Surface engineering of SrTiO 3 (111) substrates for the epitaxial growth of BLT films
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Surface engineering of SrTiO3 (111) substrates for the epitaxial growth of BLT films Ju Hyung Suh, Yong Seok Lee and Chan-Gyung Park Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea ABSTRACT The optimized surface termination of SrTiO3 (111) substrates was investigated and the effects of the terminated SrTiO3 (111) substrate on the growth characteristics of epitaxial Bi4xLaxTi3O12 (BLT) films were evaluated. It was found that etching in buffered HF (BOE) solution for 2min provides a stable etching condition for SrTiO3 (111) substrates and that etching is an important factor for the formation of terrace structures. The microstructure of BLT films grown on the terminated substrate revealed a flat surface morphology and welldefined interfacial structure in comparison with BLT films grown on non-terminated substrate. Therefore, surface termination is a crucial factor for determining the quality of film morphology and interfacial structure. INTRODUCTION Since SrTiO3 (STO) has similar structural characteristics with metal electrodes and other oxide capacitors, it can be applied to the growth of various epitaxial thin films [1-3]. STO (111) substrates were used for the epitaxial growth of layered perovskite films along the non c-axis orientation [4]. In the growth of epitaxial films, surface termination of substrates is a very important process for determining epitaxial film growth characteristics. Surface termination usually means the formation of step-like surface structure with uniform chemical composition. For instance, misfit dislocations of SrRuO3 films grown on as-received STO (001) substrates were not observed and a number of anti-phase boundaries (APBs) were formed [5]. However, there are few results about the formation and effects of terminated STO (111) substrates on film growth [6]. Therefore, the focus of our research is to investigate the optimum conditions for surface termination and to fabricate terminated substrates with welldefined terraces and finally to evaluate the effects of terminated surfaces on the epitaxial film growth. EXPERIMENTAL The surface termination of SrTiO3 (STO) (111) substrates was performed to fabricate the epitaxial BLT thin films. The STO (111) substrates revealed slightly rough or flat surface states. HCl-HNO3 (3:1) [6] and buffered HF solutions (NH4F:HF = 7:1) [7, 8] were applied as the etchants of STO (111) substrates. Etching time was varied from 2min to 15min and the etched substrates were successfully annealed at 850oC in 1atm O2 atmosphere over 45min. In
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order to evaluate the effect of surface termination on the film microstructures, BLT films were grown on the STO (111) substrates using r.f. magnetron sputtering at 750 oC over 1 hr. Surface morphology was precisely investigated by using atomic force microscopy (AFM, Digital Instruments). Field-emission transmission electron microscopy (FE-TEM, JEM2010F) was applied to evaluate the cross-sectional film morphology and interfacial str
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