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Atomic Structure and Defects in Oxides

 

Defects in thin film oxides

Defects in thin film oxides figure 1

Thin films of HfO2 on silicon were probed via conductive, contact afm and defects were identified by high leakage currents. (a) The same region was imaged over a range of tip bias to create a map of leakage current and identify defects. (b) High-resolution image of one defect. Features <5 nm in size can be resolved. (c ) A-B cross section from (b).

Defects in thin film oxides figure 2

Analysis of the higher-than-expected spatial resolution showed that stress concentrations in the contact zone yielded a “focusing” of the current. Stresses under the tip lead to increased conductivity below the surface, as shown in (d). If the combination of tip an load are in the gray region of the chart, then the silicon undergoes a semiconducting-t0-metal transition. Alternatively, pressure-induced bandgap narrowing can increase conductivity if the dopant density in the substrate is sufficiently high. (e) Another high-resolution leakage current map.

[Nikiforov, Brukman, and Bonnell, Applied Physics Letters 93, 181101(2008)]


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TiO2 (110) experiment

M.Wagner#  D.A.Bonnell*  and  M.Rühle#

#Max-Planck-Institut für Metallforschung, Seestraße 92, D-70174 Stuttgart, Germany
*Dept. of Mat. Sci. & Eng., University of Pennsylvania, Philadelphia, PA 19104, USA.

A number of reconstructions have been observed on the (110) surface of TiO2 in the rutile modification that accommodate oxygen deficiency. We present atomically resolved STM images that describe a new reconstruction with (3x2) symmetry. A model for the observed reconstruction is discussed where this symmetry is achieved by removing 1/3 or 2/3 of the oxygen in the bridging oxygen rows such that a shift in position of missing oxygen by one lattice space vector along the [001] direction occurs in every third row. This structure contrasts those reported previously in which entire rows are modified or removed.

3x2lg  3x2hr 3X2CDB1


TiO2 (110) theory

The task of fully characterizing transition metal oxide surfaces is complicated substantially by the ability of many of these compounds to accommodate high degrees of nonstoichiometry.  As a result, a large variety of stable or metastable surface structures, many with large unit cells and low symmetry, have been observed.

stmtheory1_part1
15 nm x 22 nm

stmtheory2_part1
6.5 nm x 9.0 nm


G. S. Rohrer, V. Henrich, D. A. Bonnell Science 250 (1991).

Oxides Adopt Complex Surface Structures

Reconstruction or Surface Stabilized Phases!!!

Constant current STM images of crystallographic shear structures on Ti02 (110)

 

stmtheory1_part3

srti16c


Y. Liang, D. Bonnell J. Am. Cer. Soc. 78 (1995), Surf. Sci. 310 (1994) 128

Constant current STM imaging of the stabilization of Ruddleson Popper Phases on SrTiO3 (100

One successful approach to the interpretation of images of TiO2 (110) is the use of first principles pseudo-potential calculations of spatially resolved surface electronic structure of states relevant to tunneling.  However, the ability to perform first principle calculations is often removed from the experimentalist and the formidable cost and slow running time of such calculations make them impractical as a tool for in-situ image interpretation of complex surfaces.

STM Theory

The approach shown here makes use of first principles results for ideal or unreconstructed surfaces to develop a basis that is then extended to the much larger number of relatively complex reconstructed surfaces. Since Hamiltonian diagonalizations are avoided for all reconstructions, it extends the applicability of image simulation techniques to arbitrarily complex surfaces. This method is applied to the case of TiO2(110) 2x3 for which several atomic structures can be invoked to explain STM image contrast.

