Wei-Chi Lee

Current Spreading and Blocking Designs for Improving Light Output Power from the Vertical-Structured GaN-Based Light-Emitting Diodes

In this work, use of localized Ti deposition associated with a transparent indium-zinc-oxide (IZO) layer is proposed to serve as Schottky current blocking and current spreading layer, respectively. In addition, an inductively coupled plasma (ICP) mesa etching on the surface layer (n-GaN) of regular vertical-conducting metal-substrate GaN-based light-emitting diodes (VM-LEDs) is also proposed to further enhance current spreading of the device. Through a two-dimensional device simulator, the calculated results indicate that significant avoidance of the current-crowding effect under cathode contact pad could be obtained once the n-GaN layer etching depth and width, IZO thickness, and Schottky current blocking width have been optimized. In experiments, 1000 m 1000 m GaN-based blue LEDs with an ICP mesa etching of 250 m in width and 2 m in depth on the surface n-GaN layer, 200 m in Schottky current blocking width, and a 300-nm-thick IZO layer have the been successfully fabricated. As compared to the regular VM-LEDs without the use of the present technology, typical improvement in light emission uniformity and light output power by about 6% and 38% at an injection current of 350 mA have been obtained.

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.

Enhanced Performance of Vertical GaN-Based Light-Emitting Diodes with a Current-Blocking Layer and Electroplated Nickel Substrate

The performance of vertical-structure metallic-substrate GaN-based light-emitting diodes (VM-LEDs) with a patterned SiO2 film as the current-blocking layer (CBL) was investigated. From theoretical calculations of current and light distributions and experimental results on current–voltage (I–V) and light output power–current (L–I) characteristics, we found that SiO2 CBL inserted under the n-pad electrode increases light output power by 35.4% at 20 mA as compared with VM-LEDs without CBL. Such an improvement is attributed to the insulated CBL structure, which provides better current spreading and less photon absorption and/or reflection at the n-electrode.

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

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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To equalize the resistance of all possible current paths in regular vertical-conducting metal-substrate GaN-based light-emitting diodes (VM-LEDs), an anisotropic laser etching to the surface layer (n-GaN) of 40mil VM-LEDs for improving light emission uniformity and light output power is proposed and demonstrated. The feasibility of the proposed scheme was verified by current and light emission distribution as well as light extraction rate simulations. In conjunction with a nonuniform excimer laser beam irradiation through a mask and rotation of the epitaxy wafer, VM-LEDs with a concave-surface n-GaN layer were also fabricated. Typical improvement in light output power by 38%–26% at an injection current of 350mA as compared to the one without anisotropic etching has been obtained.

-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.

Use of anisotropic laser etching to the top n-GaN layer to alleviate current- crowding effect in vertical-structured GaN-based light-emitting diodes

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.