Browsing by Author "Lo, Ming-Hua"

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AlGaN/GaN multiple quantum wells grown by using atomic layer deposition technique
(2008)
Lo, Ming-Hua
;
Li, Zhen-Yu
;
Chen, Shih-Wei
;
Hong, Jhih-Cang
;
Lu, Ting-Chang
We report the successful growth of high quality ultraviolet (UV) AlGaN/GaN multiple quantum wells (MQWs) structure using atomic layer deposition (ALD) technique. The AlGaN/GaN MQW sample grown on the sapphire substrate consisted of three GaN QWs and four AlGaN barriers comprised AlN/GaN superlattices (SLs). From atomic force microscope measurement, the root-mean-square value of the surface morphology was only 0.35 nm, and no crack was found on the surface. The dislocation density was estimated to be as low as 2x10(8) cm(-2). X-ray and transmission electron microscope data show the grown MQW has shape interface with good periodicity. The sample has a deep UV photoluminescence emission at 334 nm (3.71 eV) with a very narrow linewidth of 45 meV at 13K. The cathodoluminescence image show fairly uniform luminescence pattern at room temperature. In conclusion, the AlGaN/GaN MQW grown by ALD technique should be useful for providing high crystalline quality AlGaN/GaN-based MQW for fabrication of AlGaN/GaN-based UV light emitting devices such as light emitting diodes and lasers.
Carrier localization degree of In(0.2)Ga(0.8)N/GaN multiple quantum wells grown on vicinal sapphire substrates
(2009-01-01)
Li, Zhen-Yu
;
Lo, Ming-Hua
;
Chiu, Ching-Hua
;
Lin, Po-Chun
;
Lu, Tien-Chang
In this work, we have grown In(0.2)Ga(0.8)N/GaN multiple quantum well (MQWs) epitaxial structure on vicinal sapphire substrates by low pressure metal-organic chemical vapor deposition and investigated the relationship between carrier localization degree and vicinal angles of sapphire substrates. The optical analysis confirmed that the In(0.2)Ga(0.8)N/GaN MQWs grown on 0.2 degrees-off sapphire substrate exhibited the smallest carrier localization degree and more ordered In(0.2)Ga(0.8)N/GaN MQW structure. In addition, mechanisms for carrier localization in In(0.2)Ga(0.8)N/GaN MQWs grown on vicinal substrate were discussed based on the results obtained from the power and temperature dependent photoluminescence measurements. The Raman spectrum showing the in-plane compressive stress of the GaN epitaxial structures grown on vicinal sapphire substrates revealed the relation between the dislocation density and the carrier localization degree in MQWs. From transmission electron microscopy images, the threading dislocation density (TDD) of In(0.2)Ga(0.8)N/GaN MQWs grown on 0.2 vicinal sapphire substrate at the bottom of n-GaN layer was about 9.4 X 10(8) cm(-2) and reduced to 3.0 X 10(8) cm(-2) at the top of n-GaN layer. We also obtained the TDD of 5.6 X 10(7) cm(-2) in the MQW region and only 1.0 X 10(7) cm(-2) in the p-GaN region. Based on the results mentioned above, 0.2 degrees-off substrate can offer In(0.2)Ga(0.8)N/GaN MQW blue light-emitting diode structures with benefits, such as high crystal quality, low defects, and small carrier localization degree. (C) 2009 American Institute of Physics. (DOI: 10.1063/1.3055264)
Carrier localization degree of In0.2Ga0.8N/GaN multiple quantum wells grown on vicinal sapphire substrates
(2009-01-01)
Li, Zhen-Yu
;
Lo, Ming-Hua
;
Chiu, Ching-Hua
;
Lin, Po-Chun
;
Lu, Tien-Chang
In this work, we have grown In0.2Ga0.8N/GaN multiple quantum well (MQWs) epitaxial structure on vicinal sapphire substrates by low pressure metal-organic chemical vapor deposition and investigated the relationship between carrier localization degree and vicinal angles of sapphire substrates. The optical analysis confirmed that the In0.2Ga0.8N/GaN MQWs grown on 0.2 degrees-off sapphire substrate exhibited the smallest carrier localization degree and more ordered In0.2Ga0.8N/GaN MQW structure. In addition, mechanisms for carrier localization in In0.2Ga0.8N/GaN MQWs grown on vicinal substrate were discussed based on the results obtained from the power and temperature dependent photoluminescence measurements. The Raman spectrum showing