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Now showing 1 - 8 of 8

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.
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]
Large vacuum Rabi splitting in ZnO-based hybrid microcavities observed at room temperature
(2009-02-09)
Chen, Jun-Rong
;
Lu, Tien-Chang
;
Wu, Yung-Chi
;
Lin, Shiang-Chi
;
Liu, Wei-Rein
Wide-band gap ZnO semiconductors are attractive materials for the investigation of microcavity exciton polaritons due to the large exciton binding energy and oscillator strength. We report the growth and characterization of bulk ZnO-based hybrid microcavity. The phenomenon of strong exciton-photon coupling at room temperature has been observed in the ZnO-based hybrid microcavity structure, which consists of 30 pair epitaxially grown AlN/AlGaN distributed Bragg reflector (DBR) on the bottom side of the 3/2 lambda thick ZnO cavity and 9 pair SiO(2)/HfO(2) DBR as the top mirror. The cavity quality factor is about 221. The experimental results show good agreement with theoretically calculated exciton-polariton dispersion curves based on transfer matrix method. From the theoretical and experimental exciton-polariton dispersion curves with two different cavity-exciton detuning values, the large vacuum Rabi splitting is estimated to be about 58 meV in the ZnO-based hybrid microcavity.
High output power density from GaN-based two-dimensional nanorod light-emitting diode arrays
(2009-04-06)
Lee, Ya-Ju
;
Lin, Shawn-Yu
;
Chiu, Ching-Hua
;
Lu, Tien-Chang
;
Kuo, Hao-Chung
Here we propose and realize a scheme for making a direct contact to a two-dimensional nanorod light-emitting diode (LED) array using the oblique-angle deposition approach. And, more importantly, we demonstrate highly efficient electrical carrier injection into the nanorods. As a result, we show that at a 20 mA dc current injection, the light output power density of our nanorod LED array is 3700 mW cm(-2). More general, this contact scheme will pave the ways for making direct contacts to other kinds of nanoscale optoelectronic devices.
Nanorod epitaxial lateral overgrowth of a-plane GaN with low dislocation density
(2009-06-22)
Ling, Shih-Chun
;
Chao, Chu-Li
;
Chen, Jun-Rong
;
Liu, Po-Chun
;
Ko, Tsung-Shine
The crystal quality of a-plane GaN films was improved by using epitaxial lateral overgrowth on a nanorod GaN template. The investigation of x-ray diffraction showed that the strain in a-plane GaN grown on r-plane sapphire could be mitigated. The average threading dislocation density estimated by transmission electron microscopy was reduced from 3x10(10) to 3.5x10(8) cm(-2). From the temperature-dependent photoluminescence, the quantum efficiency of the a-plane GaN was enhanced by the nanorod epitaxial lateral overgrowth (NRELOG). These results demonstrated the opportunity of achieving a-plane GaN films with low dislocation density and high crystal quality via NRELOG.
Low Droop Nonpolar GaN/InGaN Light Emitting Diode Grown on m-Plane GaN Substrate
(2010)
Chang, Shih-Pang
;
Lu, Tien-Chang
;
Zhuo, Li-Fu
;
Jang, Chung-Ying
;
Lin, Da-Wei
The low droop nonpolar m-plane InGaN/GaN light emitting diode (LED) has been fabricated and investigated. The external quantum efficiency for a 300x300 mu m square LED chip only drops about 18% from maximum at an operation current of 22 A/cm(2) (20 mA) to 330 A/cm(2) (300 mA) dc operations at room temperature. In addition, the internal quantum efficiency has been extracted by temperature-dependent photoluminescence measurements, and there is no droop observed as the carrier density increases. The small droop in efficiency of m-plane LEDs could be due to the lack of polarization effects that enhances the carrier confinement under high current density operation. The polarization anisotropy is clearly observed in the m-plane LED, and the degree of polarization is 68%.
Highly-directional emission patterns based on near single guided mode extraction from GaN-based ultrathin microcavity light-emitting diodes with photonic crystals
(2010-07-05)
Lai, Chun-Feng
;
Kuo, Hao-Chung
;
Yu, Peichen
;
Lu, Tien-Chang
;
Chao, Chia-Hsin
This study investigates the distribution of highly-directional far-field emission on GaN-based ultrathin microcavity light-emitting diodes (uMCLEDs) with photonic crystals (PhCs). The ultrathin 550 nm cavity, PhC lattice constant of 370 nm, and hole depth of 250 nm in the GaN PhC uMCLED provide near single guided mode extraction and a pattern of high directionality radiation. Angular-spectral-resolved electroluminescence measurements reveal photon-band structure agreement with the fundamental mode effective refractive index dispersion curve. In addition, GaN PhC uMCLED increase the output power extraction efficiency by 145.9% (similar to 2.46x) compared with GaN non-PhC uMCLED, and a directional far-field emission pattern at half intensity of nearly +/- 15 degrees. (C) 2010 American Institute of Physics. [doi:10.1063/1.3459970]

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