Y. Jia

Band alignment between (100)Si and complex rare earth∕transition metal oxides

The electron energy band alignment between (100)Si and several complex transition∕rare earth (RE) metal oxides (LaScO3, GdScO3, DyScO3, and LaAlO3, all in amorphous form) is determined using a combination of internal photoemission and photoconductivity measurements. The band gap width is nearly the same in all the oxides (5.6–5.7eV) yielding the conduction and valence band offsets at the Si∕oxide interface of 2.0±0.1 and 2.5±0.1eV, respectively. However, band-tail states are observed and these are associated with Jahn-Teller relaxation of transition metal and RE cations which splits their d* states.

Amorphous lanthanum lutetium oxide thin films as an alternative high-κ gate dielectric

Lanthanum lutetium oxide thin films were grown on (100) Si by pulsed laser deposition. Rutherford backscattering spectrometry, atomic force microscopy, x-ray diffraction, and x-ray reflectometry were employed to investigate the samples. The results indicate the growth of stoichiometric and smooth LaLuO3 films that remain amorphous up to 1000°C. Internal photoemission and photoconductivity measurements show a band gap width of 5.2±0.1eV and symmetrical conduction and valence band offsets of 2.1eV. Capacitance and leakage current measurements reveal C-V curves with a small hysteresis, a dielectric constant of ≈32, and low leakage current density levels.

Epitaxial growth and properties of metastable BiMnO3 thin films

Epitaxial thin films of BiMnO3 were deposited on single-crystal substrates of (100)-oriented SrTiO3 by pulsed-laser deposition. Structural analysis by x-ray diffraction, electron diffraction, and transmission electron microscopy (TEM) indicated that the films were monoclinic and twinned with two dominant orientation relationships. The first is (111) BiMnO3 ∥ (100) SrTiO3 and ∼[101] BiMnO3 ∥ 〈010〉 SrTiO3; the second is (101) BiMnO3 ∥ (100) SrTiO3 and ∼[121] BiMnO3 ∥ 〈010〉 SrTiO3. High-resolution TEM images revealed that there is no reaction or appreciable interdiffusion at the substrate/film interface, despite the high temperature of the substrate during deposition (∼1000 K). Magnetic characterization was carried out (both magnetization versus temperature and hysteresis loops) and the results agree with previous reports of a ferromagnetic transition with TC∼105 K. The actual value of TC in the films is a few degrees lower than the bulk material, the discrepancy being attributed to strain, nonstoichiometr...

Relaxor ferroelectricity in strained epitaxial SrTiO3 thin films on DyScO3 substrates

The ferroelectric properties of 500A thick strained, epitaxial SrTiO3 films grown on DyScO3 substrates by reactive molecular-beam epitaxy are reported. Despite the near 1% biaxial tensile strain, the x-ray rocking curve full widths at half maximum in ω are as narrow as 7arcsec (0.002°). The films show a frequency-dependent permittivity maximum near 250K that is well fit by the Vogel-Fulcher equation. A clear polarization hysteresis is observed below the permittivity maximum, with an in-plane remanent polarization of 10μC∕cm2 at 77K. The high Tmax is consistent with the biaxial tensile strain state, while the superimposed relaxor behavior is likely due to defects.

Epitaxial growth of (001)-oriented and (110)-oriented SrBi2Ta2O9 thin films

Epitaxial SrBi2Ta2O9 thin films have been grown with (001) and (110) orientations by pulsed laser deposition on (001) LaAlO3–Sr2AlTaO6 and (100) LaSrAlO4 substrates, respectively. Four-circle x-ray diffraction and transmission electron microscopy reveal nearly phase pure epitaxial films. Minimization of surface mesh mismatch between the film and substrate (i.e., choice of appropriate substrate material and orientation) was used to stabilize the desired orientations and achieve epitaxial growth.

