Gallium nitride (GaN) based light-emitting diodes (LEDs) with chirped multiple quantum well (MQW) structures have been investigated experimentally and numerically in this paper. Compared to conventional LEDs wit...Gallium nitride (GaN) based light-emitting diodes (LEDs) with chirped multiple quantum well (MQW) structures have been investigated experimentally and numerically in this paper. Compared to conventional LEDs with uniform quantum wells (QWs), LEDs with chirped MQW structures have better internal quantum efficiency (IQE) and carrier injection efficiency. The droop ratios of LEDs with chirped MQW structures show a remarkable improvement at 600 mA/mm2, reduced down from 28.6% (conventional uniform LEDs) to 23.7% (chirped MQWs-a) and 18.6% (chirped MQWs-b), respectively. Meanwhile, the peak IQE increases from 76.9% (uniform LEDs) to 83.7% (chirped MQWs-a) and 88.6% (chirped MQWs-b). The reservoir effect of chirped MQW structures is the significant reason as it could increase hole injection efficiency and radiative recombination. The leakage current and Auger recombination of chirped MQW structures can also be suppressed. Furthermore, the chirped MQWs-b structure with lower potential barriers can enhance the reservoir effect and obtain further improvement of the carrier injection efficiency and radiative recombination, as well as further suppressing efficiency droop.展开更多
We have investigated the factors affecting the current spreading length(CSL) in GaN-based light-emitting diodes(LEDs) by deriving theoretical expressions and performing simulations with APSYS.For mesa-structure LE...We have investigated the factors affecting the current spreading length(CSL) in GaN-based light-emitting diodes(LEDs) by deriving theoretical expressions and performing simulations with APSYS.For mesa-structure LEDs,the effects of both indium tin oxide(ITO) and n-GaN are taken into account for the first time,and a new Q factor is introduced to explain the effects of different current flow paths on the CSL.The calculations and simulations show that the CSL can be enhanced by increasing the thickness of the ITO layer and resistivity of the n-GaN layer,or by reducing the resistivity of the ITO layer and thickness of the n-GaN layer.The results provide theoretical support for calculating the CSL clearly and directly.For vertical-structure LEDs,the effects of resistivity and thickness of the CSL on the internal quantum efficiency(IQE) have been analyzed.The theoretical expression relating current density and the parameters(resistivity and thickness)of the CSL is obtained,and the results are then verified by simulation.The IQE under different current injection conditions is discussed.The effects of CSL resistivity play a key role at high current injection,and there is an optimal thickness for the largest IQE only at a low current injection.展开更多
We report a method for measuring diffusion coefficient D of liquids by using an aplanatic and asymmetric cylinder lens with a liquid core, which is designed as both a diffusion pool and the main imaging element. The p...We report a method for measuring diffusion coefficient D of liquids by using an aplanatic and asymmetric cylinder lens with a liquid core, which is designed as both a diffusion pool and the main imaging element. The precision is better than 10-4 RIU in measuring refractive index. The D values of ethylene glycol (EG) in water are measured for various EG concentrations at 25℃, and Dinf = 1.043 × 10^-5 cm^2/s under the condition of infinite dilution is obtained. The method is characterized by observing the diffusion process directly, faster measurement and obtaining the D value under the condition of infinite dilution.展开更多
Microcavity photon dynamics in curved space is an emerging interesting area at the crossing point of nanophotonics,chaotic science,and non-Euclidean geometry.We report the sharp difference between the regular and chao...Microcavity photon dynamics in curved space is an emerging interesting area at the crossing point of nanophotonics,chaotic science,and non-Euclidean geometry.We report the sharp difference between the regular and chaotic motions of cavity photons subjected to the varying space curvature.While the island modes of regular motion rise in the phase diagram in the curved space,the chaotic modes show special mechanisms to adapt to the space curvature,including the fast diffusion of ray dynamics,and the localization and hybridization of the Husimi wavepackets among different periodic orbits.These observations are unique effects enabled by the combination of the chaotic trajectory,the wave nature of light,and the non-Euclidean orbital motion,and therefore make the system a versatile optical simulator for chaotic science under quantum mechanics in curved space-time.展开更多
Modulation bandwidth and the emission region are essential features for the widespread use of visible light communications(VLC).This paper addresses the contradictory requirements to achieve broadband and proposes ult...Modulation bandwidth and the emission region are essential features for the widespread use of visible light communications(VLC).This paper addresses the contradictory requirements to achieve broadband and proposes ultrafast,asymmetric pyramids grown on adjacent deep concave holes via lateral overgrowth.Multicolor emission with an emission region between 420 nm and 600 nm is obtained by controlling the growth rate at different positions on the same face,which also can provide multiple subcarrier frequency points for the employment of wavelength division multiplexing technology.The spontaneous emission rate distinction is narrowed by lowering the number of the crystal plane,ensuring a high modulation bandwidth over broadband.More importantly,the residual stress and dislocation density were minimized by employing a patterned substrate,and lateral overgrowth resulted in a further enhancement of the recombination rate.Finally,the total modulation bandwidth of multiple subcarriers of the asymmetric pyramids is beyond GHz.These ultrafast,multicolor microLEDs are viable for application in VLC systems and may also enable applications for intelligent lighting and display.展开更多
