Progress with GaN-based light emitting diodes （LEDs） that incorporate nanostructures is reviewed, especially the re- cent achievements in our research group. Nano-patterned sapphire substrates have been used to grow...Progress with GaN-based light emitting diodes （LEDs） that incorporate nanostructures is reviewed, especially the re- cent achievements in our research group. Nano-patterned sapphire substrates have been used to grow an A1N template layer for deep-ultraviolet （DUV） LEDs. One efficient surface nano-texturing technology, hemisphere-cones-hybrid nanostruc- tures, was employed to enhance the extraction efficiency of InGaN flip-chip LEDs. Hexagonal nanopyramid GaN-based LEDs have been fabricated and show electrically driven color modification and phosphor-free white light emission because of the linearly increased quantum well width and indium incorporation from the shell to the core. Based on the nanostruc- tures, we have also fabricated surface plasmon-enhanced nanoporous GaN-based green LEDs using AAO membrane as a mask. Benefitting from the strong lateral SP coupling as well as good electrical protection by a passivation layer, the EL intensity of an SP-enhanced nanoporous LED was significantly enhanced by 380%. Furthermore, nanostructures have been used for the growth of GaN LEDs on amorphous substrates, the fabrication of stretchable LEDs, and for increasing the 3-dB modulation bandwidth for visible light communication.展开更多
Current diffusion is an old issue, nevertheless, the relationship between the current diffusion and the efficiency of light emitting diodes(LEDs) needs to be further quantitatively clarified. By incorporating current ...Current diffusion is an old issue, nevertheless, the relationship between the current diffusion and the efficiency of light emitting diodes(LEDs) needs to be further quantitatively clarified. By incorporating current crowding effect(CCE) into the conventional ABC model, we have theoretically and directly correlated the current diffusion and the internal quantum efficiency(IQE), light extraction efficiency(LEE), and external quantum efficiency(EQE) droop of the lateral LEDs.However, questions still exist for the vertical LEDs(V-LEDs). Here firstly the current diffusion length L_s(I) and L_s(II) have been clarified. Based on this, the influence of CCE on the EQE, IQE, and LEE of V-LEDs were investigated. Specifically to our V-LEDs with moderate series resistivity, L_s(III) was developed by combining L_s(I) and L_s(II), and the CCE effect on the performance of V-LEDs was investigated. The wall-plug efficiency(WPE) of V-LEDs ware investigated finally. Our works provide a deep understanding of the current diffusion status and the correlated efficiency droop in V-LEDs, thus would benefit the V-LEDs' chip design and further efficiency improvement.展开更多
p-GaN surfaces axe nano-roughened by plasma etching to improve the optical performance of GaN-based light emitting diodes （LEDs）. The nano-roughened GaN present a relaxation of stress. The light extraction of the LE...p-GaN surfaces axe nano-roughened by plasma etching to improve the optical performance of GaN-based light emitting diodes （LEDs）. The nano-roughened GaN present a relaxation of stress. The light extraction of the LEDs with nano-roughened surfaces is greatly improved when compared with that of the conventional LEDs without nano-roughening. PL-mapping intensities of the nano-roughened LED epi-wafers for different roughening times present two to ten orders of enhancement. The light output powers are also higher for the nano-roughened LED devices, This improvement is attributed to that nano-roughened surfaces can provide photons multiple chances to escape from the LED surfaces,展开更多
Graphene on gallium nitride （GaN） will be quite useful when the graphene is used as transparent electrodes to improve the performance of gallium nitride devices. In this work, we report the direct synthesis of graph...Graphene on gallium nitride （GaN） will be quite useful when the graphene is used as transparent electrodes to improve the performance of gallium nitride devices. In this work, we report the direct synthesis of graphene on GaN without an extra catalyst by chemical vapor deposition. Raman spectra indicate that the graphene films are uniform and about 5-6 layers in thickness. Meanwhile, the effects of growth temperatures on the growth of graphene films are systematically studied, of which 950 ℃ is found to be the optimum growth temperature. The sheet resistance of the grown graphene is 41.1 Ω/square, which is close to the lowest sheet resistance of transferred graphene reported. The mechanism of graphene growth on GaN is proposed and discussed in detail. XRD spectra and photoluminescence spectra indicate that the quality of GaN epi-layers will not be affected after the growth of graphene.展开更多
