Browsing by Author "Li, Z. Y."

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AlGaN/GaN Multiple Quantum Wells Grown By Atomic Layer Deposition
(2008)
Lo, M. H.
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Chen, S. W.
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Li, Z. Y.
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Lu, T. C.
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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.
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Li, Z. Y.
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Chen, J. R.
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Ko, T. S.
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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.
Characteristics of efficiency droop in GaN-based light emitting diodes with an insertion layer between the multiple quantum wells and n-GaN layer
(2010-12-20)
Chang, S. P.
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Wang, C. H.
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Chiu, C. H.
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Li, J. C.
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Lu, Y. S.
We have studied the characteristics of efficiency droop in GaN-based light emitting diodes (LEDs) with different kinds of insertion layers (ILs) between the multiple quantum wells (MQWs) layer and n-GaN layer. By using low-temperature (LT) (780 degrees C) n-GaN as IL, the efficiency droop behavior can be alleviated from 54% in reference LED to 36% from the maximum value at low injection current to 200 mA, which is much smaller than that of 49% in LED with InGaN/GaN short-period superlattices layer. The polarization field in MQWs is found to be smallest in LED with InGaN/GaN SPS layer. However, the V-shape defect density, about 5.3 x 10(8) cm(-2), in its MQWs region is much higher than that value of 2.9 x 108 cm(-2) in LED with LT n-GaN layer, which will lead to higher defect-related tunneling leakage of carriers. Therefore, we can mainly assign this alleviation of efficiency droop to the reduction of dislocation density in MQWs region rather than the decrease of polarization field. (C) 2010 American Institute of Physics. [doi:10.1063/1.3531957]
Characterization of 380 nm UV-LEDs grown on free-standing GaN by atmospheric-pressure metal-organic chemical vapor deposition
(2014-01-01)
Shieh, C. Y.
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Li, Z. Y.
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Kuo, H. C.
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Chang, J. Y.
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Chi, G. C.
We reported the defects and optical characterizations of the ultraviolet light-emitting diodes grown on free-standing GaN substrate (FS-GaN) and sapphire. Cross-sectional transmission electron microscopy (TEM) images showed that the total defect densities of grown UV LEDs on FS-GaN and sapphire including edge, screw and mixed type were 3.6x10(6) cm(-2) and 5.5x10(8) cm(-2). When substrate of UV LEDs was changed from sapphire to FS-GaN, it can be clearly found that the crystallography of GaN epilayers was drastically different from that GaN epilayers on sapphire. Besides, the microstructures or indium clustering can be not observed at UV LEDs on FS-GaN from TEM measurement. The internal quantum efficiency of UVLEDs on FS-GaN and sapphire were 34.8 % and 39.4 % respectively, which attributed to indium clustering in multi-layers quantum wells (MQWs) of UV LEDs on sapphire. The relationship between indium-clustering and efficiency droop were investigated by temperature-dependent electroluminescence (TDEL) measurements.
Design and fabrication of a InGaN vertical-cavity surface-emitting laser with a composition-graded electron-blocking layer
(2014-08-01)
Lin, B. C.
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Chang, Y. A.
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Chen, K. J.
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Chiu, C. H.
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Li, Z. Y.
In this study, the design and fabrication of a InGaN vertical-cavity surface-emitting laser (VCSEL) with a composition-graded electron blocking layer (GEBL) are revealed experimentally and theoretically. It has been demonstrated that laser output performance is improved by using a GEBL when compared to the typical VCSEL structure of a rectangular EBL. The output power obtained at 20 kA cm(-2) is enhanced by a factor of 3.8 by the successful reduction of threshold current density from 12.6 to 9.2 kA cm(-2) and the enlarged slope efficiency. Numerical simulation results also suggest that the improved laser output performances are due mainly to the reduction of electron leakage current and the enhanced hole injection efficiency in the multiple-quantum-well (MQW) active region.
Efficiency droop alleviation in InGaN/GaN light-emitting diodes by graded-thickness multiple quantum wells
(2010-11-01)
Wang, C. H.
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Chang, S. P.
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Chang, W. T.
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Li, J. C.
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Lu, Y. S.
InGaN/GaN light-emitting diodes (LEDs) with graded-thickness multiple quantum wells (GQW) was designed and grown by metal-organic chemical vapor deposition. The GQW structure, in which the well-thickness increases along [0001] direction, was found to have superior hole distribution as well as radiative recombination distribution by performing simulation modeling. Accordingly, the experimental investigation of electroluminescence spectrum reveals additional emission from the narrower wells within GQWs. Consequently, the efficiency droop can be alleviated to be about 16% from maximum at current density of 30 to 200 A/cm(2), which is much smaller than that for conventional LED (32%). Moreover, the light output power was enhanced from 18.0 to 24.3 mW at 20 A/cm(2). (C) 2010 American Institute of Physics. [doi:10.1063/1.3507891]
Efficiency Enhancement in Single-Junction InGaP Solar Cells by Using Self-Assembled Nanospheres
(2011-06-01)
Chen, H. C.
