Aarne Kasikov

Refractive index gradients in TiO2 thin films grown by atomic layer deposition

Transmission spectra of TiO2 films grown by atomic layer deposition on fused silica have been analysed by using Lorentz dispersion and a model consisting of two sublayers inside a film. It has been shown that the deposition process parameters significantly influenced the refractive index gradient in the film growth direction. The films grown at a lower flow of the carrier gas showed a 35 nm thick sublayer with low refractive index at the silica substrate and a layer with a higher refractive index at the film surface. The films deposited at higher carrier gas flow had a higher refractive index at the substrate and a lower refractive index at the film surface. The results also demonstrate that by using a parameter modelling one can obtain some information about the internal structure of a film even if there are no clearly defined interference fringes in a transmission spectrum.

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.

Impact of plasma treatment on electrical properties of TiO2/RuO2 based DRAM capacitor

In this work, we systematically studied the influence of the plasma treatment (PT) on the structural and electrical properties of Pt/rutile-TiO2/RuO2 metal–insulator–metal capacitors. The leakage current of the 12 nm thick TiO2 dielectrics prepared by atomic layer deposition was reduced below 10−7 A cm−2 while the capacitance equivalent thickness was kept below 0.5 nm using oxygen PT of the bottom RuO2 electrode. Reflection high energy electron diffraction, transmission electron microscopy, atomic force microscopy and x-ray photoelectron spectroscopy analyses allowed the conclusion that O2 plasma smoothened the RuO2 surface and increased its oxygen content through plasma induced surface reconstruction. The nucleation of TiO2 on the plasma-treated surface was faster while the thickness of the capacitor dead layer at the TiO2/RuO2 interface was reduced.

Electrochemical Modification of Gold Electrodes with Azobenzene Derivatives by Diazonium Reduction

An electrochemical study of Au electrodes electrografted with azobenzene (AB), Fast Garnet GBC (GBC) and Fast Black K (FBK) diazonium compounds is presented. Electrochemical quartz crystal microbalance, ellipsometry and atomic force microscopy investigations reveal the formation of multilayer films. The elemental composition of the aryl layers is examined by X-ray photoelectron spectroscopy. The electrochemical measurements reveal a quasi-reversible voltammogram of the Fe(CN)6 (3-/4-) redox couple on bare Au and a sigmoidal shape for the GBC- and FBK-modified Au electrodes, thus demonstrating that electron transfer is blocked due to the surface modification. The electrografted AB layer results in strongest inhibition of the Fe(CN)6 (3-/4-) response compared with other aryl layers. The same tendencies are observed for oxygen reduction; however, the blocking effect is not as strong as in the Fe(CN)6 (3-/4-) redox system. The electrochemical impedance spectroscopy measurements allowed the calculation of low charge-transfer rates to the Fe(CN)6 (3-) probe for the GBC- and FBK-modified Au electrodes in relation to bare Au. From these measurements it can be concluded that the FBK film is less compact or presents more pinholes than the electrografted GBC layer.

Investigation of possible replacement of protective magnesium oxide layer in plasma display panels by barium ternary oxides

The firing voltage (FV) of gas discharge in a test cell of plasma display materials was investigated for standard protective layers of MgO deposited by electron beam, and ternary BaY2O4 and BaGa2O4 oxides grown by pulsed laser deposition on the special dielectric coated glass substrates. The determined FVs for MgO (160 V), BaY2O4 (210 V) and BaGa2O4 (257 V) lead to the conclusion that a replacement of MgO by these ternary oxides is not expedient for plasma display panels. Using results from luminescence spectroscopy, values for the energy gap Eg ≈ 6.2 and 5.8 eV were estimated for BaY2O4 and BaGa2O4, respectively. The main reason for the observed high FVs is attributed to strong electron affinities χ, where χ (BaY2O4) < χ (BaGa2O4).

Nanoscale Characterization of TiO2 Films Grown by Atomic Layer Deposition on RuO2 Electrodes

Topography and leakage current maps of TiO2 films grown by atomic layer deposition on RuO2 electrodes using either a TiCl4 or a Ti(O-i-C3H7)4 precursor were characterized at nanoscale by conductive atomic force microscopy (CAFM). For both films, the leakage current flows mainly through elevated grains and not along grain boundaries. The overall CAFM leakage current is larger and more localized for the TiCl4-based films (0.63 nm capacitance equivalent oxide thickness, CET) compared to the Ti(O-i-C3H7)4-based films (0.68 nm CET). Both films have a physical thickness of ∼20 nm. The nanoscale leakage currents are consistent with macroscopic leakage currents from capacitor structures and are correlated with grain characteristics observed by topography maps and transmission electron microscopy as well as with X-ray diffraction.

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.

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.

Spectrophotometric and Raman spectroscopic characterization of ALD grown TiO2 thin films

Properties of titanium dioxide thin films grown by atomic layer deposition from TiCl4 and H2O on SiO2 substrates were characterized using Raman spectroscopy and spectrophotometry methods. Raman spectroscopy revealed transformation of the film structure from amorphous to anatase, to anatase/rutile mixture and then back to anatase with the increase of deposition temperature from 100 to 680oC. Variations in the growth rate, refractive index and extinction index accompanied these structural changes. Analysis of the transmission curves demonstrated that differently from amorphous films, the crystalline films were optically inhomogeneous in the growth direction.

Optical properties of crystalline Al 2 0 3 thin films grown by atomic layer deposition

Atomic layer deposition of Al203 thin films on Si and Si02 substrates was investigated in the temperature range of 470- 1030 K. The films grown at 870-1030 K contained crystalline phase while those deposited at temperatures below 870 K were amorphous. The growth rate was 0.03 nm per cycle at 1030 K but it increased with decreasing temperature and reached 0.13 nm per cycle at 470 K. The densities of crystalline films were 3660-3670 kg/m3 while those of amorphous films ranged from 2820 to 3140 kg/m3. The refractive indices of crystalline films (1.75-1.77 at the wavelength of 500 nm) were significantly higher than the refractive indices of amorphous films. A disadvantage of crystalline films compared with amorphous films was somewhat higher surface roughness. Nevertheless, the extinction coefficient of the former films did not exceed 0.004 at the wavelength of 500 nm. Excitation of crystalline films at the wavelengths shorter than 175 nm resulted in a broadband photoluminescence peaking at 350-360 nm. The excitation spectrum indicated dominant excitation of this emission via interband transition. The intrinsic absorption edges of 7.2 and 6.6 eV were estimated for crystalline and amorphous films, respectively, from the long-wavelength edges of the excitation spectra measured at 10 K.