Browsing by Author "Lo, M. H."

Now showing 1 - 13 of 13

AlGaN/GaN Multiple Quantum Wells Grown By Atomic Layer Deposition
(2008)
Lo, M. H.
;
Chen, S. W.
;
Li, Z. Y.
;
Lu, T. C.
;
Kuo, H. C.
Three-pair AlGaN/GaN multiple quantum well structure with superlattice was grown using metal-organic chemical vapor deposition system. The AlGaN barrier and GaN well of the MQW were grown by atmoic layer deposition and conventional growth, respectively. (C) 2008 Optical Society of America
AlGaN/GaN multiple quantum wells grown by atomic layer deposition - art. no. 68941V
(2008)
Lo, M. H.
;
Li, Z. Y.
;
Chen, J. R.
;
Ko, T. S.
;
Lu, T. C.
A three-pair AlGaN/GaN multiple quantum well (MQW) structure with superlattices (SLs) was grown on c-plane sapphire using metal organic chemical vapor deposition (MOCVD) system. The AlGaN barrier and GaN well of the MQW structure were grown by atomic layer deposition (ALD) and conventional growth, respectively. The HRTEM and HRXRD results show the grown structure has shape interface between SLs layers and QWs with good periodicity. The AFM and SEM data show smooth surface morphology with low RMS value and low defect density. The CL measurements also indicate uniform luminescence pattern at room temperature. The AlGaN/GaN MQW with AlN/GaN SLs structure grown by ALD could be used to improve the surface morphology by effectively suppress the threading dislocation.
Density-dependent energy relaxation of hot electrons in InN epilayers
(2009-01-01)
Yang, M. D.
;
Liu, Y. W.
;
Shen, J. L.
;
Chen, C. W.
;
Chi, G. C.
This work investigates the dependence of the hot-electron energy relaxation in InN epilayers on electron density. From the high-energy tail of photoluminescence, the electron temperature of the hot electrons was determined. Acoustic phonons have an important role in the energy relaxation of the hot electrons. The density-dependent electron energy loss rate in InN can be explained by a combination of longitudinal optical and acoustic phonon emissions. A slowing of energy loss rate at high electron densities was observed and attributed to piezoelectric coupling to acoustic phonons. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3056383]
Efficiency enhancement of UV/blue light emitting diodes via nanoscaled epitaxial lateral overgrowth of GaN on a SiO(2) nanorod-array patterned sapphire substrate
(2008-11-15)
Chiu, C. H.
;
Li, Zhen-Yu
;
Chao, C. L.
;
Lo, M. H.
;
Kuo, H. C.
High-quality GaN layer was successfully grown on a SiO(2) nanorod-array patterned sapphire substrate (NAPSS) using metal-organic chemical vapor deposition by a nanoscaled epitaxial lateral overgrowth (NELO) method. From tunneling electron microscope images, well coalescence and turned dislocations are parallel to the surface direction were clearly observed. The Raman shift measurement shows the residual stress in GaN was greatly reduced from 1.359 to 0.652 GPa when compared to a GaN on flat sapphire substrate. Also, a high-efficiency GaN-based light-emitting diodes (LEDs) were demonstrated on the NELO NAPSS GaN, the respective output power and external quantum efficiency of the NELO NAPSS LEDs were estimated to be 22 mW and 40.2% at an injection current of 20 mA, showing an enhancement factor of 52% over conventional LEDs. The significant improvement resulted from both the enhanced light extraction assisted by the NAPSS, and the reduced dislocation densities using the NELO method. (c) 2008 Published by Elsevier B.V.
Efficiency enhancement of UV/blue light emitting diodes via nanoscaled epitaxial lateral overgrowth of GaN on a SiO2 nanorod-array patterned sapphire substrate
(2008-11-15)
Chiu, C. H.
;
Li, Zhen-Yu
;
Chao, C. L.
;
Lo, M. H.
;
Kuo, H. C.
