Session M23: Optical and Dielectric Properties

Temperature-Dependent Cathodoluminescence of Disordered SiO2 Layers

Optical coatings of disordered thin film SiO2/SiOx dielectric samples on reflective metal substrates exhibited electron-induced luminescence (cathodoluminescence) under electron beam irradiation. These experiments provided measurements of the absolute radiance and emission spectra as functions of incident electron energy, flux and power over a range of sample temperatures (\textless 40 K to \textgreater 300 K). The overall luminescent intensity increased linearly with increasing power, plateaued, then fell off approximately exponentially. Spectrometer data revealed four spectral bands. The structural defects associated with three of the four bands have been identified. Temperature dependence of the peak intensity and central position differs for the lower and higher energy bands. These results are interpreted with a model of the band structure of highly disordered trapped states within the band gap of SiO2, used to describe the excitation of electrons from the valence band to the conduction band and subsequent relaxation into trapped states. The cathodoluminescence model describes these experimental observations, providing a fundamental basis for understanding the dependence of cathodoluminescence on irradiation time and accumulated charge, incident flux and energy, and sample thickness and temperature.   

Mechanical properties of highly porous low-k dielectric nano-films: A Brillouin light scattering study

To reduce RC time delays in micro-electronic devices, the semiconductor industry has pursued low dielectric constant (k) hybrid organic-inorganic interconnect layers with controlled levels of porosity. However, increased porosity as well as reduced film thicknesses (\textless\ 100nm) could reduce mechanical and thermal stability thereby degrading device functionality. Such structural characteristics present limitations with traditional measurement techniques as nanoindentation to characterize the mechanical properties of these highly compact and porous structures. We report on Brillouin light scattering measurements to determine the independent elastic constants, and thus the mechanical properties, of dielectric films with thicknesses as low as 25 nm and porosity levels up to 45{\%}, the highest in the industry. The frequency dispersion and associated light scattering intensities of longitudinal and transverse acoustic standing mode type excitations were utilized to determine Poisson's Ratio ($\nu$) and Young's Modulus (E). Significant modifications were found in $\nu $ and E of these highly porous carbon-doped SiO$_{2}$(Si-O-C-H) and amorphous carbon(a-C:H) materials compared to traditional SiO$_{2}$ and non-porous low-k materials.   

Power and Charge Deposition and Electron Transport in Disordered SiO2 Layers Under Electron Bombardment

Power and charge deposition in multilayer dielectrics from electron bombardment is dependent on the flux and energy-dependent electron penetration depth of the electron beam. Using the Continuous Slow Down Approximation (CSDA), a composite analytical formula has been developed to approximate the electron range which can be related to the dose rate, deposited power and Radiation Induced Conductivity (RIC). Based on the constituent layer geometry and material, the deposited charge can also be inferred. Three separate pulsed electron beam experiments were conducted to measure charge deposition, power dependent cathodoluminescence and RIC. The power and charge deposition experiments measured the net surface potential, electrode currents and electron induced luminescence of disordered SiO2 multilayer dielectrics with a grounded or floating conductive middle layer, using beam energies from 200 eV to 25 keV at \textless 40 K to room temperature. These results showed that the power and charge deposition's dependence on electron beam flux and incident energy compare favorably with the model predictions. The RIC experiments measured electrode currents using disordered SiO2 layers from \textless 40 K to \textgreater 320 K with dose rates from 10-5 Gy/s to 10-1 Gy/s. The onset of RIC in the energy-dependant depth of the RIC region provides an explanation for observed retrograde charging.   

Structural evolution of nanoporous ultra-low k dielectrics under voltage stress

High speed interconnects in advanced integrated circuits require ultra-low-k dielectrics. Reduction of the dielectric constant is achieved via incorporation of nanopores in structures containing silicon, carbon, oxygen and hydrogen (SiCOH). We study nanoporous SiCOH films of k=2.5 and thicknesses of 40 - 400 nm. Leakage currents develop in the films under long-term voltage stress, eventually leading to breakdown and chip failure. Previous work* has shown the build-up of trap states as dielectric breakdown progresses. Using FTIR spectroscopy we have tracked the reorganization of the bonds in the SiCOH networks induced by voltage stress. Our results indicate that the cleavage of the Si-C and SiC-O bonds contribute toward increase in the density of bulk trapping states as breakdown is approached. AC conductance and capacitance measurements have also been carried out to describe interfacial and bulk traps and mechanisms. Comparison of breakdown properties of films with differing carbon content will also be presented to further delineate the role of carbon. *Atkin, J.M.; Shaw, T.M.; Liniger, E.; Laibowitz, R.B.; Heinz, T.F. Reliability Physics Symposium (IRPS), 2012 IEEE International   