the in-plane compressive stress of the GaN epitaxial structures grown on vicinal sapphire substrates revealed the relation between the dislocation density and the carrier localization degree in MQWs. From transmission electron microscopy images, the threading dislocation density (TDD) of In0.2Ga0.8N/GaN MQWs grown on 0.2 vicinal sapphire substrate at the bottom of n-GaN layer was about 9.4 X 10(8) cm(-2) and reduced to 3.0 X 10(8) cm(-2) at the top of n-GaN layer. We also obtained the TDD of 5.6 X 10(7) cm(-2) in the MQW region and only 1.0 X 10(7) cm(-2) in the p-GaN region. Based on the results mentioned above, 0.2 degrees-off substrate can offer In0.2Ga0.8N/GaN MQW blue light-emitting diode structures with benefits, such as high crystal quality, low defects, and small carrier localization degree. (C) 2009 American Institute of Physics. (DOI: 10.1063/1.3055264)
Enhanced Stimulated Emission from Optically Pumped Gallium Nitride Nanopillars
(2011-02-01)
Lo, Ming-Hua
;
Cheng, Yuh-Jen
;
Kuo, Hao-Chung
;
Wang, Shing-Chung
;
光電工程學系
We report the observation of an enhanced stimulated emission from optically pumped GaN nanopillars. The nanopillars were fabricated from an epitaxial wafer by patterned etching followed by crystalline regrowth. When the sample was optically excited, a strong stimulated emission peak emerged from a broad spontaneous emission background. The emission was attributed to electron-hole plasma gain at high carrier density. The emission slope efficiency was greatly enhanced by 20 times compared with a GaN substrate under the same pumping configuration. We remark that the enhancement is due to better gain and photon interaction from the multiple scattering of photons among nanopillars. (C) 2011 The Japan Society of Applied Physics
Enhancing the Emission Efficiency of In(0.2)Ga(0.8)N/GaN MQW Blue LED by Using Appropriately Misoriented Sapphire Substrates
(2009-01-01)
Li, Zhen-Yu
;
Uen, Wu-Yih
;
Lo, Ming-Hua
;
Chiu, Ching-Hua
;
Lin, Po-Chun
In this paper, we report the fabrication of an In(0.2)Ga(0.8)N/GaN multiple-quantum-well (MQW) structure on vicinal sapphire substrates with a very small offset angle of 0-1 degrees by low-pressure metallorganic chemical vapor deposition. Our study demonstrates that the quality of the In(0.2)Ga(0.8)N/GaN MQW structure is very sensitive to the offset angle of the vicinal substrate. High-resolution X-ray diffraction analyses demonstrated high-order satellite peaks and clear fringes between them for all MQW structures fabricated, from which the interface roughness (IRN) was estimated. The IRN of the In(0.2)Ga(0.8)N/GaN MQW structure fabricated on 0.2 degrees off sapphire substrate was determined as 1.36% of the quantum well layer period. Besides, reciprocal lattice mapping was employed to examine the strain status of the MQW. The lattice relaxation of the same specimen mentioned above was estimated to be 7.4x10(-5). It is therefore manifested that an In(0.2)Ga(0.8)N/GaN MQW structure with abrupt interfaces and good layer periodicity was grown. From it, a shortest radiative lifetime of 14.2 ns and a lowest fluctuation of 5.6 meV in the emission energy of micro-photoluminescence mapping were achieved. In addition, superior material qualities of the whole film fabricated on 0.2 degrees off substrate were recognized by cross-sectional transmission electron microscopy. Based on the results mentioned above, a high-quality In(0.2)Ga(0.8)N/GaN MQW blue light, emitting diode (LED) has been fabricated on 0.2 degrees off substrate, which demonstrated a strong room-temperature electroluminescence emission at the wavelength of 465 nm and with the full width at half maximum of only 19 nm. The same device also showed an output power of 13.4 mW and an external quantum efficiency of 19.2%. Both these characteristics are improved drastically compared with the devices fabricated on the substrates with other offset angles. Conclusively, the use of an appropriately misoriented sapphire substrate is suggested to be effective for elevating the emission efficiency of In(0.2)Ga(0.8)N/GaN MQW blue LED fabricated thereon.
