Chih-Yen Chen

National Taiwan University, T’ai-pei, Taipei, Taiwan

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Publications (20)49.79 Total impact

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    ABSTRACT: A reverse-biased voltage is applied to either device in the vertical configuration of two light-emitting diodes (LEDs) grown on patterned and flat Si (110) substrates with weak and strong quantum-confined Stark effects (QCSEs), respectively, in the InGaN/GaN quantum wells for independently controlling the applied voltage across and the injection current into the p-i-n junction in the lateral configuration of LED operation. The results show that more carrier supply is needed in the LED of weaker QCSE to produce a carrier screening effect for balancing the potential tilt in increasing the forward-biased voltage, when compared with the LED of stronger QCSE. The small spectral shift range in increasing injection current in the LED of weaker QCSE is attributed not only to the weaker QCSE, but also to its smaller device resistance such that a given increment of applied voltage leads to a larger increment of injection current. From a viewpoint of practical application in LED operation, by applying a reverse-biased voltage in the vertical configuration, the applied voltage and injection current in the lateral configuration can be independently controlled by adjusting the vertical voltage for keeping the emission spectral peak fixed.
    Optics Express 04/2014; 22(7):8367-75. · 3.55 Impact Factor
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    ABSTRACT: The technique of depositing a temperature-graded AlN buffer layer for crack-free GaN growth on Si substrate is demonstrated. Compared with the previously reported methods of buffer growth, this technique has the advantages of a thinner buffer layer for effectively producing compressive stress and simple binary growth without the need of changing its composition. By combining with an inter-layer structure of three-period GaN/AlN superlattice, crack-free growth of thick GaN layers of 3.7 μm in total thickness is implemented. Also, the different emission behaviors of overgrown InGaN/GaN quantum wells (QWs) under different stress conditions with different graded temperature numbers are illustrated. It is found that the QW sample with the highest graded temperature number for depositing the AlN buffer has the weakest residual tensile stress, the shortest emission wavelength, the weakest carrier localization effect, the weakest quantum-confined Stark effect, and the highest internal quantum efficiency, even though the total indium content is about the same as those of other samples with significantly stronger residual stresses.
    Journal of Crystal Growth 01/2014; 396:1–6. · 1.55 Impact Factor
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    ABSTRACT: The fundamental phenomena, basic principles, and device fabrication and characterization of surface plasmon coupled InGaN/GaN quantum-well light-emitting diode are reviewed, including experimental demonstrations and theoretical/numerical studies.
    Asia Communications and Photonics Conference; 11/2013
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    ABSTRACT: For enhancing the light extraction of a light-emitting diode, surface grating fabrication based on a simple method of combining photoelectrochemical (PEC) etching with phase mask interferometry has been demonstrated. To understand the optimum grating period in forming a surface grating on a vertical light-emitting diode (VLED), we construct a Llyod's interferometer within PEC electrolyte (KOH) to fabricate surface gratings of various periods on VLEDs for comparing their light extraction efficiencies. Also, to compare the effectiveness of light extraction enhancement between surface grating and rough surface, VLEDs with the rough surfaces fabricated with two different KOH wet etching methods are fabricated. The comparisons of VLED characterizations show that among those grating VLEDs, the light extraction is more effective in a VLED of a smaller grating period. Also, compared with VLEDs of rough surfaces, the grating VLEDs of short grating periods (<2 μm) have the higher light extraction efficiencies, even though the root-mean-square roughness of the rough surface is significantly larger than the grating groove depth.
    Optics Express 07/2013; 21(15):17686-94. · 3.55 Impact Factor
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    ABSTRACT: The cross-sectional sizes of the regularly patterned GaN nanorods (NRs) and InGaN/GaN quantum-well (QW) NRs of different heights and different hexagon orientations, which are grown on the patterned templates of different hole diameters, pitches, and crystal orientations, are compared. It is found that the cross-sectional size of the GaN NR, which is formed with the pulsed growth mode, is mainly controlled by the patterned hole diameter, and the thickness of the sidewall QW structure is mainly determined by the NR height. The cross-sectional size variation of GaN NR is interpreted by the quasi-three-dimensional nature of atom supply amount for precipitating a two-dimensional disk-shaped NR segment. The variation of the sidewall QW structure is explained by the condition of constituent atom supply in the gap volume between the neighboring NRs. Also, we compare the cathodoluminescence emission wavelengths among those samples of different growth conditions. Generally speaking, the QW NR with a smaller height, a larger cross-sectional size, or a larger pitch has a longer emission wavelength.
