Dong-Soo Shin

Hanyang University, Sŏul, Seoul, South Korea

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Publications (29)37.85 Total impact

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    ABSTRACT: High piezoelectric field in green light-emitting diodes (LEDs) has been a challenge for direct measurement for a long time. In this work, we report the direct experimental measurement of the large piezoelectric field in green LEDs by using single- and double-quantum-well samples with the electroreflectance spectroscopy at room temperature. Comparison with the theoretical prediction indicates that there is an influence of defects in reducing the piezoelectric field in green LED samples.
    Japanese Journal of Applied Physics 08/2014; 53(9):098002. · 1.07 Impact Factor
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    ABSTRACT: The efficiency droop in light-emitting diodes (LEDs) represents a gradual decrease of the internal quantum efficiency (IQE) with increasing current. Experimentally, the IQE droops are strong functions of material, epitaxial and chip structures, and operating temperature. Recently, we have proposed an IQE droop model as the saturation of the radiative recombination rate at low current and subsequent increase in the nonradiative recombination rates at high current. Once the radiative recombination rate begins to saturate at an active region, the carrier density as well as the nonradiative recombination rate rapidly increase there. Eventually, the IQE droop appears from the increase in the nonradiative recombination rate being much larger than that in the radiative one. A dominant nonradiative recombination process is not solely determined for each LED chip, but it could vary with current level and operating temperature. As temperature decreases, in general, the IQE droop becomes larger with the peak IQE occurring at an extremely small current level. We test the droop model by investigating the radiative and nonradiative recombination processes separately from the cryogenic to room temperature. The characterization methods include comparative efficiency study between photoluminescence (PL) and electroluminescence (EL), open-circuit voltage under resonant PL excitation, interrelations of current-voltage-light characteristics, and EL spectra of color-coded quantum wells (QWs). Although a sudden increase of the nonradiative recombination rate is an apparent cause of the IQE droop, the saturation of the radiative recombination rate is the common trigger behind the IQE droop issue.
    02/2014;
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    ABSTRACT: Two kinds of InGaN-based light-emitting diodes (LEDs) are investigated to understand the nonradiative carrier recombination processes. Various temperature-dependent measurements such as external quantum efficiency, current-voltage, and electroluminescence spectra are utilized from 50 to 300 K. Based on these experimental results, we analyze the dominant nonradiative recombination mechanism for each LED device. We also analyze the effect of the dominant nonradiative recombination mechanism on the efficiency droop. On the basis of correlation between the efficiency droop and nonradiative recombination mechanisms, we discuss an approach to reducing the efficiency droop for each LED device.
    Applied Physics Letters 01/2014; 104(15):151108-151108-4. · 3.79 Impact Factor
  • Journal- Korean Physical Society 09/2013; 63(6):1218-1221. · 0.51 Impact Factor
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    ABSTRACT: We investigate the impacts of quantum-well (QW) shapes on the performance of InGaN/GaN light-emitting diodes (LEDs) grown on c-plane sapphire substrates. Three different QW shapes are utilized, namely, rectangular, staircase, and trapezoidal QWs of thicknesses of 4, 5, and 6 nm. Various optoelectronic measurements are conducted on these samples to determine the correlation of the effect of piezoelectric field with device performances. It is found that the staircase QW consistently shows the reduced effect of piezoelectric field, which agrees well with the simulation results of the increased electron--hole overlap integral over the conventional rectangular QW. The nonconventional QW can reduce the effect of piezoelectric field and improve the LED performance accordingly.
    Japanese Journal of Applied Physics 08/2013; 52(8). · 1.07 Impact Factor
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    ABSTRACT: Dominant nonradiative recombination mechanisms as a function of nonradiative current were investigated in InGaN blue light-emitting diodes (LEDs). Each radiative and nonradiative current components were separated from the total current by using the information of the internal quantum efficiency (IQE), obtained from the temperature-dependent electroluminescence measurement. By analyzing voltage and light output power as functions of nonradiative current, we were able to understand that the dominant nonradiative mechanisms of the LEDs vary with the competing mechanisms of Shockley--Read--Hall or tunneling recombination at low current density to the carrier overflow at high current density, inducing the IQE droop.
    Applied Physics Express 05/2013; 6(5):2105-. · 2.73 Impact Factor
  • Dong-Soo Shin, Jong-Ik Lee, Jong-In Shim
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    ABSTRACT: We demonstrate a method of systematic analysis for the photocurrent spectroscopy on InGaN/GaN light-emitting diodes (LEDs). By normalizing photocurrent data at a photon energy of 3.2 eV for blue LEDs, we show that accurate comparison of active quantum wells is possible. Bias-dependent photocurrent measurements reveal that there are fixed points in photocurrent data from which an effective bandgap energy can be determined. The method presented in this paper can be useful when one needs to compare the LEDs fabricated at different times or by different processes.
