Band Excitation Piezoresponse Force Microscopy Adapted for Weak Ferroelectrics: On-the-Fly Tuning of the Central Band Frequency.

New interest in microscopic studies of ferroelectric materials with low piezoelectric coefficient, $d_{33}^\ast$, has emerged after the discovery of ferroelectric properties in HfO2 thin films, which are the main candidate for the next generation of nonvolatile ferroelectric memory. The study of the microscopic structure of ferroelectric HfO2 capacitors is crucial to get insights into the device behavior and performance. However, a small $d_{33}^\ast$ of ferroelectric HfO2 films leads to a low piezoresponse, especially in band excitation piezoresponse force microscopy (BE-PFM). In this work, we have implemented the BE-PFM technique with an increased scanning rate, thus improving this versatile tool for weak ferroelectrics. The acceleration of measurement was achieved by focusing excitation into a narrow frequency band and tuning the central frequency on-the-fly using an online real-time model estimation by fitting a complex BE response. The tracking of the contact resonance frequency was implemented using a pure mechanical cantilever response acquired in BE atomic force acoustic microscopy. To obtain optimal excitation parameters, we perform statistical analysis by minimizing estimator variance. The measurement precision of several PFM techniques was compared both by the simulation and experimentally using a Hf0.5Zr0.5O2-based ferroelectric capacitor.

Broadband mid-IR chalcogenide fiber couplers.

We demonstrate a broadband As2S3-based fiber coupler operating up to the 5.4 µm wavelength range developed by using a fused biconical tapering technique. During the manufacturing process, real-time data monitoring of the coupling ratio was at 2.64 µm. The measurement of minimal excess loss was at less than 1 dB in the range of 5-5.4 µm. Also, fiber bend loss was numerically analyzed to determine optimal coupler geometric parameters.

High resolution heterodyne spectroscopy of the atmospheric methane NIR absorption.

The paper describes the concept of a compact, lightweight heterodyne NIR spectro-radiometer suitable for atmospheric sounding with solar occultations, and the first measurement of CO2 and CH4 absorption near 1.65 µm with spectral resolution λ/δλ~10(8). A highly stabilized DFB laser was used as local oscillator, while single model silica fiber Y-coupler served as a diplexer. Radiation mixed in the single mode fiber was detected by a balanced couple of InGaAs p-i-n diodes within the bandpass of ~3 MHz. Wavelength coverage of spectral measurement was provided by sweeping local oscillator frequency in the range of 1.1 cm(-1). With the exposure time of 10 min, the absorption spectrum of the atmosphere over Moscow has been recorded with S/N ~120, limited by shot noise. The inversion algorithm applied to this spectrum resulted in methane vertical profile with a maximum mixing ratio of 2148 ± 10 ppbv near the surface and column density 4.59 ± 0.02·10(22) cm(-2).

Unravelling the nanoscale mechanism of polarization reversal in a Hf0.5Zr0.5O2-based ferroelectric capacitor by vector piezoresponse force microscopy.

Ferroelectricity is in demand in many device concepts in electronics, energy and microsystem engineering. The performance of ferroelectrics-based devices is determined by either out-of-plane or in-plane polarization, or out-of-plane or in-plane piezoelectric strain. Real prospects for the practical implementation of innovative devices opened up after the discovery of ferroelectricity in ultrathin hafnium oxide films, due to their perfect compatibility with silicon technology. Ferroelectric properties of this material have been assigned to an orthorhombic structural phase with a single polar axis, but the spatial orientation of the polarization vector and the tensorial piezoelectric behaviour, which are inextricably coupled, still remain unknown. Herein, the rotation of the polarization vector in a Hf0.5Zr0.5O2 (10 nm) capacitor during polarization switching and the spatial distribution of longitudinal and shear piezoelectric coefficients are elucidated at the nanoscale using operando vector piezoresponse force microscopy. In most of the capacitor, a 180°-flipping of the polarization vector is observed, which is consistent with the orthorhombic phase structure. However, a rather large fraction of the capacitor is also occupied by nanoregions of ferroelastic (non-180°) switching, which is explained by the effect of the local mechanical stress. To quantify the three-dimensional piezoresponse, a novel approach exploiting the Poisson effect in artificially created non-ferroelectric regions is proposed and it shows that the shear piezoelectric coefficient is twice the longitudinal coefficient. The experimental insights entail an important step in fundamental understanding of the ferroelectric and piezoelectric properties of hafnium oxide and have great potential to trigger new versions of ferroelectric-based devices.

Nanoscale Doping and Its Impact on the Ferroelectric and Piezoelectric Properties of Hf0.5Zr0.5O2.

