Maria R. Catalano

Perovskite CaCu3Ti4O12 thin films for capacitive applications: From the growth to the nanoscopic imaging of the permittivity

The physical properties of CaCu3Ti4O12 (CCTO) thin films grown by metal organic chemical vapor deposition on LaAlO3 substrates have been investigated. The structural, compositional, and optical characteristics have been evaluated, and all the collected data demonstrated that in the obtained (001) epitaxial CCTO thin films, a low defect density is present. The electrical behavior of the deposited thin films has been studied from both micro- and nanoscopic points of view and compared with the properties reported in the literature. The electrical measurements on large area capacitors indicated that in the investigated work frequency range (102–106 Hz), the CCTO films possess dielectric constants close to the theoretically predicted “intrinsic” value and almost independent of the frequency. The nanoscopic dielectric investigation demonstrated that the deposited CCTO films possess n-type semiconducting nature and that a colossal extrinsic behavior can be locally achieved.

Nanoscale imaging of permittivity in giant-κ CaCu3Ti4O12 grains

The possibility to image the local permittivity in giant-κ dielectrics by scanning probe microscopy was demonstrated. In particular, the microstructure and the dielectric properties of CaCu3Ti4O12 (CCTO) films grown on (001) LaAlO3 substrates were studied. CCTO amorphous layers obtained by metal-organic chemical vapor deposition have been crystallized by subsequent rapid thermal treatments at 1100 °C. X-ray diffraction measurements demonstrated the growth of CCTO and CaTiO3 phases. As a novelty, large square CCTO grains (a few microns) were formed with a very high permittivity. The local impedance modulus, phase, and morphology were simultaneously acquired, and the local permittivity (around 8000 at 90 kHz) of each isolated CCTO grain was calculated.

Multi-Scale-Porosity TiO2 scaffolds grown by innovative sputtering methods for high throughput hybrid photovoltaics.

We propose an up-scalable, reliable, contamination-free, rod-like TiO2 material grown by a new method based on sputtering deposition concepts which offers a multi-scale porosity, namely: an intra-rods nano-porosity (1–5 nm) arising from the Thornton’s conditions and an extra-rods meso-porosity (10–50 nm) originating from the spatial separation of the Titanium and Oxygen sources combined with a grazing Ti flux. The procedure is simple, since it does not require any template layer to trigger the nano-structuring, and versatile, since porosity and layer thickness can be easily tuned; it is empowered by the lack of contaminations/solvents and by the structural stability of the material (at least) up to 500 °C. Our material gains porosity, stability and infiltration capability superior if compared to conventionally sputtered TiO2 layers. Its competition level with chemically synthesized reference counterparts is doubly demonstrated: in Dye Sensitized Solar Cells, by the infiltration and chemisorption of N-719 dye (∼1 × 1020 molecules/cm3); and in Perovskite Solar Cells, by the capillary infiltration of solution processed CH3NH3PbI3 which allowed reaching efficiency of 11.7%. Based on the demonstrated attitude of the material to be functionalized, its surface activity could be differently tailored on other molecules or gas species or liquids to enlarge the range of application in different fields.

Morphology-controlled synthesis of NiO films: the role of the precursor and the effect of the substrate nature on the films' structural/optical properties

NiO thin films were grown through metalorganic chemical vapour deposition (MOCVD) on quartz and LaAlO3 (001) single crystal substrates. Two different volatile and thermally stable nickel β-diketonate adducts, Ni(hfa)2·tmeda and Ni(tta)2·tmeda [H-hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentandione; tmeda = N,N,N′,N′-tetramethylethylendiamine, Htta = 2-thenoyltrifluoroacetone], were applied as precursors for NiO film growth. The comprehensive study, applying two different precursors and changing the processing parameters, allowed for morphological control of the deposited NiO films. The AFM analyses indicated different values of roughness for NiO films obtained from the two different precursors and those from Ni(tta)2·tmeda showed a lower roughness. In addition, UV-Vis and ellipsometric measurements on NiO films grown from the Ni(tta)2·tmeda show higher transparency, fewer defects/impurities, better crystallinity and a higher refractive index than films deposited using the Ni(hfa)2·tmeda source. Raman spectroscopy confirmed the antiferromagnetic nature of the NiO layers. Work functions, around 5.1 eV, were measured by Kelvin Probe Force Microscopy for NiO films grown from Ni(tta)2·tmeda. The relationship between the precursor/substrate nature and film properties allowed us to define the best precursor and conditions for the MOCVD growth of good quality NiO films.

Upconverting Er3+,Yb3+ activated β-NaYF4 thin films: a solution route using a novel sodium β-diketonate polyether adduct

A new tetraglyme adduct of sodium hexafluoroacetylacetonate [Na(hfa)·tetraglyme] has been synthesized and successfully used, together with the RE(hfa)3·diglyme (RE = Y, Yb, Er) complexes, in the synthesis of hexagonal phase NaYF4:Yb3+,Er3+ upconverting films through a facile sol–gel method.

