Aleks Aidla

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 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.

Titanium isopropoxide as a precursor for atomic layer deposition: characterization of titanium dioxide growth process

Abstract Atomic layer deposition (ALD) of titanium oxide from titanium isopropoxide (Ti(OCH(CH 3 ) 2 ) 4 ) and water as well as from Ti(OCH(CH 3 ) 2 ) 4 and hydrogen peroxide (H 2 O 2 ) was studied. According to data of real-time quartz crystal microbalance (QCM) measurements, adsorption of Ti(OCH(CH 3 ) 2 ) 4 was a self-limited process at substrate temperatures 100–250°C. At 200–250°C, the growth rate was independent of whether water or H 2 O 2 was used as the oxygen precursor. Insufficient reactivity of water vapor hindered the film growth at temperatures 100–150°C. Incomplete removal of the precursor ligands from solid surface by water pulse was revealed as the main reason for limited deposition rate. The growth rate increased significantly and reached 0.12 nm per cycle at 100°C when water was replaced with H 2 O 2 . The carbon contamination did not exceed 1 at.% and the refractive index was 2.3 in the films grown at temperatures as low as 100°C.

In situ study of atomic layer epitaxy growth of tantalum oxide thin films from Ta(OC2H5)5 and H2O

Ta2O5 thin films have been deposited in atomic layer epitaxy process from Ta(OC2H5)5 and H2O. A quartz crystalline mass-sensor was exploited to detect the adsorption processes at the gas–solid interface during the film growth. It is suggested that Ta(OC2H5)5 reacts with surface hydroxyls producing intermediate surface species (-O)nTa(OC2H5)5−n where n varies with the reactor temperature. During the subsequent water pulse these species react further converting the surface back to the hydroxyl-terminated one. The uncontrolled deposition due to the temperature-induced decomposition of tantalum ethoxide with the activation energy of 100±6 kJ/mol contributes to the film growth above 275°C. The value of the diffusion coefficient D=0.0075 m2/s for gas-phase Ta(OC2H5)5 has been calculated at 250°C. Estimated sticking coefficient of Ta(OC2H5)5 is about one order of magnitude higher than that of H2O and nearly one order of magnitude lower than that of TaCl5.

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.

Atomic layer deposition of zirconium oxide from zirconium tetraiodide, water and hydrogen peroxide

Atomic layer deposition of zirconium oxide from zirconium tetraiodide, water and hydrogen peroxide

Atomic Layer Chemical Vapor Deposition of TiO2 Low Temperature Epitaxy of Rutile and Anatase

Atomic layer chemical vapor deposition of TiO2: Low-temperature epitaxy of rutile and anatase

Ti 2p and O 1s X-ray absorption of TiO2 polymorphs

Abstract We present and discuss the Ti 2p and O 1s X-ray absorption spectra (XAS) and resonant photoelectron emission spectra (RPES) of the various polymorphic forms (TiO2-II, rutile and anatase) of TiO2. The largest differences between the Ti 2p XAS of polymorphs of TiO2 are observed in the region of the double peak structure of the L3 edge. A strong delocalization of the excited 3d electron in this region is concluded from the RPES at the L3 threshold of Ti. The differences in XAS are attributed to increasing strength of the crystal field caused by delocalization of the excited 3d electron.

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.