Ahti Niilisk

Atomic scale optical monitoring of the initial growth of TiO2 thin films

The initial atomic-layer-chemical-vapor-deposition growth of titanium dioxide from TiCl 4 and water on quartz glass substrates is monitored in real time by incremental dielectric reflection. An interesting means for bringing the growth from the very beginning into a time-homogeneous mode is proposed and preliminarily studied. It consists in an in situ TiCl 4 -treatment procedure. The crystal structure and surface morphology of the prepared ultrathin films are characterized.

Structural study of TiO2 thin films by micro-Raman spectroscopy

The Raman spectroscopy method was used for structural characterization of TiO2 thin films prepared by atomic layer deposition (ALD) and pulsed laser deposition (PLD) on fused silica and single-crystal silicon and sapphire substrates. Using ALD, anatase thin films were grown on silica and silicon substrates at temperatures 125–425 °C. At higher deposition temperatures, mixed anatase and rutile phases grew on these substrates. Post-growth annealing resulted in anatase-to-rutile phase transitions at 750 °C in the case of pure anatase films. The films that contained chlorine residues and were amorphous in their as-grown stage transformed into anatase phase at 400 °C and retained this phase even after annealing at 900 °C. On single crystal sapphire substrates, phase-pure rutile films were obtained by ALD at 425 °C and higher temperatures without additional annealing. Thin films that predominantly contained brookite phase were grown by PLD on silica substrates using rutile as a starting material.

Monitoring of atomic layer deposition by incremental dielectric reflection

Abstract An optical method for in situ diagnostics of atomic layer deposition, based on the measuring of the reflectivity of a p-polarized light in fully transparent systems, is proposed and analyzed by using a classical four-phase approximation. The method is applied to the monitoring of TiO2 growth. It is shown that the sensitivity of the method is high if one uses the angles of incidence close to the Brewster angle of the substrate. The method is called incremental dielectric reflection.

Atomic layer deposition of HfO2 on graphene from HfCl4 and H2O

Atomic layer deposition of HfO2 on unmodified graphene from HfCl4 and H2O was investigated. Surface RMS roughness down to 0.5 nm was obtained for amorphous, 30 nm thick hafnia film grown at 180°C. HfO2 was also deposited in a two-step temperature process where the initial growth of about 1 nm at 170°C was continued up to 10–30 nm at 300°C. This process yielded uniform, monoclinic HfO2 films with RMS roughness of 1.7 nm for 10–12 nm thick films and 2.5 nm for 30 nm thick films. Raman spectroscopy studies revealed that the deposition process caused compressive biaxial strain in graphene, whereas no extra defects were generated. An 11 nm thick HfO2 film deposited onto bilayer graphene reduced the electron mobility by less than 10% at the Dirac point and by 30–40% far away from it.

Surface of TiO2 during atomic layer deposition as determined by incremental dielectric reflection

Abstract We show that the measuring of the reflectance changes in transparent systems allows one to optically characterize the surface of films growing under the conditions of atomic partial-monolayer deposition. In the model, a continuous layer with effective optical parameters describes the growth front. Growing amorphous TiO 2 thin films from TiCl 4 and H 2 O at 115°C is used in demonstration experiments.

Spectroscopic ellipsometry of TiO2/Si

In order to characterize TiO2 films in terms of the overall optical response, spectroscopic ellipsometry studies of the system TiO2/Si were carried out. The films were grown by the atomic-layer chemical vapor deposition on Si(111) substrates. Optical measurements were performed by means of a photometric ellipsometer with rotating analyzer. Experimental results have been analyzed using multilayer and pseudodielectric function approximations.

Highly sensitive NO2 sensors by pulsed laser deposition on graphene

Graphene as a single-atomic-layer material is fully exposed to environment and has therefore a great potential for creating of sensitive gas sensors. However, in order to realize this potential for different polluting gases, graphene has to be functionalized - adsorption centers of different type and with high affinity to target gases have to be created at its surface. In this present work, modification of graphene by small amounts of laser ablated materials is introduced for this purpose as a versatile and precise tool. The approach was demonstrated with two very different materials chosen for pulsed laser deposition (PLD), a metal (Ag) and a dielectric oxide (ZrO2). It was shown that the gas response and its recovery rate can be significantly enhanced by choosing the PLD target material and deposition conditions. The response to NO2 gas in air was amplified up to 40 times in case of PLD-modified graphene in comparison with pristine graphene and reached 7-8% at 40 ppb of NO2 and 20-30% at 1 ppm of N2. These results were obtained after PLD in gas environment (5 x 10-2 mbar oxygen or nitrogen) and atomic areal densities of deposited materials of were about 10 15 cm-2. The ultimate level of NO2 detection in air, as extrapolated from the experimental data obtained at room temperature under mild UV-excitation, was below 1 ppb.

