C. D. Theis

Oxide nano-engineering using MBE

Abstract Molecular beam epitaxy (MBE) has achieved unparalleled control in the integration of semiconductors at the nanometer level; its use for the integration of oxides with similar nanoscale customization appears promising. This paper describes the use of reactive MBE to synthesize layered oxide heterostructures, including new compounds and metastable superlattices, involving monolayer-level integration of the dielectric and ferroelectric oxides SrO, SrTiO 3 , BaTiO 3 , PbTiO 3 , and Bi 4 Ti 3 O 12 . The controlled synthesis of such layered oxide heterostructures offers great potential for tailoring the dielectric and ferroelectric properties of materials. Oxide nano-engineering is accomplished by supplying the incident species in the desired layering sequence with submonolayer composition control. Comparisons between the growth of compound semiconductors and oxides by MBE are made.

Epitaxial growth of the first five members of the Srn+1TinO3n+1 Ruddlesden–Popper homologous series

The first five members of the Srn+1TinO3n+1 Ruddlesden–Popper homologous series, i.e., Sr2TiO4, Sr3Ti2O7, Sr4Ti3O10, Sr5Ti4O13, and Sr6Ti5O16, have been grown by reactive molecular beam epitaxy. A combination of atomic absorption spectroscopy and reflection high-energy electron diffraction intensity oscillations were used for the strict composition control necessary for the synthesis of these phases. X-ray diffraction and high-resolution transmission electron microscope images confirm that these films are epitaxially oriented and nearly free of intergrowths. Dielectric measurements indicate that the dielectric constant tensor coefficient e33 increases from a minimum of 44±4 in the n=1(Sr2TiO4) film to a maximum of 263±2 in the n=∞(SrTiO3) film.

RHEED Intensity Oscillations for the Stoichiometric Growth of SrTiO3 Thin Films by Reactive Molecular Beam Epitaxy

The growth of high quality multicomponent oxide thin films by reactive molecular beam epitaxy (MBE) requires precise composition control. We report the use of in situ reflection high-energy electron diffraction (RHEED) for the stoichiometric deposition of SrTiO3 (1 0 0) from independent strontium and titanium sources. By monitoring changes in the RHEED intensity oscillations as monolayer doses of strontium and titanium are sequentially deposited, the Sr:Ti ratio can be adjusted to within 1% of stoichiometry. Furthermore, the presence of a beat frequency in the intensity oscillation envelope allows the adjustment of the strontium and titanium fluxes so that a full monolayer of coverage is obtained with each shuttered dose of strontium or titanium. RHEED oscillations have also been employed to determine the doping concentration in barium- and lanthanum-doped SrTiO3 films.

Abrupt PbTiO3/SrTiO3 superlattices grown by reactive molecular beam epitaxy

PbTiO3/SrTiO3 superlattices were grown on (001) SrTiO3 substrates by reactive molecular beam epitaxy (MBE). Sharp superlattice reflections were observed by x-ray diffraction. High-resolution transmission electron microscopy of a [(PbTiO3)10/(SrTiO3)10]15 superlattice revealed that the PbTiO3/SrTiO3 interface structure is atomically sharp. The superlattice interfaces are fully coherent; no misfit dislocations or other crystal defects were observed in the superlattice by transmission electron microscopy. Selected area electron diffraction patterns indicated that the PbTiO3 layers are oriented with the c axis parallel to the growth direction. The dimensional control and interface abruptness achieved in this model system indicate that MBE is a viable method for constructing oxide multilayers on a scale where enhanced dielectric effects are expected.

Adsorption-controlled growth of PbTiO3 by reactive molecular beam epitaxy

Epitaxial lead titanate films have been grown on (001) SrTiO3 substrates by reactive molecular beam epitaxy (MBE). Growth of stoichiometric phase-pure epitaxial PbTiO3 films is achieved by supplying a continuous excess of lead and ozone to the surface of the depositing film. Results obtained from RBS composition analysis, measured film thicknesses and flux measurements using atomic absorption spectroscopy (AA) indicate that the film growth rate is completely determined by the incident titanium flux supplied to the surface. Atomic force microscopy (AFM) results show that films grow smoothly in a layer-by-layer fashion with an RMS roughness of <0.5 nm. The sticking coefficient of titanium is determined to be approximately unity while the excess lead, lead oxide and ozone desorb. Lead and ozone beam equivalent pressures have been measured in the MBE environment. Thermodynamic analysis is used to help describe the processes that prevent the incorporation of PbO into films under adsorption-controlled growth conditions.

