Session Y42: Dielectrics: Optical and Bulk Properties

High-Resolution Thermal Expansion Measurements of H$_{2}$O Ice

Water is one of the most important substances in nature. Surprisingly, little detail is known about its thermal expasion due to the low-resolution of past measurements. The goal of this research is to measure the thermal expansion of single crystalline H$_{2}$O ice Ih with $\sim$$10^{4}$ times greater relative resolution than has previously been done. This presentation will discuss single-crystal growth and characterization, our high-resolution thermal expansion technique, some of the challenges we faced in carrying out the measurements, and briefly present preliminary measurements on the thermal expansion and heat capacity of polycrystalline ice. The thermal expansion in the vicinity of the glass transition at $\sim$110K [1] will be discussed. \\[4pt] [1] Suga, H., \textit{Thermochimica Acta}, \textbf{300}, pp. 117-126, 1997.   

Transverse Light Localization in waveguide arrays with random absorption or amplification

We investigate the possibility to induce transverse localization of light in an array of waveguides with randomness pertaining to the imaginary part of the dielectric constant. Although this new set-up is distinct from the traditional Anderson scenario, where localization emerges due to multiple scattering from a real random potential, we find that disordered amplification/attenuation can also lead to exponential localization. We quantify the degree of localization of the Floquet-Bloch modes of our system via their participation number, which is shown to satisfy a one-parameter scaling theory. The effects of this transverse localization of the normal modes in the paraxial beam propagation are theoretically predicted and confirmed by numerical experiments.   

Attosecond spectroscopy of band-gap dynamics excited by the electric field of light

The basis of modern electronics and information processing is the control of the electric properties of semiconductors with microwave fields. Speeding up electronics requires extending this control to optical frequencies. We apply attosecond solid state spectroscopy to investigate and compare light field induced ultrafast carrier dynamics in a prototypical semiconductor (silicon) and dielectric (SiO$_{2})$. After excitation by a highly intense few-cycle visible laser pulse, a time-delayed extreme ultraviolet attosecond pulse centered around the Silicon L-edge transition maps the conduction band population and thus probes the unfolding electronic dynamics with sub femtosecond resolution. While the induced changes in SiO$_{2}$ appear only in the presence of the strong light field, the experiment on silicon measures a permanent population transfer into the conduction band triggered by the electric field of light as well as ultrafast renormalization of the band structure.   

Small Polaron Conduction in Glasses: Established Physics and New Data

Electronic and optical phenomena in crystals and glasses can be usefully analyzed in the band-structure framework, which determines the ways electrons are allowed to behave inside these materials. Especially interesting is the wide array of phenomena that can be observed in glasses. Disorder in glasses leads to high rates of electron scattering, large band-gaps, and, for strong enough atomic interactions, the localization of electrons. Modes of conduction, such as site hopping, still exist for these trapped electrons; resistivity can be greatly changed depending on the environment created by the inclusion of various ions. In this presentation, I will begin with the band-structure model for crystals and then glasses. I will discuss the ways electrons propagate in these materials and how optical processes depend on the allowed energy states of those electrons. I will also explain how the mixing of different ions in glasses leads to striking changes in their properties. Finally, I will present hot off the alumina substrate data indicating the curious existence of trapped electron hopping in bismuth containing borosilicate glasses.   

Leakage current mechanisms in PECVD-grown amorphous hydrogenated boron carbide thin films

Thin-film amorphous hydrogenated boron carbide (a-BxC:Hy), grown by plasma-enhanced chemical vapor deposition (PECVD) from orthocarborane (C2B10H12), has emerged as a promising semi-insulating, moderately high bandgap (2--4 eV), p-type material for direct-conversion solid-state neutron detector and low-dielectric-constant (low-$\kappa )$ intra/interlayer dielectric (ILD) applications. Attaining a complete understanding of the electrical transport properties for amorphous semiconductors is challenging, but essential for material maturation and optimization. For the above applications, understanding leakage current mechanisms, in both the low and high field regimes, is particularly relevant. This contribution will shed light on the charge transport mechanisms in a-BxC:Hy and discuss the role played by Urbach energy and band gap. Current density (J) as a function of field (E) was measured for a range of films grown with different PECVD parameters, and the resulting J--E curves were analyzed. The band gap and Urbach energy were measured by fitting absorption coefficient data obtained from transmission UV-Vis spectroscopy in the Tauc and exponential regions. We will discuss how optimizing bandgap and Urbach energy can be used to improve leakage current.   

