C. R. Aita

Correlation between titania film structure and near ultraviolet optical absorption

Titania is a material with structural flexibility, and as a result, readily forms both crystalline polymorphs and an amorphous structure in thin films grown near room temperature. The goal of this study is to correlate fundamental optical absorption edge characteristics with the phase constituency of titania films. To that end, films with coexistent rutile, anatase, and amorphous constituents were sputter deposited onto fused silica and 〈111〉-Si substrates. The films were then subjected to cyclic annealing in air at moderate temperature (700 and 1000 °C) to affect phase changes. Bragg–Brentano x-ray diffraction was used for phase identification and near ultraviolet-visible transmission and reflection spectrophotometry was used to determine the optical absorption coefficient at the onset of interband transitions. The optical absorption coefficient was modeled within the framework of the coherent potential approximation (CPA), with Gaussian site disorder introduced into the valence and conduction bands of a...

Short-range order in ultrathin film titanium dioxide studied by Raman spectroscopy

It is demonstrated here that nonresonant Raman spectroscopy can be used for unequivocal determination of short-range order in ultrathin films, using different structures of titanium dioxide as the model system. Titania films as thin as 7 nm sputter deposited on 〈111〉 Si have been analyzed and their phase content determined.

Near-edge optical absorption behavior of sputter deposited hafnium dioxide

The absorption behavior at the onset of the ultraviolet optical edge of sputter deposited hafnium dioxide is reported. X-ray diffraction showed that films grown at room temperature on fused silica were nanocrystalline monoclinic, and that after air annealing at 1273 K for 24 h, they were well-crystallized monoclinic with a strong (11–1) texture. Spectrophotometry was used to measure transmission and reflection in the 190–1100 nm wavelength range. The absorption coefficient, α(E), as a function of energy, E, was determined from spectrophotometric data. Two persistent features were identified. Feature I is the rapid rise in α(E) above 6.24 eV that is unchanged upon annealing. A linear α(E)1/2 vs E dependence associates this feature with indirect interband transitions. Feature II initiates at ∼5.65 eV and saturates at 5.94 eV. It appears as a low energy shoulder on the absorption edge of the as-grown films and develops into a discrete and more intense band in the annealed films, in which the coordination of ...

Fundamental optical absorption edge of undoped tetragonal zirconium dioxide

The high-frequency optical absorption edge of pure tetragonal ZrO2, isolated in a ZrO2–Al2O3 nanolaminate film structure, was determined using transmission spectrophotometry. The functional dependence of the absorption coefficient on photon energy shows two interband transitions: an initial indirect transition at 5.22 eV (i.e., the band gap) followed by a direct transition at 5.87 eV. The edge structure is associated with O 2p→Zr 4d electron states and discussed in terms of ab initio calculations reported in the literature.

Infrared response of vitreous titanium dioxide films with anatase short-range order

The polarization-dependent optical response of vitreous titanium dioxide films was determined by infrared reflection–absorption spectroscopy (IRAS). The predominant nearest-neighbor coordination (short-range order) in the films was characteristic of crystalline anatase. A dielectric function e(ν) was constructed, based on random orientational disorder of anataselike, octahedral TiO6 units relative to the substrate. This dielectric function, which is the directional average of the dielectric function for an anatase crystal in the E⊥c and E∥c orientations, was used to calculate energy loss functions from which theoretical IRAS spectra were obtained. Experimental absorption band frequencies are in good agreement with peaks in the calculated transverse optic loss function, Im[e(ν)], at 261 and 436 cm−1, and in the calculated longitudinal optic loss function, Im[−1/e(ν)], at 840 cm−1. Agreement (i.e., polarization-dependent behavior, band frequency, and relative intensity) between the experimental and theoreti...

Crystallization and (Al,Ti)-oxide growth in annealed TiO2–Al2O3 multilayers

A family of TiO2–Al2O3 multilayers (Λ=2–72nmTiO2∕7nmAl2O3) is sputter deposited on fused silica substrates, sequentially annealed at 973 and 1273K, and analyzed by x-ray diffraction. The goal is to examine crystallization behavior upon annealing at temperatures at which thermodynamically stable mixed-cation phases should not form. The results show: (1) After the 973K anneal, films with Λ=18–72nmTiO2∕7nmAl2O3 weakly crystallize with a preferred (110) rutile orientation. In addition, enhancement of (200) rutile diffraction increases with increasing TiO2 layer thickness. (2) Significant crystallization occurs in films after the 1273K anneal. In films with Λ=36–72nmTiO2∕7nmAl2O3, a metastable pseudobrookite phase, Al0.95Ti2.05O5, crystallizes along with (110)r. However, only rutile TiO2 and α-Al2O3 crystallize in films with thinner TiO2 layers. An architecture-sensitive crystallization model is presented in which the first step common to all architectures is diffusive amorphization of TiO2 by Al2O3 at 973K to...

