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Investigation of Efficiency Droop for UV-LED with N-type AlGaN Layer
(2012-01-01)
Tu, Po-Min
;
Chang, Jet-Rung
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Huang, Shih-Cheng
;
Yang, Shun-Kuei
;
Lin, Ya-wen
The efficiency droop in InGaN-based 380nm UV light emitting device (LED) with n-GaN and n-AlGaN underlayer grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD) was investigated. From simulation result of high resolution x-ray diffraction (HRXRD) omega-2 theta. curve by using dynamical diffraction theory, the Al composition in the n-AlGaN layer was determined to be about 3%. The experimental results of temperature dependent photoluminescence (PL) demonstrated that the internal quantum efficiency (IQE) of n-GaN and n-AlGaN UV-LEDs are 43% and 39%, respectively, which are corresponding to an injected carrier density of 8.5 x 10(17) #/cm(3). It could be explained that the crystal quality of n-GaN is better than of n-AlGaN. In addition, the observation of pit density from atomic force microscopy (AFM) surface morphology is consistent with the interpretation. It was well-known that the pits appearing on the surface in the virtue of the threading dislocations. Thus, it means that defects induce the non-radiative centers and deteriorate the IQE of the UV-LED with n-AlGaN underlayer. Therefore, the light output power of n-GaN UV-LED is slightly higher below the forward current of 250 mA. Nevertheless, the output power was enhanced about 22% as the injection current was increased to 600 mA. Furthermore, the external quantum efficiency (EQE) of n-AlGaN UV-LED was nearly retained at the 600 mA (only 20% droop), whereas the UV-LED with n-GaN exhibits as high as 33%. We attributed this improvement to the less self-absoption by replacing n-GaN underlayer with n-AlGaN.
High Electron Mobility AlGaN/AlN/GaN HEMT Structure With a Nano-scale AlN Interlayer
(2012-01-01)
Huang, Shih-Chun
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Chen, Wen-Ray
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Hsu, Yu-Ting
;
Lin, Jia-Ching
;
Chang, Kuo-Jen
Epitaxies of AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with different thickness of nano-scale AlN interlayers have been realized by metalorganic chemical vapor deposition (MOCVD) technology. After epitaxy, high resolution X-ray diffraction (HRXRD), temperature-dependent Hall Effect and atomic force microscopy (AFM) measurements were used to characterize the properties of these samples. First, it was found that the Al composition of AlGaN layer increases from 21.6 to 34.2% with increasing the thickness of AlN interlayer from 0 to 5 nm under the same AlGaN growth conditions. This result may due to the influences of compressive stress and Al incorporation induced by the AlN interlayer. Then, we also found that the room-temperature (RT) electron mobility stays higher than 1500 cm(2)/Vs in the samples within AlN interlayer thickness range of 1.5 nm, on the other hand, the low-temperature (80K) electron mobility drops dramatically from 8180 to 5720 cm(2)/Vs in the samples with AlN interlayer thickness increasing from 1 to 1.5 nm. Furthermore, it was found that the two-dimensional electron gas (2DEG) density increases from 1.15x10(13) to 1.58x10(13) cm(-2) beyond the AlN interlayer thickness of 1 nm. It was also found that the temperature-independent 2DEG densities are observed in the samples with AlN interlayer thickness of 0.5 and 1 nm. The degenerated characteristics of the samples with AlN thickness thicker than 1.5 nm show the degraded crystalline quality which matched the observation of surface defects and small cracks formations from their AFM images. Finally, the 2DEG mobilities of the proposed structures can be achieved as high as 1705 and 8180 cm(2)/Vs at RT and 80K, respectively.
Abnormal PL spectrum in InGaN MQW surface emitting cavity
(2008-01-01)
Chu, J. T.
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Cheng, Y. -J.
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Kuo, H. C.
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Lu, T. C.
;
Wang, S. C.
