Shao-Hua Huang

Defect reduction and efficiency improvement of near-ultraviolet emitters via laterally overgrown GaN on a GaN/patterned sapphire template

An approach to improve the defect density and internal quantum efficiency of near-ultraviolet emitters was proposed using a combination of epitaxial lateral overgrowth (ELOG) and patterned sapphire substrate (PSS) techniques. Especially, a complementary dot array pattern corresponding to the underlying PSS was used for the ELOG-SiO2 mask design. Based on the transmission-electron-microscopy and etch-pit-density results, the ELOG∕SiO2∕GaN∕PSS structure can reduce the defect density to a level of 105cm−2. The internal quantum efficiency of the InGaN-based ELOG-PSS light-emitting diode (LED) sample showed three times in magnitude as compared with that of the conventional GaN/sapphire one. Under a 20mA injection current, the output powers of ELOG-PSS, PSS, and conventional LED samples were measured to be 3.3, 2.9, and 2.5mW, respectively. The enhanced output power could be due to a combination of the reduction in dislocation density (by ELOG) and improved light extraction efficiency (by PSS). Unlike the previ...

High-power GaN-mirror-Cu light-emitting diodes for vertical current injection using laser liftoff and electroplating techniques

A large-area (1 /spl times/ 1 mm) vertical conductive GaN-mirror-Cu light-emitting diode (LED) fabricated using the laser liftoff and electroplating techniques is demonstrated. Selective p-GaN top area was first electroplated by the thick copper film, and then an excimer laser was employed to separate the GaN thin film from the sapphire substrate. The luminance intensity of the vertical conductive p-side-down GaN-mirror-Cu LED presented about 2.7 times in magnitude as compared with that of the original GaN-sapphire LED (at 20 mA). The light output power for the GaN-mirror-Cu LED was about twofold stronger (at 500 mA). A more stable peak wavelength shift under high current injection was also observed.

Improved Light Extraction of Nitride-Based Flip-Chip Light-Emitting Diodes Via Sapphire Shaping and Texturing

A novel flip-chip structure of GaN-sapphire light-emitting diodes (LEDs) was developed to improve the external quantum efficiency, where the sapphire substrate was textured and shaped with beveled sidewalls using a wet etching technique. The forward voltage of the conventional flip-chip and shaped flip-chip GaN LEDs were 2.84 and 2.85 V at 20 mA, respectively. This indicates that the GaN LED was not destroyed during the sapphire wet etching process. It was found that the output power increased from 9.3 to 14.2 mW, corresponding to about 52% increases in the external quantum efficiency. The results agree well with the simulation data that the shaped flip-chip GaN LED can provide better light extraction efficiency than that of the conventional flip-chip sample

GaN/Mirror/Si Light-Emitting Diodes for Vertical Current Injection by Laser Lift-Off and Wafer Bonding Techniques

A p-side-up GaN/mirror/Si light-emitting diode (LED) for vertical current injection has been fabricated by laser lift-off and wafer bonding techniques. A variety of metallic mirrors (Au, Al, and Ag) were chosen to improve the optical reflectivity and contact resistance with n-GaN. The GaN/mirror/Si LED with a silver mirror achieved a maximum luminance intensity of 45 mcd (20 mA) with a low forward voltage of 3.5 V. This luminance intensity is over two times that of the original planar GaN/sapphire LED. Under high current injection, the GaN/mirror(Ag)/Si LED also showed a more stable emission wavelength than the planar GaN/sapphire LED. This can be explained by the fact that the Si substrate provides a good heat sink and alleviates the joule heating problem. On the basis of these results, the p-side-up structure confirms the possibility of the simultaneous realization of a lower contact resistance and higher reflectivity for GaN/mirror/Si LEDs.

Efficiency Improvement of GaN-Based LEDs with ITO Texturing Window Layers Using Natural Lithography

In conventional GaN light-emitting diodes (LEDs), a significant gap exists between the internal and external efficiencies owing to the narrow escape cone for light in high refractive index semiconductors. In this paper, p-side-up GaN/sapphire LEDs with surface-textured indium-tin-oxide (ITO) window layers were investigated using natural lithography with polystyrene spheres as the etching mask. Under optimum etching conditions, the surface roughness of the ITO film can reach 140 nm, while the polystyrene sphere on the textured ITO surface is maintained at about 250-300 nm in diameter. The LEDs fabricated using the surface-textured ITO provided an output power that exceeded that of the planar-surface LED by about 30% and 40% at 20 and 400 mA current injection, respectively. After calculating, the extraction quantum efficiency of ITO/GaN LEDs with and without textured surface is 22.6% and 17.4%, respectively. There is about 5.3% improvement in the extraction quantum efficiency

Fabrication and efficiency improvement of micropillar InGaN∕Cu light-emitting diodes with vertical electrodes

We present a micropillar surface structure based on the enhancement of the light extraction efficiency of the near-ultraviolet (409nm) vertical-conducting InGaN light-emitting diode (LED) with an electroplated Cu substrate. The micropillar InGaN∕Cu LED (chip size: 1×1mm2) was fabricated using a combination of patterned sapphire substrate (PSS), laser lift-off, and copper electroplating processes. The PSS and Cu substrate can offer the advantages of dislocation reduction and thermal heat sink, respectively. It was found that the light output power (at 350mA) of the micropillar InGaN∕Cu LED sample can be improved by 39% as compared with that of the conventional InGaN∕Cu LED one. This significant enhancement in output power could be attributed to the increase of the extraction efficiency which is a result of the increase in photon escaping probability caused by scattering the emission light at the micropillar surface. The light extraction efficiency can be further optimized by tuning the micropillar spacing,...

