第二章、 文獻回顧
2.3 氧化鋅磊晶薄膜
由於現階段 ZnO 單晶仍不普及，因而，大多數 ZnO 磊晶皆藉由 異質磊晶的方式，於其他較易於取得的基板上進行磊晶成長。在極性 面(0001)磊晶方面，ZnO 磊晶成長於矽[2.64]、藍寶石[2.68]、碳化矽 [2.69]、鉮化鎵[2.70, 2.71]、氟化鈣[2.72]、ScAlMgO4[2.73]、LiNbO3[2.74]
等基板皆已被實現，其中，由於藍寶石成本相對便宜，且化學性質穩 定，最常被使用；儘管藍寶石與 ZnO 的晶格不匹配高達 18.3%，但是 經過眾多團隊多年來的研究，高品質的極性面 ZnO 磊晶薄膜已可被 實現[2.75]，然而，非極性面的 ZnO 磊晶技術卻是進十年來才開始受 到重視。
所謂非極性 ZnO 磊晶膜，包含常見的棱柱面(1120)、(1100)，以及 其他(hki0)平面，只要其其極性方向(c 軸)平躺於試片表面，就可稱之 為非極性面 ZnO 磊晶。
不若極性面磊晶面的成長，由於 ZnO 為六方晶結構對稱性的關 係，當其 c 軸平躺於試片表面，必然會存在一般極性面磊晶不須考慮 的異向性問題，包含著結構、擴散、熱膨脹係數等，尤其是與基板表 面的晶格匹配異向性，對於非極性面 ZnO 來說，這是其磊晶成長需
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克服的首要課題。
目前已被實現的非極性面 ZnO 磊晶薄膜與其基板的磊晶關係整 理於表 2-4；由於晶體對稱性的關係，ZnO 的非極性面具有較低的對 稱性，若成長於較高對稱性的基板表面，例如 SrTiO3 (001)[2.66, 2.77]
與 MgO (001)[2.78, 2.79]，將會存在超過一種成長方向，形成晶域，
因而使 ZnO 磊晶膜出現產生多餘的晶界[2.80]。
表 2-4 非極性 ZnO 磊晶基板與磊晶關係
ZnO Plane Substrate Epitaxial relationship Ref.
)
LiAlO2[2.81 – 2.83]、LiGaO2[2.84]分別為正方晶與斜方晶，由於 其較低的晶體對稱性，且有著相對低的晶格不匹配度(各方向皆小於 4%)，故可成長出高品質(XRC ~0.16)的非極性 ZnO 磊晶膜；然而由
30
於基板中鋰原子於高溫的不穩定性，當製程溫度超過 600C 時，將會 產生鋰原子擴散至 ZnO 磊晶膜的現象，進而影響磊晶膜本身的特性。
LaAlO3為菱方晶，但是若以其內部金屬離子描述其晶格，其晶 體可簡化為 a = b = c， = = = 90.12的結構，故一般亦常以立方
晶的角度對其進行詮釋(表 2-4 中 LaAlO3即為採用 pseudo-cubic, pc 座 標)。與 SrTiO3、MgO 相同，以其(001)PC進行成長，亦存在 ZnO 可 沿著多個方向成長的問題[2.80, 2.85]。然而，藉由基板工程，採用(112) 甚至(114)晶面進行成長，由於其基板表面的對稱性從四軸對稱降低為 二軸對稱，如此將可侷限 ZnO 沿著單一方向成長[2.86 – 2.88]
。
除了上述基板外，由於菱方晶的晶體對稱性較低，其(1100)
、
(1012)等亦可成功成長出單一成長方向的 ZnO 磊晶膜[2.89, 2.90]，其中，又 以藍寶石基板(sapphire, Al2O3)最為常見。
藍寶石結構如圖 2-10 所示，為菱方結構，空間群為 R3̅c，晶格 常數 a = 5.132Å，
= 55.3(綠色座標軸)，若以擬立方晶型(pseudocubic)
描述其結構，其鋁原子可視為 a = 3.5 Å ， = 85.7(藍色座標軸)的 morphological rhombohedral 結構[2.91]。一般為了方便，大多將其置 入六方晶格內，以 a = 4.76 Å ，c = 12.99 Å (黑色座標軸)的六方晶結構31
描述之[2.92]，本研究亦以六方晶格座標描述藍寶石之結構。
