Cosmin S. Sandu

Ferroelectric domains and piezoelectricity in monocrystalline Pb(Zr,Ti)O3 nanowires

Monocrystalline lead zirconate titanate nanowires were grown by a polymer assisted solvothermal technique. X-ray and electron diffractions confirmed tetragonal perovskite structure and a [001] orientation along the wire axis, respectively. Raman scattering was used to analyze the structure and composition of single wires. Ferroelectric/ferroelastic domain walls were imaged by transmission electron microscopy, showing some domains with polarization directions along the wire axis and some perpendicular to it. The domain walls disappeared upon heating above the ferroelectric phase transition at 460°C. Ferroelectric switching, as well as piezoelectric activity and hysteresis, were shown locally using piezoelectric force microscopy.

Piezoelectric Al1−xScxN thin films: A semiconductor compatible solution for mechanical energy harvesting and sensors

The transverse piezoelectric coefficient e31,f of Al1-xScxN thin films was investigated as a function of composition. It increased nearly 50% from x = 0 to x = 0.17. As the increase of the dielectric constant was only moderate, these films are very suitable for energy harvesting, giving a 60% higher transformation yield (x = 0.17) as compared to pure AlN. A higher doping might even lead to a 100% augmentation. The thickness strain response (d33,f) was found to increase proportionally to the ionic part of the dielectric constant. The e-type coefficients (stress response), however, did not augment so much as the structure becomes softer. As a result, the transverse voltage/strain response (h31,f-coefficient) was raised only slightly with Sc doping. The low dielectric loss obtained at all compositions suggests also the use of Al1−xScxN thin films in sensors.

Effect of substrate roughness on c-oriented AlN thin films

(001)-textured AlN thin films as needed for bulk acoustic wave devices exhibit large mechanical stress variations as a function of growth substrate properties. We studied the relationship between stress and the surface morphology of a thermally oxidized silicon substrate that was modified by a thin amorphous silicon layer. A rms roughness of 0.1–1.1 nm of the latter resulted in an increase in mechanical stress in the subsequently sputtered AlN thin film going from −700 to +200 MPa. At the same time, the x-ray rocking curve width of AlN increased from 1.3° to 2.3°. The roughness of the Si interlayer was controlled by the Ar sputter pressure. Interestingly, the maximal roughness is obtained at an intermediate pressure. This is explained by an interplay of nucleation and diffusion phenomena governed by the kinetics of impinging atoms and ions. The Si interlayer was essential to avoid cracking of membranes exhibiting mixed Pt and SiO2 surfaces below the AlN film.

Controlled stripes of ultrafine ferroelectric domains

In the pursuit of ferroic-based (nano)electronics, it is essential to minutely control domain patterns and domain switching. The ability to control domain width, orientation and position is a prerequisite for circuitry based on fine domains. Here, we develop the underlying theory towards growth of ultra-fine domain patterns, substantiate the theory by numerical modelling of practical situations and implement the gained understanding using the most widely applied ferroelectric, Pb(Zr,Ti)O3, demonstrating controlled stripes of 10 nm wide domains that extend in one direction along tens of micrometres. The observed electrical conductivity along these thin domains embedded in the otherwise insulating film confirms their potential for electronic applications.

Highly piezoelectric AlN thin films grown on amorphous, insulating substrates

AlN thin films were grown by reactive sputtering on amorphous SiO2 thin films. Film texture, x-ray rocking curve width, mechanical stress, and the clamped piezoelectric constant d33,f were studied as a function of rf bias power and substrate roughness. A high d33,f of 5.0 pm/V was achieved at low substrate roughness and low mechanical AlN film stress. Increasing substrate roughness and stress leads to a deterioration of d33,f, which is correlated with a higher density of opposite polarity grains detected by piezoresponse force microscopy. Extrapolating to 100% uniform polarity, a d33,f of 6.1 pm/V is derived as highest possible value, probably corresponding to the d33,f=e33/c33E of perfect single crystalline material. Growth mechanisms are proposed and underlined by high resolution transmission electron microscopy to explain the observed phenomena.