High-quality GaN layer was successfully grown on a SiO2 nanorod-array patterned sapphire substrate (NAPSS) using metal-organic chemical vapor deposition by a nanoscaled epitaxial lateral overgrowth (NELO) method. From tunneling electron microscope images, well coalescence and turned dislocations are parallel to the surface direction were clearly observed. The Raman shift measurement shows the residual stress in GaN was greatly reduced from 1.359 to 0.652 GPa when compared to a GaN on flat sapphire substrate. Also, a high-efficiency GaN-based light-emitting diodes (LEDs) were demonstrated on the NELO NAPSS GaN, the respective output power and external quantum efficiency of the NELO NAPSS LEDs were estimated to be 22 mW and 40.2% at an injection current of 20 mA, showing an enhancement factor of 52% over conventional LEDs. The significant improvement resulted from both the enhanced light extraction assisted by the NAPSS, and the reduced dislocation densities using the NELO method. (c) 2008 Published by Elsevier B.V.
High efficiency light emitting diode with anisotropically etched GaN-sapphire interface
(2009-07-27)
Lo, M. H.
;
Tu, P. M.
;
Wang, C. H.
;
Hung, C. W.
;
Hsu, S. C.
We report the fabrication and study of high efficiency ultraviolet light emitting diodes with inverted micropyramid structures at GaN-sapphire interface. The micropyramid structures were created by anisotropic chemical wet etching. The pyramid structures have significantly enhanced the light output efficiency and at the same time also improved the crystal quality by partially relieving the strain and reducing the dislocation defects in GaN. The electroluminescent output power at normal direction was enhanced by 120% at 20 mA injection current and the output power integrated over all directions was enhanced by 85% compared to a reference sample.
Improvement of efficiency and ESD characteristics of ultraviolet light-emitting diodes by inserting AlGaN and SiN buffer layers
(2007-07-01)
Huang, G. S.
;
Kuo, H. C.
;
Lo, M. H.
;
Lu, T. C.
;
Tsai, J. Y.
We have studied the effect of the AlGaN insertion and SiN buffer layers of ultraviolet light-emitting diodes (UVLEDs) on efficiency and electrostatic discharge (ESD) characteristics of UVLEDs grown by metalorganic chemical vapor deposition. The etching pit density was reduced from 3.6 x 10(8) cm(-2) in conventional samples to 1.6 x 10(7) cm(-2) in UVLEDs grown with the AlGaN insertion layer and SiN buffer layer. During the ESD tests at the negative biases of 3000 V, the live percentage of UVLEDs increased from 67% in conventional samples to 86% in samples with AlGaN insertion layers and SiN buffer layers. In addition, the device shows a low leakage current of 1.1 x 10(-8) A at -5 V and the light output power was 70% higher than the conventional UVLEDs at an injection current of 20 mA. (c) 2007 Elsevier B.V. All rights reserved.
Observation of Laser Action from Gallium Nitride nanorods under optical pumping
(2010)
Lo, M. H.
;
Cheng, Y. -J.
;
He, Y. N.
;
Kuo, H. C.
;
Lu, T. C.
We report the observation of optically pumped room temperature lasing action from GaN nanopillars. The lasing action occurs at threshold pumping power density of 122 MW/cm(2) with a linewidth of 0.38 nm at 363 nm.
Optical characteristics of a-plane InGaN/GaN multiple quantum wells with different well widths
(2007-04-30)
Ko, T. S.
;
Lu, T. C.
;
Wang, T. C.
;
Lo, M. H.
;
Chen, J. R.
a-plane InGaN/GaN multiple quantum wells of different widths ranging from 3 to 12 nm were grown on r-plane sapphire by metal organic chemical vapor deposition for investigation. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3 to 12 nm. Low temperature (9 K) time-resolved PL study shows that the sample with 3-nm-thick wells has a better optical property with a fast exciton decay time of 0.57 ns. The results of cathodoluminescence and micro-PL scanning images for samples of different well widths further verify the more uniform and stronger luminescence intensity distribution observed for the samples of thinner quantum wells, indicating that the important growth parameters for a-plane InGaN/GaN multiple quantum wells could be dominated by the In fluctuation and crystal quality during the epitaxial growth. (c) 2007 American Institute of Physics.
Optical properties of a-plane InGaN/GaN multiple quantum wells on r-plane sapphire substrates with different indium compositions
(2009-03-15)
Chiu, C. H.
;
Kuo, S. Y.
;
Lo, M. H.
;
Ke, C. C.
