Hugo Mändar

Atomic layer deposition of titanium dioxide from TiCl4 and H2O: investigation of growth mechanism

Abstract Atomic layer deposition (ALD) of titanium dioxide from TiCl 4 and H 2 O was studied at substrate temperatures of 100–400°C. Using a real-time quartz crystal microbalance method, it was demonstrated that although the surface reactions were self-limited, the growth rate depended on the temperature and development of the thin film structure. Relatively low growth rate which was obtained in the TiCl 4 /H 2 O ALD process, was found to be a result of a significant chlorine amount adsorbed during the TiCl 4 pulse. Surface intermediates formed in the initial stage of TiCl 4 adsorption were unstable and weakly bonded to the surface. Desorption and decomposition of these species additionally influenced the deposition rate and, especially, its dependence on the precursor pulse times.

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.

Spectroscopic study of nanocrystalline TiO2 thin films grown by atomic layer deposition

Abstract Photoluminescence characteristics of nanocrystalline TiO 2 films grown by atomic layer deposition were studied. In dependence of growth conditions the films consisted of anatase, rutile and TiO 2 -II phases. A broad band and two sharp peaks were observed in the photoluminescence spectra measured under continuous-wave Ar + laser excitation at temperatures 5–165 K. At 5 K the maximum of the broad band was at 2.24 eV in the anatase films, and at 2.37–2.40 eV in the rutile and TiO 2 -II films. The intensity of sharp lines peaking at 3.31 and 3.37 eV depended on the crystal structure of the films and increased significantly after X-ray irradiation. The temperature dependence and decay times of different emission bands were also investigated. The data obtained allowed a defect-trapped-exciton interpretation of the sharp peaks although the free-exciton origin of the 3.31 eV peak could still be argued. The broad-band emission at 2.24–2.40 eV was obviously due to self-trapped exciton recombination.

Influence of single and double deposition temperatures on the interface quality of atomic layer deposited Al2O3 dielectric thin films on silicon

An electrical characterization of Al2O3 based metal-insulator-semiconductor structures has been carried out by using capacitance-voltage, deep level transient spectroscopy, and conductance-transient (G-t) techniques. Dielectric films were atomic layer deposited (ALD) at temperatures ranging from 300 to 800 °C directly on silicon substrates and on an Al2O3 buffer layer that was grown in the same process by using 15 ALD cycles at 300 °C. As for single growth temperatures, 300 °C leads to the lowest density of states distributed away from the interface to the insulator [disorder-induced gap states (DIGS)], but to the highest interfacial state density (Dit). However, by using 300∕500°C double growth temperatures it is possible to maintain low DIGS values and to improve the interface quality in terms of Dit. The very first ALD cycles define the dielectric properties very near to the dielectric-semiconductor interface, and growing an upper layer at higher ALD temperature produces some annealing of interfacial s...

AXES1.9: new tools for estimation of crystallite size and shape by Williamson–Hall analysis

Dialogues for the estimation of crystallite size, shape and lattice strain are designed and included in an X-ray powder diffraction (XRPD) program, named AXES. They implement peak fitting and Voigt analysis followed by a Williamson–Hall plot (WHP). Eight different peak-shape functions can be used for individual peak fitting. Volume-weighted crystallite size and effective lattice deformation are calculated from linear interpolation of the WHP. Actual dimensions (diameters of spheres, diameters and heights of cylinders) are calculated, assuming spherical or cylindrical shapes of the crystallites. Results of size–shape analyses can be visualized in the form of a WHP and as an arrow diagram (HPGL format), which shows distribution of observed apparent and true sizes of crystallites with a diffraction vector. The program has been written in Borland Pascal 7.0 for MS-DOS. The executable code is available via the World Wide Web.

Refractive index gradients in TiO2 thin films grown by atomic layer deposition

Transmission spectra of TiO2 films grown by atomic layer deposition on fused silica have been analysed by using Lorentz dispersion and a model consisting of two sublayers inside a film. It has been shown that the deposition process parameters significantly influenced the refractive index gradient in the film growth direction. The films grown at a lower flow of the carrier gas showed a 35 nm thick sublayer with low refractive index at the silica substrate and a layer with a higher refractive index at the film surface. The films deposited at higher carrier gas flow had a higher refractive index at the substrate and a lower refractive index at the film surface. The results also demonstrate that by using a parameter modelling one can obtain some information about the internal structure of a film even if there are no clearly defined interference fringes in a transmission spectrum.

