A combinatorial chemical beam vapour deposition approach to tune the electrical conductivity of Nb:TiO2 films via Si co-doping

Chemical beam vapour deposition (CBVD) is a thin film deposition technique operated under high vacuum conditions in which film growth occurs through the thermally activated chemical decomposition of precursor molecules at the substrate surface. This technique was used in a combinatorial mode to investigate the influence of Si doping on the properties of Nb-doped TiO2 films, a well-known transparent conductive oxide material. The ternary oxide system (Si, Nb, Ti, O) displays good intermiscibility between the elements. By adding a Si precursor flow gradient to homogeneous Ti and Nb precursor flows, it was demonstrated that the resistivity of deposited films increased by over 5 orders of magnitude (from 1 to 100,000 Omega . cm) with Si doping levels between 2 and 21at.%. Meanwhile, only a slight variation of the refractive index of about 10% was observed. A fundamental film morphology study showed that the conductivity variation was due to Si segregation at the grain boundaries of the conductive Nb:TiO2 structure. (C) 2016 Elsevier B.V. All rights reserved.

Selective light induced chemical vapour deposition of titanium dioxide thin films

Estelle Wagner

Light Induced Chemical Vapour Deposition (LICVD) of titanium dioxide thin films is studied in this work. It is shown that this technique enables to deposit locally and selectively a chosen crystalline phase with a precise controlled thickness at low substrate temperature, allowing even the use of polymer substrates. A home made LICVD reactor was set up, consisting of a main chamber in which the substrate was placed on a temperature controlled plate and could be irradiated perpendicularly through a window by a 250 ns long pulse XeCl excimer laser (308 nm) with a controlled fluence (energy per surface area per pulse). A liquid precursor (titanium tetra-isopropoxide) was brought by an oxygen carrier gas flow in the chamber and a nitrogen flow was added to prevent deposition on the window. The total pressure in the chamber was maintained at 10 mbar. Projection of a mask image on the substrate was realized. Titanium dioxide films could be deposited locally by this technique with a thickness ranging from few nanometers to several micrometers. Deposits were realized at substrate temperatures as low as room temperature and with fluences between 1 and 400 mJ/cm2. The deposited films were characterized by profilometry, Atomic Force Microscopy, optical microscopy, Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray diffraction, Raman spectroscopy, X-ray Photoelectron Spectroscopy, UV-vis spectroscopy and ellipsometry. A detailed and systematic study of the influence of some selected process parameters (substrate temperature and irradiation dose principally) on the deposition rate and material properties was realized on glass and silicon substrates. An empirical equation of the growth rate as a function of the different experimental parameters was derived and was tested successfully to obtain a precise control of the deposited thickness (a precision better than 10 nm was demonstrated for films below 300 nm), with growth rates up to 300 nm/min. From this equation, a deposition mechanism involving both a photolytic reaction and a thermal contribution of the substrate temperature was proposed. The deposited material was shown to consist of TiO2 with a possible very low additional carbon contamination. Depending on the deposition conditions, amorphous, anatase or rutile material was deposited. One parameter explaining the variation of the crystalline state of the material was the variation of the laser induced temperature rise which was simulated theoretically in parallel. Surface temperature rises in the order of several hundreds of degrees were calculated during the laser pulse, varying with the substrate, the fluence and the film thickness. The optical properties of the deposited material vary in a correlated way with the crystalline state of the material, and high indexes of refraction (up to 2.6 at 633 nm) were measured. Selective deposition in the irradiated area was studied. Provided substrate temperature was kept below 150°C, no thermal deposition was observed, and deposits were strictly located in the irradiated area. The edge resolution (around 10 micrometers) was demonstrated to be due to the optical aberrations of the mask projection set up. No effect of substrate temperature and irradiation conditions on the resolution could be evidenced, but resolution decreased with increasing deposited thickness due to optical effects. Thanks to the low substrate temperature required for the deposition process, deposition on thermo- fragile substrates (such as polymers, in particular PMMA) was demonstrated. However, in the case of polymer substrates, the fluence had to be kept below 15 mJ/cm2 not to damage or ablate the polymers, resulting in very low growth rates (in the order of 10-3 nm/pulse). In these conditions, only amorphous films with about 10% of carbon contamination could be obtained. Additionally, films were shown to crack above a certain deposited thickness, which is attributed to thermal effects. As a matter of fact, calculations showed a high temperature rise up to 70°C of the polymer/oxide interface. Deposition on polymers coated with transparent conductive oxide was also demonstrated.