Metallic nanotextured reflectors have been widely used in light emitting diodes(LEDs) to enhance the light extraction efficiency. However, the light absorption loss for the metallic reflectors with nanotexture structu...Metallic nanotextured reflectors have been widely used in light emitting diodes(LEDs) to enhance the light extraction efficiency. However, the light absorption loss for the metallic reflectors with nanotexture structure is often neglected. Here, the influence of absorption loss of metallic nanotextured reflectors on the LED optoelectronic properties were studied. Two commonly used metal reflectors Ag and Al were applied to green GaN-based LEDs. By applying a Ag nanotextured reflector, the light output power of the LEDs was enhanced by 78% due to the improved light extraction. For an Al nanotextured reflector, however,only a 6% enhancement of the light output power was achieved. By analyzing the metal absorption using finite-difference timedomain(FDTD) and the metal reflectivity spectrum, it is shown that the surface plasmon(SP) intrinsic absorption of metallic reflectors with nanotexture structure play an important role. This finding will aid the design of the high-performance metal nanotextured reflectors and optoelectronics devices.展开更多
Wafer-scale SiO2 photonic crystal （PhC） patterns （SiO2 air-hole PhC, SiO2-pillar PhC） on indium tin oxide （ITO） layer of GaN-based light-emitting diode （LED） are fabricated via novel nanospherical-lens lithogr...Wafer-scale SiO2 photonic crystal （PhC） patterns （SiO2 air-hole PhC, SiO2-pillar PhC） on indium tin oxide （ITO） layer of GaN-based light-emitting diode （LED） are fabricated via novel nanospherical-lens lithography. Nanoscale polystyrene spheres are self-assembled into a hexagonal closed-packed monolayer array acting as convex lens for expo- sure using conventional lithography instrument. The light output power is enhanced by as great as 40.5% and 61% over those of as-grown LEDs, for SiO2-hole PhC and SiO2-pillar PhC LEDs, respectively. No degradation to LED electrical properties is found due to the fact that SiO2 PhC structures are fabricated on ITO current spreading electrode. For SiO2- pillar PhC LEDs, which have the largest light output power in all LEDs, no dry etching, which would introduce etching damage, was involved. Our method is demonstrated to be a simple, low cost, and high-yield technique for fabricating the PhC LEDs. Furthermore, the finite difference time domain simulation is also performed to further reveal the emission characteristics of LEDs with PhC structures.展开更多
A low-temperature GaN(LT-GaN)nucleation layer is grown on a patterned sapphire substrate(PSS)using metal-organic chemical vapor deposition(MOCVD).The surface morphology of the LT-GaN is investigated and the selective ...A low-temperature GaN(LT-GaN)nucleation layer is grown on a patterned sapphire substrate(PSS)using metal-organic chemical vapor deposition(MOCVD).The surface morphology of the LT-GaN is investigated and the selective nucleation phenomenon in the growth process of the LT-GaN nucleation layer is discovered.Meanwhile,effects of thickness of the LT-GaN and the annealing process on the phenomenon are also discussed.A pattern model is also proposed to analyze the possible mechanisms in atomic scale.展开更多
We report a new method for the fabrication of two-dimensional photonic crystal （PhC） hole arrays to improve the light extraction of GaN-based light-emitting diodes （LEDs）. The PhC structures were realized using na...We report a new method for the fabrication of two-dimensional photonic crystal （PhC） hole arrays to improve the light extraction of GaN-based light-emitting diodes （LEDs）. The PhC structures were realized using nanospherical-lens photolithography and the selective-area epitaxy method, which ensured the electrical properties of the LEDs through leaving the p-GaN damage-free. At a current of 350 mA, the light output power of LEDs with PhC hole arrays of 450 nm and 600 nm in diameter with the same lattice period of 900 nm were enhanced by 49.3% and 72.2%, respectively, compared to LEDs without a PhC. Furthermore, the LEDs with PhC hole structures showed an obviously smaller divergent angle compared with conventional LEDs, which is consistent with the results of finite-difference time-domain simulation.展开更多
