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Photogeneration of coherent shear phonons in orientated wurtzite semiconductors by piezoelectric coupling
(2009-11-01)
Wen, Yu-Chieh
;
Ko, Tsung-Shine
;
Lu, Tien-Chang
;
Kuo, Hao-Chung
;
Chyi, Jen-Inn
Using piezoelectric wurtzite semiconductor with accurate control of the crystal cut, we investigate the photogeneration of coherent shear acoustic phonons through anisotropic piezoelectric coupling. Theoretical study suggests the dominant contribution of the piezoelectric effect to the shear phonon generation in a-plane wurtzite GaN, which is also confirmed by the picosecond ultrasonic experiments with different dopant and excitation conditions. Piezoelectric transduction thus explains the observed remarkable coherent shear phonon signal. The influence of the ultrafast screening of surface electric field by photocarriers on the intensity and waveform of the generated shear acoustic pulse are discussed.
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.
Characteristics of emission polarization in a-plane nanorods embedded with InGaN/GaN multiple quantum wells
(2010-09-15)
Li, J. C.
;
Lu, T. C.
;
Huang, H. M.
;
Chan, W. W.
;
Kuo, H. C.
Nonpolar (a-plane) GaN nanorod arrays with embedded In(x)Ga(1-x)N/GaN (x=0.09, 0.14, 0.24, and 0.30) multiple quantum wells (MQWs) grown on r-plane sapphire have been fabricated successfully by self-assembled Ni nanomasks and subsequent inductively coupled plasma reactive-ion etching. After nanorod fabrications, the polarization ratio of the emission from MQWs with lower indium composition (x=0.09 and 0.14) slightly decreases but apparently increases by at most 79% for the samples with higher indium composition (x=0.24 and 0.30). Competition between the effect of multiple scattering, strain relaxation and reduction in localized centers, expected in a-plane MQW samples, are attributed to the variations in the polarization ratios after the nanorod formation. (c) 2010 American Institute of Physics. [doi:10.1063/1.3483239]
Physical properties of amorphous InGaZnO4 films doped with Mn
(2009-03-02)
Liu, Shiu-Jen
;
Fang, Hau-Wei
;
Su, Shih-Hao
;
Li, Chia-Hung
;
Cherng, Jyh-Shiarn
Amorphous InGaZnO4 (a-IGZO) films doped with various concentrations of Mn have been fabricated by using pulsed-laser deposition technique. Optical, electrical, and magnetic properties of the prepared Mn-doped a-IGZO films were investigated. The resistivity, carrier concentration, and carrier mobility of the a-IGZO films were found to be, respectively, increased, decreased, and enhanced by Mn doping. Moreover, the optical transmission is slightly increased in the visible range and the optical band gaps are not affected in the Mn-doped films. Room-temperature ferromagnetism has been observed in the field-dependent magnetization measurements.
Particular Failure Mechanism of GaN-Based Alternating Current Light-Emitting Diode Induced by GaOx Oxidation
(2010-08-01)
Yen, Hsi-Hsuan
;
Kuo, Hao-Chung
;
Yeh, Wen-Yung
;
光電工程學系
;
光電工程研究所
This investigation describes the unique failure mechanism of the Wheatstone bridge circuit-type alternating current light-emitting diode (WB AC-LED). The micro-LEDs in WB AC-LED rectifying branches were reverse biased, and therefore, positive charges (holes) accumulated in the n-type GaN layer under the active region and combined with the GaN material and OH- ions to generate GaOx oxidation grains. The GaOx generation speed was fast with high reverse voltage applied to micro-LEDs, and the expansion of GaOx dimensions degraded the opto-electrical characteristics and eventually caused failure of the WB AC-LED. The root-mean-square reverse voltage dropped across each micro-LED from -13.1 to -6.7 V with different micro-LEDs array arrangements extended the WB AC-LED life-time from being less than 650 h to more than 1600 h, respectively.
