Browsing by Author "Wu, Yuh-Renn"

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3D Finite Element Strain Analysis of V-Shaped Pits in Light Emitting Diodes
(2015)
Hsu, Chung-Cheng
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Wu, Chen-Kuo
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Li, Chi-Kang
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Lu, Tien-Chang
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Wu, Yuh-Renn
In this paper, we have analyzed the strain distribution of the V-pits in light emitting diodes (LEDs). The special geometry and non-uniform distribution introduce the complicated strain and piezoelectric distribution, where it might affect the carrier injection. In the future, we will combine the calculated piezoelectric field with our in-house developed 3D drift-diffusion model to figure out how the carrier inject into LEDs with V-pits.
3D numerical modeling of the carrier transport and radiative efficiency for InGaN/GaN light emitting diodes with V-shaped pits
(2016-05-01)
Li, Chi-Kang
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Wu, Chen-Kuo
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Hsu, Chung-Cheng
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Lu, Li-Shuo
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Li, Heng
In this paper, influence of a V-pit embedded inside the multiple quantum wells (MQWs) LED was studied. A fully three-dimensional stress-strain solver and Poisson-drift-diffusion solver are employed to study the current path, where the quantum efficiency and turn-on voltage will be discussed. Our results show that the hole current is not only from top into lateral quantum wells (QWs) but flowing through shallow sidewall QWs and then injecting into the deeper lateral QWs in V-pit structures, where the V-pit geometry provides more percolation length for holes to make the distribution uniform along lateral MQWs. The IQE behavior with different V-pit sizes, threading dislocation densities, and current densities were analyzed. Substantially, the variation of the quantum efficiency for different V-pit sizes is due to the trap-assisted nonradiative recombination, effective QW ratio, and ability of hole injections. (C) 2016 Author(s).
Abnormal polarization switching phenomenon in a-plane Al(x)Ga(1-x)N
(2010-10-11)
Huang, Huei-Min
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Huang, Hung-Hsun
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Wu, Yuh-Renn
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Lu, Tien-Chang
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光電工程學系
The optical polarization properties of a-plane Al(x)Ga(1-x)N films have been investigated by polarization-dependent photoluminescence (PL). The degree of polarization decreased with increasing the Al composition, and the main optical polarization direction switched from epsilon perpendicular to c to epsilon // c at about x = 0.07 due to the valence band switching, representing that the optical transition energy of epsilon // c is surpassing that of epsilon perpendicular to c. However, with the Al composition larger than x = 0.1, the higher energy optical transitions of epsilon // c exhibited the stronger PL intensity, opposite to the normal situations that higher energy states commonly have weaker PL intensity than the lower energy states. We utilized the 6 x 6 k.p model and the lambertian-like radiation pattern assumption to explain this abnormal optical polarization switching behavior in the a-plane Al(x)Ga(1-x)N layers and obtained good agreement with the experimental results. (C) 2010 Optical Society of America
Abnormal polarization switching phenomenon in a-plane Al(x)Ga(1-x)N
(2011-01-01)
Huang, Huei-Min
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Huang, Hung-Hsun
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Wu, Yuh-Renn
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Lu, Tien-Chang
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Kuo, Hao-Chung
The a-plane Al(x)Ga(1-x)N layers show abnormal optical polarization switching behavior. This phenomenon can be well characterized by the assumption of the lambertian-like radiation pattern and simulated by utilizing the 6 x 6 k . p model. (C) 2010 Optical Society of America
Abnormal polarization switching phenomenon in a-plane AlxGa1-xN
(2010-10-11)
Huang, Huei-Min
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Huang, Hung-Hsun
;
Wu, Yuh-Renn
;
Lu, Tien-Chang
;
照明與能源光電研究所
