Exploring the facets of “soft crystals” using an Atomic Force Microscope

We obtained monocrystalline droplets in a thermotropic cubic phase, of approximate size 100μm, deposited on a flat surface. The facets of these soft crystals are explored using both an optical microscope and an AFM. The height of individual steps on the principal facets and the lateral distance between steps in vicinal facets are measured using AFM in imaging (tapping) mode. Moreover, the elastic modulus is measured locally, using the AFM tip (in contact mode) as a local rheological probe.     

Epitaxial growth of Sc-doped ZnO films on Si by sol-gel route

The epitaxial growth of doped ZnO films is of great technological importance. Present paper reports a detailed investigation of Sc-doped ZnO films grown on (1 0 0) silicon p-type substrates. The films were deposited by sol-gel technique using zinc acetate dihydrate as precursor, 2-methoxyethanol as solvent and monoethanolamine (MEA) as a stabilizer. Scandium was introduced as dopant in the solution by taking 0.5 wt%¹ of scandium nitrate hexahydrate. The effect of annealing on structural and photoluminescence properties of nano-textured Sc-doped films was investigated in the temperature range of 300-550 °C. Structural investigations were carried out using X-ray diffraction, scanning electron microscopy and atomic force microscopy. X-ray diffraction study revealed that highly c-axis oriented films with full-width half maximum of 0.21° are obtained at an annealing temperature of 400 °C. The SEM images of ZnO:Sc films have revealed that coalescence of ZnO grains occurs due to annealing. Ostwald ripening was found to be the dominant mass transport mechanism in the coalescence process. A surface roughness of 4.7 nm and packing density of 0.93 were observed for the films annealed at 400 °C. Room temperature photoluminescence (PL) measurements of ZnO:Sc films annealed at 400 °C showed ultraviolet peak at about (382 nm) with FWHM of 141 meV, which are comparable to those found in high-quality ZnO films. The films annealed below or above 400 °C exhibited green emission as well. The presence of green emission has been correlated with the structural changes due to annealing. Reflection high energy electron diffraction pattern confirmed the nearly epitaxial growth of the films.     

Formation of nanocrystalline TiO2 by 100 MeV Au

Nanocrystalline TiO₂ structures are formed by irradiation of 100 MeV Au⁸⁺ ion beam on amorphous thin films of TiO₂. Surface morphology of the nanocrystals is studied by Atomic Force Microscopy (AFM). Amorphous to nanocrystalline phase transformation is identified by Glancing Angle X-ray Diffraction (GAXRD) and Raman spectroscopic studies. Optical characterization is carried out by UV-VIS spectroscopy technique. Blue shift observed in absorption band edge indicates the formation of nanophase TiO₂ after irradiation. The impinging swift heavy ion (100 MeV Au⁸⁺) induces nucleation of nanoparticles along the ion trajectory through inelastic collisions of the projectile with electrons of the material. It is observed that the shape and size of nanoparticles formed is dependant on the irradiation fluence.     

Structural, electrical and piezoelectric properties of LiNbO3 thin films for surface acoustic wave resonators applications

In this work, 0.30 μm thick LiNbO₃ layers have been deposited by sputtering on nanocrystalline diamond/Si and platinised Si substrates. The films were then analyzed in terms of their structural and optical properties. Crystalline orientations along the (0 1 2), (1 0 4) and (1 1 0) axes have been detected after thermal treatment at 500 °C in air. The films were near-stoichiometric and did not reveal strong losses or diffusion in lithium during deposition or after thermal annealing. Pronounced decrease of the roughness on top of the LiNbO₃ layer and at the interface between LiNbO₃ and diamond was also observed after annealing, compared to the bare nanocrystalline diamond on Si substrate. Furthermore, ellipsometry analysis showed a better density and a reduced thickness of the surface layer after post-deposition annealing. The dielectric constant and losses have been measured to 50 and less than 3.5%, respectively, for metal/insulator/metal structures with 0.30 μm thick LiNbO₃ layer. The piezoelectric coefficient d₃₃ was found to be 7.1 pm/V. Finally, we succeeded in switching local domain under various positive and negative voltages.     

Growth of AgInSe<Subscript>2</Subscript> on Si(100) substrate by thermal evaporation technique

AgInSe₂ films were prepared by a thermal evaporation technique onto Si(100) substrates at a pressure of 10⁻⁵ mbar. Structural and optical properties of films deposited at 300 and 473 K have been investigated. The film composition was studied by energy dispersive analysis through X-rays. X-ray diffraction patterns indicate that AgInSe₂ films have chalcopyrite structure with strong preferred orientation in the (112) direction. Average vertical crystallite size of 25 nm was observed. The optical energy gaps of 1.20 and 1.90 eV were obtained due to the fundamental absorption edge and a transition originating from crystal field splitting, respectively. Field emission scanning electron microscopy shows loosely packed grains of spherical symmetry with some facets.     

