Enhancement of Light Extraction Efficiency of LED by Bionic Moth-Eye Structure with Frustum of a Cone
1 引言
发光二极管(LED)因其光电转换效率高、使用寿命长、节能环保等优点而被广泛应用[1-2]。LED的光提取效率指单位时间内发射到自由空间的光子数与单位时间内有源层发出的光子数的比值[3-4]。由于芯片顶层钝化材料折射率约为2.0,与空气折射率差值较大而发生全反射,致使有源层发出的光大部分被半导体材料吸收转变为热能耗散,导致表面无微结构的蓝光LED的光提取效率仅为3%~5%,光提取效率低成为伴随LED发展而一直存在的主要问题[5]。近年来国内外学者在提高LED光提取效率方面做了许多研究,提出了图形化衬底[6-7]、表面粗化[8]、光子晶体[9-11]、微结构[12-13]等方法。如Ye等[14]通过在垂直结构LED芯片的p-GaN表面制备ZnO纳米棒结构使光提取效率提高了3倍。一般而言,制作微结构破坏材料界面的全反射是直接提高光提取效率的方法,但是在芯片的p-GaN层或氧化铟锡(ITO)层刻蚀往往会影响器件的电学特性,导致器件的漏电电流较高[15-16]。而在可减小漏电电流、增加芯片可靠性的钝化层适度刻蚀将不会出现上述现象。由于蛾眼微结构具有天然的增透特性及可在任何材料上制作等优点[17],从而非常适宜用于提高半导体器件的光提取效率。
本文以等效介质理论设计了一种制作于GaN基LED钝化层表面的截头圆锥形仿生蛾眼结构,以期提高光提取效率。通过模拟结构底面占空比、底面直径、高度、倾角对LED光提取效率的影响确定最佳组合参数。通过纳米球刻蚀制作钝化层表面具有微结构的LED芯片,与无表面微结构的LED芯片进行电致发光对比测试,并分析了表面微结构对LED光提取效率的作用。
2 设计与模拟
从等效介质理论可知,当光线通过蛾眼微结构时可等效于多层均匀介质。二维亚波长周期结构的等效折射率neff表达式为:
式中f2=fxfy为体积占空比,其中fx、fy为亚波长结构在x、y方向上的占空比;
为基底折射率,在此指SiN折射率。截头圆锥形二维阵列周期结构表达式为[18]:
式中T为结构周期,h为结构高度,R(x,y)为顶点坐标(x0,y0)位于单一周期中心时结构表面坐标点到顶点的距离。截头圆锥形阵列微结构排列方式及钝化层加入该阵列结构的正装LED仿真结构如图1所示。
图 1. (a)截头圆锥形微结构排列方式及(b)加入该阵列结构的LED示意图
Fig. 1. (a) Arrangement of frustum of a cone microstructure and (b) schematic of LED with the array structure
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由于所用结构特征尺寸为亚波长量级,须使用等效介质理论或矢量衍射理论进行分析[19]。时域有限差分(FDTD)法是矢量分析法中主要的分析方法之一,该算法通过Yee元胞方式可将麦克斯韦方程组离散转化为一组适合计算机编程的差分方程,并逐步推进求解空间磁场。由于GaN基LED量子阱载流子复合发光过程可等效为点光源[20],以及LED光源的非相干特性,在FDTD模拟时以单个电偶极子源模拟有源层(MQW),设置中心波长为465 nm,仿真区域大小为2000 nm×4600 nm。进行仿真时,将x-y平面四周设置为周期性边界条件(PBC),同时在z轴两端设置完美匹配层(PML)。监视器放置在距离截头圆锥微结构顶部一个波长的位置。通过计算监视器接收到的表面有微结构的LED光辐射功率与表面无微结构的LED光辐射功率的比值,即可算出LED的光提取效率改变量。
2.1 底面占空比对光提取效率的影响
根据LED芯片及钝化层厚度和吸收特性,假定截头圆锥的底面半径r为220 nm,倾角为70°,高度为245 nm,计算不同周期T即不同底面占空比时的光提取效率,模拟结果如图2所示。占空比的改变致使截头圆锥形蛾眼结构层等效折射率发生变化,导致光提取效率呈现先上升后下降的趋势。由图2可以看出,当占空比为0.55时器件的光提取效率最大。
图 2. 光提取效率随微结构底面占空比的变化
Fig. 2. Light extraction efficiency as a function of the bottom fill factor of the microstructure
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2.2 底面半径对光提取效率的影响
