# Bridging classical and quantum interpretation of chemical state analysis by XPS/HAXPES to resolve short-range order in amorphous alumina films

## TL;DR

This study combines XPS/HAXPES and simulations to understand the structure and chemistry of amorphous alumina films.

## Contribution

The integration of XPS/HAXPES with advanced simulations provides new insights into the local structure of amorphous oxides.

## Key findings

- Auger parameter shifts reveal local coordination environments in amorphous alumina.
- Cryo-XPS can isolate intrinsic structural building blocks in amorphous oxides.
- Thermal fluctuations and structural disorder influence extra-atomic relaxation energies.

## Abstract

Probing the local structure and chemistry of wide-bandgap amorphous oxide thin films remains challenging due to the limitations of lab-based spectroscopy. This work integrates X-ray photoelectron spectroscopy (XPS), hard X-ray photoemission spectroscopy (HAXPES), molecular dynamics simulations using machine-learning interatomic potentials, density-functional theory (DFT) calculations, and classical electrostatic modeling of final-state core-ionization effects in Al atoms to uncover the structure and chemistry of amorphous alumina polymorphs made with atomic layer deposition (ALD). DFT calculations using the ΔKohn–Sham method supported the interpretation of final-state effects and validated electrostatic model assumptions. Shifts in the measured Auger parameters were interpreted as extra-atomic relaxation energies, revealing sensitivity to the local coordination environment. Structural disorder and thermal fluctuations were found to govern the distribution of extra-atomic relaxation energies, suggesting that cryo-XPS can isolate and reveal intrinsic structural building blocks of amorphous oxides. Simulated heating and annealing demonstrated that Auger parameter shifts can serve as indicators of phase decomposition in H-supersaturated ALD amorphous alumina. These findings provide a pathway for comprehensive interpretation and predictive modeling of XPS spectra in amorphous wide-bandgap oxides.

Combined XPS/HAXPES and atomistic simulations revealed that thermally driven bonding rearrangements are linked to the emergence of nanoscale gas transport pathways in alumina.