Magnetic-Field Dependent IR Active Modes in CrOCl

CrOCl is a magnetoelastic material in which a correlation between an antiferromagnetic transition at 13.5K and a lattice structural distortion was discovered in the absence of the magnetic field. An applied magnetic field is known to induce a spin-flip transition at around 4T, while a correlation between this magnetic transition and the lattice order has not been observed. Greenish leaves-like CrOCl crystals are carefully laid upon the Scotch tape to create a film-like sample for IR transmittance and reflectance measurements at 4K in the magnetic field as high as 35T. Several sharp optical absorptions have been observed and they can be attributed to the IR-active phonon modes in CrOCl lattice. Among them, several modes exhibit a strong correlation to the spin-flip transition, implying a magnetic-field induced structural transition. Moreover, evolution of these modes agrees with the magnetic hysteresis of the spin-flip transition. Implications of the strong magnetoelastic coupling in CrOCl will be discussed in the presentation.   

Ferroelectric nanoparticles and their use in disparate optical devices

The fabrication [1] and ``harvesting'' [2] of stressed ferroelectric nanoparticles and the characterization of these materials will be discussed. Due to the induced surface stress in \textless 10 nm size nanoparticles, a strong spontaneous polarization is achieved (4-5 times greater than found in the bulk for the case of BaTiO$_{3})$ [3,4]. These materials have been characterized in both isotropic and anisotropic liquids. The benefits of using these nanoparticles have been demonstrated by a significant enhancement in the field sensitivity (display) and optical gain (hybrid photorefractive) liquid crystal systems.\\[4pt] [1] H. Atkuri, G. Cook, D. R. Evans, C.-I. Cheon, A. Glushchenko, V. Reshetnyak, Yu. Reznikov, J. West, K. Zhang, \textit{Journal of Optics A: Pure and Applied Optics}, \textbf{11,} 024006 (2009).\\[0pt] [2] G. Cook, J. L. Barnes, R. F. Ziolo, A. Ponce, V. Yu. Reshetnyak, A. Glushchenko, S. A. Basun, P. P. Banerjee, D. R. Evans, \textit{J. Appl. Phys. }\textbf{108,} 064309 (2010).\\[0pt] [3] S. A. Basun, G. Cook, V. Yu. Reshetnyak, A. V. Glushchenko, and D. R. Evans. \textit{Phys. Rev. B}, \textbf{84,} 024105 (2011).\\[0pt] [4] D. R. Evans, S. A. Basun, G. Cook, I. P. Pinkevych, and V. Yu. Reshetnyak, \textit{Phys. Rev. B}, \textbf{84,} 174111 (2011).   

Dielectric function of NiO and Si from 25 meV to 6 eV: What's the difference?

Using spectroscopic ellipsometry, we determined the dielectric function of bulk NiO and Si from 25 meV to 6 eV to compare their lattice dynamics and electronic structure. From 0.7 to 6.6 eV, we studied the temperature dependence of the dielectric function from 77 to 800 K. In the visible and UV spectral region, both materials have a remarkably similar dielectric function: Both materials are transparent in the near-infrared. A slow rise of the absorption with increasing photon energies throughout the visible is followed by a sharp peak at 3.4 eV (Si) and 3.8 eV (NiO). In Si, this peak is caused by transitions from the highest valence band to the lowest conduction band along the (111) direction of the Brillouin zone. In NiO, this peak is associated with the charge-transfer gap. In both materials, the peaks broaden and redshift with increasing temperature, due to electron phonon interactions. Many recent band structure calculations for NiO focus on this main charge-transfer absorption peak (3.8 eV) and ignore the absorption below the main peak. In Si, this absorption rises smoothly (due to indirect transitions), but several peaks appear in the visible absorption of NiO, which apparently do not depend on temperature. In the infrared, the lattice absorption of Si is small because of inversion symmetry. In NiO, we find strong TO phonon absorption, which is modified by the antiferromagnetic ordering.   