Epitaxial lead titanate grown by MBE

Abstract Epitaxial PbTiO 3 films have been grown by reactive molecular beam epitaxy (MBE) on (100) LaAlO 3 substrates. Lead is supplied from a conventional effusion cell. Titanium is sublimated from a Ti-Ball™, and oxygen is supplied in the form of purified ozone. Atomic layer-by-layer composition control is obtained using real-time atomic absorption spectroscopy (AA) feedback. The titanium flux is controlled with a shutter directly coupled to the titanium AA feedback to deliver a burst corresponding to one monolayer of titania. Similarly, lead is monitored in situ using an atomic absorption signal. An adsorption-controlled growth mechanism leads to the use of a lead overpressure to insure films with proper stoichiometries. Film structure is studied during growth using in situ RHEED. 4-circle X-ray diffraction analysis indicates that films grown on LaAlO 3 are epitaxial and are mixed a - and c -axis oriented.

Probing domain microstructure in ferroelectric Bi4Ti3O12 thin films by optical second harmonic generation

The domain microstructure in an epitaxial thin film of Bi4Ti3O12 on a SrTiO3(001) substrate is studied by second harmonic generation measurements. The input polarization dependence of the second harmonic signal exhibits spatial symmetries that reflect the presence of eight different domain variants present in the film. A theoretical model is presented that explains the observed symmetries and extracts quantitative information on the nonlinear optical coefficients of the material and statistics of domain variants present in the film area being probed. The following ratios of nonlinear coefficients and birefringence was determined: d12/d11=−3.498±0.171, |d26/d12|=0.365±0.010, |d26/d11|=1.273±0.036, and |nb−na|=0.101±0.018 (at 532 nm).

Size Effects and Domains in Ferroelectric Thin Film Actuators

Ferroelectric thin films typically differ from bulk ceramics in terms of both the average grain size and the degree of stress imposed on the film by the substrate. Studies on bulk ceramics have demonstrated that the number of domain variants within grains depends on the grain size for sizes 3 films are ferroelectric at 77K down to thicknesses as low as ˜ 60nm. Data on the low and high field electrical properties are reported as a function of temperature, the film crystallinity, and film thickness for representative perovskite films.

Abrupt PbTiO{sub 3}/SrTiO{sub 3} superlattices grown by reactive molecular beam epitaxy

PbTiO3/SrTiO3 superlattices were grown on (001) SrTiO3 substrates by reactive molecular beam epitaxy (MBE). Sharp superlattice reflections were observed by x-ray diffraction. High-resolution transmission electron microscopy of a [(PbTiO3)10/(SrTiO3)10]15 superlattice revealed that the PbTiO3/SrTiO3 interface structure is atomically sharp. The superlattice interfaces are fully coherent; no misfit dislocations or other crystal defects were observed in the superlattice by transmission electron microscopy. Selected area electron diffraction patterns indicated that the PbTiO3 layers are oriented with the c axis parallel to the growth direction. The dimensional control and interface abruptness achieved in this model system indicate that MBE is a viable method for constructing oxide multilayers on a scale where enhanced dielectric effects are expected.

Domain structure of epitaxial Bi4Ti3O12 thin films grown on (001) SrTiO3 substrates

The domain structure of epitaxial (001) Bi4Ti3O12 thin films grown on (001) SrTiO3 substrates by reactive molecular beam epitaxy was studied using transmission electron microscopy. It was found that the Bi4Ti3O12 thin films contain randomly distributed rotation domains of two different types, which are related by a 90° rotation around the c axis of Bi4Ti3O12. These domains result from the difference in crystallographic symmetry between the Bi4Ti3O12 (001) plane and the SrTiO3 (001) surface. Moreover, out-of-phase boundaries were frequently observed in the epitaxial Bi4Ti3O12 films. Detailed quantitative high-resolution transmission electron microscopy studies showed that the growth of epitaxial Bi4Ti3O12 film on the SrTiO3 (001) surface begins with the energetically favorable central TiO2 layer in the middle of the triple perovskite block within Bi4Ti3O12. As a result, a number of out-of-phase domain boundaries are formed at the atomic steps on the substrate surface. These studies suggest that Bi4Ti3O12 f...