Layer-by-layer heteroepitaxial growth process of a BaO layer on SrTiO₃(001) as investigated by molecular dynamics

Layer-by-layer heteroepitaxial growth processes of a BaO layer on SrTiO₃(001) were simulated in order to predict an appropriate buffer layer for a YBa₂Cu₃O_7-x(YBCO)/SrTiO₃(001) heterojunction by using our crystal growth molecular dynamics (MD) simulation code. The SrTiO₃(001) terminated by a TiO₂ atomic plane was employed as the substrate in the present simulations. BaO molecules were continuously deposited on the SrTiO₃(001) one by one, and finally a two-dimensional and epitaxial growth of a BaO layer was observed at 700 K. Moreover, the constructed BaO layer was atomically flat and smooth without defects, retaining a NaCl-type structure and (001) oriented configuration. However, the stress of the BaO/SrTiO₃(001) heterojunction gradually increased and finally reached to approximately 1.2 GPa during the epitaxial growth process. It is expected that the large stress disturbs the subsequent fabrication of the uniform YBCO/SrTiO₃(001) heterojunction. We also simulated the epitaxial growth process of a BaO layer on a [single SrO layer/SrTiO₃(001)]. An atomically flat and smooth BaO layer without defects was also obtained at 700 K. Surprisingly, the stress of the BaO/[single SrO layer/SrTiO₃(001)] heterojunction was almost 0.0 GPa after epitaxial growth. We have already suggested that [BaO layers/single SrO layer] are suitable buffer layers for the YBCO/SrTiO₃(001) heterojunction on the basis of regular MD simulations [M. Kubo et al., Phys. Rev. B 56, 13535 (1997)]. From the present crystal growth simulations, we confirmed that the above atomically uniform and smooth BaO/SrO/SrTiO₃(001) can be fabricated and almost no heterointerface stress was induced after the epitaxial growth. Moreover, the effect of substrate temperature on the heteroepitaxial growth process of the BaO layer on the SrO/SrTiO₃(001) was discussed.