Session Y11: Glassy and Amorphous Systems, Including Quasicrystals followed by Epitaxial Growth and Structure of Oxides

Hydrogen microstructure of amorphous silicon via inversion of nuclear magnetic resonance spectra: A moment-based approach

We present an inverse approach for reconstructing hydrogen microstructure in amorphous silicon (a-Si). The approach consists of generating a prior distribution (of spins) by inverting experimental nuclear magnetic resonance (NMR) data, which is subsequently superimposed on a network of a-Si. The resulting network is then relaxed using a total-energy functional to obtain a stable, low-energy configuration such that the initial spin distribution is minimally disturbed. The efficacy of this approach is demonstrated by generating model configurations that not only have the correct NMR spectra but also satisfy simultaneously the experimental structural, electronic and vibrational properties of hydrogenated amorphous silicon.   

Antiferromagnetic order in the Cd$_6$\textit{R} (\textit{R} $=$ rare earth) quasicrystal approximants

Many theoretical treatments of spins on aperiodic lattices support the notion of long-range antiferromagnetic order. However, to date, there has been no experimental confirmation of long-range magnetic order in quasicrystalline systems. The absence of long-range magnetic order extends to crystalline approximant phases of the icosahedral structures as well. Surprisingly, the 1/1 approximant to the Cd-Mg-R icosahedral phases, Cd$_{6}R$, appears to be an exception to the rule. Here, we report on the results of x-ray resonant magnetic scattering measurements on Cd$_{6}R$ approximants which show that long range antiferromagnetic order is, indeed, realized. For $R \quad =$ Tb and Ho, viewing the structure as a body-centered cubic packing of Tsai clusters, we find that the $R$ ions associated with the icosahedral cluster at the corner of the unit cell are antiferromagnetically correlated with the $R$ ions associated with the icosahedral cluster at the body-center of the unit cell.   

Fragility, slow homogenization and Intermediate Phase in the Si$_x$Ge$_x$Te$_{100-2x}$ ternary

Small sized (0.5g) melts were synthesized by reacting pure elements in 5mm ID quartz tubes at 950C, and examined after 1 week and then 2 weeks of reaction. Bulk glass formation is realized in 6{\%}\textless x\textless16{\%} range with Tg(x) increasing linearly in 6{\%}\textless x\textless12{\%} range, and decreasing thereafter (x\textgreater12{\%}). The enthalpy of relaxation at Tg shows a flat bottomed minimum in 7.5{\%}\textless x\textless9.0{\%} range with the term increasing sharply at x\textgreater9{\%} and at x\textless7.5{\%}. We identify the 7.5{\%}\textless x\textless9.0{\%} range with the Intermediate Phase. Fragility(m) of melts were established in complex Cp measurements, and show a global minimum (m\textless30) in the IP range, and a value of m=26 at x=8.5{\%}. The slow homogenization of Telluride melts results from the \textit{strong} character of IP melts. Raman scattering, excited using low power density of 785nm radiation, shows evidence of a broad mode near 160cm$^{-1}$ (characteristic of a-Te chains) and a narrower one near 127cm$^{-1}$ (group IV crosslinking units). The scattering strength of the 127cm$^{-1}$ mode increases at the expense of the 160cm$^{-1}$ mode as x increases. The nature of structure evolution with glass composition will be commented upon.   

Slow kinetics of melt homogenization and strong nature of intermediate phase melts in chalcogenides

The strong-fragile classification of melts is manifested in the T-dependence of viscosity. Strong (fragile) melts possess a T-independent (dependent) activation energy of viscosity leading to an Arrhenian (non-Arrhenian) behavior reflecting the robust (weak) nature of network structure. We have now measured [1] complex C$_p$ of binary Ge$_x$Se$_{100-x}$ glasses as a function of x, and find that in dry and homogeneous melts, fragility (m(x)) shows a global minimum (m \textless\ 20) in the Intermediate Phase (IP) compositions (19.5{\%} \textless\ x \textless\ 26{\%}) but increases rapidly outside the IP. These findings have a direct bearing on synthesis of non-stoichiometric melt compositions at elevated temperatures in which IP melt compositions serve as a bottleneck [1] to homogenize [2] batches globally. The physical properties of dry and homogeneous glasses differ significantly from their inhomogeneous counterparts, and have led, in general, to differences in results reported by various groups.\\[4pt] [1] K. Gunasekera et al, ``Fragility and kinetics of melt homogenization of network glasses''(In preparation).\\[0pt] [2] S. Bhosle et al., Solid. St. Comm. 151, 1851-1855 (2011).   

