Pei-Ren Wang

Enhanced Light Output of GaN-Based Vertical-Structured Light-Emitting Diodes With Two-Step Surface Roughening Using KrF Laser and Chemical Wet Etching

A two-step roughening process that uses a KrF excimer laser and KOH chemical etching for the n-GaN layer surface of vertically structured GaN-based light-emitting diodes (VLEDs) to yield circular protrusions with hexagonal cones atop for light extraction enhancement is demonstrated. A possible mechanism of the formation of the circular protrusions commenced by laser irradiation with nonuniform etching rates at sites with various dislocation densities was investigated. An improvement in light output power of about 95% at 350-750 mA compared to that of flat VLEDs was obtained for the two-step roughened VLEDs, which is attributed to the increase in surface emission area and dimensions of roughness, and, in particular, the decrease in the n-GaN layer thickness.

A Sn-based metal substrate technology for the fabrication of vertical-structured GaN-based light-emitting diodes

Through the use of tin (Sn) based solder balls and patterned laser lift-off technique, a metal substrate technology was proposed for the fabrication of vertical-structured metal substrate GaN-based light-emitting diodes (VM-LEDs). Advantages including reserving the merits of metallic substrate and simplifying the fabrication processes of vertical-structured GaN-based LEDs were demonstrated. As compared to conventional sapphire substrate GaN-based LEDs, the fabricated VM-LEDs with an emission area of 620×620μm2 show an increase in light output power about 145.36% at 350mA with a significant decrease in forward voltage from 4.51to3.46V.

Use of Elastic Conductive Adhesive as the Bonding Agent for the Fabrication of Vertical Structure GaN-Based LEDs on Flexible Metal Substrate

Through the use of elastic conductive adhesive (ECA) as the bonding agent and patterned laser lift-off technology, a flexible metal substrate technology for the fabrication of vertical structured GaN-based light-emitting diodes (flex-LEDs) was proposed and demonstrated. It showed that the flex-LEDs have negligible changes in dominant wavelength-current and light output intensity-current-voltage characteristics when subjected to an external bending stress, indicating that the ECA used in the present technology performed well as a buffer to external stresses. As compared with conventional sapphire substrate GaN-based LEDs, Flex-LEDs with a chip size of 600 x 600 mum2 showed an increase in light output intensity (power) about 216% (80%) at 120 mA with an essential decrease in forward voltage from 3.51 to 3.3 V.

Enhanced Performance of Vertical GaN-Based LEDs With Highly Reflective

Through the use of polystyrene nano-spheres as a 2-D mask for the patterned-deposition of indium-zinc-oxide (IZO) and annealed Pt-Al-Pt as a high reflectivity p -ohmic/mirror layer, vertical GaN-LEDs with atop periodic IZO nano-wells (NW-VLEDs) were fabricated. At 350 mA, NW-VLEDs exhibited a crucial VF reduction of 0.1 V with an enhancement of 87% in light output and 92% in power conversion efficiency as compared to regular vertical GaN-LEDs, which should be attributed to the combination of the effectiveness of high-reflectivity ohmic contact, IZO current spreading layer, and the enhanced light extraction efficiency from the periodic nano-wells.

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The use of a refractive index matching (RIM) structure with indium zinc oxide (IZO) transparent conduction layer and SiO2 nanotube (SiO2-NT) arrays to improve light extraction of vertical structure KOH-etched GaN-based light emitting diodes (VLEDs) is demonstrated. Compared to regular VLED with KOH-roughened surface, it shows considerable gains in light emitted critical angle and light output power by 21.3° and 103% at 350 mA, respectively. These improvements could be attributed to the effectiveness of the IZO/SiO2-NT RIM scheme in ameliorating current crowding and significantly minimizing the total internal reflection effect.

