Browsing by Author "Lu S."
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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 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 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 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.Item Open Access On the origin of the electron blocking effect by an n-type AlGaN electron blocking layer(AIP Publishing, 2014) Zhang Z.-H.; Ji Y.; Liu W.; Tan S.T.; Kyaw, Z.; Ju, Z.; Zhang X.; Hasanov N.; Lu S.; Zhang, Y.; Zhu B.; Sun, X. W.; Demir, Hilmi VolkanIn this work, the origin of electron blocking effect of n-type Al 0.25Ga0.75N electron blocking layer (EBL) for c+ InGaN/GaN light-emitting diodes has been investigated through dual-wavelength emission method. It is found that the strong polarization induced electric field within the n-EBL reduces the thermal velocity and correspondingly the mean free path of the hot electrons. As a result, the electron capture efficiency of the multiple quantum wells is enhanced, which significantly reduces the electron overflow from the active region and increases the radiative recombination rate with holes. © 2014 AIP Publishing LLC.Item Open Access Simultaneous enhancement of electron overflow reduction and hole injection promotion by tailoring the last quantum barrier in InGaN/GaN light-emitting diodes(AIP Publishing, 2014-04-24) Kyaw, Z.; Zhang Z.-H.; Liu W.; Tan S.T.; Ju, Z. G.; Zhang, X. L.; Ji Y.; Hasanov N.; Zhu B.; Lu S.; Zhang, Y.; Teng, J. H.; Wei, S. X.; Demir, Hilmi VolkanA three-step graded undoped-InGaN layers embedded between the GaN last quantum barrier layer and the p-AlGaN electron blocking layer was proposed and its effect on the performance of InGaN/GaN light-emitting diodes was investigated both experimentally and theoretically. In the proposed structure, the electron leakage is found to be effectively reduced, while the hole injection efficiency is simultaneously increased significantly, hence enabling a greatly enhanced radiative recombination rate within the active region. As a result, improvements of 12.25% in the optical output power and 11.98% in the external quantum efficiency are obtained from the proposed device with the respect to the reference device.