Hyun Kyong Cho

Light extraction enhancement from nano-imprinted photonic crystal GaN-based blue light-emitting diodes.

The nano-imprint lithography method was employed to incorporate wide-area (375 x 330 mum(2)) photonic-crystal (PC) patterns onto the top surface of GaN-based LEDs. When the 280-nm-thick p-GaN was partly etched to ~140 nm, the maximal extraction-efficiency was observed without deteriorating electrical properties. After epoxy encapsulation, the light output of the PC LED was enhanced by 25% in comparison to the standard LED without pattern, at a standard current of 20 mA. By three-dimensional finite-difference time-domain method, we found that the extraction efficiency of the LED tends to be saturated as the etch-depth in the GaN epitaxial-layer becomes larger than the wavelength of the guided modes.

Laser Liftoff GaN Thin-Film Photonic Crystal GaN-Based Light-Emitting Diodes

We fabricated a thin-film vertical-injection light-emitting diode (LED), which uses a highly efficient coherent external scattering of trapped light by photonic crystal (PhC). The PhC patterns (a = 1200 nm) were formed on top of the n-GaN surface after laser liftoff of the sapphire substrate. This light extraction structure just above micron scale was prepared by a conventional photolithography (lambdac = 405 nm). The Si-gel-encapsulated thin-film LED with PhC patterns (a = 1 200 nm), which had a depth of 500 nm, demonstrated up to 76% improvement in light output power at a forward current of 60 mA, compared with the nonpatterned thin film LED.

Efficient GaN slab vertical light-emitting diode covered with a patterned high-index layer

We demonstrate the enhancement of light extraction from a wide-area (500×500μm2) GaN slab light-emitting diode (LED) that results from covering it with a TiO2-patterned layer. To fabricate this device, a Cu supporter is electroplated onto the p-GaN face followed by detaching the sapphire substrate with a laser lift-off process. At the standard current of 60mA, the wall-plug efficiency of the TiO2-patterned LED is ∼14.8%, i.e., the efficiency is enhanced by a factor of ∼1.8 over that of nonpatterned LEDs. Our three-dimensional finite-difference time-domain computations confirm that this output increases with the index of the patterned layer.

Deep level characteristics in n-GaN with inductively coupled plasma damage

The effects of energetic ion-induced damage on deep traps in n-GaN have been investigated using deep level transient spectroscopy. The energetic ions were produced in an inductively coupled plasma reactive ion etching (ICP-RIE) system. The electrons captured at the trap levels E1 (0.25 eV) and E2 (0.62 eV), in a control sample, were found to depend logarithmically on the duration of the filling pulse, indicating a relationship to dislocations. The dramatic increase in the concentration of deep level E1 traps, as a function of etching-bias voltage, is thought to indicate the introduction of a VN-related complex. On the other hand, the concentration of deep level E2 traps shows an initial increase at an etching-bias of −50 V, followed by a decrease at higher etching-bias voltages. This trend was also observed in the room-temperature yellow luminescence spectra and x-ray photoelectron spectroscopy, which suggests that the deep level E2 is associated with point defects in the form of VGa-impurity complexes.

Metal mirror assisting light extraction from patterned AlGaInP light-emitting diodes

We demonstrate light extraction from metal reflector-based AlGaInP photonic crystal (PhC) light-emitting diodes (LEDs). The photons reflected by a high-reflectivity, small-absorption, bottom Ag mirror steadily interact with the PhC, and thus enhanced light extraction is achieved. The square lattice PhC patterns are fabricated on an upper n-doped AlGaInP surface with a depth of 500 nm. An optical power measurement using an integration sphere shows that the extraction efficiency of the PhC LED is ∼1.8 times larger than that of the nonpatterned LED. A three-dimensional finite difference time domain simulation is performed to understand the output enhancement extracted by the PhC and the effect of internal absorption.We demonstrate light extraction from metal reflector-based AlGaInP photonic crystal (PhC) light-emitting diodes (LEDs). The photons reflected by a high-reflectivity, small-absorption, bottom Ag mirror steadily interact with the PhC, and thus enhanced light extraction is achieved. The square lattice PhC patterns are fabricated on an upper n-doped AlGaInP surface with a depth of 500 nm. An optical power measurement using an integration sphere shows that the extraction efficiency of the PhC LED is ∼1.8 times larger than that of the nonpatterned LED. A three-dimensional finite difference time domain simulation is performed to understand the output enhancement extracted by the PhC and the effect of internal absorption.

