Temperature‐Dependent Phase Transitions in Hf_xZr_1‐xO₂ Mixed Oxides: Indications of a Proper Ferroelectric Material

Abstract

Knowledge about phase transitions in doped HfO2 and ZrO2‐based films is crucial for developing future ferroelectric devices. These devices should perform in ambient temperature ranges with no degradation of device performance. Here, the phase transition from the polar orthorhombic to the nonpolar tetragonal phase in thin films is of significant interest. Detailed electrical and structural characterization is performed on 10 nm mixed HfxZr1‐xO2 binary oxides with different ZrO2 in HfO2 and small changes in oxygen content. Both dopant and oxygen content directly impact the phase transition temperature between the polar and nonpolar phase. A first‐order phase transition with thermal hysteresis is observed from the nonpolar to the polar phase with a maximum in the dielectric constant. The observed phase transition temperatures confirm trends as obtained by DFT calculations. Based on the outcome of the measurements, the classification of the ferroelectric material is discussed.

Influence of the Ozone Dose Time during Atomic Layer Deposition on the Ferroelectric and Pyroelectric Properties of 45 nm-Thick ZrO2 Films

“…Based on these measurements, the extracted in-plane tensile stress decreases from 1.9 to 0.6 GPa (Figure c) with a longer ALD ozone dose time. Previous work by our group reported on the ozone dose time influence on 10 nm ZrO 2 films, but the sin 2 Ψ method was not carried out. , The 10 nm ZrO 2 thin films previously reported are measured with the sin 2 Ψ method, and the stress drops from 3.4 to 1.6 GPa with a longer ozone dose time. Here, the 10 nm ZrO 2 thin films are thinner and deposited with a different ALD tool, which might lead to a different impact of the ozone dose time on the film properties, but still, a longer ozone dose time leads to a lower in-plane tensile stress.…”

“…Previous work by our group reported on the ozone dose time influence on 10 nm ZrO 2 films, but the sin 2 Ψ method was not carried out. 25,26 The 10 nm ZrO 2 thin films previously reported are measured with the sin 2 Ψ method, and the stress drops from 3.4 to 1.6 GPa with a longer ozone dose time.…”

“…Ozone is a widely used oxygen source in ALD, and tuning the ozone dose time influences the ferroelectricity and Curie temperature in the HZO-based thin films by changing the defect concentration. The longer ozone dose time results in fewer oxygen vacancies and carbon impurities, stabilizing the m-phase formation and suppressing the t-phase formation. , The formation of the ferroelectric o-phase is favorable only with an intermediate ozone dose time to enhance the ferroelectric behavior in the fluorite-structured thin films. − …”

Highly Stable Epitaxially Crystallized Ferroelectric Hf0.5Zr0.5O2 Films

“…7,9 Indeed, it is reported that the fraction of the monoclinic phase increases for lower heating and cooling rates. 10 It is also reported 11 that stabilization of the orthorhombic phase in undoped or lightly Zr-doped HfO 2 films requires a high quench rate, while the effect of the quench rate is less relevant for higher dopant concentration. On the other hand, kinetics also conditions the transformation between the tetragonal and orthorhombic phases since the orthorhombic phase has lower energy than the tetragonal phase at room temperature but not at high temperatures.…”

Discovery of Nanoscale Electric Field‐Induced Phase Transitions in ZrO2

“…[19][20][21][22][23][24][25] The first-order nature of the phase transition is predicted by Landau theory and supported by experimental observations of temperatureinduced phase transitions of the ferroelectric o-phase to the nonpolar t-phase. [18,26,27] The electric field-induced phase transitions generating AFE behavior, however, have remained hidden from view despite theoretical and supporting experimental evidence. Direct physical observations are needed for greater exploitation of this nonlinear phenomenon to pave the way for next-generation steep-slope transistors, antiferroelectric-based memories, supercapacitors, neuromorphic devices, and transducers.…”