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

Simon Gramatte, Wang, Xing+9 more|arXiv (Cornell University)|Aug 15, 2024

Glass properties and applications4 citations

TL;DR

This study combines machine learning interatomic potentials and electrostatic modeling to predict XPS/Auger parameter shifts in amorphous alumina films with varying hydrogen content from atomic layer deposition. It reveals that hydrogen primarily binds as hydroxyl ligands, and the measured Auger parameter shifts correlate strongly with local Al coordination (4–6-fold), bond angles, and ligand types, enabling precise structural assignment of short-range order in amorphous oxides.

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 Delta Kohn-Sham method supported the interpretation of final-state effects and validated electrostatic model assumptions. Shifts in the measured Auger parameter 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.

Motivation & Objective

• To resolve the local chemical bonding states of hydrogen in amorphous alumina films, which remain elusive with conventional experimental techniques.
• To establish a quantitative link between measured Auger parameter shifts and the short-range order in amorphous alumina with variable hydrogen content.
• To validate theoretical predictions against experimental XPS data using atomistic simulations and electrostatic modeling.
• To enable accurate structural assignment of amorphous oxides by correlating electronic spectroscopy data with atomic-scale coordination environments.
• To support the rational design of high-performance hydrogen barrier films, membranes, and energy materials by clarifying H incorporation effects.

Proposed method

• Employed a universal machine learning interatomic potential to model amorphous alumina polymorphs with varying H-content and density from ALD-grown films.
• Simulated annealing of highly defective crystalline hydroxide structures at ALD growth temperatures to reproduce experimentally observed amorphous phases.
• Calculated Auger parameter shifts using the Auger electron spectroscopy (AES) formalism, incorporating core-level shifts from DFT and electrostatic contributions.
• Classified Al cations by their nearest-neighbor ligands (O and OH) using a 1.2 Å cutoff, enabling statistical analysis of coordination environments.
• Performed molecular dynamics simulations at 10 K to reduce thermal noise and improve resolution of electronic transitions in XPS/AES.
• Correlated experimental Auger parameter distributions with simulated contributions from specific Al coordination groups (nO, nOH) to identify dominant structural motifs.

Experimental results

Research questions

• RQ1 How does hydrogen incorporation affect the local chemical bonding environment of Al cations in amorphous alumina?
• RQ2 What is the dominant chemical form of hydrogen (e.g., OH, H2O, H+) in ALD-grown amorphous alumina, and how does it influence electronic structure?
• RQ3 To what extent do variations in Al coordination (4-, 5-, 6-fold), bond lengths, and bond angles correlate with measured Auger parameter shifts?
• RQ4 Can atomistic and electrostatic modeling accurately reproduce experimental XPS/Auger parameter shifts across a range of H/Al ratios?
• RQ5 How does cryogenic XPS improve the resolution of local chemical and structural features in amorphous oxides?

Key findings

• Hydrogen in amorphous alumina films is predominantly present as hydroxyl (OH) ligands, not molecular H2 or interstitial H.
• The measured Auger parameter shifts increase with decreasing oxide density and increasing H-content, primarily due to changes in Al coordination and ligand type.
• For H/Al = 2 (25 °C ALD), the dominant Al environment is six-coordinate with six OH ligands (nO=0, nOH=6), contributing to the lowest-energy Auger peak.
• For H/Al = 1 (50 °C ALD), the main contributions come from four- and five-coordinated Al with mixed O and OH ligands, particularly nO=2, nOH=3 and nO=3, nOH=2.
• For H/Al = 0.2 (120 °C ALD), the structure is dominated by four- and five-fold coordinated Al with O ligands (nO=4, nOH=0 and nO=5, nOH=0), corresponding to the highest Auger parameter energy.
• The splitting of the Auger parameter distribution at 10 K is directly linked to distinct Al coordination groups, with OH-rich environments shifting peaks to lower energies due to higher ligand polarizability.

Figure 1: (a) Visualization of the KL 23 L 23 Auger process: In a first step, an incoming photon excites a 1s core-level photoelectron. The binding energy of this photoelectron is determined by the difference between the incident X-ray energy and the detected kinetic energy of the emitted photoelect