Fragility and slow kinetics of melt homogenization in the As-Se binary

Two gram sized As$_{x}$Se$_{100-x}$ batches at various As content x were synthesized using pure Se and As$_{2}$Se$_{3}$ as starting materials that were reacted at 700$^{o}$C. Such melts typically took 3-12 days to homogenize, as monitored in punctuated, off-line FT-Raman line profiling\footnote{S. Bhosle et al., Sol. St. Commun.151, 1851-1855 (2011)} experiments. We have now undertaken mDSC experiments as a function of modulation frequency to establish the compositional dependence of complex C$_{p}$(x), and deduce the variation of fragility m(x). We find the fragility to be rather low, m $<$ 20, across the 22\% $<$ x $<$ 38\% range, and to rapidly increase at x $<$ 22\% to acquire a value of 43 near x = 3\%. We show that the slow melt homogenization is a direct consequence of the ``strong'' character of melts that serves as a bottleneck in melt-mixing at high temperatures. Once homogenized, physical properties of glasses, such as density, glass transition temperature T$_{g}$(x), the Intermediate phase, and variation of enthalpy of relaxation at T$_{g}$(x) differ significantly from their inhomogeneous counterparts.   

Quantitative description of orientational order in non-graphitic carbons

The key factor that determines the ability to graphtize of a non-graphitic material is believed to be the level of orientational disorder which indicates how well the elemental structures are aligned. To characterize the disorder, we have developed a correlation function with multiple variable dependencies, such as radial distance and zenith angle. Through the characteristic parameter of the function, the ability to graphitize can be determined given the structure of a carbon material. The model is applied to a set of non-graphitic structures, which is generated systematically in non-conventional methods that emphasize to represent the orientational order of the carbon material rather than to match the radial distribution function.   

Localization and percolation in random elastic networks

We consider a minimal model for a disordered phonon system that shows rich behavior in the localization properties of the phonons. We use a percolation analysis to argue for a localization/delocalization transition of the phonon modes and predict the speed of sound in the delocalized region, with comparison to numerics. We show that in contrast to the behavior in electronic systems (cf. Anderson localization), the transition exists for arbitrarily large disorder, albeit with an exponentially small critical frequency. The structure of the modes reflects a divergent percolation length that arises from the disorder in the springs without being explicitly present in the definition of our model. We calculate the critical frequency as a function of density and test the prediction numerically using a recursive Green function method. We further explore the existence of delocalized states in the two-dimensional version of this model.   

Stoichiometric SrTiO3 Films via High Pressure Oxygen Sputter Deposition

Defect management in epilayers of semiconducting complex oxides such as SrTiO$_{3}$ is a topic of considerable contemporary interest. Recent work has shown that sufficiently precise control over stoichiometry and defects in SrTiO$_{3}$ enables facile$ n$-type doping, record high mobilities, and even simultaneous observation of quantum oscillations and superconductivity. Such progress has typically been made using techniques such as oxygen/LASER MBE or high-temperature PLD. In this work we demonstrate, via homoepitaxy on SrTiO$_{3}$(001), that RF high pressure oxygen sputtering from a ceramic target is similarly capable of growth of high-quality, stoichiometric SrTiO$_{3}$ films. We show that optimization of the deposition temperature (above 750 $^{\circ}$C) and oxygen pressure (above 2.5 mBar) leads to the deposition of films indistinguishable from the substrate via grazing incidence and wide-angle x-ray scattering. The importance of a pre-treatment of the substrates in oxygen above 900 $^{\circ}$C is emphasized. The defect density/stoichiometry was further probed via the transport properties of vacuum annealed samples with controlled O vacancy density. Finally, we also demonstrate that the stoichiometry and defect density of films deposited under non-optimal conditions can be remarkably improved via post-deposition heat treatment.   

