Väino Sammelselg

Morphology and structure of TiO2 thin films grown by atomic layer deposition

Abstract Atomic layer deposition of TiO 2 films from TiCl 4 and H 2 O was studied at reactor temperatures 100–500°C. Amorphous films grew at temperatures below 165°C, anatase structure was observed in the films grown at 165–350°C while rutile dominated in the films obtained at temperatures above 350°C. The surface morphology and optical losses of the films are related to the film structure. The highest surface roughness was observed at the temperature of amorphous to crystalline phase transition. The smoothest surfaces were recorded for amorphous films. The amorphous films grown at 100°C had optical losses below 100 cm −1 in the visible range of the spectrum.

Effect of crystal structure on optical properties of TiO2 films grown by atomic layer deposition

Abstract The dependence of optical characteristics on the structure of atomic-layer-deposited titania (TiO 2 ) thin films is studied. Amorphous films grown at 100°C have an optical band gap of 3.3 eV while the refractive index values of these films range from 2.2 to 2.3 at a wavelength of 633 nm. The refractive index can be increased up to 2.65 by using the growth temperatures about 300°C. Composition and structure studies reveal that the formation of preferentially oriented crystal (anatase) structure contributes to this increase of refractive index most significantly.

Atomic Layer Deposition of Iridium Thin Films

Thin films of metallic iridium were grown by atomic layer deposition (ALD) in a wide temperature range of 225-375°C from tris(2,4-pentanedionato)iridium [Ir(acac) 3 ] and oxygen. The films had low resistivity and low impurity contents and good adhesion to the substrate. The film growth rate saturated to a constant value as the precursor pulse times were increased, thus verifying the self-limiting growth mechanism. In addition, the film thickness depended linearly on the number of deposition cycles. The development of the surface morphology with increasing film thickness was studied by atomic force microscopy (AFM). AFM and X-ray reflectivity analysis showed that the films had smooth surfaces. The films showed a preferred (111) orientation as studied by X-ray diffraction. The results show that high-quality iridium films can be grown by ALD.

Properties of tantalum oxide thin films grown by atomic layer deposition

Thin tantalum oxide films were deposited using atomic layer deposition from TaCl5 and H2O at temperatures in the range 80–500 °C. The films deposited at temperatures below 300 °C were predominantly amorphous, whereas those grown at higher temperatures were polycrystalline containing the phases TaO2 and Ta2O5. The oxygen to tantalum mass concentration ratio corresponded to that of TaO2 at all growth temperatures. The optical band gap was close to 4.2 eV for amorphous films and ranged from 3.9 to 4.5 eV for polycrystalline films. The refractive index measured at λ = 550 nm increased from 1.97 to 2.20 with an increase in growth temperature from 80 to 300 °C. The films deposited at 80 °C showed low absorption with absorption coefficients of less than 100 cm−1 in the visible region.

Antennal sensilla of the ground beetle Bembidion properans Steph. (Coleoptera, Carabidae)

The arrangement of antennal sensilla was studied in female and male ground beetles Bembidion properans Steph. (Coleoptera, Carabidae) using scanning electron microscopy. The filiform antennae, 1.8-1.9 mm in length, consist of the scape, pedicel and nine flagellomeres. In both sexes, three types of sensilla chaetica, two types of sensilla trichodea, six types of sensilla basiconica, one type of sensilla coeloconica and one type of sensilla campaniformia were distinguished. The possible function of the sensilla is discussed and three types of sensilla are considered olfactory, sensilla trichodea type 2 and sensilla basiconica types 1 and 2. Olfactory sensilla form dorsal and/or ventral sensillar fields on the flagellomeres and occur sparsely or not at all outside these areas. No sexual differences in the types, number and distribution of antennal sensilla were found.

