However, conventional inorganic thin-fi lm light-emitting diodes (LEDs) emit only a single color that is determined by the quantum well layer thickness and composition. Achieving multiple color generation from inorganic LEDs on a substrate is a major obstacle to using inorganic semiconductors in fullcolor displays. To overcome this obstacle, we used multifacetted gallium nitride (GaN) nanorod arrays with In x Ga 1 − x N/GaN multiple quantum wells (MQWs) anisotropically formed on the nanorod tips and sidewalls. For various electroluminescence (EL) colors, current injection paths were controlled through a continuous p-GaN layer depending on the applied bias voltage. Here, we report on the fabrication and characteristics of monolithic, full-color, tunable LEDs, whose EL color can be tuned continuously from red to blue by adjusting the external electric bias. The basic strategy for epitaxial growth of multifacetted GaN nanostructures and fabrication of color-tunable LEDs is shown in Figure 1 a. To obtain the LED structure, the GaN nanorod arrays were grown on n + -GaN/Al 2 O 3 (0001) substrates with a submicrometer-hole-patterned SiO 2 growth-mask layer using catalyst-free, selective metal–organic vapor-phase epitaxy (MOVPE). As shown in the scanning electron microscopy (SEM) image in Figure 1 b, a vertically aligned GaN nanorod array exhibited excellent uniformity, with a mean length, dia meter, and neighbor spacing of 520, 220, and 550 nm, respectively, all of which could be controlled by changing the lithographic

We report on the enhanced light output of GaN-based flip-chip light-emitting diodes (LEDs) fabricated with SiO2/TiO2 distributed Bragg reflector (DBR) on mesa sidewall. At the wavelength of 400 nm, five pairs of SiO2/TiO2 DBR coats on the GaN layer showed a normal-incidence reflectivity as high as 99.1%, along with an excellent angle-dependent reflectivity. As compared to the reference LED, the LED fabricated with the DBR-coated mesa sidewall showed an increased output power by a factor of 1.32 and 1.12 before and after lamp packaging, respectively. This could be attributed to an efficient reflection of the laterally guided mode at the highly reflective mesa sidewall, enhancing the subsequent extraction of light through the sapphire substrate.

We have investigated the In/indium tin oxide (ITO) scheme for obtaining high-quality Ohmic contacts to Ga-face and N-face nGaN for InGaN-based light-emitting diodes (LEDs). The In/ITO contacts to Ga-face n-GaN become Ohmic with specific contact resistances of 1.8 x 10(-3) Omega cm(2) when annealed at 30 degrees C for I min in a N-2 ambient The resistance of the In/ITO contacts to N-face n-GaN is shown to be much lower than that of the contacts to Ga-face n-GaN. This result indicates that the In/ITO scheme can serve as a highly-promising n-type Ohmic contact for vertical LEDs.

We report on the enhanced quantum efficiency of GaN-based light-emitting diodes (LEDs) fabricated on inverted hexagonal pyramid dielectric mask (IHPDM)-embedded structure. The ray-tracing calculation showed that the extraction efficiency of LEDs fabricated on IHPDM-embedded structure could be enhanced up to 56%. Compared to the reference, the n-GaN template grown on IHPDM-embedded structure also showed a reduction in the dislocation density by 57%, leading to an increase in photoluminescence intensity by 82%. The LED fabricated on IHPDM-embedded structure exhibited a reduction in the forward leakage current by one order of magnitude (<1.5 V) and an enhancement in the output power by 41%.

An InGaN/InGaN multiquantum well (MQW) structure with a reduced internal electric field is grown, and compared with a conventional InGaN/GaN MQW structure. Time-integrated and time-resolved photoluminescence (PL) are measured as a function of an external bias voltage. The flatband condition, in which the external bias voltage completely compensates the internal electric field, is found by a measurement of PL peak energy as a function of bias voltage. From the measurement of the integrated PL intensity and the PL lifetime, we observe that tunneling has an important role in the carrier decay process of the biased MQW structure. Using the flatband condition, the internal electric field is calculated to be 1.75 and 2.15 MV/cm for InGaN/InGaN and InGaN/GaN structures, respectively.

Enhancement of light extraction in a GaInN light-emitting diode (LED) employing a conductive omnidirectional reflector (ODR) consisting of GaN, an indium-tin oxide (ITO) nanorod low-refractive-index layer, and an Ag layer is presented. An array of ITO nanorods is deposited on p-type GaN by oblique-angle electron-beam deposition. The refractive index of the nanorod ITO layer is 1.34 at 461nm, significantly lower than that of dense ITO layer, which is n=2.06. The GaInN LEDs with GaN∕low-n ITO/Ag ODR show a lower forward voltage and a 31.6% higher light-extraction efficiency than LEDs with Ag reflector. This is attributed to enhanced reflectivity of the ODR that employs the low-n ITO layer.

We have demonstrated the enhancement of the output power of ultraviolet GaN-based light-emitting diodes (LEDs) by using one-dimensionally nanopatterned Cu-doped indium oxide(CIO)/indium tin oxide (ITO) p-type contact layers. The one-dimensional (1D) nanopatterns (250nm in width and 100nm in depth) are defined using a TiO2 1D nanomask fabricated by means of a surface relief grating technique. When fabricated with the nanopatterned p-contact layers, the output power of LEDs is improved by 40 and 63% at 20mA as compared to those fabricated with the unpatterned CIO/ITO and conventional Ni∕Au contacts, respectively.

