Ta-Cheng Hsu

Enhancing the output power of GaN-based LEDs grown on wet-etched patterned sapphire substrates

GaN-based light-emitting diodes (LEDs) with emitting wavelength of 450 nm were grown on patterned sapphire substrates (PSSs) fabricated by chemical wet etching. The crystallography-etched facet was {1-102} R-plane with a 57deg against {0001} C-axis and had superior capability for enhancing light extraction efficiency. The light output power of the PSS LED was 1.15 times higher than that of the conventional LED at an injection current of 20 mA. The output power and external quantum efficiency were estimated to be 9 mW and 16.4%, respectively. The improvement was attributed not only to geometrical shapes of {1-102} crystallography-etched facets that efficiently scatter the guided light to find escape cones, but also to dislocation density reduction by adopting the PSS growth scheme

Increasing the extraction efficiency of AlGaInP LEDs via n-side surface roughening

An n-side-up AlGaInP-based light-emitting diode (LED) with a triangle-like surface morphology was fabricated using the adhesive layer bonding technique, followed by wet etching to roughen the surface. The light output power of the roughened-surface LED was 1.6 times higher than that of a flat-surface LED at an injection current of 20 mA, i.e., a significant improvement attributed to the ability of the roughened surface to not only reduce the internal reflection between the rear mirror system and the semiconductor-air interface, but also to effectively scatter the light outside the LED device.

Three dimensional numerical study on the efficiency of a core-shell InGaN/GaN multiple quantum well nanowire light-emitting diodes

This paper presents the findings of investigating core-shell multiple quantum well nanowire light-emitting diodes (LEDs). A fully self-consistent three dimensional model that solves Poisson and drift-diffusion equations was employed to investigate the current flow and quantum-confined stark effect. The core-shell nanowire LED showed a weaker droop effect than that of conventional planar LEDs because of a larger active area and stronger recombination in nonpolar quantum wells (QWs). The current spreading effect was examined to determine the carrier distribution at the sidewall of core-shell nanowire LEDs. The results revealed that a larger aspect ratio by increasing the nanowire height could increase the nonpolar-active area volume and reduce the droop effect at the same current density. Making the current spreading length exceed a greater nanowire height is critical for using the enhancement of nonpolar QWs effectively, when an appropriate transparent conducting layer might be necessary. In addition, this...

Improvement in light-output efficiency of AlGaInP LEDs fabricated on stripe patterned epitaxy

Quaternary AlGaInP light-emitting diodes (LEDs) operating at a wavelength of 630 nm with a stripe-patterned omni-directional reflector (ODR) were fabricated. It is demonstrated that the geometrical shape of stripe-patterned structure improves the light extraction efficiency by increasing the extraction of guided light. The optical and electrical characteristics of stripe-patterned ODR LEDs are presented and compared to typical ODR and distributed Bragg reflector (DBR) LEDs with the same epitaxial structure and emitting wavelength. It is shown that the output power of the stripe-patterned ODR LED exceeds that of the typical ODR and DBR LEDs by a factor of 1.15 and 2 times, respectively, and with an acceptable forward voltage of about 2.2 V.

Electrically driven nanopyramid green light emitting diode

An electrically driven nanopyramid green light emitting diode (LED) was demonstrated. The nanopyramid arrays were fabricated from a GaN substrate by patterned nanopillar etch, pillar side wall passivation, and epitaxial regrowth. Multiple quantum wells were selectively grown on the facets of the nanopyramids. The fabricated LED emits green wavelength under electrical injection. The emission exhibits a less carrier density dependent wavelength shift and higher internal quantum efficiency as compared with a reference c-plane sample at the same wavelength. It shows a promising potential for using nanopyramid in high In content LED applications.

GaN-based LEDs with Al-deposited V-shaped sapphire facet mirror

A GaN-based light-emitting diodes (LEDs) with V-shaped sapphire facet reflector was fabricated using the double transferred scheme and sapphire chemical wet etching. The {1-102} R-plane V-shaped facet reflector with a 57/spl deg/ against {0001} C-axis has the superior capability for enhancing the light extraction efficiency. The light output power of the V-shaped sapphire facet reflector LED was 1.4 times higher than that of a flat reflector LED at an injection current of 20 mA. The significant improvement is attributable to the geometrical shape of sapphire facet reflector that efficiently redirects the guided light inside the chip toward to the top escape-cone of the LED surface.

