引用本文
崔林, 汪桂根, 张化宇, 周福强, 韩杰才. 用于GaN基发光二极管的蓝宝石图形衬底制备进展. 无机材料学报, 2012, 27(9): 897-905
CUI Lin, WANG Gui-Gen, ZHANG Hua-Yu, ZHOU Fu-Qiang, HAN Jie-Cai. Progress in Preparation of Patterned Sapphire Substrate for GaN-based Light Emitting Diodes. Journal of Inorganic Materials, 2012, 27(9): 897-905  
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用于GaN基发光二极管的蓝宝石图形衬底制备进展
崔林1, 汪桂根1, 张化宇1, 周福强1, 韩杰才1,2
1. 哈尔滨工业大学 深圳研究生院, 深圳518055
2. 哈尔滨工业大学 复合材料与结构研究所, 哈尔滨150080
汪桂根, 副教授. E-mail:wanggghit@163.com

崔林(1983-), 男, 博士研究生. E-mail:cuilin0512@gmail.com

基金:国家自然科学基金(50902028, 51172054); 深圳市基础研究计划项目(JC200903120169A); 哈尔滨工业大学科研创新基金(HIT.NSFIR.2011123);
摘要

近几年, 图形化蓝宝石衬底因其作为GaN基发光二极管外延衬底, 不仅能降低GaN外延薄膜的线位错密度, 还能提高LED的光提取效率而引起国内外许多科研机构的广泛研究兴趣. 本文综述了图形化蓝宝石衬底提高GaN基发光二极管性能的作用机理, 重点评述了目前图形化蓝宝石衬底的制备方法(湿法刻蚀、干法刻蚀、固相反应)和图形尺寸(微米图形化、纳米图形化), 分析比较了不同制备方法和图形尺寸制备蓝宝石图形衬底对GaN基发光二极管性能改善, 最后针对蓝宝石图形衬底制备存在的问题对其今后的发展方向做出了展望.

关键词: 图形化蓝宝石衬底; 氮化镓; 发光二极管; 横向外延过生长; 综述
中图分类号:TN304   文献标志码:A    文章编号:1000-324X(2012)09-0897-09
Progress in Preparation of Patterned Sapphire Substrate for GaN-based Light Emitting Diodes
CUI Lin1, WANG Gui-Gen1, ZHANG Hua-Yu1, ZHOU Fu-Qiang1, HAN Jie-Cai1,2
1. Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China
2. Center for Composite Materials, Harbin Institute of Technology, Harbin 150080, China
Fund:National Natural Science Foundation of China (50902028, 51172054); Basic Research Plan Program of Shenzhen City in 2009 (JC200903120169A); Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (HIT.NSFIR.2011123);
Abstract

GaN-based light emitting diodes are extensively used for light emitting diodes in the green to ultraviolet (UV) wavelength region and have already been widely used in traffic signals, outdoor displays, full-color displays and back lighting in liquid-crystal displays. Although GaN-based light emitting diodes are commercially available, it is still difficult to manufacture highly efficient GaN-based light emitting diodes due to the high dislocation density and the low light extraction efficiency. Patterned sapphire substrates for GaN-based light-emitting diodes have attracted much interest in recent years because it can not only reduce the threading dislocation density of epitaxial GaN films, but also improve the light extraction efficiency of GaN-based light-emitting diodes. A comprehensive review is presented on the mechanisms responding for performance enhancement of GaN-based light emitting diodes on patterned sapphire substrates, methods of preparing patterned sapphire substrates and pattern-size of patterned sapphire substrates. What is more, improved performance of GaN-based light-emitting diodes on patterned sapphire substrates prepared by different methods and pattern-size are further discussed in detail. In view of the existing problems, the prospects for future development of patterned sapphire substrates are also proposed.

Keyword: patterned sapphire substrate; GaN; LED; epitaxial lateral overgrowth; review

近年来高亮、大功率GaN基发光二极管(LED)深受重视, 广泛应用于交通信号灯[ 1, 2, 3]、LCD背光 源[ 4, 5]、固态照明[ 6, 7]、全彩显示屏等[ 8, 9, 10]. 这些商业应用要求LED在亮度和发光效率方面具有优良的性能. 目前, 由于高质量、成本合适的GaN同质衬底很难制备, 而蓝宝石具有化学和物理性质稳定、透光性好、成本合适等优点[ 11], 因此被广泛用做GaN基发光二极管外延衬底. 但GaN外延薄膜与底部的蓝宝石衬底的晶格常数失配(16%)和热膨胀系数失配(34%)很大, 导致在GaN外延薄膜产生高达109~1010cm-2的线位错密度. 高的线位错密度将影响外延薄膜的光学和电学特性, 从而使器件的可靠性和内量子效率降低. 另一方面, GaN的折射率( n=2.5)大于空气的折射率( n=1)和蓝宝石衬底的折射率( n=1.78), 因此光逃逸角锥的临界角(~23°)非常小, 造成有源层产生的光子只有~4%从表面溢出, 而大部分光子逐渐消失于内部全反射, 并转化成热能.

