Improving InGaN-LED performance by optimizing the patterned sapphire substrate shape

The epitaxial growths of GaN films and GaN-based LEDs on various patterned sapphire substrates(PSSes) with different values of fill factor(f) and slanted angle(θ) are investigated in detail.The threading dislocation(TD) density is lower in the film grown on the PSS with a smaller fill factor,resulting in a higher internal quantum efficiency(IQE).Also the ability of the LED to withstand the electrostatic discharge(ESD) increases as the fill factor decreases.The illumination output power of the LED is affected by both θ and f.It is found that the illumination output power of the LED grown on the PSS with a lower production of tan θ and f is higher than that with a higher production of tan θ and f.     

掺杂PEDOT ∶PSS对聚合物太阳能电池性能影响的研究

研究了掺杂后poly(3,4-ethylene dioxythiophene):poly(styrenesulphonic acid)(PEDOT ∶PSS)电导率的变化以及掺杂PEDOT ∶PSS薄膜对聚合物太阳能电池器件性能的影响. 实验发现,向PEDOT ∶PSS中掺入极性溶剂二甲基亚砜(DMSO)明显提高了薄膜的电导率,掺杂后的电导率最大值达到1.25 S/cm,比未掺杂时提高了3个数量级. 将掺杂的PEDOT ∶PSS薄膜作为缓冲层应用于聚合物电池 (ITO/PEDOT ∶PSS/P3HT ∶PCBM/LiF/Al) 中,发现高电导率的PEDOT ∶PSS降低了器件的串联电阻,增加了器件的短路电流,从而提高了器件的性能. 最好的聚合物太阳能电池在100 mW/cm²的光照下,开路电压(V_oc)为0.63 V,短路电流密度(J_sc)为11.09 mA·cm^-2,填充因子(FF)为63.7%,能量转换效率(η)达到4.45%. 关键词： PEDOT ∶PSS 电导率 聚合物太阳能电池 能量转换效率     

全息技术制作二维光子晶体蓝宝石衬底提高发光二极管外量子效率

为了提高GaN基发光二极管(LED)的外量子效率,在蓝宝石衬底制作了二维光子晶体.衬底上的二维光子晶体结构采用激光全息技术和感应耦合等离子体(ICP)干法刻蚀技术制作,然后采用金属氧化物化学气相沉积(MOCVD)技术在图形蓝宝石衬底(PSS)上生长2μm厚的n型GaN层,4层量子阱和200nm厚的p型GaN层,形成LED结构.衬底上制作的二维光子晶体为六角晶格结构,晶格常数为3.8μm,刻蚀深度为800nm.LED器件光强输出测试结果显示,在PSS上制作的LED(PSS-LED)的发光强度普遍高于蓝宝石平 关键词： 全息 发光二极管 图形蓝宝石衬底 外量子效率     

基于双自由曲面的LED大角度光学透镜设计

针对直下式LED背光源的均匀照明系统,采用双自由曲面组合,设计了一种大角度光学透镜结构。通过近光源面的自由曲面将光发散成的c/cos3(θ)型光场分布,再利用远光源面实现目标面的均匀分布。这样可以在短距离条件下实现大面积的均匀照明,相对于传统的单自由曲面设计,有效地避免了全反射的发生,提高了照明区域的面积。采用光线追迹软件对所设计的结构进行仿真,通过对模拟结果的分析,在灯箱厚度为15mm时,单透镜均匀照明面积可以达到60mm。采用正三角阵列分布,整个目标面均匀度达到87.5%。相对于传统的大功率器件的直下式光源方式,提高了照明的均匀度,同时大大减少了箱体的厚度。     

Daylight perceptive lighting and data fusion with artificial neutral network

The basic concept of daylight perceptive is that the artificial light automatically dims to the appropriate level to keep constant indoor illumination when sufficient daylight is not available. Energy efficient character of daylight perceptive lighting was analyzed and a method to evaluate energy saving was present; according to the presentation, the more energy is saved if the longer T_last—duration of indoor illumination being is less than threshold. A daylight perceptive controller was developed and the output power of LED light source under different PWM was measured, according to the measurement, work current and output power of LED is proportional to duty cycle. With two sensors, a data fusion method was put forward to exclude interference and improve lighting comfort. With more than two sensors, data fusion based on neutral network was discussed.     