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Lin, C. C.
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Wang, H. W.
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Tsai, M. A.
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Tseng, P. C.
A single-junction InGaP solar cell with polystyrene (PS) nanospheres on the surface has been developed. The self-assembly of PS nanospheres is one of the simplest and the fastest methods with which to build a 2-D closely packed periodic structure. Due to the scattering of the PS nanospheres, the light path length can be increased when compared to the InGaP solar cells without PS nanospheres on the top. An increase short-circuit current from 8.93 to 10.31 mA is improved when a single-junction InGaP solar cell is coated with PS nanospheres. The conversion efficiency measured can also be improved from 8.79% to 9.71%. The single-junction InGaP solar cell with PS nanospheres was achieved.
Energy transfer from InGaN quantum wells to Au nanoclusters via optical waveguiding
(2011-03-14)
Shu, G. W.
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Lin, C. C.
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Lin, H. T.
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Lin, T. N.
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Shen, J. L.
We present the first observation of resonance energy transfer from InGaN quantum wells to Au nanoclusters via optical waveguiding. Steady-state and time-resolved photoluminescence measurements provide conclusive evidence of resonance energy transfer and obtain an optimum transfer efficiency of similar to 72%. A set of rate equations is successfully used to model the kinetics of resonance energy transfer. (C) 2011 Optical Society of America
GaN-Based LEDs Grown on HVPE Growth High Crystalline Quality Thick GaN Template
(2011)
Lin, D. W.
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Lin, C. C.
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Chiu, C. H.
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Lee, C. Y.
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Yang, Y. Y.
In this study, high crystalline quality 30 mu m thick gallium nitride (GaN) films were grown by hydride vapor phase epitaxy (HVPE) on sapphire substrate, and the thick GaN films were used for developing high performance light-emitting diodes (LEDs). By using high-resolution X-ray diffraction, the full width at half-maximum (FWHM) of the rocking curve shows that this 30 mu m thick GaN template had high crystalline quality. In addition, the transmission electron microscopy (TEM) images suggest that threading dislocation densities (TDDs) are almost free in multiple quantum wells (MQWs) for LEDs grown on 30 mu m thick GaN template. Compared with conventional LEDs grown on sapphire, LEDs grown on 30 mu m thick GaN template exhibit 26% enhancement of light output at 20 mA. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.003111jes] All rights reserved.
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]
Improved output power of GaN-based ultraviolet light-emitting diodes with sputtered AlN nucleation layer
(2015-03-15)
Chiu, C. H.
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Lin, Y. W.
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Tsai, M. T.
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Lin, B. C.
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Li, Z. Y.
In this work, the ultraviolet light emitting diodes (UV -LEDs) at 380 no were grown on patterned sapphire substrate (PSS) by atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). A sputtered AlN nucleation layer was utilized on the PSS to enhance the quality of the epitaxial layer. By using high resolution X-ray diffraction, the full width at half maximum of the rocking curve shows that the UV-LEDs with sputtered AlN nucleation layer had better crystalline quality when compared to conventional GaN nucleation samples. From the scanning electron microscope (SEM) and transmission electron microscopy (TEM) images, it can be observed that the Lip and sidewall portion of the pattern was smooth using the sputtered AlN nucleation layer. The threading dislocation densities (TDDs) are reduced from 6 x 107 cm(-2) to 2.5 x 107 cm(-2) at the interface between the u-GaN layers for conventional and AlN PSS devices, respectively. As a result, a much higher light output povver was achieved. The light output power at an injection current of 20 mA was enhanced by 30%. Further photoluminescence (PL) measurement and numerical simulation confirm that this increase or output power can be attributed to the improvement of material quality and light extraction. (C) 2014 Elsevier B.V. All rights reserved.
Improvement in efficiency droop of GaN-based light-emitting diodes by optimization of active regions
(2011)
Wang, C. H.
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Lin, D. W.
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Chiu, C. H.
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Chang, S. P.
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Li, Z. Y.
