Browsing by Author "Ji Y."
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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 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 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 Fluorophore-doped core-multishell spherical plasmonic nanocavities: resonant energy transfer towards a loss compensation(American Chemical Society, 2012-06-12) Peng, B.; Zhang, Q.; Liu, X.; Ji Y.; Demir, Hilmi Volkan; Huan, C. H. A.; Sum, T. C.; Wiong, Q.Plasmonics exhibits the potential to break the diffraction limit and bridge the gap between electronics and photonics by routing and manipulating light at the nanoscale. However, the inherent and strong energy dissipation present in metals, especially in the near-infrared and visible wavelength ranges, significantly hampersthe applications in nanophotonics. Therefore, it is amajor challengetomitigatethe losses. One way to compensate the losses is to incorporate gain media into plasmonics. Here, we experimentally show that the incorporation of gain material into a local surface plasmonic system (Au/silica/silica dye core multishell nanoparticles) leads to a resonant energy transfer from the gain media to the plasmon. The optimized conditions for the largest loss compensation are reported. Both the coupling distance and the spectral overlap arethe key factorsto determinetheresulting energy transfer. Theinterplay of these factors leadsto a non-monotonous photoluminescence dependence as a function of the silica spacer shell thickness. Nonradiativetransferrate is increased by morethan 3 orders of magnitude attheresonant condition, which is key evidence of the strongest coupling occurring between the plasmon and the gain material.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 On the effect of step-doped quantum barriers in InGaN/GaN light emitting diodes(IEEE, 2013) Zhang Z.-H.; Tan S.T.; Ju, Z.; Liu W.; Ji Y.; Kyaw, Z.; Dikme, Y.; Sun, X. W.; Demir, Hilmi VolkanInGaN/GaN light-emitting diodes (LEDs) make an important class of optoelectronic devices, increasingly used in lighting and displays. Conventional InGaN/GaN LEDs of c-orientation exhibit strong internal polarization fields and suffer from significantly reduced radiative recombination rates. A reduced polarization within the device can improve the optical matrix element, thereby enhancing the optical output power and efficiency. Here, we have demonstrated computationally that the step-doping in the quantum barriers is effective in reducing the polarization-induced fields and lowering the energy barrier for hole transport. Also, we have proven experimentally that such InGaN/GaN LEDs with Si step-doped quantum barriers indeed outperform LEDs with wholly Si-doped barriers and those without doped barriers in terms of output power and external quantum efficiency. The consistency of our numerical simulation and experimental results indicate the effects of Si step-doping in suppressing quantum-confined stark effect and enhancing the hole injection, and is promising in improving the InGaN/GaN LED performance.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 On the origin of the redshift in the emission wavelength of InGaN/GaN blue light emitting diodes grown with a higher temperature interlayer(AIP Publishing, 2012-03-20) Ju, Z. G.; Tan S.T.; Zhang Z.-H.; Ji Y.; Kyaw, Z. B.; Dikme, Y.; Sun, X. W.; Demir, Hilmi VolkanA redshift of the peak emission wavelength was observed in the blue light emitting diodes of InGaN/GaN grown with a higher temperature interlayer that was sandwiched between the low-temperature buffer layer and high-temperature unintentionally doped GaN layer. The effect of interlayer growth temperature on the emission wavelength was probed and studied by optical, structural, and electrical properties. Numerical studies on the effect of indium composition and quantum confinement Stark effect were also carried out to verify the experimental data. The results suggest that the redshift of the peak emission wavelength is originated from the enhanced indium incorporation, which results from the reduced strain during the growth of quantum wells.Item Open Access Polarization self-screening in [0001] oriented InGaN/GaN light-emitting diodes for improving the electron injection efficiency(AIP Publishing, 2014) Zhang Z.-H.; Liu W.; Ju, Z.; Tan S.T.; Ji Y.; Zhang X.; Wang, L.; Kyaw, Z.; Sun, X. W.; Demir, Hilmi VolkanInGaN/GaN light-emitting diodes (LEDs) grown along the [0001] orientation inherit very strong polarization induced electric fields. This results in a reduced effective conduction band barrier height for the p-type AlGaN electron blocking layer (EBL) and makes the electron blocking effect relatively ineffective and the electron injection efficiency drops. Here, we show the concept of polarization self-screening for improving the electron injection efficiency. In this work, the proposed polarization self-screening effect was studied and proven through growing a p-type EBL with AlN composition partially graded along the [0001] orientation, which induces the bulk polarization charges. These bulk polarization charges are utilized to effectively self-screen the positive polarization induced interface charges located at the interface between the EBL and the last quantum barrier when designed properly. Using this approach, the electron leakage is suppressed and the LED performance is enhanced significantly.Item Open Access Self-screening of the quantum confined Stark effect by the polarization induced bulk charges in the quantum barriers(AIP Publishing, 2014) Zhang Z.-H.; Li, W.; Ju, Z.; Tan S.T.; Ji Y.; Kyaw, Z.; Zhang X.; Wang, L.; Sun, X. W.; Demir, Hilmi VolkanInGaN/GaN light-emitting diodes (LEDs) grown along the polar orientations significantly suffer from the quantum confined Stark effect (QCSE) caused by the strong polarization induced electric field in the quantum wells, which is a fundamental problem intrinsic to the III-nitrides. Here, we show that the QCSE is self-screened by the polarization induced bulk charges enabled by designing quantum barriers. The InN composition of the InGaN quantum barrier graded along the growth orientation opportunely generates the polarization induced bulk charges in the quantum barrier, which well compensate the polarization induced interface charges, thus avoiding the electric field in the quantum wells. Consequently, the optical output power and the external quantum efficiency are substantially improved for the LEDs. The ability to self-screen the QCSE using polarization induced bulk charges opens up new possibilities for device engineering of III-nitrides not only in LEDs but also in other optoelectronic devices.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.