STM Theory


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Surface reconstruction of BaTiO3 (001)

Whether spatially ordered surface structure interacts with ferroelectric ordering is very important and remains an open question. In order to study the interactions between ferroelectric domains and surface reconstruction, STM and non-contact AFM were used to investigate (001) surface of single crystal BaTiO3. BaTiO3 (001) undergoes dramatic morphological evolution upon UHV annealing. The uppermost atoms rearrange into a variety of periodic structures at this surface depending on the thermo-chemical history. Superstructures, including the c(2x2), (2x1), c(4x4), (2x2), (r5xr5)R26.6o and a series that evolves into (3x1) or (3x2) nano-stripes have been identified by LEED. Atomic resolved STM images of (r5×r5)-R26.6o, (3×1) and (3×2) surface superstructures have been obtained for the first time. A mechanism based on ordered Ba-adatom is proposed to explain the formation of various surface reconstructions. The reconstruction has not been found to affect the ferroelectric nature of the surface to the first order.

Surface reconstruction of BaTiO3 (001)


Atomic structure of SrTiO3 (100) upon surface reduction

Due to concerns over problems with sample conductivity, the first atomic resolution STM images on oxides were obtained on severely reduced crystals and therefore produced surface structures that differed from the ideal termination of the lattice. The first oxide imaged was TiO2 (110) on which the periodic surface structures were attributed to stabilization of phases at the surface based on crystallography. Similarly, the first images of atomic scale structure on SrTiO3 were obtained on significantly reduced (001) surfaces, figure 8.Large area scans illustrate the row-like structure observed on polished and annealed surfaces. The structure consists of corrugations of several distinct periodicities, in this case 4 Å and 12 Å, but always multiples of 4 Å. Corrugation amplitudes vary between 0.5 Å and 3 Å and atomic detail is observed in some regions. A small region imaged with high resolution emphasizes the structural detail of the corrugations in a region containing 12 Å and 20 Å corrugations. The variations in corrugation periodicity on this surface cannot be described in terms of missing oxygen rows because it was found that not only had oxygen left the surface but the Sr:Ti ratio changed. The structures were described in terms of surface stabilized Ruddleson Popper phases, known to be stable in bulk perovskite, and occurring as coherent intergrowths of lamellae of reduced Srn+1TinO3n+1. The structures are related to perovskite as shown below. The corrugation periods for the structure in the figure are those of Sr2TiO4 and Sr3Ti2O7.These trends in stoichiometry were confirmed by AES, and Rutherford back scattering spectroscopy (RBS).

Atomic structure of SrTiO3 (100)

Atomic structure of SrTiO3 (100)


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Structure and charge density waves of blue bronze

Variable temperature constant current STM measurements in the figure, the “apparent” surface structure changes as the temperature is lowered from 300K to 100K. At 300K only one periodic structure is present on the (20figure image) surface,with lattice vectors 1 = 1.2 nm and b = 0.76 nm. The modulus of the lattice vector b (see inset in fig. A) agrees with the corresponding X-ray diffraction-determined lattice parameter in the bulk of 0.756 nm. The modulus of vector 1 matches the known bulk distance between (112) planes, and the angle between vectors 1 and b also agree with the bulk angle. At 140K (below the Peierls transition), two periodic structures are evident from peak splitting in the FFT, Fig. B. One structure has the lattice parameters of the atomic structure at 300K, consistent with the small change in bulk lattice parameters (<1%) in this temperature range. The second structure is smaller (vector 1 = 1.2 nm, vector b = 0.71 nm) with slightly different angles between the surface unit vectors. Lowering the temperature down to 100K results in vanishing of FFT peak splitting (fig. C) indicating the transition from incommensurate to commensurate CDW.

We attribute this structure to the CDWs on the surface. The corrugation associated with the CDW is not distinguished in the profile, but the effect of the additional intensity at the atomic sites is clear.  The superposition of the two results in an underlying low frequency component with a period 11 times that of the lattice.  This represents a lattice mismatch of  about  9%, which  agrees with bulk measurements (~ 9% at 140K).  The separation of the two lattices is evident (on Fig. E) and illustrates mismatch on the order of 10% along the b direction and almost commensurate alignment along the a + b + 2c direction.

blue bronze


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Evolution of Structure and Thermodynamic Stability of the BaTiO3(001)

