Browsing by Author "Tan S.T."
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Item Open Access Advances in the LED Materials and Architectures fro Energy-Saving Solid State Lighting towards Lighting Revolution(IEEE, 2012) Tan S.T.; Sun, X. W.; Demir, Hilmi Volkan; DenBaars, S. P.In this paper, we review the recent developments (in years 2010–2011) of energysaving solid-state lighting. The industry of white light-emitting diodes (LEDs) has made significant progress, and today, white LED market is increasing (mostly with increasing LED screen and LED TV sales). The so-called Blighting revolution[ has not yet really happened on a wide scale because of the lighting efficiency at a given ownership cost. Nevertheless, the rapid development of the white LEDs is expected to soon trigger and expand the revolution.Item Open Access Advantages of the Blue InGaN/GaN Light-Emitting Diodes with an AlGaN/GaN/AlGaN Quantum Well Structured Electron Blocking Layer(American Chemical Society, 2014-03-21) Ju, Z. G.; Liu W.; Zhang Z.-H.; Tan S.T.; Ji Y.; Kyaw, Z.; Zhang, X. L.; Lu, S. P.; Zhang, Y. P.; Zhu B.; Hasanov N.; Sun, X. W.; Demir, Hilmi VolkanInGaN/GaN light-emitting diodes (LEDs) with p-(AlGaN/GaN/AlGaN) quantum well structured electron blocking layer (QWEBL) are designed and grown by a metal− organic chemical-vapor deposition (MOCVD) system. The proposed QWEBL LED structure, in which a p-GaN QW layer is inserted in the p-AlGaN electron blocking layer, not only leads to an improved hole injection but also reduces the electron leakage, thus enhancing the radiative recombination rates across the active region. Consequently, the light output power was enhanced by 10% for the QWEBL LED at a current density of 35 A/cm2. The efficiency droop of the optimized device was reduced to 16%. This is much smaller than that of the conventional p-AlGaN electron blocking layer LED, which is 31%.Item Open Access Comparative study of field-dependent carrier dynamics and emission kinetics of InGaN/GaN light-emitting diodes grown on (11 2-2) semipolar versus (0001) polar planes(AIP Publishing, 2014) Ji Y.; Liu W.; Erdem, T.; Chen R.; Tan S.T.; Zhang Z.-H.; Ju, Z.; Zhang X.; Sun, H.; Sun, X. W.; Zhao Y.; DenBaars, S. P.; Nakamura, S.; Demir, Hilmi VolkanThe characteristics of electroluminescence (EL) and photoluminescence (PL) emission from GaN light-emitting diodes (LEDs) grown on (11 (2) over bar2) semipolar plane and (0001) polar plane have been comparatively investigated. Through different bias-dependent shifting trends observed from the PL and time-resolved PL spectra (TRPL) for the two types of LEDs, the carrier dynamics within the multiple quantum wells (MQWs) region is systematically analyzed and the distinct field-dependent emission kinetics are revealed. Moreover, the polarization induced internal electric field has been deduced for each of the LEDs. The relatively stable emission behavior observed in the semipolar LED is attributed to the smaller polarization induced internal electric field. The study provides meaningful insight for the design of quantum well (QW) structures with high radiative recombination rates.Item Open Access Critical role of CdSe nanoplatelets in color-converting CdSe/ZnS nanocrystals for InGaN/GaN light-emitting diodes(OSA - The Optical Society, 2016) Hasanov N.; Sharma, V. K.; Martinez, P. L. H.; Tan S.T.; Demir, Hilmi VolkanHere we report CdSe nanoplatelets that are incorporated into color-converting CdSe/ZnS nanocrystals for InGaN/GaN light-emitting diodes. The critical role of CdSe nanoplatelets as an exciton donor for the color conversion was experimentally investigated. The power conversion efficiency of the hybrid light-emitting diode was found to increase by 23% with the incorporation of the CdSe nanoplatelets. The performance enhancement is ascribed to efficient exciton transfer from the donor CdSe nanoplatelet quantum wells to the acceptor CdSe/ZnS nanocrystal quantum dots through F�rster-type nonradiative resonance energy transfer.Item Open Access Decoupling contact and mirror: an effective way to improve the reflector for flip-chip InGaN/GaN-based light-emitting diodes(Institute of Physics Publishing, 2016) Zhu B.; Liu W.; Lu S.; Zhang, Y.; Hasanov N.; Zhang X.; Ji Y.; Zhang Z.