Growth of nanoscale BaTiO₃/SrTiO₃ superlattices by molecular-beam epitaxy

Commensurate BaTiO₃/SrTiO₃ superlattices were grown by reactive molecular-beam epitaxy on four different substrates: TiO₂-terminated (001) SrTiO₃, (101) DyScO₃, (101) GdScO₃, and (101) SmScO₃. With the aid of reflection high-energy electron diffraction (RHEED), precise single-monolayer doses of BaO, SrO, and TiO₂ were deposited sequentially to create commensurate BaTiO₃/SrTiO₃ superlattices with a variety of periodicities. X-ray diffraction (XRD) measurements exhibit clear superlattice peaks at the expected positions. The rocking curve full width half-maximum of the superlattices was as narrow as 7 arc s (0.002°). High-resolution transmission electron microscopy reveals nearly atomically abrupt interfaces. Temperature-dependent ultraviolet Raman and XRD were used to reveal the paraelectric-to-ferroelectric transition temperature (T_C). Our results demonstrate the importance of finite size and strain effects on the T_C of BaTiO₃/SrTiO₃ superlattices. In addition to probing finite size and strain effects, these heterostructures may be relevant for novel phonon devices, including mirrors, filters, and cavities for coherent phonon generation and control.