Growth kinetics and structure formation of ZrO2 thin films in chloride-based atomic layer deposition process

It was demonstrated that ZrO2 thin films can be grown in the ZrCl4-based atomic layer deposition process at substrate temperatures of 180–600 °C. The films grown on silicon and silica substrates were amorphous or polycrystalline dependently on the growth temperature and film thickness. Crystalline films grown at 180–210 °C contained a cubic phase of ZrO2. Tetragonal ZrO2 was the main crystalline phase in the films that were 10–50 nm in thickness and were deposited at 300–600 °C. In thicker films, grown at 300–600 °C, the monoclinic phase appeared and started to dominate with the further increase of film thickness. No significant difference was observed in the phase composition and preferential orientation of films that were grown at the same process parameters on single crystal silicon and amorphous silica substrates. Neither the growth mechanism nor the phase composition of films depended considerably on whether H2O or the commercial aqueous solution of H2O2 (35%) was used as the oxygen precursor.

Electrochemical reduction of oxygen on thin-film Au electrodes in acid solution

The reduction of oxygen has been studied on thin-film gold electrodes using the rotating disk electrode (RDE) technique. Thin films of gold were prepared by vacuum evaporation onto glassy carbon and highly oriented pyrolytic graphite substrates. The surface morphology of the films was examined by atomic force microscopy. The kinetic parameters of O2 reduction have been determined and a Tafel slope of (−112±8)mVdec−1 was observed. The specific activity of the electrodes was almost independent of film thickness.

TiO2 thin films by atomic layer deposition: a case of uneven growth at low temperature

Abstract Atomic layer deposition of TiO 2 thin films from TiCl 4 and H 2 O at 150°C was found to result in nonhomogeneous film growth as a function of time. The unevenness of the growth was ascertained by in situ optical interference technique. In order to obtain a more detailed understanding of the growth behavior, AFM, SEM, EPMA, RHEED, XRD, and XPS measurements were performed on a series of films of different thickness. The data obtained allow us to relate the peculiarities of the growth to the appearance of anatase inclusions in the amorphous base layer of TiO 2 followed by their development into crystalline particles. We managed to delimit the nucleation stage of the growth, the stage of the full coverage of the substrate with the amorphous film, the stage of the origination of the anatase inclusions, and the final stage at which the polycrystalline film is deposited. The increase of crystallinity and roughness was correlated with the increase of the film thickness and quantitatively determined. It was found that in our films the composition of both the crystalline and amorphous part corresponds to TiO 2 stoichiometric phase, and that the films contain 0.3 mass% of Cl.

Characterization of titanium dioxide atomic layer growth from titanium ethoxide and water

Abstract Atomic layer growth of titanium dioxide from titanium ethoxide and water was studied. Real-time quartz crystal microbalance measurements revealed that adsorption of titanium ethoxide is a self-limited process at substrate temperatures 100–250°C. A relatively small amount of precursor ligands was released during titanium ethoxide adsorption while most of them was exchanged during the following water pulse. At temperatures 100–150°C, incomplete reaction between surface intermediates and water hindered the film growth. Nevertheless, the deposition rate reached 0.06 nm per cycle at optimized precursor doses. At substrate temperatures above 250°C, the thermal decomposition of titanium ethoxide markedly influenced the growth process. The growth rate increased with the reactor temperature and titanium ethoxide pulse time but it insignificantly depended on the titanium ethoxide pressure. Therefore reproducible deposition of thin films with uniform thickness was still possible at substrate temperatures up to 350°C. The films grown at 100–150°C were amorphous while those grown at 180°C and higher substrate temperature, contained polycrystalline anatase. The refractive index of polycrystalline films reached 2.5 at the wavelength 580 nm.

Control of thin film structure by reactant pressure in atomic layer deposition of TiO2

Abstract Atomic layer growth of TiO 2 films from TiCl 4 and H 2 O was examined. It was established that polycrystalline films of pure rutile, pure TiO 2 -II, or a mixture of them could be obtained at the same growth temperature (400°C) and at the same TiCl 4 vapor pressure (0.34 Pa) applying different water vapor pressures during the water pulse. Possible growth mechanisms leading to formation of different structures are discussed.