Enhancing the Emission Efficiency of In0.2Ga0.8N/GaN MQW Blue LED by Using Appropriately Misoriented Sapphire Substrates
(2009-01-01)
Li, Zhen-Yu
;
Uen, Wu-Yih
;
Lo, Ming-Hua
;
Chiu, Ching-Hua
;
Lin, Po-Chun
In this paper, we report the fabrication of an In0.2Ga0.8N/GaN multiple-quantum-well (MQW) structure on vicinal sapphire substrates with a very small offset angle of 0-1 degrees by low-pressure metallorganic chemical vapor deposition. Our study demonstrates that the quality of the In0.2Ga0.8N/GaN MQW structure is very sensitive to the offset angle of the vicinal substrate. High-resolution X-ray diffraction analyses demonstrated high-order satellite peaks and clear fringes between them for all MQW structures fabricated, from which the interface roughness (IRN) was estimated. The IRN of the In0.2Ga0.8N/GaN MQW structure fabricated on 0.2 degrees off sapphire substrate was determined as 1.36% of the quantum well layer period. Besides, reciprocal lattice mapping was employed to examine the strain status of the MQW. The lattice relaxation of the same specimen mentioned above was estimated to be 7.4x10(-5). It is therefore manifested that an In0.2Ga0.8N/GaN MQW structure with abrupt interfaces and good layer periodicity was grown. From it, a shortest radiative lifetime of 14.2 ns and a lowest fluctuation of 5.6 meV in the emission energy of micro-photoluminescence mapping were achieved. In addition, superior material qualities of the whole film fabricated on 0.2 degrees off substrate were recognized by cross-sectional transmission electron microscopy. Based on the results mentioned above, a high-quality In0.2Ga0.8N/GaN MQW blue light, emitting diode (LED) has been fabricated on 0.2 degrees off substrate, which demonstrated a strong room-temperature electroluminescence emission at the wavelength of 465 nm and with the full width at half maximum of only 19 nm. The same device also showed an output power of 13.4 mW and an external quantum efficiency of 19.2%. Both these characteristics are improved drastically compared with the devices fabricated on the substrates with other offset angles. Conclusively, the use of an appropriately misoriented sapphire substrate is suggested to be effective for elevating the emission efficiency of In0.2Ga0.8N/GaN MQW blue LED fabricated thereon.
Fabrication and lasing characteristics of GaN nanopillars
(2011)
Lo, Ming-Hua
;
Cheng, Yuh-Jen
;
Kuo, Hao-Chung
;
Wang, Shing-Chung
;
光電工程學系
We report the fabrication of GaN nanopillars and their laser action characteristics under optical pumping measurement. The nanopillars were fabricated from a GaN epitaxial wafer by self-assembled Ni nanomasked etching, followed by epitaxial regrowth to form crystalline facets on the etched nanopillars. The regrowth process is intended to reduce surface defects created during ICP-RIE etching. The density of etched GaN nanopillars is about 8.5x10(8)/cm(2) and the diameter and height of GaN nanopillars are about 250 nm and 650 nm, respectively. The as grown GaN nanopillars exhibit a random distribution with hexagonal pillar geometry. The sample is optically excited by frequency tripled Nd:YAG pulsed laser. The Gaussian waist of pumping spot is 1.8 um. At low pumping intensity, the emission has a broad spontaneous emission spectrum with maximum at 363 nm. As pump intensity increases, a narrow peak at 363 nm emerges quickly from the broad spontaneous emission back ground. The lasing action occurs at threshold pump power density of 122 MW/cm(2). The emission linewidth decreases with pumping power across threshold and reaches a lowest value of about 0.38 nm above threshold. The excitation-power-dependent spectra show that the lasing wavelength has a slight blue shift as pump power increases. We remark that this is due the band filling of the increasing excited carrier density.
