Nanoscale Studies of the Ferroelectric and Electromechanical Properties of Hafnia-based Capacitors

The work presented in this dissertation aims to provide nanoscopic insights into the electrical and electromechanical behavior of the recently discovered ferroelectric HfO2 or hafnia-based capacitors. Hafnia-based ferroelectrics are highly promising for technological applications due to compatibility with the existing Si technology. To realize the full potential of hafnia, however, requires comprehensive understanding of its properties. In this regard, this dissertation hopes to bridge a gap between an understanding of the nanoscopic and macroscopic properties of hafnia by performing combined high-resolution piezoresponse force microscopy (PFM) and pulse switching studies. More specifically, the dynamics of domain nucleation and wall motion during polarization reversal in hafnia was investigated. Polarization reversal was found to occur mainly via nucleation of new domains, albeit with limited expansion and sluggish domain wall motion, following the nucleation limited switching (NLS) model at low fields. At high fields, close to the thermodynamic activation fields, a convergence of the NLS and the Kolmogorov-Avrami-Ishibashi switching models was observed, signifying a uniform domain-less polarization reversal process. Furthermore, negative d33 was demonstrated for the first time in hafnia after careful calibration of the PFM phase signal, providing confirmation of a theoretically predicted negative d33. However, the sign was found to be strongly sample dependent. Depending on the film thickness, electrode materials, deposition method used, or state of the capacitors (pristine vs field-cycled), hafnia-based capacitors exhibited either a uniformly negative or positive d33 response or a mixture of both positive and negative d33 responses. In addition, a unique imprint behavior was identified in hafnia that was found to strongly depend on the switching pre-history. Our measurements highlight the critical role played by injected charges and mobile charges/defects in the imprint behavior of hafnia-based devices. Finally, application of PFM spectroscopy to ZrO2-based capacitors revealed dramatically different PFM amplitude response compared to hafnia that could be attributed to the divergence of dielectric susceptibility during field-induced antiferroelectric - ferroelectric phase transitions, providing a microscopic confirmation of antiferroelectricity in ZrO2. Adviser: Alexei Gruverma

Spontaneous Polarization in an Ultrathin Improper-Ferroelectric/Dielectric Bilayer in a Capacitor Structure at Cryogenic Temperatures

To determine the effect of depolarization and the critical thickness in improper-ferroelectric hexagonal-ferrite thin films, we investigate the polarization switching of a ferroelectric/dielectric bilayer in capacitor structures at 20 K. Experimentally, we show that the spontaneous polarization persists throughout the studied thickness range (3 to 80 unit cell), even with a thick (10-nm) dielectric layer, suggesting no practical thickness limit for applications. By fitting the effect of depolarization using the phenomenological theory, we show that the spontaneous polarization remains finite when the thickness of the ferroelectric layer approaches zero, providing a hint for the absence of critical thickness. We also find that the interfacial effects limit the multidomain formation and govern the polarization switching mechanisms

Scaling of electroresistance effect in fully integrated ferroelectric tunnel junctions

Systematic investigation of the scalability for tunneling electroresistance (TER) of integrated Co/BaTiO3/SrRuO3 ferroelectric tunnel junctions (FTJs) has been performed from micron to deep submicron dimensions. Pulsed measurements of the transient currents confirm the ferroelectric switching behavior of the FTJs, while the hysteresis loops measured by means of piezoresponse force microscopy verify the scalability of these structures. Fully integrated functional FTJ devices with the size of 300×300 nm2 exhibiting a tunneling electroresistance (TER) effect of the order of 2.7×104% have been fabricated and tested. Measured current density of 75 A/cm2 for the ON state and a long polarization retention time of ON state (\u3e10 h) show a lot of promise for implementation of high-density BaTiO3-based FTJ memory devices in future

Duality of switching mechanisms and transient negative capacitance in improper ferroelectrics

The recent discovery of transient negative capacitance has sparked an intense debate on the role of homogeneous and inhomogeneous mechanisms in polarizations switching. In this work, we report observation of transient negative capacitance in improper ferroelectric h-YbFeO3 films in a resistor-capacitor circuit, and a concaved shape of anomaly in the voltage wave form, in the early and late stage of the polarizations switching respectively. Using a phenomenological model, we show that the early-stage negative capacitance is likely due to the inhomogeneous switching involving nucleation and domain wall motion, while the anomaly at the late stage, which appears to be a reminiscent negative capacitance is the manifestation of the thermodynamically unstable part of the free-energy landscape in the homogeneous switching. The complex free-energy landscape in hexagonal ferrites may be the key to cause the abrupt change in polarization switching speed and the corresponding anomaly. These results reconcile the two seemingly conflicting mechanisms in the polarization switching and highlight their different roles at different stages. The unique energy-landscape in hexagonal ferrites that reveals the dual switching mechanism suggests the promising application potential in terms of negative capacitance.Comment: 14 pages,5 figure

