Transition-Metal-Incorporated Aluminum Oxide Thin Films: Toward Electronic Structure Design in Amorphous Mixed-Metal Oxides

Abstract

Amorphous metal oxides have numerous applications spanning from electronics to catalysis. The ability to rationally design the electronic structure of such materials would thus have significant impact. Previous work has shown that the amorphous structure allows for uniformly incorporating a large number of different metal cations into ultrasmooth thin films. Here we investigate how the atomic structure, electronic structure, and optical absorption properties of amorphous aluminum oxide (a-Al 2 O 3 ) thin films are modulated when metal cations of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, or Ga are added to form M y Al 1−y O x where y varies from 0.1 to 0.7. We use X-ray absorption spectroscopy (XAS) to analyze oxidation state and local bonding around the incorporated cations (e.g., coordination number), valence-band X-ray photoelectron spectroscopy (VB-XPS) to assess the impact of the cations on the valence band electronic structure, and optical absorption spectroscopy to assess the presence and energies of new unoccupied electronic states. Cations with partially filled d-orbitals (e.g., Fe 3+ , Cr 3+ , Co 2+ , Ni 2+ , Cu 2+ , etc.) add filled 3d electronic states at the valence band edge of a-Al 2 O 3 as well as unfilled 3d electronic states within the a-Al 2 O 3 band gap. Cations with the d 10 electronic configuration, such as Zn 2+ and Ga 3+ , introduce filled 3d electronic states at energies well below the valence band edge in addition to new unfilled electronic states in the a-Al 2 O 3 band gap. Cations with unfilled d-orbitals (e.g., those with a d 0 configuration such as V 5+ and Cr 6+ ) do not modify the valence band of a-Al 2 O 3 , but affect the optical properties by inserting unfilled 3d electronic states within the band gap. These results provide guidance for the rational design of the electronic structure of amorphous mixed-metal oxides.

Understanding Surface Reactivity of Amorphous Transition-Metal-Incorporated Aluminum Oxide Thin Films

“…This increase for mixed compositions may further arise from changes in coordination of Fe in the amorphous oxide. The coordination of Fe drops from 6 in Fe 2 O 3 to ∼4 in Al (1– x ) Fe x O y , and the Fe–O bond length shortens, suggesting more tetrahedral than octahedral Fe 3+ ions . Because methoxy and OH formation are maximum at Al6Fe4 and do not increase with further 4-fold coordinated Fe sites in Al4Fe6, this suggests both Al and Fe participate in methanol dissociation.…”

“…Different ratios of 0.2 M aqueous solutions (prepared in 18.2 MΩ cm water) of transition-metal nitrate salts (Fe­(NO 3 ) 3 and Mn­(NO 3 ) 2 ) were mixed with 0.2 M aq Al­(NO 3 ) x (OH) 3– x clusters, prepared by bulk electrolysis . The solution was spun coat onto a Si wafer, previously coated with Ti and Ir by electron-beam evaporation to mitigate potential electrostatic charging, and annealed at 450 °C for 30 min . The final film thickness was targeted to be 5 nm, and previous physical characterization of films prepared by the same technique illustrated near-atomic roughness (root-mean-squared roughness of ∼0.15 nm) by atomic force microscopy .…”

“…Alumina presents an ideal amorphous host that can incorporate a wide range of transition metal cations, and the resulting material is of great interest in applications including catalysis ,, and solar cells . As a wide-band-gap material, the addition of new electronic states into the band gap from transition metals can be probed spectroscopically; for example, the incorporation of Fe leads to new filled states at the top of the valence band . Missing, however, is an understanding of how these new electronic states change the functionality of the oxide surface, including its acidity, basicity, reactivity, and stability.…”

“…11 As a wide-band-gap material, the addition of new electronic states into the band gap from transition metals can be probed spectroscopically; for example, the incorporation of Fe leads to new filled states at the top of the valence band. 12 Missing, however, is an understanding of how these new electronic states change the functionality of the oxide surface, including its acidity, basicity, reactivity, and stability.…”

Modeling of Diffusion and Incorporation of Interstitial Oxygen Ions at the TiN/SiO2 Interface

“…There is a wealth of previous literature concerned with tackling just this problem. [69][70][71][72][73][74][75] Here we use the approach given by Emami et al 76 as they show excellent agreement between the experimental and the theoretical surface OH concentration. To facilitate the analysis of the results for oxygen incorporation energies and migration barriers we also employ a crystalline SiO 2 model to give context to the a-SiO 2 calculations.…”