High quality ultraviolet AlGaN/GaN multiple quantum wells with atomic layer deposition grown AlGaN barriers
(2008-09-29)
Li, Zhen-Yu
;
Lo, Ming-Hua
;
Hung, C. T.
;
Chen, Shih-Wei
;
Lu, Tien-Chang
Low dislocation density ultraviolet (UV) AlGaN/GaN multiple quantum well (MQW) structure was grown using atomic layer deposition (ALD) technique. The AlGaN/GaN MQW grown on the sapphire substrate consisted of three GaN QWs and four AlGaN barriers formed by ALD grown AlN/GaN superlattices. The as-grown sample showed smooth surface morphology with a root-mean-square roughness value of only 0.35 nm, and no surface cracks were found. The dislocation density was estimated to be as low as 3.5 x 10(7) cm(-2). X-ray and transmission electron microscope data showed the MQW had sharp interfaces with good periodicity. The sample had an UV photoluminescence emission at 334 nm (3.71 eV) with a very narrow linewidth of 47 meV at 13 K. The cathodoluminescence image revealed a fairly uniform luminescence pattern at room temperature. The AlGaN/GaN MQW grown by ALD technique should be useful for providing high crystalline quality for fabrication of various optical devices. c 2008 American Institute of Physics.
HRTEM investigation of high-reflectance AlN/GaN distributed Bragg-reflectors by inserting AlN/GaN superlattice
(2009-05-01)
Li, Zhen-Yu
;
Lu, Tien-Chang
;
Kuo, Hao-Chung
;
Wang, Shing-Chung
;
Lo, Ming-Hua
A high-quality AlN/GaN distributed Bragg-reflectors (DBR) was successfully grown on sapphire substrate by low-pressure metal-organic chemical vapor deposition using ultra-thin AlN/GaN super-lattice insertion layers (SLILs). The reflectivity of AlN/GaN DBR with ultra-thin AlN/GaN SLIL was measured and achieved blue peak reflectivity of 99.4% at 462 nm. The effect of ultra-thin AlN/GaN super-lattice insertion layer was examined in detail by transmission electron microscopy, and indicated that the crack of AlN/GaN DBR can be suppress by inserting AlN/GaN SLIL. For electronic properties, the turn on voltage is about 4.1 V and CW laser action of vertical-cavity surface-emitting laser (VCSEL) was achieved at a threshold injection current of 1.4 mA at 77 K, with an emission wavelength of 462 nm. (C) 2009 Elsevier B.V. All rights reserved.
Lasing at exciton transition in optically pumped gallium nitride nanopillars
(2011-09-12)
Lo, Ming-Hua
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Cheng, Yuh-Jen
;
Liu, Mei-Chun
;
Kuo, Hao-Chung
;
Wang, Shing Chung
We report the observation of room temperature lasing action in optically pumped GaN nanopillars. The nanopillars were fabricated by patterned etching and crystalline regrowth from a GaN substrate. When nanopillars were optically excited, a narrow emission peak emerged from the broad spontaneous emission background. The increasing rate is nine times faster than that of the spontaneous emission background, showing the onset of lasing action. The lasing occurs right at the center of spontaneous emission rather than the often reported redshifted wavelength. A spectroscopic ellipsometry analysis indicates that the gain of lasing action is provided by exciton transition. (C) 2011 Optical Society of America
Light Output Enhancement of UV Light-Emitting Diodes With Embedded Distributed Bragg Reflector
(2011-05-15)
Cheng, Bo-Siao
;
Chiu, Ching-Hsueh
;
Lo, Ming-Hua
;
Wu, Yun-Lin
;
Kuo, Hao-Chung
We demonstrate the output power enhancement of ultraviolet light-emitting diodes (UVLEDs) by epitaxial lateral over-growth (ELOG) on a distributed Bragg reflector (DBR) patterned substrate. The patterned DBR mesas are used as ELOG masks to improve material quality as well as reflectors to enhance the light output coupling. Compared with the conventional UVLEDs, the surface pit density of UVLEDs at regions right above embedded patterned DBR were reduced from 2.5 x 10(6) to 1.6 x 10(6) cm(-2). The improved light extraction efficiency is verified by ray tracing simulation. The luminous intensity of this novel structure is enhanced by 75% compared to that of reference UVLED structure at the wavelength 390 nm.
