成長於鋁酸鑭基板上之非極性氧化鋅薄膜顯微結構分析

Abstract

本研究以電子顯微鏡進行觀察與分析三種不同面向的非極性ZnO薄膜整體的顯微結構，其中包括晶向關係，界面，晶體缺陷與成長機制。非極性ZnO薄膜則以脈衝雷射沉積法分別成長於(001)、(112)以及(114)面向的鋁酸鑭(LaAlO3, LAO)基板上。 經由橫截面穿透式電子顯微鏡(cross-sectional transmission electron microscopy, XTEM)的選區繞射(selected area diffraction, SAD)圖形發現於(001)LAO上所得到的a面ZnO薄膜是由兩組互相垂直的晶區所組成。根據SAD可得出a面ZnO第一組晶區與(001)LAO間的晶體方為關係為[0001]ZnO-I//[110]LAO與 ZnO-I// LAO，而第二組晶區與(001)LAO的關係則為[0001]ZnO-II// LAO與 ZnO-II// LAO。在(112)LAO上所成長之ZnO則呈現m面( )磊晶薄膜的行為，其磊晶薄膜與LAO的晶體關係為[0001]ZnO// LAO與 ZnO// LAO。至於沉積在(114)LAO上的ZnO薄膜則呈現出接近 面向的磊晶，該ZnO與LAO間的晶體方位關係只具有單軸向平行，其關係為[0001]ZnO// LAO。 成長在(001)LAO上的a面ZnO薄膜顯微結構方面， XTEM影像的結果顯示，a面ZnO晶區以柱狀晶的形態形成，內部差排皆由其異質界面垂直向上延伸貫穿至薄膜表面。而在平面TEM(plan-view TEM, PVTEM)的結果中發現，a面ZnO晶區呈現L字型的形貌，且L字型雙臂的延伸方向為 ZnO與 ZnO(接近平行LAO的±[100]與±[010]方向)。藉由高解析PVTEM(high-resolution PVTEM, HR PVTEM)可以定義出ZnO晶區界面主要分為反轉晶界，近45°或45°界面與局部調整出現的r面( )雙晶界面。在晶體缺陷方面，該薄膜內部主要有差排、基面疊差(basal stacking fault, BSFs)與堆疊錯合界面(stacking mismatch boundary, SMB)，而SMB是由m面與r面串連組成。(001)LAO上的a面ZnO薄膜內部總差排密度約略為5×1010 cm-2，BSF的密度則約5×105 cm-1，至於SMB的密度大約3×105 cm-1。 對於(112)與(114)LAO上的非極性ZnO磊晶薄膜而言，其異質界面行為經由HR XTEM與其模擬影像比對後，可大至瞭解該異質界面的結構行為。模擬影像結果顯示在同一個異質界面中可能存在一種以上的界面結構，其原因主要是由於界面中晶體缺陷的影響，如錯位差排或BSF。而在異質磊晶界面上，HR XTEM顯示BSF的產生可能對於界面應力釋放具有某種程度上的幫助。在非極性ZnO磊晶薄膜內主要的晶體缺陷同樣為差排、BSFs與SMBs。根據PVTEM的分析，非極性ZnO磊晶薄膜中的SMBs會轉折甚至會迴轉形成迴圈，SMBs的形成主要是由於兩個平行的晶體在邊界包含數量不均的BSFs接合後所造成的界面。在(112)LAO上的m面ZnO磊晶薄膜中，總差排密度約為5.1×1010 cm-2，BSF的密度則約4.3×105 cm-1。至於在(114)LAO上的 面ZnO磊晶薄膜中總差排密度大約3.8×1010 cm-2，BSF的密度則約略為3.1×105 cm-1。 非極性ZnO薄膜的成長機制在藉由自主裝直立式化學氣相沉積系統的非極性ZnO磊晶薄膜成長實驗結果獲得佐證。非極性ZnO薄膜成長過程中，m面與r面在薄膜成長初期便已存在島狀晶粒表面，其中以m面數量最多，而薄膜成長過程中ZnO島狀晶粒的接合便是靠m面與r面進行接合。在a面ZnO晶區的接合過程中，由於存在於島狀晶粒上的r面，使得a面ZnO晶區的界面為了降低界面能而形成近45°或45°界面與r面雙晶界面。在(112)與(114)LAO上的非極性ZnO磊晶薄膜方面，薄膜成長初期島狀晶粒的m面以及r面會和延伸至晶粒邊緣的BSFs交會，最終便形成SMBs。而對於a面ZnO晶區中的SMBs，其形成方式也與非極性ZnO磊晶薄膜中的SMBs相同。由於ZnO島狀晶粒表面上m面所佔比例最大，因而會使其晶粒成為屋脊狀的形貌，薄膜沉積完畢後由m面所構成的屋脊形貌會有些微保留在薄膜表面。由掃描式電子顯微鏡便可觀察到ZnO薄膜表面形貌具有許多平行c軸的線條。