Optical design of a freeform TIR lens for LED streetlight

Secondary optics is a critical component determining the output efficiency and light distribution of the LED streetlight. A good roadway illumination requires the LED streetlight have a rectangular light pattern just cover the roadway and without excess spill light creeping into yards and shining into windows of neighbor residents. The adoption of the unsymmetrical freeform optics with different light distributions along X and Y directions is necessary. The whole LED streetlight can be constructed if only mounts these LED modules on the planar PCB plate. The mechanism, thermal management, and the power control electronics of the LED streetlight become very simple. In this paper, an optical design approach of a freeform TIR (total internal reflection) lens for the LED streetlight is investigated. With these freeform lenses, high output efficiency, even light distribution, and batwing far field beam angle distribution can be achieved.     

Influence of the spectral power distribution of a LED on the illuminance responsivity of a photometer

The measurement accuracy in the photometric quantities measured through photometer head is determined by the value of the spectral mismatch correction factor (c(S_t,S_s)), which is defined as a function of spectral power distribution of light sources, besides illuminance responsivity of the photometer head used. This factor is more important when photometric quantities of the light-emitting diode (LED) style optical sources, which radiate within relatively narrow spectral bands as compared with that of other optical sources, are being measured. Variations of the illuminance responsivities of various V(λ)-adopted photometer heads are discussed. High-power-colored LEDs, manufactured by Lumileds Lighting Co., were used as light sources and their relative spectral power distributions (RSPDs) were measured using a spectrometer-based optical setup. Dependences of the c(S_t,S_s) factors of three types of photometer heads (f₁′=1.4%, f₁′=0.8% and f₁′=0.5%) with wavelength and influences of the factors on the illuminance responsivities of photometer heads are presented.     

Electric and dielectric properties of solution-gas interface grown amorphous AgCl films

Electric and dielectric properties of solution-gas interface grown AgCl thin film capacitors (Al/AgCl/Al) of various thicknesses have been studied in the frequency range 10¹–10⁶ Hz at various temperatures (303–393 K). I–V characteristics show ohmic, space-charge-limited, and thermionic emission conduction mechanisms to operate at low, intermediate and high voltages respectively. Capacitance decreases with increasing film thickness and applied frequency while it increases with increase of temperature. Loss factor (tanδ), which shows a pronounced minimum with frequency, increases with the rise of temperature and (tanδ)_min shifts to a higher frequency. The large values of capacitance and dielectric constant (ɛ) in the low frequency region indicate the possibility of an interfacial polarization mechanism to operate in this region while electronic and ionic polarizations dominate in the high frequency region.     

GaAs/GaAlAs双异质结发光管退化特性的研究

本文研究了GaAs/GaAlAs双异质结发光管的退化特性。有快、慢二种退化类型。器件的慢退化是由于有源区有暗缺陷(DSD)产生和长大,引起光功率下降。文中研究了老化过程中I-V特性和I-P特性与EL图象的变化规律,并与相同结构的InP/InGaAsP双异质结发光管的退化特性进行了比较,结果表明:它们有着不同的退化机理。     

Comparison of InGaN/GaN light emitting diodes grown on m ‐plane and a ‐plane bulk GaN substrates

InGaN/GaN‐based light emitting diodes (LEDs) grown on m ‐plane, a ‐plane and off‐axis between m ‐ and a ‐plane GaN bulk substrates were investigated. A smooth surface was obtained when a ‐plane substrate was applied; however, large amounts of defects were observed. Photoluminescence measurements of the LEDs with a well thickness of 2.5 nm revealed that all the LEDs showed the peak emission wavelength at 389 nm. The PL intensity of the a ‐plane LED is one order of magnitude lower than that of the m ‐plane LED. The a ‐plane LEDs showed significant lower electroluminescence output powers than m ‐plane LEDs, suggesting that excitons are trapped by the defects, which act as non‐radiative recombination centers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

用CARS技术确定拉曼退偏比的一种数据处理新方法

Yuika等人利用偏振CARS技术可以准确地确定分子的拉曼退偏比.其方法是，首先对不同检偏角_d所对应CARS谱峰的频率分布进行数学模拟，然后由所得系数随检偏角φ_d的变化求得使CARS信号中共振项消失的偏振角φ^０_d，最后由消失条件ρ＝-1/(tanθtanφ⁰_d)求出退偏比ρ，θ为产生CARS光的Pump光与Stokes光偏振方向的夹角.本文提出的数据处理方法，即交点法.同Yuika等人处理数据的方法相比，交点法毋需关于谱峰频率分布的知识，做法也更为简便.     

