利用橫截面掃描穿隧式顯微鏡解析氧化鍶鈦極化表面構造

Abstract

氧化晶成鈣鈦礦 ( perovskite oxides ) 結構，相較於塊材，其薄膜系統更能展現有趣的物理現象，譬如多鐵性磁電薄膜材料，典型的例子便是室溫鐵電反鐵磁體 BiFeO3 ( 以下縮寫為BFO ) 的磁電效應。特殊面向的晶成鈣鈦礦表面 ( 例如，BFO(111) ) 的製程方式，像是雷射沉積法 ( PLD )，無法提供掃描穿隧式顯微鏡 ( scanning tunneling microcopy, STM ) 合適的研究表面，卻可利用在超真空環境橫切晶體的方式達成。此論文以最常見的基體SrTiO3 ( 以下縮寫為STO ) 做為實驗模型，成功於室溫中利用橫截面掃描穿隧式顯微鏡 ( cross-sectional scanning tunneling microcopy, XSTM ) 量測STO在極化(110)斷面，發現多樣性的表面結構，雖少數可被定性描述。但我們依然於 STO(110) 斷面上發現二個單位晶格高的檯階式表面結構。相較於擁有原子尺度平整檯階的非極化 STO(100) 斷面，STO(110) 在在顯示因為表面極化特性所造成的不穩性，並於理論預測相符。未來可嘗試於低溫下進行斷面實驗。

Aside from the bulk properties, interesting phenomena can be revealed in thin films of perovskite oxides, such as magnetoelectric switching effects in multiferroic BiFeO3 (BFO) films. Certain interesting perovskite surfaces (e.g., BFO(111)) normally pre- pared by methods such as pulsed laser deposition, but with a resulting morphology unsuitable for STM studies, can be made accessible by crystal cleavage. Instead of taking bottom-up way to get a surface, in this work, using the common substrate as our model system, we demonstrate the first observation of (110) polar surfaces of fractured Nb-doped SrTiO3 (Nb:STO) utilizing cross-sectional scanning tunneling microscopy (XSTM) at room temperature. A rich variety of surface morphologies have been observevd, but few are well characterised. Terraced STO(110) surfaces with two unit-cell height could be found by in situ cleaving. Comparing the results of atomically flat terraces with STO(100) non-polar surfaces, indicates the existence of polar instability on fractured STO(110) surfaces, in agreement with theoretical predictions. Future advances in understanding these surfaces will be accompanied by fracturing at low temperature.