;
Wang, T. C.
A-plane In(x)Ga(1-x)N/GaN (x=0.09, 0.14, 0.24, and 0.3) multiple-quantum-wells (MQWs) samples, with a well width of about 4.5 nm, were achieved by utilizing r-plane sapphire substrates. Optical quality was investigated by means of photoluminescence (PL), cathodoluminescence, and time resolved PL measurements (TRPL). Two distinguishable emission peaks were examined from the low temperature PL spectra, where the high- and low-energy peaks were ascribed to quantum wells and localized states, respectively. Due to an increase in the localized energy states and absence of quantum confined Stark effect, the quantum efficiency was increased with increasing indium composition up to 24%. As the indium composition reached 30%, however, pronounced deterioration in luminescence efficiency was observed. The phenomenon could be attributed to the high defect densities in the MQWs resulted from the increased accumulation of strain between the InGaN well and GaN barrier. This argument was verified from the much shorter carrier lifetime at 15 K and smaller activation energy for In(0.3)Ga(0.7)N/GaN MQWs. In addition, the polarization-dependent PL revealed that the degree of polarization decreased with increasing indium compositions because of the enhancement of zero-dimensional nature of the localizing centers. Our detailed investigations indicate that the indium content in a-plane InGaN/GaN MQWs not only has an influence on optical performance, but is also important for further application of nitride semiconductors.
Optical properties of In0.3Ga0.7N/GaN green emission nanorods fabricated by plasma etching
(2007-08-22)
Chiu, C. H.
;
Lo, M. H.
;
Lai, C. F.
;
Lu, T. C.
;
Huang, H. W.
In this study, we have fabricated In0.3Ga0.7N/GaN green emission nanorods and demonstrated optical enhancement by photoluminescence (PL) measurements. An enhancement factor of 3.5 and an emission peak blue-shift of 6.6 nm were observed at 300 K for the green emission nanorods structure in comparison to the as-grown flat surface structure. The blue-shift phenomenon from the nanorod structure could be caused by a partial reduction of the internal piezoelectric field. However, the similar carrier decay time for the green emission nanorod structure and the as-grown flat surface structure observed in low-temperature time-resolved PL measurements indicates that the dominant optical enhancement mechanism of the green emission nanorod structure could be mainly resulting from the large emission surface areas and the multiple scattering paths between the nanorods.
Optical studies of InN epilayers on Si substrates with different buffer layers
(2007-12-01)
Yang, M. D.
;
Shen, J. L.
;
Chen, M. C.
;
Chiang, C. C.
;
Lan, S. M.
We have investigated the photoluminescence (PL) and time-resolved PL from the InN epilayers grown on Si substrates with different buffer layers. The narrowest value of the full width at half maximum of the PL peak is 52 meV with the AlN/AlGaN/GaN triple buffer layer, which is better than previous reports on similar InN epilayers on Si substrates. Based on the emission-energy dependence of the PL decays, the localization energy of carriers is also the least for the InN with a triple buffer layer. According to the x-ray diffraction measurements, we suggest that the reduced lattice mismatch between the InN epilayer and the top buffer layer is responsible for improvement of sample quality using the buffer-layer technique. (c) 2007 American Institute of Physics.
Optical study of a-plane InGaN/GaN multiple quantum wells with different well widths grown by metal-organic chemical vapor deposition
(2008-11-01)
Ko, T. S.
;
Lu, T. C.
;
Wang, T. C.
;
Chen, J. R.
;
Gao, R. C.
a-plane InGaN/GaN multiple quantum wells of different widths ranging from 3 to 12 nm grown on r-plane sapphire by metal-organic chemical vapor deposition were investigated. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3 to 12 nm. Low temperature (9 K) time-resolved PL (TRPL) study shows that the sample with 3-nm-thick wells has the best optical property with a fastest exciton decay time of 0.57 ns. The results of cathodoluminescence and micro-PL scanning images for samples of different well widths further verify that the more uniform and stronger luminescence intensity distribution are observed for the samples of thinner quantum wells. In addition, more effective capturing of excitons due to larger localization energy E(loc) and shorter radiative lifetime of localized excitons are observed in thinner well width samples in the temperature dependent TRPL. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.3013435]