Alar Gerst

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 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.

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 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.

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.

Influence of annealing on atomic layer deposited Cr2O3-TiO2 thin films

Thin films in the Cr-Ti-O system were atomic layer deposited from CrO2Cl2, TiCl4, and CH3OH on Si(1 0 0), fused SiO2, and a-Al2O3(0 1 2) substrates at 420 oC. The proportion between Ti and Cr resulted from the ratio of the CrO2Cl2/CH3OH and TiCl4/CH3OH pulsing. The films were grown up to the thickness of about 70 nm. Annealing of the films was performed in O2 at 1000 oC. A notable dependence of their microstructure, conductance, and conductometric response to CO, H2, and CH4 in dry air on the substrates, Ti content, and annealing has been demonstrated. The films were polycrystalline on Si and SiO2, and epitaxial on a-Al2O3. At temperatures above 400 oC, the films had a conductance, advantageous from the point of view of semiconductor gas sensors. In response to a 30-ppm CO exposure at 450 oC, an annealed film on the a-Al2O3(0 1 2) substrate, distinguished by a relatively high Ti/Cr atomic ratio, showed a 16-% conductance decrease in 20 s, with a 120-s recovery.