Polarisation reversal in ferroelectric PVDF and PZT films

Ferroelectrics are characterised by a spontaneous polarisation that can be reversed by an external electric field. The stability of the polarisation states and the possibility for controlled switching between the states render ferroelectric materials very attractive for memory applications, in which polarisation states are associated to a binary information "0" or "1". Fast, dense, and reliable data storage has gained enormous importance in our information technology based society. In this context, the fundamental physics of ferroelectric materials attracted great interest with the ultimate goal of reaching memory elements with shortest access times and highest densities. Two model systems of ferroelectric materials were studied in this thesis on the polarisation reversal: an inorganic, classical perovskite and an organic polymer. Despite the fundamental structural differences between lead zirconate titanate [Pb(Zr,Ti)O3] and the investigated polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE)] copolymer, the switching kinetics in these materials can be described by a similar formalism. Our approach of a combination of macroscopic and microscopic techniques enables an insight into the mechanisms of the polarisation reversal and allows for a better understanding of macroscopic effects governed by mechanisms occurring on the nanoscale. The experimental techniques developed for the ferroelectric capacitor structures have been successfully applied for studying a first hybrid PVDF/silicon ferroelectric transistor for non-volatile information storage. The major accomplishments of this thesis are the following: We demonstrated for the first time a piezoelectric scanning probe study on the cross section of ferroelectric thin films. In combination with switching pulses, this allowed us to directly image the polarisation reversal in ferroelectric films quasi-dynamically. The utility of cross-sectional domain imaging as a powerful tool for the study of the polarisation reversal in ferroelectrics has been corroborated. The apparent contradiction between observations interpreted as a fast sideways growth and the classical scenario of a fast forward growth could be explained by an oblique polarisation direction with respect to the film plane. A surface-stimulated nucleation of reverse domains, with preference on the electrode with a negative potential was observed. It was interpreted in terms of a built-in field due to a depletion layer at the ferroelectric-electrode interface. A combination of microscopic and macroscopic methods used for the first time in organic ferroelectric thin films enabled a better insight into polarisation reversal of the copolymer of vinylidene fluoride and trifluoroethylene P(VDF-TrFE). The polarisation reversal was found to be impeded by a restricted domain growth. This limits the applicability of the conventional Kolmogorov-Avrami model as well as the nucleation-limited switching model with a broad distribution of nucleation times. We demonstrated that an interface-adjacent passive layer may impact on the switching properties and gives rise to a retardation of the polarisation reversal. Furthermore, an extraordinary dielectric constant increase was observed in the films with a passive layer due to an additional domain wall contribution. We fabricated for the first time a hybrid silicon/ferroelectric polymer ferroelectric single transistor memory element with non-volatile operation. The observed difference of the polarisation reversal kinetics of the ferroelectric in the simple capacitor and in the transistor structure was explained by the gate oxide buffer layer in the transistor. Our previously developed tools and approaches enabled a better understanding of retention loss mechanism. A semi-quantitative retention model was suggested that explains the observed exponential retention loss. The investigation of relevant fundamental aspects of ferroelectric materials and the development of novel characterisation techniques in this work can be of interest for further development of non-volatile memory devices.