The optical polarization properties of a-plane AlxGa1-xN films have been investigated by polarization-dependent photoluminescence (PL). The degree of polarization decreased with increasing the Al composition, and the main optical polarization direction switched from epsilon perpendicular to c to epsilon // c at about x = 0.07 due to the valence band switching, representing that the optical transition energy of epsilon // c is surpassing that of epsilon perpendicular to c. However, with the Al composition larger than x = 0.1, the higher energy optical transitions of epsilon // c exhibited the stronger PL intensity, opposite to the normal situations that higher energy states commonly have weaker PL intensity than the lower energy states. We utilized the 6 x 6 k.p model and the lambertian-like radiation pattern assumption to explain this abnormal optical polarization switching behavior in the a-plane AlxGa1-xN layers and obtained good agreement with the experimental results. (C) 2010 Optical Society of America
Abnormal polarization switching phenomenon in a-plane AlxGa1-xN
(2011-01-01)
Huang, Huei-Min
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Huang, Hung-Hsun
;
Wu, Yuh-Renn
;
Lu, Tien-Chang
;
Kuo, Hao-Chung
The a-plane AlxGa1-xN layers show abnormal optical polarization switching behavior. This phenomenon can be well characterized by the assumption of the lambertian-like radiation pattern and simulated by utilizing the 6 x 6 k . p model. (C) 2010 Optical Society of America
Analysis of Strain Relaxation and Emission Spectrum of A Free-Standing GaN-based Nanopillar
(2008-01-01)
Wu, Yuh-Renn
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Yu, Peichen
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Chiu, C. H.
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Chang, Cheng-Yu
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Kuo, H. C.
We have made a, GaN-based single nanopillar with a, diameter of 300nm using the focused ion beam (FB) technique. The micro-photoluminescence (mu-PL) from the embedded GaN/InGaN multi-quantum wells reveals a blue shift of 68.3 meV in energy. In order to explain the spectrum shift, we have developed a valence force field model to study the strain relaxation mechanism in a, single GaN-based nanopillar structure. The strain distribution and strain induced polarization effect inside the multiple quantum wells is added to our self-consistent Poisson, drift-diffusion, and Schrodinger solver to study the spectrum shift of mu-PL.
Back-Contacted Thin-Film GaAs Solar Cells
(2016-01-01)
Hong, Chung-Yu
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Lin, Yu-Chen
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Ho, Kuan-Ying
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Tsai, Jia-Ling
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Chien, Zhan-Tien
In this work, we present a thin-film GaAs solar cell employing via contacts for forming a back-contacted solar cell to eliminate light blocking effects and enhance photon recycling at the same time. The fabrication procedure and the characterization of the fabricated devices are discussed in this paper. From our measurement, a conversion efficiency of 6.5%, open circuit voltage of 0.95 V, and a current density of 8.6mA/cm(2) under one-sun illumination were obtained in a 5x5 mm cell.
Characteristics of large-scale nanohole arrays for thin-silicon photovoltaics
(2014-04-01)
Chen, Ting-Gang
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Yu, Peichen
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Chen, Shih-Wei
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Chang, Feng-Yu
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Huang, Bo-Yu
Nanostructured crystalline silicon is promising for thin-silicon photovoltaic devices because of reduced material usage and wafer quality constraint. This paper presents the optical and photovoltaic characteristics of silicon nanohole (SiNH) arrays fabricated using polystyrene nanosphere lithography and reactive-ion etching (RIE) techniques for large-area processes. A post-RIE damage removal etching is subsequently introduced to mitigate the surface recombination issues and also suppress the surface reflection due to modifications in the nanohole sidewall profile, resulting in a 19% increase in the power conversion efficiency. We show that the damage removal etching treatment can effectively recover the carrier lifetime and dark current-voltage characteristics of SiNH solar cells to resemble the planar counterpart without RIE damages. Furthermore, the reflectance spectra exhibit broadband and omnidirectional anti-reflective properties, where an AM1.5 G spectrum-weighted reflectance achieves 4.7% for SiNH arrays. Finally, a three-dimensional optical modeling has also been established to investigate the dimension and wafer thickness dependence of light absorption. We conclude that the SiNH arrays reveal great potential for efficient light harvesting in thin-silicon photovoltaics with a 95% material reduction compared to a typical cell thickness of 200 mu m. Copyright (c) 2012 John Wiley & Sons, Ltd.