Densification of In2O3:Sn multilayered films elaborated by the dip-coating sol–gel route

Abstract Indium Tin Oxide (ITO) thin films have been deposited by the Sol–Gel Dip-Coating technique, the starting solutions being prepared from chlorides. These multilayered films were crystallized by means of a classical heat treatment at temperatures ranging from 500 to 600 °C. Five stacked layers are necessary to obtain a global electrical resistivity value of 2.9×10 −3 Ω cm, for 500 °C annealed film. The paper focuses on the study of the structure of such multilayered deposits, and on the densification process, using transmission electron microscopy, Rutherford Back-scattering Spectrometry and electrical resistivity measurements. This analysis reveals structural inhomogeneities and different crystallite growth processes as a function of annealing temperature and number of deposited layers.

Densification and crystallization of SnO2:Sb sol–gel films using excimer laser annealing

Abstract We have successfully applied laser annealing to sol–gel deposited SnO2:Sb thin films in order to achieve their crystallization. The as-deposited films are quasi-amorphous and electrically non-conductive. After laser annealing they crystallize and become conductive. This paper presents a comparative study of the laser annealed films and shows the influence of the irradiation parameters on the crystallization process and the electrical behavior of the films. Our results are quite promising in view of applying this kind of treatment to films deposited on thermally sensitive substrates (e.g. polymers).

Large-scale fabrication of titanium-rich perovskite PZT submicro/nano wires and their electromechanical properties

We report a 2-step approach to prepare tetragonal perovskite PbZr0.1Ti0.9O3 submicro/nano wires in gram scale and with over 95% wire content. Non-perovskite precursor wires were first fabricated by hydrothermal processing. A subsequent annealing in a PbO atmosphere at 600degC converted these wires into perovskite structures which retain the one-dimensional shape. Binding of the perovksite nanowires to a conductive substrate could be achieved by a similar heat treatment of the non-perovskite precursor wires on a flat Pt coated substrate. Taking advantage of the strong mechanical attachment and good electrical contact between the wires and the metallic layer, piezoresponse force microscopy (PFM) was used to measure the local piezoelectric and ferroelectric properties of the individual wires. Enhanced piezoelectric response relative to sputtered epitaxial PbZr0.2Ti0.8O3 film and squared hysterisis loop with sharp switching indicate pronounced electro- mechanical and ferroelectric behavior. The 90deg domain structure of the as-prepared perovskite PZT wires was confirmed by both PFM and transmission electron microscopy investigations.

Epitaxial growth of Ba0.3Sr0.7TiO3 thin films on Al2O3(0001) using ultrathin TiN layer as a sacrificial template

Epitaxial Ba0.3Sr0.7TiO3 (BST) films with significantly improved tunable performance were grown on c-plane sapphire [Al2O3(0001)] substrates using ultrathin TiN seed layers by pulsed laser deposition. A very thin (111) epitaxial TiN layer deposited at room temperature acted as a sacrificial epitaxial template for the BST film in the initial growth stage. The BST films grew with (111) orientation having BST[21¯1¯]‖Al2O3[112¯0] and BST[21¯1¯]‖Al2O3[21¯1¯0] epitaxial relationships. Planar capacitors fabricated on the epitaxial BST film showed significantly higher tunability as compared to those on a polycrystalline BST film directly deposited on the substrate.

Negative-pressure-induced enhancement in a freestanding ferroelectric.

Ferroelectrics are widespread in technology, being used in electronics and communications, medical diagnostics and industrial automation. However, extension of their operational temperature range and useful properties is desired. Recent developments have exploited ultrathin epitaxial films on lattice-mismatched substrates, imposing tensile or compressive biaxial strain, to enhance ferroelectric properties. Much larger hydrostatic compression can be achieved by diamond anvil cells, but hydrostatic tensile stress is regarded as unachievable. Theory and ab initio treatments predict enhanced properties for perovskite ferroelectrics under hydrostatic tensile stress. Here we report negative-pressure-driven enhancement of the tetragonality, Curie temperature and spontaneous polarization in freestanding PbTiO3 nanowires, driven by stress that develops during transformation of the material from a lower-density crystal structure to the perovskite phase. This study suggests a simple route to obtain negative pressure in other materials, potentially extending their exploitable properties beyond their present levels.