Session Y24: Insulators: pt. Defects, Dielectrics, and Scintillators/Phosphors

Defect diffusion in CaF$_{2}$ for optical applications

Single crystal calcium fluoride (CaF$_{2}$) is an important lens material for deep-ultraviolet optics and a key material for 193 nm lithography. The exposure to high radiation densities requires extreme laser-stability of the material, which is to a large part ensured by a high purity level. However, for long exposure times the optical quality of CaF$_{2}$ is affected by radiation-induced point defects, namely F- and H-centers. The migration and agglomeration of these point defects play an important role in understanding laser-damage processes on a microscopic level. We use ab-initio methods to investigate the stabilization of laser-induced point defects. As stabilization processes involve defect migration, we also focus on diffusion properties of defects. We present results for the diffusion barrier and details of the transition state of the migrating F-center, which shows good agreement with experimental results.   

Transient absorption centers in CaF$_{2}$ under 157-nm irradiation

With an 11.8 eV band gap, CaF$_{2}$ is nominally transparent at 157 nm. However, exposure to x-rays, vacuum ultraviolet photons, or energetic electrons can produce defects that degrade its optical properties. We report transient VUV and UV absorption features centered at 153 nm, 220 nm, and 375 nm that appear during pulsed 157-nm irradiation. We attribute the 153-nm feature to substitutional hydride ions (H$_{s}$$^{-}$ ); the 220- nm feature to electrons trapped at Ca$^{2+}$ interstitials; and the 375-nm feature to F-centers. Annealing in hydrogen strengthens the transient feature at 153 nm, consistent with the H$_{s}$$^{-}$ absorption. During prolonged exposure, the transmission at these wavelengths drops by 1-3\%. When irradiation ceases, the transmission partially recovers. When irradiation is resumed, however, transmission drops rapidly to the previous low value. We propose that these defects are created by the laser and that charge transfer to and from these defects is responsible for the resulting absorption and recovery. A model incorporating bimolecular recombination provides a good description of the kinetics of transient absorption and recovery.   

Study of the Ta substitutionality in pulsed laser deposited Ta doped TiO$_{2}$ films by Rutherford backscattering-ion channeling spectroscopy

In the present work we report on the growth of high quality epitaxial TiO$_{2}$ thin films after 0-7.5{\%} Ta doping at 500-800$^{\circ}$C in an oxygen partial pressure of 10$^{-1}$ -10$^{-6}$ torr on (001) LaAlO$_{3}$ substrate by pulsed laser deposition (PLD) technique. Rutherford backscattering-Ion channeling analysis directly reveals quantitatively amount of Ta substitution in Ti sites along with the degree of disorder at the Ti site indicated by the channeling minimum yield, $\chi _{min}$. Under optimum condition $\chi _{min}$ is of the order of 2{\%} implying that greater than 98{\%} of the Ta substitute in the Ti site. On the one hand $\chi _{min}$ increases with increasing O$_{2}$ partial pressure and decreasing deposition temperature. On the other hand, it decreases with increasing Ta concentration. These variations of the channeling yields are directly correlated with the conductivities of these films which suggest that Ta is substituting in a 5+ state at the Ti4+ sites.   

Identification and lattice location of oxygen impurities in $\alpha$-Si$_3$N$_4$

For over 40 years impurities have been believed to stabilize the ceramic $\alpha$-Si$_3$N$_4$, but there is no direct evidence for their identity or lattice location. In bulk materials electron microscopy can generally image heavy impurities. Here we report direct imaging of N columns in $\alpha$-Si$_3$N$_4$ that suggests the presence of excess light elements in specific N columns. First-principles calculations rule out Si or N interstitials and suggest O impurities, which are then confirmed by atomically-resolved electron energy-loss spectroscopy. The result provides a possible explanation for the stability of $\alpha$-Si$_3$N$_4$ with implications for the design of next-generation structural ceramics.   

