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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
;
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.
AlN Surface Passivation of GaN-Based High Electron Mobility Transistors by Plasma-Enhanced Atomic Layer Deposition
(2017-04-27)
Tzou, An-Jye
;
Chu, Kuo-Hsiung
;
Lin, I-Feng
;
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.
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
;
Chen, Szu-Hung
;
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-Efficiency InGaN/GaN Core-Shell Nanorod Light-Emitting Diodes With Low-Peak Blueshift and Efficiency Droop
(2017-03)
Tzou, An-Jye
;
Hsieh, Dan-Hua
;
Hong, Kuo-Bin
;
Lin, Da-Wei
;
Huang, Jhih-Kai
In this study, novel three-dimensional (3-D) nanoscale structures and methodology are demonstrated for application in high-efficiency core-shell nanorod (NR) light-emitting diodes (LEDs). The key to our successful growth of the structures is the introduction of passivation, which can be used to selectively grow active layers in desired structures. Through the fabrication methodology, core-shell NR green LEDs exhibiting large nonpolar active region and homogeneous indium distributions without a point tip shape were achieved. Stable light emission at a central wavelength of 518 nm was achieved as the injected current increased to more than 40 mA. The improved NR LEDs exhibited a stable luminescence emission wavelength (blueshift of 62 nm) and low-efficiency droop (18.1%) at 200 mA. Our scheme is scalable and compatible with current technologies, which provides a new perspective for developing high-performance, 3-D nanoscale optoelectronic devices.
Non-thermal alloyed ohmic contact process of GaN-based HEMTs by pulsed laser annealing
(2016-05)
Tzou, An-Jye
;
Hsieh, Dan-Hua
;
Chen, Szu-Hung
;
Li, Zhen-Yu
;
Chang, Chun-Yen
We have demonstrated Si implantation incorporation into GaN HEMTs with a non-alloyed ohmic contact process. We optimized the power density of pulsed laser annealing to activate implanted Si dopants without a thermal metallization process. The experimental results show that the GaN surface will be reformed under the high power density of the illumination conditions. It provides a smooth surface for following contact engineering and leads to comparable contact resistance. The transmission line model (TLM) measurement shows a lower contact resistance to 6.8. x. 10(-7) Omega . cm(2) via non-alloyed contact technology with significantly improved surface morphology of the contact metals. DC measurement of HEMTs shows better current and on-resistance. The on-resistance could be decreased from 2.18 to 1.74 m Omega-cm(2) as we produce a lower contact resistance. Pulsed laser annealing also results in lower gate leakage and smaller dispersion under a pulse I-V measurement, which implies that the density of the surface state is improved.
Coreshell InGaN/GaN MQW Nanorod Photovoltaic Device
(2015)
Li, Yun-Jing
;
Tsai, Yu Lin
;
Chang, Jet-Rung
;
Chang, Shih-Pang
;
Lin, Chien-Chung
We report the fabrication and study of coreshell InGaN/GaN multiple quantum well (MQW) nanorod photovoltaic device. The nanorods were fabricated from a GaN substrate by top-down etch, followed by InGaN/GaN MQW growth. The 3D geometry allows higher Indium incorporation and extends the solar absorption spectral range as compared with a planar reference sample. The proof-of-concept coreshell nanorod photovoltaic device has a fill factor of about 54.2%, the short current density of 1.16 mA/cm(2), the open circuit voltage of 0.68 V, and the power conversion efficiency of 0.38%, respectively.
Efficiency improvement of GaN-based ultraviolet light-emitting diodes with reactive plasma deposited AlN nucleation layer on patterned sapphire substrate
(2014-09-16)
Lee, Chia-Yu
;
Tzou, An-Jye
;
Lin, Bing-Cheng
;
Lan, Yu-Pin
;
Chiu, Ching-Hsueh
The flip chip ultraviolet light-emitting diodes (FC UV-LEDs) with a wavelength of 365 nm are developed with the ex situ reactive plasma deposited (RPD) AlN nucleation layer on patterned sapphire substrate (PSS) by an atmospheric pressure metal-organic chemical vapor deposition (AP MOCVD). The ex situ RPD AlN nucleation layer can significantly reduce dislocation density and thus improve the crystal quality of the GaN epitaxial layers. Utilizing high-resolution X-ray diffraction, the full width at half maximum of the rocking curve shows that the crystalline quality of the epitaxial layer with the (RPD) AlN nucleation layer is better than that with the low-temperature GaN (LT-GaN) nucleation layer. The threading dislocation density (TDD) is estimated by transmission electron microscopy (TEM), which shows the reduction from 6.8 x 10(7) cm(-2) to 2.6 x 10(7) cm(-2). Furthermore, the light output power (LOP) of the LEDs with the RPD AlN nucleation layer has been improved up to 30 % at a forward current of 350 mA compared to that of the LEDs grown on PSS with conventional LT-GaN nucleation layer.