A full structure 290 nm ultraviolet light emitting diode(UV-LED)with a nanoporous n-AlGaN underlayer was fabricated by top via hole formation followed by high-voltage electrochemical etching.The 20 to 120 nm nanopores...A full structure 290 nm ultraviolet light emitting diode(UV-LED)with a nanoporous n-AlGaN underlayer was fabricated by top via hole formation followed by high-voltage electrochemical etching.The 20 to 120 nm nanopores were prepared in regular doped n-AlGaN by adjusting the etching voltage.The comparison between the Raman spectrum and the photoluminescence wavelength shows that the biaxial stress in the nanoporous material is obviously relaxed.The photoluminescence enhancement was found to be highly dependent on the size of the pores.It not only improves the extraction efficiency of top-emitting transverse-electric(TE)-mode photons but also greatly improves the efficiency of side emitting tran sverse-magnetic(TM)-mode photons.This leads to the polarization change of the side-emitting light from-0.08 to-0.242.The intensity of the electroluminescence was increased by 36.5%at 100 mA,and the efficiency droop at high current was found to decrease from 61%to 31%.展开更多
Multimode and random directionalities are major issues restricting the application of whispering gallery mode microcavity lasers.We demonstrated a 40μm diameter microring with an off-centered embedded hole and warped...Multimode and random directionalities are major issues restricting the application of whispering gallery mode microcavity lasers.We demonstrated a 40μm diameter microring with an off-centered embedded hole and warped geometry from strained III-nitride quantum well multilayers.Single-mode directional whispering gallery mode lasing was achieved by the warped structure and high-order mode suppression induced by the off-centered hole.In addition,the introduction of the off-centered hole reduced the lasing threshold from 3.24 to 2.79 MW/cm^(2) compared with the warped microdisk without an embedded hole while maintaining a high-quality factor of more than 4000.Directional light emission in 3D was achieved and attributed to the warped structure,which provides a vertical component of the light emission,making it promising for building multifunctional coherent light sources in optoelectronic integration.展开更多
基金supported by the National High Technology Research and Development Program of China(Grant No.2014AA032608)the Key Laboratory for Mechanical Behavior of Material of Xi’an Jiaotong University,China(Grant No.20121201)the Fundamental Research Funds for the Central Universities,China
文摘Gallium nitride (GaN) based light-emitting diodes (LEDs) with chirped multiple quantum well (MQW) structures have been investigated experimentally and numerically in this paper. Compared to conventional LEDs with uniform quantum wells (QWs), LEDs with chirped MQW structures have better internal quantum efficiency (IQE) and carrier injection efficiency. The droop ratios of LEDs with chirped MQW structures show a remarkable improvement at 600 mA/mm2, reduced down from 28.6% (conventional uniform LEDs) to 23.7% (chirped MQWs-a) and 18.6% (chirped MQWs-b), respectively. Meanwhile, the peak IQE increases from 76.9% (uniform LEDs) to 83.7% (chirped MQWs-a) and 88.6% (chirped MQWs-b). The reservoir effect of chirped MQW structures is the significant reason as it could increase hole injection efficiency and radiative recombination. The leakage current and Auger recombination of chirped MQW structures can also be suppressed. Furthermore, the chirped MQWs-b structure with lower potential barriers can enhance the reservoir effect and obtain further improvement of the carrier injection efficiency and radiative recombination, as well as further suppressing efficiency droop.
基金Project supported by the National High Technology Research and Development Program of China(Grant No.2014AA032608)the National Natural Science Foundation of China(Grant No.61404101)the China Postdoctoral Science Foundation(Grant No.2014M562415)
文摘We have investigated the factors affecting the current spreading length(CSL) in GaN-based light-emitting diodes(LEDs) by deriving theoretical expressions and performing simulations with APSYS.For mesa-structure LEDs,the effects of both indium tin oxide(ITO) and n-GaN are taken into account for the first time,and a new Q factor is introduced to explain the effects of different current flow paths on the CSL.The calculations and simulations show that the CSL can be enhanced by increasing the thickness of the ITO layer and resistivity of the n-GaN layer,or by reducing the resistivity of the ITO layer and thickness of the n-GaN layer.The results provide theoretical support for calculating the CSL clearly and directly.For vertical-structure LEDs,the effects of resistivity and thickness of the CSL on the internal quantum efficiency(IQE) have been analyzed.The theoretical expression relating current density and the parameters(resistivity and thickness)of the CSL is obtained,and the results are then verified by simulation.The IQE under different current injection conditions is discussed.The effects of CSL resistivity play a key role at high current injection,and there is an optimal thickness for the largest IQE only at a low current injection.