-H.; Tan S.T.; Liu, H.; Demir, Hilmi VolkanIn the conventional fabrication process of the widely-adopted Ni/Ag/Ti/Au reflector for InGaN/GaN-based flip-chip light-emitting diodes (LEDs), the contact and the mirror are entangled together with contrary processing conditions which set constraints to the device performance severely. Here we first report the concept and its effectiveness of decoupling the contact formation and the mirror construction. The ohmic contact is first formed by depositing and annealing an extremely thin layer of Ni/Ag on top of p-GaN. The mirror construction is then carried out by depositing thick layer of Ag/Ti/Au without any annealing. Compared with the conventional fabrication method of the reflector, by which the whole stack of Ni/Ag/Ti/Au is deposited and annealed together, the optical output power is improved by more than 70% at 350 mA without compromising the electrical performance. The mechanism of decoupling the contact and the mirror is analyzed with the assistance of contactless sheet resistance measurement and secondary ion mass spectrometry (SIMS) depth profile analysis. © 2016 IOP Publishing Ltd.Item Open Access An efficient non-Lambertian organic light-emitting diode using imprinted submicron-size zinc oxide pillar arrays(AIP, 2013) Liu, S. W.; Wang, J. X.; Divayana, Y.; Dev, K.; Tan S.T.; Demir, Hilmi Volkan; Sun, X. W.We report phosphorescent organic light-emitting diodes with a substantially improved light outcoupling efficiency and a wider angular distribution through applying a layer of zinc oxide periodic nanopillar arrays by pattern replication in non-wetting templates technique. The devices exhibited the peak emission intensity at an emission angle of 40° compared to 0° for reference device using bare ITO-glass. The best device showed a peak luminance efficiency of 95.5 ± 1.5 cd/A at 0° emission (external quantum efficiency - EQE of 38.5 ± 0.1%, power efficiency of 127 ± 1 lm/W), compared to that of the reference device, which has a peak luminance efficiency of 68.0 ± 1.4 cd/A (EQE of 22.0 ± 0.1%, power efficiency of 72 ± 1 lm/W). © 2013 American Institute of Physics.Item Open Access Enhanced hole transport in InGaN/GaN multiple quantum well light-emitting diodes with a p-type doped quantum barrier(Optical Society of America, 2013) Ji Y.; Zhang, Z. -H.; Tan S.T.; Ju, Z. G.; Kyaw, Z.; Hasanov N.; Liu W.; Sun X. W.; Demir, Hilmi VolkanWe study hole transport behavior of InGaN/GaN light-emitting diodes with the dual wavelength emission method. It is found that at low injection levels, light emission is mainly from quantum wells near p-GaN, indicating that hole transport depth is limited in the active region. Emission from deeper wells only occurs under high current injection. However, with Mg-doped quantum barriers, holes penetrate deeper within the active region even under low injection, increasing the radiative recombination. Moreover, the improved hole transport leads to reduced forward voltage and enhanced light generation. This is also verified by numerical analysis of hole distribution and energy band structure. © 2013 Optical Society of America.Item Open Access Facile synthesis of luminescent AgInS2–ZnS solid solution nanorods(Wiley-VCH Verlag, 2013-04-16) Yang, X.; Tang, Y.; Tan S.T.; Bosman, M.; Dong, Z.; Leck K.S.; Ji Y.; Demir, Hilmi Volkan; Sun, X. W.Highly luminescent semiconducting AgInS2–ZnS solid solution nanorods are successfully prepared by a facile one-pot solvothermal method. The resulting solid solution nanorods with length of 32 ± 5 nm are formed by fast growth of the AgInS2-rich solid