新穎磊晶技術－－側向編織式磊晶薄膜

Fabrication of twisted lateral homostructures–Weave epitaxy

在薄膜材料研究與相關應用中，磊晶技術一直是高品質薄膜製程中重要的環節。在製程中，藉由晶格常數的匹配性與否即能很大程度操控目標薄膜。使用與目標薄膜的晶格常數匹配的基板，能有效降低功能性薄膜中的缺陷濃度並提升其功能性；反之，則能提供目標薄膜相對應的應力，對強關聯電子系統材料而言，這意味著能高程度操控材料的物理特性。這樣直觀卻又具高影響力的因子被廣泛應用於不同材料系統的製備與研究，如異質結構、奈米結構、垂直陣列結構、以及超晶格結構等等。
傳統薄膜磊晶的方式主要為沿膜面法線方向的垂直堆疊，在選定與目標薄膜匹配的基板後，就能得到擁有對應晶格結構、晶向與對稱性的磊晶薄膜。然而，建構沿平行膜面方向的磊晶結構的相關製程長期以來並沒有被系統性提出。為解決此問題，成功大學物理系楊展其老師以及其研究團隊利用開發的超薄獨立自支撐薄膜，來實現建構側向磊晶結構的概念，其方法被命名為編織式磊晶(weave epitaxy)，在本文中會詳細敘述製程細節及其相關應用。
此方法是先透過脈衝雷射磊晶技術先製作犧牲層(La0.7Sr0.3MnO3, LSMO)以及與單晶基板相同的目標薄膜(SrTiO3, STO)，接著使用酸性蝕刻液進行蝕刻，便能得到與基板相同的獨立自支撐薄膜(freestanding STO, FS-STO)，接著即可將FS-STO以任意角度轉移至任意基板上，形成具備界面角度可控性的雙晶基板。隨後二次進行目標氧化物的磊晶成長，便能得到混相雙晶結構。作者分別透過多鐵性的鐵酸鉍(BiFeO3, BFO)與高溫超導性的釔鋇銅氧(YBa2Cu3O7, YBCO)，來驗證其所提出的方法可廣泛應用於不同材料系統。楊老師團隊所提出的新穎磊晶技術，不僅突破了單晶基板的侷限，也在介面物理上面提供一個可以幫助解決問題的方案，讓薄膜材料的設計上添加了更多的自由度與可行性。

In the realm of materials research and its practical applications, the fabrication of high crystallinity thin films holds significant significance. Epitaxial growth, in particular, relies on the careful selection of a substrate with a suitable lattice constant to either enhance or manipulate the physical properties of functional thin films. This conventional epitaxial approach is currently employed to fabricate and explore various material systems, including heterostructures, vertical epitaxial arrays, superlattices, and so on. Thin film epitaxy, a widely utilized concept in materials science, which mainly involves the vertical stacking of thin films. During the selection of the substrate, crucial properties of the thin films, including crystal structure, orientation, and symmetry, can be determined. Despite this progress, the challenge of achieving lateral epitaxy remains unresolved. Addressing this issue, Prof. Jan-Chi Yang and his research group have developed an innovative method known as weave epitaxy, which enables the creation of twisted homostructures along the lateral direction. This article will demonstrate the effectiveness of their lateral epitaxy approach.
The cutting-edge process commences with the fabrication of a heterostructure thin film comprising a sacrificial layer, LaSrMnO3 (LSMO), and an ultrathin target layer, SrTiO3 (STO), utilizing pulsed laser deposition (PLD) techniques. Following the dissolution of the LSMO layer in an acid solution, a freestanding STO thin film (FS-STO) is obtained, allowing for its transfer onto templates with arbitrary angles to create a bicrystal substrate. To showcase the versatility of the proposed method, the classical multiferroic material bismuth ferrite (BiFeO3, BFO), as well as the high-temperature superconductor, yttrium barium copper oxide (YBa2Cu3O7, YBCO), were employed and validated. The concept of weave epitaxy offers more than just the degree of freedom to overcome the limitations of single crystal substrates and design innovative architectures using complex oxides. It also opens up new avenues for exploring the physics of interfaces, providing an additional option for researchers in their quest for deeper understanding.