First principle investigations of the dielectric properties of Si3N4 thin films

We have investigated the dielectric properties of silicon nitride thin films with thickness below 10 nm, by using first-principles density functional theory calculations. We find a substantial decrease of the static dielectric constant as the size is reduced, and the variation of the response in proximity of the film surfaces play a key role in the observed decrease. In addition, amorphization of the films can bring further reduction of both the static and optical dielectric constants.   

Brillouin light scattering studies on the mechanical properties of ultrathin, porous low-K dielectric films

Low K dielectrics have predominantly replaced silicon dioxide as the interlayer dielectric material for interconnects in state of the art integrated circuits. To further reduce interconnect resistance-capacitance (RC) delays, additional reductions in the K for these low-K materials is being pursued by the introduction of controlled levels of porosity. The main challenge for porous low-K dielectrics is the substantial reduction in mechanical properties that is accompanied by the increased pore volume content needed to reduce K. We report on the application of the nondestructive Brillouin light scattering technique to monitor and characterize the mechanical properties of these porous films at thicknesses well below 200 nm that are pertinent to present applications. Observation of longitudinal and transverse standing wave acoustic resonances and the dispersion that accompany their transformation into traveling waves with finite in-plane wave vectors provides for the principal elastic constants that completely characterize the mechanical properties of these porous films. The mode amplitudes of the standing waves, their variation within the film, and the calculated Brillouin intensities account for most aspects of the spectra. The resulting elastic constants are compared with corresponding values obtained from other experimental techniques.   

Mass Density and Composition of low-k SiC$_{x}$:H Thin Film Dielectrics as Determined by Fourier Transform Infrared Spectroscopy

Fourier Transform Infrared (FT-IR) Spectroscopy has long been utilized as an analytical technique for qualitatively determining the presence of various different chemical bonds in gases, liquids, and thin dielectric films. Quantitative measurements of the concentration or density of different chemical bonds have also been achieved utilizing FTIR via cross calibrating the intensity for particular absorption band against a known standard. In this paper, we demonstrate that FT-IR can be further extended to quantitatively determine the concentration of multiple chemical bonds in dielectric thin films of interest to the microelectronics industry. In turn, knowledge of the full bond concentrations allows accurate determination of the mass density and full elemental composition of the film. This capability is demonstrated for a-SiC$_{x}$:H thin films deposited by plasma enhanced chemical vapor deposition (PECVD). The resulting FTIR mass density and compositional analysis determined by this technique shows an excellent correlation to similar measurements by X-ray Reflectivity, X-ray Photoelectron Spectroscopy, and Scanning Ion Mass Spectroscopy.   

Bulk and Surface Excitations in Gd$_{2}$O$_{3}$: Electron Energy Loss Spectroscopy Study

Gd$_{2}$O$_{3}$ with its high dielectric constant ($\kappa \quad \sim $ 14), large band gap (5.4 eV)and thermodynamic stability has featured prominently in the literature as an effective passivation in GaAs substrate to fabricate the metal-oxide-semiconductor field-effect transistors (MOSFETs) and promising candidates for future scaling of CMOS technology. Here, we report studies of electronic excitations of Gd$_{2}$O$_{3}$ in cubic phase by electron energy-loss spectroscopy (EELS). EELS spectra in bulk Gd$_{2}$O$_{3}$ reveal several broad spectral features above the optical band gap at $\sim $7.5, $\sim $15, $\sim $17.5, $\sim $27.5, $\sim $31.5 and $\sim $36 eV. We have obtained the dielectric function by performing Kramers-Kr\"onig analysis. The 15 eV peak is identified as bulk-plasmon excitation. The 13.6 eV peak, which is visibly enhanced at thinner areas, arises from excitation of surface-plasmon. The other features at $\sim $7.5, $\sim $17.5, $\sim $27.5, $\sim $31.5 and $\sim $36 eV result from bulk interband transitions. Moreover, we note that the 7.5-eV peak associated with interband transition also bears a strong character of surface excitations, as evidenced by measurements carried out in aloof geometry. Detailed characteristics of this unconventional surface excitation will also be discussed.   