solution head, followed by slow growth of the ZnS-rich solid solution tail. Photoluminescence studies on the solid solution nanorods reveal strong photoluminescence with peak emission wavelengths tunable from 650 to 700 nm.Item Open Access Full Visible Range Covering InP/ZnS Nanocrystals with High Photometric Performance and Their Application to White Quantum Dot Light-Emitting Diodes(Wiley-VCH Verlag, 2012-04-30) Yang, X.; Zhao, D.; Leck K.S.; Tan S.T.; Tang, Y. X.; Zhao, J.; Demir, Hilmi Volkan; Sun, X. W.High-quality InP/ZnS core–shell nanocrystals with luminescence tunable over the entire visible spectrum have been achieved by a facile one-pot solvothermal method. These nanocrystals exhibit high quantum yields (above 60%), wide emission spectrum tunability and excellent photostability. The FWHM can be as narrow as 38 nm, which is close to that of CdSe nanocrystals. Also, making use of these nanocrystals, we further demonstrated a cadmium-free white QD-LED with a high color rendering index of 91. The high-performance of the resulting InP/ZnS NCs coupled with their low intrinsic toxicity may further promote industrial applications of these NC emitters.Item Open Access High brightness formamidinium lead bromide perovskite nanocrystal light emitting devices(Nature Publishing Group, 2016) Perumal, A.; Shendre, S.; Li, M.; Tay, Y. K. E.; Sharma, V. K.; Chen, S.; Wei, Z.; Liu, Q.; Gao, Y.; Buenconsejo, P. J. S.; Tan S.T.; Gan, C. L.; Xiong, Q.; Sum, T. C.; Demir, Hilmi VolkanFormamidinium lead halide (FAPbX3) has attracted greater attention and is more prominent recently in photovoltaic devices due to its broad absorption and higher thermal stability in comparison to more popular methylammonium lead halide MAPbX3. Herein, a simple and highly reproducible room temperature synthesis of device grade high quality formamidinium lead bromide CH(NH2)2 PbBr3 (FAPbBr3) colloidal nanocrystals (NC) having high photoluminescence quantum efficiency (PLQE) of 55-65% is reported. In addition, we demonstrate high brightness perovskite light emitting device (Pe-LED) with these FAPbBr3 perovskite NC thin film using 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) commonly known as TPBi and 4,6-Bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PYMPM) as electron transport layers (ETL). The Pe-LED device with B3PYMPM as ETL has bright electroluminescence of up to 2714 cd/m2, while the Pe-LED device with TPBi as ETL has higher peak luminous efficiency of 6.4 cd/A and peak luminous power efficiency of 5.7 lm/W. To our knowledge this is the first report on high brightness light emitting device based on CH(NH2)2 PbBr3 widely known as FAPbBr3 nanocrystals in literature. © The Author(s) 2016.Item Open Access High-efficiency and low-loss gallium nitride dielectric metasurfaces for nanophotonics at visible wavelengths(American Institute of Physics Inc., 2017) Emani, N. K.; Khaidarov, E.; Paniagua-Domínguez, R.; Fu, Y. H.; Valuckas, V.; Lu S.; Zhang X.; Tan S.T.; Demir, Hilmi Volkan; Kuznetsov, A. I.The dielectric nanophotonics research community is currently exploring transparent material platforms (e.g., TiO2, Si3N4, and GaP) to realize compact high efficiency optical devices at visible wavelengths. Efficient visible-light operation is key to integrating atomic quantum systems for future quantum computing. Gallium nitride (GaN), a III-V semiconductor which is highly transparent at visible wavelengths, is a promising material choice for active, nonlinear, and quantum nanophotonic applications. Here, we present the design and experimental realization of high efficiency beam deflecting and polarization beam splitting metasurfaces consisting of GaN nanostructures etched on the GaN epitaxial substrate itself. We demonstrate a polarization insensitive beam deflecting metasurface with 64% and 90% absolute and relative efficiencies. Further, a polarization beam splitter with an extinction ratio of 