We report the observation of an abnormal photoluminescent (PL) spectrum from a HeCd laser pumped InGaN multiple quantum well (MQW) vertical cavity. The device is fabricated using standard MOCVD deposition on a (0001)-oriented sapphire substrate. The layer structures are: 10nm nucleation layer, a 4um bulk GaN layer, InGaN MQWs, and a final 200nm GaN cap layer. The InGaN MQWs consist of 10 pairs of 5 nm GaN barrier and 3 run In0.1Ga0.9N well. The peak emission of the as-grown MQWs sample was similar to 420nm. A dielectric distributed Bragg reflectors (DBR) were then coated on the top layer, followed by a laser lift off from sapphire substrate, and subsequently another DBR coated on the bulk GaN bottom surface. The cavity has a quality factor of similar to 520 from 400-490nm. The device was pumped by a focused CW HeCd laser from the bulk GaN side. When the laser is focused onto the InGaN MQWs, a photoluminescent spectrum centered at the designed MOW wavelength was observed as expected. However, when the focused position was moved toward the bulk GaN region, an additional abnormal PL peak around 460nm was observed. This is far outside the designed MQW wavelength.
Analysis of Strain Relaxation and Emission Spectrum of A Free-Standing GaN-based Nanopillar
(2008-01-01)
Wu, Yuh-Renn
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Yu, Peichen
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Chiu, C. H.
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Chang, Cheng-Yu
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Kuo, H. C.
We have made a, GaN-based single nanopillar with a, diameter of 300nm using the focused ion beam (FB) technique. The micro-photoluminescence (mu-PL) from the embedded GaN/InGaN multi-quantum wells reveals a blue shift of 68.3 meV in energy. In order to explain the spectrum shift, we have developed a valence force field model to study the strain relaxation mechanism in a, single GaN-based nanopillar structure. The strain distribution and strain induced polarization effect inside the multiple quantum wells is added to our self-consistent Poisson, drift-diffusion, and Schrodinger solver to study the spectrum shift of mu-PL.
Light emission characteristics of nonpolar a-plane GaN-based photonic crystal defect cavities
(2015-01-01)
Kao, Tsung Sheng
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Wu, Tzeng-Tsong
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Tsao, Che-Wei
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Lin, Jyun-Hao
;
Lin, Da-Wei
In this paper, nonpolar a-plane GaN-based photonic crystals (PCs) with different defect cavities have been demonstrated. By using a micro-photoluminescence (mu-PL) system operated at 77 K, the dominant resonant modes of the GaN-based PC defect cavities show high quality factor (Q) values in the light emission performance which can be up to 4.3x10(3). Moreover, the degree of polarization (DOP) of the light emission from the nonpolar GaN-based PC defect cavities was measured to achieve around 64 % along the m crystalline direction.
3D structural construction of GaN-based light emitting diode by confocal micro-Raman spectroscopy
(2016-01-01)
Li, Heng
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Chang, Chiao-Yun
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Cheng, Hui-Yu
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Chen, Wei-Liang
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Huang, Yi-Hsin
The key issue for light emission strength of GaN-based LEDs is the high defect density and strain in MQWs causing the electric polarization fields. In this work, we construct 3D confocal microspectroscopy to clarify strain distribution and the relationship between photoluminescence (PL) intensity and pattern sapphire substrate (PSS). From 3D construction of E-2(high) Raman and PL mapping, the dislocation in MQW can be traced to the cone tip of PSS and the difference in E-2(high) Raman mapping between substrate and surface is also measured. The ability to measure strain change in 3D structure nondestructively can be applied to explore many structural problems of GaN-based optoelectronic devices.
AlN Surface Passivation of GaN-Based High Electron Mobility Transistors by Plasma-Enhanced Atomic Layer Deposition
(2017-04-27)
Tzou, An-Jye
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Chu, Kuo-Hsiung
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Lin, I-Feng
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Ostreng, Erik
;
Fang, Yung-Sheng
We report a low current collapse GaN-based high electron mobility transistor (HEMT) with an excellent thermal stability at 150 degrees C. The AlN was grown by N-2-based plasma enhanced atomic layer deposition (PEALD) and shown a refractive index of 1.94 at 633 nm of wavelength. Prior to deposit AlN on III-nitrides, the H-2/NH3 plasma pretreatment led to remove the native gallium oxide. The X-ray photoelectron spectroscopy (XPS) spectroscopy confirmed that the native oxide can be effectively decomposed by hydrogen plasma. Following the in situ ALD-AlN passivation, the surface traps can be eliminated and corresponding to a 22.1% of current collapse with quiescent drain bias (V-DSQ) at 40 V. Furthermore, the high temperature measurement exhibited a shift-free threshold voltage (V-th), corresponding to a 40.2% of current collapse at 150 degrees C. The thermal stable HEMT enabled a breakdown voltage (BV) to 687 V at high temperature, promising a good thermal reliability under high power operation.
