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Abnormal PL spectrum in InGaN MQW surface emitting cavity
(2008-01-01)
Chu, J. T.
;
Cheng, Y. -J.
;
Kuo, H. C.
;
Lu, T. C.
;
Wang, S. C.
We report the observation of an abnormal photoluminescent (PL) spectrum from a HeCd laser pumped InGaN multiple quantum well (MQW) vertical cavity. The device is fabricated using standard MOCVD deposition on a (0001)-oriented sapphire substrate. The layer structures are: 10nm nucleation layer, a 4um bulk GaN layer, InGaN MQWs, and a final 200nm GaN cap layer. The InGaN MQWs consist of 10 pairs of 5 nm GaN barrier and 3 run In0.1Ga0.9N well. The peak emission of the as-grown MQWs sample was similar to 420nm. A dielectric distributed Bragg reflectors (DBR) were then coated on the top layer, followed by a laser lift off from sapphire substrate, and subsequently another DBR coated on the bulk GaN bottom surface. The cavity has a quality factor of similar to 520 from 400-490nm. The device was pumped by a focused CW HeCd laser from the bulk GaN side. When the laser is focused onto the InGaN MQWs, a photoluminescent spectrum centered at the designed MOW wavelength was observed as expected. However, when the focused position was moved toward the bulk GaN region, an additional abnormal PL peak around 460nm was observed. This is far outside the designed MQW wavelength.
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.
Improving Light Output Power of the GaN-Based Vertical-Injection Light-Emitting Diodes by Mg+ Implanted Current Blocking Layer
(2009-05-01)
Tsai, Min-An
;
Yu, Peichen
;
Chen, J. R.
;
Huang, J. K.
;
Chiu, C. H.
A method for forming a current blocking layer (CBL) by ion implantation in GaN-based vertical-injection light-emitting diodes (VI-LEDs) was proposed. It was found that the use of CBL in VI-LEDs can effectively reduce the current crowding effect and enhance the light output power. The uniform emission intensity distribution of VI-LEDs with CBL was demonstrated by electroluminescence measurements. Experimental results show that the wall-plug efficiency was enhanced by 12.3% at an injection current of 20 mA, compared to that of VI-LEDs without CBL, and by 56.2% compared to that of conventional LEDs. The device simulation results reveal that the current path can be blocked by CBL, resulting in high light extraction efficiencies and large current densities within the effective emission region of active layers.
Abnormal PL spectrum in InGaN MQW surface emitting cavity - art. no. 69080L
(2008)
Chu, J. T.
;
Cheng, Y. -J.
;
Kuo, H. C.
;
Lu, T. C.
;
Wang, S. C.
We report the observation of an abnormal photoluminescent (PL) spectrum from a HeCd laser pumped InGaN multiple quantum well (MQW) vertical cavity. The device is fabricated using standard MOCVD deposition on a (0001)-oriented sapphire substrate. The layer structures are: 10nm nucleation layer, a 4um bulk GaN layer, InGaN MQWs, and a final 200nm GaN cap layer. The InGaN MQWs consist of 10 pairs of 5 nm GaN barrier and 3 run In0.1Ga0.9N well. The peak emission of the as-grown MQWs sample was similar to 420nm. A dielectric distributed Bragg reflectors (DBR) were then coated on the top layer, followed by a laser lift off from sapphire substrate, and subsequently another DBR coated on the bulk GaN bottom surface. The cavity has a quality factor of similar to 520 from 400-490nm. The device was pumped by a focused CW HeCd laser from the bulk GaN side. When the laser is focused onto the InGaN MQWs, a photoluminescent spectrum centered at the designed MOW wavelength was observed as expected. However, when the focused position was moved toward the bulk GaN region, an additional abnormal PL peak around 460nm was observed. This is far outside the designed MQW wavelength.
The Lasing Characteristics of GaN-based Two-dimensional Photonic Crystal Surface Emitting Lasers
(2010)
Chen, S. W.
;
Kao, T. T.
;
Wu, T. T.
;
Lu, T. C.
;
Kuo, H. C.
GaN-based photonic crystal surface-emitting lasers(PCSELs) with AlN/GaN distributed Bragg reflectors were fabricated and analyzed. Different lasing characteristics of GaN-based PCSEL has been determeined and demonstrated by the PC lattice constants. The laser emission behavior covered the whole PC patterns of 50 mu m in diameter. Under the optical pumping at room temperature, the PCSEL with PC lattice constant of 230nm shows a threshold energy density of about 2.7 mJ/cm(2). Above the threshold, one dominated peak emits at 420.11 nm with a linewidth of 1.1 angstrom. The lasing wavelength emitted from PC lasers with different lattice constants occurs at the calculated band-edges provided by the PC patterns which further shows different polarization angles due to the light diffracted in specific directions, corresponding exactly to Gamma, K, and M directions in the K-space. The PCSEL also shows a characteristic temperature of 148K and a spontaneous emission coupling efficiency beta of about 5x10(-3). Besides, the coupled-wave model in the PC hexagonal-lattice is applied to distinguish the discrepancy in threshold power and the corresponding coupling coefficient. The results show the lasing actions within Gamma, K, and M modes have the substantial relation between the threshold energy density and the coupling coefficient.
