Shiu Jen Liu

Anomalous magnetic ordering in b-axis-oriented orthorhombic HoMnO3 thin films

Orthorhombic HoMnO3 films with well-aligned crystallographic orientations were deposited on LaAlO3(110) single crystal substrates by using pulsed laser deposition. The nearly perfect b-axis-oriented films provide the opportunity of investigating the orientation-dependent physical property of this material. The temperature dependent magnetization evidently displays an antiferromagnetic ordering near 42K, irrespective to the direction of applied field. Furthermore, the theoretically expected lock-in transition was clearly observed at around 30K when field was applied along the c axis and was undetectable along the a and b axes. The 30K transition was suppressed to 26K when the applied field increased from 100to500Oe.

Physical properties of amorphous InGaZnO4 films doped with Mn

Amorphous InGaZnO4 (a-IGZO) films doped with various concentrations of Mn have been fabricated by using pulsed-laser deposition technique. Optical, electrical, and magnetic properties of the prepared Mn-doped a-IGZO films were investigated. The resistivity, carrier concentration, and carrier mobility of the a-IGZO films were found to be, respectively, increased, decreased, and enhanced by Mn doping. Moreover, the optical transmission is slightly increased in the visible range and the optical band gaps are not affected in the Mn-doped films. Room-temperature ferromagnetism has been observed in the field-dependent magnetization measurements.

Transport properties of CrO2 (110) films grown on TiO2 buffered Si substrates by chemical vapor deposition

Epitaxial CrO2 (110)-oriented films were fabricated on Si (100) substrates buffered by rutile TiO2 derived from oxidation of a pulsed-laser-deposited TiN layer. The epitaxial films of CrO2 were prepared by chemical vapor deposition in a two-zone furnace with oxygen flow from a CrO3 precursor. The transport measurements show that the CrO2 films are metallic with a Curie temperature of about 380 K. The temperature dependence of resistivity was best described by a phenomenological expression ρ(T)=ρ0+AT2e(−Δ/T) over the range of 5–350 K with Δ=94 K. The magnetic measurements show the in-plane coercive fields are about 30 and 60 Oe at 300 and 5 K, respectively. The temperature dependent spontaneous magnetization follows Bloch’s T3/2 law and the slope suggests a critical wavelength of λΔ∼30.6 A beyond which spin-flip scattering becomes important.

Effects of postdeposition annealing on the metal–insulator transition of VO2−x thin films prepared by RF magnetron sputtering

We report the fabrication of textured VO2−x films on c-cut sapphire substrates by postdeposition annealing of V2O3 films prepared by RF magnetron sputtering using V2O3 as the target. Although the prepared VO2−x films are expected to be oxygen-deficient, overoxidation on the film surface was revealed by X-ray photoelectron spectroscopy. The metal–insulator transition (MIT) characteristics of the VO2−x films were investigated. MIT parameters including the transition temperature, transition sharpness, and hysteresis width of the VO2−x films were manipulated by varying the oxygen pressure during postdeposition annealing. The suppression of optical transmittance in the near-infrared region was observed by increasing the temperature through the MIT.

Metal–insulator transition characteristics of Mo- and Mn-doped VO2 films fabricated by magnetron cosputtering technique

Mo- and Mn-doped VO2 thin films have been grown on c-cut sapphire substrates by the magnetron co-sputtering technique. The effects of Mo and Mn doping on the structure and metal–insulator transition of the doped VO2 thin films were studied. An enlargement of the out-of-plane lattice constant of the film caused by Mo doping was observed. As expected, the transition temperature (TMI) is reduced by Mo doping. However, the valence of the Mo ions doped in the VO2 films is determined by X-ray photoelectron spectroscopy to be 6+ on the surface, but 4+ and 3+ in the bulk part of the films. The reduction in TMI observed in this study is attributed to the variation in the band structure resulting from the incorporation of Mo4+ into the VO2 lattice. The optical transmission is remarkably enhanced by low-concentration Mo doping and then monotonically decreases with increasing Mo content. On the other hand, the out-of-plane lattice constant and TMI are not affected by Mn doping. The transmission is enhanced and then monotonically increases with increasing Mn concentration. The thermochromism of doped films is suppressed by Mo and Mn doping.