GaN-based vertical light-emitting-diodes （V-LEDs） with an improved current injection pattern were fabricated and a novel current injection pattern of LEDs which consists of electrode-insulator-semiconductor （E1S） ...GaN-based vertical light-emitting-diodes （V-LEDs） with an improved current injection pattern were fabricated and a novel current injection pattern of LEDs which consists of electrode-insulator-semiconductor （E1S） structure was proposed. The EIS structure was achieved by an insulator layer （20-nm Ta2O5） deposited between the p-GaN and the ITO layer. This kind of EIS structure works through a defect-assisted tunneling mechanism to realize current injection and obtains a uniform current distribution on the chip surface, thus greatly improving the current spreading ability of LEDs. The appearance of this novel current injection pattern of V-LEDs will subvert the impression of the conventional LEDs structure, including simplifying the chip manufacture technology and reducing the chip cost. Under a current density of 2, 5, 10, and 25 A/cm2, the luminous uniformity was better than conventional structure LEDs. The standard deviation of power density distribution in light distribution was 0.028, which was much smaller than that of conventional structure LEDs and illustrated a huge advantage on the current spreading ability of EIS-LEDs.展开更多
To achieve high quality lighting and visible light communication(VLC)simultaneously,Ga N based white light emitting diodes(WLEDs)oriented for lighting in VLC has attracted great interest.However,the overall bandwidth ...To achieve high quality lighting and visible light communication(VLC)simultaneously,Ga N based white light emitting diodes(WLEDs)oriented for lighting in VLC has attracted great interest.However,the overall bandwidth of conventional phosphor converted WLEDs is limited by the long lifetime of phosphor,the slow Stokes transfer process,the resistance-capacitance(RC)time delay,and the quantum-confined Stark effect(QCSE).Here by adopting a self-assembled In Ga N quantum dots(QDs)structure,we have fabricated phosphor-free single chip WLEDs with tunable correlated color temperature(CCT,from 1600 K to 6000 K),a broadband spectrum,a moderate color rendering index(CRI)of 75,and a significantly improved modulation bandwidth(maximum of150 MHz)at a low current density of 72 A∕cm^2.The broadband spectrum and high modulation bandwidth are ascribed to the capture of carriers by different localized states of In Ga N QDs with alleviative QCSE as compared to the traditional In Ga N/Ga N quantum well(QW)structures.We believe the approach reported in this work will find its potential application in Ga N WLEDs and advance the development of semiconductor lighting-communication integration.展开更多
Commercial white LEDs (WLEDs) are generally limited in modulation bandwidth due to a slow Stokes process,long lifetime of phosphors,and the quantum-confined Stark effect. Here we report what we believe is a novel plas...Commercial white LEDs (WLEDs) are generally limited in modulation bandwidth due to a slow Stokes process,long lifetime of phosphors,and the quantum-confined Stark effect. Here we report what we believe is a novel plasmonic WLED by infiltrating a nanohole LED (H-LED) with quantum dots (QDs) and Ag nanoparticles(NPs) together (M-LED). This decreased distance between quantum wells and QDs would open an extra non-radiative energy transfer channel and thus enhance Stokes transfer efficiency. The presence of Ag NPs enhances the spontaneous emission rate significantly. Compared to an H-LED filled with QDs (QD-LED),the optimized M-LED demonstrates a maximum color rendering index of 91.2,a 43% increase in optical power at 60 m A,and a lowered correlated color temperature. Simultaneously,the M-LED exhibits a data rate of 2.21 Gb/s at low current density of 96 A∕cm2(60 m A),which is 77% higher than that of a QD-LED. This is mainly due to the higher optical power and modulation bandwidth of the M-LED under the influence of plasmon,resulting in a higher data rate and higher signal-to-noise ratio under the forward error correction.We believe the approach reported in this work should contribute to a WLED light source with increased modulation bandwidth for a higher speed visible light communication application.展开更多
A versatile nanosphere composite lithography（NSCL） combining both the advantages of multiple-exposure nanosphere lens lithography（MENSLL） and nanosphere template lithography（NSTL） is demonstrated. By well contro...A versatile nanosphere composite lithography（NSCL） combining both the advantages of multiple-exposure nanosphere lens lithography（MENSLL） and nanosphere template lithography（NSTL） is demonstrated. By well controlling the development, washing and the drying processes, the nanosphere monolayer can be well retained on the substrate after developing and washing. Thus the NSTL can be performed based on MENSLL to fabricate nanoring, nanocrescent and hierarchical multiple structures. The pattern size and the shape can be systemically tuned by shrinking nanospheres by using dry etching and adjusting the tilted angle. It is a natural nanopattern alignment process and possesses a great potential in the scope of nano-science due to its low cost,simplicity, and versatility for variuos nano-fabrications.展开更多