Ferroelectric gate on AlGaN/GaN heterostructures

Lisa Malin

The capability of switching the spontaneous polarisation under an applied electric field in ferroelectric materials can be exploited for the use in low power, non-volatile, re-writable memory devices. Currently available commercially is ferroelectric random access memory, FeRAM, which allows for high speed, low voltage and greater write-erase endurance compared to its two main competitors, Flash and EEPROM, when using the one transistor – one capacitor configuration. However, it is desired to further optimise the configuration in order to obtain better densification, faster access time and better reliability. One way to do such is to pass from the ferroelectric capacitors and develop ferroelectric field effect transistors. Exploiting the phenomenon of ferroelectricity and integrating ferroelectrics with the semiconductor technology has not been simple. The FeFET has been demonstrated using a silicon-based transistor, however commercial devices are not available. Challenges arise mainly due to the high temperature deposition of perovskite ferroelectrics causing the degradation of the ferroelectric/semiconductor interface due to inter-diffusion. Acquiring long term retention of the transistor behavior has also been problematic due to phenomenons such as charge injection and depolarisation. In this thesis a new approach to the problem of semiconductor devices with a ferroelectric gate is explored. Instead of using a silicon-based device, semiconductor heterostructures are investigated. Combining the high mobility channel existing in semiconductor heterostructures, with the non-volatile switching of the polarisation in the ferroelectric gate can pave the way to novel future devices. The AlGaN/GaN semiconductor heterostructure was chosen for two main reasons. The first is a two dimensional electron gas, 2DEG, located at the AlGaN/GaN interface which possesses better transport properties than a single layered semiconductor. Secondly, GaN and its alloys are known to have large chemical and temperature stability making them ideal to withstand the high temperature deposition process of perovskite ferroelectrics. The deposition of two ferroelectric layers onto the AlGaN heterostructure were investigated. Lead zirconium titanate, PZT, a traditional perovskite ferroelectric deposited at high temperature, was chosen for its high remanent polarisation and low coercive field. An alternative ferroelectric gate, the co-polymer poly(vinylidene fluoride/trifluoroethylene), P(VDF/TrFE)(70:30) was deposited and of interest due its low crystallisation temperature and low dielectric constant. Its remanent polarisation is smaller and coercive field larger than that of PZT, but were determined sufficient to observe the depletion effect in the two dimensional electron gas. The goals accomplished in this research were: Development of Ferroelectric Gate Processing: Deposition processes of the ferroelectric layers were developed and optimised in order to obtain a high quality ferroelectric, while maintaining the original transport properties of AlGaNs 2DEG. The processing of HfO2 and MgO buffer layers were developed and investigated for their effects on limiting unwanted inter-diffusion and charge injection. PZT Gate on Al0.3Ga0.7N: The first successful development and observation of a PZT gate on a Al0.3Ga0.7N/GaN heterostrucutre was accomplished. A (111) oriented PZT(40:60) ferroelectric gate on the Al0.3Ga0.7N heterostructure depleted the sheet resistance of the 2DEG by a factor of three when poling the PZT layer with –40V directly to the conductive cantilever used in a piezoresponse force microscope. This decrease in sheet resistance was stable for more than three days. P(VDF/TrFE)(70:30) Gate on Al0.3Ga0.7N: The ferroelectric co-polymer P(VDF/TrFE) (70:30) was investigated as a gate on the Al0.3Ga0.7N heterostructure. When the P(VDF/TrFE) was poled with –30V to a top electrode the sheet resistance was modulated by a factor three, however there was no retention of this modulation. Ferroelectric Spontaneous Polarisation on a Semiconductor Heterostructure: It was theoretically derived that when depositing a ferroelectric layer onto a semiconductor heterostructure its spontaneous polarisation is dramatically reduced due to the depolarisation field and may be suppressed completely. This decay in spontaneous polarisation can be minimised by using a ferroelectric layer with a small dielectric constant, which justifies the use of ferroelectric polymers as the gate material. Domain Writing:The technique of domain writing using piezoresponse force microscopy showed that it was possible to create domain patterning on the ferroelectric layers that were deposited onto the Al0.3Ga0.7N heterostructure with sub-micron line resolution, on the order of 300nm or better. Using ferroelectrics on semiconductors is not only limited to ferroelectric memory devices. Other applications could be ferroelectric nano-lithography, where a ferroelectric pattern is written with a piezoresponse force microscope, PFM, and this pattern is projected onto the transistors channel, potentially creating quantum/ballistic devices. The challenge of integrating ferroelectrics with semiconductors has yet to be fully understood. One of the main advancements that was reached with this research is the understanding of the decrease of the spontaneous polarisation when a ferrroelectric layer is deposited onto a semiconductor heterostructure of finite thickness. The maximisation of the ferroelectrics polarisation on 20nm of Al0.3Ga0.7N can be obtained when minimising the dielectric constant of the ferroelectric layer. The current challenge is to get a better physical understanding of the limitation of the retention of the spontaneous polarisation, while discovering methods for its improvement.

EPFL2007

Ferroelectric Polymer Gates for Persistent Field Effect Control of Ferromagnetism in (Ga,Mn)As Layers

