Abstract
The complex structure of Ta2O5 led to the development of various structural models. Among them, superstructures represent the most stable configurations. However, their formation requires kinetic activity and long-range ordering processes, which are hardly present during physical vapor deposition. Based on nano-beam X-ray diffraction and concomitant ab initio studies, a new metastable orthorhombic basic structure is introduced for Ta2O5 with lattice parameters a=6.425Å, b=3.769Å and c=7.706Å. The unit cell containing only 14 atoms, i.e. two formula unit blocks in the c direction, is characterized by periodically alternating the occupied oxygen site between two possible positions in succeeding 002-planes. This structure can be described by the space group 53 (Pncm) with four Wyckoff positions, and exhibits an energy of formation of −3.209eVatom−1. Among all the reported basic structures, its energy of formation is closest to those of superstructures. Furthermore, this model exhibits a 2.5eV band gap, which is closer to experimental data than the band gap of any other basic-structure model. The sputtered Ta2O5 films develop only a superstructure if annealed at temperatures >800°C in air or vacuum. Based on these results and the conveniently small unit cell size, it is proposed that the basic-structure model described here is an ideal candidate for both structure and electronic state descriptions of orthorhombic Ta2O5 materials.
Highlights
Tantalum pentoxide (Ta2O5) is an important semiconductor material exhibiting a high dielectric constant and refractive index, and it is commonly used for capacitors or optical coatings [1,2]
Tantalum oxide films were deposited at 500 °C on (1 0 0) oriented silicon substrates (20 Â 7 Â 0.35 mm3) on a laboratory scale, magnetically unbalanced, Leybold Heraeus magnetron sputtering system
Tantalum pentoxide Ta2O5 thin films were prepared by reactive sputter deposition at 500 °C for detailed studies of their growth morphology, structure and mechanical properties
Summary
Tantalum pentoxide (Ta2O5) is an important semiconductor material exhibiting a high dielectric constant and refractive index, and it is commonly used for capacitors or optical coatings [1,2]. The orthorhombic structure can transform into a tetragonal structure as the b and c lattice constants of both structures are similar (3.66 vs 3.88 A ) R. Hollerweger et al / Acta Materialia 83 (2015) 276–284 the ion positions can influence the resulting structure of Ta2O5 and its properties. Hollerweger et al / Acta Materialia 83 (2015) 276–284 the ion positions can influence the resulting structure of Ta2O5 and its properties This could be the reason for a large variety of different reported basic and super-structures. The “visualization for electronic and structural analysis” software [27] was used to visualize the various crystal structures
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