Shih-Chun Ling

Low efficiency droop in blue-green m-plane InGaN/GaN light emitting diodes

We investigated the electroluminescence and relatively external quantum efficiency (EQE) of m-plane InGaN/GaN light emitting diodes (LEDs) emitting at 480 nm to elucidate the droop behaviors in nitride-based LEDs. With increasing the injection current density to 100 A/cm2, the m-plane LEDs exhibit only 13% efficiency droop, whereas conventional c-plane LEDs suffer from efficiency droop at very low injection current density and the EQE of c-plane LEDs decrease to as little as 50% of its maximum value. Our simulation models show that in m-plane LEDs the absence of polarization fields manifest not only the hole distribution more uniform among the wells but also the reduction in electron overflow out of electron blocking layer. These results suggest that the nonuniform distribution of holes and electron leakage current due to strong polarization fields are responsible for the relatively significant efficiency droop of conventional c-plane LEDs.

Dislocation reduction in GaN grown on stripe patterned r-plane sapphire substrates

Extended defect reduction in GaN can be achieved via direct growth on stripe patterned (11¯02) r-plane sapphire substrates by metal organic chemical vapor deposition. The striped mesa is along [112¯0] with two etched sides in {0001} and {11¯01} faces. GaN grown on both etched facets in epitaxy exhibit different crystallographic relationships with sapphire substrate which are (11¯02)sapphire‖(112¯0)GaN and [112¯0]sapphire‖[1¯100]GaN, and (0001)sapphire‖(0001)GaN and [112¯0]sapphire‖[1¯100]GaN, respectively. The dislocation densities can be significantly reduced through epitaxial growth on the inclined lateral faces of mesas. Dislocation density in the order of ∼107cm−2 can be achieved in the tilted GaN.

Nanorod epitaxial lateral overgrowth of a-plane GaN with low dislocation density

The crystal quality of a-plane GaN films was improved by using epitaxial lateral overgrowth on a nanorod GaN template. The investigation of x-ray diffraction showed that the strain in a-plane GaN grown on r-plane sapphire could be mitigated. The average threading dislocation density estimated by transmission electron microscopy was reduced from 3×1010 to 3.5×108 cm−2. From the temperature-dependent photoluminescence, the quantum efficiency of the a-plane GaN was enhanced by the nanorod epitaxial lateral overgrowth (NRELOG). These results demonstrated the opportunity of achieving a-plane GaN films with low dislocation density and high crystal quality via NRELOG.

Characteristics of a-Plane Green Light-Emitting Diode Grown on r-Plane Sapphire

In this work, we have successfully grown a-plane green light-emitting diodes (LEDs) on r-plane sapphire and investigated the device characteristics of a-plane green LEDs. The apparent emission polarization anisotropy was observed and the polarization degree was as high as 67.4%. In addition, the electroluminescence (EL) spectra first revealed a wavelength blue-shift with increasing drive current to 20 mA, which could be attributed to the band-filling effect, and then the EL peak become constant. The current-voltage curve showed the forward voltage of a-plane LED grown on r-plane sapphire substrate was 3.43 V and the differential series resistance was measured to be about 24 Omega as 20-mA injected current. Furthermore, the output power was 240 muW at 100-mA drive current.

Extraction Efficiency Enhancement of GaN-Based Light-Emitting Diodes by Microhole Array and Roughened Surface Oxide

The light-output power of GaN-based light-emitting diodes (LEDs) was enhanced by microhole array pattern and roughened GaOx film grown on the exposed surface. The GaOx film was grown by photoelectrochemical (PEC) oxidation via H2O and formed a naturally rough oxide surface and GaOx/GaN interface. Compared with that of conventional broad-area LEDs, the output power of the microhole array LED and the surface-oxidized microhole array LED increased by 1.38 and 1.82 times at 20-mA forward current, respectively. The results show that the microhole array pattern with the roughened surface oxide method could significantly enhance light extraction efficiency and be a candidate for manufacturing high-efficient low-cost GaN-based LEDs.