为了提高GaN基发光二极管的输出功率, 必须改善LED光提取效率和内部量子效率. 采用横向外延过生长(ELOG)[ 12, 13, 14, 15]、悬臂外延生长(PE)[ 16, 17, 18]等多种生长方法, 来改善GaN外延薄膜性能. ELOG技术被普遍认为是极有效的一种减少线位错密度的生长方法, 然而, ELOG技术是一种耗时工艺, 并且经常要求两步生长, 这就会引入非故意掺杂或污染. 最近报道表明, 采用图形化蓝宝石作为GaN基发光二极管外延衬底, 不仅能有效降低GaN外延薄膜的线位错密度, 还能提高LED的光提取效率[ 19, 20, 21, 22]. 另外, 图形化蓝宝石衬底上生长氮化物外延薄膜工艺, 属于单步生长工艺, 不发生任何生长中断, 具有产量高的特点, 从而引起国内外广泛关注. 目前, 图形化蓝宝石衬底已经成功地用于制备大功率GaN基发光器件, 成为国内外研究机构的主要研究课题. 本文主要对近几年用于GaN基发光二极管性能的图形化蓝宝石衬底制备进展进行综述.

1 图形化蓝宝石衬底作用机理
1.1 降低GaN外延薄膜线位错密度

蓝宝石图形衬底制备GaN外延层中位错的侧向生长过程演示, 如图1所示[ 23]. 在高温条件下, 在蓝宝石图形衬底上制备GaN外延层, 通过增大Ⅴ、Ⅲ元素比, 使GaN的横向生长速度大于纵向生长速度, 发生横向生长. 当横向生长达到一定程度后, 便会使两翼在蓝宝石相邻图形之间处聚合, 得到全覆盖的GaN外延层. 由于横向外延生长, 使蓝宝石图形上方GaN外延层线位错弯曲90°, 使线位错不能到达薄膜表面, 这样可以大大降低GaN外延薄膜的线位错密度[ 24, 25, 26, 27, 28].

图1 蓝宝石图形衬底制备GaN外延层中位错的侧向生长过程[ 19]Fig. 1 Lateral growth process of dislocation in GaN epitaxial layers on patterned sapphire substrates[ 19]

1.2 提高GaN基发光二极管发光效率

以图2制备在图形化蓝宝石衬底(虚线)和普通蓝宝石衬底(实线)上GaN基发光二极管中发光射线跟踪原理图[ 29]来进行说明, 由于GaN的折射率( n=2.5)大于空气的折射率( n=1)和蓝宝石衬底的折射率( n=1.78), 根据斯涅尔定律[ 30, 31, 32, 33], 其全内反射角则只有23°, 所以大部份从有源区所发射的光线, 将被局限于GaN内部, 这种被局限的光有可能会被较厚的基板所吸收, 转化为热能. 图2中a射线跟踪路径图, 表明普通蓝宝石衬底上GaN基发光二极管, 当入射光线的入射角大于逃逸角锥的临界角, 则会产生全内反射, 强度将逐渐被减小, 直至被完全吸收. 图2中b和c射线跟踪路径图, 表明蓝宝石图形衬底上GaN基发光二极管, 当入射光线的入射角大于逃逸角锥的临界角, 在蓝宝石图形侧面发生反射可以改变入射光线方向, 使在GaN/空气界面处入射光线的入射角小于逃逸角锥的临界角, 在GaN表面被提取出来, 从而可以大大提高GaN基发光二极管的发光效率.