Piezoelectricity in hafnia

Because of its compatibility with semiconductor-based technologies, hafnia (HfO2) is today’s most promising ferroelectric material for applications in electronics. Yet, knowledge on the ferroic and electromechanical response properties of this all-important compound is still lacking. Interestingly, HfO2 has recently been predicted to display a negative longitudinal piezoelectric effect, which sets it apart from classic ferroelectrics (e.g., perovskite oxides like PbTiO3) and is reminiscent of the behavior of some organic compounds. The present work corroborates this behavior, by first-principles calculations and an experimental investigation of HfO2 thin films using piezoresponse force microscopy. Further, the simulations show how the chemical coordination of the active oxygen atoms is responsible for the negative longitudinal piezoelectric effect. Building on these insights, it is predicted that, by controlling the environment of such active oxygens (e.g., by means of an epitaxial strain), it is possible to change the sign of the piezoelectric response of the material

Piezoelectricity in hafnia

Because of its compatibility with semiconductor-based technologies, hafnia (HfO2) is today’s most promising ferroelectric material for applications in electronics. Yet, knowledge on the ferroic and electromechanical response properties of this all-important compound is still lacking. Interestingly, HfO2 has recently been predicted to display a negative longitudinal piezoelectric effect, which sets it apart from classic ferroelectrics (e.g., perovskite oxides like PbTiO3) and is reminiscent of the behavior of some organic compounds. The present work corroborates this behavior, by first-principles calculations and an experimental investigation of HfO2 thin films using piezoresponse force microscopy. Further, the simulations show how the chemical coordination of the active oxygen atoms is responsible for the negative longitudinal piezoelectric effect. Building on these insights, it is predicted that, by controlling the environment of such active oxygens (e.g., by means of an epitaxial strain), it is possible to change the sign of the piezoelectric response of the material

Electrically induced cancellation and inversion of piezoelectricity in ferroelectric Hf0.5Zr0.5O2

HfO2-based thin films hold huge promise for integrated devices as they show full compatibility with semiconductor technologies and robust ferroelectric properties at nanometer scale. While their polarization switching behavior has been widely investigated, their electromechanical response received much less attention so far. Here, we demonstrate that piezoelectricity in Hf0.5Zr0.5O2 ferroelectric capacitors is not an invariable property but, in fact, can be intrinsically changed by electrical field cycling. Hf0.5Zr0.5O2 capacitors subjected to ac cycling undergo a continuous transition from a positive effective piezoelectric coefficient d33 in the pristine state to a fully inverted negative d33 state, while, in parallel, the polarization monotonically increases. Not only can the sign of d33 be uniformly inverted in the whole capacitor volume, but also, with proper ac training, the net effective piezoresponse can be nullified while the polarization is kept fully switchable. Moreover, the local piezoresponse force microscopy signal also gradually goes through the zero value upon ac cycling. Density functional theory calculations suggest that the observed behavior is a result of a structural transformation from a weakly-developed polar orthorhombic phase towards a well-developed polar orthorhombic phase. The calculations also suggest the possible occurrence of a non-piezoelectric ferroelectric Hf0.5Zr0.5O2. Our experimental findings create an unprecedented potential for tuning the electromechanical functionality of ferroelectric HfO2-based devices

Persistent opto-ferroelectric responses in molecular ferroelectrics

Persistent photoresponses require optical excitations to metastable states, which are rare of ionic origin due to the indirect photon-ion interaction. In this work, we explore the photoinduced metastable proton states in the proton-transfer type molecular ferroelectric croconic acid. We observe that, after the photoexcitation, the changes of structural and ferroelectric properties relax in ∼10^3s, indicating persistent photoresponses of ionic origin. In contrast, the photoconductivity relaxes within 1 s. The 10^3s timescale suggests that the ionic metastable states result from proton transfer both along and out of the hydrogen bonds. This discovery unveils an ionic mechanism for the phototunability, which offers persistent opto-ferroelectric control for proton-transfer type molecular ferroelectrics

Voltage controlled Néel vector rotation in zero magnetic field

Multi-functional thin films of boron (B) doped Cr2O3 exhibit voltage-controlled and nonvolatile Néel vector reorientation in the absence of an applied magnetic field, H. Toggling of antiferromagnetic states is demonstrated in prototype device structures at CMOS compatible temperatures between 300 and 400 K. The boundary magnetization associated with the Néel vector orientation serves as state variable which is read via magnetoresistive detection in a Pt Hall bar adjacent to the B:Cr2O3 film. Switching of the Hall voltage between zero and non-zero values implies Néel vector rotation by 90 degrees. Combined magnetometry, spin resolved inverse photoemission, electric transport and scanning probe microscopy measurements reveal B-dependent TN and resistivity enhancement, spin-canting, anisotropy reduction, dynamic polarization hysteresis and gate voltage dependent orientation of boundary magnetization. The combined effect enables H = 0, voltage controlled, nonvolatile Néel vector rotation at high-temperature. Theoretical modeling estimates switching speeds of about 100 ps making B:Cr2O3 a promising multifunctional single-phase material for energy efficient nonvolatile CMOS compatible memory applications