於鋁酸鑭單晶基板成長非極性氧化鋅薄膜之研究

Abstract

摘要 本論文主要研究以脈衝雷射鍍膜法(Pulsed Laser Deposition, PLD)異質磊晶成長非極性晶面氧化鋅薄膜。所採用之基板為過去常用於合成高溫超導氧化物薄膜之鋁酸鑭(LaAlO3, LAO)單晶。本研究除了於(001)面成長氧化鋅外，為得到較佳之晶格匹配度，以成長非極性晶面(a或 m面)，利用基板工程觀念設計出不同偏切(miscut)面，(114)及(112)面，其偏切角分別為19.47° 及35.26°。接著運用PLD鍍膜技術, 實際成長氧化鋅磊晶薄膜。並驗證上述基板工程觀念之設計其可行性。 本論文主要可分為三個主題： 於第一個主題中，主要探討以PLD於(001)LaAlO3單晶基板成長出非極性a面氧化鋅之研究。其中針對成長溫度調變發現，300~850℃可成功成長出純相a面氧化鋅薄膜，而成長溫度為150℃之樣品則呈現c面與a面氧化鋅共存之現象，這可能是基板表面原子排列有關。以X光繞射(XRD)之θ-2θ及φ角掃瞄、反射式高能電子繞射(RHEED)以及原子力顯微鏡(AFM)掃瞄，發現呈現L-型特徵之雙晶域(dual domains)結構。橫截面穿透式電子顯微鏡(TEM)影像以及繞射分析均顯示於此基板成長之a面氧化鋅，為相互垂直之兩組晶向成長。其薄膜內面(in-plane)之成長排列關係為：[0001]ZnO // [1-10]LAO 以及 [1-100]ZnO // [1-10]LAO。雙晶域結構之成長，主要是立方晶LAO之(100)面具有四軸對稱。以光致螢光光譜(Photoluminescence, PL)分析發現此試片於3.31eV有一強度高之能帶邊緣放射訊號(Near Band edge Emission, NBE)存在，並且無明確之綠光放射訊號。 (001)LaAlO3因與a面氧化鋅之晶格較匹配之故(晶格差異3~5%)，雖有雙晶向成長結構，仍有不錯之光學性質。 第二個主題，主要是從如何克服雙晶域成長結構的出發點，利用基板工程觀念，研究如何選擇適當之LAO晶面做為基板。考慮將LaAlO3單晶沿著<110>方向作偏切，以延伸單方向基板表面原子間距，消除基板之方形對稱性，以期成長出非極性之氧化鋅磊晶。當調整LaAlO3單晶偏切角至19.47°，即(114)晶面，以此計算可得最低之晶格不匹配度，分別為3 % (//CZnO)及1.02%(⊥CZnO)。以PLD技術於(114)LaAlO3單晶基板成長氧化鋅薄膜，經AFM分析之發現表面呈現長條狀成長。同時以RHEED進行臨場分析顯示其為二重對稱，皆表示此氧化鋅為單晶向成長。由RHEED繞射圖可明顯看出，此氧化鋅雖然c軸以平行於基板表面成長，但其a軸與基板垂直方向呈一傾斜角。經ω/2θ XRD分析其氧化鋅與基板之關係，可以確定為：(001)LAO//(11-20)ZnO且(112)LAO//(10-10)ZnO。另外，以橫截面TEM分析氧化鋅與基板之成長關係，可確定此非極性氧化鋅之晶面為(13-40)。這是首先被觀察到，異於a面及m面之全新非極性晶面。由室溫PL分析測得此非極性晶面之NBE峰值為3.29 eV，且半高寬在成長條件未優化下可達87.5 meV。顯示(114) LaAlO3單晶基板，可成長出光學性質不錯之非極性(13-40) ZnO磊晶薄膜。 於最後一個主題，主要討論如何以LaAlO3單晶基板成長m面氧化鋅。沿用基板工程觀念，設計出偏切角35.26°，即(112)晶面。此時基板與m面氧化鋅之晶格不匹配度僅為2.9 % (//CZnO)及0.9% (⊥CZnO)。同樣以PLD技術於(112) LaAlO3單晶基板成長氧化鋅薄膜。經AFM分析確定表面形貌呈現條狀，且RHEED分析顯示二重對稱，表示所成長之氧化鋅薄膜為單晶向。藉由X光繞射(XRD)之θ-2θ與ψ角掃描分析，確定所成長之氧化鋅薄膜為純相之(10 0)面磊晶。此外，以橫截面TEM分析氧化鋅與基板之磊晶成長關係，可確定此氧化鋅之晶面為(10 0)。 由室溫PL分析測得此非極性晶面氧化鋅之NBE峰值為3.28 eV。顯示(112) LaAlO3單晶是成長非極性(10-10) ZnO磊晶薄膜深俱應用潛力之基板。