FABRICATION AND MODELING OF LARGE-SCALE SILICON NANOWIRE SOLAR CELLS FOR THIN-FILM PHOTOVOLTAICS
(2012)
Cheng, Yu-Chih
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Chen, Ting-Gang
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Chang, Feng-Yu
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Huang, Bo-Yu
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Pan, Huai-Te
Nanostructured crystalline silicon is promising for thin photovoltaic devices due to reduced material usage and wafer quality constraint. In this study, a simple and cost-effective method is presented for producing large-area silicon nanowire (SiNW) arrays with various lengths by employing the polystyrene nanosphere lithography and the metal-assisted chemical etching (MACE). The nanowire structure exhibits broadband and omnidirectional antireflection properties, where the lowest AM1.5G-spectrum-weighted reflectance of SiNW arrays is 4.38%, compared to 8.84% of the conventional single layer antireflection coating (SLARC). Next, we made SiNW array structures into solar cells and its power efficiency achieved 10.8%. Afterward, the SiNW array solar cells were analyzed by external and internal quantum efficiency measurement which shows the device performance primarily limited by direct and indirect interfacial recombination of charge carriers. We further develop a modeling technique based on three-dimensional (3) optical and two-dimensional (20) electrical simulations which are tailored for the nanostructured silicon photovoltaic devices. The optical simulation employs a rigorous couple wave analysis (RCWA) technique to calculate the distribution of electromagnetic field in order to obtain the generation rate within the nanostructure. Next, an electrical calculation is based on a 20 self-consistent drift-diffusion and Poisson solver using a finite element method (FEM). We investigate the influence to the device performance of carrier diffusion length with three different structures. The simulation results show that a short diffusion length does not seriously deteriorate the short-circuit current density and power conversion efficiency of a core-shell PN junction solar cell compared to the conventional planar structure. Our work suggests that SiNW solar cells can achieve efficient light harvesting and photoelectric conversion with less and lower-grade materials than the conventional technology, which is promising for thin silicon photovoltaics.
Mechanisms of the Asymmetric Light Output Enhancements in a-Plane GaN Light-Emitting Diodes With Photonic Crystals
(2014-12-01)
Li, Hsiang-Wei
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Yin, Yu-Feng
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Chang, Chen-Yu
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Tsai, Chen-Hung
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Hsu, Yen-Hsiang
The unique properties of nonpolar GaN light-emitting diodes (LEDs) have the advantages of generating polarized light emission. The employment of asymmetric 2-D photonic crystals (PhCs) can further enhance the light polarization ratio. In addition, it was generally recognized that the Purcell effect can increase the internal quantum efficiency of the LEDs with PhCs. In this paper, we study the properties of optical modes from different crystal planes. The Purcell effect is analyzed based on the PhCs and material crystal orientations. With different transition probability of the polarized photons in valence bands, the corresponding Purcell effect enhancement on the quantum efficiency varies.
Mode-Hopping Phenomena in the InGaN-Based Core-Shell Nanorod Array Collective Lasing
(2018-07-01)
Huang, Chia-Yen
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Tai, Tzu-Ying
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Lin, Jing-Jie
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Chang, Tsu-Chi
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Liu, Che-Yu
A two-dimensional monolithic InGaN-based core-shell it array was excited under a varying temperature. We observed an abrupt dominant mode hopping from 3.29 to 141 eV in the collective lasing as the temperature decreased from T = 240 to 225 K, Photoluminescence under a near-threshold pumping density revealed the splitting of spontaneous emission. between the core and the shell with a decreasing temperature. The bandgap evolution of GaN and InGaN showed opposite trends due to the interaction of temperature effects and bandgap renormalization effects. Theoretical simulation revealed the differences of gain spectra evolution between room lire and to temperature due to the difference the carrier dynamics, In the optically coupled nanorod array, the dominant lasing;mode was not only determined by it structure, but also strongly influenced by external operating conditions. Rattier than singular microcavity lasers the coupled nanorod array demonstrated a broader parameter space for on-chip wavelength manipulation.
