Electronic-Structural Phase Correlations in Oxygen-Deficient Hafnia Nanocrystals

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.

Keywords: FTIR spectroscopy; hafnia; low temperature phase transition; oxygen vacancies; pyroelectric effect.

Figure 1

Hf 4f high‐resolution XPS spectra of a) HfO₂‐0.6, b) (Hf,Zr)O₂‐0.6, c) HfO₂‐2.2, d) (Hf,Zr)O₂‐2.2 e) Evolution of the oxide share versus (TEMA)M/ H₂O flow ratio f) Zr 3d high‐resolution XPS spectra of (Hf,Zr)O₂‐2.2.

Figure 2

Capacitance versus Voltage characteristics at RT for a) Si/Mo/AlN/HfO₂‐0.6 (black), Si/Mo/AlN/(Hf,Zr)O₂‐2.2 (magenta), and b) Si/Mo/AlN/HfO₂‐2.2 (red); 100 kHz, 0.5 V AC signal.

Figure 3

TEM image at low magnification and the corresponding SAED pattern of the a, c) Si/SiO₂/ HfO₂‐2.2 and b, d) Si/SiO₂/AlN/HfO₂‐2.2 heterostructures; TEM image at high magnification of the e) Si/SiO₂/ HfO₂‐2.2 and f) Si/SiO₂/AlN/ HfO₂‐2.2 heterostructures.

Figure 4

The frequency dependence of the pyroelectric signal for a) Si++/SiO₂ (gray line), Si++/SiO₂/AlN, b) Si++/SiO₂/HfO₂‐2.2, and c) Si++/SiO₂/AlN/HfO₂‐2.2 devices.

Figure 5

Capacitance versus Temperature measured at 100 kHz with AC signal of 0.5 V for a) Si++/SiO₂/ HfO₂‐2.2 and b) Si++/SiO₂/AlN/ HfO₂‐2.2 devices; 100 kHz, 0.5 V AC signal.

Figure 6

GIXRD patterns of the HfO₂‐2.2 on AlN at various temperatures: a) 100 K, b) 150 K, c) 200 K, d) 250 K, e) 300 K and f) zoom‐in of the XRD pattern measured at 300 K.

Figure 7

The FTIR‐ATR spectra of the HfO₂‐2.2 and AlN; a) investigation domain: 100–900 cm⁻¹, b) zoom‐in: 100–600 cm⁻¹; absorbance (blue line) and first derivative (red line for HfO₂‐2.2 and gray line for AlN).

Figure 8

Schematic of the employed DLI‐ALD system.