Formation of anodic films on sputtering-deposited Al–Hf alloys

Abstract

a b s t r a c tThe growth of barrier-type anodic films at high efficiency on a range of sputtering-deposited Al-Hf alloys, containing from 1 to 95 at.% Hf, has been investigated in ammonium pentaborate electrolyte. The alloys encompassed nanocrystalline and amorphous structures, the latter being produced for alloys containing from 26 to 61 at.% Hf. Except at the highest hafnium content, the films were amorphous and contained units of HfO 2 and Al 2 O 3 distributed relatively uniformly through the film thickness. Boron species were confined to outer regions of the films. The boron distributions suggest that the cation transport number decreases progressively with increasing hafnium concentration in the films, from ∼0.4 in anodic alumina to ∼0.2 for a film on an Al-61 at.% Hf alloy. The distributions of Al 3+ and Hf 4+ ions in the films indicate their similar migration rates, which correlates with the similarity of the energies of Al 3+ -O 2− and Hf 4+ -O 2− bonds. For an alloy containing ∼95 at.% Hf, the film was largely nanocrystalline, with a thin layer of amorphous oxide, of non-uniform thickness, at the film surface. The formation ratios for the films on the alloys changed approximately in proportion to the hafnium content of the films between the values for anodic alumina and anodic hafnia, ∼1.2 and 1.8 nm V −1 respectively.

Growth and field crystallization of anodic films on Ta–Nb alloys

“…The finding is characteristic for amorphous anodic oxides on binary alloys. A similar compositional dependence was reported for amorphous oxide films on Al-Ta [46][47][48] and Al-Hf [49] alloys. The compositional dependence of the properties of "mixed" amorphous oxides contrasts with that of crystalline anodic oxides formed on binary alloys.…”

“…The slower migration of Ta 5+ ions with respect to Nb 5+ ions is correlated with the stronger Ta 5+ -O bond (347 kJ mol −1 ) compared with the Nb 5+ -O bond (329 kJ mol −1 ). The good correlation between the relative migration rates of the two cations and their metal-oxygen bond strengths has been found for many amorphous anodic oxides formed on binary alloys, including Al-Ta, Al-Zr, Al-Hf, Al-Sm, Al-Mo, Al-W, Ti-W, Ti-Mo, Ti-Si, and Nb-Si alloys [47,49,52,[54][55][56][57][58][59][60][61][62][63]. The correlation suggests the importance of breakage of the metal-oxygen bonds in the ionic transport in growing amorphous anodic oxides under the high electric field.…”

Enhanced Capacitance of Composite Anodic ZrO2 Films Comprising High Permittivity Oxide Nanocrystals and Highly Resistive Amorphous Oxide Matrix

“…However, there is a positive empirical correlation between  ox and  (Figure 1). Valve metals such as aluminum, [7][8][9][10][11][12][13] niobium, [14][15][16][17][18] and titanium [19][20][21][22][23][24][25] were alloyed in order to control the properties of anodic oxide films and improve their dielectric properties. The general trend in amorphous anodic oxide films is that their properties on binary valve metal alloys-including the relative permittivity and formation ratio-are the compositional averages of the respective alloy-constituting metals.…”

Electrochemical Oxidation of Hf–Nb Alloys as a Valuable Route to Prepare Mixed Oxides of Tailored Dielectric Properties

“…Anodizing of Hf proceeds differently with respect to Nb. For hafnium, the growth of the anodic oxide is mainly sustained by inward migration of O 2− ion from the oxide/electrolyte interface to the metal/oxide interface, since the transport number of Hf 4+ is 0.05, and the corresponding anodic oxides are crystalline . Moreover, for high formation voltage, an electronic current circulates across the oxide, leading to O 2 evolution, whose presence can be detrimental for a possible application of HfO 2 as dielectric.…”