Optimization of All-Back-Contact GaAs Solar Cells
(2015)
Ho, Kuan-Ying
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Hong, Chung-Yu
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Yu, Peichen
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Wu, Yuh-Renn
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光電工程學系
The optimized condition of all-back-contact solar cells is studied to eliminate the problem of light blocking by the front contacts in conventional solar cells. The influences of layer thickness and pitch distance are studied. The optimized structure is obtained with a 2 mu m thick base layer and a 631 mu m contact pitch, and the efficiency can reach to 26.15%.
Optimized Design of Back-Contact Thin-Film GaAs Solar Cells
(2017-01-01)
Tsai, Jia-Ling
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Hong, Chung-Yu
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Zhan, Tien-Chien
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Wu, Yuh-Renn
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Lin, Albert
In this work, we attempt to find the optimal design parameters for the doping concentration, thickness of the base region, the length and pitch of back electrodes by employing a validated Sentaurus TCAD model. Through current-voltage characteristics, one could determine best power conversion efficiency and the correlation between design parameters. While optical shadowing is eliminated in the back-contact design, it is found that electrical shading affects the cell performance and is sensitive to the electrode length and pitch. Through minimizing the length of anode, the efficiency can be elevated significantly. Moreover, a relatively wide cathode results in efficient carrier separation and superior transportation properties. Based on these simulation outcomes, we are able to optimize the design of back-contact back-junction GaAs solar cells
Polarization ratio enhancement of a-plane GaN light emitting diodes by asymmetric two-dimensional photonic crystals
(2014-05-21)
Chou, Yen
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Li, Hsiang-Wei
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Yin, Yu-Feng
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Wang, Yu-Ting
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Lin, Yen-Chen
Fabricating photonic crystals (PhCs) on GaN based non-polar light emitting diodes (LEDs) is an effective way to increase light extraction and meanwhile to preserve or improve polarization ratio. In this work, a-plane GaN LEDs with two-dimensional PhCs were demonstrated. With the E // m polarized modes (which mean the optical polarization with the electric field parallel to m-axis) as the target of diffraction, we matched E//m modes to the photonic bands and aligned E//c modes to fall within the photonic band gap. The results show stronger E//m but weaker E//c mode diffractions on both c- and m-axes. At the vertical direction, the polarization ratio is enhanced from 45.8% for the planar device to 52.3% for the LEDs with PhCs. (C) 2014 AIP Publishing LLC.
Projected Efficiency of Polarization-Matched p-InxGa1-xN/i-InyGa1-yN/n-GaN Double Heterojunction Solar Cells
(2013-07-01)
Wang, Hsun-Wen
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Yu, Peichen
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Wu, Yuh-Renn
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Kuo, Hao-Chung
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Chang, Edward Yi
Traditional p-GaN/i-InGaN/n-GaN double heterojunction solar cells have limited power conversion efficiency due to large polarization charges that accumulate at the heterojunction interfaces, leading to severe band bending that, in turn, hinders the carrier transport. In this study, we proposed the use of a p-type InGaN layer to reduce the polarization field and projected the power conversion efficiencies of p-InxGa1-xN/i-InyGa1-yN/n-GaN double heterojunction solar cells that are grown on a c-facet sapphire substrate with various indium components. Numerical simulations predict that a maximal power conversion efficiency that is close to 7% with a short-circuit current density of 4.05 mA/cm(2) and an open-circuit voltage of 1.94 V can be achieved with a p-In0.2Ga0.8N/i-In0.2Ga0.8N/n-GaN structure due to a polarization-matched p-i interface. Further efficiency enhancement with a higher indium composition over 20% is also possible via the redistribution of the built-in potential with n-GaN doping.