8.6/1 (6.2/1) and a transmission of 73% (67%) for p-polarization (s-polarization) is implemented to demonstrate the broad functionality that can be realized on this platform. The metasurfaces in our work exhibit a broadband response in the blue wavelength range of 430-470 nm. This nanophotonic platform of GaN shows the way to off- and on-chip nonlinear and quantum photonic devices working efficiently at blue emission wavelengths common to many atomic quantum emitters such as Ca+ and Sr+ ions.Item Open Access Highly Efficient Visible Colloidal Lead-Halide Perovskite Nanocrystal Light-Emitting Diodes(American Chemical Society, 2018) Yan, F.; Xing, J.; Xing, G.; Quan, L.; Tan S.T.; Zhao, J.; Su, R.; Zhang, L.; Chen, S.; Zhao Y.; Huan, A.; Sargent, E. H.; Xiong, Q.; Demir, Hilmi VolkanLead-halide perovskites have been attracting attention for potential use in solid-state lighting. Following the footsteps of solar cells, the field of perovskite light-emitting diodes (PeLEDs) has been growing rapidly. Their application prospects in lighting, however, remain still uncertain due to a variety of shortcomings in device performance including their limited levels of luminous efficiency achievable thus far. Here we show high-efficiency PeLEDs based on colloidal perovskite nanocrystals (PeNCs) synthesized at room temperature possessing dominant first-order excitonic radiation (enabling a photoluminescence quantum yield of 71% in solid film), unlike in the case of bulk perovskites with slow electron-hole bimolecular radiative recombination (a second-order process). In these PeLEDs, by reaching charge balance in the recombination zone, we find that the Auger nonradiative recombination, with its significant role in emission quenching, is effectively suppressed in low driving current density range. In consequence, these devices reach a maximum external quantum efficiency of 12.9% and a power efficiency of 30.3 lm W-1 at luminance levels above 1000 cd m-2 as required for various applications. These findings suggest that, with feasible levels of device performance, the PeNCs hold great promise for their use in LED lighting and displays.Item Open Access A hole modulator for InGaN/GaN light-emitting diodes(American Institute of Physics, 2015) Zhang, Z-H.; Kyaw, Z.; Liu W.; Ji Y.; Wang, L.; Tan S.T.; Sun, X. W.; Demir, Hilmi VolkanThe low p-type doping efficiency of the p-GaN layer has severely limited the performance of InGaN/GaN light-emitting diodes (LEDs) due to the ineffective hole injection into the InGaN/GaN multiple quantum well (MQW) active region. The essence of improving the hole injection efficiency is to increase the hole concentration in the p-GaN layer. Therefore, in this work, we have proposed a hole modulator and studied it both theoretically and experimentally. In the hole modulator, the holes in a remote p-type doped layer are depleted by the built-in electric field and stored in the p-GaN layer. By this means, the overall hole concentration in the p-GaN layer can be enhanced. Furthermore, the hole modulator is adopted in the InGaN/GaN LEDs, which reduces the effective valance band barrier height for the p-type electron blocking layer from ∼332meV to ∼294 meV at 80 A/cm2 and demonstrates an improved optical performance, thanks to the increased hole concentration in the p-GaN layer and thus the improved hole injection into the MQWs.Item Open Access Improving hole injection efficiency by manipulating the hole transport mechanism through p-type electron blocking layer engineering(Optical Society of America, 2014) Zhang, Zi-Hui; Ju, Z.; Liu W.; Tan S.T.; Ji Y.; Kyaw, Z.; Zhang X.; Hasanov N.; Sun, X. W.; Demir, Hilmi VolkanThe p-type AlGaN electron blocking layer (EBL) is widely used in InGaN/GaN light-emitting diodes (LEDs) for electron overflow suppression. However, a typical EBL also reduces the hole injection efficiency, because holes have to climb over the energy barrier