标题: 湿蚀刻蓝宝石图形化基板形貌演化对氮化镓磊晶的影响
Effect of the Evolution of Wet-etched Patterned Sapphire on the GaN Epitaxial Behavior
作者: 陈俞中
Chen, Yu-Chung
吴耀铨
材料科学与工程学系所
关键字: 发光二极体;图形化蓝宝石基板;湿蚀刻;氮化镓;Light-emitting diode;Patterned sapphire substrate;Wet etching;Gallium nitride
公开日期: 2013
摘要: 发光二极体 (LED) 因为其高效率、寿命长、小尺寸、环保等优点,扮演下一代照明重要的角色。很多技术已被研发用来改善氮化镓LED的内部量子效率 (IQE) 和外部量子效率 (EQE),像是侧向磊晶成长、表面粗化、镜面反射层和图形化蓝宝石基板 (PSS)。近期,由于PSS技术可以大量生产受到瞩目。此外,该技术也可让LED的IQE以及LEE同时提升。制作PSS可使用干蚀刻与湿蚀刻两种制程。在干蚀刻部分,因为基板表面受到离子轰击的伤害,容易产生贯穿差排而影响发光层的晶粒品质。但在湿蚀刻方面,它不会有这现象,此外还可批量制作以降低制造成本。进行湿蚀刻时,是以二氧化矽当作蚀刻遮罩覆盖在蓝宝石基板上,而蚀刻液是硫酸与磷酸的混合液。在蚀刻后,会出现许多晶面。这些晶面在其他文献中被定义成类n面、类r面和类m,r,a的混合面。此外在磊晶后,除了wurtzite结构的氮化镓,zinc-blende结构的氮化镓也会在这些图形化蓝宝石基板上成长。因此在这论文中,将以点状、线状以及特殊方向三角形图案遮罩蚀刻后的蓝宝石晶面做分析,探讨这些晶面是如何演变以及氮化镓在这些晶面上的磊晶状况。在点状图形化蓝宝石基板部分,圆形的二氧化矽被制作成蚀刻遮罩。蚀刻后会出现许多晶面。一个称作两阶段湿蚀刻的制程被用来探讨这些晶面的演变。蓝宝石基板以这些圆形点状的二氧化矽当作遮罩,接着以摄氏270度硫酸与磷酸的混合液体当作蚀刻液进行不同长度的蚀刻。结果发现当二氧化矽遮罩还附着在基板上时,也就是第一阶段的蚀刻,图形化蓝宝石基板会出现六面平顶锥的表面形貌,这六个面为6B面{1 3 4 ̅ 7}。当二氧化矽被移除后的第二阶段蚀刻,三个3T面 {1 1 ̅ 0 5} 首先会出现在图形的顶端,当蚀刻持续进行,六个L6B面 {1 4 5 ̅ 30} 会出现在图案底端。若第二阶段的蚀刻到达三分钟后,三个L3T面 {1 1 ̅ 0 10} 会出现在图案顶端。最后到达五分钟时,大部份的图形几乎已消失在蓝宝石基板上。而6B、3T、L6B和L3T面在此条件的蚀刻液中,蚀刻率分别为每分钟0、0.22、0.09和0.20微米。在线状图形化蓝宝石中,长方形条状二氧化矽图形被用来当作蚀刻遮罩,而倾斜角度有0、15和30度三种。蚀刻后,图形共会出现A、B、B1、B2、D1、D2和E这七类面。其中A、B和E这三类是属于表面平滑的面,面指数分别为 {1 3 4 ̅ 7}、 {1 1 ̅ 0 4} 和 {1 2 3 ̅ 5}。其中A面与点状图形化蓝宝石基板出现的6B面是相同的。另一类为B1、B2、D1和D2面,其表面皆呈粗糙的线条状,似乎是A、B和E面的交界所构成。而线状遮罩的蓝宝石在经过第二阶段的蚀刻后,结果与点状遮罩相同,会依序出现3T、L6B和L3T面。最后将观察氮化镓在点状、线状与特定方向三角状图形化蓝宝石上的磊晶行为。发现俱zinc-blende氮化镓是从E面开始成核成长,而非A (或6B) 面。其他不同长晶方向或结构的氮化镓也会出现在3T、L6B和L3T面上,但其细节仍不明。
Light-emitting diodes (LEDs) are expected to play an important role in next-generation light source due to their advantages of high efficiency, long life, small size, environmental protection, various colors and wide applications. Many techniques have been developed for improving GaN-based LEDs internal quantum efficiency (IQE) and light extraction efficiency (LEE), such as epitaxial lateral overgrowth, surface roughing, metal mirror reflect layer and patterned sapphire substrate (PSS). Currently, the PSS technique has attracted much attention for its high production yield. Besides, using the PSS technique can improve both IQE and LEE. Two kinds of etching methods have been used to fabricate PSS : (1) dry etching and (2) wet etching. In dry etching, the ion bombardment caused damages on PSS surface and resulted in an increased of threading dislocations propagating through the GaN epitaxial layers. On the other hand, wet etching did not have this ion damage problem. Besides, wet etching method can also reduce the fabrication cost of PSS and simplify the