(502f) Understanding Interface Formation Interactions Between BTO/MgO/Si By Epitaxial Growth of BaTiO3 Thin Film On Si (100)

Effective integration of functional oxides (magnetic, ferroelectric, piezoelectric and other multi-functional materials) with semiconductors will lead to next-generation devices such as: new architectures that enable multiple and simultaneous interactions with the environment for multifunctional active sensors and controllers; and integrated nonvolatile memories for harsh environments; Many of the functional properties of complex oxides are directional, thus requiring a specific plane alignment in a device, and most oxides have multiple stable structures for a given stoichiometry. In addition, small changes in stoichiometry with the same unit cell structure can produce easily measurable differences in performance properties. This is particularly true for the interfaces and near-interface layers between the semiconductor and oxide as well as between subsequent functional oxide layers, because surface quality impacts chemistry, structure, and morphology of next-layer film deposition that, in turn, impacts the functionality of the film layer and the coupling effects across layers through critical interfaces.

For the BTO/MgO/Si system, this means MgO as the buffer layer is needed to overcome the large lattice mismatch between the oxide and Si in addition to reducing the thermal strain upon cooling as a result of the differences on the coefficient of thermal mismatch. Also it prevents the possible interface reaction. Limited work was carried out on the growth of BTO on compound semiconductors using MBE. Most of the reported literature involved pulsed laser deposition of the oxide films on buffer layers such as MgO on GaAs.

  Through molecular beam epitaxy (MBE), the use of a magnesium oxide (MgO) template layer and the interface formation mechanisms of an oxygen bridge have been investigated for effective heteroepitaxy of high-quality ferroelectric barium titanate (BTO) and ferrimagnetic barium hexaferrite (BaM) on 6H-SiC.  Understanding from these results, has allowed us to elucidate interface formation mechanisms for BTO/MgO/Si heterostructures and influence BTO orientation and crystal quality.

6H-SiC(0001) substrates are cleaned in an ex-situ hydrogen furnace which produces and atomically smooth, stepped surface with a √3×√3 R30° surface reconstruction, verified by reflection high energy electron diffraction (RHEED) and x-ray photoelectron spectroscopy (XPS). High quality, single crystalline MgO(111) is obtained with a smooth surface (RMS<0.5 nm) and a stepped morphology conformal to the underlying 6H-SiC morphology, but is inherently twinned due to the ionic nature of a (111) oriented rock salt structure. The smooth, conformal 2-D growth of MgO requires the presence of atomic oxygen in a Mg-adsorbsion controlled mechanism, and grows in tension with a 3.3% lattice mismatch.  The engineered MgO surface is both effective and necessary to promote the pseudo-hexagonal heteroepitaxy of BTO(111) resulting in a BTO{111}//MgO{111}//6H-SiC{0001} out-of-plane relationship and a BTO{11(bar)0}//MgO{11(bar)0}//6H-SiC{002(bar)1} in-plane relationship.  The relative flux relationship between Ba and Ti must be controlled, and the relationship between temperature, relative fluxes, and surface composition and structure are being explored and will be discussed. The BTO layer of the heterostructure has ferroelectric properties with a saturated polarization around 4.7 mC/cm² and an apparent striped domain structure. Tunneling Electron Microscope analysis reveals subtle bond angle differences in the first layer of atoms at the interface that lead to subsequent micron-scale features on the BTO surface.  We are currently expanding these results to attain controlled orientation of BTO films on Si (100), and will share these results as well.