Time-resolved \textit{in-situ} X-ray Study of Homoepitaxial SrTiO$_3$ Growth Using Reactive Molecular-Beam Epitaxy

Functional materials based on complex oxides in thin film form offer new and exciting strategies for meeting many energy challenges. Unfortunately, synthesis of such oxide films can be a major challenge even when utilizing reactive molecular-beam epitaxy (MBE). To understand the fundamental physics of complex oxide thin film growth, we have developed the world's first reactive MBE system with {\it in-situ} synchrotron x-ray scattering capability at the Advanced Photon Source (APS). Here we present the results of {\it in-situ} surface x-ray scattering measurements taken during homoepitaxial growth of SrTiO$_3$ on (001) SrTiO$_3$ substrates. We compare the shuttered growth technique with codeposition to understand the nature of the distinctly different approaches. Work at the APS, Argonne is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.   

STM Studies of Sub-monolayer SrO and LaAlO$_{3}$ Film Growth on SrTiO$_{3}$(001) Substrate Surfaces

We report atomic-scale observations of initial growth of sub-monolayer SrO and LaAlO$_{3}$ (LAO) films on the atomically-ordered ($\sqrt{13}$$\times$$\sqrt{13}$)-$\textit{R}$33.7$^{\circ}$ SrTiO$_{3}$ (STO) (001) substrate surfaces using scanning tunneling microscopy/spectroscopy (STM/STS). We found that the growth processes depend strongly on the film compositions and the investigations unveil complex chemistry of thin-film oxides. These findings will provide microscopic insights into the understanding of transport properties at the LAO/STO interface, which is known to exhibit conducting and insulating behavior depending on the termination structures of STO substrates, namely, whether ``TiO$_{2}$-" or ``SrO-terminated" surfaces, respectively. Controlling the interface structure genuinely with atomic precision will eventually lead to the creation of exotic electronic phenomena and functionalities at the complex oxide interfaces.   

In-situ Surface X-ray Diffraction Study of Ruddlesden-Popper Series Thin Film Growth

The layered Ruddlesden-Popper phases of A$_{n+1}$B$_{n}$O$_{3n+1}$, such as Sr$_{2}$TiO$_{4}$ and La$_{2}$NiO$_{4}$, have attracted much attention as potential materials for solid-oxide fuel cell cathodes and thermoelectrics. To understand the fundamentals of this class of layered oxide thin films, we studied the growth of (001)-oriented Sr$_{2}$TiO$_{4}$ and La$_{2}$NiO$_{4}$ on SrTiO$_{3}$ substrates by using oxide molecular beam epitaxy with in-situ surface x-ray diffraction. For Sr$_{2}$TiO$_{4}$, the synthesis of the double SrO layer followed by TiO$_{2}$ dynamically reconstructs back into the SrTiO$_{3}$ phase, which demonstrates that during thin film deposition other pathways under growth conditions can give rise to new structural arrangements. In contrast with Sr$_{2}$TiO$_{4}$, the growth of La$_{2}$NiO$_{4}$ involves the stacking of polar LaO$^{+}$ and NiO$_{2}^{-}$ layers. This raises the question of how polarity mismatch at the interface with the SrTiO$_{3}$ substrate will influence the growth process. A detail comparison of these two cases will be discussed. Work at the Advanced Photon Source, Argonne is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.   

Atomic Resolution and First Principles Study of the Electronic Structure at SrTiO$_{3}$/GaAs Hetero-interfaces

We examined ultra-thin SrTiO$_{3}$ films deposited on As-terminated GaAs (001) using molecular beam epitaxy under various O$_{2}$ partial pressures. Atomic-resolution Z-contrast images of different SrTiO$_{3}$ films were obtained using the aberration-corrected JEOL JEM-ARM200CF operated at 80 kV. Using atomic-column resolved EELS, our analysis of the Ti and O near-edge fine structure reveals different bonding configurations at the interface resulting from different growth methods. These results strongly suggest that a Ti pre-layer deposition alleviates the oxidation of the substrate and consequently the Fermi level pinning at the interface, as reported before. We also examined BaTiO$_{3}$ thin films grown on GaAs (001) with an ultrathin SrTiO$_{3}$ buffer layer. Interfacial charge distribution related to the polarization of BaTiO$_{3}$ thin film will be studied using atomic-resolution Z-contrast images, annular bright field images and EELS. Using first-principles DFT calculations, we analyze the formation energies of Ti-related impurity defects in different GaAs surface reconstructions to help interpret the electron microscopy experiments.   