Light Output Enhancement of UV Light-Emitting Diodes With Embedded Distributed Bragg Reflector
(2011-05-15)
Cheng, Bo-Siao
;
Chiu, Ching-Hsueh
;
Lo, Ming-Hua
;
Wu, Yun-Lin
;
Kuo, Hao-Chung
We demonstrate the output power enhancement of ultraviolet light-emitting diodes (UVLEDs) by epitaxial lateral over-growth (ELOG) on a distributed Bragg reflector (DBR) patterned substrate. The patterned DBR mesas are used as ELOG masks to improve material quality as well as reflectors to enhance the light output coupling. Compared with the conventional UVLEDs, the surface pit density of UVLEDs at regions right above embedded patterned DBR were reduced from 2.5 x 10(6) to 1.6 x 10(6) cm(-2). The improved light extraction efficiency is verified by ray tracing simulation. The luminous intensity of this novel structure is enhanced by 75% compared to that of reference UVLED structure at the wavelength 390 nm.
Metal organic chemical vapor deposition growth of GaN-based light emitting diodes with naturally formed nano pyramids
(2008-04-01)
Chiu, Ching-Hua
;
Lee, Chia-En
;
Lo, Ming-Hua
;
Huang, Hung-Wen
;
Lu, Tien-Chang
GaN-based light-emitting diodes (LEDs) with naturally formed nano pyramids roughened surfaces grown by metal organic chemical vapor deposition (MOCVD) were demonstrated. In this study, Mg-treatment, a growth-.interruption step and a surface treatment using biscyclopentadienyl magnesium (CP(2)Mg), was performed to form the nano pyramids on the surface of a p-type cladding layer, and then a p-type contact layer was grown on the p-type cladding layer, so as to create a p-type contact layer with a rough surface. Assisted by the nano pyramids surface roughening process, the light output power of the LEDs reached 11.3 and 9.7 mW with 10 and 5 min Mg-treatment at a driving current of 20 mA. The light outputs were increased by 48 and 27%, respectively, compared with the results from the LED without Mg-treatment.
Nano-processing techniques applied in GaN-Based light-emitting devices with self-assembly Ni nano-masks
(2008-05-01)
Chiu, Ching-Hua
;
Lo, Ming-Hua
;
Lu, Tien-Chang
;
Yu, Peichen
;
Huang, H. W.
We have developed a simple method to fabricate nanoscale masks by using self-assembly Ni clusters formed through a rapid thermal annealing (RTA) process. The density and dimensions of the Ni nano-masks could be precisely controlled. The nano-masks were successfully applied to GaN-based light-emitting diodes (LEDs) with nano-roughened surface, GaN nanorods, and GaN-based nanorod LEDs to enhance light output power or change structure properties. The GaN-based LED with nano-roughened surface by Ni nano-masks and excimer laser etching has increased 55% light output at 20 mA when compared to that without the nano-roughened process. The GaN nanorods fabricated by the Ni nano-masks and ICP-RIE dry etching showed 3.5 times over the as-grown sample in photoluminescence (PL) intensity. The GaN-based nanorod LEDs assisted by photo-enhanced chemical (PEC) wet oxidation process were also demonstrated. The electroluminescence (EL) intensity of the GaN-based nanorod LED with PEC was about 1.76 times that of the as-grown LED. The fabrication, structure properties, physical features, and the optical and electrical properties of the fabricated devices will be discussed.
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.