依据图2的结果,取底面占空比为0.55,另取倾角为70°,高度为245 nm,在此条件下光提取效率随底面半径变化的模拟结果如图3所示。当微结构底面半径在90~300 nm变化时,器件的光提取效率均有较大提升,当底面半径大于140 nm时,光提取效率随底面半径的变化趋缓;当底面半径为220 nm时芯片的光提取效率最优。
图 3. 光提取效率随微结构底面半径的变化
Fig. 3. Light extraction efficiency as a function of the bottom radius of the microstructure
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2.3 高度对光提取效率的影响
当底面占空比为0.55、底面半径为220 nm、倾角为70°时,光提取效率随截头圆锥高度的变化如图4所示。模拟结果表明,光提取效率随微结构高度增大的变化趋势较为明显,可描述为先上升后下降;同时考虑结构厚度对光提取效率的影响,微结构的最佳高度选为245 nm。
图 4. 光提取效率随微结构高度的变化
Fig. 4. Light extraction efficiency as a function of the height of the microstructure
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2.4 倾角对光提取效率的影响
当底面占空比为0.55、底面半径为220 nm、高度为245 nm时,光提取效率随截头圆锥微结构侧面倾角的变化如图5所示。模拟结果表明,微结构的侧面倾角对光提取效率影响较为明显,当倾角为70°时光提取效率最大,其值是无表面微结构器件的4.85倍。
图 5. 光提取效率随微结构侧面倾角的变化
Fig. 5. Light extraction efficiency as a function of the side slope angle of the microstructure
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综合上述仿真结果,当LED芯片钝化层表面截头圆锥微结构的底面占空比为0.55、底面半径为220 nm、高度为245 nm、侧面倾角为70°时得到的光提取效率最优,与无微结构的芯片相比光提取效率可提高4.85倍。
3 实验制作
实验选取相同条件下生长的外延片,使用的芯片结构自上而下依次是约300 nm的SiNx钝化层,约150 nm的ITO层,约200 nm的p-GaN层,5个周期的有源层InGaN/GaN多量子阱结构,2 μm的n-GaN层,3.5 μm的非掺杂GaN层,最下层是双抛型蓝宝石衬底,样品的发光波长在465 nm左右,以无表面微结构的LED芯片为参考样品。实验通过纳米球刻蚀制作具有表面微结构的LED芯片,基本步骤为先制作单层六角密排的纳米球层,再进行反应离子刻。考虑到纳米球掩膜刻蚀工艺的特点及材料特性,选用直径为500 nm的SiO2纳米球。
制作单层密排的纳米球步骤为:亲水处理—匀SiO2纳米球—自组装。首先将器件浸入H2O/NH4OH/H2O2配比为50∶10∶3的溶液进行亲水处理,然后再将器件用去离子水冲洗。匀SiO2纳米球使用的是Smartcoater 100型匀胶机,首先在器件表面滴上约10 μL的10%(体积分数)固体单分散SiO2纳米球乳液,先以1000 r/min的低速旋涂5 s,使乳液均匀旋涂于器件钝化层表面,再用2500 r/min的高速旋转将SiO2球在钝化层表面旋涂为单层,得到单层规则排列的纳米球阵列结构。但此时的SiO2纳米球排列并非密排,需将旋涂有SiO2球的器件放入滴有表面活性剂十二烷基硫酸钠的溶液中,使SiO2纳米球自组装呈六角密排阵列结构,形成单层微球膜重新沉积于器件表面。最后将钝化层表面附着SiO2纳米球的器件放在干燥箱中烘烤固化。图6是直径为500 nm的SiO2纳米球在大面积分布内的扫描电镜(SEM)图,可以看出,纳米球排列无双层或多层现象,虽偶见缺陷球形和较小缝隙,但总体范围内SiO2纳米球是六角单层密排的。
图 6. 大面积直径为500 nm的SiO2纳米球阵列SEM图
Fig. 6. SEM image of the large area SiO2 nanosphere array with the diameter of 500 nm
