Brice Gautier

Geometric conductive filament confinement by nanotips for resistive switching of HfO2-RRAM devices with high performance

Filament-type HfO2-based RRAM has been considered as one of the most promising candidates for future non-volatile memories. Further improvement of the stability, particularly at the “OFF” state, of such devices is mainly hindered by resistance variation induced by the uncontrolled oxygen vacancies distribution and filament growth in HfO2 films. We report highly stable endurance of TiN/Ti/HfO2/Si-tip RRAM devices using a CMOS compatible nanotip method. Simulations indicate that the nanotip bottom electrode provides a local confinement for the electrical field and ionic current density; thus a nano-confinement for the oxygen vacancy distribution and nano-filament location is created by this approach. Conductive atomic force microscopy measurements confirm that the filaments form only on the nanotip region. Resistance switching by using pulses shows highly stable endurance for both ON and OFF modes, thanks to the geometric confinement of the conductive path and filament only above the nanotip. This nano-engineering approach opens a new pathway to realize forming-free RRAM devices with improved stability and reliability.

Electromechanical response of amorphous LaAlO3 thin film probed by scanning probe microscopies

The electromechanical response of a 3 nm thick amorphous LaAlO3 layer obtained by molecular beam epitaxy has been studied using scanning probe microscopies. Although this kind of sample is not ferroelectric due to its amorphous nature, the resulting images are identical to what is generally obtained on truly ferroelectric samples probed by piezoresponse force microscopy: domains of apparently opposite polarisation are detected, and perfect, square shaped hysteresis loops are recorded. Moreover, written patterns are stable within 72 h. We discuss in the general case the possible origins of this behaviour in terms of charge injection, ionic conduction and motion of oxygen vacancies. In the case presented in this paper, since the writing process has been conducted with applied voltages lower than the injection threshold measured by conductive atomic force Microscopy, allowing to withdraw the hypothesis of charge injection in the sample, we propose that a bistable distribution of oxygen vacancies is responsib...

Room temperature multiferroicity in Ga0.6Fe1.4O3:Mg thin films

We report on the multiferroic behavior of 2%-magnesium-doped Ga0.6Fe1.4O3 thin film at room temperature. The sample was grown by pulsed laser deposition on a Pt-coated Yttrium-Stabilized Zirconia substrate. Magnetic measurements indicate a net magnetization of 105 emu/cm3 at 295 K, and the persistence of magnetic ordering above room temperature. Ferroelectric measurements show clear polarization switching with negligible contribution from leakage currents, with a polarization of 0.2 μC/cm−2 and a coercive field of 133 kV/cm. Scanning probe microscopy confirms the low leakage current and detects a stable piezoelectric signal. This could open original perspectives for the application of single-phased multiferroic systems.

Ferroelectric Pb(Zr,Ti)O3 epitaxial layers on GaAs

Ferroelectric epitaxial Pb(Zr,Ti)O3 (PZT) layers were grown by pulsed laser deposition on SrTiO3/GaAs templates fabricated by molecular beam epitaxy. The templates present an excellent structural quality and the SrTiO3/GaAs is abrupt at the atomic scale. The PZT layers contain a- and c-domains, as shown by X-Ray diffraction analyses. Piezoforce microscopy experiments and macroscopic electrical characterizations indicate that PZT is ferroelectric. A relative dielectric permittivity of 164 is extracted from these measurements.

Epitaxial inversion on ferromagnetic (Fe,Zn)3O4 /ferroelectric BiFeO3 core-shell nanodot arrays using three dimensional nano-seeding assembly

We demonstrate an advanced fabrication method based on standard surface diffusion theory for preparation of core-shell nano-heterostructure arrays consisting of ferromagnetic (Fe,Zn)3O4 (FZO) and ferroelectric BiFeO3 (BFO) using a 3D nano-seeding-assembly technique. By adapting epitaxial spinel FZO nanodot arrays on a perovskite SrTiO3 substrate to templates, well-positioned selective epitaxial growth of FZO cores and BFO shells was spontaneously stimulated. This technique resolves the longstanding issues of the precise positional alignment and configuration inversion of materials that conventional self-assembly growth has faced.