Epitaxial growth of non-c-oriented SrBi2Nb2O9 on (111) SrTiO3

Epitaxial SrBi2Nb2O9 thin films have been grown with a (103) orientation on (111) SrTiO3 substrates by pulsed-laser deposition. Four-circle x-ray diffraction and transmission electron microscopy reveal nearly phase-pure epitaxial films. Epitaxial (111) SrRuO3 electrodes enabled the electrical properties of these (103)-oriented SrBi2Nb2O9 films to be measured. The low-field relative permittivity was 185, the remanent polarization was 15.7 μC/cm2, and the dielectric loss was 2.5% for a 0.5-μm-thick film.

Dielectric and Optical Properties of Epitaxial Rare-Earth Scandate Films and Their Crystallization Behavior

Rare-earth scandates (ReScO3, with Re=Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, i.e., the entire series for which the individual oxides are chemically stable in contact with Si) were deposited in a temperature-gradient pulsed laser deposition system onto LaAlO3 substrates. The crystallization temperature depends monotonically on the Re atomic number and the Goldschmidt tolerance factor, with crystallization temperatures as low as 650°C for LaScO3 and PrScO3. The dielectric constants of the crystalline films K≈30 (determined by microwave microscopy) are significantly larger than those of their amorphous counterparts. In combination with the large observed band gaps (Eg>5.5eV, determined by ellipsometry), these results indicate the potential of these materials as high-K dielectrics for field-effect transistor applications.

Growth of (103) fiber-textured SrBi2Nb2O9 films on Pt-coated silicon

(103) fiber-textured SrBi2Nb2O9 thin films have been grown on Pt-coated Si substrates using a SrRuO3 buffer layer. High-resolution transmission electron microscopy reveals that the fiber texture arises from the local epitaxial growth of (111) SrRuO3 grains on (111) Pt grains and in turn (103) SrBi2Nb2O9 grains on (111) SrRuO3 grains. The films exhibit remanent polarization values of 9 μC/cm2. The uniform grain orientation (fiber texture) should minimize grain-to-grain variations in the remanent polarization, which is important to continued scaling of ferroelectric memory device structures.

Morphology, structure, and nucleation of out-of-phase boundaries (OPBs) in epitaxial films of layered oxides

Out-of-phase boundaries (OPBs) are translation boundary defects characterized by a misregistry of a fraction of a unit cell dimension in neighboring regions of a crystal. Although rarely observed in the bulk, they are common in epitaxial films of complex crystals due to the physical constraint of the underlying substrate and a low degree of structural rearrangement during growth. OPBs can strongly affect properties, but no extensive studies of them are available. The morphology, structure, and nucleation mechanisms of OPBs in epitaxial films of layered complex oxides are presented with a review of published studies and new work. Morphological trends in two families of layered oxide phases are described. The atomic structure at OPBs is presented. OPBs may be introduced into a film during growth via the primary mechanisms that occur at film nucleation (steric, nucleation layer, a-bmisfit, and inclined-cmisfit) or after growth via the secondary nucleation mechanism (crystallographic shear in response to loss of a volatile component). Mechanism descriptions are accompanied by experimental examples. Alternative methods to the direct imaging of OPBs are also presented.

Epitaxial growth of SrBi2Nb2O9 on (110) SrTiO3 and the establishment of a lower bound on the spontaneous polarization of SrBi2Nb2O9

Epitaxial SrBi2Nb2O9 thin films have been grown on (110) SrTiO3 substrates by pulsed laser deposition. Four-circle x-ray diffraction and transmission electron microscopy reveal nearly phase pure epitaxial films with the c axis of the films at 45° with respect to the substrate normal. Electrical characterization is presented for films grown on epitaxial SrRuO3 electrodes. The low-field relative permittivity was 235, the remanent polarization was 11.4 μC/cm2, and the dielectric loss was 3.0% for 0.3-μm-thick films. From the remanent polarization and an understanding of the epitaxial geometry, a lower bound of 22.8 μC/cm2 was determined for the spontaneous polarization of SrBi2Nb2O9.