Effect of oxygen content on optical, structural, and dielectric properties of Al$_x$Ta$_y$O$_z$$ thin films

TL;DR

This work demonstrates that oxygen content in Al_xTa_yO_z thin films governs dielectric breakdown, driven by oxygen-deficiency–induced metal–metal bonding motifs that create in-gap defect states near the valence band. A combined experimental–theoretical framework—comprising reactive sputtering, XPS, optical spectroscopy, XRD/SEM, and melt‑quench DFT with a PBE0 functional—shows that substoichiometric compositions host localized metal-derived states which coincide with visible-range absorption and reduced dielectric strength. The highest breakdown strength measured is 231 V/μm for a high-O composition, and ab initio results link defect-state formation to bonding topology rather than crystallinity. Overall, the study provides a mechanistic route to tailor high-k oxide dielectric performance in amorphous Al–Ta–O systems by tuning oxygen content and bonding motifs, with implications for durable dielectric coatings and optical applications.

Abstract

This study reports on the optical, structural, and dielectric properties of aluminum tantalum oxide (Al$_x$Ta$_y$O$_z$) thin films deposited at low temperature on silicon and steel substrates by pulsed direct current reactive magnetron sputtering of a target containing 80 at.% aluminum and 20 at.% tantalum in Ar/O$_2$ atmosphere. Oxygen flow rates ranging from 5.0 to 20 sccm corresponded to O content changes from 57.7 to 69.6 at.% and resulted in large differences in dielectric behavior, from films with no measurable dielectric strength to a dielectric strength of 231 V$μ$m$^{-1}$, respectively. Ab initio calculations were employed to explain the large property changes, and we show that a decrease in the dielectric strength can be linked to the formation of metal-metal bonds in the material, when the O content is less than what would correspond to a stoichiometric Ta$_2$O$_5$ and Al$_2$O$_3$ mixture. The electronic states corresponding to the metal--metal bonds are located in the band gap close to the top of the valence band, leading to an effective band gap reduction, which is directly supported by X-ray photoelectron spectroscopy valence band measurements and by a broad optical absorption in the visible region.

Figures (9)

• Figure 1: Typical cross-section SEM image and EDX spectrum of Al_xTa_yO_z thin films deposited on a silicon substrate by pulsed-DC reactive magnetron sputtering at an oxygen flow of 5. The EDX spectrum corresponds to the indicated region.
• Figure 2: XRD patterns of Al_xTa_yO_z thin films deposited by pulsed-DC reactive magnetron sputtering at different oxygen flows. Black dots indicate reflections from the sample holder; reference patterns are shown below.
• Figure 3: Dielectric strengths of Al_xTa_yO_z thin films deposited by pulsed-DC reactive magnetron sputtering at different oxygen flow.
• Figure 4: a) XPS valence band spectra. b) Density of states calculated for the amorphous Al_xTa_yO_z structures with variable O content. Gaussian broadening of 0.3 was applied. DOS plots were aligned by the valence band edge for visualization purposes.
• Figure 5: Real a) and imaginary b) parts of the complex dielectric function in the NIR-VUV spectral region with reference spectra of amorphous Ta2O5 and Al2O3.