Structural study and ferroelectricity of epitaxial BaTiO3 films on silicon grown by molecular beam epitaxy

Integration of epitaxial complex ferroelectric oxides such as BaTiO3 on semiconductor substrates depends on the ability to finely control their structure and properties, which are strongly correlated. The epitaxial growth of thin BaTiO3 films with high interfacial quality still remains scarcely investigated on semiconductors; a systematic investigation of processing conditions is missing although they determine the cationic composition, the oxygen content, and the microstructure, which, in turn, play a major role on the ferroelectric properties. We report here the study of various relevant deposition parameters in molecular beam epitaxy for the growth of epitaxial tetragonal BaTiO3 thin films on silicon substrates. The films were grown using a 4 nm-thick epitaxial SrTiO3 buffer layer. We show that the tetragonality of the BaTiO3 films, the crystalline domain orientations, and SiO2 interfacial layer regrowth strongly depend on the oxygen partial pressure and temperature during the growth and on the post-de...

Phase transitions in [001]-oriented morphotropic PbZr0.52Ti0.48O3 thin film deposited onto SrTiO3-buffered Si substrate

An 85 nm-thick morphotropic PbZr0.52Ti0.48O3 (PZT) film grown epitaxially and [001]-oriented onto a SrTiO3-buffered Si-wafer is investigated using temperature dependent X-ray diffraction. Two phase transitions, at Trt ∼ 500 K and Tc ∼ 685 K, are evidenced and are attributed to structural phase transitions from monoclinic-like to tetragonal-like phase and from tetragonal to paraelectric phase, respectively. The stronger upper shift of Trt value with respect to the bulk one and the weakly affected Tc (Tc bulk ∼ 665 K) are explained assuming misfit strain changes when crossing Trt. This finding opens new perspectives for piezoelectric PZT films in harsh applications.

Study of the physical and electrical degradation of thin oxide films by atomic force microscope

The hillocks created by the application of ramped voltage stress on thin oxide films have been imaged using different modes of the atomic force microscope (AFM) and using conductive or insulating tips, leading to the conclusion that these anomalous hillocks correspond to real (physical) modification of the oxide’s surface. Electric force microscopy has also been used, which shows that negative charges are trapped in the oxide layer after the ramps and contribute to the contrast of AFM images although their role may not be predominant. Comparisons between ramps operated in air and in dry atmosphere or vacuum emphasize the role of the water layer covering the sample in the apparition of the hillock. The authors’ results tend to accredit that the formation of the hillock is a complex phenomenon involving a chemical (oxidation), electrical (trapped charges), and physical (electrothermal effect) mechanisms.

Interpretation of multiscale characterization techniques to assess ferroelectricity: The case of GaFeO3

In this paper, we propose a thorough experimental procedure to assess the ferroelectricity of thin films, and apply this procedure to Pulsed Laser Deposition grown GaFeO3 thin films at the macroscale by means of Polarisation-Voltage hysteresis and at the nanoscale by Piezoresponse Force Microscopy. GaFeO3 is a serious candidate for the multiferroicity at room temperature, being ferrimagnetic and possibly ferroelectric. However, the non-ambiguous measurement of ferroelectric polarisation of such thin films remains a challenge. We show that although doped to decrease the leakage currents, the samples remain too leaky to allow any detection of a polarisation current, whereas Piezoresponse Force Microscopy images are indeed obtained in certain conditions. Nevertheless, the images obtained from scanning probe methods must be questioned in that context. This is why we propose to obtain PFM images at much higher frequencies to discriminate between artefactual images and true ferroelectric behaviour. The application of the method combined with the comparison with results obtained on a PbZrTiO3 sample allow to rule out the ferroelectricity of our GaFeO3 samples. Beyond the problem of GaFeO3, our objective is to propose a method which enables to assess objectively the ferroelectricity of any leaky film.

A new technique based on current measurement for nanoscale ferroelectricity assessment: Nano-positive up negative down

In this paper, we propose a new procedure which aims at measuring the polarisation switching current at the nanoscale on ferroelectric thin films with the atomic force microscope tip used as a top electrode. Our technique is an adaptation of the so-called positive up negative down method commonly operated on large electrodes. The main obstacle that must be overcome to implement such measurement is the enhancement of the signal to noise ratio, in a context where the stray capacitance of the sample/tip/lever/lever holder system generates a dielectric displacement current several orders of magnitude higher than the current to be measured. This problem is solved by the subtraction of the displacement current through a reference capacitance. For the first time, we show an example of nanoscale positive up negative down measurement of the polarisation charge on a PbZrTiO3 thin film and compare the measured value with paraelectric samples. From the comparison with macroscopic measurement, we deduce the effective area of contact between the tip and the sample.