Tantalum oxide films produced from anodic oxidation and thermal oxidation at 400 and 800°C have been investigated with respect to long-and short-range structure by X-ray diffraction and X-ray absorption spectroscopy, respectively. Films formed at 800°C show orthorhombic structure (-Ta 2 O 5 ) with greater disorder compared to the -Ta 2 O 5 powder reference, where six O atoms were observed at an average radial distances of 1.957 Ϯ 0.014 (Ta 2 O 5 powder͒ and 1.971 Ϯ 0.007 Å ͑800°C Ta oxide film͒. Lower temperature films consist primarily of metastable TaO x (Ta 2 O) and appear to be relatively stable for extended times of treatment with a first shell not unlike that of -Ta 2 O 5 , based on a linear combination analysis. Furthermore, using a linear combination of -Ta 2 O 5 and ␣-Ta metal, the amorphous anodic oxide films studied as a function of thickness were observed to exhibit a relatively ordered local structure consistent with the high-temperature ͑i.e., 800°C͒ phase. This work demonstrates that the nanoscale oxide films formed on tantalum exhibit an ordered local structure, reflecting the very compact nature that enhances its corrosion resistance.High-temperature and anodic tantalum oxide films have been studied extensively in the field of corrosion. 1,2 Ta possesses a high melting point ͑2996°C͒, is highly ductile, and is corrosion resistant in aggressive environments. 3 The low thermal expansion and the predictability of tantalum oxidation are additional characteristics that make tantalum attractive for high-temperature applications. In addition, with a dielectric constant greater than 20, tantalum pentoxide (Ta 2 O 5 ) has received significant attention in the semiconductor industry. 4 Although Ta itself is highly reactive, the stable, dense nanometer oxide film formed provides an effective barrier to corrosive environments.Thermodynamically, tantalum pentoxide (Ta 2 O 5 ) is the only stable high-temperature oxidation product. 1 However, thin films of Ta 2 O 5 have several different structures depending on the system temperature. 5-10 The principal structures of crystalline Ta 2 O 5 are orthorhombic ͑␣ and  phases͒ 5-7 and hexagonal ͑␦ phase͒. 9,10 Fukumoto and Miwa 10 reported that the hexagonal structure of Ta 2 O 5 has the space group P6/mmm with lattice constants of a ϭ 7.191 Å and c ϭ 3.831 Å and eight oxygen atoms in the first shell. The orthorhombic structure occurs in two forms with a reversible transition reported at approximately 1360°C: 5-7 the high-temperature ␣-phase (H-Ta 2 O 5 ) and the low-temperature -phase (L-Ta 2 O 5 ). A recent study 7 on the low-temperature oxide showed that -Ta 2 O 5 has unit-cell parameters of a ϭ 6.217 Å, b ϭ 3.677 Å, and c ϭ 7.794 Å with the space group Pccm. Voltovich et al. 11 reported that over the temperature range of 600-1000°C -Ta 2 O 5 was produced. However, the formation of metastable tantalum oxide phases occurs between 300 and 800°C. 12 Steeb and Renner 13 found that at approximately 400°C, platelets of metastable metallic oxide phases are nuclea...
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