图2 制备在图形化蓝宝石衬底(虚线)和普通蓝宝石衬底(实线)上GaN基发光二极管中发光射线跟踪原理图[ 29]Fig. 2 A schematic ray tracing of light for the LEDs grown on planar substrate (solid line) and patterned substrate (dash line)[ 29]

2 用于GaN基发光二极管的蓝宝石图形衬底制备进展

蓝宝石图形衬底按图形尺寸划分为: 微米图形化和纳米图形化. 蓝宝石图形衬底制备工艺大体分为两种: 刻蚀法和固相反应法. 采用刻蚀法制备图形化蓝宝石衬底, 首先在蓝宝石衬底上制备刻蚀用的掩膜, 利用干法或湿法刻蚀技术刻蚀蓝宝石, 并去掉掩膜材料. 采用固相反应法制备图形化蓝宝石衬底, 先在蓝宝石衬底溅射金属铝膜, 采用标准光刻工艺形成图形化光刻胶, 接着在图形化光刻胶上溅射金属铝膜, 剥离工艺制备图形化金属铝膜, 最后采用固相反应使图形化铝膜转化为图形化单晶Al2O3, 获得蓝宝石图形衬底. 蓝宝石图形衬底对GaN基发光二极管性能改善主要体现在两个方面: GaN外延薄膜线位错密度的降低和GaN基发光二极管的光提取效率的提高.

2.1 微米图形化蓝宝石衬底

2.1.1 湿法刻蚀

湿法刻蚀微米图形化蓝宝石衬底一般用图形化SiO2膜作为湿法刻蚀掩膜, 采用标准光刻工艺制备图形化SiO2刻蚀掩膜[ 34, 35, 36, 37, 38, 39, 40, 41, 42, 43].

Feng等[ 40]将InGaN多量子阱结构GaN基蓝光LED通过金属有机物化学气相淀积(MOVPE)制备在图形化蓝宝石衬底上. 标准的光刻工艺制备图形化SiO2掩膜, 采用BCl3和Cl2刻蚀气体, 电感耦合等离子体(ICP)技术对蓝宝石衬底进行刻蚀, 获得图形化蓝宝石衬底. 刻蚀结果表明, 刻蚀速率大约 90 nm/min, 氧化物和蓝宝石的刻蚀选择比为2, 图形化蓝宝石衬底表面粗糙度为0.28 nm. 图形化蓝宝石衬底上制备LED在465 nm电致发光峰强比普通蓝宝石衬底上制备LED的发光峰强提高. 在室温20 mA电流驱动下, 图形化蓝宝石衬底上制备的LED功率较普通蓝宝石衬底上制备的LED功率提高25%.

Gao等[ 41]为了提高LED的光输出功率, 将InGaN/GaN结构LED制备在锥体图形化蓝宝石衬底上(图3). 首先PECVD(等离子体增强化学气相沉积)淀积 SiO2膜, 采用标准光刻工艺图形化光刻胶, 接着以图形化光刻胶为掩膜, 在BOE(缓冲氧化层刻蚀液)中对SiO2膜进行湿法刻蚀, 接着用 V(H2SO4): V(H2O2)=2:1刻蚀液去除光刻胶, 以图形化SiO2掩膜, 3H2SO4:1H3PO4混合刻蚀液中对蓝宝石衬底进行湿法刻蚀, 用HF液去除SiO2掩膜. 在锥体图形化蓝宝石衬底上, 采用金属有机物化学气相淀积(MOCVD)技术制备InGaN/GaN结构LED. 在20 mA电流驱动下, 制备在锥体图形化蓝宝石衬底InGaN/GaN结构LED的光输出功率较普通蓝宝石衬底InGaN/GaN结构LED的光输出功率提高37%.

图3 湿法刻蚀图形化蓝宝石衬底图形低放大倍数(a)和高放大倍数(b)的SEM照片[ 41]Fig. 3 SEM images at different magnifications of sapphire substrate patterned by wet etching[ 41]

Yao等[ 42]采用MOCVD在图形化蓝宝石衬底外延低位错密度的GaN层. 首先PECVD淀积SiO2膜, 采用标准光刻工艺制备图形化SiO2掩膜, 在280℃, 3H2SO4:1H3PO4混合刻蚀液中对蓝宝石衬底进行湿法刻蚀, 最后用缓冲氧化层刻蚀液(BOE)去除SiO2膜, 获得图形化蓝宝石衬底. 图形化蓝宝石衬底上外延GaN薄膜(0002)面和(10 2)面XRD摇摆曲线的FWHM(半峰宽)分别为312.8 acrsec和298.08 acrsec. 图形化蓝宝石衬底上外延GaN薄膜的均方根表面粗糙度为0.233 nm.

