Analysis of Plasmon Loss Peaks of Oxides and Semiconductors with the Energy Loss Function.

This paper highlights the use and applications of the energy loss function (ELF) for materials analysis by using electron energy loss spectroscopy (EELS). The basic Drude-Lindhart theory of the ELF is briefly presented along with reference to reflection electron energy loss (REELS) data for several dielectric materials such as insulating high-k binary oxides and semiconductors. Those data and their use are critically discussed. A comparison is made to the available ab initio calculations of the ELF for these materials. Experimental, high-resolution TEM-EELS data on Si, SiC, and CeO2 obtained using a high-resolution, double-Cs-corrected transmission electron microscope are confronted to calculated spectra on the basis of the ELF theory. Values of plasmon energies of these three dielectric materials are quantitatively analyzed on the basis of the simple Drude's free electron theory. The effects of heavy ion irradiation on the TEM-EELS spectra of Si and SiC are addressed. In particular, the downward shifts of plasmon peaks induced by radiation damage and the subsequent amorphization of Si and SiC are discussed. TEM-EELS data of CeO2 are also analyzed with respect to the ELF data and with comparison to isostructural ZrO2 and PuO2 by using the same background and with reference to ab initio calculations.

Virtual photon approach of cathodoluminescence and ion-beam induced luminescence of solids.

A novel analysis of cathodoluminescence (CL) and ion-beam induced luminescence (IBIL) is presented on the basis of virtual photon spectra (VPS) produced by charged particles (electrons or ions) passing by luminescent species such as defects or impurities, in wide band-gap ionic-covalent solids. A discussion is provided for irradiations in a wide range of charged particle kinetic energy by using the Weizsäcker-Williams theory. The computed VPS are found to decay rapidly as a function of virtual photon (VP) energy regardless of particle energy, for close or distant collisions. The electron-energy dependence of experimental CL spectra of sapphire (α-Al2O3) is discussed in relation to the computed VPS for the primary and secondary electrons. The experimental IBIL spectra ofα-Al2O3are also analyzed in this framework for protons and helium ions in the MeV energy range. The variations of stopping power are consistent with the variation of the number of emitted VPs. The decay of IBIL yield versus ion stopping power is discussed on the basis of the variation of the computed VPS, and ionization and excitation induced by primary ions and secondary electrons. This decay is accounted for by a decrease of the yield of low-energy secondary electrons with the subsequent VP emission.

Color-center production and recovery in electron-irradiated magnesium aluminate spinel and ceria.

Single crystals of magnesium aluminate spinel (MgAl2O4) with (1 0 0) or (1 1 0) orientations and cerium dioxide or ceria (CeO2) were irradiated by 1.0 MeV and 2.5 MeV electrons in a high-fluence range. Point-defect production was studied by off-line UV-visible optical spectroscopy after irradiation. For spinel, regardless of both crystal orientation and electron energy, two characteristic broad bands centered at photon energies of 5.4 eV and 4.9 eV were assigned to F and F(+) centers (neutral and singly ionized oxygen vacancies), respectively, on the basis of available literature data. No clear differences in color-center formation were observed for the two crystal orientations. Using calculations from displacement cross sections by elastic collisions, these results are consistent with a very large threshold displacement energy (200 eV) for oxygen atoms at room temperature. A third very broad band centered at 3.7 eV might be attributed either to an oxygen hole center (V-type center) or an F2 dimer center (oxygen di-vacancy). The onset of recovery of these color centers took place at 200 °C with almost full bleaching at 600 °C. Activation energies (~0.3-0.4 eV) for defect recovery were deduced from the isochronal annealing data by using a first-order kinetics analysis. For ceria, a sub-band-gap absorption feature, which peaked at ~3.1 eV, was recorded for 2.5 MeV electron irradiation only. Assuming a ballistic process, we suggest that the latter defect might result from cerium atom displacement on the basis of computed cross sections.

Swift heavy ion-beam induced amorphization and recrystallization of yttrium iron garnet.

Pure and (Ca and Si)-substituted yttrium iron garnet (Y3Fe5O12 or YIG) epitaxial layers and amorphous films on gadolinium gallium garnet (Gd3Ga5O12, or GGG) single crystal substrates were irradiated by 50 MeV (32)Si and 50 MeV (or 60 MeV) (63)Cu ions for electronic stopping powers larger than the threshold value (~4 MeV µm(-1)) for amorphous track formation in YIG crystals. Conductivity data of crystalline samples in a broad ion fluence range (10(11)-10(16) cm(-2)) are modeled with a set of rate equations corresponding to the amorphization and recrystallization induced in ion tracks by electronic excitations. The data for amorphous layers confirm that a recrystallization process takes place above ~10(14) cm(-2). Cross sections for both processes deduced from this analysis are discussed in comparison to previous determinations with reference to the inelastic thermal-spike model of track formation. Micro-Raman spectroscopy was also used to follow the related structural modifications. Raman spectra show the progressive vanishing and randomization of crystal phonon modes in relation to the ion-induced damage. For crystalline samples irradiated at high fluences (⩾10(14) cm(-2)), only two prominent broad bands remain like for amorphous films, thereby reflecting the phonon density of states of the disordered solid, regardless of samples and irradiation conditions. The main band peaked at ~660 cm(-1) is assigned to vibration modes of randomized bonds in tetrahedral (FeO4) units.