generated at the p-AlGaN/p-GaN interface before entering the quantum wells. In this work, to address this problem, we report the enhancement of hole injection efficiency by manipulating the hole transport mechanism through insertion of a thin GaN layer of 1 nm into the p-AlGaN EBL and propose an AlGaN/GaN/AlGaN-type EBL outperforming conventional AlGaN EBLs. Here, the position of the inserted thin GaN layer relative to the p-GaN region is found to be the key to enhancing the hole injection efficiency. InGaN/ GaN LEDs with the proposed p-type AlGaN/GaN/AlGaN EBL have demonstrated substantially higher optical output power and external quantum efficiency.Item Open Access Influence of n-type versus p-type AlGaN electron-blocking layer on InGaN/GaN multiple quantum wells light-emitting diodes(AIP Publishing, 2013-08-01) Ji Y.; Zhang Z.-H.; Kyaw, Z.; Tan S.T.; Ju, Z. G.; Zhang, X. L.; Liu W.; Sun, X. W.; Demir, Hilmi VolkanThe effect of n-AlGaN versus p-AlGaN electron-blocking layers (EBLs) on the performance of InGaN/GaN light-emitting diodes is studied in this work. Experimental results suggest that the n-type EBL leads to higher optical output power and external quantum efficiency, compared to the devices with p-AlGaN EBL, which is commonly used today. Numerical simulations on the carrier distribution and energy band diagram reveal that the n-AlGaN EBL is more efficient in preventing electron overflow, while not blocking the hole injection into the active region, hence leading to higher radiative recombination rate within the multiple quantum wells active region. © 2013 AIP Publishing LLC.Item Open Access InGaN/GaN light-emitting diode with a polarization tunnel junction(American Institute of Physics, 2013) Zhang Z.-H.; Tan S.T.; Kyaw, Z.; Ji Y.; Liu W.; Ju, Z.; Hasanov N.; Sun, X. W.; Demir, Hilmi VolkanWe report InGaN/GaN light-emitting diodes (LED) comprising in situ integrated p(+)-GaN/InGaN/n(+)-GaN polarization tunnel junctions. Improved current spreading and carrier tunneling probability were obtained in the proposed device architecture, leading to the enhanced optical output power and external quantum efficiency. Compared to the reference InGaN/GaN LEDs using the conventional p(+)/n(+) tunnel junction, these devices having the polarization tunnel junction show a reduced forward bias, which is attributed to the polarization induced electric fields resulting from the in-plane biaxial compressive strain in the thin InGaN layer sandwiched between the p(+)-GaN and n(+)-GaN layers. (C) 2013 AIP Publishing LLC.Item Open Access InGaN/GaN multiple-quantum-well light-emitting diodes with a grading InN composition suppressing the Auger recombination(AIP Publishing, 2014) Zhang Z.-H.; Liu W.; Ju, Z.; Tan S.T.; Ji Y.; Kyaw, Z.; Zhang, X.; Wang, L.; Sun, X. W.; Demir, Hilmi VolkanIn conventional InGaN/GaN light-emitting diodes (LEDs), thin InGaN quantum wells are usually adopted to mitigate the quantum confined Stark effect (QCSE), caused due to strong polarization induced electric field, through spatially confining electrons and holes in small recombination volumes. However, this inevitably increases the carrier density in quantum wells, which in turn aggravates the Auger recombination, since the Auger recombination scales with the third power of the carrier density. As a result, the efficiency droop of the Auger recombination severely limits the LED performance. Here, we proposed and showed wide InGaN quantum wells with the InN composition linearly grading along the growth orientation in LED structures suppressing the Auger recombination and the QCSE simultaneously. Theoretically, the physical mechanisms behind the Auger recombination suppression are also revealed. The proposed LED structure has experimentally demonstrated significant improvement in optical output power and efficiency droop, proving to be an effective solution to this important problem of Auger recombination.Item Open Access Investigation of p-type depletion doping for InGaN/GaN-based light-emitting diodes(American Institute of Physics Inc., 2017) Zhang, Y.; Zhang Z.