Metal-Organic Chemical Vapor Deposition (MOCVD) Synthesis of Heteroepitaxial Pr0.7Ca0.3MnO3 Films: Effects of Processing Conditions on Structural/Morphological and Functional Properties

Calcium-doped praseodymium manganite films (Pr0.7Ca0.3MnO3, PCMO) were prepared by metal-organic chemical vapor deposition (MOCVD) on SrTiO3 (001) and SrTiO3 (110) single-crystal substrates. Structural characterization through X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM) analyses confirmed the formation of epitaxial PCMO phase films. Energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) characterization was used to confirm lateral and vertical composition and the purity of the deposited films. Magnetic measurements, obtained in zero-field-cooling (ZFC) and field-cooling (FC) modes, provided evidence of the presence of a ferromagnetic (FM) transition temperature, which was correlated to the transport properties of the film. The functional properties of the deposited films, combined with the structural and chemical characterization collected data, indicate that the MOCVD approach represents a suitable route for the growth of pure, good quality PCMO for the fabrication of novel spintronic devices.

CaCu3Ti4O12 Thin Films for Capacitive Applications: MOCVD Synthesis and Nanoscopic/Microscopic Characterization

The increasing need to miniaturize capacitor devices is demanding for progressively higher dielectric constant materials. Recent studies have demonstrated that the calcium copper titanate, CaCu3Ti4O12 (CCTO), single crystals and ceramics possess impressive dielectric constant values of about 10 working at 1 MHz which remain constant in the 100-600K temperature range and slightly depend upon work frequency in the 10-10 Hz range. Fabrication of thin films becomes now a required issue to understand their properties and evaluate potentialities for device integration. The synthesis of a multi-element oxide phase often requires considerable efforts to define the appropriate deposition conditions for the fabrication of high quality thin films and in particular the MOCVD growth process depends critically on the availability of volatile, thermally stable precursors. CaCu3Ti4O12 (CCTO) thin films have been successfully deposited on LaAlO3 (100) single substrates and on Pt/TiO2/SiO2/Si(100) stacks using Metal Organic Chemical Vapor Deposition technique, a suitable method even for large-scale production. A novel approach based on a molten multi-component precursor source has been applied. The molten mixture consists of the Ca(hfa) 2 •tetraglyme, Ti(tmhd)2(i-Opr)2, and Cu(tmhd)2 [Hhfa= 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; tetraglyme= 2,5,8,11,14-pentaoxapentadecane; Htmhd= 2,2,6,6-tetramethyl-3,5-heptandione; i-Opr = isopropoxide] precursors. 6-8 Film complete structural and morphological characterizations have been carried out by several techniques [X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM)]. The in-situ deposited CCTO (001) films on LaAlO3 (001) substrates are of high quality as evidenced by their inplane and out-of-plane alignments investigated using Xray and electron transmission diffraction patterns. CCTO films grown on Pt/TiO2/SiO2/Si(100) industrial substrates are polycrystalline (Figure 1) and present more disordered morphologies (Figure 2). Moreover, the electrical characterization at micro (Figure 2) and nanometer scale has been performed. The permittivity imaging provided at nanometer scale by Scanning Impedance Microscopy provided a correlation between the physical properties and structural characteristics. The nano impedance modulus and phase have been detected and the permittivity of the single CCTO layer has been calculated to be about 8000 at 90 kHz. Figure 1. XRD of CCTO polycrystalline films on Pt electrode and CCTO unit cell as inset.

Metal-Organic Chemical Vapor Deposition of BiFeO3 Based Multiferroics

BiFeO3 films undoped and doped with Ba and/or Ti have been fabricated through Metal-Organic Chemical Vapor Deposition (MOCVD) on SrTiO3 (100), SrTiO3:Nb (100) and YSZ (100) substrates. Films have been deposited using a multi-metal source, consisting of the Bi (phenyl)3, Fe (tmhd)3, Ba (hfa)2•tetraglyme and Ti (tmhd)2(O-iPr)2 (phenyl= -C6H5, H-tmhd=2,2,6,6-tetramethyl-3,5-heptandione; O-iPr= iso-propoxide; H-hfa=1,1,1,5,5,5-hexafluoro-2,4-pentanedione; tetraglyme = CH3O(CH2CH2O)4CH3) precursor mixture. The structural and morphological characterization of films has been carried out using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Chemical compositional studies have been performed by energy dispersive X-ray (EDX) analysis. Structural and morphological characterizations point to the formation of crystalline phases and homogeneous surfaces for both undoped and doped BiFeO3 films. Piezoresponse force microscopy (PFM) and piezoresponce force spectroscopy (PFS) have been applied to study the piezoelectric and ferroelectric properties of the films.

Optimization of Ca Precursor Transport for High Vacuum Chemical Vapor Deposition (HV-CVD)

The fabrication of high-k thin films of CaCu3Ti4O12 (CCTO) using HV-CVD is of high interest for various applications such as future high density capacitors for radio frequency circuits and integrated antenna used in wireless electronics. Fabrication of CCTO requires a precursor for Calcium transport in an HVCVD setup. Existing Calcium precursors have low volatility and low thermal stability. This imposes significant difficulties for transportation of precursors from the reservoir to the substrate. This work investigates various process parameters in order to determine ideal transport conditions for an HV-CVD setup.