濕蝕刻藍寶石圖形化基板形貌演化對氮化鎵磊晶的影響

Abstract

發光二極體 (LED) 因為其高效率、壽命長、小尺寸、環保等優點，扮演下一代照明重要的角色。很多技術已被研發用來改善氮化鎵LED的內部量子效率 (IQE) 和外部量子效率 (EQE)，像是側向磊晶成長、表面粗化、鏡面反射層和圖形化藍寶石基板 (PSS)。近期，由於PSS技術可以大量生產受到矚目。此外，該技術也可讓LED的IQE以及LEE同時提升。製作PSS可使用乾蝕刻與濕蝕刻兩種製程。在乾蝕刻部分，因為基板表面受到離子轟擊的傷害，容易產生貫穿差排而影響發光層的晶粒品質。但在濕蝕刻方面，它不會有這現象，此外還可批量製作以降低製造成本。進行濕蝕刻時，是以二氧化矽當作蝕刻遮罩覆蓋在藍寶石基板上，而蝕刻液是硫酸與磷酸的混合液。在蝕刻後，會出現許多晶面。這些晶面在其他文獻中被定義成類n面、類r面和類m,r,a的混合面。此外在磊晶後，除了wurtzite結構的氮化鎵，zinc-blende結構的氮化鎵也會在這些圖形化藍寶石基板上成長。因此在這論文中，將以點狀、線狀以及特殊方向三角形圖案遮罩蝕刻後的藍寶石晶面做分析，探討這些晶面是如何演變以及氮化鎵在這些晶面上的磊晶狀況。在點狀圖形化藍寶石基板部分，圓形的二氧化矽被製作成蝕刻遮罩。蝕刻後會出現許多晶面。一個稱作兩階段濕蝕刻的製程被用來探討這些晶面的演變。藍寶石基板以這些圓形點狀的二氧化矽當作遮罩，接著以攝氏270度硫酸與磷酸的混合液體當作蝕刻液進行不同長度的蝕刻。結果發現當二氧化矽遮罩還附著在基板上時，也就是第一階段的蝕刻，圖形化藍寶石基板會出現六面平頂錐的表面形貌，這六個面為6B面{1 3 4 ̅ 7}。當二氧化矽被移除後的第二階段蝕刻，三個3T面 {1 1 ̅ 0 5} 首先會出現在圖形的頂端，當蝕刻持續進行，六個L6B面 {1 4 5 ̅ 30} 會出現在圖案底端。若第二階段的蝕刻到達三分鐘後，三個L3T面 {1 1 ̅ 0 10} 會出現在圖案頂端。最後到達五分鐘時，大部份的圖形幾乎已消失在藍寶石基板上。而6B、3T、L6B和L3T面在此條件的蝕刻液中，蝕刻率分別為每分鐘0、0.22、0.09和0.20微米。在線狀圖形化藍寶石中，長方形條狀二氧化矽圖形被用來當作蝕刻遮罩，而傾斜角度有0、15和30度三種。蝕刻後，圖形共會出現A、B、B1、B2、D1、D2和E這七類面。其中A、B和E這三類是屬於表面平滑的面，面指數分別為 {1 3 4 ̅ 7}、 {1 1 ̅ 0 4} 和 {1 2 3 ̅ 5}。其中A面與點狀圖形化藍寶石基板出現的6B面是相同的。另一類為B1、B2、D1和D2面，其表面皆呈粗糙的線條狀，似乎是A、B和E面的交界所構成。而線狀遮罩的藍寶石在經過第二階段的蝕刻後，結果與點狀遮罩相同，會依序出現3T、L6B和L3T面。最後將觀察氮化鎵在點狀、線狀與特定方向三角狀圖形化藍寶石上的磊晶行為。發現俱zinc-blende氮化鎵是從E面開始成核成長，而非A (或6B) 面。其他不同長晶方向或結構的氮化鎵也會出現在3T、L6B和L3T面上，但其細節仍不明。

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.