Spectroscopic analysis of erbium doped laser-induced crystals for fiber-laser applications

Laser induced crystallization of glasses is a highly spatially selective process which could be used to produce crystalline-core optical fibers for fiber-laser applications. Toward this goal, single crystal lines were ``written'' in Er:LaBGeO$_{5}$ glass using a femtosecond pulsed laser. These structures were analyzed using micro-Raman and luminescence spectroscopy in order to determine their viability as waveguiding laser gain media. Two-dimensional scans reveal that the erbium fluorescence is inhomogeneous over the cross-section of the crystal and lacks spatial coordination with the Raman emission, implying a physical ion accumulation in addition to enhancement due to the crystal field. Additionally, erbium fluorescence spectra taken at low temperatures from polycrystals with varying concentrations of erbium were compared to those from the laser-induced crystal lines. Significant differences in the emission energies and intensity ratios of the erbium peaks were observed. These differences may be due to the presence of strain, grain boundaries, or charge resulting from the different crystallization processes used.   

Luminescence and Local Structure Correlation of Er-doped Glasses and Composites

Er-doped (0.05{\%} to 3{\%}) Ga2O3 containing silicate glasses and composites have been prepared by rapid coolong from the melt (glasses), followed by annealing at various temperatures from 800C to 1100C (composites). The Er luminescence has been measures and will be correlated to the llcal structural properties of the Er atoms as measured by x-ray absorption spectroscopy (XAS) at the MRCAT (Sector 10) beamline at the Advanced Photon Source. Preliminary analysis of the XAS data indicates that the Er is in an octahedral environment in both the glasses and composites. The glasses show no clustering of Er atoms which would lead to quenched lumineacence.   

Large change in dielectric constant of CaCu$_3$Ti$_4$O$_{12}$ under violet laser

This work reports the influence of light illumination on the dielectric constant of CaCu$_{3}$Ti$_{4}$O$_{12}$ (CCTO) polycrystals which exhibit giant dielectric constant. When the CCTO samples were exposed to 405-nm laser light, the enhancement in capacitance as high as 22{\%} was observed for the first time, suggesting application of light-sensitive capacitance devices. To understand this change better microscopically, we also performed electronic-structure measurements using photoemission spectroscopy, and measured the electrical conductivity of the CCTO samples under different conditions of light exposure and oxygen partial pressure. All these measurements suggest that this large change is driven by oxygen vacancy induced by the irradiation.   

Giant dielectric constant in CaCu$_3$Ti$_4$O$_{12}$-MgB$_2$ composites near the percolation threshold

We have investigated the enhancement of the dielectric constant K in CaCu$_{3}$Ti$_{4}$O$_{12}$ (CCTO)-MgB$_{2}$ composite near the percolation threshold. To optimize the dielectric properties of pure CCTO we have sintered the samples at variuos temperatures. We will present the results of the measurements of $K$ in a broad frequency for pure CCTO for the samples sintered at 1100$^{\circ}$C and 500$^{\circ}$C. Commercially available MgB$_{2}$ powder was mixed with different weight fractions of CCTO and the pressure of 1GPa was applied to form composite pellets. Near the percolation threshold P$_{\mathrm{C}}$, CCTO/MgB$_{2}$ composite system exhibit a dramatic increase of the dielectric constant K by several orders of magnitude, compared to pure CCTO. We will also discuss the magnetic field dependence of the capacitance of CCTO composite powders.   