Atomic layer deposition of aluminum oxide films on graphene

Seed-layer approach was studied to initiate atomic layer deposition (ALD) of Al2O3 films on graphene. Low-temperature ALD and electron beam evaporation (EBE) were applied for seed-layer preparation before deposition of the dielectric at 200 °C using trimethyl-aluminum and water or ozone as precursors. To characterize nucleation of the films and possible influence of the ALD processes on the quality of graphene, properties of graphene and Al2O3 films were investigated by Raman spectroscopy, X-ray fluorescence and X-ray photoelectron spectroscopy methods. The results suggest that seed layer formation by low-temperature ALD was more efficient in the O3-based process than in the H2O-based one while EBE seed layer provided fastest growth of Al2O3 together with minimum incubation period.

Atomic Layer Deposition of High-k Oxides on Graphene

Graphene that is a single hexagonal layer of carbon atoms with very high intrinsic charge carrier mobility (more than 200 000 cm2/Vs at 4.2 K for suspended samples; Bolotin, et al., 2008) attracts attention as a promising material for future nanoelectronics. During last few years, significant advancement has been made in preparation of large-area graphene. The lateral sizes of substrates for graphene layers have been increased up to 3⁄4 m (Bae et al., 2010) and continuous roll-to-roll deposition of graphene has been published (Hesjedal, 2011). This kind of progress might allow one to apply similar planar technologies for fabricating graphene-based devices in future as currently used for processing of siliconbased structures. After very first experiments (Novoselov et al., 2004), in which the electrical properties of isolated graphene sheets were characterized, a lot of attention has been paid to the similar studies, i.e. investigation of uncovered graphene flakes deposited on oxidized silicon wafers that served as back gates. However, in order to realize graphene-based devices, a highquality dielectric on top of graphene is required for electrostatic gates as well as for tunnel barriers for spin injection. For efficient control of charge carrier movement dielectric layers deposited on graphene should be very thin, a few nanometers thick, and of very uniform thickness without any pinholes. At the same time, the dielectric should possess high dielectric constant, high breakdown voltage and low leakage current even at a small thickness. And, of course, it is expected that the high mobility of charge carriers in graphene should not be markedly affected by the dielectric layer. In order to make top-gated graphene-based Field Effect Transistor (FET), Lemme et al. (2007) applied evaporation techniques for preparation of a gate stack with ~20 nm thick SiO2 dielectric layer on graphene. They used p-type Si(100) wafers with a boron doping concentration of 1015 cm-3, which were oxydized to a SiO2 thickness of 300 nm. On these wafers, micromechanically exfoliated graphene flakes were sticked. The Ti/Au source and drain electrodes were prepared using optical lift-off lithography. Next, electron beam lift-off lithography was applied to define a top gate electrode on top of the graphene flake covered with the dielectric (Fig. 1a). Lemme et al. were first to demonstrate that the combined effect of back and top gates can be applied to graphene devices. However, measurements of the back-gate characteristics before

Atomic layer deposition of epitaxial TiO2 II on c-sapphire

Using atomic layer deposition technique, epitaxial titania polymorph TiO2 II was grown on α-Al2O3(0 0 1) (c-sapphire) substrates. TiCl4 and H2O served as precursors. The growth temperature ranged from 350 to 680 °C. Raman scattering and high-resolution x-ray diffraction and reflection measurements were applied to characterize the films. It appeared that the films contained, in addition to TiO2 II, anatase and/or rutile phase. The dependence of the film properties on the growth temperature and the film thickness was explored. The growth of the TiO2 II phase was shown to be controlled by the α-Al2O3 substrate orientation. This phase did not grow when the substrate was (0 1 2) oriented (r-sapphire). The epitaxial relationship was determined to be (1 0 0)[0 1¯ 0]TiO2 II ∥ (0 0 1)[1 2 0]sapphire, (1 0 0)[0 0 1]TiO2 II ∥ (0 0 1)[1 0 0]sapphire.