An improved non-alloyed ohmic contact Cr/Ni/Au to n-type GaN with surface treatment

The Cr/Ni/Au non-alloyed ohmic contact resistance on n-type GaN is obtained by chemical surface treatment of n-type GaN films following the laser lift-off of the sapphire substrate. The effects of n-GaN surface treatments on the metal/GaN interface were studied using x-ray photoelectron spectroscopy. Nitrogen vacancies at the n-type GaN surface are therefore produced and act as donors for electrons, improving the non-alloyed ohmic contact resistance induced by the reduction in native oxygen by the surface treatment of chemical solutions. In addition, the n-GaN surface treatment reduces the forward voltage (Vf) of the vertical LEDs.

Experimental study of light output power for vertical GaN-based light-emitting diodes with various textured surface and thickness of GaN layer

The light output power (LOP) of vertical-type GaN-based light emitting diodes (LED) with surface roughness (texture) can be changed by texture size, density, and thickness of GaN film or by the combined effects of texture formation and thickness of GaN film. We have investigated these changes experimentally and note that the enhancement of the LOP by a factor of 2.4 can be improved with optimum texturing parameters as compared to that without texturing. In addition, the LOP of GaN-based LEDs under the same texture density increase slightly as thickness of GaN film decreases. Base on these results, we have evidently demonstrated that the enhancement factors of LOP are related to the correlation between texture size (density) and thickness of GaN film.

Two inch large area patterning on a vertical light-emitting diode by nano-imprinting technology

A vertical light-emitting diode (LED) with a chip size of 500 × 500 µm2 was fabricated by the laser lift-off (LLO) process of an InGaN-based blue LED wafer. After the LLO process, photonic crystal patterns by UV nano-imprint lithography were formed on the n-GaN top layer of the vertical LED over the entire area with a diameter of 2 inches. As the result of n-GaN patterning, light output power of the vertical LED with photonic crystals was increased by up to 44% compared to that of the vertical LED without a photonic crystal at a driving current of 1000 mA.

Angle-tuned, evanescently-decoupled reflector for high-efficiency red light-emitting diode

We propose and demonstrate evanescently-decoupled, solid-angle-optimized distributed Bragg reflectors (DBRs) for AlGaInP light-emitting diodes (LEDs). The thickness of each DBR layer is tuned to the wavelength slightly longer than the emission peak of the active medium in order to maximize the radiated power integrated over the top surface. In addition, to increase the horizontal radiation through the side facets, the glancing-angle reflectivity at the AlInP/AlAs interface is improved by employing an AlAs layer thicker than the attenuation length of the evanescent field. With the improved DBR, the integrated output power of AlGaInP LEDs is enhanced by a factor of 1.9 in comparison to those of LEDs with conventional DBRs. Additional 1.25-fold enhancement is observed by incorporating an square-lattice hole array (a=1200nm) into the top GaP surface by a conventional photolithography.

Hole and interface traps in Mg-doped Al0.1Ga0.9N/GaN grown by metalorganic chemical vapor deposition

Deep level traps in Mg-doped Al0.1Ga0.9N/undoped GaN heterostructures grown by metalorganic vapor deposition with different Cp2Mg molar flow rates are studied by deep level transient spectroscopy (DLTS). Two distinct hole traps were observed, one with an activation energy of 0.18 eV (AD1) and the other with an activation energy of 0.88 eV (AD2) above the valence band maximum. From an analysis of the voltage dependence of DLTS spectra, the AD1 trap is associated with a defect in the Al0.1Ga0.9N/GaN interface based on the Poole–Frenkel model to interpret the enhancement of the thermal emission rate due to potential barrier lowering. The DLTS peak intensity of the AD2 trap steadily decreases with decreasing the Cp2Mg molar flow rate. It appears that the AD2 hole trap is strongly related to Mg-related deep levels, which contribute to the strong blue emission observed by photoluminescence measurements.