Bor-Wen Liou

Use of patterned laser liftoff process and electroplating nickel layer for the fabrication of vertical-structured GaN-based light-emitting diodes

The fabrication process and performance characteristics of a vertical-structured GaN-based light-emitting diode (VM-LED) employing nickel electroplating and patterned laser liftoff techniques are presented. As compared to regular LED, the forward voltage drop of the VM-LED at 20–80 mA is about 10%–21% lower, while the light output power (Lop) is more than twice in magnitude. Especially, the Lop exhibits no saturation or degradation at an injection current up to 520 mA which is about 4.3 times higher than that of the regular one. Substantial improvements in the VM-LEDs performances are mainly attributed to the use of metallic substrate which results in less current crowding, larger effective area, and higher thermal conductivity.

The influence of oxygen content in the sputtering gas on the self-synthesis of tungsten oxide nanowires on sputter-deposited tungsten films

The self-synthesis of tungsten oxide (W18O49) nanowires on sputter-deposited W films prepared under different O2/Ar flow rate ratios (OAFRRs) in the sputtering gas is reported. After thermally annealing at 700?850??C in N2 ambient for 15?min, dense and well crystalline W18O49 (010) nanowires or nanobelts were obtained depending on the oxygen content in the sputtering gas. Experimental results show that the annealing temperature required for the full growth of W18O49 nanowires reduced when the OAFRR in the sputtering gas was increased. It is found that the oxygen absorbed in the surface region is responsible for the growth of nanowires. As the OAFRR was increased to (8?sccm)/(24?sccm), which resulted in a saturated oxygen content of about 55 at.% inside the W film, large-scale nanobelts or nanosheets of W18O49 were grown. The possible growth mechanism which governs the evolution from nanowires to nanobelts as the OAFRR was changed is also discussed.

Preparation of tungsten oxide nanowires from sputter-deposited WCx films using an annealing/oxidation process

The self-synthesis of tungsten oxide (W18O49) nanowires on sputter-deposited WCx films using a simple annealing/oxidization process was reported. It was found that thermal annealing of WCx films at 680°C for 30min in nitrogen followed by oxidation at 450°C for 30min in pure oxygen would yield dense and well-crystallized monoclinic W18O49 (010) nanowires with a typical length/diameter of about 0.15–0.2μm∕10–20nm. The formation of W18O49 nanowires is attributed to the nuclei of immature W2C nanowires experiencing a regrowth process, accompanied by carbon depletion and the oxidization of tungsten during the subsequent oxidization process.

Effect of Surface Treatment on the Performance of Vertical-Structure GaN-Based High-Power Light-Emitting Diodes with Electroplated Metallic Substrates

Large-area (0.6 ×0.6 and 1 ×1 mm2) highly-efficient GaN-based light-emitting diodes (LEDs) with a vertical-conducting structure (VM-LEDs), using a patterned laser lift-off technique and a Ni electroplating process as well as a surface treatment of the top n-GaN epilayer by plasma and chemical etching, were successfully fabricated and investigated. Compared to regular LEDs of the same size, both the forward voltage drop and the light output power (Lop) of the VM-LED were substantially improved. With inductively coupled plasma (ICP) etching followed by an additional KOH etching and an HF/HCl treatment on the n-GaN layer, an increase in Lop by 227% (195%) at 350 (800) mA has been achieved for the (1 ×1 mm2)-sized VM-LEDs.

High-Power GaN-Based Light-Emitting Diodes with Transparent Indium Zinc Oxide Films

Large-area (0.6 mm×0.6 mm–1.5 mm×1.5 mm), high-power GaN-based blue-light-emitting diodes (LEDs) with an indium zinc oxide (IZO) overlay as a transparent conduction layer (TCL) and the effects of the overlayer on light output power (Lop) improvement are investigated. Experimental results show that sputter-deposited IZO TCLs with thicknesses in the range of 100–500 nm have a low resistivity ranging from 12.1–3.1×10-4 Ω-cm and a transparency ≥80% in the visible light range. The benefit obtained from the use of an IZO TCL is much more profound in LEDs of larger chip size; in addition, the optimum IZO TCL thickness is approximately 300 nm. As compared to the case without an IZO layer, under an injection current of 60–1000 mA, a 39–90% improvement in Lop has been achieved from LEDs (1.5 mm×1.5 mm) with a 300-nm-thick IZO TCL.

