E. E. Hoppe

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.

Suppression of near-edge optical absorption band in sputter deposited HfO2–Al2O3 nanolaminates containing nonmonoclinic HfO2

Nanolaminates of polycrystalline (tetragonal+orthorhombic) HfO2 and amorphous Al2O3 are sputter deposited on unheated fused SiO2, air annealed at 573–1273K, and analyzed by x-ray diffraction and spectrophometry. Significant O 2p→Hf 5d interband absorption occurs in all films at energy E⩾6.2eV. For E<6.2eV, films annealed below 1273K retain a featureless optical absorption edge despite further crystallization. A band with a 5.65eV onset concurrently develops with m-HfO2 crystallization after a 1273K anneal, indicating this phase and not nanocrystallinity per se is responsible for increased absorption.

Initial phases in sputter deposited HfO2–Al2O3 nanolaminate films

Nanolaminate films of crystalline HfO2 and amorphous Al2O3 were grown by reactive sputter deposition on unheated fused SiO2 and the surface oxide of ⟨111⟩ Si. X-ray diffraction showed the amount of monoclinic (m) HfO2 decreased with decreasing HfO2 layer thickness, consistent with a finite crystal size effect. High resolution transmission electron microscopy of individual crystallites detected tetragonal (t) and orthorhombic (o) HfO2 as the initial phases formed. Whereas the t→m transition is accomplished by a shear mechanism, we demonstrate the important role of polysynthetic twinning for the o→m transition.

Zirconia-alumina nanolaminate for perforated pitting corrosion protection of stainless steel

The subject of this article is control of perforated pit growth in 316L stainless steel by application of an artificial overlayer film. The protection afforded by 250 nm thick films with two ZrO2–Al2O3 nanolaminate architectures, single-layer ZrO2, and single-layer Al2O3 under cyclic polarization in saline electrolyte is reported here. The post-exposure morphology is characterized by scanning electron microscopy with in situ electron energy dispersive spectroscopy. The films’ diverse behavior is analyzed in terms of a model for perforated pit growth that requires partial pit occlusion until an autocatalytic geometry is established. The results show that the key property a film must have to arrest the development of autocatalytic geometry is the capability to fracture locally so that the electrolyte from the pit freely mixes with the bulk electrolyte and the pit interior becomes passivated. We show how a nanolaminate with 5.0 nm tetragonal ZrO2–5.0 nm amorphous Al2O3 bilayers sustains local fracture (blow holes) without widespread cracking and is protective against perforated pit growth.The subject of this article is control of perforated pit growth in 316L stainless steel by application of an artificial overlayer film. The protection afforded by 250 nm thick films with two ZrO2–Al2O3 nanolaminate architectures, single-layer ZrO2, and single-layer Al2O3 under cyclic polarization in saline electrolyte is reported here. The post-exposure morphology is characterized by scanning electron microscopy with in situ electron energy dispersive spectroscopy. The films’ diverse behavior is analyzed in terms of a model for perforated pit growth that requires partial pit occlusion until an autocatalytic geometry is established. The results show that the key property a film must have to arrest the development of autocatalytic geometry is the capability to fracture locally so that the electrolyte from the pit freely mixes with the bulk electrolyte and the pit interior becomes passivated. We show how a nanolaminate with 5.0 nm tetragonal ZrO2–5.0 nm amorphous Al2O3 bilayers sustains local fracture (blow ...

Ti-catalyzed HfSiO4 formation in HfTiO4 films on SiO2 studied by Z-contrast scanning electron microscopy

Hafnon (HfSiO4) as it is initially formed in a partially demixed film of hafnium titanate (HfTiO4) on fused SiO2 is studied by atomic number (Z) contrast high resolution scanning electron microscopy, x-ray diffraction, and Raman spectroscopy and microscopy. The results show exsoluted Ti is the catalyst for hafnon formation by a two-step reaction. Ti first reacts with SiO2 to produce a glassy Ti-silicate. Ti is then replaced by Hf in the silicate to produce HfSiO4. The results suggest this behavior is prototypical of other Ti-bearing ternary or higher order oxide films on SiO2 when film thermal instability involves Ti exsolution.