图形硅衬底GaN基发光二极管薄膜去除衬底及AlN缓冲层后单个图形内微区发光及 应力变化的研究

研究了图形硅衬底上外延生长的氮化镓(GaN)基发光二极管(LED)薄膜、去除硅衬底后的无损自由状态LED薄膜以及去除氮化铝(AlN)缓冲层后的自由状态LED薄膜单个图形内的微区光致发光(PL)性能, 用荧光显微镜与扫描电镜观测了去除AlN缓冲层前后LED薄膜断面弯曲状况的变化. 研究结果表明: 1)去除硅衬底后, 自由支撑的LED薄膜朝衬底方向呈柱面弯曲状态, 且相邻图形的柱面弯曲方向不一致, 当进一步去除AlN缓冲层后薄膜会由弯曲变为平整; 2)LED薄膜在去除硅衬底前后同一图形内不同位置的PL谱具有显著差异, 而当去除AlN缓冲层后不同位置的PL谱会基本趋于一致; LED薄膜每一位置的PL 谱在去除硅衬底后均出现明显红移, 进一步去除AlN缓冲层后PL谱出现程度不一的微小蓝移; 3)自由支撑的LED薄膜去除AlN缓冲层后, PL光强随激光激发密度变化的线性关系增强, 光衰减得到改善.     

p-GaN插入层调控InGaN基黄绿双波长LED发光光谱

采用MOCVD技术在硅衬底上生长了含有7个黄光量子阱和1个绿光量子阱的混合有源区结构的InGaN基黄绿双波长LED外延材料,研究了电子阻挡层前p-GaN插入层厚度对黄绿双波长LED载流子分布及外量子效率(EQE)的影响。通过LED变温电致发光测试系统对LED光电性能进行了表征。结果表明,100 K小电流时随着电流密度的增大,三组样品的绿光峰与黄光峰相对强度的比值越来越大,且5.5 A·cm^-2的电流密度下,随着温度从300 K逐步降低至100 K,三组样品的绿光峰与黄光峰相对强度的比值也越来越大,说明其载流子都在更靠近p型层的位置发生辐射复合。三组样品的p-GaN插入层厚度为0,10,30 nm时,EQE峰值依次为29.9%、29.2%和28.2%,呈现依次减小的趋势,归因于p-GaN插入层厚度越大,p型层越远离有源区,空穴注入也越浅。电子阻挡层前p-GaN插入层可以有效减小器件EL光谱中绿光峰随着电流密度增加时峰值波长的蓝移(33 nm),实现了对低温发光光谱的调控。     

退火对ZnS/PS复合体系光致发光特性的影响

用脉冲激光沉积(PLD)技术以多孔硅(PS)为衬底生长了ZnS薄膜,分别测量了ZnS、PS以及ZnS/PS复合体系在室温下的光致发光(PL)光谱。结果发现,ZnS/PS复合体系的PL光谱中PS的发光峰位相对于新制备的PS有所蓝移。把该ZnS/PS样品分成三块,在真空400℃分别退火10,20,30 min,研究不同退火时间对ZnS/PS复合体系光致发光特性的影响。发现退火后样品的PL光谱中都出现了一个新的绿色发光带,归结为ZnS的缺陷中心发光。随着退火时间的延长,PS的发光强度逐渐降低且峰位红移。把ZnS的蓝、绿光与PS的红光相叠加,整个ZnS/PS复合体系在可见光区450~700 nm形成一个较宽的光致发光谱带,呈现较强的白光发射。     

硅衬底氮化镓基LED薄膜转移至柔性黏结层基板后其应力及发光性能变化的研究

本文将硅(Si)衬底上外延生长的氮化镓(GaN)基发光二极管(LED)薄膜转移至含有柔性黏结层的基板上, 获得了不受衬底和支撑基板束缚的LED薄膜. 利用高分辨率X射线衍射仪(HRXRD)研究了薄膜转移前后的应力变化, 同时对其光致发光(PL)光谱的特性进行了研究. 结果表明: 硅衬底GaN基LED薄膜转移至柔性基板后, GaN受到的应力会由转移前巨大的张应力变为转移后微小的压应力, InGaN/GaN量子阱受到的压应力则增大; 尽管LED薄膜室温无损转移至柔性基板其InGaN阱层的In组分不会改变, 然而按照HRXRD倒易空间图谱通用计算方法会得出平均铟组发生了变化; GaN基LED薄膜从外延片转移至柔性基板时其PL谱会发生明显红移.     