Annealing Effect on the Microstructure and Optical Characterization of Zn2SiO4 Thin Film Sputtered on Quartz Glass

A thin ZnO film (approximately 500 nm in thickness) was deposited on quartz glass by sputtering a ZnO target (purity of 99.995%) using a radio frequency (RF) power source in the magnetron sputter. Annealing of the specimen at elevated temperatures (i.e., 1000, 1100, and 1200 °C) was conducted to clarify the effects of annealing on the microstructure and optical characteristics of the film. It was found that silicon ions diffused faster than zinc and oxygen ions in the system containing the oxide film and quartz glass substrate Zn2SiO4 whose thickness depended on the annealing temperature and duration. The surface and cross-sectional morphologies of the as-sputtered film and films annealed at various temperatures were examined by field emission scanning electron microscopy (FE-SEM) and in more detail by high-resolution transmission electron microscopy (HRTEM). Line scans obtained by scanning transmission electron microscopy (STEM) showed that the composition (wt %) of the film annealed at 1200 °C was Zn (70%), Si (20%), and O (10%). These findings indicated that ZnO completely transferred into the Zn2SiO4 film to act as a luminescence center in the case of postannealing at 1200 °C for 2 h. The amounts of ZnO transferred to Zn2SiO4 were 38.5% at 1000 °C and 31.1% at 1100 °C for the same duration. The specimens annealed at 1000, 1100, and 1200 °C showed no photoluminescence (PL) emission as determined by measurement.

Fabrication and Characterization of Oxide Heterojunctions Prepared by Electrodepositing Cuprous Oxide on Conductive Glasses

The properties of oxide heterojunctions prepared by electrodepositing Cu2O on commercial F-doped SnO2 (FTO) and Sn-doped In2O3 (ITO) glasses were studied. The X-ray diffraction patterns showed that Cu2O films grown on both FTO and ITO glasses are mainly (111)-oriented. The optical band gap of the electrodeposited Cu2O films grown on FTO glass determined from the absorption spectrum is about 2 eV. A peak with a wavelength of 622 nm that could be correlated to the near band edge emission was observed in the photoluminescence spectrum. Nonlinear and rectifying behaviors were observed in the current–voltage measurements of these fabricated heterojunctions. Furthermore, the high turn-on voltage of 4.7 V indicates the absence of defect states at the interface of the Cu2O/FTO heterojunction. Moreover, thermal annealing on Cu2O/FTO heterojunctions may cause Cu diffusion into the n-type layers.

Influence of Oxygen Vacancies on Transport Properties of La2/3Ca1/3MnO3-y Films Grown on SrTiO3 Substrates

We prepared La2/3Ca1/3MnO3-y films with various oxygen contents on SrTiO3 substrates and measured the temperature-dependent resistivity of the manganite films under tensile stress due to the lattice mismatch between manganites and single-crystal substrates. The effect of oxygen vacancies on the transport properties of the films was investigated. The experimental results reveal that the dependence of temperature on the resistivity of fully oxygenated films can be attributed to the adiabatic small-polaron conduction and unconventional one-magnon scattering at temperatures above and far below the insulator–metal (I–M) transition temperature, respectively. However, for highly oxygen-deficient films, the temperature-dependent resistivity can be ascribed to Motts variable-range-hopping and small polaron metallic conduction at temperatures above and far below the I–M transition temperature, respectively.

Specific heat studies on the charge and magnetic ordering in manganites

Abstract Specific heat (C) studies with a wide range of temperatures (T=80– 300 K ) on polycrystalline manganites Pr1−xCaxMnO3 (113-PCMO) with x=0.3–0.4, La1−yCayMnO3 (113-LCMO) with y=0.3 and 0.5, and La2−2zSr1+2zMn2O7 (327-LSMO) with z=0.3 and 0.5 are reported. Clear anomalies in C are identified which are associated with charge ordering (CO), antiferromagnetic (AFM) and ferromagnetic (FM) transitions. The anomalies in 113-PCMO and 113-LSMO are bigger than those of 327-LSMO. The possible explanations are discussed on the basis of dimensionality effects.

Growth and Transport Properties of (110)-Oriented La2/3Sr1/3MnO3 Thin Films on TiO2-Buffered Si (100) Substrates

The growth and transport properties of (110)-oriented La2/3Sr1/3MnO3 thin films on TiO2-buffered silicon substrates by pulsed-laser deposition are reported. An insulator-metal transition associated with a ferromagnetic transition was observed at about 360 K. Magnetic measurements showed that in-plane coercive fields are about 60 Oe and 300 Oe at 300 K and 5 K, respectively. A magnetoresistance with a Δρ/ρ(H=0) ratio of -20% in a magnetic field of 5 T was observed not only near the insulator-metal transition temperature but also below 30 K. Moreover, a T2- to T3-dependence crossover of resistivity was observed around 60 K, indicating that an unconventional one-magnon scattering may have occurred.