Abstract In this thesis, hetero-epitaxial growth of non-polar ZnO on LaAlO3 substrate by pulsed laser deposition method (PLD) has been investigated. LaAlO3 (LAO) single crystal has a pseudocubic structure at room temperature with lattice parameter of 3.791 Å, and it has been widely used to grow various dielectric oxide and high-temperature superconductor oxide films in the past. In this study, a-plane ZnO growth on (001)LaAlO3 has been demonstrated. In addition, the concept of substrate engineering has been applied to design (114) and (112) planes of LaAlO3 as substrates for growth of nonpolar ZnO, which can have a miscut angle of 19.47° and 35.26°, respectively, from (001) LAO. Thereafter, PLD was used to realize the growth of nonpolar ZnO. Furthermore, the results support the feasibility of the design from the concept of substrate engineering. The thesis is mainly composed of three topics: In the first topic, the study of non-polar ZnO growth on (001)LaAlO3 is presented. In the range of growth temperature from 300 to 850°C, pure a-plane ZnO can be successfully grown due to small lattice mismatch (3~5%), while growth at 150°C results in formation of c-plane ZnO co-existed with a-plane ZnO. As analyzed by x-ray diffraction (XRD) θ-2θ and ψ-scan, reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM), the microstructure of the a-plane ZnO films actually consists of L-shaped dual domains due to the cubic symmetry of atomic configuration of LAO substrate surface. Cross-sectional transmission electron microscopy (TEM) with selected-area diffraction (SAD) reveals that the a-plane ZnO film consists of two types of growth domains that are perpendicular to one another. The in- plane growth relationships of a-plane ZnO with LAO (001) substrate are: [0001]ZnO // [1 0]LAO and [1 00]ZnO // [1 0]LAO. Besides, a near band edge emission (NBE) of 3.30eV was measured by room temperature photoluminescence (PL) on this sample. As no evidence of green line emission was observed, the a-plane ZnO grown on LaAlO3 (001) has good optical characteristics, even though there are the dual- domain structure with boundaries. The second topic starts with the issue how to eliminate the dual domain structure of nonpolar ZnO on cubic symmetric substrate surface. It can be shown, from the concept of substrate engineering by enlarging the lattice distance of substrate along one direction, that (114) plane of LaAlO3 with a miscut angle of 19.47° from (001) is a proper one for nonpolar ZnO growth. The lattice mismatch of (114) LaAlO3 with a-plane ZnO is as small as 3 % in the direction //CZnO and 1.02% ⊥CZnO. After growth on this substrate by PLD, the ZnO thin film as examined by AFM as well as by RHEED exhibits a stripe-like morphology and two-fold symmetry, suggesting that it has a single domain structure on (114)LaAlO3. Examination of the RHEED pattern along [0001]ZnO azimuth, it is found that the c-axis lies on the substrate but the a-axis is tilted with an angle to the substrate normal. The epitaxial relationships of ZnO with substrate can be determined by XRD ω/2θ scan as (001)LAO//(11 0)ZnO and (112)LAO//(10 0)ZnO. In fact, cross-sectional TEM and SAD reveal that the growth plane of ZnO on (114) LaAlO3 is (13 0) which is an unconventional nonpolar plane observed for the first time. The room temperature PL spectrum of the ZnO film exhibits NBE at 3.29eV with FWHM of 87.5meV, suggesting that the (13 0)ZnO has good optical characteristics. In the last topic, the growth of m-plane ZnO on LaAlO3 substrate has been explored. Based on atomic arrangement of the (112) LAO which has a miscut angle of 35.26° from (001), it is shown that (112) LAO has better fit with m-plane ZnO by the same concept of substrate engineering as presented for (114) LAO with (13 0)ZnO. The lattice mismatch of (112) LAO with m-plane ZnO is small as 2.9 % in the direction //CZnO and 0.9%⊥CZnO. Structural characterization by AFM, RHEED and XRD 2θ and ψ-scan shows that the growth of m-plane ZnO on (112)LaAlO3 by PLD is in epitaxy with good crystallinity. The room temperature PL data also illustrate NBE at 3.28 eV. Therefore, it is suggested that (112) LaAlO3 can be a promising substrate for (1 00) ZnO growth.