Tsung-Han Yang

Semiconductor-metal transition characteristics of VO2 thin films grown on c- and r-sapphire substrates

We have made a comparative study of epitaxial growth of VO2 thin films on c-cut (0001) and r-cut (11¯02) sapphire substrates, and the semiconductor to metal transition (SMT) characteristics of these films have been correlated with their structural details. On c-sapphire, VO2 grows epitaxially in (002) orientation. These (002) oriented VO2 films have 60° twin boundaries due to three equivalent in-plane orientations. The epitaxial VO2 films on r-sapphire consisted of two orientations, namely (200) and (2¯11). The coexistence of these two orientations of VO2 has been explained on the basis of similarity of atomic arrangements in (200) and (2¯11) planes. The thermal hysteresis (ΔH), sharpness of the transition (ΔT), and the transition temperature for VO2 films on c-sapphire were found to be 4.8, 8.5, and 72.6 °C, respectively, which were higher than the corresponding values of 3.3, 5.4, and 60.3 °C for films on r-sapphire. The SMT temperature for VO2 films on c-sapphire was close to the bulk value of 68.0 °C....

Role of interfacial transition layers in VO2/Al2O3 heterostructures

Epitaxial VO2 films grown by pulsed laser deposition (PLD) on c-cut sapphire substrates ((0001) Al2O3) were studied by aberration-corrected scanning transmission electron microscopy (STEM). A number of film/substrate orientation relationships were found and are discussed in the context of the semiconductor-metal transition (SMT) characteristics. A structurally and electronically modified buffer layer was revealed on the interface and was attributed to the interface free-energy minimization process of accommodating the symmetry mismatch between the substrate and the film. This interfacial transition layer is expected to affect the SMT behavior when the interfacial region is a significant fraction of the VO2 film thickness.

Defect-mediated room temperature ferromagnetism in vanadium dioxide thin films

High quality epitaxial undoped vanadium oxide (VO2) thin films on c-plane sapphire (0001) substrate have been grown using pulsed laser deposition technique. The as-grown films exhibited excellent structural and transport properties without requiring further annealing treatments for these oxygen-deficient oxide films. The epitaxial growth has been achieved via domain matching epitaxy, where matching of integral multiples of planes occurs across the film-substrate interface. The magnetic properties of vanadium oxide (VO2) films investigated at different temperatures in the range of 10–360 K showed significant magnetic hysteresis as well as saturation of the magnetic moment. The origin of ferromagnetic properties with an estimated Curie temperature above 500 K is discussed in the absence of magnetic impurities in VO2 thin films as determined by x-ray photoelectron spectroscopy, x-ray diffraction, and transmission electron microscopy.

On growth of epitaxial vanadium oxide thin film on sapphire (0001)

We report the characteristics of epitaxial growth and properties of vanadium oxide (VO 2 ) thin films on sapphire (0001) substrates. Pulsed laser deposition was used to grow (002) oriented VO 2 films on sapphire (0001). Transmission electron microscopy studies showed that the orientation relationship between the substrate and the thin film is: (002) f2 ∥(0006) sub3 and [010] f2 ∥ sub . It was also established that VO 2 has three different orientations in the film plane which are rotated by 60° from each other. The epitaxial growth of vanadium oxide on sapphire (0001) has been explained in the framework of domain matching epitaxy (DME). Electrical resistivity measurements as a function of temperature showed a sharp transition with a hysteresis width ˜5 °C, and large resistance change (˜1.5 × 10 4 ) from the semiconductor phase to the metal phase. It is interesting to note that in spite of large angle twin boundaries in these VO 2 films, the SMT characteristics are better than those observed for polycrystalline films. The higher width of thermal hysteresis for the VO 2 film on c -sapphire compared to a bulk single VO 2 crystal and a single-crystal VO 2 film on r -sapphire can be attributed to the existence of these large-angle twin grain boundaries. These findings can provide insight into the phase transformation characteristics of VO 2 , which has important applications in switching and memory devices.

Role of twin boundaries in semiconductor to metal transition characteristics of VO2 films

Epitaxial (002) VO2 films were grown on c-sapphire with Ga:ZnO and ZnO buffer layers. We investigated the influence of twin boundaries on the semiconductor-to-metal transition (SMT) characteristics of VO2, when current flows parallel and perpendicular to the twin boundaries. The structure of the twin boundary was kept the same for these two configurations. The hysteresis in SMT characteristics is considerably reduced when current flows parallel to the boundaries compared to that in the normal direction of the boundaries. We present a model to explain these observations and discuss the role of these boundaries on the SMT characteristics, and the importance of grain boundary engineering in the design of VO2 based devices.

Epitaxial VO2/Cr2O3/sapphire heterostructure for multifunctional applications

In this letter, we report integration of magnetic and ultrafast-transition properties of VO2 films with antiferromagnetic (AFM) Cr2O3 template layer in the epitaxial VO2/Cr2O3/Al2O3 heterostructure The Cr2O3 is an AFM material, which can pin the spin momentum of ferromagnetic VO2, in addition to providing epitaxial template. Thus, the magnetic properties of VO2 films grown with Cr2O3 buffer layer can be improved for multifunctional magnetic tunnel junctions and sensor applications. Electrical resistivity measurements as a function of temperature showed a sharp transition width (1.94 °C), with a small hysteresis width (5.7 °C), and large resistance change (∼3.8×104) across the semiconductor to metal transition (SMT). We discuss the correlations of the magnetic properties and SMT characteristics with epitaxial growth and formation of twins.

Effect of annealing on atomic ordering of amorphous ZrTaTiNbSi alloy

In this letter, we have reported on initial stages of atomic ordering in ZrTaTiNbSi amorphous films during annealing. The atomic ordering and structure evolution were studied in Zr17Ta16Ti19Nb22Si26 amorphous films as a function of annealing temperature in the temperature range from 473 to 1173 K. Up to annealing temperature of 1173 K, the films retained amorphous structure, but the degree of disorder is increased with the increase in temperature. The formation of Si–M covalent bonds, which contributed to the local atomic arrangement, occurred in the initial stages of ordering. The bonding reactions between Si and other metal species explain the anomalous structural changes which were observed in x-ray diffraction and transmission electron microscopy. We discuss the stages of phase transformation for amorphous films as a function of annealing temperature. From these results, we propose that annealing leads to formation of random Si–M4 tetrahedron, and two observed rings, a first and second in the electron...

Epitaxial growth and Ohmic contacts in MgxZn1−xO∕TiN∕Si(111) heterostructures

In this work, the electronic properties of Mg0.1Zn0.9O∕TiN∕Si(111) heterostructures processed using pulsed laser deposition were examined. X-ray diffraction and transmission electron microscopy studies demonstrated epitaxial growth of the titanium nitride buffer layer and the Mg0.1Zn0.9O thin film. Transmission electron microscopy demonstrated a thin (∼5nm) spinel layer along the magnesium zinc oxide/titanium nitride interface. Current-voltage measurements revealed Ohmic contact behavior through the magnesium zinc oxide/titanium nitride interface. These results suggest that the titanium nitride buffer layer in the MgxZn1−xO∕TiN∕Si(111) heterostructure provides a buffer layer for integrating magnesium zinc oxide thin films with silicon substrates, which both enable epitaxial growth and serve as an Ohmic electrode for the magnesium zinc oxide thin film.