    Journal of Applied Physics 02/2013; 113(5). · 2.21 Impact Factor
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    ABSTRACT: A vertical light-emitting diode (VLED) with the CdZnO/n-ZnO quantum wells and n+-ZnO capping layer grown with molecular beam epitaxy and the p-GaN layer grown with metalorganic chemical vapor deposition, is fabricated and characterized. Its performances are compared with those of a lateral LED based on the same epitaxial structure to show the significantly lower device resistance, smaller leakage current, weaker output intensity saturation, relatively lower defect emission, and stronger emissions from the p-GaN and n-ZnO layers in the VLED.
    IEEE Photonics Technology Letters 02/2013; 25(3):317-319. · 2.04 Impact Factor
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    ABSTRACT: The comparisons of the morphology, material property, and optical characteristics of a crack-free InGaN/GaN quantum-well light-emitting diode (LED) structure of completely coalesced overgrowth on an a-axis-oriented one-dimensional trench-patterned Si (110) substrate with the other two samples grown on flat Si (110) and Si (111) substrates are demonstrated. This sample on patterned Si (110) substrate shows the highest crystal quality, weakest tensile strain, largest internal quantum efficiency, strongest LED output intensity, lowest device resistance, and smallest spectral shift range in increasing injection current. The small spectral shift range indicates the weak quantum-confined Stark effect. The advantages of this sample are attributed to the small lattice mismatch between Si and GaN along the m-axis, the reduced thermal stress along the m-axis, and the minimized upward-propagating dislocation density.
    Applied Physics Letters 01/2013; 103(14):141914-141914-4. · 3.52 Impact Factor
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    ABSTRACT: A low-cost large-area effective sapphire substrate liftoff method based on the photoelectrochemical (PEC) etching technique is demonstrated. By preparing patterned sapphire substrate (PSS) with 1-D periodic grooves and an epitaxial structure with the grooves preserved to form tunnels, PEC electrolyte can flow along the tunnels to etch the bottom of the GaN layer for separating the PSS from the wafer-bonded epitaxial layer. Assisted by the device isolation procedure, the PSS liftoff of a quarter-wafer sample can be completed in 8 min. After a smoothing process of the exposed N-face surface after liftoff, a vertical light-emitting diode (LED) is fabricated for comparing its characteristics with those of a conventional LED.
    IEEE Photonics Technology Letters 10/2012; 24(19):1775-1777. · 2.04 Impact Factor
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    ABSTRACT: CdZnO/ZnO quantum well (QW) samples are grown on GaN and ZnO templates with plasma-assisted molecular beam epitaxy under different conditions of substrate temperature, Cd effusion cell temperature, and O<sub>2</sub> flow rate for emission characteristics comparison. It is found that the Cd incorporation on the ZnO template is generally lower, when compared with that on the GaN template, such that the O<sub>2</sub> flow rate needs to be reduced for stoichiometric CdZnO/ZnO QW growth on the ZnO template. Besides the wurtzite (wt) CdZnO structure, the rock-salt (rs) CdZnO structure exists in the CdZnO well layers when the total Cd content is high. The rs structure may dominate over the wt structure in photoluminescence intensity when the total Cd content is high. In either group of samples on the GaN and ZnO templates, the emission efficiency first increases and then decreases with increasing total Cd content. The low emission efficiency at low (high) Cd content is attributed to the weaker quantum confinement (the poorer crystal quality) of the QWs. The emission efficiencies of the QW samples on the GaN template are generally higher than those on the ZnO template. The carrier localization behavior in a CdZnO/ZnO QW, grown on either GaN or ZnO template, is significantly weaker than that in an InGaN/GaN QW. The strength of the quantum-confined Stark effect generally increases with increasing Cd content in either group of samples on the GaN and ZnO templates.
    Optics Express 09/2012; 20(20):21860-74. · 3.55 Impact Factor
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    ABSTRACT: With the nano-imprint lithography and the pulsed growth mode of metalorganic chemical vapor deposition, a regularly-patterned, c-axis nitride nanorod (NR) array of quite uniform geometry with simultaneous depositions of top-face, c-plane disc-like and sidewall, m-plane core-shell InGaN/GaN quantum well (QW) structures is formed. The differences of geometry and composition between these two groups of QW are studied with scanning electron microscopy, cathodoluminescence, and transmission electron microscopy (TEM). In particular, the strain state analysis results in TEM observations provide us with the information about the QW width and composition. It is found that the QW widths are narrower and the indium contents are higher in the sidewall m-plane QWs, when compared with the top-face c-plane QWs. Also, in the sidewall m-plane QWs, the QW width (indium content) decreases (increases) with the height on the sidewall. The observed results can be interpreted with the migration behaviors of the constituent atoms along the NR sidewall from the bottom.
