Kaupo Kukli

Tailoring the dielectric properties of HfO2–Ta2O5 nanolaminates

Dielectric thin films applicable, for instance, as insulating layers in electroluminescent display devices have been studied. In order to improve dielectric characteristics HfO2–Ta2O5 nanolaminates were prepared by atomic layer epitaxy at 325 °C. The nanolaminates were evaluated in capacitance and current–voltage measurements. By optimizing the layer thicknesses in the nanolaminate structures the dielectric properties, especially leakage current densities, could be tailored remarkably. The best nanolaminates showed charge storage factors improved up to 8 times when compared with those of the single oxide films. The presence of nanosize crystallites of monoclinic and metastable tetragonal HfO2 was observed by x‐ray diffraction analysis.

Properties of Ta2 O 5‐Based Dielectric Nanolaminates Deposited by Atomic Layer Epitaxy

Dielectric thin films and multilayers suitable for application as insulating layers in electroluminescent display devices have been studied. In this work, ZrO 2 -Ta 2 O 5 and Al 2 O 3 -Ta 2 O 5 nanolaminates with improved dielectric characteristics were grown by atomic layer epitaxy. The films were evaluated by capacitance and current-voltage measurements. The pure Ta 2 O 5 , Al 2 O 3 , and ZrO 2 films possessed charge-storage factors up to 8, 16, and 19 nC/mm 2 , respectively, at a leakage current density of 1 μA/cm 2 . The Al 2 O 3 -Ta 2 O 5 nanolaminates were completely amorphous and their storage factors did not exceed 30 nC/mm 2 . The ZrO 2 -Ta 2 O 5 nanolaminates showed remarkably improved dielectric properties when compared with those of the pure oxide films, especially when the interlayer thicknesses were optimized. Nanosize crystallites of monoclinic and metastable tetragonal ZrO 2 were observed in the nanolaminates by x-ray diffraction. The ZrO 2 -Ta 2 O 5 nanolaminates possessed high charge-storage factors up to 64 nC/mm 2 and showed superior stability of the dielectric properties.

Atomic Layer Deposition of Hafnium Dioxide Films from Hafnium Tetrakis(ethylmethylamide) and Water

HfO 2 films were produced from Hf[N(CH 3 )(C 2 H 5 )] 4 and H 2 O, on borosilicate glass, indium-tin-oxide (ITO), and Si(100) substrates, in the temperature range 150-325 °C, using atomic layer deposition (ALD). In the temperature range 200-250 °C, the growth rate of the HfO 2 films was 0.09 nm per cycle, but increased with both increasing and decreasing temperatures. The self-limiting adsorption of Hf[N(CH 3 )(C 2 H 5 )] 4 at 250 °C was verified. The films were stoichiometric dioxides with an O/Hf ratio of 2.0 ± 0.1. The concentrations of residual carbon, nitrogen, and hydrogen, determined using ion beam analysis, were 0.3-0.6 at.-%, 0.1-0.2 at.-%, and 2-3 at.-%, respectively. The films crystallized at growth temperatures exceeding 150-175 °C, and consisted mainly of the monoclinic HfO 2 phase. The refractive index of the films varied between 2.08 and 2.10. The effective permittivities of the HfO 2 films grown in the temperature range 200-300°C varied between 11 and 14.

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 (Nb1-xTax)2O5 solid solutions and (Nb1-xTax)2O5-ZrO2 nanolaminates grown by atomic layer epitaxy

(Nb1 − xTax)2O5 solid solution films and (Nb1 − xTax)2O5-ZrO2 nanolaminates have been deposited by Atomic Layer Epitaxy. Amorphous (Nb1 − xTax)2O5 films were obtained by sequential pulsing of tantalum and niobium ethoxide and water. The composition of the films was checked by Energy Dispersive X-ray Spectroscopy. (Nb1 − xTax)2O5 films exhibited 1.6–4 times higher permittivity than that of Ta2O5 films. Leakage currents through the (Nb1 − xTax)2O5 films were reduced drastically by adding intermediate ZrO2 layers with thickness of 5–10 nm i.e. by constructing nanolaminate structures. The ZrO2 interlayers contained nanosize tetragonal ZrO2 crystallites.

Development of Dielectric Properties of Niobium Oxide, Tantalum Oxide, and Aluminum Oxide Based Nanolayered Materials

Nb 2 O 5 , Ta 2 O 5 , and Al 2 O 3 solid solutions and nanolaminates were grown using atomic layer deposition technique. Electrical properties of the materials deposited were comparatively characterized by studying the behavior of Al/niobium-aluminum (tantalum) oxide/indium-tin oxide capacitor structures. The films with high Nb content demonstrated high polarizability and leakage current density. The films with high Al content demonstrated low leakage current densities. The leakage currents in Nb-based films were reduced by depositing thin alternate layers of Al 2 O 3 or Ta 2 O 5 and Nb 2 O 5 , thereby increasing the number of interfaces between distinct oxide layers. The permittivity of Nb 2 O 5 :Al 2 O 3 films could be increased with Nb 2 O 5 concentration without considerable loss in resistivity.

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.

Influence of growth temperature on properties of zirconium dioxide films grown by atomic layer deposition

ZrO2 films were grown by atomic layer deposition from ZrCl4 and H2O or a mixture of H2O and H2O2 on Si(100) substrates in the temperature range of 180–600  °C. The films were evaluated in the as-deposited state, in order to follow the effect of deposition temperature on the film quality. The rate of crystal growth increased and the content of residual impurities decreased with increasing temperature. The zirconium-to-oxygen atomic ratio, determined by ion-beam analysis, corresponded to the stoichiometric dioxide regardless of the growth temperature. The effective permittivity of ZrO2 in Al/ZrO2/Si capacitor structures increased from 13–15 in the films grown at 180  °C to 19 in the films grown at 300–600  °C, measured at 100 kHz. The permittivity was relatively high in the crystallized films, compared to the amorphous ones, but rather insensitive to the crystal structure. The permittivity was higher in the films grown using water. The leakage current density tended to be lower and the breakdown field highe...

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

Properties of hafnium oxide films grown by atomic layer deposition from hafnium tetraiodide and oxygen

Polycrystalline monoclinic HfO2 films were atomic layer deposited on Si(100) substrates by a nonhydrous carbon-free process of HfI4 and O2. The oxygen to hafnium ratio corresponded to the stoichiometric dioxide within the limits of accuracy of ion beam analysis. A 1.5–2.0 nm thick SiO2 interface layer formed between the HfO2 films and Si substrates. Hysteresis of the capacitance–voltage curves was observed in Al/HfO2/p-Si(100) structures with oxide grown in the substrate temperature range of 570–755 °C. The hysteresis ceased with an increase in O2 pressure. The effective permittivity of the dielectric layers varied between 12 and 16. The breakdown voltages were found to be lower in the case of higher oxygen doses and higher HfO2 deposition temperatures.