Temperature-driven irreversible phase transition of Sr template for epitaxial SrTiO$_{3}$ growth on vicinal Si (001)

Strontium titanate (STO) grown epitaxially on silicon has been an area of interest both for its own properties as a high-k dielectric and its capacity to act as a substrate for other crystalline oxides. In this study, we investigate STO growth on a 4$^{\circ}$ miscut Si (001) surface with double atomic steps to enhance our understanding of submonolayer Sr deposition and STO growth. It is well-known that a half-ML of Sr on the Si surface is a necessary prerequisite for crystalline growth; however, detailed study of reflection high-energy electron diffraction (RHEED) pattern during Sr deposition at various substrate temperatures reveals two distinct surface reconstructions at half-ML coverage. At temperatures below 350$^{\circ}$C, the 2x1 pattern is nearly identical to that of clean Si, but as the temperature is increased, we see the irreversible appearance of a 2x spot parallel to the step edge while the 2x spot perpendicular to the step edge dims. We also find that crystalline STO can be grown on both of these high- and low-temperature templates, with identical RHEED and band alignment as determined by XPS, showing that this previously unexplored low-temperature template can provide an alternative route for STO growth on Si.   

Correlation effects on the different crystal structures of AO2 (A=Na, K, and Ba)

In alkali superoxide (A=Na, and K), the structural phase transition from high symmetry to low symmetry structures occurs upon cooling. On the other hand, in BaO$_2$ peroxide, the crystal structure is always the high symmetry tetragonal structure of KO$_2$, independent of temperature. To resolve these different crystal structures of AO$_2$ (A=Na, K, and Ba), we have calculated phonon dispersions of AO$_2$, assuming the high symmetry tetragonal structure of KO$_2$ with first-principle band structure method in the generalized gradient approximation (GGA) incorporating the Coulomb interaction U (GGA+U). From softened phonon modes, we have shown that, in KO$_2$ and NaO$_2$, the degeneracy of the incomplete pi anti-bonding level is lifted with the symmetry lowering such as Jahn-Teller effect with help of Coulomb correlation U. In contrast, in BaO$_2$, the pi anti-bonding level of the peroxide is completely filled without degeneracy. Thus, U is not effective on the phonon structure so that the structural instability does not occur in BaO$_2$.   

Fragility, Intermediate Phase and Polaronic conductivity in heavy metal oxides

The (B$_{2}$O$_{3})_{5}$(TeO$_{2})_{95-x}$(V$_{2}$O$_{5})_{x}$ ternary forms bulk glasses over a wide range of compositions, 18{\%} \textless\ x \textless\ 35{\%}. Complex C$_{p}$(x) measurements as a function of modulation frequency reveal that melt fragility (m) show a global minimum (m $=$ 52(2)) in the 23{\%} \textless\ x \textless\ 26{\%} range with m \textgreater\ 65 outside that window. These results suggest more stable network structure in the window than outside it. The fragility window coincides with a global minimum of the non-reversing enthalpy of relaxation at T$_{g}$, the reversibility window (23{\%} \textless\ x \textless\ 27{\%}), a behavior also found in chalcogenide glasses. Conductivity ($\sigma )$ data show three regimes of variation; a low $\sigma $ at x \textless\ 23{\%}, a plateau in 23{\%} \textless\ x \textless\ 27{\%}, and an exponential increase at x \textgreater\ 27{\%}. The reduced activation energy for conductivity at x \textgreater\ 27{\%} is consistent with increased polaronic mobility as the network becomes flexible. These findings show glasses at x \textless\ 23{\%} are \textit{stressed-rigid}, in 23{\%} \textless\ x \textless\ 27{\%} range in the \textit{Intermediate Phase}, and at x \textgreater\ 27{\%} to be \textit{flexible}.