    IEEE Journal of Quantum Electronics 01/2013; 49(12):1062-1065. · 2.11 Impact Factor
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    ABSTRACT: An origin of the efficiency droop has been suggested as the saturation of the radiative recombination rate in InGaN quantum well at low current and subsequent increase in the nonradiative recombination rates at high current.
    Lasers and Electro-Optics Pacific Rim (CLEO-PR), 2013 Conference on; 01/2013
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    ABSTRACT: InGaN-based blue and green light-emitting diodes are studied by temperature-dependent electroluminescence (EL) from 300 to 50 K to elucidate the effects of carrier overflow and the saturation in radiative recombination rate on the efficiency droop. Severe efficiency droop at cryogenic temperatures is attributed to the carrier overflow, which is confirmed by the EL spectra. The degree of overflow is thought to be related to the reduced effective active volume and the subsequent saturation in radiative recombination rate. Carrier transport and indium clustering in the active region are discussed in relation to the reduced effective active volume.
    Applied Physics Letters 04/2012; 100(15). · 3.79 Impact Factor
  • Han-Youl Ryu, Dong-Soo Shin, Jong-In Shim
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    ABSTRACT: In InGaN quantum wells (QWs), effective active volume can be greatly reduced due to carrier localization in In-rich region and inhomogeneous carrier distribution. The authors investigate the efficiency droop of InGaN-based light-emitting diodes (LEDs) based on the carrier rate equation including the influence of the reduced effective active volume. It is found that efficiency droop characteristics can be modeled well without employing a large Auger recombination coefficient by assuming that only a small portion of the QWs is effectively used as active region. The presented model is expected to provide insight into the realization of droop-free operation in nitride LEDs.
    Applied Physics Letters 03/2012; 100(13). · 3.79 Impact Factor
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    ABSTRACT: At room temperature, AlGaInP pn-junction light-emitting diodes (LEDs) emitting at 630 nm do not exhibit an efficiency droop. However, upon cooling the AlGaInP LEDs to cryogenic temperatures, they show a pronounced efficiency droop. We attribute the efficiency droop in AlGaInP LEDs to electron-drift-induced reduction in injection efficiency (i.e., carrier leakage out of the active region) mediated by the asymmetry of the pn junction, specifically the disparity between electron and hole concentrations and mobilities, with the concentration disparity exacerbated at low temperatures.
    Applied Physics Letters 03/2012; 100(11). · 3.79 Impact Factor
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    ABSTRACT: Thermal circuit modeling is successfully established and utilized for analyzing the temperature distribution within each epitaxial film of InGaN/GaN multiple-quantum-well blue light-emitting diodes (LEDs). The temperature distribution based on the infrared (IR) intensity from the LED surface is also calculated by applying the known emissivity and transmittance values for each epitaxial material to the simulated results. We measure the temperature distribution by using an IR camera on the LED surface and demonstrate that the simulated and measured temperature distributions have very similar distribution tendencies.
    IEEE Transactions on Electron Devices 01/2012; 59(6):1799-1802. · 2.06 Impact Factor
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    ABSTRACT: The internal electric field due to the piezoelectric effect in an InGaN/GaN multiple quantum-well structure is measured via photoluminescence (PL), electroreflectance (ER), and photocurrent (PC) spectroscopies, and the measurement results are compared with each other and with theoretically calculated values. The flat-band voltage is estimated by measuring the applied bias voltage that induces the maximum in PL peak energy by the quantum-confined Stark effect, 180$^{\circ}$ phase shift in the ER spectrum, and the smallest band tail state in the PC spectrum. The internal electric fields estimated by the PL, PC, and ER spectra are ${-}{1.81}$, ${-}{\rm 2.12}\pm 0.14$, and ${-}{\rm 2.04}~{\rm MV}/{\rm cm}$, respectively. The measured piezoelectric fields are in good agreement with theoretically calculated values. Possible factors affecting piezoelectric field measurements are discussed from various perspectives.
    IEEE Journal of Quantum Electronics 01/2012; 48(4):500-506. · 2.11 Impact Factor
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    ABSTRACT: We have proposed an efficiency droop model which can comprehensively explain experimental IQE droop phenomena occurring at different temperatures, materials, and active structures. In our model, carriers are located and recombined both radiatively and nonradiatively inside randomly distributed In-rich areas of InGaN-based QWs and the IQE droop originates from the saturated radiative recombination rate and the monotonically increasing nonradiative recombination rate there. Due to small effective active volume and small density of states of In-rich areas, carrier density is rapidly increased even at low current density and the radiative recombination rate is easily saturated by different distributions of electrons and holes in the momentum k-space. A measurement method that can separately estimate the radiative and nonradiative carrier lifetimes just at room temperature is theoretically developed by analyzing the time-resolved photoluminescence (TRPL) response. The method is applied to a blue InGaN/GaN QW LED. The experimental results show that the radiative carrier lifetime increases and the nonradiative carrier lifetime saturates with increasing TRPL laser power, which is one of direct evidences validating our IQE droop model.
