# Electronic–Structural Phase Correlations in Oxygen‐Deficient Hafnia Nanocrystals

## TL;DR

This study explores the structural and electronic properties of oxygen-deficient hafnia nanocrystals and their phase transitions, which could lead to better lead-free sensors.

## Contribution

The paper reveals a phase transition in hafnia nanocrystals and shows how oxygen vacancies influence their electronic and pyroelectric behavior.

## Key findings

- HfO2 films under reducing conditions show sub-oxide phases and p-type semiconducting behavior.
- A tetragonal-to-orthorhombic phase transition occurs near 200 K, with room-temperature stabilization of polar phases.
- An AlN interlayer significantly enhances the pyroelectric performance of HfO2-based devices.

## Abstract

Layers of HfO2 and (Hf,Zr)O2 crystalline nano‐particles are synthesized via direct liquid injection atomic layer deposition, and a comprehensive set of structural, chemical, and electrical characterizations is employed to elucidate their phase composition and functional behavior. X‐ray photoelectron spectroscopy revealed a compositional contrast between the films: (Hf,Zr)O2 layers contained up to 45% stoichiometric oxide, while pure HfO2 films are dominated by sub‐oxides, especially under strongly reducing conditions, in which exclusively sub‐oxide phases and p‐type semiconducting behavior is revealed. Electrical measurements indicated room‐temperature stabilization of polar phases and tetragonal‐to‐orthorhombic phase transition with a Curie temperature near 200 K. FTIR spectroscopy confirmed the presence of tetragonal and orthorhombic HfO2 phases, providing insight into minor features observed ≈30° (2θ) in X‐ray diffraction patterns. Notably, devices incorporating an AlN interlayer demonstrated a significant enhancement in pyroelectric performance, suggesting this strategy to advance the pyroelectric performance of HfO2‐based materials, supporting their development for lead‐free sensor technologies.

Layers of HfO2 nanoparticles are fabricated by DLI‐ALD. The resulting devices exhibit a pyroelectric response, contrary to expectations based on SAED and XRD results. A phase transition from orthorhombic to tetragonal phase (above 200 K) is revealed. The investigation indicates that the tetragonal phase transforms into a polar phase, a phenomenon conditioned by the presence of oxygen vacancies.