2014 TMS RF Mehl Medal Symposium on Frontiers in Nanostructured Materials and Their Applications: Poster Session
Sponsored by: TMS Electronic, Magnetic, and Photonic Materials Division, TMS: Thin Films and Interfaces Committee
Program Organizers: Nuggehalli Ravindra, New Jersey Institute of Technology; Ramki Kalyanaraman, University of Tennessee; Haiyan Wang, Texas A & M University; Yuntian Zhu, North Carolina State University; Justin Schwartz, North Carolina State University; Amit Goyal, Oak Ridge National Laboratories

Monday 6:30 PM
February 17, 2014
Room: Sails Pavilion
Location: San Diego Convention Center

Session Chair: Sudhakar Nori, North Carolina State University; Ravindra Nuggehalli, New Jersey Institute of Technology

K20: Effect of Growth Parameters on Electrical and Optical Properties of Ga and Al Doped Transparent Conducting Zinc Oxide Thin Films: Structure-property Correlations: Namik Temizer1; Sudhakar Nori1; Jagdish Narayan1; 1North Carolina State University
    The optoelectronic properties of ZnO thin films depend highly on the deposition and growth conditions as these properties change significantly with the dopant, the oxygen flux during film deposition and substrate temperature. Here, we present a systematic study of the structure-property correlations in Ga and Al doped ZnO thin films on c-sapphire where the microstructure varies from nano-crystalline to single crystal films. We have achieved films with lowest resistivity (~110 μΩ-cm) and highest optical transmittance (~90%) characteristics. The films grown at 5x10-2 Torr of ambient PO2 and at growth temperatures ranging from room temperature to 600oC show semiconducting behavior, whereas films grown at PO2 of 1x10-3 Torr show metallic behavior. The most striking feature is that the resistivity minima occur at high temperatures ~75K. We propose that formation of Oxygen vacancy-Zinc interstitial defect complex (VO-IZn) is responsible for the significant concurrent enhancements in n-type conductivity and optical transparency.

K21: Fabrication of Single crystalline NiO based P-N junctions by KrF Laser Treatment: Structure and Photochemical Properties.: Roya Molaei1; M.Reza Bayati1; Jay Narayan1; 1NC State University
    We report the formation of NiO based single crystalline p-n junctions with enhanced photocatalytic activity induced by pulsed laser irradiation. The NiO epilayers were grown on Si(001) substrates buffered with cubic yttria-stabilized zirconia (c-YSZ) by using pulsed laser deposition. Microstructural studies conducted by X-ray diffraction (θ-2θ and φ techniques) and high resolution TEM. The p-type electrical characteristics of the pristine NiO epilayers turned to an n-type behavior and the electrical conductivity was increased by one order of magnitude after laser treatment. Photocatalytic activity of the pristine (p-NiO/c-YSZ/Si) and the laser-annealed (n-NiO/p-NiO/c-YSZ/Si) heterostructures were assessed by measuring the decomposition rate of 4-chlorophenol under UV light. The enhanced photocatalytic efficiency was attributed to the suppressed charge carrier recombination in the NiO based p-n junctions and higher electrical conductivity.

K22: Interfacial Modeling and Photochemical Properties of Rutile TiO2/Sapphire Epitaxial Heterostructures: Mohammad Reza Bayati1; Roya Molaei1; Roger Narayan1; Jay Narayan1; 1North Carolina State University
    TiO2 epitaxial thin films were integrated with on c-sapphire, m-sapphire, and r-sapphire substrates. Using XRD (θ/2θ and φ scans) technique, the epitaxial alignment at the film/substrate interfaces was established as [001](100)rutile||[10-10](0001)c-sapphire, [010](001)rutile||[0001](10-10)m-sapphire, and [010](101)rutile||[2-1-10](01-12)r-sapphire. Based on our STEM results, it was found that the rutile film initially grows pseudomorphically on sapphire as Ti2O3 and, after a few monolayers, it grows tetragonally on the Ti2O3/sapphire platform. Formation of the Ti2O3 transition layer was attributed to the symmetry and chemistry mismatch between tetragonal structure of TiO2 and hexagonal structure of alumina. In addition, the decomposition rate of 4-chlorophenol by the rutile/sapphire heterostructures under ultraviolet illumination was measured. The relative photocatalytic activity of different faces of rutile TiO2 was shown to be in the following sequence: (001) > (101) > (100). The difference in the photochemical characteristics was attributed to the atomic arrangement on different crystallographic surface planes.

K23: Inverse Spin Hall Effect Studies on ZnO Thin Films: Megan Prestgard1; Gene Siegel1; Shiang Teng1; Ashutosh Tiwari1; 1University of Utah
    In this talk we will present our work on the inverse spin Hall Effect (ISHE) in ZnO thin films. ISHE refers to the generation of an electric voltage from a spin-current. Although it is a well-studied effect, ISHE has never been investigated in ZnO. For testing ISHE in ZnO films, we designed a novel test structure consisting of a ZnO thin film channel deposited over sapphire substrate, an intermediate MgO tunnel barrier, and a top NiFe layer. The MgO tunnel barrier allows a spin-current to be injected from the NiFe to the ZnO as confirmed by Hanle measurements. A keithley nanovoltmeter was used to measure voltage in ZnO in a direction perpendicular to both the injected spin-current and the spin-direction of the injected carriers. Measurement of this voltage sheds light on the extent of ISHE and stands to establish the potential of ZnO in a wide array of spintronics applications.

