Browsing by Author "Ke, C. C."

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Hole injection and efficiency droop improvement in InGaN/GaN light-emitting diodes by band-engineered electron blocking layer
(2010-12-27)
Wang, C. H.
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Ke, C. C.
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Lee, C. Y.
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Chang, S. P.
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Chang, W. T.
A graded-composition electron blocking layer (GEBL) with aluminum composition increasing along the [0001] direction was designed for c-plane InGaN/GaN light-emitting diodes (LEDs) by employing the band-engineering. The simulation results demonstrated that such GEBL can effectively enhance the capability of hole transportation across the EBL as well as the electron confinement. Consequently, the LED with GEBL grown by metal-organic chemical vapor deposition exhibited lower forward voltage and series resistance and much higher output power at high current density as compared to conventional LED. Meanwhile, the efficiency droop was reduced from 34% in conventional LED to only 4% from the maximum value at low injection current to 200 A/cm(2). (C) 2010 American Institute of Physics. [doi:10.1063/1.3531753]
Internal Quantum Efficiency Measurement in InGaN/GaN UV LEDs with Patterned Sapphire Substrate by Photoluminescence and Electroluminescence Method
(2010)
Wang, C. H.
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Chiu, C. H.
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Ke, C. C.
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Kuo, H. C.
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Lu, T. C.
Internal quantum efficiency (IQE) of InGaN-based ultraviolet light emitting diodes (LED) grown on patterned sapphire substrate (PSS) and flat sapphire were measured by photoluminescence and electroluminescence methods. The IQE improvement of PSS LED is significant. (C)2010 Optical Society of America
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.
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Kuo, S. Y.
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Lo, M. H.
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Ke, C. C.
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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.
Study of the internal quantum efficiency of InGaN/GaN UV LEDs on patterned sapphire substrate using the electroluminescence method
(2011-01-15)
Wang, C. H.
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Ke, C. C.
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Chiu, C. H.
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Li, J. C.
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Kuo, H. C.
Internal quantum efficiency (IQE) of InGaN/GaN UV LEDs with patterned sapphire substrates (PSS) was investigated using electroluminescence (EL) and photoluminescence (PL) methods. The physical mechanisms that affect temperature-dependent EL efficiency as a function of injected carrier density were deduced. In order to reduce the density of non-radiative recombination centers to improve quantum efficiency, improvement of crystal quality and reduction in the number of defects are necessary. PSS LEDs showed better EL characteristics than non-PSS LEDs because of the improved epitaxial quality. Contrary to PL IQE, EL IQE was significantly affected by carrier injection efficiency, especially at low temperature when high bias voltage was applied to the p-n junction. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.