Joonmo Park

Doping selective lateral electrochemical etching of GaN for chemical lift-off

An electrochemical etching based on oxalic acid was developed for use in the chemical lift-off of GaN epitaxial structures. It was shown that only the Si-doped n-GaN layer was etched away, while the p-type and undoped GaN layers were not etched at all. The etch rate and the remaining structure were analyzed for various doping concentrations and etching voltages. A lateral etch rate of 12 μm/min was achieved under 60 V for n-type doping concentration of 8×1018 cm−3. This doping selective etching was used to lift-off a GaN epitaxial layer patterned into 300×300 μm2 squares.

Improved Light Output of Photonic Crystal Light-Emitting Diode Fabricated by Anodized Aluminum Oxide Nano-Patterns

A photonic crystal (PC) light-emitting diode (LED) was fabricated using an anodized aluminum oxide (AAO) nano-pattern technique, which has potential applications to mass production. The AAO nano-pattern, which was anodized in 0.3-M oxalic acid at 70 V, was transferred to the n-GaN bottom cladding through reactive ion etching and covered during the regrowth of GaN to form periodic air holes. The PC was embedded into the bottom cladding, which resulted in a low operating voltage. For the PC LED, the light output was enhanced by more than 20%, while the operating voltage increased only slightly. The radiation pattern of the PC LED showed two lobes at approximately plusmn30deg, which originated from the diffraction by PC.

High Diffuse Reflectivity of Nanoporous GaN Distributed Bragg Reflector Formed by Electrochemical Etching

A GaN distributed Bragg reflector (DBR) based on nanoporous GaN was fabricated by doping selective electrochemical etching. Twenty-one pairs of undoped-GaN/n-GaN were grown and only n-GaN was converted into a nanoporous layer during the etching. From spectral reflectance measurement, clear stopbands were observed, but the maximum reflectivity was limited to 87% at ~450 nm in specular reflection geometry. It was attributed to significant scattering in the DBR caused by the non-uniform formation of nanopores. However, the diffuse reflectivity of nanoporous DBR was much higher (~100%), which could be advantageous for a diffuse reflection mirror in a light-emitting diode.

Reduction of interpore distance of anodized aluminum oxide nano pattern by mixed H3PO4:H2SO4 electrolyte.

A self-formed and ordered anodized aluminum oxide (AAO) nano pattern has generated considerable interest in both scientific research and commercial application. However, the interpore distance obtainable by AAO is limited by 40-500 nm depending on electrolyte and anodizing voltage. Its believed that below-30 nm AAO pattern is a key technology in the fabrication semiconductor nano structures with enhanced quantum confinement effect, so we worked on the reduction of interpore distance of AAO with a novel electrolyte. AAO nano patterns were fabricated with mixed H2SO4 and H3PO4 as an electrolyte for various voltages and temperatures. The interpore distance and pore diameter of AAO were decreased with reduced anodizing voltage. As a result, an AAO nano pattern with the interpore distance of 27 nm and the pore diameter of 7 nm was obtained. This is the smallest pattern, as long as we know, reported till now with AAO technique. The fabricated AAO pattern could be utilized for uniform and high density quantum dots with increased quantum effect.

Reduced Indium Fluctuation in InGaN Quantum Well Grown on GaN Air Bridge

A GaN air bridge was fabricated by electrochemical etching and was used as a template for the regrowth of an InGaN quantum well (QW). Raman spectroscopy confirmed that the GaN membrane on the air bridge relieved part of its compressive strain, and its effect on the InGaN growth was studied. A μ-photoluminescence (PL) measurement showed a large blueshift of band-to-band emission for the QW grown on the air bridge. The small residual strain reduced the indium fluctuation during the growth of the InGaN QW and resulted in a low defect density and low activation energy of emission intensity. As a result, the QW grown on the air bridge had a better internal quantum efficiency as measured by temperature-dependent PL.

Selective lateral anodic etching of n-doped GaN without photo-assistance for lift-off application

An anodic etching technique for selective lateral wet etching of n-doped GaN without the photo-assistance was described. N-doped GaN was successfully etched with 0.3 M oxalic acid at 40 V.

Enhancement of Light Extraction Efficiency of Light-Emitting Diode with Hexagonal Photonic Crystal Layer

The efficiency of light emitting diode (LED) is limited because large amount of generated light is confined inside of it by total internal reflection. A photonic crystal (PC) layer embedded in LED structure substantially modifies the guiding properties inside the chip and prevents the lateral propagation of light, so that it largely increases the output power of an LED. In this paper, we present both numerical and experiment studies on the enhancement of light extraction of GaN-based light-emitting diodes (LEDs) with hexagonal PC layer. By finite difference time domain (FDTD) simulation, the PC parameters were varied in order to evaluate the enhancement. Best extraction efficiency was obtained with the lattice constant of 400 - 600 nm, the PC thickness of 150 - 200 nm and the ratio of hole radius to lattice constant of 0.3 - 0.4 for the 465 nm LED based on GaN. Furthermore, hexagonal PC GaN-based LED was fabricated using anodic aluminum oxide (AAO) method. The PC layer is located below quantum well active layer and the efficiency was improved more than 20%. It was shown that these numerical results agree reasonably well with the experimental results.