Internal quantum efficiency of GaN-based light-emitting diodes grown on silicon substrates determined from rate equation analyses

We present a convenient and reliable method for determining the internal quantum efficiency (IQE) in GaN-based blue light-emitting diodes (LEDs) grown on Si(111) substrates based on the carrier rate equation model. By using the peak point of the efficiency curve in photoluminescence (PL) measurements as the parameter of the rate equation analysis, the IQE can be unambiguously determined without any pre-assumed parameters. The theoretical IQE model is used to fit the measured PL efficiency curves and the IQE of LED samples are determined. The maximum IQE of the LED sample grown on the Si substrate was obtained to be 0.74, which is found to agree well with the results obtained by conventional temperature-dependent PL measurements.     

纳米柱高度对GaN基绿光LED光致发光谱的影响

纳米柱结构是释放高In组分InGaN/GaN绿光LED量子阱层应变的有效方法。本文采用自组装的聚苯乙烯微球掩模、感应耦合等离子体干法刻蚀和KOH溶液的湿法腐蚀,在GaN基绿光LED外延片上制备了3种高度的纳米柱结构,通过扫描电子显微镜观察纳米柱结构的形貌,并测试了常温和10 K低温时的光致发光谱(PL)。结果表明:应变释放对压电场的影响显著,使得纳米柱结构样品的内量子效率(IQE)提高,PL谱峰值波长蓝移;应变在量子阱中的不均匀分布还使得PL谱半高全宽(FWHM)展宽。与普通平面结构相比,高度为747 nm的纳米柱结构可使得IQE提升917%,PL谱峰值波长蓝移18 nm、FWHM展宽7 nm。另外,纳米柱结构样品的有源区有效面积减小可使得PL谱FWHM减小。     

Forming the graded-refractive-index antireflection layers on light-emitting diodes to enhance the light extraction

Distributed antireflection (AR) layers with different composition ratios of ITO and SiO(2) formed on an ITO electrode of GaN-based LEDs provide substantial enhancement in light-extraction efficiency. By using the coradio frequency magnetron sputtering deposition, four 50 nm thick AR layers with graduated refractive indices were fabricated. The effect of the AR layers on enhancing the efficiency of the LED device was analyzed by electroluminescence (EL) and I-V measurements. As a result, the EL intensity of the LED device grown on the patterned sapphire substrate with AR layers was increased by up to 13% compared to the conventional patterned sapphire substrate-applied LED device without AR layers at a drive current of 20 mA. The AR layers on top of the LED device gradually changed the refractive indices between ITO (n=2.1) and air (n=1.0), which minimized the total internal reflection of generated light. And no degradation in the electrical characteristic of the LEDs was observed according to the I-V measurements.     

高温固相法制备ZnO∶Zn绿色荧光粉及其在近紫外LEDs中的应用研究

采用高温固相法在温度900～1 100 ℃,时间3 h的条件下制备出系列ZnO∶Zn荧光粉样品,并进行了封装应用研究。利用X射线衍射分析(XRD)、场发射扫描电子显微镜(FESEM)、荧光分光光度计(PL)和可见光光谱分析系统等手段对样品分别进行了表征分析。研究结果表明：所制备的荧光粉样品均具有ZnO晶型的六方纤锌矿结构,样品可以有效地被近紫外光激发,所发射的绿光光谱具有宽谱发射特征,峰位于502 nm归属于氧空位发射,同时940 ℃条件下制备的荧光粉样品具有最高的发光强度。结合近紫外LED芯片和RGB荧光粉分别制备出绿光LED和白光LED,其中绿光LED在不同驱动电流(250～500 mA)泵浦下表现出稳定的光谱发射特性,发射光谱谱型和色坐标基本未变,发射强度随电流升高而增加,所制备的白光LED在色温3 212 K时,显色指数达到94.1,发光效率为85.6 lm·W-1(@300 mA,9.3 V),在(250～500 mA)驱动电流泵浦下同时也表现出稳定的光谱发射特性,所制备的ZnO∶Zn绿色荧光粉对于制备高显色、高品质近紫外白光LED具有潜在的应用价值。     

Luminescent Characteristics of Near Ultraviolet InGaN/GaN MQWs Grown on Grooved Sapphire Substrates Fabricated by Wet Chemical Etching

InGaN/GaN multiple quantum wells （MQWs） are grown on planar and maskless periodically grooved sapphires by metal organic vapour phase epitaxy （MOCVD）. High-resolution x-ray rocking curves and transmission electron microscopy （TEM） are adopted to characterize the film quality. Compared with the MQWs grown on planar sapphire, the sample grown on grooved sapphire shows better crystalline quality： a remarkable reduction of dislocation densities is achieved. Meanwhile, the MQWs grown on grooved sapphire show two times larger PL intensity at room temperature. Temperature-dependent PL measurements are adopted to investigate the luminescence properties. The luminescence thermal quenching based on a fit to the Arrhenius plot of the normalized integrated PL intensity over the measured temperature range suggests that the nonradiative recombination centres （NRCs） are greatly reduced for the sample grown on grooved sapphire. We assume that the reduction of dislocations which act as NRCs is the main reason for the sample grown on pattern sapphire having higher PL intensity.     