    Optics Express 07/2012; 20(14):15859-71. · 3.55 Impact Factor
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    ABSTRACT: The counteraction between the increased carrier localization effect due to the change of composition nanostructure in the quantum wells (QWs), which is caused by the thermal annealing process, and the enhanced quantum-confined Stark effect in the QWs due to the increased piezoelectric field, which is caused by the increased p-type layer thickness, when the p-type layer is grown at a high temperature on the InGaN/GaN QWs of a high-indium light-emitting diode (LED) is demonstrated. Temperature- and excitation power-dependent photoluminescence (PL) measurements are performed on three groups of sample, including 1) the samples with both effects of thermal annealing and increased p-type thickness, 2) those only with the similar thermal annealing process, and 3) those with increased overgrowth thickness and minimized thermal annealing effect. From the comparisons of emission wavelength, internal quantum efficiency (IQE), spectral shift with increasing PL excitation level, and calibrated activation energy of carrier localization between various samples in the three groups, one can clearly see the individual effects of thermal annealing and increased p-type layer thickness. The counteraction leads to increased IQE and blue-shifted emission spectrum with increasing p-type thickness when the thickness is below a certain value (20-nm p-AlGaN plus 60-nm p-GaN under our growth conditions). Beyond this thickness, the IQE value decreases and the emission spectrum red shifts with increasing p-type thickness.
    Optics Express 05/2012; 20(10):11321-35. · 3.55 Impact Factor
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    ABSTRACT: A CdZnO/n-ZnO multiple-quantum-well (QW) light-emitting diode (LED), with the QWs and ${\rm n}^{+}$-ZnO capping layer grown with molecular beam epitaxy on p-GaN, which is grown with metal–organic chemical vapor deposition, is fabricated and characterized. Because of the weak carrier localization mechanism in the ZnO-based LED, its defect emission is quite strong and dominates the LED output when injection current is low. The blue shift of the LED output spectrum in applying a forward-biased voltage and the large blue-shift range in increasing injection current show the different behaviors of such a ZnO-based LED from those of a nitride LED.
    IEEE Photonics Technology Letters 01/2012; 24(11):909-911. · 2.04 Impact Factor
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    ABSTRACT: The spiral deposition of InGaN with a quasiperiodical distribution of indium content along the growth direction for forming InGaN nanoneedles (NNs) with the vapor-liquid-solid (VLS) growth mode is demonstrated. The VLS growth is implemented by using Au nanoparticles (NPs) as the catalyst in metalorganic chemical vapor deposition. The Au NPs on a GaN template are generated through pulsed laser irradiation. The observation of spiral deposition is based on the analyses of the scanning results in the high angle annular dark field and energy dispersive X-ray measurements of transmission electron microscopy. In the measurements, the composition variations along and perpendicular to the growth direction (the c-axis) are illustrated. The alternating indium content along the growth direction is attributed to a quasiperiodically pulsed behavior of indium supersaturation process in the melted Au NP at the top of an InGaN NN. The spiral deposition of InGaN is due to the formation of an NN at the location of an Au NP with a screw-type dislocation beneath in the GaN template, at which the growth of a quasi-one-dimensional structure can be easily initiated.
    Journal of Nanomaterials 01/2012; 2012. · 1.55 Impact Factor
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    ABSTRACT: Based on the fabrication of Ag nanoparticles (NPs) with controlled geometry and surface density on an InGaN/GaN quantum well (QW) epitaxial structure, which contains indium-rich nano-clusters for producing localized states and free-carrier (delocalized) states in the QWs, and the characterization of their localized surface plasmon (LSP) coupling behavior with the carriers in the QWs, the interplay behavior of LSP coupling with carrier delocalization in the QWs is demonstrated. By using the polystyrene nanosphere lithography technique with an appropriate nanosphere size and adjusting the post-fabrication thermal annealing condition, the induced LSP resonance wavelength of the fabricated Ag NPs on the QW sample can match the QW emission wavelength for generating the coherent coupling between the carriers in the QWs and the induced LSP. The coupling leads to the enhancement of radiative recombination rate in the QWs and results in increased photoluminescence (PL) intensity, red-shifted PL spectrum, reduced PL decay time, and enhanced internal quantum efficiency. It is found that the observed effects are mainly due to the LSP coupling with the delocalized carriers in the QWs.