GaN-based light-emitting diodes directly grown on sapphire substrate with holographically generated two-dimensional photonic crystal patterns

As an approach to enhance light extraction from GaN-based light-emitting diodes (LEDs), we inserted a submicron period photonic crystal (PC) pattern at the interface between GaN epilayer and sapphire substrate. A two-dimensional square-lattice pillar array of 600-nm period was produced directly onto the sapphire substrate by a combination of laser holography and inductively-coupled-plasma etching. A standard GaN LED heterostructure was grown on top of the nano-patterned substrate, which was then processed to conventional bottom-emitting LED chips. At the drive current of 20 mA, the PC-LED produced surface-normal output power about 40% higher than that of the reference LED (with no PC integrated). Temperature-dependent photoluminescence measurement indicated that the emission enhancement was solely a structural effect by the integrated PC pattern.     

Efficiency characteristics of a silicon oxide passivation layer on p-type crystalline silicon solar cell at low illumination

Silicon dioxide (SiO₂) is widely used to improve the surface passivation properties of silicon solar cells. To minimize solar cell potential-induced degradation when the PV module is installed outdoors, a silicon oxide film is widely used as an insulator. However, experiments have confirmed that solar cells with a silicon oxide (SiO₂) film have a lower efficiency than solar cells without a silicon oxide (SiO₂) film at low illumination (<0.4 sun). Actually, the efficiency in the low illumination condition affects the average power output per day because the PV module mostly operates when the solar irradiation dose is less than 1 sun. To maximize the performance of the PV module, the output at a low light intensity level should also be considered. Shunt resistance (R_shunt) is known to cause a decrease in solar cell efficiency under low illumination conditions. PC1D simulation was used to analyze parameters, such as the series resistance, parallel resistance, and surface recombination, that affect the characteristics of the solar cell at low light intensity. In this study, we confirmed how the SiO₂ layer affected the low illumination properties of solar cells, even though these cells were more efficient at 1 sun. Silicon solar cells with a SiN_x/SiO₂ bilayer or a SiN_x single film were fabricated, and their characteristics were evaluated. Passivation characteristics were measured using the quasi-steady-state photoconductance (QSSPC) technique to evaluate the minority carrier lifetime and the implied open-circuit voltage (V_OC), and capacitance-voltage measurements were used to analyze the fixed charges. The values of the shunt resistance and series resistance in solar cells with different passivation layers were compared, and the cause of the decrease in the efficiency under low illumination was also analyzed via fill factor calculation.     

A novel 4H–SiC MESFET with modified channel depletion region for high power and high frequency applications

In this paper, a novel 4H–SiC metal semiconductor field effect transistor (MESFET) with modified depletion region is introduced. The key idea in this work is modifying the depletion region in the channel for improving the electrical performances. The proposed structure consists of upper and lower gates. Also, the lower gate is divided into a number (N) of smaller step-shaped sections. Therefore, we have called the proposed structure multiple-recessed 4H–SiC MESFET (MR-MESFET). DC and RF characteristics of the MR-MESFET structure with various lower gate segments are analyzed by 2D numerical simulation. The simulated results show that as the number of the lower gate sections increases, the channel depletion region is modified and the drain current (I_D) enhances. Also, by increasing the number of the lower gate sections, the breakdown voltage (V_BR) enhances, too. Improvement of the I_D and V_BR leads to a further increase in the output power density of the device. Also, cut-off frequency (f_T), maximum oscillation frequency (f_max), and maximum available gain (MAG) improvements are achieved for the MR-MESFET structure with further number of the lower gate sections. The results show that the MR-MESFET structure with higher number of the lower gate segments has superior electrical characteristics and performances in comparison with the MR-MESFET structure with fewer number of the lower gate sections.     

Observability of Charged Higgs Contribution in t-channel Single Top at LHC

In this paper, the charged Higgs contribution in t-channel single top production is studied. The production process is a t-channel single top event with charged Higgs exchange. Therefore the signal is similar with Standard Model single top production in terms of the final state. In the first step, the signal cross section is calculated and compared to the other main production processes which are used for a heavy charged Higgs search at LHC, i.e., \(pp\rightarrow t\bar {b}H^{-}\) and \(pp\rightarrow H^{+} \rightarrow t\bar {b}\). In the next step, an event generation and analysis is applied on signal and background events, in order to estimate the signal significance. The signal cross section is typically smaller than the associated production (\(t\bar {b}H^{-}\)) and resonance production (\(t\bar {b}\)) by a factor of 10^?3 and ranges from 10 f b to 1 f b for charged Higgs mass from 200 to 500 GeV at tanβ = 50. Due to the small cross section of signal events and large SM background, the signal significance is small even after a dedicated kinematic analysis and selection of events, however, tanβ values above 120 can be excluded at an integrated luminosity of 3000 f b ^?1.     