We had demonstrated several novel methods to improve efficiency droop behavior in GaN-based light-emitting diodes (LEDs). LEDs with different kinds of insertion layers (ILs) between the multiple quantum wells (MQWs) layer and n-GaN layer were investigated. By using low-temperature (LT, 780. C) n-GaN as IL, the efficiency droop behavior can be alleviated from 54% in reference LED to 36% from the maximum value at low injection current to 200 mA, which is much smaller than that of 49% in LED with InGaN/GaN short-period superlattices (SPS) layer. The polarization field in MQWs is found to be smallest in LED with InGaN/GaN SPS layer. However, the V-shape defect density, about 5.3x10(8) cm(-2), in its MQWs region is much higher than that value of 2.9x10(8) cm(-2) in LED with LT n-GaN layer, which will lead to higher defect-related tunneling leakage of carriers. Therefore, we can mainly assign this alleviation of efficiency droop to the reduction of dislocation density in MQWs region rather than the decrease of polarization field. At second part, LEDs with graded-thickness multiple quantum wells (GQW) was designed and found to have superior hole distribution as well as radiative recombination distribution by simulation modeling. Accordingly, the experimental investigation of electroluminescence spectrum reveals additional emission from the previous narrower wells within GQWs. Consequently, the efficiency droop can be alleviated to be about 16% from maximum at current density of 30 A/cm(2) to 200 A/cm(2). Moreover, the light output power is enhanced by 35% at 20 A/cm(2).
Influence of free-standing GaN substrate on ultraviolet light-emitting-diodes by atmospheric-pressure metal-organic chemical vapor deposition
(2013-01-01)
Shieh, C. Y.
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Li, Z. Y.
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Chiu, C. H.
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Tu, P. M.
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Kuo, H. C.
We reported the influence of free-standing (FS) GaN substrate on ultraviolet light-emitting-diodes (UV LEDs) by atmospheric-pressure metal-organic chemical vapor deposition (APMOCVD). The Raman spectrum shows the in-plane compressive stress of the GaN epitaxial structures grown on FS GaN substrate. Besides, the Raman spectrum reveals the relation between the crystal quality and the carrier localization degree in multi-quantum wells (MQWs). High resolution X-ray diffraction (HRXRD) analysis results show that the In0 025Ga0 975N/Al0 08Ga0 92N MQWs grown on FS GaN substrate has higher indium mole fraction than sapphire at the same growth conditions. The higher indium incorporation is corresponding with the red-shift 6 nm (387 nm) of the room temperature photoluminescence (PL) peak. The full widths at half maximum (FWHM) of omega-scan rocking curve in (002) and (102) reflectance on FS GaN substrate (83 arcsec and 77 arcsec) are narrower than UV LEDs grown on sapphire (288 arcsec and 446 arcsec). This superior quality may attribute to homoepitaxial growth structure and better strain relaxation in the FS GaN substrate. An anomalous temperature behavior of PL in UV LEDs designated as an S-shaped peak position dependence and W-shaped linewidth dependence indicate that exciton/carrier motion occurs via photon-assisted tunneling through localized states, what results in incomplete thermalization of localized excitons at low temperature. The Gaussian broadening parameters of carrier localization is about 16.98 meV from the temperature dependent photoluminescence (TDPL) measurement. The saturation temperature from the TDPL linewidth of UV LEDs on FS GaN substrate at about 175 K represents a crossover from a nonthermalized to thermalized energy distribution of excitons.
Morphology and optical properties of zinc oxide thin films grown on Si (100) by metal-organic chemical vapor deposition
(2009-01-01)
Lan, S. M.
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Uen, W. Y.
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Chan, C. E.
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Chang, K. J.
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Hung, S. C.
Undoped zinc oxide (ZnO) thin films were deposited on Si (100) substrates by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD). After a nucleation layer of ZnO about 500 A... was deposited, top ZnO films with 320-440 nm in thickness were fabricated at temperatures varying from 450 to 600 A degrees C, respectively. X-ray diffraction analyses demonstrate that all the films grown have polycrystalline wurtzite structure. Scanning electron microscopy observations indicate that ZnO films grow with c-axis aligned grains when the deposition temperature is lower than 600 A degrees C. However, 3D microstructures without c-axis alignment were found when the film was deposited at 600 A degrees C. Room temperature photoluminescence (PL) spectra peaking at 3.27-3.29 eV were observed for the ZnO films grown at 450-600 A degrees C. However, only the spectrum from the film grown at 600 A degrees C could present a high intensity ratio of the ultraviolet band to deep-level emissions, which is above 392 and show a minimum full-width at half-maximum of 99 meV for the ultraviolet band. Temperature-dependent PL measurements from 20 to 300 K exhibited the luminescent mechanism of the present ZnO films to be near band excitonic emission.
Nanoscale epitaxial lateral overgrowth of GaN-based light-emitting diodes on a SiO(2) nanorod-array patterned sapphire template
(2008-08-25)
Chiu, C. H.