Kissinger等[ 43]用MOCVD单步生长工艺将InGaN多量子阱结构LED制作在一个凸透镜图形化蓝宝石衬底上. 首先将作为刻蚀掩膜的SiO2膜用PECVD淀积在蓝宝石衬底上, 接着采用ICP刻蚀SiO2膜, Cl2和Ar刻蚀气体, 在蓝宝石衬底上获得凸透镜图形SiO2掩膜. 在130℃, V(H2SO4): V(H2O2)=2:1刻蚀液刻蚀蓝宝石衬底. 图形化蓝宝石衬底被清洗后在H2环境下加热到1040℃进行热刻蚀. 室温下, 在20 mA正向电流驱动下, 制备在凸透镜图形化蓝宝石衬底蓝光LED器件的输出功率和发光强度分别为69.3 μW和159.2 mcd. 在20 mA正向电流驱动下, 凸透镜图形化蓝宝石衬底上蓝光LED器件光输出功率较普通蓝宝石衬底上LED器件光输出功率提高50%. 在20 mA正向电流驱动下, 凸透镜图形化蓝宝石衬底上蓝光LED器件发射角度是161.46°, 较普通蓝宝石衬底上LED器件发射角度提高1.17倍. 另外, 凸透镜图形化蓝宝石衬底上制备GaN的HR-XRD曲线的FWHM减小, 结果表明凸透镜图形化蓝宝石衬底有效地改善了GaN晶体质量.

2.1.2 干法刻蚀

干法刻蚀微米图形化蓝宝石衬底常采用光刻 胶[ 44, 45, 46, 47, 48, 49]、金属Ni[ 50]、SiN x[ 51]、SiO2[ 52], 微米聚苯乙烯球[ 53]等作为干法刻蚀蓝宝石衬底掩膜材料.

Kim等[ 48]采用AZ9260光刻胶作为掩膜, 利用BCl3基电感耦合等离子体对φ2 inch的(0001)面蓝宝石衬底进行高速率刻蚀. 分析调节刻蚀气体组分, 电感功率, 直流偏压对蓝宝石刻蚀的影响. 实验结果表明, BCl3/Cl2组成刻蚀气体, 可以获得380 nm/min刻蚀速率. BCl3/HBr组成刻蚀气体, 可以获得各向异性刻蚀; 电感功率和直流偏压增加, 几乎线性的增加蓝宝石和光刻胶的刻蚀速率. BCl3/HBr/Ar组成刻蚀气体, 1400 W电感功率, -800 V直流偏压下, 可以获得 550 nm/min的最高刻蚀速率, 刻蚀断面夹角75°, 光刻胶的刻蚀选择比大约是0.87.

为了改善外量子效率, Lee等[ 49]通过MOCVD技术在CSPSS(圆锥体蓝宝石图形衬底)上生长了高质量InGaN/GaN膜(图4). 首先光刻胶被旋涂在(0001)面蓝宝石衬底上, 标准光刻工艺形成间距不同的方形光刻胶图形, 在140℃硬烘烤下变成圆锥型光刻胶, 接着Cl2作为刻蚀气体, 对蓝宝石衬底进行ICP刻蚀, 获得圆锥体图形化蓝宝石衬底. 在20 mA电流驱动下, 圆锥体图形化蓝宝石衬底上制备LED输出功率是16.5 mW, 相对于普通蓝宝石衬底上制备LED输出功率提高35%.

图4 圆锥体蓝宝石图形衬底制备工艺过程的SEM照片[ 49]Fig. 4 SEM images of the fabricated CSPSS process[ 49]

Hsu等[ 50]在φ2 inch (0001)面蓝宝石衬底淀积金属Ni, 接着利用标准光刻技术制备沿着<1 00>方向的金属Ni图形, Cl2/BCl3/CH2Cl2 组成刻蚀气体, 利用ICP对蓝宝石衬底进行刻蚀研究. 在电感功率600 W, 射频功率150 W, Cl2和BCl3组成刻蚀气体, 工作压力0.7 Pa下, 获得刻蚀蓝宝石最高速率 100 nm/min. 调节刻蚀工艺条件可以获得高各向异性刻蚀轮廓粗糙的边墙, 这种结构可以减少线位错, 并且增加光输出功率和器件寿命.

Chang等[ 51]将460 nm InGaN基LED制备在次微米间距半球形图形化蓝宝石衬底上. 首先PECVD淀积SiN x膜, 利用紫外光刻技术在SiN x膜

上形成图形聚甲基丙烯酸甲酯(PMMA), 然后通过热回流技术减小PMMA图形间的间距, 形成半球形图形PMMA, 通过反应离子刻蚀技术, 使光刻胶图形转移到SiN x膜上, BCl3刻蚀气体, 用ICP对蓝宝石进行干法刻蚀. 分别在次微米间距半球形图形化蓝宝石衬底和普通蓝宝石衬底上MOCVD外延GaN层( )面摇摆曲线的FWHM(半峰宽)是 262 arcsec 和480 arcsec. 在20 mA电流驱动下, 在次微米间距半球形图形化蓝宝石衬底和普通蓝宝石衬底上LED的光输出功率分别为5.86和4.05 mW, 表明次微米间距半球形图形化蓝宝石衬底上LED的光输出功率较普通蓝宝石衬底上LED的光输出功率提高44%.