-H.; Tan S.T.; Hernandez-Martinez, P. L.; Zhu B.; Lu S.; Kang, X. J.; Sun, X. W.; Demir, Hilmi VolkanDue to the limitation of the hole injection, p-type doping is essential to improve the performance of InGaN/GaN multiple quantum well light-emitting diodes (LEDs). In this work, we propose and show a depletion-region Mg-doping method. Here we systematically analyze the effectiveness of different Mg-doping profiles ranging from the electron blocking layer to the active region. Numerical computations show that the Mg-doping decreases the valence band barrier for holes and thus enhances the hole transportation. The proposed depletion-region Mg-doping approach also increases the barrier height for electrons, which leads to a reduced electron overflow, while increasing the hole concentration in the p-GaN layer. Experimentally measured external quantum efficiency indicates that Mg-doping position is vitally important. The doping in or adjacent to the quantum well degrades the LED performance due to Mg diffusion, increasing the corresponding nonradiative recombination, which is well supported by the measured carrier lifetimes. The experimental results are well numerically reproduced by modifying the nonradiative recombination lifetimes, which further validate the effectiveness of our approach.Item Open Access Light extraction efficiency enhancement of colloidal quantum dot light-emitting diodes using large-scale nanopillar arrays(Wiley-VCH Verlag, 2014) Yang, X.; Dev, K.; Wang, J.; Mutlugun, E.; Dang, C.; Zhao Y.; Liu, S.; Tang, Y.; Tan S.T.; Sun, X. W.; Demir, Hilmi VolkanA colloidal quantum dot light-emitting diode (QLED) is reported with substantially enhanced light extraction efficiency by applying a layer of large-scale, low-cost, periodic nanopillar arrays. Zinc oxide nanopillars are grown on the glass surface of the substrate using a simple, efficient method of non-wetting templates. With the layer of ZnO nanopillar array as an optical outcoupling medium, a record high current efficiency (CE) of 26.6 cd/A is achieved for QLEDs. Consequently, the corresponding external quantum efficiency (EQE) of 9.34% reaches the highest EQE value for green-emitting QLEDs. Also, the underlying physical mechanisms enabling the enhanced light-extraction are investigated, which leads to an excellent agreement of the numerical results based on the mode theory with the experimental measurements. This study is the first account for QLEDs offering detailed insight into the light extraction efficiency enhancement of QLED devices. The method demonstrated here is intended to be useful not only for opening up a ubiquitous strategy for designing high-performance QLEDs but also with respect to fundamental research on the light extraction in QLEDs.Item Open Access Modulating ohmic contact through InGaxNyOz interfacial layer for high-performance InGaN/GaN-based light-emitting diodes(Institute of Electrical and Electronics Engineers Inc., 2016) Zhu B.; Tan S.T.; Liu W.; Lu S.; Zhang, Y.; Chen, S.; Hasanov N.; Kang, X.; Demir, Hilmi VolkanWe report the improved performance of InGaN/GaN-based light-emitting diodes (LEDs) through the design and the formation of the InGaxNyOz interfacial layer, which maintains high reflectivity of silver and forms good ohmic contact between pristine silver and p-GaN. The interfacial layer was designed and formed by depositing a thin layer of indium tin oxide (ITO) on top of p-GaN, followed by thermal annealing, to enable the interdiffusion and the intermixing of In, Sn, Ga, O, and N atoms. Both electrical and optical performances of the LED with the optimized InGaxNyOz interfacial layer are improved, thus achieving the highest wall-plug efficiency, compared with those LEDs with and without ITO layers at operation current.