Nicolas Vaxelaire

We present a hard x-ray photoemission (HAXPES) study of the oxygen vacancy (VO) concentration profile in 6 nm Hf0.5Zr0.5O2 ferroelectric capacitors as a function of field cycling at three operational voltages, VOP, namely 1.5, 2.0, and 2.5 V. Pristine capacitors show a marked pinched hysteresis loop and a double peak I–V response consistent with domain pinning and/or an antiferroelectric phase. The capacitors are woken up by field cycling at all three VOP, as shown by the increase in remanent polarization 2PR and the partial merging of the ferroelectric switching current peaks. At higher VOP, fatigue sets in earlier. Non-destructive, synchrotron radiation-induced HAXPES was used to quantify the evolution of the VO concentration with cycling. Field cycling induces VO migration, unpinning domains during the ferroelectric wake-up with a concomitant reduction in coercive voltage with increasing VOP. The VO concentration profile is directly correlated with the internal field due to polarization, as evidenced by the Hf 3d binding energies. Above 105 cycles, there is evidence for additional VO generation, which can be associated with the onset of ferroelectric fatigue. Finally, imprint appears to be mainly due to charge injection with only a weak direct dependence on the VO concentration profile.

Switching spectroscopy piezoresponse force microscopy (SSPFM) is widely used to probe local polarization reversal in ferroelectric thin films, but resonance-enhanced implementations remain difficult to apply to large datasets because they typically rely on repeated frequency sweeps, leading to longer acquisition times, higher noise levels, and fitting-related instability. Here, we introduce an approach that combines SSPFM with real-time dual-frequency resonance tracking (DFRT), allowing the excitation frequency to remain locked to the instantaneous contact-resonance frequency throughout the spectroscopy sequence. This strategy extends the benefits of real-time resonance tracking, commonly exploited in continuous measurements, to pulsed switching spectroscopy, while avoiding repeated reconstruction of the full resonance peak at each voltage step. The method is evaluated on a Pb(Zr0.2Ti0.8)O3 thin film and compared with two conventional SSPFM approaches: sweep-based and fixed-frequency measurements. At short segment durations, sweep-based SSPFM becomes increasingly affected by phase noise, phase-inversion artifacts, and fitting errors, whereas SSPFM-DFRT preserves low noise and measurement stability. In addition, the conversion of raw data into hysteresis loops is substantially faster because no repeated resonance fitting is required. Compared with fixed-frequency SSPFM, it also compensates resonance shifts, which become critical during prolonged measurements such as point matrix acquisitions and would otherwise reduce measurement repeatability and robustness. Finally, SSPFM-DFRT is demonstrated in mapping mode on a 50 × 30 point matrix, where it enables robust extraction of switching parameters with high fit quality across the dataset. Overall, this approach provides a practical route to faster, more stable, and more robust resonance-enhanced SSPFM for spectroscopy and mapping, particularly when short measurement times and reliable large-scale measurements are required.

Bulk CsPbBr3 is an emerging semiconductor that has shown unprecedented increase in performance over the last decade for optoelectronic applications. However, further development of devices based on CsPbBr3 is hampered by their poor electrical stability under operation. Migration and accumulation of native ions (Cs+, Pb2+, and Br-) under electric fields has been suggested by many groups to be responsible for the observed device instabilities, although direct experimental evidence of ionic motion during operation has been seldom reported. In our study, ion migration has been probed by grazing incidence x-ray fluorescence (GIXRF) in CsPbBr3 polycrystalline layers grown in vapor phase. Our findings indicate that both Cs and Br experience ionic migration under electric field, suggesting that these ions are responsible for the measured current instability in our devices. In the timescale of a few hours, the ionic drifting rates of mobile Cs and Br under the top electrode were found to be similar, as high as ∼ 20 ppm h−1 V−1 mm. This work paves the way for a better understanding of ion motion issues that play a key role in the optoelectronic properties of CsPbBr3 devices.

Nowadays, the growing worldwide electrification requires new materials for power management. SiC currently dominates the market thanks to excellent energy efficiency and broad operating capabilities. The present paper proposes an experimental study of the Ni-SiC backside ohmic contact formation using 308 nm nanosecond laser annealing (NLA). After Nickel (80 nm) sputtering over 4H-SiC wafers, various laser conditions are investigated, with energy density (ED) ranging from 2.4 to 5.4 J/cm², pulse number from 1 to 20 and chuck temperature from 25 °C (RT) to 400 °C. For all series, a common scenario is noticed as the ED increases, with first solid-state reactions, then local melt and, finally, complete top layer melt and de-wetting at high ED. An in-depth understanding of the impact of laser conditions on these stages is achieved, based on electrical data, Raman spectroscopy, optical microscopy, Scanning Electron Microscopy (SEM) and Scanning Transmission Electron Microscopy (STEM). Results reveal that both high pulse numbers and the use of a hot chuck enable to significantly reduce the ED needed to form low resistance contacts. In addition, sheet resistances and contact resistivities are linked to the microstructure evolution upon NLA exposure. As a proof-of-concept, an acceptable process point yields a contact resistivity around 5×10^-5 Ω cm² when the wafer is processed at 25 °C and a value as low as 10^-5 Ω cm² for 400 °C processing. The mechanisms involved and discussed in the present work may very likely pave the way for other contact formation with limited thermal budget.

