Search

Ferroelectric hafnium zirconium oxide holds great promise for a broad spectrum of complementary metal-oxide-semiconductor (CMOS) compatible and scaled microelectronic applications, including memory, low-voltage transistors, and infrared sensors, among others. An outstanding challenge hindering the implementation of this material is polarization instability during field cycling. In this study, the nanoscale phenomena contributing to both polarization fatigue and wake-up are reported. Using synchrotron X-ray diffraction, the conversion of non-polar tetragonal and polar orthorhombic phases to a non-polar monoclinic phase while field cycling devices comprising noble metal contacts is observed. This phase exchange accompanies a diminishing ferroelectric remanent polarization and provides device-scale crystallographic evidence of phase exchange leading to ferroelectric fatigue in these structures. A reduction in the full width at half-maximum of the superimposed tetragonal (101) and orthorhombic (111) diffraction reflections is observed to accompany wake-up in structures comprising tantalum nitride and tungsten electrodes. Combined with polarization and relative permittivity measurements, the observed peak narrowing and a shift in position to lower angles is attributed, in part, to a phase exchange of the non-polar tetragonal to the polar orthorhombic phase during wake-up. These results provide insight into the role of electrodes in the performance of hafnium oxide-based ferroelectrics and mechanisms driving wake-up and fatigue, and demonstrate a non-destructive means to characterize the phase changes accompanying polarization instabilities.

Doped ferroelectric HfO2 is highly promising for integration into complementary metal-oxide semiconductor (CMOS) technology for devices such as ferroelectric nonvolatile memory and low-power field-effect transistors (FETs). We report the direct measurement of the energy barriers between various metal electrodes (Pt, Au, Ta, TaN, Ti/Pt, Ni, Al) and hafnium zirconium oxide (Hf0.58Zr0.42O2, HZO) using internal photoemission (IPE) spectroscopy. Results are compared with valence band offsets determined using the three-sample X-ray photoelectron spectroscopy (XPS) as well as the two-sample hard X-ray photoelectron spectroscopy (HAXPES) techniques. Both XPS and IPE indicate roughly the same dependence of the HZO barrier on metal work function with a slope of 0.8 ± 0.5. XPS and HAXPES-derived barrier heights are on average about 1.1 eV smaller than barrier heights determined by IPE, suggesting the presence of negative charge in the HZO.

Ferroelectric oxides interfaced with transition metal dichalcogenides (TMDCs) offer a promising route toward ferroelectric-based devices due to lack of dangling bonds on the TMDC surface leading to a high-quality and abrupt ferroelectric/TMDC interface. In this work, the thermal stability of this interface is explored by first depositing hafnium zirconium oxide (HZO) directly on geological MoS2 and as-grown WSe2, followed by sequential annealing in ultra-high vacuum (UHV) over a range of temperatures (400-800 {\deg}C), and examining the interface through X-ray photoelectron spectroscopy (XPS). We show that the nucleation and stability of HZO grown through atomic layer deposition (ALD) varied depending on functionalization of the TMDC, and the deposition conditions can cause tungsten oxidation in WSe2. It was observed that HZO deposited on non-functionalized MoS2 was unstable and volatile upon annealing, while HZO on functionalized MoS2 was stable in the 400-800 {\deg}C range. The HZO/WSe2 interface was stable until 700 {\deg}C, after which Se began to evolve from the WSe2. In addition, there is evidence of oxygen vacancies in the HZO film being passivated at high temperatures. Lastly, X-ray diffraction (XRD) was used to confirm crystallization of the HZO within the temperature range studied., Comment: 33 pages, 11 figures

Recent reports of superconductivity and magnetism in single crystals of [Li1−xFexOH]FeS show unexplained variations in both superconducting and magnetic properties. We investigate the effect of hydrothermal growth conditions on these properties and find that increasing the growth temperature systematically increases the superconducting transition temperature (Tc), sharpens the magnetic transition, and decreases the scattering rate (Γ). The slow rate of Tc suppression with increasing Γ indicates a conventional s-wave superconducting state according to the Abrikosov-Gorkov expression. Samples with higher scattering rate show broader magnetic transitions and a stronger temperature dependence in the magnetic susceptibility. These results identify disorder, due to interstitial iron impurities, as the unique internal parameter responsible for the unexplained variations in Tc and magnetic ordering. We demonstrate the optimal hydrothermal growth conditions to minimize disorder and maximize Tc in [Li1−xFexOH]FeS crystals.

Electrochemical CO2 reduction can convert CO2 into fuels and valuable chemicals using renewable electricity, which provides a prospective path toward large-scale energy storage. Au nanostructured e...