Study on the impedance of aligned carbon microcoils embedded in silicone rubber matrix

This paper reports that carbon microcoils are grown through a chemical vapour deposit process, they are then embedded in silicone rubber, and manipulated to parallel with each other along their axes in the resulting composite. The impedance |Z| as well as phase angle θ of both the original carbon microcoil sheets and the aligned carbon microcoil/silicone rubber composites are measured. The results illustrate that carbon microcoils in different forms show different alternating current electric properties. The aligned carbon microcoils in the composites show stable parameters for f<10⁴ Hz but a sharp decrease in both |Z| and θ for frequencies >10⁴ Hz, which will also change as the carbon microcoils are extended. But, the original sheets have a pure resistance with their parameters stable throughout the entire alternating current frequency range investigated.     

Characteristic enhancement of white LED lamp using low temperature co-fired ceramic-chip on board package

In this paper, we propose low temperature co-fired ceramic-chip on board (LTCC-COB) package with improved thermal characteristics; no insulation layer exists between the LED chip and metal base. In actual measurement as well as in thermal simulation, the proposed LED lamp structure showed excellent thermal properties, compared with surface mound device-printed circuit board (SMD-PCB) package LED lamp. The optical output power, thermal distribution, current-voltage (I-V) and electroluminescence (EL) were measured and compared to analyze the characteristics of LTCC-COB package LED lamp with SMD-PCB package LED lamp. EL peak intensity of LTCC-COB package LED lamp is 1.75 times better than that of SDM-PCB package LED lamp. The thermal resistance between packing area and air was found to be 7.3 K/W and 7.9 K/W for LTCC-COB package and SMD-PCB package respectively. The proposed LTCC-COB packaged LED lamp is not only suitable for high power LED package due to its low thermal resistance but also a promising solution for illumination modules.     

Primary isotopic effect on the magnetic shielding of tin nuclei (117Sn and 119Sn)

The Kirkwood-Buff statistical mechanical theory of surface tension γ for monatomic fluids is extended to molecular fluids. A rigorous expression for γ is derived in terms of the angular pair distribution function f(z ₁ R ₁₂θ₁θ₂) of an equilibrium fluid-fluid system (liquid-gas, liquid-liquid, or gas-gas). The Fowler approximation is applied for the liquid-gas case, and a simple expression for γ is derived in terms of the bulk liquid angular pair correlation function g(R ₁₂θ₁θ₂). Thermodynamic perturbation theory for g(R ₁₂θ₁θ₂) is also used to calculate γ theoretically. The theoretical results are compared with experimental values.     

Fabrication of 160-nm T-gate metamorphic AlInAs/GaInAs HEMTs on GaAs substrates by metal organic chemical vapour deposition

The fabrication and performance of 160-nm gate-length metamorphic AlInAs/GaInAs high electron mobility tran-sistors (mHEMTs) grown on GaAs substrate by metal organic chemical vapour deposition (MOCVD) are reported. By using a novel combined optical and e-beam photolithography technology, submicron mHEMTs devices have been achieved. The devices exhibit good DC and RF performance. The maximum current density was 817 mA/mm and the maximum transconductance was 828 mS/mm. The non-alloyed Ohmic contact resistance Rc was as low as 0.02 Ω- mm. The unity current gain cut-off frequency (fT) and the maximum oscillation frequency (fmax) were 146 GHz and 189 GHz, respectively. This device has the highest fT yet reported for a 160-nm gate-length HEMTs grown by MOCVD. The output conductance is 28.9 mS/mm, which results in a large voltage gain of 28.6. Also, an input capacitance to gate-drain feedback capacitance ratio, Cgs/Cgd, of 4.3 is obtained in the device.     

磁控反应溅射AlN缓冲层对GaN基LED器件性能的影响

以直流磁控反应溅射法(RMS)在图形化蓝宝石衬底上制备的AlN薄膜作为缓冲层,采用金属有机化学气相沉积法(MOCVD)外延生长了GaN基LED。与MOCVD生长的低温GaN缓冲层相比,RMS制备的AlN缓冲层具有表面更平整、颗粒更小的形核岛,有利于促进GaN外延的横向生长,减少了形核岛合并时的界面数量和高度差异,降低了缺陷和位错产生的几率。研究结果表明,溅射AlN缓冲层取代传统低温GaN缓冲层后,外延生长的GaN材料具有更高的晶体质量,LED器件在亮度、漏电和抗静电能力等光电特性上均有明显提升。