Fabrication of Dicing-Free Vertical-Structured High-Power GaN-Based Light-Emitting Diodes With Selective Nickel Electroplating and Patterned Laser Liftoff Techniques

Through the use of selective nickel (Ni) electroplating, patterned laser liftoff technique, and surface roughing of the top n-GaN epilayer, a novel process for the fabrication of vertical-structured metal-substrate GaN-based light-emitting diodes (VM-LEDs) to avoid difficulties in Ni substrate dicing and improve device yield was proposed and demonstrated. In conjunction with a sidewall passivation with SiO2 and keeping the size of epilayer smaller than that of Ni island, a considerable improvement in yield and device performance were shown. As compared to conventional lateral-structured GaN-based LEDs, VM-LEDs show an increase in light output power about 174% at 350 mA with a significant decrease in forward voltage from 3.5 to 3.17 V

The use of transparent conducting indium-zinc oxide film as a current spreading layer for vertical-structured high-power GaN-based light-emitting diodes

In this study, the performance of vertical-structured high-power GaN-based light-emitting diodes (VM-LEDs) with a transparent and low-resistant indium-zinc oxide (IZO) film as a current spreading layer (CSL) was investigated. Nickel electroplating and patterned laser liftoff techniques were employed for the transfer of sapphire substrate to nickel substrate. The novel IZO CSL atop n-side-up VM-LEDs offering benefits of superior current spreading ability, larger extraction efficiency, and lower forward voltage drop was demonstrated. As compared to the regular LED without IZO CSL, the use of an IZO CSL with an optimum thickness of around 300 nm leads to an increase in light output power by 97.1 (67.8)% and a decrease in forward voltage drop by 4.9 (15.5)% under an injection current of 350 (800) mA

The evolution of tungsten oxide nanostructures from nanowires to nanosheets

The self-synthesis of tungsten oxide (W18O49) nanowires/nanosheets on sputtering-deposited tungsten films was obtained by thermal annealing in nitrogen under the thermal budgets of 750 °C for ≥2 h or ≥850 °C for 0.5 h. Experimental evidence of the nanomorphology transformation of tungsten oxide nanostructures from nanowires to nanosheets was presented, which can be attributed to the formation and re-crystallization of an amorphous interface layer between two neighboring parallel-growth nanowires linked together with a low angle misalignment along their growth directions.

Development of Gas Sensors Based on Tungsten Oxide Nanowires in Metal/SiO2/Metal Structure and Their Sensing Responses to NO2

In this work, the fabrication of gas sensors utilizing self-synthesized tungsten oxide nanowires (TONWs) and their response to NO2 are reported. The gas sensor is based on a sputter-deposited WCx/SiO2/WCx triple-layer structure with the periphery of the SiO2 layer etched chemically. Self-synthesized TONWs with crystalline W18O49(010) were grown by simple thermal annealing in nitrogen ambient, which linked, in parallel, the upper and lower WCx electrodes for gas sensing. The TONW-based sensors increased in resistance in NO2 because the surface of TONWs comprised oxygen adsorbates and the adjacent space charge region was electron-depleted. The amount of enlargement in resistance increased with increasing temperature. To improve the detectability of the parallel-connected TONW-based sensor, a connection of several individual sensors in series was proposed to enlarge the number of TONWs for gas sensing. For the 8-series-connected sensor, a sensitivity as high as 9.3, a response time as low as about 9 s, and a detectability as low as 2 ppm for NO2 were obtained.

Improvement of field emission characteristics of tungsten oxide nanowires by hydrogen plasma treatment

The use of hydrogen plasma (H-plasma) treatment to improve field emission (FE) characteristics of self-synthesized tungsten oxide nanowires (TONWs) is reported. With a H-plasma treatment under a working power of 200 W and a pressure of 500 mtorr for 20 s, improved FE characteristics with a turn-on field (4.7 V/μm at 10 μA/cm2) lower than those of the as-grown case by 23% and a reduction in the effective emission barrier of 0.72 eV were obtained, which is attributed to the reduction in oxygen adsorption, decrease in the wire length and density, and transition of TONWs surfaces from well crystalline into the amorphous phase.