Wuu等[ 52]将近紫外氮化物基LED分别制备在湿法刻蚀获得的锥体图形化蓝宝石衬底和干法刻蚀获得的柱体图形化蓝宝石衬底. PECVD淀积SiO2膜, 采用标准光刻工艺图形化SiO2膜. 在280℃下, V(H2SO4): V(H3PO4)=3:1刻蚀液中, 以图形SiO2膜为掩膜对蓝宝石衬底进行湿法刻蚀, 获得锥体图形化蓝宝石衬底. 以图形SiO2膜为掩膜采用Cl2和BCl3刻蚀气体电感耦合等离子体刻蚀技术对蓝宝石衬底进行干法刻蚀, 获得柱体图形化蓝宝石衬底. 制备在锥体图形化、柱体图形化、普通蓝宝石衬底上 403 nm InGaN基LED的光输出功率分别是8.82、7.85和6.88 mW, 表明锥体图形化蓝宝石衬底上LED的光输出功率较普通蓝宝石衬底上LED的光输出功率提高25%.

Soh等[ 53]将InGaN/GaN量子阱的蓝光LED结构制备在微米半球形图形化蓝宝石衬底. 微米半球形图形化蓝宝石衬底的制备工艺: 首先单分子层的PS(聚苯乙烯)球被旋涂在蓝宝石衬底上, 接着用PS球做刻蚀掩膜, 采用BCl3和Cl2刻蚀气体ICP(电感耦合等离子体)技术对蓝宝石衬底进行刻蚀. 制备在微米半球形图形化蓝宝石衬底和普通蓝宝石衬底的InGaN/GaN量子阱的内量子效率分别为56%和50%.

2.2 纳米图形化蓝宝石衬底

2.2.1 湿法刻蚀

湿法刻蚀纳米图形化蓝宝石衬底一般用图形化SiO2作为湿法刻蚀掩膜, SiO2纳米球[ 54]和金属Ni纳米岛[ 55]等材料制备图形化SiO2刻蚀掩膜.

Chan等[ 54]为了提高GaN基LED输出功率, GaN基LED被制备在纳米图形化蓝宝石衬底上. 首先PECVD淀积SiO2膜, 通过提拉技术在SiO2膜制备φ750 nm SiO2单层纳米球, SiO2纳米球作为掩膜采用BCl3和Ar刻蚀气体在ICP系统中对SiO2膜进行刻蚀, 以图形化SiO2膜为掩膜, 用H2SO4和H3PO4刻蚀液对蓝宝石衬底进行湿法刻蚀, 最后用HF溶液去除SiO2. 纳米图形化蓝宝石衬底制备LED的输出功率较普通蓝宝石衬底制备LED的输出功率提高76%. 纳米图形化蓝宝石衬底上制备GaN(0002)面和(10 2)面的 ω扫面曲线的FWHM减小, 表明纳米图形化蓝宝石衬底改善GaN晶体质量.

Yan等[ 55]采用化学湿法刻蚀工艺制备了纳米图形化蓝宝石衬底(图5). PECVD淀积SiO2膜, 接着 15 nm的金属Ni层通过电子束蒸发在SiO2膜, 接着金属Ni层被在850℃下快速热退火(RAT)1 min, 获得金属Ni纳米岛. 金属Ni纳米岛作为SiO2膜的掩膜, C4F8刻蚀气体对SiO2膜进行ICP刻蚀,接着以SiO2掩膜, 在高温的 V(H2SO4): V(H3PO4)=3:1化学刻蚀液中刻蚀蓝宝石, 残余的SiO2掩膜用HF溶液去除. 在20 mA电流驱动下, 纳米图形化蓝宝石衬底上制备LED和普通宝石衬底上制备LED的光输出功率分别为13.78和9.28 mW. 因此, 纳米图形化蓝宝石衬底上制备LED的输出功率较普通宝石衬底上制备LED的输出功率提高了大约46%.