process. In wet etching, the sapphire substrate covered with SiO2 hard mask is usually etched by a mixed solution of hot H2SO4 and H3PO4. After etching, several etched facets were exposed. These facets have been identified differently as n-like plane, r-like plane, and mixture of m-, r- and a-like plane. It has been found beside normal wurtzite GaN, zinc-blende GaN has also on these planes of PSS after epitaxy. Therefore, in this dissertation, the formation of planes after etching and the properties of GaN epitaxy were investigated by using dot-, line- and specific triangular PSS.In dot-PSS, a circular-shaped SiO2 pattern as etched mask. Several etched facets were exposed on the dot-PSS after etching. In this study, a two-step wet etching process was employed to investigate the evolution of these facets on the dot-PSS. The substrate with the SiO2 mask was immersed in a H2SO4 and H3PO4 mixed etchant at 270˚ for various durations. When the SiO2 mask remained on the top c-plane, PSS has hexagonal pyramid structures covered with six 6B facets {1 3 4 ̅ 7}. When the SiO2 mask was etched away, three 3T facets {1 1 ̅ 0 5} were found on the top. With increase in etching time, other six L6B facets {1 4 5 ̅ 30} appeared on the bottom of pyramids. When the second-step etching time reached 3 min, another three L3T facets {1 1 ̅ 0 10} were found on the top. Finally, when the etching time reached around 5 min, most of pyramids on the PSS disappeared. The etching rates of 6B, 3T, L6B and L3T were about 0, 0.22, 0.09 and 0.20 μm/min, respectively.In line-PSS, the rectangle-shaped SiO2 hard masks with three orientations (0˚, 15˚ and 30˚) were employed to find the facets (A, B, B1, B2, D1, D2 and E) that appearing after etching. The surfaces of A, B and E-facets were smooth. Their plane indexes were {1 3 4 ̅ 7}, {1 1 ̅ 0 4} and {1 2 3 ̅ 5}, respectively. The A and 6B facet were the same. On the other hand, the surfaces of B1, B2, D1 and D2-facets were not smooth, with some ambiguous stripes, which seem to be the border of A, B and E-facets. The 3T, L6B and L3T facets appeared in order after the SiO2 mask was removed like the dot-PSS.At last, dot-PSS, line-PSS and specific triangular PSS were used to investigate the GaN epitaxial behavior. It was found that most of the growth of zinc-blende GaN was initiated not from A(6B) but E-facets. Others GaN crystalline types were observed on 3T, L6B and L3T-facets, but the structures were not still clearly.
URI: http://140.113.39.130/cdrfb3/record/nctu/#GT079718508
http://hdl.handle.net/11536/74133
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