We report a series of new surface reconstructions on BaTiO3(001) as a function of environmental conditions, determined via scanning tunneling microscopy and low energy electron diffraction. Using density functional theory calculations and thermodynamic modeling, we construct a surface phase diagram and determine the atomic structures of the thermodynamically stable phases. Excellent agreement is found between the predicted phase diagram and experiment. The results enable prediction of surface structures and properties under the entire range of accessible environmental conditions.

figure


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Atomic and Electronic Structure of the BaTiO(001) (√5 x √5) R26.6 Surface Reconstruction

figureUsing both experimental scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements, we were able to further narrow down the candidate structures, and conclude that the surface is either TiO2 –Ti3/5, TiO2 –Ti4/5 , or some combination, where Ti adatoms occupy hollow sites of the TiO surface. DFT indicates that the defect states close to the valence band are from Ti adatom 3d orbitals (≈1.4 eV below the conduction band edge) in agreement with STS measurements showing defect states 1.56 ± 0.11 eV below the conduction band minimum (1.03 ± 0.09 eV below ).  STM measurements show electronic contrast between empty and filled states images. The calculated local density of states at the surface shows that Ti 3d states below and above explain the difference in electronic contrast in the experimental STM images by the presence of electronically distinctive arrangements of Ti adatoms. This work provides and interesting contrast with the related oxide SrTiO, for which the (001) surface reconstruction is reported to be the TiO surface with Sr adatoms.

 


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On the Relationship Between Surface Reconstructions and Step Edge Stability on BaTiO3 (001)

figureThe step orientations on four BaTiO3 (001) surfaces were quantified from Scanning Tunneling Microscopy images in order to determine the relationship between surface reconstructions and step stability.   The (1x1), c(2x2), (√5x√5) R26.6°, and (√13x√13) R33.7° surface reconstructions are compared.  A well-known procedure that renders non-polar and low energy surface energies is applied to consider model step face structures, taking into account for the first time the adatoms of the surface reconstruction.   A comparison of the number of steps with two crystallographically equivalent step face structures but different reconstruction termination shows that the surface adatoms affect the stability of steps.  Except for the (1x1) surface Ti adatoms influence the choice of crystallographic direction of the step edge in all the surfaces mentioned above.  The observed step edge directions are in accordance with general energetic principles where the facets exposed are most favorable surface planes in the cubic system.

 


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A Kinetic Product, Thermodynamic Stability, and Phase Boundaries on the c(2x2) and c(4x4) reconstructions of BaTiO3(001)

figureThe step orientations on four BaTiO3 (001) surfaces were quantified from Scanning Tunneling Microscopy images in order to determine the relationship between surface reconstructions and step stability.   The (1x1), c(2x2), (√5x√5) R26.6°, and (√13x√13) R33.7° surface reconstructions are compared.  A well-known procedure that renders non-polar and low energy surface energies is applied to consider model step face structures, taking into account for the first time the adatoms of the surface reconstruction.   A comparison of the number of steps with two crystallographically equivalent step face structures but different reconstruction termination shows that the surface adatoms affect the stability of steps.  Except for the (1x1) surface Ti adatoms influence the choice of crystallographic direction of the step edge in all the surfaces mentioned above.  The observed step edge directions are in accordance with general energetic principles where the facets exposed are most favorable surface planes in the cubic system.

A combined experimental and theoretical study elucidates the  c(2x2) and c(4x4) surfaces of BaTiO3 (001).   Scanning Tunneling Microscopy (STM) proves the coexistence of these two phases where Density Functional Theory calculations asserts that the c(4x4) is a thermodynamically stable phase with the coexistence of the c(2x2) due to a more kinetic favorable path of formation.    The boundaries between the two phases were successfully calculated and match STM atomically resolved images.  The stoichiometry of the c(2x2) and c(4x4) surfaces are TiO2-(TiO)1/2 and TiO2-(Ti3O3)1/8 (TiO)1/8 , respectively.  Other than the size of the surface unit cell we find that the main difference between the c(2x2) and c(4x4) is that the latter introduces a Ti-O cluster for its stabilization.

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