    Proc SPIE 02/2011;
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    ABSTRACT: We present our approach to measure the profile of nonuniformly bent GaN epi-wafers grown on sapphire substrates. By using a laser displacement sensor, the position of the epi-wafer is accurately measured and mapped. From the measured profile data, analysis of stress distributions over the nonuniformly bent wafer is performed by using a theoretical model. We show the result of theoretical analysis of how the stress tensors distribute over a wafer. The estimated stress tensors are related with optical properties such as photoluminescence of the wafer.
    Proc SPIE 02/2011;
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    ABSTRACT: Current crowding effect is detrimental for the performance of light-emitting diodes (LEDs), causing non-uniform light emission and local heat generation. In particular, heat generated by non-uniform current distribution can badly influence the performance of LED devices. In this paper, we examine the temperature distributions of lateral InGaN/GaN multiple-quantum-well LEDs in relation to current crowding, using both simulation and experimental results. Simulation results are obtained from a 3-dimensional electrical circuit model consisting of resistances and intrinsic diodes. Temperature and luminance distributions are investigated by images taken by an infrared camera and a charge-coupled-device camera, respectively. Finally, the internal quantum efficiency is taken for each device and compared. We show that the thermal property in the lateral LED is affected by the current crowding due to the local Joule heating nearby electrodes. Therefore, uniform current spreading is very important not only for uniform luminance distribution but also for good thermal property in the LED device. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 01/2010; 7:2133-2135.
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    ABSTRACT: A measurement method has been developed that can estimate carrier lifetimes and internal quantum efficiency (IQE) in semiconductor materials at room temperature. From the analysis of time-resolved photoluminescence (TRPL) response based on the carrier rate equation, the physical meaning of the TRPL response is clarified and expressions for carrier lifetimes and IQE are obtained. It is found that the final stage of the TRPL response is mainly governed by the non-radiative recombination carrier lifetime. The proposed analysis model is applied to the TRPL measurement results on InGaN-based quantum-well structures, and the non-radiative carrier lifetime and IQE of the measured samples are determined.
    Japanese Journal of Applied Physics 01/2010; 49. · 1.07 Impact Factor
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    ABSTRACT: We analyze the stress distribution in the nonuniformly bent GaN epilayers grown on a sapphire substrate. By using theoretical analysis combined with an analytical formula describing the realistic shape for the wafer bending of GaN epiwafers, we examine the effect of nonuniformity in the wafer bending on the stress-value variation over the entire wafer. We show that the stress on the GaN thin film can deviate by ~1 MPa from the value obtained by the simple Stoney's formula that is typically used for the uniformly bent wafer. We also show that the maximum value of the stress linearly increases with the bow difference along the horizontal and vertical directions.
    Journal of Applied Physics 01/2010; 107. · 2.21 Impact Factor
  • Dong-Hyun Jang, Jong-In Shim, Dong-Soo Shin
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    ABSTRACT: We have demonstrated both theoretically and experimentally that a lozenge-shaped light-emitting diode (LED) enhances light extraction efficiency compared with a conventional rectangular LED. The total light output power of the lozenge-shaped LED on a transmitter optical can (TO-can)-type package shows an increase of 12% at an injection current of 20 mA when compared with that of a rectangular LED. Moreover, the series resistance and the forward voltage of the lozenge-shaped LED slightly decrease compared with those of the rectangular LED. The far-field emission pattern shows that the light escaping from the lozenge-shaped LED along the horizontal direction is larger than that from the rectangular LED.
    IEEE Photonics Technology Letters 07/2009; · 2.04 Impact Factor
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    ABSTRACT: A built-in potential exists in the reflective liquid-crystal-on-silicon (LCoS) microdisplay cell due to the work-function difference between the aluminum and indium-tin-oxide electrodes. As a consequence, the flicker is generated unless the dc offset voltage is applied to compensate the built-in potential. In this paper, we present the experimental result that the dc offset voltage changes with time as the display is operated and the ions are generated in the liquid crystal (LC). To understand the experimental result, we simulate the ion motion in the LCoS cell by considering both the drift and diffusion. We discuss how the ion concentration in the LC affects the screening of the internal electric field in the bulk LC region and subsequently the built-in potential in the LCoS cell. We show that for an ion concentration higher than the value required to fully compensate the initial built-in potential, the polarity of the built-in potential is changed due to the high electric field near electrodes. By matching the experimental and simulation results, we predict how the ion concentration in the LC increases as a function of operation time.
    Japanese Journal of Applied Physics 01/2009; 48. · 1.07 Impact Factor