In this paper, the ferroelectric properties of TiN/Ti/Si:HfO2/TiN stacks are shown to be modulated by the Ti oxygen scavenging layer, leading to improved remanent polarization and wake-up at low switching fields. Hard x-ray photoelectron spectroscopy is used to measure the oxygen vacancy (VO) concentration in Si-doped HfO2 based capacitors. This VO engineered ferroelectric stack is then assessed at the wafer scale within 16 kbit 1T-1C FeRAM arrays integrated into 130 nm CMOS. The introduction of a Ti oxygen scavenging layer at the top interface of Si:HfO2-based BEOL-integrated metal/ferroelectric/metal capacitors enhances their ferroelectric (FE) behavior (+100% remanent polarization 2.Pr), confirming the positive role of modest concentrations of VO in orthorhombic phase crystallization at low thermal budgets. It is statistically demonstrated at 16 kbit array level that VO-rich FE Si:HfO2 facilitates 2.5 V FeRAM array operation with improved memory window. The field cycling dependence of the memory window is correlated with the VO concentration, and an endurance of 108 cycles is measured at the array level.

Switching Spectroscopy Piezoresponse Force Microscopy (SSPFM) stands out as a powerful method for probing ferroelectric properties within materials subjected to incremental polarization induced by an external electric field. However, the dense data processing linked to this technique is a critical factor influencing the quality of obtained results. Furthermore, meticulous exploration of various artifacts, such as electrostatics, which may considerably influence the signal, is a key factor in obtaining quantitative results. In this paper, we present a global methodology for SSPFM data processing, accessible in open-source with a user-friendly Python application called PySSPFM. A ferroelectric thin film sample of potassium sodium niobate has been probed to illustrate the different aspects of our methodology. Our approach enables the reconstruction of hysteresis nano-loops by determining the PR as a function of applied electric field. These hysteresis loops are then fitted to extract characteristic parameters that serve as measures of the ferroelectric properties of the sample. Various artifact decorrelation methods are employed to enhance measurement accuracy, and additional material properties can be assessed. Performing this procedure on a grid of points across the surface of the sample enables the creation of spatial maps. Furthermore, different techniques have been proposed to facilitate post-treatment analysis, incorporating algorithms for machine learning (K-means), phase separation, and mapping cross correlation, among others. Additionally, PySSPFM enables a more in-depth investigation of the material by studying the nanomechanical properties during poling, through the measurement of the resonance properties of the cantilever–tip–sample surface system.

Piezoelectric thin films are particularly difficult to model at the grain scale. Moreover, this problem is sparsely experimentally documented due to the lack of adequate methods. Here, an original methodology is proposed to study the behavior of single grains during in situ biasing. Pb(Zr,Ti₎O₃ films have been evaluated thanks to an in situ biasing x-ray diffraction technique performed on a synchrotron source with a sub-micronic x-ray beam. Small capacitors have been biased with DC voltage between 0 and ±20 V and at each step of bias a set of spotty Debye rings have been recorded. By selecting an appropriate region of interest, the evolution of a single grain peak has been followed. The fine analysis of these peaks allows to calculate the effective piezoelectric coefficient d_33,eff, the proportion of each domain variants, the tetragonality inside each grain, as well as their evolution during the electrical biasing. A higher heterogeneity in samples with morphotropic phase boundary composition is observed. This study contributes to a better understanding of the local behaviors in piezo/ferroelectric polycrystalline films and in the improvement of their performance for different applications.

We present a study of the effect of electrically induced cracks on both the ferroelectric and piezoelectric properties of Pt/PbZr_0.52Ti_0.48O₃ (PZT)/Pt capacitors and correlations with domain structures of PZT films. Above a threshold bipolar electric field, cracks appear inside the PZT layer thickness leading to an increase in the ferroelectric polarization (+50% for the remnant polarization, from 16 to 25 μC/cm²) and the longitudinal piezoelectric coefficient d_33,f (from ∼150 to ∼220 pm/V). The use of x-ray diffraction during in situ biasing provides direct evidence for a modification of the PZT crystalline structure as well as the a/c domain configuration. After cracking, the fraction of c-domains is strongly increased, thus contributing to higher polarization and larger strain in the out-of-plane direction.

Electrical aging in lead zirconate titanate (PbZr_xTi_1−xO₃) thin films has been intensively studied from a macroscopic perspective. However, structural origins and consequences of such degradation are less documented. In this study, we have used synchrotron radiation to evaluate the behavior of ferroelectric domains by X-ray diffraction (XRD). The sample was loaded with an AC triangular bias waveform between ±10 V with a number of cycle varying from one up to 10⁸. At each step of the aging procedure, XRD spectra had been collected in situ during the application of an electric field on a capacitor. The fine analysis of the (200) pseudo-cubic peak structure allows to separate the evolution of the volume of a/c tetragonal and rhombohedral domains along the electrical biasing. Throughout the aging, both intrinsic and extrinsic responses of tetra and rhombohedral domains are altered, the behavior depending on the observed phase. This methodology opens up new perspectives in the comprehension of the aging effect in ferroelectric thin film.