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Yen, H. H.
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Chao, C. L.
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Li, Z. Y.
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Yu, Peichen
High efficiency GaN-based light-emitting diodes (LEDs) are demonstrated by a nanoscale epitaxial lateral overgrowth (NELO) method on a SiO(2) nanorod-array patterned sapphire substrate (NAPSS). The transmission electron microscopy images suggest that the voids between SiO(2) nanorods and the stacking faults introduced during the NELO of GaN can effectively suppress the threading dislocation density. The output power and external quantum efficiency of the fabricated LED were enhanced by 52% and 56%, respectively, compared to those of a conventional LED. The improvements originated from both the enhanced light extraction assisted by the NAPSS and the reduced dislocation densities using the NELO method. (C) 2008 American Institute of Physics.
Performance enhancement of GaN-based flip-chip ultraviolet light-emitting diodes with a RPD AlN nucleation layer on patterned sapphire substrate
(2014-08-01)
Lin, B. C.
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Chiu, C. H.
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Lee, C. Y.
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Han, H. V.
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Tu, P. M.
In this work, flip-chip ultraviolet light-emitting diodes (FCUV-LEDs) on patterned sapphire substrate (PSS) at 375 nm were grown by an atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). A specialized reactive plasma deposited (RPD) AlN nucleation layer was utilized on the PSS to enhance the quality of the epitaxial layer. By using high-resolution X-ray diffraction, the full-width at half-maximum of the rocking curve shows that the FCUV-LEDs with RPD AlN nucleation layer had better crystalline quality when compared to conventional GaN nucleation samples. From the transmission electron microscopy (TEM) image, it can be observed that the tip and incline portion of the pattern was smooth using the RPD AlN nucleation layer. The threading dislocation densities (TDDs) are reduced from 7 x 10(7) cm(-2) to 2.5 x 10(7) cm(-2) at the interface between the u-GaN layers for conventional and AlN PSS devices, respectively. As a result, a much higher light output power was achieved. The improvement of light output power at an injection current of 20 mA was enhanced by 30%. Further photoluminescence measurement and numerical simulation confirm such increase of output power can be attributed to the improvement of material quality and light extraction. (C) 2014 Optical Society of America
Reduction of Efficiency Droop in Semipolar (1(1)over-bar01) InGaN/GaN Light Emitting Diodes Grown on Patterned Silicon Substrates
(2011-01-01)
Chiu, C. H.
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Lin, D. W.
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Lin, C. C.
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Li, Z. Y.
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Kuo, H. C.
The semi-polar InGaN-based LEDs exhibits low efficiency droop because the reduction of the polarization field not only made the band diagram smoother but also restricted electron overflow to the p-GaN layer as shown in simulations. (C) 2010 Optical Society of America
Structural and optical characterizations of GaN-based green light-emitting diodes growth using TiN buffer layer
(2013-01-01)
Shieh, C. Y.
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Li, Z. Y.
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Kuo, H. C.
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Chi, G. C.
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光電工程學系
The present study investigated the structural and optical characterizations of the growth of GaN-based green light-emitting diodes using a TiN buffer layer. The purpose of growing GaN-based green LEDs on the TiN interlayer was to produce the naturally occurring hexagonal pattern structure on the surface of undoped-GaN. Then dislocations of the grown InGaN/GaN MQWs green LEDs structure on the uGaN template with the TiN interlayer produced base plane staking faults through epitaxial lateral overgrowth. Cross-section transmission electron microscope images showed that the dislocation density of green LEDs was decreased from 5 x 10(8) cm(-2) to 7 x 10(7) cm(-2), and that the dislocations in the green LEDs structure were reproduced. The full widths at half maximum of the omega-scan rocking curves in (002) and (102) reflectance on the GaN-based green LEDs were 334 and 488 arcsec, respectively. As the injection current was increased from 5 mA to 40 mA, the electroluminescence peak wavelength of the GaN-based green LEDs was shifted from 508 nm to 481 nm, a blue-shift of 27 nm. The forward voltage measured at an injection current of 20 mA was 4.9 V for the GaN-based green LEDs according to the current-voltage characteristics. Due to an increase in the In mole fraction of the GaN-based green LEDs on the uGaN template with the TiN interlayer, the strain and phase separation were increased, and the multiple quantum wells structural quality and device performances of the GaN-based green LEDs were decayed. A yellow band with a wavelength of 551nm was thereby produced according to room temperature photoluminescence measurement. Meanwhile, cross-section transmission electron microscope images indicated V-defects in multiple quantum wells structures of the green LEDs.