Junjun Jia

Experimental observation on the Fermi level shift in polycrystalline Al-doped ZnO films

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 dev...

Thermal Conductivity of Amorphous Indium–Gallium–Zinc Oxide Thin 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 cm2V-1s-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 m2s-1 and was almost independent of the carrier density. The average thermal conductivity was 1.4 Wm-1K-1.

In situ analyses on negative ions in the indium-gallium-zinc oxide sputtering process

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.

Origin of carrier scattering in polycrystalline Al-doped ZnO films

We observed the carrier transport phenomena in polycrystalline Al-doped ZnO (AZO) films with carrier densities ranging from 2.0 × 1019 to 1.1 × 1021 cm−3. A comparison of the optical carrier density and Hall carrier density indicates that the conduction band in AZO films is nonparabolic above 2.0 × 1020 cm−3. A transition from grain boundary scattering to ionized impurity scattering is observed at a doping level of ~4.0 × 1020 cm−3. The trap density at the grain boundary increases with increasing Al concentration in the films, implying that the doping level plays a decisive role in the trap density. The excellent fitting of the optical mobility and carrier density using the Brooks–Herring model shows that the acceptor concentration increases with increasing doping level.

Temperature dependence of thermal conductivity of VO2 thin films across metal–insulator transition

Thermal conductivity of a 300-nm-thick VO2 thin film and its temperature dependence across the metal–insulator phase transition (TMIT) were studied using a pulsed light heating thermoreflectance technique. The VO2 and Mo/VO2/Mo films with a VO2 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 VO2 films were deposited by reactive rf magnetron sputtering using a V2O3 target and an Ar–O2 mixture gas at 645 K. The VO2 films consisted of single phase VO2 as confirmed by X-ray diffraction and electron beam diffraction. With increased temperature, the electrical resistivity of the VO2 film decreased abruptly from 6.3 × 10−1 to 5.3 × 10−4 Ω cm across the TMIT of around 325–340 K. The thermal conductivity of the VO2 film increased from 3.6 to 5.4 W m−1 K−1 across the TMIT. This discontinuity and temperature dependence of thermal conductivity can be explained by the phonon heat conduction and the Wiedemann–Franz law.

Direct observation of the band gap shrinkage in amorphous In2O3–ZnO thin films

We investigated the dependence of valence- and core-level photoemission spectra of amorphous In2O3–ZnO (a-IZO) films on carrier density by using hard x-ray photoemission spectroscopy (hν=8000 eV). The valence band edge distinctly shifts toward high binding energy with the increase in carrier density from 0.80 to 3.96 × 1020 cm−3, and an abrupt jump for the shift of the valence band edge from high to low binding energy occurs at a carrier density of 4.76×1020 cm−3. After considering the effect of nonparabolic bandstructure, the shifts are still less than the width of the occupied conduction band, providing direct evidence for the band gap shrinkage. Our calculation results indicate that the contribution of the band gap shrinkage increases as the carrier density increases, which accords with the observations in doped conducting crystal materials, such as Sn doped In2O3. Moreover, it is found that the conduction electrons of a-IZO films are strongly perturbed by the ionization of core levels, which leads to ...

Amorphous indium-tin-zinc oxide films deposited by magnetron sputtering with various reactive gases: Spatial distribution of thin film transistor performance

This work presents the spatial distribution of electrical characteristics of amorphous indium-tin-zinc oxide film (a-ITZO), and how they depend on the magnetron sputtering conditions using O2, H2O, and N2O as the reactive gases. Experimental results show that the electrical properties of the N2O incorporated a-ITZO film has a weak dependence on the deposition location, which cannot be explained by the bombardment effect of high energy particles, and may be attributed to the difference in the spatial distribution of both the amount and the activity of the reactive gas reaching the substrate surface. The measurement for the performance of a-ITZO thin film transistor (TFT) also suggests that the electrical performance and device uniformity of a-ITZO TFTs can be improved significantly by the N2O introduction into the deposition process, where the field mobility reach to 30.8 cm2 V–1 s–1, which is approximately two times higher than that of the amorphous indium-gallium-zinc oxide TFT.

Thermal Boundary Resistance of W/Al2O3 Interface in W/Al2O3/W Three-Layered Thin Film and Its Dependence on Morphology

We investigated the dependence of the thermal boundary resistance of the W/Al2O3 interface in W/Al2O3/W three-layered thin films on the interface morphology. The layered structures, Al2O3 thin layers with thicknesses from 1 to 50 nm covered by top and bottom W layers with a thickness of 100 nm, were fabricated by magnetron sputtering using a W target (99.99%) and an Al2O3 target (99.99%). The fabrication of polycrystalline W and amorphous Al2O3 films was confirmed by structural analysis. The morphology of the bottom W layer/Al2O3 layer and Al2O3 layer/top W layer interfaces showed a wavelike structure with a roughness of about 1 nm. Thermophysical properties and thermal boundary resistance were measured by a pulsed light heating thermoreflectance technique. The thermal boundary resistance of the W/Al2O3 interface was 1.9?10-9 m2 K W-1, which corresponds to the thermal resistance of a 3.7-nm-thick Al2O3 film or a 120-nm-thick W film.

Crystallization behavior of amorphous indium–gallium–zinc-oxide films and its effects on thin-film transistor performance

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 (Id–Vg) characteristics of the IGZO thin-film transistor (TFT) showed that the threshold voltage (Vth) and subthreshold swing decreased markedly after the post-annealing at 300 °C. The TFT using the totally crystallized films also showed the decrease in Vth, whereas the field-effect mobility decreased considerably.

Tailoring the crystal structure of TiO2 thin films from the anatase to rutile phase

TiO2 films with various Sn concentrations were deposited on quartz substrates using rf reactive magnetron sputtering. The crystal structure was investigated by using x-ray diffraction, Raman spectroscopy, and transmission electron microscopy, and the chemical states of Ti and Sn were analyzed by x-ray absorption near edge structure (XANES) spectroscopy. Without Sn doping, TiO2 films change the crystal structure from rutile to anatase as the total gas pressure increases in the sputtering deposition. On the other hand, Sn doping induces the transformation of TiO2 crystalline structure from anatase to rutile phase, where the XANES spectra implied that Sn substitutes into Ti site of rutile TiO2. Atomic force microscope analyses revealed that the Sn-doped TiO2 films exhibited a flat surface with the roughness of approximately 2 nm.