The elastic moduli of amorphous and crystalline atomic layer-deposited Hf1-xZrxO2 (HZO, x = 0, 0.31, 0.46, 0.79, 1) films prepared with TaN electrodes on silicon substrates were investigated using picosecond acoustic measurements. The moduli of the amorphous films were observed to increase between 211 ± 6 GPa for pure HfO2 and 302 ± 9 GPa for pure ZrO2. In the crystalline films, it was found that the moduli increased upon increasing the zirconium composition from 248 ± 6 GPa for monoclinic HfO2 to 267 ± 9 GPa for tetragonal ZrO2. Positive deviations from this increase were observed for the Hf0.69Zr0.31O2 and Hf0.54Zr0.46O2 compositions, which were measured to have moduli of 264 ± 8 GPa and 274 ± 8 GPa, respectively. These two compositions contained the largest fractions of the ferroelectric orthorhombic phase, as assessed from polarization and diffraction data. The biaxial stress states of the crystalline films were characterized through sin2( ψ) x-ray diffraction analysis. The in-plane stresses were all found to be tensile and observed to increase with the increasing zirconium composition, between 2.54 ± 0.6 GPa for pure HfO2 and 5.22 ± 0.5 GPa for pure ZrO2. The stresses are consistent with large thermal expansion mismatches between the HZO films and silicon substrates. These results demonstrate a device-scale means to quantify biaxial stress for investigation on its effect on the ferroelectric properties of hafnia-based materials.

We introduce an Atomic Layer Deposition (ALD) technique referred to here as Sequential, No-Atmosphere Processing (SNAP) to fabricate ferroelectric Hf0.5Zr0.5O2 capacitors in Metal–Ferroelectric–Metal (MFM) structures. SNAP involves the ALD of each layer sequentially while maintaining the sample under vacuum process conditions without ambient exposure during the entire sequential deposition processes. We first use plasma enhanced ALD to fabricate 002-textured TiN films and study the degree of texture and quality of the film by X-ray Diffraction (XRD), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and transmission electron microscopy. Building upon the textured TiN film, we fabricate MFM capacitors with 10-nm-thick Hf0.5Zr0.5O2 via SNAP deposition and observe an unexpectedly large remanent polarization (2Pr = 54.2 μC/cm2). We report that annealing at T

Composition dependence of second harmonic generation, refractive index, extinction coefficient, and optical bandgap in 20 nm thick crystalline Hf1−xZrxO2 (0 ≤ x ≤ 1) thin films is reported. The refractive index exhibits a general increase with increasing ZrO2 content with all values within the range of 1.98–2.14 from 880 nm to 400 nm wavelengths. A composition dependence of the indirect optical bandgap is observed, decreasing from 5.81 eV for HfO2 to 5.17 eV for Hf0.4Zr0.6O2. The bandgap increases for compositions with x > 0.6, reaching 5.31 eV for Hf0.1Zr0.9O2. Second harmonic signals are measured for 880 nm incident light. The magnitude of the second harmonic signal scales with the magnitude of the remanant polarization in the composition series. Film compositions that display near zero remanent polarizations exhibit minimal second harmonic generation while those with maximum remanent polarization also display the largest second harmonic signal. The results are discussed in the context of ferroelectric phase assemblage in the hafnium zirconium oxide films and demonstrate a path toward a silicon-compatible integrated nonlinear optical material.

We report on the thermal resistances of thin films (20 nm) of hafnium zirconium oxide (Hf1–xZrxO2) with compositions ranging from 0 ≤ x ≤ 1. Measurements were made via time-domain thermoreflectance and analyzed to determine the effective thermal resistance of the films in addition to their associated thermal boundary resistances. We find effective thermal resistances ranging from 28.79 to 24.72 m2 K GW−1 for amorphous films, which decreased to 15.81 m2 K GW−1 upon crystallization. Furthermore, we analyze the heat capacity for two compositions, x = 0.5 and x = 0.7, of Hf1–xZrxO2 and find them to be 2.18 ± 0.56 and 2.64 ± 0.53 MJ m−3 K−1, respectively.

Magnetic van der Waals (vdW) materials are the centerpiece of atomically thin devices with spintronic and optoelectronic functions. Exploring new chemistry paths to tune their magnetic and optical properties enables significant progress in fabricating heterostructures and ultracompact devices by mechanical exfoliation. The key parameter to sustain ferromagnetism in 2D is magnetic anisotropy-a tendency of spins to align in a certain crystallographic direction known as easy-axis. In layered materials, two limits of easy-axis are in-plane (XY) and out-of-plane (Ising). Light polarization and the helicity of topological states can couple to magnetic anisotropy with promising photoluminescence or spin-orbitronic functions. Here, a unique experiment is designed to control the easy-axis, the magnetic transition temperature, and the optical gap simultaneously in a series of CrCl3-x Brx crystals between CrCl3 with XY and CrBr3 with Ising anisotropy. The easy-axis is controlled between the two limits by varying spin-orbit coupling with the Br content in CrCl3-x Brx . The optical gap, magnetic transition temperature, and interlayer spacing are all tuned linearly with x. This is the first report of controlling exchange anisotropy in a layered crystal and the first unveiling of mixed halide chemistry as a powerful technique to produce functional materials for spintronic devices.