GaN-based Vertical Light-emitting Diodes Research Articles

Improvement of interfacial adhesion in vertical GaN-based LEDs by introducing O 2 plasma cleaning and intermediate layers

Interfacial adhesion between an indium tin oxide (ITO)/Ni/Ag/Ni/Au p-electrode, and Au and Ni/Au seeds in vertical GaN-based light emitting diodes (LEDs) was enhanced by O 2 plasma cleaning treatment of the Au surface in the p-electrode. However, AES and REELS analyses of the Au surface in the p-electrode detected surface damage to the p-electrode and photoresist (PR) passivation structure from O 2 plasma cleaning. W/Ni and Al/Ni adhesion layers were introduced in the Au seed to increase interfacial adhesion between Au seed and untreated PR passivation. Forward leakage current as low as 0.91 nA at 2 V was observed for the vertical LED with the Al/Ni/Au seed, for which adhesion strength to O 2 plasma-cleaned Au and untreated PR was 141.2 MPa and 62.8 MPa, respectively.

Enhanced Performance of Vertical GaN-Based Light-Emitting Diodes with a Current-Blocking Layer and Electroplated Nickel Substrate

The performance of vertical-structure metallic-substrate GaN-based light-emitting diodes (VM-LEDs) with a patterned SiO2 film as the current-blocking layer (CBL) was investigated. From theoretical calculations of current and light distributions and experimental results on current–voltage (I–V) and light output power–current (L–I) characteristics, we found that SiO2 CBL inserted under the n-pad electrode increases light output power by 35.4% at 20 mA as compared with VM-LEDs without CBL. Such an improvement is attributed to the insulated CBL structure, which provides better current spreading and less photon absorption and/or reflection at the n-electrode.

Effect of Interfacial Properties of p-GaN/Sputter-Deposited NiAg-Based Electrode on Optical Properties of Vertical GaN-Based LEDs

The influence of the interfacial adhesion and reflectance of a p-GaN/NiAgNiAu p-electrode on the optical properties of vertical GaN-based light emitting diodes (LEDs) was investigated. The thickness of the sputter-deposited Ni ohmic contact was varied from 2 to 100 nm. The p-electrode with a 2 nm thick Ni layer showed the highest adhesion strength of 160 MPa to the p-GaN and the highest reflectance of 80.79% after annealing because all the Ni atoms participated in the indiffusion into p-GaN and formed a transparent NiO. The NiAgNiAu p-electrodes with a higher reflectance led to the improved output power of the vertical LEDs regardless of the interfacial adhesion strength of p-GaN/Ni.

Fabrication of High Performance GaN-Based Vertical Light-Emitting Diodes Using a Transparent Conducting Indium Tin Oxide Channel Layer

We fabricate blue GaN-based vertical light-emitting diodes (LEDs) using transparent insulating SiO 2 and conducting indium tin oxide (ITO) channel layers connected to the passivation layer by a combined process of electroplated deposition and a laser lift-off technique. It is shown that unlike the SiO 2 layer, the ITO layer effectively serves as current spreading and injection layers. LEDs fabricated with the ITO channel layer exhibit a slightly higher reverse current when the voltage exceeds ―10 V. However, LEDs fabricated with the ITO layer produce higher output power (by ∼20% at 100 mA) compared to LEDs with the SiO 2 layers.

Enhanced Light Output Power of GaN-Based Vertical Light-Emitting Diodes by Using Highly Reflective ITO–Ag–Pt Reflectors

Highly reflective and thermally stable indium-tin-oxide (ITO)-Ag-Pt p-type reflectors for use in high-performance GaN-based light-emitting diodes (LEDs) have been investigated. The specific contact resistance of the ITO-Ag-Pt contacts was found to be 7.2 t10-5Omegamiddotcm2. The ITO-Ag-Pt contacts showed a higher reflectance after thermal annealing (82% at 460 nm), while the reflectance of the ITO-Ag contacts was reduced from 81% to 65%. In addition, surface agglomeration was drastically decreased, indicating that the Pt layer efficiently prevents the surface agglomeration of the Ag layer. The vertical LEDs (VLEDs) fabricated with the ITO-Ag-Pt contacts had a 17% higher output power (at 20 mA) than the VLEDs fabricated with the ITO-Ag contacts.

Light Output Power Enhancement of GaN-based Vertical Light Emitting Diodes via Fabricating Surface Nanorod Arrays

In this research, we demonstrated two approaches to fabricate nanorod arrays on the surface of GaN-based vertical light emitting diodes (VLEDs). The first approach was via synthesizing single-crystal ZnO nanorod arrays in aqueous solution at room temperature. The light output intensity and wall-plug efficiency of the GaN-based VLEDs with the ZnO nanorod arrays shows 38.9 and 41.2% increases, respectively, at 200 mA current injections compared to that of a VLED without ZnO nanorod arrays. The second approach was formed by spinning nano silica spheres and inductively coupled plasma-reactive ion etching. The light output power of the VLEDs with the dry etched nanorod arrays showed about 40% enhancement than that without the nanorod arrays. The large enhancement of VLED with both ZnO or GaN nanorod arrays was mainly due to the total internal reflection destruction, and the gradually changed refractive index from the nanorod arrays layer.