Blue GaN-based vertical cavity surface emitting lasers by CW current injection at 77K
(2009)
Kuo, H. C.
;
Chen, S. W.
;
Kao, T. T.
;
Kao, C. C.
;
Chen, J. R.
In the paper, we describe the fabrication and performance characteristics of GaN-based vertical-cavity surface-emitting lasers (VCSELs) by optical pumping and current injection. According to the employment of high-quality and high-reflectivity AlN/GaN DBRs in the whole structure, the lasing action of optically pumped GaN-based VCSELs with hybrid mirrors has been observed at room temperature. Due to the excellent results of optically pumped GaN-based VCSELs with hybrid mirrors, we further demonstrated the lasing behavior of GaN-based VCSELs by continuous-wave current injection at 77 K. The laser has one dominated blue wavelength located at 462 nm with a linewidth of about 0.15 nm and the threshold injection current at 1.4 mA. The divergence angle and polarization ratio of the GaN-based VCSELs with hybrid mirrors are about 11.7 degrees and 80 %, respectively. A larger spontaneous coupling efficiency of about 7.5x10(-2) was also measured.
Enhancement of InGaN-GaN indium-tin-oxide flip-chip light-emitting diodes with TiO2-SiO2 multilayer stack omnidirectional reflector
(2006-09-01)
Lin, C. H.
;
Lai, C. F.
;
Ko, T. S.
;
Huang, H. W.
;
Kuo, H. C.
Enhancement of light extraction of GaN-based flip-chip indium-tin-oxide light-emitting diodes (FC ITO LEDs) with an onmidirectional reflector (ODR) is presented. The ODR consisting of alternating layers of TiO2 and SiO2 is designed to possess a complete photonic bandgap within the blue region of interest, and it is fabricated by E-beam deposition. At a driving current of 300 mA and a chip size of 1 mm x 1 mm, the light output power of the FC ITO LEDs with the ODR reaches 156 mW. This is an enhancement of 31% when compared with the same device with an Al mirror instead. Furthermore, by examining the radiation patterns, the FC ITO LED with the ODR shows stronger enhancement around the vertical direction. Our work offers promising potential for enhancing output powers of commercial light-emitting devices.
Light-output enhancement of nano-roughened GaN laser lift-off light-emitting diodes formed by ICP dry etching
(2007-05-01)
Kao, Chih-Chiang
;
Kuo, H. C.
;
Yeh, K. F.
;
Chu, J. T.
;
Peng, W. L.
In this paper, we report the fabrication and characteristics of nano-roughened GaN laser lift-off (LLO) light-emitting diodes (LEDs) with different scale surface roughness. The surface roughness of devices was controlled by inductively coupled plasma reactive inn etching. Using this fabrication method to form nanoscaled roughness, the electrical property was almost not degraded. Furthermore, the light-output power and wall-plug efficiency of LLO LED could be both significantly enhanced about two times using this simple method.
Fabrication of microcavity light-emitting diodes using highly reflective AlN-GaN and Ta2O5-SiO2 distributed Bragg mirrors
(2007-07-01)
Huang, G. S.
;
Lu, T. C.
;
Kuo, H. C.
;
Wang, S. C.
;
Chen, Hou-Guang
We report the fabrication of microcavity light-emitting diodes (MCLEDs) with high reflectivity and crack-free AIN-GaN distributed Bragg reflector (DBR). The 5 lambda microcavity structure consists of an n-type GaN, ten pairs InGaN-GaN multiple quantum wells and p-type GaN sandwiched between the hybrid cavity mode of an AIN-GaN and a Ta2O5-SiO2 DBR. The AIN-GaN DBR has 29 periods with insertion of six AIN-GaN superlattice layers showing a crack-free surface morphology and a high peak reflectivity of 99.4% with a stopband of 21 nm. The output power of MCLED is about 11 mu W at an injection current of 7 mA. The electroluminescence has a polarization property with a degree of polarization of about 51 %.
Efficiency Enhancement and Beam Shaping of GaN-InGaN Vertical-Injection Light-Emitting Diodes via High-Aspect-Ratio Nanorod Arrays
(2009-01-01)
Tsai, Min-An
;
Yu, Peichen
;
Chao, C. L.
;
Chiu, C. H.
;
Kuo, H. C.
The enhanced light extraction and collimated output beam profile from GaN-InGaN vertical-injection light-emitting diodes (VI-LEDs.) are demonstrated utilizing high-aspect-ratio nanorod arrays. The nanorod arrays are patterned by self-assembled silica spheres, followed by inductively coupled-plasma reactive ion etching. The fabricated nanorod arrays not only provide an omnidirectional escaping zone for photons, but also serve as waveguiding channels for the emitted light, resulting in a relatively collimated beam profile. The light output power of the VI-LED with nanorod arrays is enhanced by 40 %, compared to a conventional VI-LED. The measured far-field profiles indicate that the enhancement is mainly along the surface normal direction, within a view angle of 20 degrees.

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