDs are systematically studied and compared in detail. At 20 mA, as compared with a conventional LED with naturally textured (planar) p-GaN surface, the studied device exhibits 12.2% (55.5%) enhancement in light output power. Additionally, a 28.5% (95%) increment of luminous flux is achieved. The studied device also shows 15.6% light intensity improvement of far-filed pattern. Experimentally, although power consumption and junction temperature are slightly increased because of the insertion of RIL structure, these drawbacks could be surpassed by the mentioned optical improvements. Therefore, for conventional GaN-based LEDs, light extraction efficiency could be further improved by the employment of RIL structure.

{\rm SiO}_{2}

An interesting GaN-based light-emitting diode (LED) with an aluminum (Al) metal mirror deposited on naturally textured V-shaped pits (V-pits), grown on the device surface, is fabricated and studied. The V-pits is used to limit the total internal reflection as well as enhance light extraction, and the Al metal mirror is used to prevent photons from being absorbed by the Cr/Pt/Au metal pad. As compared with a conventional LED (with V-pits while without Al mirror), at 20 mA, the studied device exhibits 13.7% enhancement in light output power as well as 14% increment in external quantum efficiency. Therefore, for a LED with V-pits on top, the light extraction efficiency could be further improved by employing an Al metal mirror.

Insulating Layers (RIL) on the Performance of a GaN-Based Light-Emitting Diode With the Naturally Textured p-GaN Surface

Enhanced light extraction efficiency (LEE) of high-power GaN-based light-emitting diodes (LEDs) is achieved by inserting a self-assembled SiO2 nanosphere monolayer between the substrate and backside reflectors. Due to the presence of concave surfaces and photonic crystal-like air voids, downward photons emitted from multiple quantum well toward 3-D backside reflectors, could be reflected, scattered, and redirected into arbitrary directions for light extraction. These textured 3-D backside reflectors with an SiO2 nanosphere monolayer could also extract the lateral light inside device into the normal direction and improve LEE. As compared with a conventional LED without a backside reflector and an LED with a planar hybrid backside reflector, at 350 mA, the studied device with a 3-D hybrid backside reflector exhibits 136.4% (165%) and 23.6% (27.4%) enhancements in light output power (luminous flux) without the degradation of electrical properties. Higher light intensities in light emission mapping image and far-field pattern are also obtained. These results show that a textured 3-D backside reflector could be easily formed by inserting an SiO2 nanosphere monolayer to significantly enhance the performance of high-power GaN-based LEDs.

On a GaN-Based Light-Emitting Diode With an Aluminum Metal Mirror Deposited on Naturally-Textured V-Shaped Pits Grown on the p-GaN Surface

A Pd/AlGaN/GaN heterostructure field-effect transistor (HFET)-type hydrogen gas sensor, based on the sensitization, activation, and electroless plating (EP) deposition processes, is fabricated and studied. Due to the used sensitization and activation approaches, a dense and uniform EP seed layer could be achieved. Good dc and microwave characteristics, including the higher turn-on voltage, lower reverse leakage current, improved thermal stability of drain current, enhanced unity current gain cutoff frequency, and maximum oscillation frequency, are obtained for a 1-m-gate-length device. Moreover, the significant hydrogen gas sensing performance, such as larger drain current variation and higher hydrogen detection sensitivity, are found under 1% and 5 ppm H2/air ambiences, respectively. Consequently, the studied EP-based Pd/AlGaN/GaN HFET gives the promise for high-performance electronic device and hydrogen gas sensor applications.

Implementation of High-Power GaN-Based LEDs With a Textured 3-D Backside Reflector Formed by Inserting a Self-Assembled

A hybrid SiO2 micro/nanospheres antireflection coating, deposited by a rapid convection deposition, acting as a passivation layer of GaN-based light-emitting diodes (LEDs) is studied in this paper. Since the critical angle could be enlarged by antireflection coating, Fresnel reflection could be reduced. In addition, due to the roughened surface of hybrid SiO2 microsphere/nanosphere antireflection coating, the scattering effect could be increased. Thus, the light extraction efficiency could be further enhanced. As compared with a conventional LED (device A), at 20 mA, the studied device C exhibits 18.7% enhancement in light output power without any degradation of electrical properties. Reduced leakage current could also be achieved. Therefore, the use of hybrid SiO2 microsphere/nanosphere antireflection coating could effectively improve the performance of GaN-based LEDs.