图5 化学刻蚀纳米图形化蓝宝石衬底表面形貌低倍放大(a)和高倍放大(b)的SEM照片[ 55]Fig. 5 SEM surface morphologies of nano-patterned sapphire substrate prepared by chemical wet etching[ 55](a) Pyramidal patterns in large-scale observed at a low magnification; (b) Pyramidal patterns observed at a relatively high magnification

2.2.2 干法刻蚀

干法刻蚀微米图形化蓝宝石衬底常采用光刻 胶[ 56, 57]、金属Al[ 58]、纳米聚苯乙烯球[ 59, 60]、聚合 物[ 61]、SiO2[ 29]、SiO2纳米球[ 62]等材料作为干法刻蚀掩膜.

Lee等[ 56]将半微米周期的光子晶体模式插入到GaN外延层和蓝宝石衬底之间, 来提高GaN基LED器件的光提取效率(图6). 一个二维600 nm晶格周期的正方晶格柱状阵列被直接制备在蓝宝石衬底, 通过全息光刻和随后的ICP刻蚀. 一个标准的GaN基LED异质结构被制备在纳米蓝宝石图形衬底上. 在20 mA电流驱动下, 光子晶体图形化蓝宝石衬底上制备LED的输出功率较普通宝石衬底上制备LED的输出功率提高了大约40%.

图6 二维光子晶体表面模式蓝宝石衬底的SEM照片: (a)平面图(b)侧面图[ 56]Fig. 6 SEM images of sapphire substrate with a 2D-PC surface pattern, (a) plan view and (b) side view[ 56]

Akihiro等[ 57]为了提高光提取效率引入一种蛾眼结构图形化蓝宝石衬底, 这种结构由周期性的锥体组成, 并且周期性锥体的间距是光波长的大小. 蛾眼结构图形蓝宝石衬底是通过低能量电子束投影光刻(LEEPL)和BCl3气体干法刻蚀制备. 利用MOVPE技术450 nm的GaInN/GaN 结构LED被分别制备在普通蓝宝石衬底和蛾眼结构图形化蓝宝石衬底上. 蛾眼结构图形化蓝宝石衬底上LED室温PL谱的发射峰强是普通蓝宝石衬底上LED室温PL谱的发射峰强的1.6倍. 在50 mA电流注入下, 蛾眼结构图形化蓝宝石衬底上LED的输出功率是普通蓝宝石衬底上LED的输出功率的3.6倍.

Lee等[ 58]应用一种新型金属接触式压印光刻技术制备图形化蓝宝石衬底. 首先通过压印技术将图形化金属铝膜图形从Si模板直接转移到蓝宝石衬底上, 接着用图形化金属铝膜做掩膜, 采用ICP对蓝宝石衬底进行刻蚀. 接触式光刻技术优点在于能够直接、容易、大面积的制备亚微米或者纳米级图形化蓝宝石衬底, 并且因为金属膜的高刻蚀选择比, 所以可以获得较深的刻蚀深度. 红光AlGaInP基 LED制备在这种通过金属接触式压印光刻技术获得图形直径400 nm蓝宝石图形衬底上的光提取效率较制备在普通蓝宝石衬底上的光提取效率提高23%.

Chen等[ 60]在通过纳米球刻蚀技术制备纳米图形化蓝宝石衬底上制备了450 nm发射波长的GaN基LED(图7). 首先旋涂直径500 nm的聚苯乙烯球刻蚀掩膜, 采用BCl3 和 Cl2刻蚀气体ICP技术对蓝宝石衬底进行刻蚀. 在20 mA电流驱动下, 纳米图形化蓝宝石衬底制备LED的输出功率较普通蓝宝石衬底和微米图形化蓝宝石衬底制备LED的输出功率分别提高1.3和1.1倍.

图7 六角形紧密堆积单层聚苯乙烯球(a)和图形化蓝宝石(b)衬底的SEM照片[ 60]Fig. 7 SEM images of (a) a hexagonal close-packed monolayer of polystyrene spheres with 500 nm diameter on top of the sapphire substrate and (b) NPSS[ 60]

Huang等[ 61]将GaN基LED制备在纳米压印刻蚀制备的纳米孔图形化蓝宝石衬底. 首先旋凃聚合物层, 把图形化模板放在聚合物层上面, 在模板上施加压力, 加热蓝宝石衬底至聚合物的玻璃转化温度, 接着蓝宝石衬底和模板冷却到室温, 剥离模板图形化聚合物作为掩膜, 采用BCl3和Ar刻蚀气体ICP技术对蓝宝石衬底进行刻蚀, 最终用O2刻蚀气体RIE(反应离子刻蚀)技术对聚合物进行去除, 获得图形化蓝宝石衬底. 在20 mA电流驱动下, 纳米压印刻蚀制备的纳米孔图形化蓝宝石衬底上制备InGaN/GaN多量子阱LED的光输出功率较普通蓝宝石衬底上制备InGaN/GaN多量子阱LED的光输出功率提高1.3倍. 在20 mA电流驱动下, 纳米压印刻蚀制备的纳米孔图形化蓝宝石衬底上制备InGaN/GaN多量子阱LED的插座效率较普通蓝宝石衬底上制备InGaN/GaN多量子阱LED的插座效率提高30%.