Investigation of Emission Polarization and Strain in InGaN–GaN Multiple Quantum Wells on Nanorod Epitaxially Lateral Overgrowth Templates

Non-polar (a-plane) InGaN-GaN multiple quantum wells (MQWs) on the GaN nanorod epitaxially lateral overgrowth templates with different nanorod height have been fabricated. The average in-plane strain in the InGaN MQWs has been determined from 2.73 × 10-2 to 2.58 × 10-2 while the nanorod height in templates increases from 0 to 1.7 μm. The polarization ratio of the emission from InGaN MQWs varies from 85 % to 53 % along with the increase of the GaN nanorod height. The reduction of polarization ratio has been attributed to the partial strain relaxation within the epitaxial structures as a result of growth on the GaN nanorod templates and the micro-size air-voids observed in the nanorod templates.

Optical Properties of A-Plane InGaN/GaN Multiple Quantum Wells Grown on Nanorod Lateral Overgrowth Templates

A-plane InGaN/GaN multiple-quantum wells (MQWs) were grown on a series of nanorod epitaxial lateral overgrowth (NRELOG) templates with varied nanorod depth. Optical properties of these samples were investigated by excitation power and temperature-dependent photoluminescence (PL). Due to the absence of quantum-confined Stark effect, the negligible PL emission peak shift and nearly identical power index for all samples were observed. In contrast to the as-grown MQWs, the thermal activation energy and internal quantum efficiency of NRELOG MQWs exhibit 1.6-fold and 4-fold increases, respectively, which are attributed to the improvement of crystal quality by NRELOG. Furthermore, the Shockley-Read-Hall nonradiative coefficient, determined from the fits of power-dependent PL quantum efficiency, is also apparently reduced while MQWs are grown on NRELOG GaN template. The results show the feasibility to fabricate high radiative efficiency a-plane devices via NRELOG.

Performance Enhancement of

We have fabricated a-plane light-emitting diodes (LEDs) with inserted InGaN/GaN superlattices. The structural characteristics and device performance of a-plane light-emitting diodes with/without superlattices were also identified. Transmission electron microscope (TEM) images revealed that the threading dislocation (TD) density for the sample using superlattices was reduced from 3 10 10 cm 2 down to 9 10 9 cm 2 . The electroluminescence (EL) intensity of the sample with InGaN/GaN superlattices was enhanced by a factor of 3.42 times to that of the conventional sample without InGaN/GaN superlattices. Furthermore, we observed that the polarization degree of a-plane LEDs with superlattices (56.3%) was much higher than that without superlattices (27.4%). A series of experiments demonstrated that the feasibility of using InGaN/GaN superlattices for the TD reduction and the improvement of luminescence performance in a-plane III–nitride devices. # 2009 The Japan Society of Applied Physics

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Non-polar (a-plane) InGaN-GaN multiple quantum wells (MQWs) on the GaN nanorod epitaxially lateral overgrowth templates with different nanorod height have been fabricated. The average in-plane strain in the InGaN MQWs has been determined from 2.73 × 10-2 to 2.58 × 10-2 while the nanorod height in templates increases from 0 to 1.7 μm. The polarization ratio of the emission from InGaN MQWs varies from 85 % to 53 % along with the increase of the GaN nanorod height. The reduction of polarization ratio has been attributed to the partial strain relaxation within the epitaxial structures as a result of growth on the GaN nanorod templates and the micro-size air-voids observed in the nanorod templates.

-Plane Light-Emitting Diodes Using InGaN/GaN Superlattices

We have fabricated a-plane light-emitting diodes (LEDs) with inserted InGaN/GaN superlattices. The structural characteristics and device performance of a-plane light-emitting diodes with/without superlattices were also identified. Transmission electron microscope (TEM) images revealed that the threading dislocation (TD) density for the sample using superlattices was reduced from 3 10 10 cm 2 down to 9 10 9 cm 2 . The electroluminescence (EL) intensity of the sample with InGaN/GaN superlattices was enhanced by a factor of 3.42 times to that of the conventional sample without InGaN/GaN superlattices. Furthermore, we observed that the polarization degree of a-plane LEDs with superlattices (56.3%) was much higher than that without superlattices (27.4%). A series of experiments demonstrated that the feasibility of using InGaN/GaN superlattices for the TD reduction and the improvement of luminescence performance in a-plane III–nitride devices. # 2009 The Japan Society of Applied Physics