{\rm SiO}_{2}

A high-power GaN-based light-emitting diode (LED) with an inductively coupled plasma (ICP)-transferred nanohemispherical hybrid backside reflector is studied. A self-assembled 100 ± 5 nm SiO2 nanosphere monolayer is drop-coated on the backside of a sapphire substrate as a mask to transfer nanohemispherical patterns onto the backside of the sapphire substrate by ICP. Nanohemispherical patterns could be transferred to the deposited backside reflector. Thus, reflected photons could be redirected and scattered into arbitrary directions for light extraction. As compared with a conventional LED without a backside reflector, at 350 mA, the studied device exhibits a 118.2% enhancement in light output power without the degradation of electrical properties. Note that the adhesion between an ICP-transferred sapphire substrate and the hybrid backside reflector is better than when directly inserting an SiO2 nanosphere monolayer in the device. Thus, the process yield could be enhanced for applying in the solid-state lighting.

Nanosphere Monolayer

A GaN-based light-emitting diode (LED) grown on a nanocomb-shaped patterned sapphire substrate (PSS) is fabricated and studied. Nanocomb-shaped patterns are transferred on a sapphire substrate using a well-ordered anodized aluminum oxide (AAO) thin film as a mask for the inductively coupled plasma etching process. This well-ordered AAO thin film with a high aspect ratio is grown on a sapphire substrate by an oxalic acid-based electrochemical system and a three-step anodization. The strain state generated during epitaxial growth could be effectively alleviated by the use of nanocomb-shaped PSS. The treading dislocation density could be reduced. Thus, the enhanced crystalline quality is obtained. In addition, due to the presence of photonic crystal-like air buffer layer, part of reflected photons upward the top side could be scattered by this layer. Therefore, more photons could be extracted outside. Experimentally, at 20 mA, as compared with a conventional LED grown on a planar sapphire substrate, the studied LED grown on a nanocomb-shaped PSS shows 53.8% and 43.7% enhancements in light output power and external quantum efficiency as well as a reduced leakage current.

On an Electroless Plating (EP)-Based Pd/AlGaN/GaN Heterostructure Field-Effect Transistor (HFET)-Type Hydrogen Gas Sensor

Interesting pseudomorphic high electron mobility transistors using an electrophoretic deposition (EPD) approach are fabricated and studied. Due to the low-temperature deposited gate structure, the studied device exhibits enhanced performance with less thermal damages and improved Schottky contact properties by EPD approach. In comparison with a thermal evaporation (TE) device, the higher turn-on voltage, lower gate current, and lower interface state density are observed for the EPD device. For the gate dimension of 1 × 100 μm2 the EPD device shows the higher maximum drain saturation current of 242.2 (231.9) mA/mm and excellent maximum extrinsic transconductance of 151.6 (132.5) mS/mm at 300 (420) K. Besides, the EPD device presents a comparable RF performance as compared with the TE one. Due to the improved device performance and advantages of low cost, simple process, flexible deposition on varied substrate, and adjustable metal grain size, the reported EPD approach shows the promise for high-performance device applications.

Study of GaN-Based LEDs With Hybrid SiO 2 Microsphere/Nanosphere AntiReflection Coating as a Passivation Layer by a Rapid Convection Deposition

The electrostatic discharge (ESD) performance of GaN-based light-emitting diodes (LEDs) with naturally-textured p-GaN contact layers grown on c-axis miscut sapphire substrates are studied. During machine model tests, the device grown on a 0.35° miscut sapphire shows the highest ESD tolerance, while the one grown on a 0.2° miscut sapphire exhibits the poorest tolerance. It is discovered that this effect correlates with the presence of maximum capacitance (Cm) values, over the difference in defect densities between LEDs. The variation in Cm values is caused by parasitic capacitance effect induced by different p-GaN surface morphologies between the studied devices. This observation gives us more reliable application in improving ESD performance based on the device grown on a 0.35° miscut sapphire.