EPFL2003

Free carriers in nanocrystalline titanium dioxide thin films

Simon Springer

This thesis analyzes the properties of heavily doped nanocrystalline titanium dioxide. Thin films were deposited by magnetron sputtering with either water vapor as reactive gas or a periodically interrupted oxygen supply. The samples were at the same time electrically conducting and transparent. They consisted of mixtures of rutile, anatase and amorphous phases. Titanium dioxide is chemically stable, hard, non-toxic, transparent, and inexpensive. Due to a high refractive index it is often found in optical applications as a thin film coating. For many purposes it is interesting to take advantage of a combination of good transparency and electrical conductivity. An even larger field of applications would open if the electric conductivity of TiO2 could be increased in a controlled manner. Conventional bulk doping is limited, however, by low solubility limits. Highly conducting nanocrystalline TiO2 thin films have been obtained recently by sputtering in presence of water vapor. The present project intends to elucidate the doping mechanisms at work in such films. In addition, an alternative process to achieve grain-boundary doping of TiO2 was explored. Spectrophotometry over a wide spectral range (0.05 to 5 eV) was the main tool in probing the mobile electrical charge carriers. The interpretation of these optical measurements required appropriate models. The models developed took many structural properties into account, such as thickness, surface roughness, density, anisotropy and crystalline phase composition. The phase and morphology of the films were analyzed by X-ray diffraction and reflection, scanning probe microscopy, and electron microscopy. The chemical analyses included electron-probe microanalyses, Rutherford backscattering and elastic recoil detection analysis. The DC electrical properties were measured between room temperature and 250 °C. Most water-deposited samples showed semiconducting behavior with an activation energy of the order of 100 meV. The optical measurements revealed a broad absorption band around 1 eV. The absorption coefficient scaled with conductivity. It was successfully modelled as a Lorentzian. Similar absorption bands have been reported for reduced rutile and anatase single crystals. Water-deposited samples containing anatase exhibited a metallic behavior. In these samples the thermal activation energy was reduced to about 30 meV and the charge carriers displayed a high mobility (3 cm2V-1s-1). The infrared absorption band was about 10 times less important than in the samples devoid of anatase. Thin films of TiO2 intercalated with the equivalent of nanometer-thick layers of Ti were prepared. By changing the period in the oxygen supply the grain size, and thereby the film properties, could be modified. The films obtained in this way had many properties in common with water-deposited films. They showed the same absorption band in the infrared, had comparable charge carrier densities and were electrically conductive (102 to 104 Sm-1 at room temperature). To our knowledge, this was the first time that such high electrical conductivities were achieved by sputtering in an oxygen plasma.

EPFL2004

Fabrication of complex oxide microstructures by combinatorial chemical beam vapour deposition through stencil masks

Giacomo Benvenuti, Scott Harada, Paul Muralt, Silviu Cosmin Sandu, Andreea Veronica Savu, Estelle Wagner

Chemical Beam Vapour Deposition is a gas phase deposition technique, operated under high vacuum conditions, in which evaporated chemical precursors are thermally decomposed on heated substrates to form a film. In the particular equipment used in this work, different chemical beams effuse from a plurality of punctual precursor sources with line of sight trajectory to the substrate. A shadow mask is used to produce 3D-structures in a single step, replicating the apertures of a stencil as deposits on the substrate. The small gap introduced between substrate and mask induces a temperature difference between both surfaces and is used to deposit selectively solely on the substrate without modifying the mask, taking advantage of the deposition rate dependency on temperature. This small gap also enables the deposition of complex patterned structures resulting from the superposition of many patterns obtained using several precursor beams from different directions through a single mask aperture. A suitable process parameter window for precursor flow and substrate temperature is evidenced to maximize resolution. (C) 2015 Elsevier B.V. All rights reserved.