Gao等[ 29]为了提高GaN基LED的性能, 采用干法刻蚀工艺制备纳米图形化蓝宝石衬底. PECVD淀积SiO2膜, 接着金属Ni层通过电子束蒸发技术被沉积在SiO2膜, 接着金属Ni层被在850℃下快速热退火(RAT)1 min, 获得金属Ni纳米岛. 金属Ni纳米岛作为SiO2膜的掩膜, C4F8刻蚀气体对SiO2膜进行电感耦合等离子体刻蚀. 最终以SiO2纳米岛作为蓝宝石衬底掩膜, 采用BCl3 和 Cl2刻蚀气体ICP技术对蓝宝石衬底进行刻蚀, 用HF溶液去除残余的SiO2掩膜. 测试结果表明, 在相同电流驱动下, 制备在纳米图形化蓝宝石衬底LED的光输出功率和插座效率较普通蓝宝石衬底LED提高很多.

2.2.3 固相反应

Park等[ 63]采用固相反应技术制备蓝宝石图形衬底. 首先在蓝宝石衬底上磁控溅射一层~10 nm铝膜, 然后采用电子束光刻技术制备出图形PMMA(聚甲基丙烯酸甲酯)掩膜, 再次溅射铝膜, 剥离铝膜, 在蓝宝石上得到周期排列的图形化铝膜, 最后两步热退火固相反应, 制备出纳米级别的蓝宝石图形衬底.

Ee等[ 64]在固相反应制备的蓝宝石图形衬底上采用金属有机物化学气相淀积(MOVPE)外延GaN,并对蓝宝石图形衬底上LED性能进行评价(图8). 固相反应制备的蓝宝石图形衬底工艺: 采用电子束光刻技术制备图形化电子束光刻胶, 在图形化电子束光刻胶上淀积100 nm左右金属铝膜, 剥离工艺制备图形化金属铝膜, 图形化金属铝膜低温热处理转化为图形化多晶Al2O3, 接着图形化多晶Al2O3高温热处理转化为单晶Al2O3, 获得纳米图形化蓝宝石衬底. 在固相反应法图形化蓝宝石衬底上制备InGaN量子阱结构LED的输出功率较普通蓝宝石衬底上制备InGaN量子阱结构LED的输出功率提高24%.

图8 铝纳米结构矩阵在450℃氧化(a)和在1200℃退火转变为单晶Al2O3(b)的SEM照片[ 64]Fig. 8 SEM images of (a) aluminum nanostructure array after oxidation at 450℃ and (b) epitaxial conversion to single crystal Al2O3 after annealed at 1200℃[ 64]

3 影响蓝宝石图形衬底作用因素

蓝宝石图形衬底对GaN基发光二极管性能改善作用主要体现在两个方面: GaN外延薄膜线位错密度的降低和GaN基发光二极管的光提取效率的提高. 从用于GaN基发光二极管的蓝宝石图形衬底制备研究进展来看, 蓝宝石图形衬底作用影响因素主要包括两点: 图形制备方法和图形尺寸.

3.1 制备方法

目前蓝宝石图形衬底制备方法主要分为: 湿法刻蚀、干法刻蚀、固相反应法.

