Catalin Harnagea

Structural and optical characteristics of bismuth oxide thin films

The formation of different phases of Bi–O system during oxidizing process of Bi films from the viewpoint of structure characteristics and optical properties, is investigated. As revealed by structure and surface investigations, the thermal oxidation, in air, of Bi films on glass results in polycrystalline and multiphase bismuth oxide films. At substrate–film interface an amorphous oxide layer forms up, to whom surface the growth processes of crystallites belonging to divers phases, with different geometric shapes occur. For nanometric up to submicronic thicknesses, BiO is predominant phase, while in case of micronic films, a-Bi2O3 and Bi2O3 are the main phase components. From absorption curves the band gap values, (Eg ¼ 2:29–3:31 eV), for direct transitions, have been determined and an energy gap of � 2 eV for amorphous phase was found. 2002 Elsevier Science B.V. All rights reserved.

Polarization imprint and size effects in mesoscopic ferroelectric structures

Piezoresponse scanning force microscopy measurements performed on lead zirconate titanate mesoscopic structures revealed a negative shift of the initial piezoelectric hysteresis loop. The shift is dependent on the size of the structure and is most probably due to the pinning of ferroelectric domains at the free lateral surface and ferroelectric–electrode interface. Considering a simple model, the thickness of the pinned domain layers is found to be about 15 and 70 nm at the ferroelectric–electrode interface and lateral free surface, respectively.

Patterning and switching of nanosize ferroelectric memory cells

A fundamental limitation on the recent development of nonvolatile ferroelectric memories in 64 Mbit–4 Gbit densities has been the ability to scale ferroelectric capacitor cell sizes below 1 μm2. In the present work, ferroelectric memory cells with lateral sizes down to 100 nm were fabricated by electron-beam direct writing. Switching of single 100 nm cells was achieved and piezoelectric hysteresis loops were recorded using a scanning probe microscope working in piezoresponse mode.

Ferroelectric epitaxial nanocrystals obtained by a self-patterning method

Lead zirconate titanate nanoislands were obtained by a self-patterning method making use of the instability of ultrathin films during high-temperature treatments. After high-temperature annealing, the as-deposited film breaks into islands with a narrow size distribution. The single-crystal nanoislands were studied by scanning and high-resolution transmission electron microscopy, atomic force microscopy, and x-ray diffraction. They show an epitaxial relationship with the Nb-doped (001) SrTiO3 substrate. The ferroelectric switching of several individual islands was investigated by piezoresponse force microscopy.

Switching properties of self-assembled ferroelectric memory cells

In this letter, we report on the switching properties of an ordered system of Bi4Ti3O12 ferroelectric memory cells of an average lateral size of 0.18 μm formed via a self-assembling process. The ferroelectricity of these cells has been measured microscopically and it has been demonstrated that an individual cell of 0.18 μm size is switching. Switching of single nanoelectrode cells was achieved via scanning force microscopy working in piezoresponse mode.

Contact resonances in voltage-modulated force microscopy

A study of the frequency dependence of the signal in piezoresponse scanning force microscopy of ferroelectric materials has been performed. It is found that, for soft cantilevers, the signal is governed by the cantilever elastic properties. Both ferroelectric-electromechanical and electrostatic interaction contributions to the overall signal were found to depend on the frequency of the testing voltage. Indications for optimal measurement regimes are given.

Piezoresponse Scanning Force Microscopy: What Quantitative Information Can We Really Get Out of Piezoresponse Measurements on Ferroelectric Thin Films

Abstract Voltage modulated scanning force microscopy in contact mode or piezoresponse scanning force microscopy is now turning into an established technique for imaging ferroelectric domains in ferroelectric thin films. The quantities measured, however, are the amplitude and phase of a locally induced piezoelectric strain, and not the ferroelectric polarization itself. As all ferroelectrics possess piezoelectric properties, the domain structure visualized does correspond to that of the ferroelectric domains whose polarization is partly normal to the film surface, but the amplitude of the signal is actually not proportional to the magnitude of the normal component of the polarization. Likewise, the shear mode of scanning force microscopy allows the imaging of domains with a polarization component in the plane of the film. Methods to relate the amplitude of the piezoresponse signal to the magnitude of the ferroelectric polarization taking into account the anisotropic nature of the piezoelectric coefficients are described and discussed for the most common ferroelectric materials.

Structural and electrical anisotropy of (001)-, (116)-, and (103)-oriented epitaxial SrBi2Ta2O9 thin films on SrTiO3 substrates grown by pulsed laser deposition

Epitaxial SrBi2Ta2O9 (SBT) thin films with well-defined (001), (116), and (103) orientations have been grown by pulsed laser deposition on (001)-, (011)-, and (111)-oriented Nb-doped SrTiO3 substrates. X-ray diffraction pole figure and φ-scan measurements revealed that the three-dimensional epitaxial orientation relation SBT(001)‖SrTiO3(001), and SBT[110]‖SrTiO3[100] is valid for all cases of SBT thin films on SrTiO3 substrates, irrespective of their orientations. Atomic force microscopy images of the c-axis-oriented SBT revealed polyhedron-shaped grains showing spiral growth around screw dislocations. The terrace steps of the c-axis-oriented SBT films were integral multiples of a quarter of the lattice parameter c of SBT (∼0.6 nm). The grains of (103)-oriented SBT films were arranged in a triple-domain configuration consistent with the symmetry of the SrTiO3(111) substrate. The measured remanent polarization (2Pr) and coercive field (2Ec) of (116)-oriented SBT films were 9.6 μC/cm2 and 168 kV/cm, respec...

Analysis of ferroelectric switching in finite media as a Landau-type phase transition

A detailed analysis of polarization reversal in ferroelectrics has been performed, in the framework of the Landau model for phase transitions. Some important characteristics of homogeneous switching have been emphasized and later used in studying the more general case of inhomogeneous switching. The two extremes of switching current correspond to the inflexion points of the dielectric hysteresis loop. Hysteresis loops of poled ferroelectric samples are expected to include negative-susceptibility regions, for high-frequency applied electric fields. The switching current minimum is eliminated by the experimental method used for recording the switching responses. Equivalent Landau coefficients and electric fields have been defined, in order to integrate the size effects and inhomogeneity contribution to switching of the global order parameter. We correlated the size effects on the critical parameters of the switching (the coercive field) and the ferroelectric-to-paraelectric phase transition (the Curie temperature). Polarization reversal in small-size ferroelectrics can be regarded as a diffuse phase transition, whereas its character is closer to normal for large-size samples. The size dependencies of the reversal speed and maximum current result from the size dependencies of the equivalent Landau coefficients and electric field inducing reversal.

Mesoscopic ferroelectric cell arrays prepared by imprint lithography

Arrays of mesoscopic ferroelectric (Pb,Zr)TiO3 cells with lateral sizes from several micrometers down to below 300 nm were prepared applying nanoimprint lithography. The ferroelectric properties of the mesoscopic cells were investigated by scanning force microscopy in piezoresponse mode. The best chemical route to obtain ferroelectric cells was found to be the sol-gel method. Using Nb-doped SrTiO3 single crystals as bottom electrodes, the crystallization into the ferroelectric phase was uniform with grain sizes in the 35 nm range.