Thermal conductivity of a 300-nm-thick VO 2 thin film and its temperature dependence across the metal-insulator phase transition (T MIT ) were studied using a pulsed light heating thermoreflectance technique. The VO 2 and Mo/VO 2 /Mo films with a VO 2 thickness of 300 nm were prepared on quartz glass substrates: the former was used for the characterization of electrical properties, and the latter was used for the thermal conductivity measurement. The VO 2 films were deposited by reactive rf magnetron sputtering using a V 2 O 3 target and an Ar-O 2 mixture gas at 645 K. The VO 2 films consisted of single phase VO 2 as confirmed by X-ray diffraction and electron beam diffraction. With increased temperature, the electrical resistivity of the VO 2 film decreased abruptly from 6.3 ' 10 %1 to 5.3 ' 10 %4 Ω cm across the T MIT of around 325-340 K. The thermal conductivity of the VO 2 film increased from 3.6 to 5.4 W m %1 K %1 across the T MIT . This discontinuity and temperature dependence of thermal conductivity can be explained by the phonon heat conduction and the Wiedemann-Franz law.

No abstract

No abstract

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.

The thermal conductivities of c- and a-axis-oriented zinc oxide (ZnO) thin films with nominal thicknesses of 100, 200, and 300 nm are investigated. The c- and a-axis-oriented ZnO thin films were synthesized by radio frequency magnetron sputtering on the c- and r-plane sapphire substrates, respectively. The epitaxial relationship between the ZnO thin film and the c-plane sapphire substrate is (0001)[11¯00] || (0001)[112¯0], and that between the ZnO thin film and the r-plane sapphire substrate is (112¯0)[11¯00] || (011¯2)[112¯0]. The c-axis-oriented ZnO thin film has a columnar structure, whereas the a-axis-oriented ZnO thin film has a single domain-like structure and a significantly flat surface. The thermal conductivity of the c-axis-oriented ZnO thin film is in the range of 18–24 W m−1 K−1, whereas for the a-axis-oriented ZnO thin film, it is in the range of 24–29 W m−1 K−1. For the c-axis-oriented ZnO thin films, the phonon scattering on both the out-of-plane and in-plane grain boundaries affects the thermal conductivity. In contrast, the thermal conductivity of the a-axis-oriented ZnO thin films decreases with the decrease of the film thickness. The distribution of the normalized cumulative thermal conductivity of the a-axis-oriented ZnO thin films suggests that the heat transport carrier mostly consists of phonons with the mean free paths between 100 nm and 1 μm.

The origin of negative ions in the dc magnetron sputtering process using a ceramic indium-gallium-zinc oxide target has been investigated by in situ analyses. The observed negative ions are mainly O− with energies corresponding to the target voltage, which originates from the target and barely from the reactive gas (O2). Dissociation of ZnO−, GaO−, ZnO2−, and GaO2− radicals also contributes to the total negative ion flux. Furthermore, we find that some sputtering parameters, such as the type of sputtering gas (Ar or Kr), sputtering power, total gas pressure, and magnetic field strength at the target surface, can be used to control the energy distribution of the O− ion flux.

Amorphous indium–gallium–zinc oxide (a-IGZO) films were deposited by DC magnetron sputtering and post-annealed in air at 300–1000 °C for 1 h to investigate the crystallization behavior in detail. X-ray diffraction, electron beam diffraction, and high-resolution electron microscopy revealed that the IGZO films showed an amorphous structure after post-annealing at 300 °C. At 600 °C, the films started to crystallize from the surface with c-axis preferred orientation. At 700–1000 °C, the films totally crystallized into polycrystalline structures, wherein the grains showed c-axis preferred orientation close to the surface and random orientation inside the films. The current–gate voltage (I d–V g) characteristics of the IGZO thin-film transistor (TFT) showed that the threshold voltage (V th) and subthreshold swing decreased markedly after the post-annealing at 300 °C. The TFT using the totally crystallized films also showed the decrease in V th, whereas the field-effect mobility decreased considerably.