Patterned and self-assembled ferroelectric nano-structures obtained by epitaxial growth and e-beam lithography

The continuous downscaling of microelectronic circuits combined with increasing interest in ferroelectric thin films for non-volatile random access memories (FeRAM) is drawing great attention to small ferroelectric thin film structures. There are various challenges and open questions related to processing and theoretical understanding. The processing must assure damage-free ferroelectric capacitors of reproducible properties. More theoretical understanding is necessary to estimate the impact of size effects on the stability of the ferroelectric polarization, domain configurations, switching properties, and other properties such as the piezoelectric response. Pb(Zr,Ti)O3 (PZT) is one of the favorite materials for FeRAMs. Today's capacitors are poly-crystalline with sizes in the micron range. Each capacitor is composed of a large number of grains, guaranteeing the averaging of electrical properties. If only a few grains were present, the scattering of properties from one capacitor to another would take place. This can be avoided using oriented single-crystalline capacitors. An important processing issue is patterning. The switchable polarization might be reduced due to etching damage, a frequently reported current fabrication problem. There is also the question of domain configuration - processing relations. The optimal solution of (001) oriented tetragonal PZT cannot be realized in large capacitors because ferroelastic 90° domains form spontaneously upon cooling through the transition temperature and are stabilized for reasons of stress compensation. In this work, we investigated two routes to fabricate small ferroelectric structures: A subtractive route and an additive route. Patterning was done by electron beam lithography (EBL) in order to access the sub 100nm range. Dry etching was done in an electron cyclotron resonance (ECR/RF) reactor working at very low pressures and ion energy. In the subtractive route, the starting point were continuous 50 to 200nm thick Pb(Zr0.4,Ti0.6)O3 films, which were deposited on conductive, Nb-doped SrTiO3 (100) substrates. The films were c-axis oriented with an a-axis fraction of about 20%. The films were patterned by means of EBL. The positive poly-methyl-methacrylate (PMMA) resist was spun on the PZT film, a dot pattern was directly written by the e-beam, and after development a Cr layer was evaporated. The obtained Cr patterns served as a hard mask for PZT dry etching. Features with an aspect ratio of up to 2 could be cut out of a 200nm thick PZT film. We have shown that dry etching in an electron ECR/RF reactor provides damage-free single crystalline sub-200nm features which were still ferroelectric. The smallest features obtained had a lateral size of 80nm. We found that the resolution of the EBL was limited by the backscattering of electrons from the high density PZT layer. Piezoelectric sensitive atomic force microscopy (PAFM) revealed an increase in the piezoelectric response when the feature's aspect ratio was increased. The measured piezoelectric coefficient increased by a factor of more than three compared to the continuous film. Un-clamping was found to account only for a portion of the increase. As major contribution, we suppose a change in the domain configuration from a to c, and unpinning of domains. This hypothesis was supported by local PAFM measurements on the continuous film, where the same behavior was observed, when the film underwent local training cycles just before the measurement using pulses with voltages well above the coercive field. It is also important to investigate non-destructive fabrication processes to reduce the negative impact of interface defects and grain boundaries. This was achieved using an additive route. The fact, that PZT nucleation on Pt surfaces is difficult but easy on TiO2 was used for the site controlled growth of PZT single crystals on patterned 2nm thick TiO2 layers serving as affinity spots. This route was carried out on Pt (111) and (100) surfaces. A 50nm thick Pt layer was epitaxially grown on single crystalline SrTiO3 (111) and MgO (100) substrates, and then covered by a 2nm thick TiO2 layer. On both substrates, the TiO2 was found to be (100) oriented. In this route, EBL was used to pattern the TiO2 layer into seed islands. As only 2nm of TiO2 layer have to be etched, a new type of negative organic mask could be used, which circumvented the lift off process. Direct deposition of Pb(Zr0.4,Ti0.6)O3 on the seed island covered Pt (111) surfaces led to the nucleation of triangular shaped crystals only on the seeds and not elsewhere. The lateral size of the triangles was between 30 and 150nm. The uniform orientation of the triangle's sidelines implies epitaxial growth of the PZT. Applying a 1nm thick PbTiO3 starting layer prior to PZT deposition increased the nucleation density. A 2nm thick PbTiO3 starting layer prior to PZT deposition led to square shaped crystals on small TiO2 seeds implying the growth of (001) PZT. On Pt, triangular crystals were obtained. For this deposition, the squares showed a uniform size distribution, but their in-plane orientation was random. In all cases, the nucleus density was found to be 60 times less on Pt than on the affinity spots. The nucleation controlled character was expressed in an observable depletion layer around larger TiO2 seeds. A theoretical nucleation model was able to well explain this behavior and the relevant parameters such as activation energies of diffusion and desorption could be derived from experiments. On the Pt (100) surface, direct deposition of PZT led to the nucleation of a non-ferroelectric phase on most parts of the substrate. A depletion around TiO2 seeds was again observable. PZT crystals were found exclusively on some TiO2 seeds. PAFM measurements revealed that all PZT crystals obtained by the additive route were ferroelectric. The ferroelectric features have been investigated by means of PAFM. Complex domain structures were identified, the smallest observed domains had diameters of 15nm. The thinnest investigated feature showing ferroelectricity was 6nm thick. An attempt was made to compare experimental findings with the Landau-Devonshire-Ginzburg phenomenological theory. The d33,f loop together with the CV-loop of a 600nm thick, mostly c-oriented film could well be described by the theory. Deviations could be identified as domain wall contributions. The critical thickness for ferroelectricity was estimated using in addition literature data for domain wall energies. The critical thickness was calculated as 1 to 2nm, which does not contradict experimental findings.

