Aivar Tarre

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.

Atomic-layer chemical vapor deposition of SnO2 for gas-sensing applications

Abstract SnO2 layers are grown from SnCl4 and H2O at 180–300°C on quartz glass substrates by atomic-layer-chemical-vapor-deposition-like process. The layers are oxygen deficient, predominantly amorphous and contain chlorine residues. Their amorphization degree decreases with the growth temperature. All the layers are sensitive to CO in air. Perfectly amorphous scanning-force-microscopy-smooth ultrathin (

Atomic layer deposition in traveling-wave reactor: In situ diagnostics by optical reflection

Abstract The traveling-wave reactor is an effective instrument for performing atomic layer deposition (ALD). In this paper, the use of a Brewster-angle interferometric reflectance technique for in situ real-time monitoring of ALD growth of transparent thin films on transparent and non-transparent substrates in suited traveling-wave reactors is proposed and analyzed on the basis of a classical four-phase approximation. An appropriate experimental reactor system is described. The technique is applied to the diagnostics of TiO2 growth from TiCl4 and H2O. Features of the reflectance signal in the conditions of time-homogeneous ALD growth are discussed.

Comparative study of low-temperature chloride atomic-layer chemical vapor deposition of TiO2 and SnO2

Abstract Peculiarities of the atomic-layer chemical vapor deposition growth of TiO2 and SnO2 thin films from chlorides and water at low (≤300°C) temperatures are in situ monitored on a submonolayer scale by incremental dielectric reflection. RHEED, AES, EPMA, SFM, and UV–VIS spectrophotometry are used for the ex situ characterization of the growth. It is shown that the main peculiarity of the TiO2 growth exhibits itself in the spontaneous crystallization at the very beginning of the deposition while the peculiarity of the SnO2 growth is connected with a low reactivity of water.

Nanoepitaxy of SnO2 on α-Al2O3(0 1 2)

Many materials exhibit interesting and novel properties when prepared as thin films. Thin film metal oxides have had an impact on the technological progress of the microelectronics mainly due to their electrical and optical properties. Since the future goes towards the nanometre scale there is an increasing demand for thin film deposition processes that can produce high quality metal oxide films in this scale with high accuracy.This thesis describes atomic layer deposition of Ta2O5, HfO2 and SnO2 thin films and chemical vapour deposition of SnO2 thin films. The films have been deposited by employing metal iodides and oxygen as precursors. All these processes have been characterised with regards to important processing parameters. The films themselves have been characterised by standard thin film analysing techniques such as x-ray diffraction, scanning electron microscopy, atomic force microscopy and transmission electron microscopy. The chemical and physical properties have been coupled to critical deposition parameters. Furthermore, additional data in the form of electrical and gas sensing properties important to future applications in the field of microelectronics have been examined.The results from the investigated processes have shown the power of the metal iodide based atomic layer deposition (ALD) and chemical vapour deposition (CVD) processes in producing high quality metal oxide thin films. Generally no precursor contaminations have been observed. In contrast to metal chloride based processes the metal iodide processes produces films with a higher degree of crystalline quality when it comes to phase purity, roughness and epitaxy. The use of oxygen as oxidising precursor allowed depositions at higher temperatures than normally employed in water based ALD processes and hence a higher growth rate for epitaxial growth was possible.

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.

Pt coated Cr2O3 thin films for resistive gas sensors

The resistive response of atomic layer deposited thin epitaxial α-Cr2O3(0 0 1) films, to H2 and CO in air, was studied. The films were covered with Pt nanoislands formed by electron-beam evaporation of a sub-monolayer amount of the material. The gas measurements were performed at 250°C and 450°C. These temperatures led to different proportion of chemical states, Pt2+ and Pt4+, to which the Pt oxidized. The modification was ascertained by the X-ray photoelectron spectroscopy method. As a result of the modification, the response was fast at 250°C, but slowed at 450°C. A disadvantageous abundance of Pt4+ arising at 450°C in air could be diminished by high-vacuum annealing thus restoring the response properties of the system at 250°C.

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.

RuO 2 /TiO 2 based MIM capacitors for DRAM application

MIM capacitors with MOCVD-grown RuO<inf>2</inf> bottom electrode, ALD-grown TiO<inf>2</inf> rutile dielectric and RuO<inf>2</inf> and Pt top electrodes were prepared and characterised by the means of C-V and J-V measurements. Dielectric constants were in the range of 140, which correspond to EOT of 0.5 nm and leakage current density as low as 1.5*10⁻⁶ A/cm² was achieved. Strong influence of TiO<inf>2</inf> stoichiometry on the leakage currents was found and analysed.

Resistive switching in RuO 2 /TiO 2 /RuO 2 MIM structures for non-volatile memory application

In this paper we describe resistive switching in RuO<inf>2</inf>/TiO<inf>2</inf>/RuO<inf>2</inf> structures. Electrodes (RuO<inf>2</inf>) were grown by metal organic chemical vapor deposition and dielectric TiO<inf>2</inf> switching layers with rutile structure were prepared by atomic layer deposition. After proper nitrogen annealing of as-grown samples followed by an electro-forming procedure and forming procedure bipolar resistive switching was observed. For various switching parameters 100 switching cycles were performed to test retention characteristics. Ratio of high to low resistance up to 10³ was obtained for reading voltage of −0.3 V.