Growth and characterization of nanostructured anatase phase TiO<Subscript>2</Subscript> thin films prepared by DC reactive unbalanced magnetron sputtering

Titanium dioxide thin films were deposited on three different unheated substrates by unbalanced magnetron sputtering. The effects of the sputtering current and deposition time on the crystallization of TiO₂ thin films were studied. The TiO₂ thin films were deposited at three sputtering current values of 0.50, 0.75, and 1.00 A with different deposition times of 25, 35, and 45 min, respectively. The surface morphology of the films was investigated by atomic force microscopy (AFM). The structure was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The film thickness was determined by field emission scanning electron microscopy (FE-SEM), and the optical property was evaluated with spectroscopic ellipsometry. The results show that polycrystalline anatase films were obtained at a low sputtering current value. The crystallinity of the anatase phase increases as the sputtering current increases. Furthermore, nanostructured anatase phase TiO₂ thin films were obtained for all deposition conditions. The grain size of TiO₂ thin films was in the range 10–30 nm. In addition, the grain size increases as the sputtering current and deposition time increase.     

Growth mechanism, microstructure and transport properties of Sr1−x La x CuO2 (x=0.10−0.15) thin films

Sr_1−x La _x CuO₂ (x=0.10−0.15) thin films with an infinite-layer type structure were grown on BaTiO₃ buffered (001) SrTiO₃ substrates by pulsed laser deposition (PLD). The evolution of the growth front was monitored, in-situ, by high-pressure reflection high-energy electron diffraction (RHEED), while the surface morphology was analyzed by means of atomic force microscopy (AFM), ex-situ. X-ray diffraction (XRD) was used to determine the evolution of the film structure with deposition and cooling parameters, as well as to study the type and level of epitaxial strain in the Sr_1−x La _x CuO₂ films. The RHEED data showed that the Sr_1−x La _x CuO₂ films grow on BaTiO₃/SrTiO₃ following a 2D or Stranski-Krastanov mechanism, depending on the La doping level. The transition point (critical thickness d _c) from layer-by-layer like (2D) to island (3D) growth depends on the film stoichiometry: decreasing the La doping concentration x from 0.15 to 0.10, the critical thickness d _c increases from ∼45 nm to ∼75 nm. In order to induce superconductivity, the Sr_1−x La _x CuO₂ films were cooled down under reduction conditions. The as-deposited films showed semiconducting or metallic behavior, the resistivity decreasing with increasing La concentration. Post-deposition vacuum annealing resulted in a superconducting transition onset (but no zero resistance down to 4.2 K) only for some of the x=0.15 Sr_1−x La _x CuO₂ films.     

Growth of thin films of TiN on MgO(100) monitored by high-pressure RHEED

Reflection high-energy electron diffraction (RHEED) operated at high pressure has been used to monitor the initial growth of titanium nitride (TiN) thin films on single-crystal (100) MgO substrates by pulsed laser deposition (PLD). This is the first RHEED study where the growth of TiN films is produced by PLD directly from a TiN target. At the initial stage of the growth (average thickness ∼2.4 nm) the formation of islands is observed. During the continuous growth the islands merge into a smooth surface as indicated by the RHEED, atomic force microscopy and field emission scanning electron microscopy. These observations are in good agreement with the three-dimensional Volmer–Weber growth type, by which three-dimensional crystallites are formed and later cause a continuous surface roughening. This leads to an exponential decrease in the intensity of the specular spot in the RHEED pattern as well.     

Structural and electrical properties of zinc oxides thin films prepared by thermal oxidation

We report on zinc oxide (ZnO) thin films (d = 55-120 nm) prepared by thermal oxidation, at 623 K, of metallic zinc films, using a flash-heating method. Zinc films were deposited in vacuum by quasi-closed volume technique onto unheated glass substrates in two arrangements: horizontal and vertical positions relative to incident vapour. Depending on the preparation conditions, both quasi-amorphous and (0 0 2) textured polycrystalline ZnO films were obtained. The surface morphologies were characterized by atomic force microscopy and scanning electron microscopy. By in situ electrical measurements during two heating-cooling cycles up to a temperature of 673 K, an irreversible decrease of electrical conductivity of as flash-oxidized Zn films was revealed. The influence of deposition arrangement and oxidation conditions on the structural, morphological and electrical properties of the ZnO films is discussed.