Kwang-Choong Kim

High Power and High External Efficiency m-Plane InGaN Light Emitting Diodes

High power and high efficiency nonpolar m-plane (1100) nitride light emitting diodes (LEDs) have been fabricated on low extended defect bulk m-plane GaN substrates. The LEDs were grown by metal organic chemical vapor deposition (MOCVD) using conditions similar to that of c-plane device growth. The output power and external quantum efficiency (EQE) of the packaged 300 ×300 µm2 was 23.7 mW and 38.9%, respectively, at 20 mA. The peak wavelength was 407 nm and <1 nm redshift was observed with change in drive current from 1–20 mA. The EQE shows a minimal drop off at higher currents.

Demonstration of Nonpolar m-Plane InGaN/GaN Laser Diodes

The first nonpolar m-plane (1-100) nitride laser diodes (LDs) have been realized on low extended defect bulk m-plane GaN substrates. The LDs were grown by metal organic chemical vapor deposition (MOCVD) using conditions similar to that of c-plane device growth. Broad area lasers were fabricated and tested under pulsed conditions. Lasing was observed at duty cycles as high as 10%. These laser diodes had threshold current densities (Jth) as low as 7.5 kA/cm2. Stimulated emission was observed at 405.5 nm, with a spectral line-width of 1 nm.

Defect reduction in nonpolar a-plane GaN films using in situ SiNx nanomask

We report on the use of in-situ SiNx nanomask for defect reduction in nonpolar a-plane GaN films, grown by metal-organic chemical vapor deposition. High-resolution x-ray diffraction analysis revealed that there was a monotonic reduction in the full width at half maximum, both on-axis and off-axis, with the increase in the SiNx thickness. Atomic force microscopy images revealed a significant decrease in the root-mean-square roughness and the density of submicron pits. Cross-section and plan-view transmission electron microscopy on the samples showed that the stacking fault density decreased from 8×105to3×105cm−1 and threading dislocation density decreased from 8×1010to9×109cm−2. Room temperature photoluminescence measurement revealed that the band-edge emission intensity increased with the insertion of the SiNx layer, which suggests reduction in the nonradiative recombination centers.

AlGaN-Cladding-Free Nonpolar InGaN/GaN Laser Diodes

We have previously demonstrated the AlGaN-cladding-free (ACF) laser diode (LD) concept over a wide visible spectral range and various nonpolar/semipolar crystallographic orientations. The benefits of this ACF epitaxial design include lower operating voltages and higher production yields. Nonpolar LDs have been demonstrated out to 500nm1 with semipolar reaching as long as 534nm2. No results have been published on nonpolar/semipolar orientations for short wavelength LDs. In this paper we report on the short wavelength limits of this epitaxial design on nonpolar bulk m-plane GaN substrates.

Study of nonpolar m-plane InGaN∕GaN multiquantum well light emitting diodes grown by homoepitaxial metal-organic chemical vapor deposition

Nonpolar m-plane (11¯00) InGaN-based light emitting diodes (LEDs) grown on low-extended defect density bulk m-plane GaN substrates offer great potential for high performance devices due to the absence of polarization-related internal electric fields. To optimize the quantum well (QW) structure, systematic sets of near blue-ultraviolet LEDs using different well widths, barrier widths, and QW periods were packaged and tested. With increasing current, high power LEDs were realized with fairly flat external quantum efficiency and blueshift-free peak wavelength for QWs with thicknesses from 8to20nm, barrier widths from 10to22nm, and QW numbers from 4 to 10.

Defect-mediated surface morphology of nonpolar m-plane GaN

The role of extended defects in determining the atomic scale surface morphology of nonpolar {11¯00} m-plane gallium nitride has been elucidated. The heteroepitaxially grown m-GaN films are commonly reported to yield striated surface morphologies (slate morphology) correlated with their high densities of basal plane stacking faults. Here, the growth window was explored to allow nonslate morphologies for hydride vapor phase epitaxy. Lateral epitaxial overgrowth was then utilized to produce m-GaN films with three regimes of different extended defect contents. Elimination of stacking faults from the m-GaN yielded step-flow features with an average step height of 4–7 ML even for slate morphology growth conditions.

Low extended defect density nonpolar m-plane GaN by sidewall lateral epitaxial overgrowth

Sidewall lateral epitaxial overgrowth (SLEO) is demonstrated by metal organic chemical vapor deposition for nonpolar {11¯00} m-plane GaN films. m-plane GaN films were grown by metal organic chemical vapor deposition on m-plane 6H SiC substrates with an AlN initiation layer. Subsequently, an SiO2 stripe dielectric pattern was formed with 2μm window openings parallel to the [112¯0] a-direction and an 8μm mask and then the m-plane GaN was etched through the window openings to reveal Ga-face (0001) and N-face (0001¯) sidewalls. The SLEO growth was achieved in two growth steps—lateral growth from the sidewalls and subsequent growth through and then over the dielectric mask openings. In comparison to planar m-plane GaN films grown on 6H SiC, the threading dislocation density was reduced from low 1010to3×108cm−2 and the stacking fault density was reduced by one order of magnitude.

Electroluminescence efficiency of (1\,0\,\bar{1}\,0) -oriented InGaN-based light-emitting diodes at low temperature

This paper discusses radiative recombination efficiency in electroluminescence of InGaN-based light-emitting diodes prepared on the (1?0??0) plane. Radiative efficiency was studied over a wide range of temperatures and drive currents on four types of LED samples with different InGaN active-layer thicknesses. Efficiency was minimally affected by active-layer thickness, yet was a strong function of temperature and current. Efficiency reduction at high current was observed on these LEDs, which confirms strain-induced electric polarization fields are not a dominant mechanism. Luminescence intensity was found to be proportional to the square root of current at low temperature. Acceptor freeze-out was suggested to induce hole depletion at increased current; shortage of holes resulted in reduced efficiency and triggered off electron injection into the p-type layer to sustain total current. Injected electrons were shown to lead to the square-root relationship by solving rate equations.

Recent Performance of Nonpolar/Semipolar GaN-based Blue LEDs/LDs

Violet InGaN/GaN light emitting diodes (LEDs) was fabricated on semipolar GaN bulk substrates. The output power and external quantum efficiency at a driving current of 20 mA were measured. The first nonpolar m-plane nitride laser diodes (LDs) were realized on low extended defect bulk m-plane GaN substrates.

High Power and High External Efficiency m-Plane InGaN LEDs

In this paper, we report on the growth and fabrication of high power and high efficiency nonpolar m-plane (11 macr00) nitride light emitting diodes (LEDs) on low defect density bulk m-plane GaN substrates. The LEDs were grown by MOCVD on free-standing bulk m -plane GaN. All MOCVD growth was performed at atmospheric pressure, at typical V/III ratios (>3000), and at typical temperature ranges (875 to 1185degC). Standard processing was used to fabricate the LEDs. After device fabrication, the LEDs were diced and packaged using standard die and wire bonding techniques and then molded with epoxy.