Ion implantation and annealing of crystalline oxides

Annealing studies combined with low temperature emission Mössbauer spectroscopy of short-lived parent isotopes: Determination of local Debye-Waller factors

An extension of the online implantation chamber used for emission Mössbauer Spectroscopy (eMS) at ISOLDE/CERN that allows for quick removal of samples for offline low temperature studies is briefly described. We demonstrate how online eMS data obtained during implantation at temperatures between 300 K and 650 K of short-lived parent isotopes combined with rapid cooling and offline eMS measurements during the decay of the parent isotope can give detailed information on the binding properties of the Mössbauer probe in the lattice. This approach has been applied to study the properties of Sn impurities in ZnO following implantation of ¹¹⁹In (T_½ = 2.4 min). Sn in the 4+ and 2+ charge states is observed. Above T > 600 K, Sn²⁺ is observed and is ascribed to Sn on regular Zn sites, while Sn²⁺ detected at T < 600 K is due to Sn in local amorphous regions. A new annealing stage is reported at T ≈ 550 K, characterized by changes in the Sn⁴⁺ emission profile, and is attributed to the annihilation of close Frenkel pairs.

Prediction of Site Preference of Implanted Transition Metal Dopants in Rock-salt Oxides

Transition metals (TMs) implanted in oxides with rock-salt crystal structures (for example MgO and BaO) are assumed to substitute cations (Mg in case of MgO) from the lattice sites. We show that not all implanted TMs substitute cations but can be stable in interstitial sites as well. Stability of TM (Sc-Zn) dopants in various charge states in MgO and BaO has been investigated in the framework of density functional theory. We propose an effective way to calculate stability of implanted metals that let us predict site preference (interstitial or substitution) of the dopant in the host. We find that two factors govern the preference for an interstitial site: (i) relative ionic radius and (ii) relative oxygen affinity of cation and the TM dopants. If the radius of the cation is much larger than TM dopant, as in BaO, TM atoms always sit at interstitial sites. On the other hand, if the radius of the cation is comparable to that of the dopant TM, as in case of MgO, the transition of the preferred defect site, from substituting lattice Mg atom (Sc to Mn) to occupying interstitial site (Fe to Zn) is observed. This transition can be attributed to the change in the oxygen affinity of the TM atoms from Sc to Zn. Our results also explain experiments on Ni and Fe atoms implanted in MgO. TM dopants at interstitial sites could show substantially different and new properties from substitutionally doped stable compounds.

Tuning bandgap and surface wettability of NiFe2O4 driven by phase transition

Stress variation induced bandgap tuning and surface wettability switching of spinel nickel ferrite (NiFe2O4, NFO) films were demonstrated and directly driven by phase transition via a post-annealing process. Firstly, the as-deposited NFO films showed hydrophilic surface with water contact angle (CA) value of 80 ± 1°. After post-annealing with designed temperatures ranged from 400 to 700 °C in air ambience for 1 hour, we observed that the crystal structure was clearly improved from amorphous-like/ nanocrystalline to polycrystalline with increasing post-annealing temperature and this phenomenon is attributed to the improved crystallinity combined with relaxation of internal stress. Moreover, super-hydrophilic surface (CA = 14 ± 1°) was occurred due to the remarkable grain structure transition. The surface wettability could be adjusted from hydrophilicity to super-hydrophilicity by controlling grain morphology of NFO films. Simultaneously, the saturation magnetization (Ms) values of NFO films at room temperature increased up to 273 emu/cm3 accompanied with transitions of the phase and grain structure. We also observed an exceptionally tunable bandgap of NFO in the range between 1.78 and 2.72 eV under phase transition driving. Meanwhile, our work demonstrates that direct grain morphology combined with the stress tuning can strongly modulate the optical, surface and magnetic characteristics in multifunctional NFO films.

Formation of oxygen vacancies and Ti(3+) state in TiO2 thin film and enhanced optical properties by air plasma treatment

This is the first time we report that simply air plasma treatment can also enhances the optical absorbance and absorption region of titanium oxide (TiO₂) films, while keeping them transparent. TiO₂ thin films having moderate doping of Fe and Co exhibit significant enhancement in the aforementioned optical properties upon air plasma treatment. The moderate doping could facilitate the formation of charge trap centers or avoid the formation of charge recombination centers. Variation in surface species viz. Ti³⁺, Ti⁴⁺, O²⁻, oxygen vacancies, OH group and optical properties was studied using X-ray photon spectroscopy (XPS) and UV-Vis spectroscopy. The air plasma treatment caused enhanced optical absorbance and optical absorption region as revealed by the formation of Ti³⁺ and oxygen vacancies in the band gap of TiO₂ films. The samples were treated in plasma with varying treatment time from 0 to 60 seconds. With the increasing treatment time, Ti³⁺ and oxygen vacancies increased in the Fe and Co doped TiO₂ films leading to increased absorbance; however, the increase in optical absorption region/red shift (from 3.22 to 3.00 eV) was observed in Fe doped TiO₂ films, on the contrary Co doped TiO₂ films exhibited blue shift (from 3.36 to 3.62 eV) due to Burstein Moss shift.