Low resistance and thermally stable n-type contacts to N-polar GaN are essentially important for vertical light emitting diodes （VLEDs）. The electrical characteristics of VLEDs with n-type contacts on a roughened an...Low resistance and thermally stable n-type contacts to N-polar GaN are essentially important for vertical light emitting diodes （VLEDs）. The electrical characteristics of VLEDs with n-type contacts on a roughened and flat N-polar surface have been compared. VLEDs with contacts deposited on a roughened surface exhibit lower leakage currents yet a higher operating voltage. Based on this, a new scheme by depositing metallization contacts on a selectively wet-etching roughened surface has been developed. Excellent electrical and optical characteristics have been achieved with this method. An aging test further confirmed its stability.展开更多
Rapid improvement in the efficiency of GaN-based LEDs not only speed up its applications for general illumination, but offer the possibilities for data transmission. This review is to provide an overview of current pr...Rapid improvement in the efficiency of GaN-based LEDs not only speed up its applications for general illumination, but offer the possibilities for data transmission. This review is to provide an overview of current progresses of GaN-based LEDs for light communications. The modulation bandwidth of GaN-based LEDs has been first improved by optimizing the LED epilayer structures and the modulation bandwidth of 73 MHz was achieved at the driving current density of 40 A/cm2 by changing the multi-quantum well structures. After that, in order to increase the current density tolerance, different parallel flip-chip micro-LED arrays were fabricated. With a high injected current density of ~7900 A/cm2, a maximum modulation bandwidth of ~227 MHz was obtained with optical power greater than 30 mW. Besides the increase of carrier concentrations, the radiative recombination coefficient B was also enhanced by modifying the photon surrounding environment based on some novel nanostructures such as resonant cavity, surface plasmon, and photonic crystals. The optical 3 dB modulation bandwidth of GaN-based nanostructure LEDs with Ag nanoparticles was enhanced by 2 times compared with GaN-based nanostructure LEDs without Ag nanoparticles.Our results demonstrate that using the QW-SP coupling can effectively help to enhance the carrier spontaneous emission rate and also increase the modulation bandwidth for LEDs, especially for LEDs with high intrinsic IQE. In addition, we discuss the progress of the faster color conversion stimulated by GaN-based LEDs.展开更多
High-quality AlN layers with low-density threading dislocations are indispensable for high-efficiency deep ultraviolet light-emitting diodes（UV-LEDs）. In this work, a high-temperature AlN epitaxial layer was grown o...High-quality AlN layers with low-density threading dislocations are indispensable for high-efficiency deep ultraviolet light-emitting diodes（UV-LEDs）. In this work, a high-temperature AlN epitaxial layer was grown on sputtered AlN layer（used as nucleation layer, SNL） by a high-yield industrial metalorganic vapor phase epitaxy（MOVPE）. The full width half maximum（FWHM） of the rocking curve shows that the AlN epitaxial layer with SNL has good crystal quality. Furthermore, the relationships between the thickness of SNL and the FWHM values of（002） and（102） peaks were also studied. Finally, utilizing an SNL to enhance the quality of the epitaxial layer, deep UV-LEDs at 282 nm were successfully realized on sapphire substrate by the high-yield industrial MOVPE. The light-output power（LOP） of a deep UV-LED reaches 1.65 mW at 20 mA with external quantum efficiency of 1.87%. In addition, the saturation LOP of the deep UV-LED is 4.31 mW at an injection current of 60 mA. Hence, our studies supply a possible process to grow commercial deep UV-LEDs in high throughput industrial MOVPE, which can increase yield, at lower cost.展开更多
The Fe-doped CuO nanopowder was synthesized by following the standard solid-state reaction method. The structure and magnetic properties of the Fe-doped CuO nanopowder were investigated. X-ray diffraction spec- tra co...The Fe-doped CuO nanopowder was synthesized by following the standard solid-state reaction method. The structure and magnetic properties of the Fe-doped CuO nanopowder were investigated. X-ray diffraction spec- tra confirmed the monoclinic structure of CuO and no secondary phase was detected, indicating that the Fe ions were incorporated into CuO. The ferromagnetism in Fe-doped CuO was studied and is believed to originate from the interaction between Fe ions and Cu ions via a super-exchange interaction or F-center mediated exchange interaction.展开更多
基金Project supported by the National Natural Science Foundation of China(Grant No.61334009)the National High Technology Research and Development Program of China(Grant Nos.2015AA03A101 and 2014BAK02B08)+1 种基金China International Science and Technology Cooperation Program(Grant No.2014DFG62280)the"Import Outstanding Technical Talent Plan"and"Youth Innovation Promotion Association Program"of the Chinese Academy of Sciences