Ferroelectric hafnium oxide thin films-the most promising materials in microelectronics' non-volatile memory-exhibit both unconventional ferroelectricity and unconventional piezoelectricity. Their exact origin remains controversial, and the relationship between ferroelectric and piezoelectric properties remains unclear. We introduce a new method to investigate this issue, which consists in a local controlled modification of the ferroelectric and piezoelectric properties within a single Hf0.5Zr0.5O2 capacitor device through local doping and a further comparative nanoscopic analysis of the modified regions. By comparing the ferroelectric properties of Ga-doped Hf0.5Zr0.5O2 thin films with the results of piezoresponse force microscopy and their simulation, as well as with the results of in situ synchrotron X-ray microdiffractometry, we demonstrate that, depending on the doping concentration, ferroelectric Hf0.5Zr0.5O2 has either a negative or a positive longitudinal piezoelectric coefficient, and its maximal value is -0.3 pm/V. This is several hundreds or thousands of times less than those of classical ferroelectrics. These changes in piezoelectric properties are accompanied by either improved or decreased remnant polarization, as well as partial or complete domain switching. We conclude that various ferroelectric and piezoelectric properties, and the relationships between them, can be designed for Hf0.5Zr0.5O2 via oxygen vacancies and mechanical-strain engineering, e.g., by doping ferroelectric films.

Nanoscale Tailoring of Ferroelectricity in a Thin Dielectric Film.

New opportunities in the development and commercialization of novel photonic and electronic devices can be opened following the development of technology-compatible arbitrary-shaped ferroelectrics encapsulated in a passive environment. Here, we report and experimentally demonstrate nanoscale tailoring of ferroelectricity by an arbitrary pattern within the nonferroelectric thin film. For inducing the ferroelectric nanoregions in the nonferroelectric surrounding, we developed a technology-compatible approach of local doping of a thin (10 nm) HfO2 film by Ga ions right in the thin-film capacitor device via focused ion beam implantation. Local crystallization of the doped regions to the ferroelectric structural phase occurs during subsequent annealing. The remnant polarization of the HfO2:Ga regions reached 13 µC/cm2 at a Ga concentration of 0.6 at. %. Piezoresponse force microscopy over the capacitor device revealed an asymmetrical switching of ferroelectric domains within written HfO2:Ga patterns after capacitor switching, which was attributed to the mechanical stress across the doped film. The lateral spatial resolution of ferroelectricity tailoring is found to be ∼200 nm, which enables diverse applications in switchable photonics and microelectronic memories.

Ferroelectricity in Hf0.5Zr0.5O2 Thin Films: A Microscopic Study of the Polarization Switching Phenomenon and Field-Induced Phase Transformations.

Because of their full compatibility with the modern Si-based technology, the HfO2-based ferroelectric films have recently emerged as viable candidates for application in nonvolatile memory devices. However, despite significant efforts, the mechanism of the polarization switching in this material is still under debate. In this work, we elucidate the microscopic nature of the polarization switching process in functional Hf0.5Zr0.5O2-based ferroelectric capacitors during its operation. In particular, the static domain structure and its switching dynamics following the application of the external electric field have been monitored with the advanced piezoresponse force microscopy (PFM) technique providing a nm resolution. Separate domains with strong built-in electric field have been found. Piezoresponse mapping of pristine Hf0.5Zr0.5O2 films revealed the mixture of polar phase grains and regions with low piezoresponse as well as the continuum of polarization orientations in the grains of polar orthorhombic phase. PFM data combined with the structural analysis of pristine versus trained film by plan-view transmission electron microscopy both speak in support of a monoclinic-to-orthorhombic phase transition in ferroelectric Hf0.5Zr0.5O2 layer during the wake-up process under an electrical stress.

Simple yet accurate noncontact device for measuring the radius of curvature of a spherical mirror.

An easily reproducible device is demonstrated to be capable of measuring the radii of curvature of spherical mirrors, both convex and concave, without resorting to high-end interferometric or tactile devices. The former are too elaborate for our purposes, and the latter cannot be used due to the delicate nature of the coatings applied to mirrors used in high-power CO(2) laser applications. The proposed apparatus is accurate enough to be useful to anyone using curved optics and needing a quick way to assess the values of the radii of curvature, be it for entrance quality control or trouble shooting an apparently malfunctioning optical system. Specifically, the apparatus was designed for checking 50 mm diameter resonator (typically flat or tens of meters concave) and telescope (typically some meters convex and concave) mirrors for a high-power CO(2) laser, but it can easily be adapted to any other type of spherical mirror by a straightforward resizing.