The structural properties of various non-polar ZnO thin films have been systematically characterized in detail by using electron microscopy. The structural properties include crystallography relationships, interfaces, crystal defects, and growth mechanism. Non-polar ZnO thin films have been grown on (001), (112), and (114)LaAlO3 (LAO) single crystal substrates by pulsed laser deposition method, respectively. The crystallography relationships between non-polar ZnO films and LAO substrate are identified according to the selected area diffraction (SAD) pattern of cross-sectional transmission electron microscopy (XTEM). On (001)LAO, a-plane ZnO thin film is realized to consist of two types of domains perpendicular to each other. The crystallography relationships between a-plane ZnO domains and (001)LAO are determined to be [0001]ZnO-I//[110]LAO and ZnO-I// LAO, whereas [0001]ZnO-II// LAO and ZnO-II// LAO for the other domain. Non-polar ZnO epitaxial film grown on (112)LAO appears in pure m-plane orientation. The crystal orientation relationships between m-plane ZnO and (112)LAO are verified to be [0001]ZnO// LAO and ZnO// LAO. For non-polar ZnO epitaxial film on (114)LAO, the surface plane of ZnO exhibits near orientation. The -plane ZnO epitaxial film presents only one direction parallel to (114)LAO, which is confirmed to be [0001]ZnO// LAO. For the microstructure of a-plane ZnO domains, XTEM images show that the domains form columnar structure. In a-plane ZnO domains, the threading dislocations extend straight perpendicular to ZnO/LAO hetero-interface. The results of plan-view TEM (PVTEM) reveal that a-plane ZnO domains appear L-like shape. The arms of L-like shape are verified to be parallel to ZnO and ZnO(±[100]LAO and ±[010]LAO). According to high-resolution PVTEM (HR PVTEM) images, domain boundaries are identified to be inversion domain boundary, near 45° or 45° boundary, and local r-plane twin boundary. In a-plane ZnO domains, the predominant crystal defects are dislocation, basal stacking fault (BSF), and stacking mismatch boundary (SMB). SMB is a planar defect consisted of m- and r-planes. The densities of dislocations, BSFs, and SMBs are 5×1010 cm-2, 5×105 cm-1, and 3×105 cm-1, respectively. For non-polar ZnO epitaxial thin films grown on (112) and (114)LAO, the hetero-interfaces have been realized by the comparison of HR XTEM images and simulation HR XTEM images. The results of the simulation images conclude that a ZnO/LAO hetero-interface might be composed of more than one type of configuration. This phenomenon can be attributed to the defects in the hetero-interface, such as BSF and misfit dislocation. The HR XTEM image also shows that the BSF generated at the hetero-interface might be able to assist the relaxation of mismatch-induced strain. In non-polar ZnO epitaxial thin films on (112) and (114)LAO, the predominant crystal defects are identical with that in a-plane ZnO domains. In non-polar ZnO epitaxial films, SMBs can extend to have U-turn profile and become loops. The appearance of SMBs results from that two crystals with penetration BSFs coalesce to form a boundary with stacking mismatch. Based on the calculation in PVTEM images, the densities of dislocations and BSFs are 5.1×1010 cm-2 and 4.3×105 cm-1 in m-plane ZnO epitaxial film. For -plane ZnO epitaxial film, the densities of dislocations and BSFs are 3.8×1010 cm-2 and 3.1×105 cm-1. The growth mechanism of non-polar ZnO films can be supported by the results of ZnO nucleation and growth experiments in home-built vertical chemical vapor deposition system. At the initial stage of thin film growth, m- and r-plane facets naturally appear on non-polar ZnO island-like growing grains. For a-plane ZnO domains, the growing grains would merge with accommodation along r-plane facets to form near 45° or 45° boundaries and local r-plane twin boundaries in order to reduce the total energy of domain boundaries. During the growth of non-polar ZnO epitaxy, the growing grains with penetration BSFs coalesce along pre-existent m- and r-planes to form SMBs. The formation of SMBs in a-plane ZnO domains is similar with that in non-polar ZnO epitaxy. After the growth of non-polar ZnO thin film, the surface exhibit stripe morphology parallel ZnO c-axis.