文摘Progress with GaN-based light emitting diodes （LEDs） that incorporate nanostructures is reviewed, especially the re- cent achievements in our research group. Nano-patterned sapphire substrates have been used to grow an A1N template layer for deep-ultraviolet （DUV） LEDs. One efficient surface nano-texturing technology, hemisphere-cones-hybrid nanostruc- tures, was employed to enhance the extraction efficiency of InGaN flip-chip LEDs. Hexagonal nanopyramid GaN-based LEDs have been fabricated and show electrically driven color modification and phosphor-free white light emission because of the linearly increased quantum well width and indium incorporation from the shell to the core. Based on the nanostruc- tures, we have also fabricated surface plasmon-enhanced nanoporous GaN-based green LEDs using AAO membrane as a mask. Benefitting from the strong lateral SP coupling as well as good electrical protection by a passivation layer, the EL intensity of an SP-enhanced nanoporous LED was significantly enhanced by 380%. Furthermore, nanostructures have been used for the growth of GaN LEDs on amorphous substrates, the fabrication of stretchable LEDs, and for increasing the 3-dB modulation bandwidth for visible light communication.
基金Project supported by the National Key Research and Development Program of China(Grant No.2018YFB0406702)the Professorship Start-up Funding(Grant No.217056)+2 种基金the Innovation-Driven Project of Central South University,China(Grant No.2018CX001)the Project of State Key Laboratory of High-Performance Complex Manufacturing,Central South University,China(Grant No.ZZYJKT2018-01)Guangzhou Science&Technology Project of Guangdong Province,China(Grant Nos.201704030106 and 2016201604030035)
文摘Current diffusion is an old issue, nevertheless, the relationship between the current diffusion and the efficiency of light emitting diodes(LEDs) needs to be further quantitatively clarified. By incorporating current crowding effect(CCE) into the conventional ABC model, we have theoretically and directly correlated the current diffusion and the internal quantum efficiency(IQE), light extraction efficiency(LEE), and external quantum efficiency(EQE) droop of the lateral LEDs.However, questions still exist for the vertical LEDs(V-LEDs). Here firstly the current diffusion length L_s(I) and L_s(II) have been clarified. Based on this, the influence of CCE on the EQE, IQE, and LEE of V-LEDs were investigated. Specifically to our V-LEDs with moderate series resistivity, L_s(III) was developed by combining L_s(I) and L_s(II), and the CCE effect on the performance of V-LEDs was investigated. The wall-plug efficiency(WPE) of V-LEDs ware investigated finally. Our works provide a deep understanding of the current diffusion status and the correlated efficiency droop in V-LEDs, thus would benefit the V-LEDs' chip design and further efficiency improvement.
文摘p-GaN surfaces axe nano-roughened by plasma etching to improve the optical performance of GaN-based light emitting diodes （LEDs）. The nano-roughened GaN present a relaxation of stress. The light extraction of the LEDs with nano-roughened surfaces is greatly improved when compared with that of the conventional LEDs without nano-roughening. PL-mapping intensities of the nano-roughened LED epi-wafers for different roughening times present two to ten orders of enhancement. The light output powers are also higher for the nano-roughened LED devices, This improvement is attributed to that nano-roughened surfaces can provide photons multiple chances to escape from the LED surfaces,
基金supported by the National Natural Science Foundation of China(Grant Nos.61274040 and 51102226)the National Basic Research Program of China(Grant No.2011CB301904)+2 种基金the National High Technology Program of China(Grant Nos.2011AA03A103 and 2011AA03A105)the National Science Foundation of China(Grant Nos.10774032 and 90921001)the Key Knowledge Innovation Project of the Chinese Academy of Sciences on Water Science Research,Instrument Developing Project of the Chinese Academy of Sciences(Grant No.Y2010031)
文摘Graphene on gallium nitride （GaN） will be quite useful when the graphene is used as transparent electrodes to improve the performance of gallium nitride devices. In this work, we report the direct synthesis of graphene on GaN without an extra catalyst by chemical vapor deposition. Raman spectra indicate that the graphene films are uniform and about 5-6 layers in thickness. Meanwhile, the effects of growth temperatures on the growth of graphene films are systematically studied, of which 950 ℃ is found to be the optimum growth temperature. The sheet resistance of the grown graphene is 41.1 Ω/square, which is close to the lowest sheet resistance of transferred graphene reported. The mechanism of graphene growth on GaN is proposed and discussed in detail. XRD spectra and photoluminescence spectra indicate that the quality of GaN epi-layers will not be affected after the growth of graphene.