Sebastian Riester

The growing demand for higher computing power and element density continuously drives the development of novel device concepts. One group of materials currently attracting a lot of interest are the magnetoelectric multiferroics, due to their potential for creating devices with novel functionalities. These materials exhibit coupled ferroelectric and ferromagnetic behavior. Although the focus is on homogeneous multiferroics like bismuth ferrite, the stronger multiferroic coupling that is possible in composite multiferroics makes them attractive for many potential applications (e.g. spintronic logic and memory elements). In particular, electric field-mediated multiferroic composites promise worthwhile compatibility with CMOS circuitry. The present work explores a multiphase multiferroic system where the ferroelectric gate controls a dilute magnetic semiconductor (DMS) channel via the nonvolatile field effect associated with the spontaneous polarization. The system consists of a copolymer of vinylidene fluoride and trifluoroethylene [P(VDF-TrFE)] as the ferroelectric gate and an ultra-thin magnetic layer of (Ga,Mn)As which is one of the most well-established DMS and is considered a chief candidate for spintronic applications. The main outcomes of this research can be summarized as follows: A widely recognized incompatibility between ferroelectrics and group III-V semiconductors has been solved by the integration of a polymer ferroelectric onto a DMS in a multiferroic transistor configuration. Nonvolatile electric field control of ferromagnetism in a (Ga,Mn)As channel has been successfully demonstrated. Several pieces of evidence including the change of hysteretic properties, ferromagnetic Curie temperature shift and magnetoresistance behavior attest to the field effect-mediated multiferroic coupling in this system. Low ferroelectric gate operation voltages below 10 V were achieved through aggressive P(VDF-TrFE) thickness reduction without diminishing the gate effect strength. Polarization screening at the semiconductor/ferroelectric interface was identified as the main issue limiting the ferroelectric gate effect. A significant enhancement of the multiferroic coupling has been reached by thinning the (Ga,Mn)As channel down to 3 nm without compromising the ferromagnetic properties. From the Curie temperature response to ferroelectric gating, that follows the same trend for samples with thicknesses ranging from 3 to 7 nm, we conclude that the 2D limit has not been reached for the thinnest channels and the 3D models describing the ferromagnetic coupling are valid for this case. The ferroelectric control of ferromagnetism has been quantitatively interpreted in terms of existing models for hole-mediated exchange in (Ga,Mn)As.

EPFL2010

Towards room-temperature deterministic ferroelectric control of ferromagnetic thin films

Zhen Huang

The persisting demand of higher computing power and faster information processing keeps pushing scientists and engineers to explore novel materials and device structures. Within emerging functional materials, there is a focus on multiferroics materials and material-systems possessing both ferroelectric and ferromagnetic orders. Multiferroics with two order parameters coupled through a magnetoelectric interaction are of particular interest for novel information processing technologies. This thesis explores a promising concept of magnetoelectric multiferroic heterostructure incorporating separate ferroelectric and ferromagnetic thin layers with field-effect-mediated coupling through the heterointerface. The major issues related to ferroelectric control of ferromagnetism in multiferroic heterostructures addressed in this thesis include non-volatile magnetoelectric coupling at room temperature, deterministic switching of magnetization via polarization reversal, ferroelectric control of dynamics of magnetic domains and integration of ferroelectric gates on magnetic channels. The major accomplishments of this thesis are the following: Non-volatile ferroelectric control of magnetic properties has been demonstrated in ultra-thin Co layers at room temperature. The ferromagnetic transition Curie temperature, magnetic anisotropy energy and magnetic coercivity are shown to be switchable via the persistent field effect associated with the ferroelectric polarization. The magnitude of the effect is comparable or exceeds the results observed using the conventional (non-ferroelectric) gates. Local control of individual magnetic domain nucleation and propagation in Co channels by creating new ferroelectric domains has been achieved at the ambient conditions. The microscopic ferroelectric domains written on poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] ferroelectric polymer gate projected onto the magnetic channel changing locally the magnetic anisotropy energy, and consequently altering magnetic domain dynamics. Therefore it was possible to promote/impede magnetic domain nucleation and significantly change the domain wall velocity using a non-destructive and reversible procedure of ferroelectric domain writing. Non-volatile control of anisotropic magnetoresistance (AMR) has been demonstrated on diluted magnetic semiconductor (Ga,Mn)(As,P) thin films with integrated P(VDF-TrFE) ferroelectric gate. The field effect induced by the ferroelectric gate has shown the capability to strongly modulate the AMR behavior. Profound qualitative changes of the nature of AMR has been observed, including complete on/off switching of the crystalline component of AMR in (Ga,Mn)(As,P). A workflow for fabrication of integrated FET-type structures comprising a magnetic metal or semiconductor magnetic channel and ferroelectric polymer gate has been successfully developed. Different approaches for enhancement of ferroelectric gate operation were explored including integration of highly resistive hydrophobic interfacial layer and change of polymer composition for lower leakage. A significant increase of the non-volatile gate effect magnitude compared to the state of art was reached via improved quality of the ferroelectric/ferromagnetic interface.

EPFL2015