Thermo-stimulated luminescence of x-ray- and alpha-irradiated yttria-stabilized zirconia.

Yttria-stabilized zirconia (ZrO2 : Y3+) single crystals (with 9.5 mol% Y2O3) were irradiated with x-rays and α particles. Thermally stimulated luminescence (TSL) data show a main broad peak centred at ∼500-550 K in the glow curves of all irradiated samples. The TSL peak maximum temperature is consistent with the characteristic recovery temperature (∼450 K) of colour centres (T centres) deduced from isochronal annealing curves measured by electron paramagnetic resonance (EPR) spectroscopy. However, the trap-depth energies (ranging between 0.8 and 1.2 eV) deduced from the initial rise of partially cleaned TSL peaks (and from a rough approximation using Urbach's formula) are much larger than the activation energies for defect recovery of 0.3 eV deduced from the EPR data. A second TSL peak centred at ∼350-450 K found in freshly irradiated samples is seen to decay substantially in aged samples. The processes involved in TSL are discussed in relation to the defect annealing processes, and available defect-level energy and TSL data.

Point defects induced in yttria-stabilized zirconia by electron and swift heavy ion irradiations.

We present an extensive study of point-defect creation in yttria-stabilized zirconia (ZrO(2):Y) exposed to 2.5 MeV electrons and various heavy ions (from C to U) covering an energy range from 100 MeV to several GeV. A synthesis of results from UV-visible optical absorption spectroscopy and electron paramagnetic resonance spectroscopy is provided with special emphasis on the respective roles of elastic collisions and electronic excitations. The colour centre production and recovery are the main focus in this survey. It is concluded that F(+)-type centres (involving singly ionized oxygen vacancies) are produced by elastic-collision processes. The large threshold displacement energy and defect volume hint that these colour centres might actually be small paramagnetic oxygen vacancy clusters, most probably divacancies (i.e. F(2)(+) centres). Such a picture is consistent with the (100) axial symmetry, inhomogeneous broadening of the optical absorption band, lack of hyperfine splitting, and weak spin-lattice coupling found for this defect.

Thermo-stimulated luminescence of ion-irradiated yttria-stabilized zirconia.

Yttria-stabilized zirconia (ZrO(2):Y(3+)) single crystals (with 9.5 mol% Y(2)O(3)) were irradiated with ions (from 1 MeV He to 2.6 GeV U). Electron paramagnetic resonance (EPR) data show that two kinds of colour centres (F(+)-type and T centres) are produced. Thermo-stimulated luminescence (TSL) data exhibit a quite strong peak at ∼ 500-550 K in the glow curves of all irradiated samples regardless of the ion species and energy. Moreover, the 3D-TSL measurements reveal that this peak is correlated with a light emission at a wavelength of ∼ 620 nm (i.e. photon energy ∼ 2 eV). The TSL peak maximum temperatures are consistent with characteristic temperatures of about 500 K of annealing stages of colour centres. However, the trap-depth energies (ranging between 0.7 and 1.4 eV) deduced from the initial rise of partially cleaned TSL peaks, or from a rough approximation using Urbach's formula, are rather larger than the activation energies for defect recovery, ranging between 0.3 and 0.7 eV, as deduced from the EPR data. The processes involved in TSL are discussed in relation to available photoluminescence and defect energy-level data.

Generation of colour centres in yttria-stabilized zirconia by heavy ion irradiations in the GeV range.

We have studied the colour centre production in yttria-stabilized zirconia (ZrO(2):Y(3 +)) by heavy ion irradiation in the GeV range using on-line UV-visible optical absorption spectroscopy. Experiments were performed with 11.4 MeV amu(-1) (127)Xe, (197)Au, (208)Pb and (238)U ion irradiations at 8 K or room temperature (RT). A broad and asymmetrical absorption band peaked at a wavelength about 500 nm is recorded regardless of the irradiation parameters, in agreement with previous RT irradiations with heavy ions in the 100 MeV range. This band is de-convoluted into two broad Gaussian-shaped bands centred at photon energies about 2.4 and 3.1 eV that are respectively associated with the F(+)-type centres (involving a singly ionized oxygen vacancy, VO· and T centres (i.e. Zr(3+) in a trigonal symmetry) observed by electron paramagnetic resonance (EPR) spectroscopy. In the case of 8 K Au ion irradiation at low fluences, six bands are used at about 1.9, 2.3, 2.7, 3.1 and 4.0 eV. The three bands near 2.0-2.5 eV can be assigned to oxygen divacancies (i.e. F(2)(+) centres). No significant effect of the irradiation temperature is found on the widths of all absorption bands for the same ion and fluence. This is attributed to the inhomogeneous broadening arising from the static disorder due to the native charge-compensating oxygen vacancies. However, the colour centre production yield is strongly enhanced at 8 K with respect to RT. When heating irradiated samples from 8 K to RT, the extra colour centres produced at low temperature do not recover completely to the level of RT irradiation. The latter results are accounted for by an electronically driven defect recovery process.