Thermal diffusivity of polycrystalline tin-doped indium oxide (ITO) films with a thickness of 200 nm has been characterized quantitatively by subnanosecond laser pulse irradiation and thermoreflectance measurement. ITO films sandwiched by molybdenum (Mo) films were prepared on a fused silica substrate by dc magnetron sputtering using an oxide ceramic ITO target (90 wt % In2O3 and 10 wt % SnO2). The resistivity and carrier density of the ITO films ranged from 2.9×10−4 to 3.2×10−3 Ω cm and from 1.9×1020 to 1.2×1021 cm−3, respectively. The thermal diffusivity of the ITO films was (1.5–2.2)×10−6 m2/s, depending on the electrical conductivity. The thermal conductivity carried by free electrons was estimated using the Wiedemann–Franz law. The phonon contribution to the heat transfer in ITO films with various resistivities was found to be almost constant (λph=3.95 W/m K), which was about twice that for amorphous indium zinc oxide films.

We investigated the thermal conductivity of 200-nm-thick amorphous indium–gallium–zinc-oxide (a-IGZO) films. Films with a chemical composition of In:Ga:Zn= 1:1:0.6 were prepared by dc magnetron sputtering using an IGZO ceramic target and an Ar–O2 sputtering gas. The carrier density of the films was systematically controlled from 1014 to >1019 cm-3 by varying the O2 flow ratio. Their Hall mobility was slightly higher than 10 cm2·V-1·s-1. Those films were sandwiched between 100-nm-thick Mo layers; their thermal diffusivity, measured by a pulsed light heating thermoreflectance technique, was ∼5.4×10-7 m2·s-1 and was almost independent of the carrier density. The average thermal conductivity was 1.4 W·m-1·K-1.

The shift of the Fermi level in polycrystalline aluminum doped zinc oxide (AZO) films was studied by investigating the carrier density dependence of the optical band gap and work function. The optical band gap showed a positive linear relationship with the two-thirds power of carrier density ne2/3. The work function ranged from 4.56 to 4.73 eV and showed a negative linear relationship with ne2/3. These two phenomena are well explained on the basis of Burstein-Moss effect by considering the nonparabolic nature of the conduction band, indicating that the shift of Fermi level exhibits a nonparabolic nature of the conduction band for the polycrystalline AZO film. The variation of work function with the carrier density reveals that the shift of the surface Fermi level can be tailored by the carrier density in the polycrystalline AZO films. The controllability between the work function and the carrier density in polycrystalline AZO films offers great potential advantages in the development of optoelectronic devices.

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The bombardment of various types of energetic ions during rf-superimposed dc magnetron sputter deposition was investigated in detail and their effects on the structural and electrical properties of Al-doped ZnO (AZO) films were analyzed. Aside from the expected energetic negative oxygen ions (i.e., O- and O2 -), various other negative ions (i.e., AlO-, AlO2 -, AlO3 -, ZnO-, and ZnO2 -) with a high energy were clearly observed. Such negative ions were found to be generated on the target surface and accelerated towards the substrate by the full cathode voltage. Furthermore, we found that the energy of these negative ions decreased with decreasing plasma impedance by superimposing rf power on dc sputtering. The resistivity of the AZO films deposited using the rf-superimposed dc sputtering was much lower than that of the films deposited using conventional dc sputtering. Such a decrease in resistivity should be attributed to reducing the damage of AZO films by suppressing the bombardment energies of various types of energetic negative ions impinging on a growing film surface.

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Visible-Light active photocatalytic tungsten trioxide (WO3) films were deposited at a substrate temperature of 800 degrees C by dc reactive magnetron sputtering using a W metal target. In addition, Platinum (Pt) was deposited on the WO3 film surfaces at room temperature, also by sputtering. In the early stages of Pt growth, formation of Pt nanoparticles could be expected because of the island structure observed in Volmer-Weber-type growth mode. The surface coverage of Pt on the WO3 films was estimated quantitatively by X-ray photoelectron spectroscopy and was found to be approximately 60% after 7 s deposition. High resolution electron microscopy (HREM) demonstrated that Pt nanoparticles with a diameter of about 2.5 nm were generated and dispersed uniformly on the entire surface area of the columnar polycrystalline WO3 films. These Pt-loaded films exhibited high photocatalytic activity in the decomposition of acetaldehyde (CH3CHO) under visible light irradiation.

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