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为保证制作的蛾眼结构接近设计的底面占空比,需用CHF3垂直刻蚀SiO2纳米球,通过控制刻蚀时间及气体流量来控制纳米球的直径。同时采用O2辉光的方式清除覆盖在纳米球表面的其他残余有机物质。然后以SiO2纳米球为掩膜,采用等离子刻蚀方法在样品钝化层表面刻蚀出截头圆锥形微结构阵列,刻蚀操作在ICP-801型反应离子刻蚀机内进行。经反复调试,当刻蚀时所用的混合气体CHF3/O2流量为20/10 sccm(标准状态下20/10 mL/min)、刻蚀速率约为7 nm/s时截头圆锥结构面形最为理想。最后通过高温热退火去除残余的SiO2纳米球。图7为最终制作完成的截头圆锥形仿生蛾眼微结构阵列SEM图,可以看出其均匀性较好,基本符合制作要求。
图 7. 截头圆锥阵列结构形貌SEM图
Fig. 7. Surface SEM image of the frustum of a cone array structure
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4 测试
良好的刻蚀形貌特征对微结构的性能十分重要,图8为原子力显微镜(AFM)测试图,图8(a)为三维扫描立体图,可以看出微结构阵列面型较为平滑;图8(b)为微结构截面轮廓数据图,由数据计算所得的微结构各项参数的测量结果与理论设计值的对比如表1所示,可以看出二者差值较小,基本符合设计要求。
为了研究微结构阵列对光提取效率的具体影响,需对LED芯片进行电致发光(EL)测试。测试使用的是在表面微结构制作完成后,再经过退火等工艺制作完整的300 μm×300 μm的小芯片,测试电源使用Keithly 2611A半导体分析仪,同时使用
图 8. AFM扫描结构图。(a) AFM 测试的三维图;(b)截面轮廓图
Fig. 8. AFM scanning structure. (a) Three dimensional diagram of AFM test; (b) section profile
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表 1. 设计参数与测试参数
Table 1. Design parameters and test parameters
Item | Fill factor | Bottom radius /nm | Height /nm | Angle /(°) |
---|
Design value | 0.55 | 220 | 245 | 70 | Real value | 0.58±0.05 | 220±6 | 245±6 | 70±3 |
|
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其测试样品的伏安特性曲线,使用ARS SP2557单色仪采集发光谱。图9(a)为分别测试无微结构参考样品和钝化层制作有微结构样品的伏安特性曲线,可以看出,制作有截头圆锥形微结构阵列样品的电压较参考样品略大,原因是由于采用扎探针方式测量光强,切入探针的位置与接触角等因素会导致一定的测量误差。图9(b)是样品的电流-光强曲线,可以看出,在相同电流的条件下,制作有截头圆锥形微结构阵列样品的出光强度较参考样品要强。例如,在电流为20 mA时增强4.41倍,而在电流为150 mA时增强4.36倍,均与模拟结果较为接近。随着电流增大光强度增强倍数减小的原因是微结构对LED的散热有一定影响,致使阻值有略微增加,在电流较大时更为明显。综合上述测试和分析结果可以得出,加入截头圆锥形仿生蛾眼阵列结构可以有效提高LED光提取效率。
图 9. 测试样品与参考样品EL测试曲线。(a)伏安特性曲线;(b)光强-电流曲线
Fig. 9. EL test curves of the test sample and the reference sample. (a) Volt ampere characteristic curve; (b) light intensity-current curve
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5 结论
研究了截头圆锥形仿生蛾眼微结构阵列对LED出光效率的提升作用。首先利用等效介质理论进行分析,并通过FDTD模拟LED钝化层表面制作截头圆锥阵列结构参数对光提取效率的影响进行仿真计算,得出微结构的底面占空比为0.55、底面半径为220 nm、高度为245 nm、侧面倾角为70°时光提取效率最优。在此基础上,采用纳米球刻蚀技术在LED钝化层表面制备该亚波长纳米结构,并与无表面微结构的LED对比测试,EL测试结果表明,制作有微结构的样品在20 mA和150 mA工作电流下较无微结构的参考样品发光效率分别增加4.41倍和4.36倍。本工作表明,加入截头圆锥形仿生蛾眼微结构可以有效提高LED光提取效率。
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