圖 2-10 藍寶石結構示意圖，綠色座標為菱方晶座標軸，藍色為擬立 方晶座標軸，黑色為六方晶格座標軸；紅色面為其 R 面。
儘管可藉由六方晶格座標對藍寶石之結構進行描述，但是仍需注 意，其結構為兩個 Al-O 八面體組成的多面體，以 ABCABC 的順序進 行堆疊(如圖 2-11 所示)，故其沿著最密堆積面之對稱性仍為 3-fold，
並非六方晶的 6-fold。
32
圖 2-11 藍寶石基板堆疊示意圖，(a)為沿著最密堆積面之俯視圖，(b) 為沿著最密堆積方向之側視圖。
33
由於藍寶石基板相對便宜、化學性質穩定，目前藍寶石基板已成 為一般極性面發光二極體元件的主要磊晶基板。在藍寶石常見、穩定 的低指數面中，C 面(0001)與 A 面(1120)可用於成長極性面(c 面)ZnO 磊晶，而 M 面(1100)與 R 面(1012)則分別適合用於成長 m 面與 A 面 ZnO 磊晶[2.17, 2-90]；其中又以 R 面成長非極性 A 面 ZnO 磊晶膜最為常 見。ZnOA 面與藍寶石 R 面的表面基本週期結構示意於圖 2-12(a)與(b)，
其磊晶關係為
[ 0001 ]
ZnO//[ 1 01 1 ]
sapphire、 [ 1 100 ]
ZnO//[ 1 2 10 ]
sapphire，除了表面週期 外，若進一步由圖 2-12(c)與(d)的 ZnOA 面與藍寶石 R 面之側視圖觀 察，可發現 ZnOA 面與藍寶石 R 面皆為 1-fold，此特性不僅可避免 ZnO 沿著多個方向成長，甚至可進一步控制 ZnO 的極性方向[2.93]。藉由 R 面藍寶石基板，單一成長方向之 A 面 ZnO 已可藉由 PLD、
sputtering、PAMBE、MOCVD 等製程方式所實現[2.90, 2.94 – 2.97]。
34
圖 2-12 ZnO 與藍寶石基板磊晶關係示意圖，(a) ZnOA 面基本週期結 構，(b)藍寶石 R 面表面基本週期結構，(c)A 面 ZnO 側視圖，(d)R 面
藍寶石側視圖。其中灰色為 Zn、藍色為 Al、紅色為 O。
35
36
排明顯的少了許多[2.17, 2.102, 2.103]，但是值得注意的是，不同於 傳統極性面磊晶膜，非極性面磊晶膜往往存在著顯著的基面疊差 [2.17, 2.103, 2.104]。
儘管過去十年來，已有許多非極性 ZnO 的相關研究，A 面 ZnO 亦已藉由 PLD、sputtering、PAMBE、MOCVD 等製程方式成長於 R 面藍寶石基板[2.90, 2.94 – 2.97]，然而，儘管已有許多研究，ZnO 薄 膜仍因過多的缺陷致使無法被有效利用。在大多數磊晶製程中，緩衝 層的利用，往往可以有效提昇其結晶品質，然而，在 A 面 ZnO 成長 於 R 面藍寶石基板的研究中，即便已藉由高品質 GaN 磊晶層或極平
37
坦的 ZnO:Co 作為緩衝層，對其結晶品質的提昇仍相當有限，甚至反 而造成結晶性的下降[2.106, 2.107]。由於晶體內的缺陷與其晶體的成 長行為息息相關，然而，非極性 ZnO 的研究卻鮮少從晶體成長的過 程進行深入研究。韓國 Lee 等人曾藉由 TEM 沿著[0001]ZnO觀察 ZnO 於 R 面藍寶石上初始成長的行為，並推斷 ZnO 於 R 面藍寶石上乃藉 由 VW 模式進行成長[2.102]，然而法國 Chauveau 等人亦於實驗中觀 察到 ZnO 於 R 面藍寶石上之成長行為並非如一般於 C 面上成長之 VW 模式[2.108]。本研究將藉由脈衝雷射蒸鍍系統搭配臨場反射式高 能電子繞射儀進行即時的磊晶製程監控，並藉由原子力顯微鏡、高解 析 X 光繞射儀與穿透式電子顯微鏡，分析不同磊晶階段之形貌、結 構特性，希望藉由深入瞭解整個磊晶製程的演變，得以改善 ZnO 磊 晶膜的品質。
38
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