Probing the mechanical properties of high-k dielectric nano-films by Brillouin light scattering study

As microelectronic transistors scale to smaller dimensions, device functionality suffers from current leakage. This problem can be overcome by using thicker gate materials with a high dielectric constant. SiO$_{\mathrm{2}}$ has been the material of choice, but becomes unsuitable due to its relatively low dielectric constant (k $=$ 3.9). Alternate materials, such as BN:H (k $=$ 5.7) and HfO$_{\mathrm{2}}$ (k $=$ 25) are promising choices to replace SiO$_{\mathrm{2}}$ to achieve the desired performance while preserving ultra-thin thickness (\textless 10 nm). Despite these promising features, one concern of including these materials, are their mechanical and thermal properties that could degrade device functionality. There is thus a growing need for non-destructive techniques to evaluate the mechanical properties of such laminar structures since traditional methods like nano-indentation are not effective at these dimensions. We report on Brillioun light scattering studies to determine the individual elastic constants and, thus the mechanical properties of BN:H and HfO$_{\mathrm{2}}$ high-k films with thicknesses as low as 24 nm. Young's modulus (E) and Poisson's ratio ($\nu )$ were determined by measuring the frequency dispersion of confined and traveling transverse and longitudinal acoustic waves as well as their associated light scattering intensities.   

Refractive Index Variation of Zn-rich BaZnO Alloys Grown by Pulsed Laser Deposition

Ba$_{x}$Zn$_{1-x}$O alloys have been grown by pulsed laser deposition on sapphire(0001) substrates. Three concentration were investigated, $x =$ 0.05, 0.1, and 0.25 . The XRD of the films, all concentrations, did not exhibit significant peaks, indicating amorphous structures yet film of $x =$ 0.05 exhibited a very weak peak representing little crystallite within the amorphous surrounding. Spectroscopic Ellipsometry measurements were carried out to probe the optical properties of the films and the topography of their structures by an optical means. It was found that the addition of Ba to the ZnO film reduced the index of refraction for the $x =$ 0.05 and 0.1. However, when Ba doping was increased the index of refraction increased. Moreover, Ba-dpoed ZnO with $x =$ 0.05 and 0.1 barium had homogenous films while at $x =$ 0.25 the film incorporated voids, as indicated by the Elipsometric analysis as well. Funding is provided by Saudi National Plan for Science and Technology; the funding {\#} is 10-NAN1197-02.   

Microscopic Insight into the Isosymmetric Jahn-Teller Bond Axis Reorientation in Na$_3$MnF$_6$

Using first-principles density functional theory calculations, we investigate the hydrostatic pressure-induced spontaneous reorientation of the Jahn-Teller bond axis in the fluoride cryolite Na$_3$MnF$_6$. We find a first-order isosymmetric transition occurs between crystallographically equivalent monoclinic structures at approximately 2.15 GPa, consistent with earlier experimental studies. Analogous calculations for Na$_3$ScF$_6$ show no evidence of a transition up to 6.82 GPa. Mode crystallography analysis of the pressure-dependent structures in the vicinity of the transition reveals a clear evolution of the Jahn-Teller bond distortions in cooperation with an asymmetrical stretching of the equatorial fluorine atoms in the MnF$_6$ octahedral units. We identify a change in orbital occupancy of the e$_g$ manifold in the d$^4$ Jahn-Teller active Mn(III) to be responsible for the transition, which stabilizes one monoclinic P2$_1$/n variant over the other. From our results, we conjecture that the same transition may be accessible in epitaxially grown thin films of Na$_3$MnF$_6$ with a modest biaxial tensile strain.