-ohmic Contact and Periodic Indium–Zinc–Oxide Nano-Wells

Through the deposition of a thin SiO2 film to sheathe hydrothermally grown (HTG) ZnO nanowires (ZnO-NWs), unveiling their top portion, and then selectively removing ZnO-NWs by wet chemical etching, SiO2 nanotubes (SiO2-NTs) with controllable inner/outer diameters and lengths were fabricated. The prepared SiO2-NTs with average inner/outer diameters and lengths of approximately 200/300 nm and 1.5 µm, respectively, exhibited a superior transmittance of 92% in the visible light spectrum. The surface roughened process using SiO2-NTs on vertical-structure GaN light-emitting diodes (VLEDs) showed additional light output improvement of about 11.6% at 350 mA and 10% at 750 mA, compared with those of VLEDs with ZnO-NWs, suggesting the effectiveness and promising applications of the proposed SiO2-NTs in optics and optoelectronics devices.

Improved light output of GaN-based vertical light emitting diodes using SiO2 nanotube arrays and transparent metal oxide current conduction layer

To further improve the performance of vertical-structured GaN-based light-emitting diodes (V-LEDs), surface roughening using a KrF laser and KOH wet chemical etching, followed by hydrothermal growth of vertically aligned ZnO nanorods on top of the n-GaN surface were investigated and discussed. Compared with that of the V-LEDs (300×300 µm2 in chip size) with only surface KOH wet etching, the formation of curved protrusions and ZnO nanorods on the n-GaN surface typically enables an increase in light output power (Lop) by 29% at 20 mA and 41% at 100 mA with a decrease in forward voltage (Vf) from 3.24 to 3.06 V at 20 mA and 3.9 to 3.7 V at 100 mA, respectively. The cumulative effect of the curved protrusions, hexagonal cones, and vertically aligned ZnO nanorods formed as a result of effectively reducing the effective thickness of the n-GaN layer, improving the ohmic contact to n-GaN, increasing the surface emission area, and enhancing the escape probability of photons was responsible for these improvements.

Preparation of SiO2 Nanotubes with Controllable Inner/Outer Diameter and Length Using Hydrothermally Grown ZnO Nanowires as Templates

A dicing-free substrate technology was proposed and demonstrated to simplify the fabrication of vertical-structured metal substrate GaN-based light-emitting diodes (VM-LEDs) using a Sn-based solder screen printing technique with patterned laser lift-off technology. As compared with conventional sapphire substrate GaN-based LEDs, VM-LEDs with an effective emission area of 1000×1000 µm2 were found to have a 0.38 (0.87) V reduction in forward voltage at 350 (700) mA. In addition, their enhancement in light output power in the current range of 350–700 mA was found to successively increase from 55 to 76%. By considering these results, the power conversion efficiency of VM-LEDs was found to be 2.14 times that of regular LEDs at 700 mA.

Enhanced Light Output of Vertical-Structured GaN-Based Light-Emitting Diode with Surface Roughening Using KrF Laser and ZnO Nanorods

In the above titled paper (ibid., vol. 20, no. 7, pp. 523-525, 1 Apr 08), there were errors in Table I. The correct table is presented here.

A Screen-Printed Sn-Based Substrate Technology for the Fabrication of Vertical-Structured GaN-Based Light-Emitting Diodes

Use of deep ultraviolet (248 nm) KrF laser irradiation to roughen vertical GaN-based LEDs surface with volcanolike protrusions for light output (Lop) improvement was proposed and demonstrated. After pulse irradiations of KrF laser (750-850 mJ/cm2), the rate of electron-hole pair recombination at sites with dislocation defects is greater than for crystalline GaN, favoring for the formation of GaOx, and in turn, resulting in a relatively lower etching rate therein and leading to a roughened surface with volcano-like protrusions. Typical diameter/height and density of protrusions are around 2~4 μm/2 μm and 106 cm-2. Through the use of KrF laser and KOH etching, an enhancement in the root-meansquare surface roughness by 250 times and an improvement in Lop by 25% at 750 mA were obtained. It is expected that the surface roughness of Gallium Nitride by KrF excimer laser technology would be a potential candidate for the fabrication of high power GaN-based LEDs for solid-state lighting in the near future.