Alkoxide-based precursors for direct drawing of metal oxide micro- and nanofibres

Abstract The invention of electrospinning has solved the problem of producing micro- and nanoscaled metal oxide fibres in bulk quantities. However, until now no methods have been available for preparing a single nanofibre of a metal oxide. In this work, the direct drawing method was successfully applied to produce metal oxide (SnO2, TiO2, ZrO2, HfO2 and CeO2) fibres with a high aspect ratio (up to 10 000) and a diameter as small as 200 nm. The sol–gel processing includes consumption of precursors obtained from alkoxides by aqueous or non-aqueous polymerization. Shear thinning of the precursors enables pulling a material into a fibre. This rheological behaviour can be explained by sliding of particles owing to external forces. Transmission (propagation) of light along microscaled fibres and their excellent surface morphology suggest that metal oxide nanofibres can be directly drawn from sol precursors for use in integrated photonic systems.

Investigation of possible replacement of protective magnesium oxide layer in plasma display panels by barium ternary oxides

The firing voltage (FV) of gas discharge in a test cell of plasma display materials was investigated for standard protective layers of MgO deposited by electron beam, and ternary BaY2O4 and BaGa2O4 oxides grown by pulsed laser deposition on the special dielectric coated glass substrates. The determined FVs for MgO (160 V), BaY2O4 (210 V) and BaGa2O4 (257 V) lead to the conclusion that a replacement of MgO by these ternary oxides is not expedient for plasma display panels. Using results from luminescence spectroscopy, values for the energy gap Eg ≈ 6.2 and 5.8 eV were estimated for BaY2O4 and BaGa2O4, respectively. The main reason for the observed high FVs is attributed to strong electron affinities χ, where χ (BaY2O4) < χ (BaGa2O4).

Structures of polynuclear complexes of palladium(II) and platinum(II) formed by slow hydrolysis in acidic aqueous solution

The aqua ions of palladium(II) and platinum(II) undergo extremely slow hydrolysis in strongly acidic aqueous solution, resulting in polynuclear complexes. The size and structures of these species have been determined by EXAFS and small angle X-ray scattering, SAXS. For palladium(II), the EXAFS data show that the Pd-O and Pd···Pd distances are identical to those of crystalline palladium(II) oxide, but the intensities of the Pd···Pd distances in the Fourier transform at 3.04 and 3.42 Å are significantly lower compared to those of crystalline PdO. Furthermore, no Pd···Pd distances beyond 4 Å are observed. These observations strongly indicate that the polynuclear palladium(II) complexes are oxido- and hydroxido-bridged species with the same core structure as solid palladium(II) oxide. Based on the number of Pd···Pd distances, as derived from the EXAFS data, their size can be estimated to be approximately two unit cells, or ca. 1.0 nm(3). For platinum(II), EXAFS data of the polynuclear species formed in the slow hydrolysis process show Pt-O and Pt···Pt distances identical to those of amorphous platinum(II) oxide, precipitating from the solution studied. The Pt···Pt distances are somewhat different from those reported for crystalline platinum(II) oxide. The polynuclear platinum(II) complexes have a similar structure to the palladium ones, but they are somewhat larger, with an estimated diameter of 1.5-3.0 nm. It has not been possible to precipitate any of these species by ultracentrifugation. They are detectable by SAXS, indicating diameters between 0.7 and 2 nm, in excellent agreement with the EXAFS observations. The number of oxido- relative to hydroxido bridges will increase with increasing size of the complex. The charge of the complexes will remain about the same, +4, at growth, with approximate formulas [Pd10O4(OH)8(H2O)12](4+) and [Pt14O8(OH)8(H2O)12](4+) for complexes with a size of 2 and 3 unit cells of the corresponding solid metal oxide, respectively. Their high ionic charge in acidic aqueous solution will result in a stabilizing hydration shell.

Afterglow and thermoluminescence of ZrO2 nanopowders

A careful study of the phosphorescence afterglow and the thermoluminescence (TL) of sol-gel-prepared m-ZrO2 nanocrystalline powders in an extended temperature range −100 to 300 °C was carried out. Wavelength-resolved TL proved the existence of a single active luminescence centre in this temperature range. A TL method based on various heating rates was used to derive more reliable trap depths of 0.75, 0.95, 1.25, 1.46 and 1.66 eV whereas deconvolution methods provided somewhat lower values. The most intense room-temperature afterglows (that were easily observable beyond 1000 s) were obtained from samples annealed at 1250 and 1500 °C, and were attributed mainly to depopulation of the 1.25 eV traps.