High-Efficiency and Crack-Free InGaN-Based LEDs on a 6-inch Si (111) Substrate With a Composite Buffer Layer Structure and Quaternary Superlattices Electron-Blocking Layers
(2014-05-01)
Li, Zhen-Yu
;
Lee, Chia-Yu
;
Lin, Da-Wei
;
Lin, Bing-Cheng
;
Shen, Kun-Ching
In this paper, a composite buffer layer structure (CBLS) with multiple AlGaN layers and grading of Al composition/u-GaN1/(AlN/GaN) superlattices/u-GaN2 and InAlGaN/AlGaN quaternary superlattices electron-blocking layers (QSLs-EBLs) are introduced into the epitaxial growth of InGaN-based light-emitting diodes (LEDs) on 6-inch Si (111) substrates to suppress cracking and improve the crystalline quality and emission efficiency. The effect of CBLS and QSLs-EBL on the crystalline quality and emission efficiency of InGaN-based LEDs on Si substrates was studied in detail. Optical microscopic images revealed the absence of cracks and Ga melt-back etching. The atomic force microscopy images exhibited that the root-meansquare value of the surface morphology was only 0.82 nm. The full widths at half maxima of the (0002) and (1012) reflections in the double crystal X-ray rocking curve were similar to 330 and 450 arcs, respectively. The total threading dislocation density, revealed by transmission electron microscopy, was <6 x 10(8) cm(-2). From the material characterizations, described above, blue and white LEDs emitters were fabricated using the epiwafers of InGaNbased LEDs on 6-inch Si substrates. The blue LEDs emitter that comprised blue LEDs chip and clear lenses had an emission power of 490 mW at 350 mA, a wall-plug efficiency of 45% at 350 mA, and an efficiency droop of 80%. The white LEDs emitter that comprised blue LEDs chip and yellow phosphor had an emission efficiency of similar to 110 lm/W at 350 mA and an efficiency droop of 78%. These results imply that the use of a CBLS and QSLs-EBL was found to be very simple and effective in fabricating high-efficiency InGaN-based LEDs on Si for solid-state lighting applications.
Study of InGaN-Based Light-Emitting Diodes on a Roughened Backside GaN Substrate by a Chemical Wet-Etching Process
(2011-10-01)
Fu, Yi-Keng
;
Chen, Bo-Chun
;
Fang, Yen-Hsiang
;
Jiang, Ren-Hao
;
Lu, Yu-Hsuan
The InGaN-based light-emitting diodes (LEDs) with a roughened backside on the N-face surface of GaN substrate were fabricated through a chemical wet-etching process to increase light-extraction efficiency. The stable crystallographic etching planes were formed as the GaN {10 (11) over bar} planes. When the near-ultraviolet and blue LED were operated as a forward current of 20 mA, the output power of LEDs was improved from 13.2 and 19.9 mW to 25.6 and 24.0 mW, respectively. The different enhanced ratio is attributed to the different transmittance as a function of wavelength is caused from hexagonal pyramid on N-face GaN substrate after wet-etching process.
Output Power Enhancement of Light Emitting Diodes with Defect Passivation layer
(2010)
Lo, Ming-Hua
;
Tu, Po-Min
;
Cheng, Yuh-Jen
;
Wang, Chao-Hsun
;
Hung, Cheng-Wei
We demonstrate high efficiency blue light emitting diodes with defect passivation layers. The defect passivation layers were formed by defect selective wet etching, SiO(2) deposition, and chemical mechanical polishing process. The process does not require photolithography patterning. The threading dislocation density of grown sample was reduced down to similar to 4x10(7) cm(-2). The defect passivated epi-wafer is used to grow light emitting diode (LED) and the output power of the fabricated chip is enhanced by 45% at 20 mA compared to a reference one without using defect passivation.

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