First Principles Calculations for Eu-doped Inorganic Scintillator Gamma Ray Detectors

This work presents the results of first principles electronic structure calculations for europium (Eu2+) doped inorganic compounds performed using the pseudopotential method based on the local spin density approximation (LSDA) and generalized gradient approximation+U (GGA+U) in density functional theory. The positions of the europium 4f and 5d states relative to the valence band maximum and conduction band minimum of the host material are determined. Qualitative predictions of the brightness of Eu2+ activated scintillation in the doped material is made based on the following criteria: (1) The size of the host material bandgap (2) The energy difference between the VBM (Valence Band Maximum) of the host material and the Eu2+ 4f level (3) The energy difference between the occupied Eu 5d excited state (Eu2+)* and the host material CBM (Conduction Band Minimum) (4) The level of localization of the excited (Eu2+)* electron state on the Eu atom. We have validated this theoretical approach on examples of known bright scintillators like SrI2 and BaI2 and non-Eu activated scintillators. We have performed calculations on new Eu doped compounds to determine if they are candidates for Eu2+ activated scintillators.   

A mid-IR phosphor using thulium-praseodymium-doped potassium lead chloride

Efficient energy transfer at room temperature from Tm$^{3+}$ to Pr$^{3+}$ has been demonstrated in co-doped KPb$_{2}$Cl$_{5}$. The energy transfer processes result in the conversion of 805 nm laser diode pump energy to a broad band of mid-IR radiation between 3500 to 5500 nm. Spectroscopic measurements show that the material is suitable as a phosphor for the 4 to 5 micron spectral range that can be optically pumped with low-cost 0.8-micron laser diodes. The mid-IR emission is enabled by the low phonon energies in KPb$_{2}$Cl$_{5}$.$_{ }$Energy transfer pathways, rates, and quantum efficiencies are evaluated.   

High excitation density, quenching, and radiative kinetics in CsI:Tl scintillator

Nonproportionality of scintillator light yield \textit{vs} particle energy is a significant issue in new-generation radiation analyzers for homeland security, medical, and astrophysical applications. To study kinetics of quenching at high excitation densities characteristic of electron track ends, we have used 0.3 ps pulses of 5.9-eV light to excite up to 5 x 10$^{20}$ e-h/cm$^{3}$ in CsI. Streak camera luminescence and transient absorption data show high-density e-h pair excitation yielding partially quenched self-trapped exciton (STE) luminescence and activator-trapped e-h population yielding Tl$^{+}$* luminescence, as a function of excitation density and Tl concentration in CsI:Tl. Observation of quenching as accelerated decay in the STE channel is consistent with results for CdWO$_{4}$, another intrinsic excitonic scintillator [1], whereas Tl$^{+}$* luminescence exhibits quenched yield without observable accelerated decay. \\[4pt] [1] M. Kirm \textit{et al}, Phys. Rev. B 79, 233103-1 (2009).   

Materials Discovery:~ Informatic Strategies for Semiconducting Radiation Detection Materials

Inorganic semiconducting materials used in gamma radiation detection applications are typically binary and ternary inorganic crystals. Performance metrics for these materials include band gap, relating to carrier concentration and thermal background current; density, relating to stopping power; and electron mobility, which limits electron transport and is typically the dominant information carrier. In this paper, we describe an information-based approach to the identification of new radiation detection materials, using the specific case of the II-VI semiconductors. Even for simple binary systems, the sheer number of potential materials considering the presence of crystal system polymorphs and higher order compositions is daunting. The key to a successful materials search is the ability to suggest promising materials and a priori eliminate unfruitful inquiry. The success of an informatics-based design program depends on the relation of materials-level properties to atomic-scale properties that change rationally with structure, and the ability to extract rules which define these mappings. A brief example of a property-level screen will be given to illustrate the materials development process. The authors gratefully acknowledge financial support from U.S. Department of Homeland Security under Contract No. HSHQDC-08-X-00872.