Visible light emission from Si/SiO2 superlattices in optical microcavities

Bright quantum confined luminescence due to band-to-band recombination can be obtained from Si/SiO₂ superlattices. Placing them in a one-dimensional optical microcavity results in a pronounced modulation of the photoluminescence (PL) intensity with emission wavelength, as a consequence of the standing wave set up between the substrate and top interfaces. For a Si substrate, absorption of light reduces the PL efficiency, but for an Al-coated glass substrate the PL intensity is twice that of a quartz substrate case. The addition of a broad-band high reflector to the superlattice surface results in enhanced narrow-band emission. These results show that a suitably designed planar microcavity can not only considerably increase the external efficiency of luminescence in Si/SiO₂ superlattices but can also be used to decrease the bandwidth and selectively tune the peak wavelength.     

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     

The influence of the coating metals with various work function on the photoluminescence of a GaN-based blue LED wafer

A GaN light emitting diodes (LED) wafer was coated with Pt, Au, Al and In. The photoluminescence (PL) excited from wafer back shows that Pt and Au coating can quench the PL while In and Al can increase the PL intensity by eight times and make the wavelength red shift. When connected to the Pt with a wire, indium can also quench the PL. The potential difference between any two kinds of the metals was measured and the PL intensity from each coated area showed a remarkable pertinence to the coating metal potentials. A built-in potential barrier model is proposed to explain results.     

Fabrication and Optical Characterization of GaN-Based Nanopillar Light Emitting Diodes

InGaN/GaN-multiple-quantum-well-based light emitting diode (LED) nanopillar arrays with a diameter of approximately 200nm and a height of 700nm are fabricated by inductively coupled plasma etching using Ni self-assembled nanodots as etching mask. In comparison to the as-grown LED sample an enhancement by a factor of four of photoluminescence (PL) intensity is achieved after the fabrication of nanopillars, and a blue shift and a decrease of full width at half maximum of the PL peak are observed. The method of additional wet etching with different chemical solutions is used to remove the etch-induced damage. The result shows that the dilute HCl (HCl:H₂O=1:1) treatment is the most effective. The PL intensity of nanopillar LEDs after such a treatment is about 3.5 times stronger than that before treatment.     

Influence of intrinsic internal field on recombination kinetics of high coverage (N11) InAs/GaAs quantum dots

We present, by means of cw and time-resolved photoluminescnce, a detailed experimental study of the optical properties of a large set of InAs self-assembled quantum dots grown on (N11)A/B GaAs substrates with different InAs coverages. Large variation of the external PL efficiency is observed, with a strong asymmetry between the A and B substrate termination. The analysis of PL time evolution leads us to exclude that the reduction of PL intensity would be associated to an increase of the non radiative recombination rates. The PL efficiency and decay times of the complete series of samples can be understood as a consequence of a large built-in electric potential associated to piezoelectric field and permanent dipole moment inside the QDs. Received 25 October 2001 and Received in final form 5 February 2002     

磁控溅射制备ZnO薄膜的结构及发光特性研究

采用射频反应磁控溅射法在玻璃衬底上制备出具有c轴高择优取向的ZnO薄膜,利用X射线衍射、扫描探针显微镜及荧光分光光度法研究了生长温度对ZnO薄膜微观结构及光致发光特性的影响。结果表明,合适的衬底温度有利于提高ZnO薄膜的结晶质量;在室温下测量样品的光致发光谱(PL),观察到波长位于400 nm左右的紫光、446 nm左右的蓝色发光峰及502 nm左右微弱的绿光峰,随衬底温度升高,样品的PL谱中紫光及蓝光强度逐渐增大,同时,绿光峰的强度也表现出一定程度的增强。经分析得出紫光应是激子发光所致,而锌填隙则是引起蓝光发射的主要原因,502 nm左右的绿光峰应该是氧的深能级缺陷造成的。此外,还测量了样品的吸收谱,并结合样品吸收谱的拟合结果对光致发光机理的分析作了进一步的验证。     

