Surface Steps on (100) MgO Foils Imaged by TEM, SEM and AFM
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SIMON KING, JASON HEFFELFINGER, MICHAEL MALLAMACI, STUART McKERNAN AND C. BARRY CARTER Department of Chemical Engineering and Materials Science, University of Minnesota, 421, Washington Av S.E., Minneapolis, MN 55455. ABSTRACT The morphology characteristic of acid-washed and annealed MgO (100) surfaces used as substrates for pulsed-laser deposition (PLD) is studied employing TEM, SEM and AFM. Each of these three microscopal techniques provides information not so readily accessible by and complementary to the other two techniques, leading to a more rounded view of the characteristic morphology of these surfaces. INTRODUCTION The importance of surface morphology, most notably of surface steps, in the formation and evolution of the microstructure of deposited thin-films is particularly evident in the phenomenon of grapho-epitaxy, in which surface steps act as sites for the nucleation of islands. Examples include the islanding of a- and y-Fe203 and YBCO deposited by pulsed-laser deposition (PLD) and chemical vapor deposition (CVD) onto (100) MgO and A1203 foils [1,2]. The general surface morphology and steps in particular are also found to be of critical importance in their effect on the microstructure, and even the phase of as-deposited titania thin-films on MgO [3-6]. Hence, microscopal studies of these substrate surfaces prior to deposition onto the studied area, which subsequently is re-analysed [3-6], play an important role in the understanding of the observed effects. In the above papers, deposition experiments onto both TEM-ready and "bulk" (- 0.5-1mm thick) specially prepared substrates were performed. The preparation treatment, which involves
acid-washing and annealing in air, yields a reproducible morphology for the comparison of different experiments. Since both TEM-ready and bulk substrates are employed, it is of interest to analyse the surface morphologies of both substrate types formed after identical preparation treatments. The thin-foils are suited to study by TEM, whilst SEM and AFM are available for bulk substrates. The relevence of the three techniques to the study of these surfaces is considered, and information available from each is employed to address the issue as to whether the surface morphology seen in TEM investigation of thin foils is representative of the surface morphology of the bulk substrates. EXPERIMENTAL The method of preparation of the TEM-ready specimens; similar to that of Norton et al [7], is given in detail elsewhere [8]. Briefly, dimpled foils are polished to a 0.06 jtm finish, perforated by ion-milling, cleaned in acid, with the active ingredient being aqua-regia, and heat treated in air. TEM foils heat-treated at 1350*C for 8.5, 10 and 30 min. are shown in this paper. The bulk specimens similarly are polished to a 0.061im finish, but then are taken directly to the acid-wash stage prior to an anneal under conditions identical to those undergone by TEM foils for 30 mins at 13501C. The acid-cleaning stage leads to an undulating specimen morphology, consisting of a number of pore
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