We report on the enhancement of the light output of near-UV (298 nm) GaN-based light-emitting diodes (LEDs) by using nanopatterned indium tin oxide (ITO) p-type contact layers. One-dimensional (1D) and two-dimensional (2D) nanopatterns are defined using a TiO2 nano-mask, fabricated by means of a surface relief grating technique. The LEDs fabricated with the I D and 2D nanopatterned p-type electrodes produce higher output powers by 33-48% (at 20 mA) as compared to those fabricated with the unpatterned contacts. The pattern-induced improvement of the output power is described in terms of the fort-nation of the sidewalls of p-type electrodes.

The external quantum efficiency of nitride light-emitting diodes (LEDs) grown on sapphire substrates is greatly influenced by the internal reflection of lights at the nitride-substrate interface. To improve the external quantum efficiency of nitride LEDs with an InGaN multi-quantum well (MQW) structure, we introduced various patterned structures at the interface. The output power was increased by 1.7 times by incorporating an optimized patterned structure in nitride LEDs. In this study, the corrugated interface substrate (CIS) pattern was fabricated by standard photolithography and subsequent reactive ion etching (RIE) process. When the flip-chip type LED grown on CIS was operated at a forward bias current 20mA at room temperature, emission wavelength, output power, and external quantum efficiency were 400 nm, 21 mW and 34%, respectively.

The external efficiency of normal LEDs is reduced by total internal reflection, which typically causes 50 to 80 percent of the emitted photons to be trapped in guided modes. We have increased the external quantum efficiency by using the corrugated interface substrate (CIS) texturing on the sapphire substrates to scatter light out of the nitride film. In order to maximize the outcoupling efficiency of the corrugation, we stimulated the outcoupling efficiency by varying the diameter, the period and the depth of the corrugations. With the use of the optimized corrugation pattern, the external quantum efficiency of 34% was achieved experimentally, without detriment to the forward and reverse electrical characteristics of LEDs. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated high-quality ohmic contacts for flip-chip light emitting diodes using Zn-Ni nanoparticles/Ag schemes. The Zn-Ni nanoparticles/Ag contacts produce specific contact resistances of 10(-5)-10(-6) omegacm2 when annealed at temperatures of 330-530 degrees C for 1 min in air ambient, which are much better than those obtained from the Ag contacts. It is shown that blue InGaN/GaN multi-quantum well light emitting diodes fabricated with the annealed Zn-Ni nanoparticles/Ag contacts give much lower forward-bias voltages at 20 mA compared with those of the multi-quantum well light emitting diodes made with the as-deposited Ag contacts. It is further presented that the multi-quantum well light emitting diodes made with the Zn-Ni nanoparticles/Ag contacts show similar output power compared to those fabricated with the Ag contact layers.

We have demonstrated the improvement of the light-output power of GaN-based light-emitting diodes (LEDs) using hole-patterned indium tin oxide (ITO) p-type electrodes. Hole patterns were defined by a laser holographic lithography combined with a postlithography deposition process. It is shown that near-UV LEDs made with the patterned ITO with a hole period of 710nm and a size of 320nm give 23% and 67% higher light-output power (at 20mA) than those of LEDs with unpatterned ITO and Ni∕Au contacts, respectively. It is further shown that the reduction of the hole period results in an additional improvement of light-output power.

The junction temperature rise of light emitting diodes due to self‐heating effects during operation of the LED is measured using the electro‐luminescence of the band‐to‐band recombination. This method is useful for the junction temperature monitoring of small geometry devices, indirectly. The junction temperature measured in InGaN/GaN multi‐quantum well LEDs with 1 mm2 device size rises to 180 °C when the input current is 380 mA. The relationship between the junction temperature and the LED efficiency is clarified with experimental results. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In-rich InGaN quantum dot structures were grown by metalorganic chemical vapor deposition. Growth at low temperature made possible the growth of InGaN layers of high In content (over 70 InN%) with negligible formation of In metal droplets. The density, average diameter and height of typical InGaN quantum dots (QDs) were estimated at 8×109/cm2, 80 and 1.2 nm, respectively. The emission wavelength from the QDs could be controlled to the near ultraviolet (UV) region by variation of the growth conditions. This work demonstrates that In-rich InGaN QD active layers are very promising device structures for application as UV light-emitting diodes.

20 nm GaNAs epilayers were grown on GaN/sapphire by metalorganic chemical vapor deposition. As growth temperature decreased from 720 to 565 °C, it was found that As concentration was increased from 1.3 × 1020 and saturated at about 5–6 × 1020 cm–3. GaAs-like GaNAs islands were formed on GaN at 530 °C. In case of GaNAs epilayers grown at 565 and 600 °C, GaN-like GaNAs phases were observed by high-resolution X-ray diffraction (XRD) and low angle XRD. By low angle XRD and high-resolution TEM with electron nano-beam diffraction patterns, the FCC-stacked region in wurtzite matrix was clearly observed in thin GaNAs layers. We propose that this structural nonuniformity in GaNAs might be caused by the nonuniform distribution of As concentration. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In-rich InGaN/GaN single quantum wells were grown by metalorganic chemical vapor deposition for the first time to the best of our knowledge. The structures consist of a 2-mum thick GaN buffer layer, a 2-nm thick In-rich InGaN single quantum well, and a 20 nm thick GaN capping layer. Single quantum well structures were examined by transmission electron microscopy. Photoluminescence emissions from the single quantum well samples were observed at wavelengths ranged from 400 nm to 500 nm depending upon the growth conditions of the InN layer. From a simple energy level calculation, we found the possibility of extremely large emission peak shift with well thickness. (C) 2003 WILEYNCH Verlag GmbH & Co. KGaA, Weinheim.