Gold ion implantation into alumina using an "inverted ion source" configuration

We describe an approach to ion implantation in which the plasma and its electronics are held at ground potential and the ion beam is injected into a space held at high negative potential, allowing considerable savings both economically and technologically. We used an "inverted ion implanter" of this kind to carry out implantation of gold into alumina, with Au ion energy 40 keV and dose (3-9) × 10(16) cm(-2). Resistivity was measured in situ as a function of dose and compared with predictions of a model based on percolation theory, in which electron transport in the composite is explained by conduction through a random resistor network formed by Au nanoparticles. Excellent agreement is found between the experimental results and the theory.

Distribution and Characterization of Iron in Implanted Silicon Carbide

Analytical electron microscopy (AEM) and Rutherford backscattering spectroscopy-ion channeling (RBS-C) have been used to characterize single crystal α-silicon carbide implanted at room temperature with 160 keV 57Fe ions to fluences of 1, 3, and 6×1016 ions/cm2. Best correlations among AEM, RBS, and TRIM calculations were obtained assuming a density of the amorphized implanted regions equal to that of crystalline SiC. No iron-rich precipitates or clusters were detected by AEM. Inspection of the electron energy loss fine structure for iron in the implanted specimens suggests that the iron is not metallically-bonded, supporting conclusions from earlier conversion electron Mössbauer spectroscopy (CEMS) studies. In-situ annealing surprisingly resulted in crystallization at 600°C with some redistribution of the implanted iron.

Modification of The Optical Properties of Al2O3 by Ion Implantation

The implantation of Au into A1203 followed by thermal annealing at 1100°C leads to dramatic changes in the optical properties. The linear and nonlinear optical properties are correlated to the presence of small Au precipitates which form during annealing.

Coherent V2O3 Precipitates in ɑ-Al2O3 Co-Implanted With Vanadium and Oxygen

The oxides of vanadium VO2 and V2O3 are of fundamental and practical interest since they undergo structural phase transitions during which large variations in their optical and electronic properties are observed. In the present work, we report the formation of buried precipitates of V2O3 in sapphire by ion implantation and thermal annealing. It was found that the co-implantation of oxygen and vanadium was required in order to form nanophase V2O3 precipitates. Additionally, these precipitates, which formed only following an anneal of the co-implanted sample under reducing conditions, are coherent with the sapphire lattice. Two epitaxial relationships were observed: (0001)V2O3//(0001) ɑ-Al2O3 and (11-20)V2O3//(0001) ɑ-Al2O3. This finding is in agreement with results obtained elsewhere for thin films of V2O3 deposited on c-axis-oriented sapphire.

Structural Criteria for Amorphization of Network Solids

Amorphization, like glass-formation, represents fundamentally a failure to crystallize. The problem is to understand how atoms can rearrange themselves, perhaps within the confines of unaffected surrounding crystal, after a local disordering event. Some ceramics, like alkali halides and oxides with the rocksalt structure, appear almost impossible to amorphize, forming localized aggregate defects (voids, dislocation loops, colloids) and even decomposing (as a response to radiation disorder), rather than structurally reordering (or disordering). Other network solids such as silicas and silicates readily amorphize. In this study, we attempt to establish topology and structural freedom as criteria for amorphization of network solids.

Synthesis, Optical Properties, and Microstructure of Semiconductor Nanocrystals Formed by Ion Implantation

High-dose ion implantation, followed by annealing, has been shown to provide a versatile technique for creating semiconductor nanocrystals encapsulated in the surface region of a substrate material. We have successfully formed nanocrystalline precipitates from groups IV (Si, Ge, SiGe), III-V (GaAs, InAs, GaP, InP, GaN), and II-VI (CdS, CdSe, CdSxSe1x, CdTe, ZnS, ZnSe) in fused silica, Al2O3 and Si substrates. Representative examples will be presented in order to illustrate the synthesis, microstructure, and optical properties of the nanostructured composite systems. The optical spectra reveal blue-shifts in good agreement with theoretical estimates of size-dependent quantum-confinement energies of electrons and holes. When formed in crystalline substrates, the nanocrystal lattice structure and orientation can be reproducibly controlled by adjusting the implantation conditions.

Quantum antidot formation and correlation to optical shift of gold nanoparticles embedded in MgO

Quantum antidots are subnanometer scale vacancy clusters, the localized electronic structure of which can significantly alter the properties of a nanomaterial. We use positron spectroscopy to study vacancy clusters generated during the formation of gold nanoparticles via ion implantation in an MgO matrix. We observed that quantum antidots are associated with the nanoparticle surfaces after annealing in an O2 atmosphere, but not after annealing in a H2 atmosphere. In the former case, the presence of quantum antidots bound to the gold nanoparticles correlates with the redshift of the gold surface plasmon resonance, thus allowing an explanation for the redshift based on the transfer of electrons away from the metal particles.

Technique for preparing cross-section transmission electron microscope specimens from ion-irradiated ceramics

The general techniques necessary to produce a high-quality cross-sectioned ceramic specimen for transmission electron microscope observation are outlined. A particularly important point is that the width of the glued region between faces of the ceramic specimen must be less than 0.2 micron to prevent loss of the near-surface region during ion milling. A recently developed vise for gluing ceramic cross-section specimens is described, and some examples of the effect of glue thickness on specimen quality are shown.