High extraction efficiency GaN-based light-emitting diodes on embedded SiO2 nanorod array and nanoscale patterned sapphire substrate
(2010-06-28)
Huang, Hung-Wen
;
Huang, Jhi-Kai
;
Kuo, Shou-Yi
;
Lee, Kang-Yuan
;
Kuo, Hao-Chung
In this paper, GaN-based LEDs with a nanoscale patterned sapphire substrate (NPSS) and a SiO2 photonic quasicrystal (PQC) structure on an n-GaN layer using nanoimprint lithography are fabricated and investigated. The light output power of LED with a NPSS and a SiO2 PQC structure on an n-GaN layer was 48% greater than that of conventional LED. Strong enhancement in output power is attributed to better epitaxial quality and higher reflectance resulted from NPSS and PQC structures. Transmission electron microscopy images reveal that threading dislocations are blocked or bended in the vicinities of NPSS layer. These results provide promising potential to increase output power for commercial light emitting devices. (C) 2010 American Institute of Physics. [doi:10.1063/1.3456385]
Continuous wave operation of current injected GaN vertical cavity surface emitting lasers at room temperature
(2010-08-16)
Lu, Tien-Chang
;
Chen, Shih-Wei
;
Wu, Tzeng-Tsong
;
Tu, Po-Min
;
Chen, Chien-Kang
We report the demonstration of the continuous wave laser action on GaN-based vertical cavity surface emitting lasers at room temperature. The laser structure consists of a ten-pair Ta(2)O(5)/SiO(2) distributed Bragg reflector (DBR), a 7 lambda-thick optical cavity, ten-pairs InGaN/GaN multiquantum wells with an AlGaN electron blocking layer, and a 29-pair AlN/GaN DBR. The laser has a threshold current of about 9.7 mA corresponding to the current density of about 12.4 kA/cm(2) and a turn-on voltage about 4.3 V at 300 K. The lasing wavelength was 412 nm with a linewidth of about 0.5 nm. A spontaneous emission coupling efficiency factor of about 5x10(-3) and the degree of polarization of about 55% were measured, respectively. The laser beam has a narrow divergence angle of about 8 degrees. (C) 2010 American Institute of Physics. [doi:10.1063/1.3483133]
High-gain complementary inverter with InGaZnO/pentacene hybrid ambipolar thin film transistors
(2010-08-23)
Liu, Po-Tsun
;
Chou, Yi-Teh
;
Teng, Li-Feng
;
Fuh, Chur-Shyang
;
光電工程學系
Ambipolar thin film transistors (TFTs) with InGaZnO/pentacene heterostructure channels are demonstrated for a high-voltage-gain complementary metal oxide semiconductor (CMOS) inverter. The ambipolar TFT exhibits a electron mobility of 23.8 cm(2)/V s and hole mobility of 0.15 cm(2)/V s for the InGaZnO and pentacene, respectively. The thermal annealing process was also studied to adjust electron concentration reducing operating voltage of the CMOS inverter. The voltage gain achieves as high as 60 obtained in the first and third quadrants of the voltage transfer characteristic. The high performance and simple manufacture of the heterostructure CMOS inverter show promise as critical components in various electrical applications. (C) 2010 American Institute of Physics. [doi:10.1063/1.3483616]
Characteristics of emission polarization in a-plane nanorods embedded with InGaN/GaN multiple quantum wells
(2010-09-15)
Li, J. C.
;
Lu, T. C.
;
Huang, H. M.
;
Chan, W. W.
;
Kuo, H. C.
Nonpolar (a-plane) GaN nanorod arrays with embedded In(x)Ga(1-x)N/GaN (x=0.09, 0.14, 0.24, and 0.30) multiple quantum wells (MQWs) grown on r-plane sapphire have been fabricated successfully by self-assembled Ni nanomasks and subsequent inductively coupled plasma reactive-ion etching. After nanorod fabrications, the polarization ratio of the emission from MQWs with lower indium composition (x=0.09 and 0.14) slightly decreases but apparently increases by at most 79% for the samples with higher indium composition (x=0.24 and 0.30). Competition between the effect of multiple scattering, strain relaxation and reduction in localized centers, expected in a-plane MQW samples, are attributed to the variations in the polarization ratios after the nanorod formation. (c) 2010 American Institute of Physics. [doi:10.1063/1.3483239]
Regrowth of High Quality Heavily Si-Doped nGaN Utilizing Nano-Rod GaN Template
(2011)
Kuo, Cheng-Huang
;
Chang, Li-Chuan
;
Chou, Hsiu-Mei
;
照明與能源光電研究所
;
Institute of Lighting and Energy Photonics
In this study, heavily Si-doped n-type GaN (n-GaN) epitaxial layers with and without nanorod GaN (NR GaN) template were grown by metal organic chemical vapor deposition (MOCVD) system. It was found that we could achieve high-qulaity heavily Si-doped n-GaN layers by using the NR GaN template. It was also found that we can reduce etching pits density in n-GaN (1.5E19 cm(-3)) epitaxial layer by a factor of 2.82 using the NR GaN template, as compared to the conventional sapphire substrate. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3617533] All rights reserved.

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