Crystallization and segregation in vitreous rutile films annealed at high temperature

Vitreous titania films with rutile short-range order were sputter deposited on unheated fused silica substrates, sequentially annealed at 973 and 1273 K, and examined by Raman microscopy, scanning electron microscopy, and x-ray diffraction. A segregated microstructure developed after the 1273 K anneal. This microstructure consists of supermicron-size craters dispersed in a matrix of submicron rutile crystals. Ti–O short-range order in the craters is characteristic of a mixture of two high pressure phases, m-TiO2 (monoclinic P21∕c space group) and α-TiO2 (tetragonal Pbcn space group). We calculated that a high average compressive stress parallel to the substrate must be accommodated in the films at 1273 K, caused by the difference in the thermal expansion coefficients of titania and fused silica. The formation of the segregated microstructure is modeled by considering two processes at work at 1273 K to lower a film’s internal energy: crystallization and nonuniform stress relief. The Gibbs–Thomson relation ...

Far-infrared spectra of amorphous titanium dioxide films

Abstract The amorphous TiO 2 film structure on an aluminum substrate was studied by far infrared reflection–absorption spectroscopic (IRAS) analysis. The transverse (TO) and longitudinal (LO) optical modes of the amorphous oxide films were studied experimentally and through simulation. These optical modes were compared to those of the corresponding crystal. While TO modes of the films correspond to those of the crystal, the LO modes do not. The best agreement between the experimental and the simulated IRAS spectra is obtained when an appropriate dielectric function e film ( ν ) is chosen. e film ( ν ) must be related to the amorphous character of the films e.g. by averaging the anisotropic crystal properties. As a consequence of our findings, the Berreman effect does apply in amorphous films to absorption bands at frequencies of the peaks of the loss function Im[−1/ e film ( ν )], which does not correspond to frequencies of the LO modes of the crystal.

(Zr, Ti)O2 interface structure in ZrO2-TiO2 nanolaminates with ultrathin periodicity

A mixed cation interfacial structure in ZrO2–TiO2 nanolaminate films with ultrathin bilayer periodicity grown by sputter deposition at 297 K was identified by x-ray diffraction and nonresonant Raman spectroscopy. This structure consists of an amorphous phase at a ZrO2-on-TiO2 bilayer interface, followed by an extensive crystalline monoclinic (Zr,Ti)O2 solid solution predicted by Vegard’s law. Monoclinic (Zr,Ti)O2 has previously been reported only once, in bulk powder of a single composition (ZrTiO4) at high pressure. Its stabilization in the nanolaminates is explained by the Gibbs–Thomson effect. This complex interfacial structure is shown to be a means of accommodating chemical mixing in the absence of a driving force for heteroepitaxy.

Optical absorption behavior of ZrO2–TiO2 nanolaminate films

The near-ultraviolet fundamental optical absorption edge of sputter-deposited ZrO2–TiO2 nanolaminate films on SiO2 substrates was studied by transmission-reflection spectrophotometry. Seven different bilayer architectures were investigated, with nominal ZrO2 volume fractions ranging from 0.10 to 0.91 (Zr atom fractions of 0.1–0.9). The absorption coefficient, α(E), was determined as a function of the incident photon energy, E, in the 3.5–5.8 eV range (350–215 nm wavelength). α(E) vs E curves show a systematic blueshift and a change in shape with an increase in the Zr atom fraction in a bilayer. Neither amalgamation nor persistence models can adequately explain the experimental results. The reason why is that an extensive and structurally complex mixed cation interfacial structure formed even during room temperature deposition. A model that takes into account contributions to α(E) from Ti–O–Ti and Zr–O–Zr linkages far from the interfaces between constituents and Ti–O–Zr linkages at these interfaces is succ...