Surface Texturing for Wafer-Bonded Vertical-Type GaN/Mirror/Si Light-Emitting Diodes

An n-side-up GaN/mirror(Pd/Au)/Si light-emitting diode (LED) with surface texturing has been fabricated by a combination of wafer-bonding, laser lift-off, and surface texturing techniques. Two concentrations of KOH solution were used to roughen the n-GaN surface. In order to obtain a uniformly roughened surface, the solution was heated instead of being subjected to photoirradiation. The GaN/Pd/Au/Si LEDs with surface texturing exhibited a maximum luminance intensity of 130 mcd (at 20 mA) with a forward voltage of 3.2 V. The luminance intensity is over two times larger than that of the original planar GaN/sapphire LEDs (at 20 mA). Under high current injection, the surface textured GaN/Pd/Au/Si LEDs also showed a more stable luminance intensity. This feature is attributed to the Si substrate providing a good heat sink and surface roughening enhancing the external quantum efficiency. Furthermore, the n-side-up GaN/mirror(Pd/Au)/Si LEDs with surface texturing have been demonstrated to have high reliability.

Thermally Stable Mirror Structures for Vertical-Conducting GaN/Mirror/Si Light-Emitting Diodes

Vertical-conducting GaN/mirror/Si light-emitting diodes (LEDs) with thermally stable mirrors have been fabricated using a combination of wafer-bonding and laser liftoff techniques. The thermal stabilities of NiO-Ag, NiO-Ag-Ni, and NiO-Au-Ag mirrors and their effects on the performance of mirror-substrate LEDs were studied. It is found that the NiO-Ag-Ni mirror presents the best performance, where the specific contact resistance and the reflectivity can achieve 5.1times10-3 Omega-cm2 and 92% at 470 nm after oxidation annealing at 500degC for 10 min. The top Ni layer could protect the Ag mirror from clustering during the thermal treatment process. The output powers of the GaN-sapphire and GaN/mirror/Si LEDs with NiO-Au-Ag and NiO-Ag-Ni mirrors show 4.5, 4.3, and 13 mW, respectively.

Enhanced Luminance Efficiency of Wafer-Bonded InGaN–GaN LEDs With Double-Side Textured Surfaces and Omnidirectional Reflectors

A p-side-up GaN-based light-emitting diode (LED) on a silicon substrate was designed and fabricated using a combination of omnidirectional reflector (ODR) and double-side textured surface (both p-GaN and undoped-GaN) structures via surface-roughening, laser lift-off (LLO) and wafer-bonding technologies. The reflectivity of the designed ODR can reach 99.1% at a wavelength of 460 nm. The textured surface of top p-GaN was achieved under low temperature (LT) conditions using metalorganic chemical vapor deposition. It was found that the GaN LED with an extra 200-nm-thick LT p-GaN layer exhibits a 50% enhancement in luminance intensity. The luminance efficiency of double-side roughened silicon-ODR-GaN LED with a small chip size of 250 mum times 500 mum can be improved from 23.2% to 28.2% at an injection current of 20 mA. For the case of 1 mm times 1 mm in chip size, the saturation behavior of the light output power is not observed when an injection current increased from 20 to 350 mA, where the luminance efficiency at 20 mA can reach 28.9%, demonstrating an enhancement by 46%, as compared with that of the conventional GaN-sapphire LEDs. These enhanced results can be attributed to higher reflectivity from the ODR and multiple chances of light emitted from the active region to escape, as well as a centralizing effect of light along the vertical direction.

Improvements of N-Side-up GaN Light-Emitting Diodes Performance by Indium?Tin-Oxide/Al Mirror

An n-side-up vertical conducting GaN/mirror/Si light-emitting diode (LED) has been fabricated using a combination of wafer-bonding, laser lift-off, and surface texturing techniques. The indium–tin-oxide (ITO)/Al and Pd mirror were chosen to improve optical reflectivity and contact resistance with p-GaN. It was found that the LEDs with higher reflectivity mirrors present better optical properties. A GaN/ITO/Al/Si LED with surface texturing presents maximum luminance intensity with five times the magnitude of the original planar GaN/sapphire LED at 20 mA. The output power characteristic of a GaN/ITO/Al/Si LED is 8.9 mW, and it is higher than that of the original GaN/sapphire (4.5 mW) LED at 20 mA. With high current injection, the surface textured GaN/ITO/Al/Si LED also shows more stable luminance intensity and output power. The junction temperature is measured by the diode forward method. As the dc forward current increases to 200 mA, the junction temperatures of the GaN/mirror (ITO/Al)/Si and GaN/sapphire LED are 87 and 158 °C, respectively. Obviously, the Si substrate provides good thermal dissipation.