Size-Dependent Strain Relaxation and Optical Characteristics of InGaN/GaN Nanorod LEDs
(2009-07-01)
Wu, Yuh-Renn
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Chiu, Chinghua
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Chang, Cheng-Yu
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Yu, Peichen
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Kuo, Hao-Chung
In this paper, InGaN/GaN nanorod LEDs with various sizes are fabricated using self-assembled Ni nanomasks and inductively coupled plasma-reactive ion etching. Photoluminescence (PL) characteristics exhibit size-dependent, wavelength blue shifts of the emission spectra from the nanorod LEDs. Numerical analyses using a valence force field model and a self-consistent Poisson, Schrodinger, and drift-diffusion solver quantitatively describe the correlation between the wavelength blue shifts and the strain relaxation of multiple quantum wells embedded in nanorods with different averaged sizes. Time-resolved PL studies confirm that the array with a smaller size exhibits a shorter carrier lifetime at low temperature, giving rise to a stronger PL intensity. However, the PL intensity deteriorates at room temperature, compared to that of a larger size, possibly due to an increased number of surface states, which decreases the nonradiative lifetime, and hence reduces the internal quantum efficiency.
Strain relaxation induced microphotoluminescence characteristics of a single InGaN-based nanopillar fabricated by focused ion beam milling
(2008-08-25)
Yu, Peichen
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Chiu, C. H.
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Wu, Yuh-Renn
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Yen, H. H.
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Chen, J. R.
A freestanding nanopillar with a diameter of 300 nm and a height of 2 mu m is demonstrated by focused ion beam milling. The measured microphotoluminescence (mu-PL) from the embedded InGaN/GaN multiple quantum wells shows a blueshift of 68 meV in energy with a broadened full width at half maximum, similar to 200 meV. Calculations based on the valence force field method suggest that the spatial variation of the strain tensors in the nanopillar results in the observed energy shift and spectrum broadening. Moreover, the power-dependent mu-PL measurement suggests that the strain-relaxed emission region exhibits a higher radiative recombination rate than that of the strained region, indicating potential for realizing high-efficiency nanodevices in the UV/blue wavelength range. (C) 2008 American Institute of Physics.
Suppression of Current Collapse in Enhancement Mode GaN-Based HEMTs Using an AIGaN/GaN/AIGaN Double Heterostructure
(2017-04-01)
Ho, Shin-Yi
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Lee, Chun-Hsun
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Tzou, An-Jye
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Kuo, Hao-Chung
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Wu, Yuh-Renn
Current collapse is a phenomenon that increases the channel resistance during the switching process when the high gate and drain voltages are applied. The effect can be found from GaN-based high electron mobility transistors (HEMTs) and is an obstacle for their applications to power electronics. There have been numerous reports on suppressing current collapse using semiconductor process technologies. In this paper, an enhancement mode (E-mode) AIGaN/GaN/AIGaN double heterostructure was proposed. The current collapse phenomena were studied on devices of E-mode MIS and Schottky gate HEMTs. The results indicate the suppression of current collapse for devices with double heterostructure.
Three dimensional characterization of GaN-based light emitting diode grown on patterned sapphire substrate by confocal Raman and photoluminescence spectromicroscopy
(2017-03-30)
Li, Heng
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Cheng, Hui-Yu
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Chen, Wei-Liang
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Huang, Yi-Hsin
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Li, Chi-Kang
We performed depth-resolved PL and Raman spectral mappings of a GaN-based LED structure grown on a patterned sapphire substrate (PSS). Our results showed that the Raman mapping in the PSS-GaN heterointerface and the PL mapping in the InxGa1-xN/GaN MQWs active layer are spatially correlated. Based on the 3D construction of E2(high) Raman peak intensity and frequency shift, V-shaped pits in the MQWs can be traced down to the dislocations originated in the cone tip area of PSS. Detail analysis of the PL peak distribution further revealed that the indium composition in the MQWs is related to the residual strain propagating from the PSS-GaN heterointerface toward the LED surface. Numerical simulation based on the indium composition distribution also led to a radiative recombination rate distribution that shows agreement with the experimental PL intensity distribution in the InxGa1-xN/GaN MQWs active layer.