湿法刻蚀制备蓝宝石图形衬底通常采用的刻蚀溶液有H3PO4、H2SO4等, 刻蚀温度一般在300~ 500℃之间, 通过调节溶液混合比例、溶液温度、刻蚀时间等来调节刻蚀速率与深度. 但是, 湿法刻蚀一般是各向同性刻蚀, 把图形化掩膜图形转移到蓝宝石衬底上的同时, 刻蚀不仅纵向进行, 也会向着横向进行, 会使图形失真, 甚至使线宽失真. 干法刻蚀技术是国际上普遍采用的制备蓝宝石图形衬底的方法. ICP刻蚀技术由于能够控制等离子体密度和轰击能量, 适于辉光放电时自动匹配网络等优点而广泛应用于制备蓝宝石图形衬底. 一般以BCl3或Cl2或两者的混合物作为化学反应气体, 以HBr、Ar等作为物理性离子轰击的辅助气体, 通过控制工作压强、反应气体流量、磁场强度和直流偏压等参数, 可以控制刻蚀速. 但是, 干法刻蚀容易对蓝宝石基片表面, 特别是台面边缘部位造成一定的污染和损伤, 不利于外延层晶体质量的进一步提高. 固相反应制备蓝宝石图形衬底是首先在蓝宝石上制备周期图形化金属铝膜, 由于金属铝膜熔点为660℃, 所以需要对图形化金属铝膜进行两步热处理实现固相反应, 获得蓝宝石图形衬底. 固相反应整套工艺不需要对蓝宝石进行刻蚀, 避免了刻蚀技术制备蓝宝石衬底的缺点.

3.2 图形尺寸

纳米图形蓝宝石衬底相对微米图形蓝宝石衬底而言, 不仅其图案尺寸更小, 能够在有限区域内更有效地降低外延位错密度, 提高外延晶体质量, 同时可增加光线反射路径, 提高光线提取效率. Gao等[ 35]采用相同工艺将InGaN/GaN结构LED分别制备在普通蓝宝石衬底、微米图形蓝宝石衬底、纳米图形蓝宝石衬底上. 其微米图形蓝宝石衬底和纳米图形蓝宝石衬底采用湿法刻蚀技术制备. 测试结果表明, 在20 mA电流驱动下, 纳米图形蓝宝石衬底和微米图形蓝宝石衬底上InGaN/GaN结构LED的输出功率较普通蓝宝石衬底上InGaN/GaN结构LED的输出功率分别提高48%和29%. Su等[ 65]采用相同工艺将InGaN/GaN结构LED分别制备在普通蓝宝石衬底、微米图形蓝宝石衬底、纳米图形蓝宝石衬底上. 其微米图形蓝宝石衬底和纳米图形蓝宝石衬底采用干法刻蚀技术制备. 测试结果表明, 在20 mA电流驱动下, 纳米图形蓝宝石衬底、2 μm图形蓝宝石衬底、3 μm图形蓝宝石衬底上InGaN/GaN结构LED的输出功率较普通蓝宝石衬底上InGaN/GaN结构LED的输出功率分别提高30%、20%、17%(图9). 由此可见, 纳米图形化蓝宝石衬底较微米图形化蓝宝石衬底更进一步提高GaN基发光二极管的性能.

图9 非图形化蓝宝石衬底LED(方形)、微米蓝宝石图形衬底LED(实三角形)、纳米蓝宝石图形衬底LED(空三角形)的光输出功率与注入电流关系曲线[ 65]Fig. 9 Light output power in relation to injection current for the PL-LED (squares), MPSS-LED(filled triangles), and NPSS- LED (open triangles)[ 65]

4 展望

理论和实践已经证明, 用于GaN基发光二极管的图形化蓝宝石衬底, 不仅能降低GaN外延薄膜的线位错密度, 还能提高LED的光提取效率. 一般采用湿法或干法刻蚀技术制备蓝宝石图形衬底. 湿法刻蚀一般是各向同性刻蚀, 把图形化掩膜图形转移到蓝宝石衬底上的同时, 刻蚀不仅纵向进行, 也会向着横向进行, 会使图形失真, 甚至使线宽失真. 而干法刻蚀容易对蓝宝石基片表面, 特别是台面边缘部位, 造成一定的污染和损伤, 不利于GaN外延层晶体质量的进一步提高. 固相反应制备蓝宝石图形衬底, 整套工艺不需要对蓝宝石进行刻蚀, 避免了刻蚀技术制备蓝宝石衬底的缺点. 纳米图形化蓝宝石衬底较微米图形化蓝宝石衬底更进一步提高GaN基发光二极管的性能. 结合固相反应技术和纳米图形化技术可以很好的克服目前制备图形化蓝宝石衬底面临的问题. 然而, 目前固相反应纳米图形化蓝宝石衬底制备要求用纳米图形化光刻胶作为掩膜, 采用电子束光刻技术制备纳米图形化光刻胶掩膜, 工艺成本高、效率低不适合商业化. 如何大量、低成本制备出纳米图形化掩膜仍是未来研究固相反应纳米图形化蓝宝石衬底的一个重点. 所以许多新的纳米图形化蓝宝石衬底制备方法和技术值得进一步探索.

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