In this study, high performance nitride-based flip-chip (FC) light-emitting diodes (LEDs) using optimized distributed bragg reflector (DBR) were fabricated and compared with conventional FC-LED using silver (Ag) reflector. Most of FCLEDs are using the silver (Ag) as reflector due to its superior reflectance at visual spectrum region. However, A silver has detrimental problems such as electro-chemical migration and agglomerations, which resulting in reliability issues such as degradation of power drop, unstable operating voltage and leakage issues. Our DBR structure was designed to have 99% at whole visible spectrum range (400~750nm), which is higher reflectance than silver reflector (90~95%). Optical power is higher than higher than the Ag-LED up to 30% @ 500mA. As the current increases up to 1A, the gap slightly decreased. Reliability test results show stable optical power, operating voltage, and leakage maintenance.
We have fabricated the near-ultraviolet (NUV) flip-chip (FC) light-emitting diodes (LEDs) with the high external quantum efficiency (EQE) using distributed Bragg reflectors (DBRs) and compared with conventional FC-LED using silver (Ag) reflector. Reflectance of Ag is very high (90 ~ 95 %) at visible spectrum region, but sharply decrease at NUV region. Therefore we used DBR composed of two different materials which have high-index contrast, such as TiO2 and SiO2. However, to achieve high-performance NUV flip-chip LEDs, we used Ta2O5 instead of TiO2 that absorbs lights of NUV region. Thus, we have designed a DBR composed of twenty pairs of Ta2O5 and SiO2 using optical coating design software. The DBR designed by our group achieves a reflectance of ~99 % in the NUV region (350 ~ 500 nm), which is much better than Ag reflector. Optical power is higher than the Ag-LED up to 22 % @ 390 nm.
Although Si and Mg impurities are essential elements for n and p type GaN, unintentional incorporation into
InGaN-multiple quantum wells (MQWs) seriously affects the optical property of LEDs. Si doping in MQWs obstructs
the hole carrier transport and induces the dead quantum wells (QWs) of MQWs. Also, Mg impurity diffusion from p-
GaN into MQWs degrades the radiative recombination rate of the QWs placed near Mg doped p-GaN layer. In this paper,
the effects of Si and Mg impurities on the optical property were systematically investigated.
ZnO has been considered as a substrate for epitaxial growth of III-Nitrides due to its close lattice and stacking order
match. This paper will cover growth of InxGa1-xN epitaxial layer on lattice-matched ZnO substrates by metal-organic
chemical vapor deposition (MOCVD). InGaN of various indium compositions from different growth temperatures were
well controlled in the InGaN films on ZnO substrates. High-resolution X-ray diffraction (HRXRD) confirmed the
epitaxial growth of InGaN film on ZnO. The optical and structural characterization of InGaN epilayer on ZnO substrates
was measured by room temperature photoluminescence, temperature-dependent photoluminescence, and field-emission
secondary electron microscope. In addition, a transition layer of Al2O3 on ZnO substrates have been employed for
InGaN growth to help prevent Zn and O diffusion into the epilayers as well as assist nitride epilayer growth. HRXRD
results show a single crystal InGaN film has been successfully grown on annealed Al2O3 coated ZnO substrates.
In this work, ZnO has been investigated as a substrate technology for GaN-based devices due to
its close lattice match, stacking order match, and similar thermal expansion coefficient. Since
MOCVD is the dominant growth technology for GaN-based materials and devices, there is a need
to more fully explore this technique for ZnO substrates. Our aim is to grow low defect density
GaN for efficient phosphor free white emitters. However, there are a number of issues that need
to be addressed for the MOCVD growth of GaN on ZnO. The thermal stability of the ZnO
substrate, out-diffusion of Zn from the ZnO into the GaN, and H2 back etching into the substrate
can cause growth of poor quality GaN. Cracks and pinholes were seen in the epilayers, leading to
the epi-layer peeling off in some instances. These issues were addressed by the use of H2 free
growth and multiple buffer layers to remove the cracking and reduce the pinholes allowing for a
high quality GaN growth on ZnO substrate.
Conference Committee Involvement (11)
Sixteenth International Conference on Solid State Lighting and LED-based Illumination Systems
9 August 2017 | San Diego, California, United States
Fifteenth International Conference on Solid State Lighting and LED-based Illumination Systems
31 August 2016 | San Diego, California, United States
Fourteenth International Conference on Solid State Lighting and LED-based Illumination Systems
12 August 2015 | San Diego, California, United States
Thirteenth International Conference on Solid State Lighting and LED-based Illumination Systems
20 August 2014 | San Diego, California, United States
LED-based Illumination Systems
26 August 2013 | San Diego, California, United States
Twelfth International Conference on Solid State Lighting and Fourth International Conference on White LEDs and Solid State Lighting
13 August 2012 | San Diego, California, United States
Eleventh International Conference on Solid State Lighting
22 August 2011 | San Diego, California, United States
Tenth International Conference on Solid State Lighting
2 August 2010 | San Diego, California, United States
Ninth International Conference on Solid State Lighting
3 August 2009 | San Diego, California, United States
Eighth International Conference on Solid State Lighting
11 August 2008 | San Diego, California, United States
Seventh International Conference on Solid State Lighting
27 August 2007 | San Diego, California, United States
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