基金supported by the National Key Research and Development Program of China (No. 2017YFB0402900)the National Natural Science Foundation of China (No. 61504132, 61505197)
文摘Metallic nanotextured reflectors have been widely used in light emitting diodes(LEDs) to enhance the light extraction efficiency. However, the light absorption loss for the metallic reflectors with nanotexture structure is often neglected. Here, the influence of absorption loss of metallic nanotextured reflectors on the LED optoelectronic properties were studied. Two commonly used metal reflectors Ag and Al were applied to green GaN-based LEDs. By applying a Ag nanotextured reflector, the light output power of the LEDs was enhanced by 78% due to the improved light extraction. For an Al nanotextured reflector, however,only a 6% enhancement of the light output power was achieved. By analyzing the metal absorption using finite-difference timedomain(FDTD) and the metal reflectivity spectrum, it is shown that the surface plasmon(SP) intrinsic absorption of metallic reflectors with nanotexture structure play an important role. This finding will aid the design of the high-performance metal nanotextured reflectors and optoelectronics devices.
基金Project supported by the National Basic Research Program of China (Grant No.2011CB301902)
文摘Wafer-scale SiO2 photonic crystal （PhC） patterns （SiO2 air-hole PhC, SiO2-pillar PhC） on indium tin oxide （ITO） layer of GaN-based light-emitting diode （LED） are fabricated via novel nanospherical-lens lithography. Nanoscale polystyrene spheres are self-assembled into a hexagonal closed-packed monolayer array acting as convex lens for expo- sure using conventional lithography instrument. The light output power is enhanced by as great as 40.5% and 61% over those of as-grown LEDs, for SiO2-hole PhC and SiO2-pillar PhC LEDs, respectively. No degradation to LED electrical properties is found due to the fact that SiO2 PhC structures are fabricated on ITO current spreading electrode. For SiO2- pillar PhC LEDs, which have the largest light output power in all LEDs, no dry etching, which would introduce etching damage, was involved. Our method is demonstrated to be a simple, low cost, and high-yield technique for fabricating the PhC LEDs. Furthermore, the finite difference time domain simulation is also performed to further reveal the emission characteristics of LEDs with PhC structures.
文摘A low-temperature GaN(LT-GaN)nucleation layer is grown on a patterned sapphire substrate(PSS)using metal-organic chemical vapor deposition(MOCVD).The surface morphology of the LT-GaN is investigated and the selective nucleation phenomenon in the growth process of the LT-GaN nucleation layer is discovered.Meanwhile,effects of thickness of the LT-GaN and the annealing process on the phenomenon are also discussed.A pattern model is also proposed to analyze the possible mechanisms in atomic scale.
基金supported by the National Natural Science Foundation of China(No.61274040)the National Basic Research Program of China(No.2011CB301902)the National High Technology R&D Program of China(Nos.2011AA03A105,2011AA03A103)
文摘We report a new method for the fabrication of two-dimensional photonic crystal （PhC） hole arrays to improve the light extraction of GaN-based light-emitting diodes （LEDs）. The PhC structures were realized using nanospherical-lens photolithography and the selective-area epitaxy method, which ensured the electrical properties of the LEDs through leaving the p-GaN damage-free. At a current of 350 mA, the light output power of LEDs with PhC hole arrays of 450 nm and 600 nm in diameter with the same lattice period of 900 nm were enhanced by 49.3% and 72.2%, respectively, compared to LEDs without a PhC. Furthermore, the LEDs with PhC hole structures showed an obviously smaller divergent angle compared with conventional LEDs, which is consistent with the results of finite-difference time-domain simulation.