Enhancement of Light Output Intensity by Integrating ZnO Nanorod Arrays on GaN-Based LLO Vertical LEDs

Enhancement of light output intensity for GaN-based vertical light-emitting diodes (LEDs), combining wafer bonding and the laser lift-off (LLO) process, employing an omnidirectional extraction surface with synthesized single-crystal ZnO nanorod arrays in aqueous solution at room temperature is presented. The light output intensity and wall-plug efficiency of the GaN-based LLO vertical LED with the omnidirectional extraction surface by ZnO nanorod arrays shows 38.9 and 41.2% increases, respectively, at 200 mA current injections compared to that of a vertical LED without ZnO nanorod arrays. The ZnO nanorod arrays not only support a current spreading layer but enhance the probability of photon escape through the omnidirectional extraction surface.

Fabrication and characteristics of thin-film InGaN–GaN light-emitting diodes with TiO2/SiO2 omnidirectional reflectors

In this paper, a novel GaN-based thin-film vertical injection light-emitting diode (LED) structure with a TiO2 and SiO2 omnidirectional reflector (ODR) and an n-GaN rough surface is designed and fabricated. The designed ODR, consisting of alternating TiO2 and SiO2, layers possesses a complete photonic band gap within the blue region of interest. The arrays of the conducting channels are integrated into the TiO2/SiO2 ODR structure for vertically spreading the current. Assisted by the laser lift-off and photo-enhanced chemically etched surface roughening process, the light output power and the external quantum efficiency of our thin-film LED with a TiO2/SiO2 ODR (at a driving current of 350 mA and with chip size of 1 mm × 1 mm) reached 330 mW and 26.7%, increased by 18% and 16%, respectively, compared with the results from the thin-film LED with an Al mirror. By examining the radiation patterns of the LEDs, the optical output power mainly increased within the 120 deg cone due to the higher reflectance of the TiO2/SiO2 ODR within the blue regime.

High Brightness GaN-Based Light-Emitting Diodes

This paper reviews our recent progress of GaN-based high brightness light-emitting diodes (LEDs). Firstly, by adopting chemical wet etching patterned sapphire substrates in GaN-based LEDs, not only could increase the extraction quantum efficiency, but also improve the internal quantum efficiency. Secondly, we present a high light-extraction 465-nm GaN-based vertical light-emitting diode structure with double diffuse surfaces. The external quantum efficiency was demonstrated to be about 40%. The high performance LED was achieved mainly due to the strong guided-light scattering efficiency while employing double diffuse surfaces

High Light-Extraction GaN-Based Vertical LEDs With Double Diffuse Surfaces

High light-extraction (external quantum efficiency ~40%) 465-nm GaN-based vertical light-emitting diodes (LEDs) employing double diffuse surfaces were fabricated. This novel LED structure includes one top transmitted diffuse surface and another diffuse omnidirectional reflector (ODR) on the bottom of a LED chip. The diffusive ODR consists of a roughened p-type GaN layer, an indium-tin-oxide (ITO) low refractive index layer, and an Al layer. The surface of the p-type GaN-layer was naturally roughened while decreasing the growth temperature to 800 degC. After flip-bonding onto a Si substrate by AuSn eutectic metal and laser lift-off processes to remove the sapphire substrate, an anisotropic etching by dilute potassium hydroxide (KOH) was employed on the N-face n-GaN layer to obtain transmitted diffuse surfaces with hexagonal-cone morphology. The double diffused surfaces LEDs show an enhancement of 56% and 236% in light output power compared to single side diffused surface and conventional LEDs, respectively. The devices also show a low leakage current in the order of magnitude of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> A at -5 V and a calculated external quantum efficiency of about 40%. The high scattering efficiency of double diffused surfaces could be responsible for the enhancement in the device light output power

A study of transparent contact to vertical GaN-based light-emitting diodes

In this study, transparent indium tin oxide (ITO) deposited by sputtering was applied to laser lift-off (LLO) GaN-based vertical light-emitting diodes (VLEDs) and the electrical and optical properties of ITO films were measured as a function of annealing conditions. The measured minimum resistivity of ITO film was about 3.78×10−4Ωcm and the measured optical transmittance at 460nm was 96.8% after the annealing process. In this condition, about 1×10−5Ωcm2 of ITO contact resistance to LLO n-GaN could be obtained. By applying the transparent ITO layer to the LLO GaN-based VLEDs, a significant decrease of the forward operating voltage from 3.3to3.8V at 20mA could be obtained.

Highly efficient vertical laser-liftoff GaN-based light-emitting diodes formed by optimization of the cathode structure

Vertical GaN-based light-emitting diodes (LEDs) were fabricated using a laser-liftoff process and the effect of the cathode processing conditions on the properties of the LEDs was investigated. Surface roughening by 10%Cl2∕90%BCl3 plasma etching improved the light emission intensity at an operating current of 20mA; however, the forward operating voltage was increased due to the thin and rough n‐GaN layer. The use of an indium tin oxide (ITO) contact on the roughened n-type GaN surface decreased the forward voltage significantly, by decreasing the spreading resistance of the n-type GaN contact without decreasing the emission intensity. Through the combination of the ITO contact and the surface roughness of the n‐GaN layer, a 100% increase in the extraction efficiency was obtained compared to that of a lateral GaN device, with maintaining a similar forward operating voltage.