    Nanotechnology 11/2011; 22(47):475201. · 3.84 Impact Factor
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    ABSTRACT: The experimental demonstrations of light-emitting diode (LED) fabrication with surface plasmon (SP) coupling with the radiating dipoles in its quantum wells are first reviewed. The SP coupling with a radiating dipole can create an alternative emission channel through SP radiation for enhancing the effective internal quantum efficiency when the intrinsic non-radiative recombination rate is high, reducing the external quantum efficiency droop effect at high current injection levels, and producing partially polarized LED output by inducing polarization-sensitive SP for coupling. Then, we report the theoretical and numerical study results of SP-dipole coupling based on a simple coupling model between a radiating dipole and the SP induced on a nearby Ag nanoparticle (NP). To include the dipole strength variation effect caused by the field distribution built in the coupling system (the feedback effect), the radiating dipole is represented by a saturable two-level system. The spectral and dipole-NP distance dependencies of dipole strength variation and total radiated power enhancement of the coupling system are demonstrated and interpreted. The results show that the dipole-SP coupling can enhance the total radiated power. The enhancement is particularly effective when the feedback effect is included and hence the dipole strength is increased.
    Optics Express 07/2011; 19 Suppl 4:A914-29. · 3.55 Impact Factor
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    ABSTRACT: The method of sapphire substrate liftoff for the fabrication of vertical nitride light-emitting diode (LED), based on the combination of the photoelectrochemical (PEC) etching and epitaxial lateral overgrowth (ELOG) techniques, is demonstrated. This method relies on the formation of connected voids during the ELOG process on a GaN template such that PEC electrolyte can approach the GaN portions above the SiO<sub>2</sub> masks. Also, the GaN template must be thin enough for the illuminating ultraviolet light to reach the GaN portions above the SiO<sub>2</sub> masks. It is shown that PEC etching starts from a very thin layer of GaN right above a SiO<sub>2</sub> mask. It then extends into the window regions of ELOG to completely separate GaN from sapphire. The performances of a vertical LED are illustrated.
    IEEE Photonics Technology Letters 06/2011; · 2.04 Impact Factor
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    ABSTRACT: InGaN epilayers exhibiting strong defect-related sub-bandgap emission, which is undesirable in epilayers and quantum well structures designed for light-emitting diodes and laser diodes, have been studied by confocal photoluminescence spectroscopy, Auger electron spectroscopy, and atomic force microscopy. Inhomogeneous spatial distribution of band-edge luminescence intensity and comparatively homogenous distribution of defect-related emission are demonstrated. It is shown that laser annealing at power densities causing the increase of the temperature at the epilayer surface high enough for indium atoms to move to the surface results in suppression of the defect-related emission.
    Journal of Luminescence 01/2011; 131(7):1322-1326. · 2.14 Impact Factor
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    ABSTRACT: The dependencies of quantum-well (QW) internal quantum efficiency (IQE) and device behaviors on the p-layer thickness in a high-indium InGaN/GaN QW light-emitting diode (LED) are demonstrated. During the high-temperature growths of the p-AlGaN and p-GaN layers, the QWs are thermally annealed to increase their IQEs and blue-shift the emission with increasing p-layer thickness. Meanwhile, the quantum-confined Stark effect is enhanced with increasing p-layer thickness to decrease the IQEs and red-shift the emission. Based on the counteraction between the two effects, the maximum IQE and the shortest emission wavelength are observed in a sample with an optimized p-layer thickness, which includes a p-AlGaN layer of 20 nm and a p-GaN layer of 60 nm in thickness under our growth conditions. The fabricated LEDs of different p-GaN thicknesses show the similar variation trends in emission efficiency and wavelength. Index Terms—Internal quantum efficiency (IQE), light-emitting diode (LED), p-GaN, quantum-confined Stark effect, thermal an- nealing.
    IEEE Photonics Technology Letters 01/2011; 23(23):1757-1759. · 2.04 Impact Factor
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    ABSTRACT: The reduction in the external quantum efficiency (EQE) droop effect of an InGaN/GaN quantum-well (QW) light-emitting diode (LED) through the mechanism of surface plasmon (SP) coupling with QW is demonstrated. With a current spreading grid pattern on the mesa surface, a smaller grid period leads to more effective carrier transport into the QW regions of Ag deposition for stronger SP–QW coupling such that the droop effect is more significantly reduced, as indicated by the increase in injection current density of maximum EQE and the decrease in drooping slope. The claim of the SP–QW coupling effect in the samples of thin p-GaN is supported by the different droop behaviors of the LED samples fabricated with another epitaxial structure of thick p-GaN, in which the SP–QW coupling effect is expected to be weak.
    Applied Physics Letters 06/2010; 96(26):261104-261104-3. · 3.52 Impact Factor
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    ABSTRACT: Further enhancement of the efficiency of an InGaN/GaN quantum well (QW) light-emitting diode (LED) through QW coupling with surface plasmons generated on Ag nano-gratings by inserting a SiO2 layer between semiconductor and metal is demonstrated.
    01/2010;