Surface morphology of AlGaN/GaN heterostructures grown on bulk GaN by MBE

In this report the influence of the growth conditions on the surface morphology of AlGaN/GaN heterostructures grown on sapphire-based and bulk GaN substrates is nondestructively investigated with focus on the decoration of defects and the surface roughness. Under Ga-rich conditions specific types of dislocations are unintentionally decorated with shallow hillocks. In contrast, under Ga-lean conditions deep pits are inherently formed at these defect sites. The structural data show that the dislocation density of the substrate sets the limit for the density of dislocation-mediated surface structures after MBE overgrowth and no noticeable amount of surface defects is introduced during the MBE procedure. Moreover, the transfer of crystallographic information, e.g. the miscut of the substrate to the overgrown structure, is confirmed. The combination of our MBE overgrowth with the employed surface morphology analysis by atomic force microscopy (AFM) provides a unique possibility for a nondestructive, retrospective analysis of the original substrate defect density prior to device processing

On the relationship between field cycling and imprint in ferroelectric Hf₀.₅Zr₀.₅O₂

Manifold research has been done to understand the detailed mechanisms behind the performance instabilities of ferroelectric capacitors based on hafnia. The wake-up together with the imprint might be the most controversially discussed phenomena so far. Among crystallographic phase change contributions and oxygen vacancy diffusion, electron trapping as the origin has been discussed recently. In this publication, we provide evidence that the imprint is indeed caused by electron trapping into deep states at oxygen vacancies. This impedes the ferroelectric switching and causes a shift of the hysteresis. Moreover, we show that the wake-up mechanism can be caused by a local imprint of the domains in the pristine state by the very same root cause. The various domain orientations together with an electron trapping can cause a constriction of the hysteresis and an internal bias field in the pristine state. Additionally, we show that this local imprint can even cause almost anti-ferroelectric like behavior in ferroelectric films

Physical Approach to Ferroelectric Impedance Spectroscopy: The Rayleigh Element

The Rayleigh law describes the linear dependence of the permittivity of a ferroelectric on the applied ac electric field amplitude due to irreversible motions of domain walls. We show that this gives rise to a new equivalent-circuit element predestined to fit the impedance spectra of ferroelectrics based on an accepted physical model. Such impedance spectroscopy is a powerful tool to obtain a dielectric and resistive representation of the entire sample structure. The superiority of the Rayleigh analysis based on impedance spectroscopy compared to the common single-frequency approach is demonstrated for a ferroelectric Si : HfO₂ thin fil

Dynamic modeling of hysteresis-free negative capacitance in ferroelectric/dielectric stacks under fast pulsed voltage operation

To overcome the fundamental limit of the transistor subthreshold swing of 60 mV/dec at room temperature, the use of negative capacitance (NC) in ferroelectric materials was proposed [1]. Due to the recent discovery of ferroelectricity in CMOS compatible HfO₂ and ZrO₂ based thin films [2], [3], the promise of ultra-low power steep-slope devices seems within reach. However, concerns have been raised about switching-speed limitations and unavoidable hysteresis in NC devices [4], [5]. Nevertheless, it was shown that NC effects without hysteresis can be observed in fast pulsed voltage measurements on ferroelectric/dielectric capacitors [6], which was recently confirmed using ferroelectric Hf₀.₅ Zr₀.₅ O₂[7], [8]. While in these works only the integrated charge after each pulse was studied, here we investigate for the first time if the transient voltage and charge characteristics are also hysteresis-free

Versatile experimental setup for FTJ characterization

Ferroelectric Tunnel Junctions (FTJ) are intriguing electron devices which can be operated as memristors and artificial synapses for hardware neural networks. In this work, two virtual–grounded amplifiers have been designed to extract the hysteretic I–V and Q–V characteristics directly, and good agreement between repeated measurements on both circuits demonstrates the accuracy and flexibility of the two setups. Optimal measurement conditions have also been assessed and, finally, wake–up, fatigue, and the preset–dependent early breakdown have been studied

Novel experimental methodologies to reconcile large- and small-signal responses of Hafnium-based Ferroelectric Tunnel Junctions

Ferroelectric Tunnel Junctions (FTJs) are promising electron devices which can be operated as memristors able to realize artificial synapses for neuromorphic computing. In this work, after a thorough validation of the in-house-developed experimental setup, novel methodologies are devised and employed to investigate the large- and small-signal responses of FTJs, whose discrepancies have proven difficult to interpret in previous literature. Our findings convey a significant insight into the contribution of the irreversible polarization switching to the bias-dependent differential capacitance of the ferroelectric–dielectric stack

Evaluation of Imprint and Multi-Level Dynamics in Ferroelectric Capacitors

Fluorite-structured ferroelectrics are one of the most promising material systems for emerging memory technologies. However, when integrated into electronic devices, these materials exhibit strong imprint effects that can lead to a failure during writing or retention operations. To improve the performance and reliability of these devices, it is cardinal to understand the physical mechanisms underlying the imprint during operation. In this work, the comparison of First-Order Reversal Curves measurements with a new gradual switching experimental approach named "Unipolar Reversal Curves" is used to analyze both the fluid imprint and the time-dependent imprint effects within a 10 nm-thick Hf0.5Zr0.5O2 capacitor. Interestingly, the application of delay times (ranging from 100 mu s up to 10 s) between the partial switching pulses of a Unipolar Reversal Curve sequence enables analysis of the connection between the two aforementioned imprint types. Based on these results, the study finally reports a unified physical interpretation of imprint in the context of a charge injection model, which explains both types of imprint and sheds light on the dynamics of multi-level polarization switching in ferroelectrics.Multi-level ferroelectric switching depends strongly on pulse timings. A hysteresis shift along the voltage axis ("imprint") occurs when a ferroelectric device is left in a particular state. Here, different pulse sequences are adopted to investigate and explain the contrasting effects of fluid (short time scales) and time-dependent imprint (long time scales) on multi-level switching in Hf0.5Zr0.5O2 capacitors. imag