High brightness AlGaInP-based light emitting diodes by adopting the stripe-patterned omni-directional reflector

An n-side-up AlGaInP-based LED operating at a wavelength of 630 nm with a stripe-patterned omni-directional reflector (ODR) was fabricated by adopting the adhesive-layer bonding scheme. It is demonstrated that the periodic and geometrical shape of the stripe-patterned array improves the light extraction efficiency by increasing the extraction of guided light. Compared to the conventional ODR LED, the stripe-patterned ODR LED significantly enhanced the output power and with only a slightly higher forward voltage. This improvement was analysed by the scanning near-field optical microscope (SNOM) and the optimized dimension of stripe patterns was also calculated on the basis of a Monte Carlo ray tracing simulation. According to the above analysis, the increase of light extraction could not only be attributed to the geometrical shape of the stripe patterns that redirect the trapped light towards the top-escaping cone of the LED surface but also to the repetitive stripe-patterned array of diffracting elements that effectively diffract the guided light outside the LED surface. Moreover, the optimized dimension of the stripe pattern is 3 µ mw ide, 2µm deep and spaced 3 µm apart, which coincides with experimental results. (Some figures in this article are in colour only in the electronic version)

AlGaInP Light-Emitting Diodes with Stripe Patterned Omni-Directional Reflector

An n-side-up AlGaInP-based light-emitting diode (LED) with a stripe-patterned omni-directional reflector (ODR) was fabricated by adopting the adhesive-layer bonding scheme. Comparing to the conventional ODR LED, the stripe-patterned ODR LED significantly enhanced the output power with an only slight higher forward voltage. This improvement was analyzed by the scanning near-field optical microscope (SNOM) and could be attributable to the geometrical shape of stripe patterns that redirects the guided light towards to the escaping cone of the LED surface. The optimized dimension of stripe patterns was also calculated by simply modeling a LED dice with a stripe-patterned ODR structure, according to optical ray-tracing method.

InGaN LEDs prepared on β-Ga2O3(-201) substrates

We fabricated InGaN LEDs prepared on β-Ga2O3 (201) single-crystal substrates. The substrates were produced by using the edge-defined film-fed growth (EFG) method. A Si-doped GaN epitaxial layer was grown on an electrically conductive β-Ga2O3 (201) substrate by metal organic chemical vapor deposition (MOCVD). The full-width at half maximum (FWHM) of (0002) and (101 1) X-ray rocking curves (XRCs) of the Si-doped GaN layer were 220 arcsec and 223 arcsec, respectively. The dark spot density measured by cathode luminescence (CL) was approximately 1.5×108 cm-2. The crystalline quality was equal to that of GaN layer on sapphire. We fabricated a vertical LED in the p-side down configuration. The peak wavelength was approximately 450 nm. The p-contact metal area was 300 ×300 μm2. The light output power did not saturate at 1000 A/cm2. This device characteristic indicates the great potential of Ga2O3 for use in high-power LEDs.

High efficient InGaN blue light emitting diode with embedded nanoporous structure

We report a highly efficient GaN-based blue light-emitting diodes (LEDs) structure with an emitting wavelength of 450nm on flat sapphire substrate by utilizing a nano-porous (NP) GaN insertion layer. Unlike the LED on patterned sapphire substrates (PSS), the presented substrate has a new morphology which not only can generate an embedded nano-dimensional void structure as a mirror layer to reflect the light from active layers for enhancing the light extraction, but can also easily enlarge the wafer size to a large scale, such as wafer diameter larger than 6 inches. With a chip size of 45 mil × 45 mil under a driving current of 350 mA, the light output powers of the NP GaN LEDs without and with encapsulation are 455 and 554 mW respectively. The light output power is improved about 2 -fold comparing to the LED on a flat sapphire substrate, and even comparable to the LED on PSS which all of them have a flat p-type GaN surface. The characterization and performance of this newly NP LED structure will be discussed in detail.