Coupling of photonic, plasmonic and electric effects in metal nanostructures

Strong photon drag was observed in thin metal films and nanostructures, with the maximum of the effect at plasmon resonance conditions. To better understand mechanism of the effect and explore the possibility to control it with nanoscale geometry, we studied photoinduced currents in gold films and nanomesh structures in the dependence on the wavelength and period of nanostructure. We showed that nanostructuring of the surface lead to significant (50-fold) increase in the magnitude of the effect. Results are discussed in terms of coupling of optical, plasmonic and electric effects   

Plasmons for Coulomb Coupled Spherical Shells

We report calculations of the collective plasmon excitations for an electron gas confined to the surface of a spherical shell. The energy spectra of the plasmons and particle-hole modes are presented as functions of the radius of the shell as well as the angular momentum quantum number $L$. We compare results for the plasma excitations for a single shell, a pair of concentric shells as well as when two shells have their centers separated by a distance which exceeds the sum of the radii of the two shells. For the single shell and pair of concentric shells, the plasma modes are labelled by the angular momentum quantum number $L$ only. However, for the pair of non-concentric shells, the plasma modes are labelled by both $L$ and $M$, the projection of angular momentum on the $z$ axis. These results have been obtained in the random phase approximation (RPA).   

High Optical Performance and Practicality of Active Plasmonic devices based on Rhombohedral BiFeO$_3$

BiFeO$_3$ is a multiferroic oxide with perovskite type structure, which has been studied extensively for its ferroelectric and magnetic behavior. The magnetoelectric coupling could potentially provide new functionalities. We have studied the electronic and optical properties of Rhombohedral BiFeO$_3$, which we show to be a very promising candidate material to build active nanophotonic devices, in particular nanoplasmonic devices. It has a strong switching modulated optical properties and a large optical birefringence $\Delta $n arising from the combination of octahedral tilts, ferroelectricity and G-type antiferromagnetism in BiFeO$_3$. A prototype of a plasmonic resonator with a Rhombohedral BiFeO$_3$ thin film layer is used as an example and shows excellent switch and modulation responses. The proposed approach provides potential opportunities to develop high performance nanophotonic devices for optical communication. We find excellent switching and modulation responses. The use of Rhombohedral BiFeO$_3$ provides an effective way to actively control optical performance of plasmonic nanostructures.   

Taming the flow of light via active magneto-optical impurities

We demonstrate that the interplay of a magneto-optical layer sandwiched between two judiciously balanced gain and loss layers which are both birefringent with misaligned in-plane anisotropy, induces unidirectional electromagnetic modes. Embedding one such optically active non-reciprocal unit between a pair of birefringent Bragg reflectors, results in an exceptionally strong asymmetry in light transmission. Remarkably, such asymmetry persists regardless of the incident light polarization. This photonic architecture may be used as the building block for chip-scale non-reciprocal devices such as optical isolators and circulators.   

Gyro-active structures: Unidirectional Reflectionless Isolators and Perfect Absorbers

We propose a novel circuit architecture that consists of gyrotropic elements sandwiched between two judiciously balanced gain and loss constituents. These structures exhibit unique transport characteristics stemming from a generalized parity-time (${\cal P{\tilde T}}$)-symmetry. Some of these features include unidirectional reflection-less isolation and perfect absorption as well as asymmetric Anderson localization when disorder is introduced. Realizations as well as applications within the framework of electronic and photonic circuitry are discussed.   

Local heating of ZnO due to the surface plasmon excitation of Au nanoparticles

Temperature dependent $E_{2}$\textit{(high)} Raman active optical phonon mode was investigated to identify the local heating of the ZnO, due to the surface plasmon excitation of the Au nanoparticles. The variation of the linewidth (FWHM) of $E_{2}$\textit{(high)} mode for ZnO was investigated from room temperature to 450 $^{\circ}$C with 25 $^{\circ}$C steps under constant 532 nm laser excitation intensity of 2.6*10$^{5}$ W/m$^{2}$. Linewidth (FWHM) was increased with the temperature and it was fitted into the theoretical model originally developed by Menendez \textit{et al}, which contains both cubic and quadratic anharmonicities. After optimizing the cubic and quadratic anharmonic coupling constants, the fit was used to estimate the local temperatures of Au/ZnO, which were irradiated with different laser intensities. The estimated local temperature for Au/ZnO was 613 $^{\circ}$C at the laser intensity of 8.1*10$^{5}$ W/m$^{2}$. ZnO without Au nanoparticles didn't show any large temperature variation under the different laser intensities. This is a clear evidence for the heat generation of Au nanoparticles due to the surface plasmon excitation.