Vanadium-Doped Hafnium Oxide: A High-Endurance Ferroelectric Thin Film with Demonstrated Negative Capacitance

Abstract

This study proposes and validates a novel CMOS-compatible ferroelectric thin-film insulator made of vanadium-doped hafnium oxide (V:HfO₂) by using an optimized atomic layer deposition (ALD) process. Comparative electrical performance analysis of metal-ferroelectric-metal capacitors with varying V-doping concentrations, along with advanced material characterizations, confirmed the ferroelectric behavior and reliability of V:HfO₂. With remnant polarization (P_r) values up to 20 μC/cm², a coercive field (E_c) of 1.5 MV/cm, excellent endurance (>10¹¹ cycles without failure, extrapolated to 10¹² cycles), projected 10-year nonvolatile retention (>100 days measured), and large grain sizes of ∼180 nm, V:HfO₂ emerges as a promising robust candidate for nonvolatile memory and neuromorphic applications. Importantly, negative capacitance (NC) effects were observed and analyzed in V:HfO₂ through pulsed measurements, demonstrating its potential for NC applications. Finally, this novel ferroelectric shows potential as a gating insulator for future 3-terminal vanadium dioxide Mott-insulator devices and sensors, achieved through an all-ALD process.

Keywords: CMOS-compatible; atomic layer deposition (ALD); ferroelectric thin film; high endurance; negative capacitance; vanadium-doped hafnium oxide (V:HfO2).

Figure 1

(a) Schematic and fabrication process flow of the MFM capacitors. (b) Schematic of the optimized ALD process and cycle structure for 5.9% ferroelectric V:HfO₂. (c) TEM and (d) high-resolution TEM images of the cross section of TiN/V:HfO₂/TiN MFM stack which was electrically woken up. Visible atomic fringes and large grains of >95 nm confirm the crystal quality. (e) Top SEM image of MFM structure after removal of the top TiN layer. Grain domains with a size of ∼200 nm are visible. (f) EBSD map of the same sample, showing an average grain size of ∼180 nm. (g) TEM EDX mapping and (h) elemental profile of the cross section of the MFM capacitor. In the ferroelectric layer, an atomic concentration of 5.6% was measured for V. (i) GXRD spectra of the annealed TiN/V:HfO₂/TiN MFM stack. Inset represents low angle peaks with longer acquisition time and better precision.

Figure 2

DC off-field PFM mapping images of amplitude and phase of local piezoresponses within same (1 μm²) area in (a and d) negatively poled state, (b and e) mixed state, and (c and f) positively poled state; prepolarization was performed using DC biases of −5, 2, and 5 V, respectively. The spatial uniformity of the phase and amplitude images over a 1 h scan time indicates robust retention of the ferroelectric. (d and h) DC off-field and DC on-field PFM loops of amplitude and phase of local piezoresponse measured at the marked point in panel a. These experiments were carried out on an MFM capacitor with a thickness of 16 nm and 5.9% V-doping level.

Figure 3

P_r–E and current density–electric field loops for different VO₂ ALD cycle ratios (top row), measured using a 6 V, 10 kHz triangular stimulation. ε_r–E curves (bottom row) are measured by a CV measurement method with a 100 kHz AC 30 mV RMS signal added to a DC voltage sweep of 6 V amplitude. All the MFM capacitors were annealed at 600 °C for 2 min. The highest remnant polarization of 17 μC/cm² and the highest ε_r change observed at a VO₂ ALD cycle ratio of 5.9%, which is considered as the optimum fabrication condition.

Figure 4

(a) P_r and ε_r at RT as a function of the VO₂ ALD cycle ratio. The error bar represents the standard deviation of five similar batches. (b) SS± and OS± retention test of 5.9% V:HfO₂ in MFM capacitor devices at 85 °C and at RT. (c) Endurance characteristic of an MFM capacitor with 5.9% V:HfO₂ layer under 1 MHz, 4.8 V (3 MV/cm), 5.2 V (3.25 MV/cm), 5.6 V (3.5 MV/cm), and 6 V (3.75 MV/cm) pulse stimulation. An endurance of up to 10¹¹ cycles without failure was observed under a pulse stimulation of 4.8 V. Empty symbols represent breakdown. (d) Benchmark study of endurance versus maximum 2P_r during the endurance test, comparing V:HfO₂ and recent reports on HfO₂-based ferroelectrics. Empty symbols represent breakdown. The data were obtained from the following references: HZO,⁻ La:HZO,^, Si:HZO, Si:HfO₂, Ga:HfO₂, La:HfO₂, Gd:HfO₂, Al:HfO₂, Al:Si:HfO₂, undoped HfO₂. (e) P_r–E hysteresis loops at different temperature conditions from 100 to 350 K, (f) P_r–E and (g) ε_r–E curves at different annealing temperatures, ranging from 450 to 700 °C. (h) 2P_r as a function of annealing and measurement temperatures, extracted from panels e and f. (i) P_r–E curves and (j–l) ε_r–E curves and frequency dispersion of 8, 12 and 24 nm V:HfO₂ layers measured at RT.