Luminous efficiency enhancement of white light-emitting diodes by using a hybrid phosphor structure
(2015-02-19)
Lin, Bing-Cheng
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Huang, Jhih-Kai
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Chen, Kuo-Ju
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Chiu, Sheng-Huan
;
Tu, Zong-Yi
This paper presents a "hybrid" structure for the coating of yellow YAG: Ce3(+) phosphor on blue GaN-based light-emitting diodes (LEDs). The luminous efficiency of the hybrid phosphor structure improved by 5.9% and 11.7%, compared with the conventional remote and conformal phosphor structures, respectively, because of the increased intensity of the yellow component. The hybrid structure also has an advantage in the phosphor usage reduction for the LEDs. Furthermore, the electric intensity of the hybrid phosphor structure was calculated for various thicknesses by conducting TFCalc32 simulation, and the enhanced utilization of blue rays was verified. Finally, the experimental results were consistent with the simulation results performed using the Monte-Carlo method. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)
An Investigation of Carbon-Doping-Induced Current Collapse in GaN-on-Si High Electron Mobility Transistors
(2016-06-01)
Tzou, An-Jye
;
Hsieh, Dan-Hua
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Chen, Szu-Hung
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Liao, Yu-Kuang
;
Li, Zhen-Yu
This paper reports the successful fabrication of a GaN-on-Si high electron mobility transistor (HEMT) with a 1702 V breakdown voltage (BV) and low current collapse. The strain and threading dislocation density were well-controlled by 100 pairs of AlN/GaN superlattice buffer layers. Relative to the carbon-doped GaN spacer layer, we grew the AlGaN back barrier layer at a high temperature, resulting in a low carbon-doping concentration. The high-bandgap AlGaN provided an effective barrier for blocking leakage from the channel to substrate, leading to a BV comparable to the ordinary carbon-doped GaN HEMTs. In addition, the AlGaN back barrier showed a low dispersion of transiently pulsed I-D under substrate bias, implying that the buffer traps were effectively suppressed. Therefore, we obtained a low-dynamic on-resistance with this AlGaN back barrier. These two approaches of high BV with low current collapse improved the device performance, yielding a device that is reliable in power device applications.
High Performance Metal-Gate/High-kappa GaN MOSFET With Good Reliability for Both Logic and Power Applications
(2016-09-01)
Yi, Shih-Han
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Ruan, Dun-Bao
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Di, Shaoyan
;
Liu, Xiaoyan
;
Wu, Yung Hsien
The gate-recessed GaN MOSFET on a Si substrate is demonstrated to achieve a record highest normalized transistor current (mu C-ox) of 335 mu A/V-2 (410 mA/mm at L-G = 5 mu m and only V-G = 4 V), I-ON/I-OFF of 9 orders of magnitude, small 79 mV/dec sub-threshold slope, a low oxide/GaN interface trap density of 1.2 x 10(10) eV(-1)/cm(2), a low on-resistance of 17.0 Omega-mm, a high breakdown voltage of 720-970 V, and excellent reliability of only 40 mV Delta V-T after 175 degrees C 1000 s stress at maximum drive current. Such excellent device integrities are due to the high-kappa gate dielectric and the high conduction band offset (Delta E-C) of SiO2/GaN. From the calculation results of self-consistent Schrodinger and Poisson equations, the good reliability of GaN MOSFET is related to the confined carrier density within the GaN channel, which is in sharp contrast to the strong wave-function penetration into the high-trap density AlGaN barrier in the AlGaN/GaN HEMT.

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