The temperature dependence of the epitaxial growth of cerium dioxide (CeO2) layers on (111) silicon substrates was studied using ultra-high-vacuum evaporation, varying the substrate temperature in the range between 150 and 900 °C. Characterization using Rutherford backscattering spectrometry and reflection high-energy electron diffraction proved that a CeO2 layer on (111) silicon has considerably good crystalline quality in the range between 200 and 850 °C. It is clarified that the epitaxy can be attained even at surprisingly low temperatures (less than 200 °C). A substrate temperature above 850 °C leads to rather poor crystallinity due to an exponential increase of residual gas pressure. Refractive indices of the epitaxial layers were measured by ellipsometry to be in the range of 2.19–2.79.
Electrical and optical properties of Nb-doped TiO 2 films deposited by dc magnetron sputtering using slightly reduced Nb-doped TiO 2 − x ceramic targets J. Vac. Sci. Technol. A 28, 851 (2010); 10.1116/1.3358153 rf magnetron sputter deposition of transparent conducting Nb-doped Ti O 2 films on Sr Ti O 3 Transparent and semitransparent conducting film deposition by reactive-environment, hollow cathode sputtering A transparent metal: Nb-doped anatase Ti O 2The authors report stoichiometry control and postdeposition annealing-free fabrication of Nb-doped transparent anatase TiO 2 ͑A-TiO 2 :Nb͒ films on alkaline-free glass substrates by helicon-wave-excited-plasma sputtering. The films tended to crystallize in the stable electrically semi-insulating rutile phase. However, although the appropriate deposition condition window was narrow, precise stoichiometry control using near-reducing ambient, namely, the deposition temperature higher than 450°C and O 2 partial pressure ͑P O 2 ͒ in the range between 5 ϫ 10 −4 and 1 ϫ 10 −2 Pa, enabled the deposition of a high refractive index semiconducting anatase phase. The electron concentration of the A-TiO 2 : Nb films increased with increasing Nb concentration up to Ti 0.907 Nb 0.093 O 2 . The results indicate that the Nb 5+ donor on the Ti 4+ site can be activated under near-reducing atmosphere where unwanted compensating defects may be passivated. As a result, anatase Ti 0.907 Nb 0.093 O 2 film deposited at 500°C and P O 2 =5ϫ 10 −4 Pa exhibited a resistivity of 3.4ϫ 10 −3 ⍀ cm and an optical transmittance higher than 90%. The refractive index of A-TiO 2 :Nb was found to be approximately 2.63 at 450 nm with spectroscopic ellipsometry, which is comparable to the InGaN alloys.
Crystal phase-selective epitaxy of Nb-doped TiO2 (TiO2:Nb) films on GaN is demonstrated using the helicon-wave-excited-plasma sputtering epitaxy method. On the clean (0001) GaN, rutile (100) TiO2:Nb was grown regardless of growth temperature (Tg). On the (0001) GaN covered with a monolayer-thick (100) or (001) β-Ga2O3, anatase (001) TiO2:Nb was grown with the in-plane relation <110>TiO2 ∥ <1120>GaN under high Tg and low O2 partial pressure conditions. The transmittance between 450 and 900 nm for the 200-nm-thick films was approximately 80%. Typical resistivity of the anatase TiO2:Nb/GaN heterostructure was as low as 3.6×10-4 Ω·cm, due to the interfacial conduction.
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