Pulsed laser deposition of (SrBa)Nb2O6 thin films and their properties

Anna Infortuna

Strontium barium niobate (SrxBa1-xNb2O6, shortly SBN) is a solid solution system with tetragonal tungsten bronze crystal structure. It exhibits a ferroelectric phase with only one polar axis and a transition temperature depending on the Sr/Ba ratio. This thesis studies the growth of SBN thin films, in order to obtain a ferroelectric thin film model system for the study of 180° domain switching close to the ferroelectric phase transition. SBN has been chosen not only because uniaxial, but because it is possible to tune the temperature of phase transition varying the Sr/Ba ratio. It is thus possible to study electric properties of the phase transition close to room temperature, where conduction phenomena related to defects are normally lower. Thin films of SBN were grown by Pulsed Laser Deposition in a vacuum system constructed during this thesis work. For the formulation of a model it is desirable to have a system as close as possible to a single crystal. Therefore deposition parameters have been optimized to grow SBN thin films with the polar axis oriented perpendicular to the film plane. The films are integrated in parallel plate capacitor structures, in order to measure dielectric properties. The stability of the ferroelectric phase is found to be sensitive to impurities; contaminants with concentration below 1% induce a lowering of the transition temperature of about 100°C. Strain as well has influence on the phase transition; in the case of substrate with thermal expansion coefficients different from the SBN ones, the SBN film is under stress. A 0.6% tensile strain induces a lowering of the transition temperature of about 100°C, this is the case of silicon single crystal substrate. Grown on strontium titanate single crystals (STO), whose thermo-mechanical properties are close to the one of SBN, the films exhibit a phase transition temperature compatible with the one measured on single crystals. The obtained films suffer from leakage that is reduced by thermal treatment in oxygen rich atmosphere. The oxygen vacancies, created during the deposition process, are considered to be responsible of leakage. Vacancies recovery upon annealing does not eliminate the problem, and measurement of polarization at room temperature is difficult. However, piezoelectric measurements performed on individual grains, with atomic force microscope, prove that these have ferroelectric properties; by these experiment has been proved that it is possible to switch the polarization. Films with a disordered structure of grains are much less leaky than the ones with columnar grains spanning the whole cross section. These observations lead to the conclusion that conduction goes trough the grain boundaries. On STO single crystals, SBN grows epitaxial in Volmer-Weber mode. With a suitable preparation of the substrate surface, it is possible to induce the growth of films with different orientations of the polar axis: in the film plane, perpendicular to it or inclined of 45°. A model is proposed, explaining the epitaxial relation with the substrate on the basis of the perovskite kernel contained in the unit cell of the tetragonal tungsten bronze structure. The film grows from the nucleation of the perovskite structure that rules the orientation of the film; the high temperature at which the substrate is kept during the deposition, assure the necessary mobility for the arriving atoms to organize in the SBN structure around these nuclei. The study of literature data allows to state that such a model is valid not only for substrates with perovskite structure, but can be extended to substrates with a cubic crystal structure, like magnesium oxide.

EPFL2005

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

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