基金supported by the Natural Science Foundation of China(Nos.61306051,61306050)the National High Technology Program of China(No.2014AA032606)
文摘GaN-based vertical light-emitting-diodes （V-LEDs） with an improved current injection pattern were fabricated and a novel current injection pattern of LEDs which consists of electrode-insulator-semiconductor （E1S） structure was proposed. The EIS structure was achieved by an insulator layer （20-nm Ta2O5） deposited between the p-GaN and the ITO layer. This kind of EIS structure works through a defect-assisted tunneling mechanism to realize current injection and obtains a uniform current distribution on the chip surface, thus greatly improving the current spreading ability of LEDs. The appearance of this novel current injection pattern of V-LEDs will subvert the impression of the conventional LEDs structure, including simplifying the chip manufacture technology and reducing the chip cost. Under a current density of 2, 5, 10, and 25 A/cm2, the luminous uniformity was better than conventional structure LEDs. The standard deviation of power density distribution in light distribution was 0.028, which was much smaller than that of conventional structure LEDs and illustrated a huge advantage on the current spreading ability of EIS-LEDs.
基金National Key Research and Development Program of China(2018YFB0406702)Professorship Startup Funding(217056)+2 种基金Innovation-Driven Project of Central South University,China(2018CX001)Project of State Key Laboratory of High-Performance Complex Manufacturing,Central South University,China(ZZYJKT2018-01)Fundamental Research Funds for the Central Universities of Central South University(2018zzts147)。
文摘To achieve high quality lighting and visible light communication(VLC)simultaneously,Ga N based white light emitting diodes(WLEDs)oriented for lighting in VLC has attracted great interest.However,the overall bandwidth of conventional phosphor converted WLEDs is limited by the long lifetime of phosphor,the slow Stokes transfer process,the resistance-capacitance(RC)time delay,and the quantum-confined Stark effect(QCSE).Here by adopting a self-assembled In Ga N quantum dots(QDs)structure,we have fabricated phosphor-free single chip WLEDs with tunable correlated color temperature(CCT,from 1600 K to 6000 K),a broadband spectrum,a moderate color rendering index(CRI)of 75,and a significantly improved modulation bandwidth(maximum of150 MHz)at a low current density of 72 A∕cm^2.The broadband spectrum and high modulation bandwidth are ascribed to the capture of carriers by different localized states of In Ga N QDs with alleviative QCSE as compared to the traditional In Ga N/Ga N quantum well(QW)structures.We believe the approach reported in this work will find its potential application in Ga N WLEDs and advance the development of semiconductor lighting-communication integration.
基金National Key Research and Development Program of China (2018YFB0406702)National Natural Science Foundation of China (61925104)+1 种基金Key Laboratory of Energy Conversion and Storage Technologies (Southern University of Science and Technology),Ministry of Education,ShenzhenFundamental Research Funds for the Central Universities of Central South University (2018zzts147)。
文摘Commercial white LEDs (WLEDs) are generally limited in modulation bandwidth due to a slow Stokes process,long lifetime of phosphors,and the quantum-confined Stark effect. Here we report what we believe is a novel plasmonic WLED by infiltrating a nanohole LED (H-LED) with quantum dots (QDs) and Ag nanoparticles(NPs) together (M-LED). This decreased distance between quantum wells and QDs would open an extra non-radiative energy transfer channel and thus enhance Stokes transfer efficiency. The presence of Ag NPs enhances the spontaneous emission rate significantly. Compared to an H-LED filled with QDs (QD-LED),the optimized M-LED demonstrates a maximum color rendering index of 91.2,a 43% increase in optical power at 60 m A,and a lowered correlated color temperature. Simultaneously,the M-LED exhibits a data rate of 2.21 Gb/s at low current density of 96 A∕cm2(60 m A),which is 77% higher than that of a QD-LED. This is mainly due to the higher optical power and modulation bandwidth of the M-LED under the influence of plasmon,resulting in a higher data rate and higher signal-to-noise ratio under the forward error correction.We believe the approach reported in this work should contribute to a WLED light source with increased modulation bandwidth for a higher speed visible light communication application.
基金supported by the National Natural Science Foundation of China(Nos.61604051,51502074 and61474109)the Natural Science of Foundation of Tianjin(Nos.16JCQNJC01000 and 16JCYBJC16200)+1 种基金the Technology Foundation for Selected Overseas Chinese Scholar by Ministry of Human Resources and Social Security of the People’s Republic of China(No.CG2016008001)the Research Award for Top Young Scientist of Excellence of Hebei Province(No.210013)
文摘A versatile nanosphere composite lithography（NSCL） combining both the advantages of multiple-exposure nanosphere lens lithography（MENSLL） and nanosphere template lithography（NSTL） is demonstrated. By well controlling the development, washing and the drying processes, the nanosphere monolayer can be well retained on the substrate after developing and washing. Thus the NSTL can be performed based on MENSLL to fabricate nanoring, nanocrescent and hierarchical multiple structures. The pattern size and the shape can be systemically tuned by shrinking nanospheres by using dry etching and adjusting the tilted angle. It is a natural nanopattern alignment process and possesses a great potential in the scope of nano-science due to its low cost,simplicity, and versatility for variuos nano-fabrications.