基金Supported by the National Natural Science Foundation of China under Grant No 11164033, and the Items on Research Team of Science and Technology in Yunnan Province.
文摘We report a method for measuring diffusion coefficient D of liquids by using an aplanatic and asymmetric cylinder lens with a liquid core, which is designed as both a diffusion pool and the main imaging element. The precision is better than 10-4 RIU in measuring refractive index. The D values of ethylene glycol (EG) in water are measured for various EG concentrations at 25℃, and Dinf = 1.043 × 10^-5 cm^2/s under the condition of infinite dilution is obtained. The method is characterized by observing the diffusion process directly, faster measurement and obtaining the D value under the condition of infinite dilution.
基金supported by the National Key R&D Program of China(Grant No.2023YFA1407100)the National Natural Science Foundation of China(Grant Nos.12074303,and 11804267)+2 种基金the Shaanxi Key Science and Technology Innovation Team Project(Grant No.2021TD-56)the Sichuan Science and Technology Program(Grant No.2022NSFSC1811)the Xiaomi Young Scholar Program。
文摘Microcavity photon dynamics in curved space is an emerging interesting area at the crossing point of nanophotonics,chaotic science,and non-Euclidean geometry.We report the sharp difference between the regular and chaotic motions of cavity photons subjected to the varying space curvature.While the island modes of regular motion rise in the phase diagram in the curved space,the chaotic modes show special mechanisms to adapt to the space curvature,including the fast diffusion of ray dynamics,and the localization and hybridization of the Husimi wavepackets among different periodic orbits.These observations are unique effects enabled by the combination of the chaotic trajectory,the wave nature of light,and the non-Euclidean orbital motion,and therefore make the system a versatile optical simulator for chaotic science under quantum mechanics in curved space-time.
基金Major State Basic Research Development Program of China(2016YFB0400801)National Natural Science Foundation of China(61774121,6190030620)+2 种基金Natural Science Foundation of Shaanxi Province(2020JQ-074)Postdoctoral Research Foundation of China(2019M653640,2019M663718)Fundamental Research Funds for the Central Universities(xjh012019038).
文摘Modulation bandwidth and the emission region are essential features for the widespread use of visible light communications(VLC).This paper addresses the contradictory requirements to achieve broadband and proposes ultrafast,asymmetric pyramids grown on adjacent deep concave holes via lateral overgrowth.Multicolor emission with an emission region between 420 nm and 600 nm is obtained by controlling the growth rate at different positions on the same face,which also can provide multiple subcarrier frequency points for the employment of wavelength division multiplexing technology.The spontaneous emission rate distinction is narrowed by lowering the number of the crystal plane,ensuring a high modulation bandwidth over broadband.More importantly,the residual stress and dislocation density were minimized by employing a patterned substrate,and lateral overgrowth resulted in a further enhancement of the recombination rate.Finally,the total modulation bandwidth of multiple subcarriers of the asymmetric pyramids is beyond GHz.These ultrafast,multicolor microLEDs are viable for application in VLC systems and may also enable applications for intelligent lighting and display.
基金National Natural Science Foundation of China(61574114,61774121)National Key Research and Development Program of China(2016YFB0400801)Fundamental Research Funds for the Central Universities(Z201805198).
文摘A full structure 290 nm ultraviolet light emitting diode(UV-LED)with a nanoporous n-AlGaN underlayer was fabricated by top via hole formation followed by high-voltage electrochemical etching.The 20 to 120 nm nanopores were prepared in regular doped n-AlGaN by adjusting the etching voltage.The comparison between the Raman spectrum and the photoluminescence wavelength shows that the biaxial stress in the nanoporous material is obviously relaxed.The photoluminescence enhancement was found to be highly dependent on the size of the pores.It not only improves the extraction efficiency of top-emitting transverse-electric(TE)-mode photons but also greatly improves the efficiency of side emitting tran sverse-magnetic(TM)-mode photons.This leads to the polarization change of the side-emitting light from-0.08 to-0.242.The intensity of the electroluminescence was increased by 36.5%at 100 mA,and the efficiency droop at high current was found to decrease from 61%to 31%.
基金Fundamental Research Funds for the Central Universities(Z201805198)National Key Research and Development Program of China(2016YFB0400801)National Natural Science Foundation of China(61574114,6190030620).
文摘Multimode and random directionalities are major issues restricting the application of whispering gallery mode microcavity lasers.We demonstrated a 40μm diameter microring with an off-centered embedded hole and warped geometry from strained III-nitride quantum well multilayers.Single-mode directional whispering gallery mode lasing was achieved by the warped structure and high-order mode suppression induced by the off-centered hole.In addition,the introduction of the off-centered hole reduced the lasing threshold from 3.24 to 2.79 MW/cm^(2) compared with the warped microdisk without an embedded hole while maintaining a high-quality factor of more than 4000.Directional light emission in 3D was achieved and attributed to the warped structure,which provides a vertical component of the light emission,making it promising for building multifunctional coherent light sources in optoelectronic integration.