Thermal stress reduction of GaAs epitaxial growth on V-groove patterned Si substrates

We investigate the thermal stresses for GaAs layers grown on V-groove patterned Si substrates by the finite-element method. The results show that the thermal stress distribution near the interface in a patterned substrate is nonuniform,which is far different from that in a planar substrate. Comparing with the planar substrate, the thermal stress is significantly reduced for the Ga As layer on the patterned substrate. The effects of the width of the V-groove, the thickness, and the width of the SiO₂ mask on the thermal stress are studied. It is found that the SiO₂ mask and V-groove play a crucial role in the stress of the Ga As layer on Si substrate. The results indicate that when the width of V-groove is 50 nm, the width and the thickness of the SiO₂ mask are both 100 nm, the Ga As layer is subjected to the minimum stress. Furthermore,Comparing with the planar substrate, the average stress of the Ga As epitaxial layer in the growth window region of the patterned substrate is reduced by 90%. These findings are useful in the optimal designing of growing high-quality Ga As films on patterned Si substrates.     

Anti-Reflection Characteristics of Si Nanowires for Enhanced Photoluminescence from CdTe/CdS Quantum Dots

CdTe/CdS quantum dots(QDs) are fabricated on Si nanowires(NWs) substrates with and without Au nanoparticles(NPs). The formation of Au NPs on Si NWs can be certified as shown in scanning electron microscopy images. The optical properties of samples are also investigated. It is interesting to find that the photoluminescence(PL) intensity of Cd Te/Cd S QD films on Si nanowire substrates with Au NPs is significantly increased,which can reach 8-fold higher than that of samples on planar Si without Au NPs. The results of finite-difference time-domain simulation indicate that Au NPs induce stronger localization of electric field and then boost the PL intensity of QDs nearby. Furthermore, the time-resolved luminescence decay curve shows the PL lifetime, which is about 5.5 ns at the emission peaks of QD films on planar, increasing from 1.8 ns of QD films on Si NWs to4.7 ns after introducing Au NPs into Si NWs.     

氧、氮掺杂硅基薄膜的荧光光谱

采用磁控溅射法制备了氧、氮掺杂的非晶硅膜 ,测量了薄膜的荧光光谱 ,获得了包括红光、绿光、蓝光及紫光和紫外的强荧光发射 ,其荧光特性受氮、氧含量及沉积时基片温度的影响。结果表明 :红光包括起源于量子限制效应的宽带及氧缺陷能级引起的分立峰 ;绿光依赖于氮的掺杂 ,起源于氮的缺陷能级跃迁 ,其峰型和峰位受基片沉积温度的影响 ;蓝光部分表现为多个分立峰的叠加 ,起源于复杂的氧缺陷能级 ;紫光部分包括一对双峰和多个宽发射带 ,发射带的强度受掺杂种类、掺杂浓度及沉积时基片温度的影响。当基片温度为750℃、中等氧氮掺杂时 ,可获得强的绿光和紫光发射。     

量子垒生长速率对InGaN基绿光LED性能的影响

利用MOCVD技术在图形化Si(111)衬底上生长了InGaN/GaN绿光LED外延材料。在GaN量子垒的生长过程中,保持NH3流量不变,通过调节三乙基镓(TEGa)源的流量来改变垒生长速率,研究了量子垒生长速率对LED性能的影响。使用二次离子质谱仪(SIMS)和荧光显微镜(FLM)分别对量子阱的阱垒界面及晶体质量进行了表征,使用电致发光测试系统对LED光电性能进行了表征。实验结果表明,垒慢速生长,在整个测试电流密度范围内,外量子效率(EQE)明显提升。我们认为,小电流密度下,EQE的提升归结为量子阱晶体质量的改善;而大电流密度下,EQE的提升则归结为阱垒界面陡峭程度的提升。     

Improved light extraction of GaN-based light-emitting diodes with surface-patterned ITO

The surface patterning of the indium tin oxide (ITO) transparent current layer has been investigated to improve the light extraction efficiency of GaN-based light-emitting diodes (LEDs). LEDs with periodic micro-hexagon patterned ITO have been fabricated utilizing standard lithography techniques and inductively coupled plasma (ICP) technology. The luminance intensity of the LED chips with patterned ITO following 160 s ICP etching was enhanced by about 50% compared to the LED chips with unpatterned ITO. Detailed processing parameters are provided. scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to examine the micro-structures. The results indicate that the surface-patterned ITO technique could have potential applications in high-power GaN-based LEDs.