基金Project supported by the Knowledge Innovation Program of ISCAS(No.08S4060000)
文摘Low resistance and thermally stable n-type contacts to N-polar GaN are essentially important for vertical light emitting diodes （VLEDs）. The electrical characteristics of VLEDs with n-type contacts on a roughened and flat N-polar surface have been compared. VLEDs with contacts deposited on a roughened surface exhibit lower leakage currents yet a higher operating voltage. Based on this, a new scheme by depositing metallization contacts on a selectively wet-etching roughened surface has been developed. Excellent electrical and optical characteristics have been achieved with this method. An aging test further confirmed its stability.
基金supported by the National Natural Science Foundation of China(Grant No.11574306)the China International Science and Technology Cooperation Program(Grant No.2014DFG62280)the National High Technology Program of China(Grant No.2015AA03A101)
文摘Rapid improvement in the efficiency of GaN-based LEDs not only speed up its applications for general illumination, but offer the possibilities for data transmission. This review is to provide an overview of current progresses of GaN-based LEDs for light communications. The modulation bandwidth of GaN-based LEDs has been first improved by optimizing the LED epilayer structures and the modulation bandwidth of 73 MHz was achieved at the driving current density of 40 A/cm2 by changing the multi-quantum well structures. After that, in order to increase the current density tolerance, different parallel flip-chip micro-LED arrays were fabricated. With a high injected current density of ~7900 A/cm2, a maximum modulation bandwidth of ~227 MHz was obtained with optical power greater than 30 mW. Besides the increase of carrier concentrations, the radiative recombination coefficient B was also enhanced by modifying the photon surrounding environment based on some novel nanostructures such as resonant cavity, surface plasmon, and photonic crystals. The optical 3 dB modulation bandwidth of GaN-based nanostructure LEDs with Ag nanoparticles was enhanced by 2 times compared with GaN-based nanostructure LEDs without Ag nanoparticles.Our results demonstrate that using the QW-SP coupling can effectively help to enhance the carrier spontaneous emission rate and also increase the modulation bandwidth for LEDs, especially for LEDs with high intrinsic IQE. In addition, we discuss the progress of the faster color conversion stimulated by GaN-based LEDs.
基金Project supported by the National Natural Sciences Foundation of China(Nos.61334009,61474109,61306050)
文摘High-quality AlN layers with low-density threading dislocations are indispensable for high-efficiency deep ultraviolet light-emitting diodes（UV-LEDs）. In this work, a high-temperature AlN epitaxial layer was grown on sputtered AlN layer（used as nucleation layer, SNL） by a high-yield industrial metalorganic vapor phase epitaxy（MOVPE）. The full width half maximum（FWHM） of the rocking curve shows that the AlN epitaxial layer with SNL has good crystal quality. Furthermore, the relationships between the thickness of SNL and the FWHM values of（002） and（102） peaks were also studied. Finally, utilizing an SNL to enhance the quality of the epitaxial layer, deep UV-LEDs at 282 nm were successfully realized on sapphire substrate by the high-yield industrial MOVPE. The light-output power（LOP） of a deep UV-LED reaches 1.65 mW at 20 mA with external quantum efficiency of 1.87%. In addition, the saturation LOP of the deep UV-LED is 4.31 mW at an injection current of 60 mA. Hence, our studies supply a possible process to grow commercial deep UV-LEDs in high throughput industrial MOVPE, which can increase yield, at lower cost.
文摘The Fe-doped CuO nanopowder was synthesized by following the standard solid-state reaction method. The structure and magnetic properties of the Fe-doped CuO nanopowder were investigated. X-ray diffraction spec- tra confirmed the monoclinic structure of CuO and no secondary phase was detected, indicating that the Fe ions were incorporated into CuO. The ferromagnetism in Fe-doped CuO was studied and is believed to originate from the interaction between Fe ions and Cu ions via a super-exchange interaction or F-center mediated exchange interaction.