K24: Microstructure and 9MeV Au+ Irradiation Effects of 9Cr-ODS(Oxide Dispersion Strengthened) Steel: Chenyang Lu1; Lumin Wang1; Zheng Lu2; 1University of Michigan; 2Northeastern University
    A kind of 9Cr-ODS(oxide dispersion strengthened) steel was produced by mechanical alloying and spark plasma sintering(SPS) at Northeastern University in China. The nominal composition of the steel is Fe-9Cr-1.5W-0.4Mn-0.1Ta-0.2V-0.3Ti-0.3Y2O3 (wt.%). Au+ ion irradiation was carried out using the 9MeV Tandem accelerator at University of Tennessee. The samples were irradiated at 673K to an ion fluence of 71016 ion/cm2. The predicted irradiation depth and displacement damage were calculated by the SRIM08 code. Focused ion beam (FIB) lift-out method was adopted for the preparation of cross-section TEM specimen by using a FEI Helios Nanolab Dualbeam. STEM analysis was conducted using a 200kV JEOL 2100F spherical aberration (Cs)-corrected Analytical Electron Microscope, including high angle annual dark field(HAADF) and bright field-STEM imaging techniques. Atom probe tomography (APT) was used to characterize the nano precipitates in the steel.

K25: Observation of the Spin Seebeck Effect in La1-xSrxMnO3 (LSMO): Gene Siegel1; Megan Prestgard1; Julia Russ2; Ashutosh Tiwari1; 1University of Utah; 2Ithaca College
    LSMO is often used in spintronics research as a spin-injection material. Most commonly, spin-injection is done via spin-polarized electric current passing into a material. In this paper we show the possibility of thermally-induced spin current by means of the spin Seebeck effect (SSE) in LSMO for use in spintronic research and devices. The SSE separates spins in a magnetic material along the direction of a thermal gradient. If a conductor is attached anywhere along the magnet, a pure spin current will flow from the magnet into the metal due to diffusion. This thermally-induced spin current can be detected using the inverse spin Hall effect (ISHE) which generates an electronic current perpendicular to the injected spin current within a strongly spin-orbit coupled material, in this case, platinum. The electronic current is easily measured using traditional electronic measurement tools, and is directly proportional to the amount of spin current that was injected.

K26: Resistance Switching Properties and Mechanism of Switching In Epitaxial Pt/ZnO/TiN Thin Film Heterojunctions Grown on Si(001) Substrate: Sandhyarani Punugupati1; Jagdish Narayan1; Frank Hunte1; 1North Carolina State University
    Zinc oxide (ZnO) is a wide band gap semiconductor with applications ranging from opto-electronics, spintronics to memristor based devices. The physical properties of ZnO films are highly sensitive to the growth parameters. We deposited epitaxial ZnO thin films on Si(001) substrate with titanium nitride (TiN) buffer layer using pulsed laser deposition (PLD) technique. To measure the electrical properties, we deposited Pt electrode on ZnO/TiN/Si by PLD using metal shadow mask technique. XRD 2θ and φ scans indicated that all the films are epitaxial in nature with fixed out of plane orientations. HRTEM images indicated that there was no reaction between various layers. We observed room temperature bi-polar resistance switching in our heterojunctions. These shown reproducible switching up to 24 cycles. We will investigate and present the mechanism of switching in the heterojunctions by imaging them using HRTEM and measuring the junction resistance as a function of temperature.

K27: Structural, Optical and Transport Properties of Room Temperature Deposited Al and Ga Doped ZnO Films: Namik Temizer1; Sudhakar Nori1; Jagdish Narayan1; 1North Carolina State University
    Here, we present interesting structural, optical, transport and magnetic properties of Al and Ga doped ZnO films deposited on sapphire and glass substrates at room temperature. The important feature of the current investigation is to obtain films grown at relatively low temperatures that are both conducting and exhibit high transparency (>80%). Low temperature processing is essential (i) to grow films with less strain due to minimized thermal misfit and (ii) for some flexible polymer-based substrates that are heat intolerant. We have found that the microstructure varies from amorphous to nano-crystalline in the above films. The films deposited at higher partial pressure of oxygen show semiconducting behavior, whereas the films grown at lower ambient oxygen partial pressures (>1x10-3 Torr) show metal-like behavior with a resistivity minimum. The formation of oxygen vacancy-Zinc interstitial defect complex (VO-IZn) could be responsible for the enhancements in n-type conductivity, optical transparency and room temperature ferromagnetism.

K28: Thin Film Epitaxy and Stress Relaxation Mechanism in Rutile/Sapphire Heterostructures: Mohammad Reza Bayati1; Roya Molaei1; Roger Narayan1; Jay Narayan1; 1North Carolina State University
    We grew TiO2 epitaxial thin films on Al2O3(0001), Al2O3(10-10), and Al2O3(01-12) substrates and studied structure and properties of the epilayers as a function of the crystallographic characteristics of the substrate. The epitaxial relationship across the film/substrate interfaces was determined to be [001](100)rutile||[10-10](0001)c-sapphire, [010](001)rutile||[0001](10-10)m-sapphire, and [010](101)rutile||[2-1-10](01-12)r-sapphire. Our HRTEM imaging and diffraction studies confirmed the established epitaxial alignments and revealed formation of atomically sharp interfaces. The origin and the relaxation mechanism of stress and strain for each heterostructure were studied in detail. It was revealed that large lattice misfit strains relax easily even if the primary slip system is not active. We also showed that even small misfit strains can relax provided that the primary slip system is active. The origin of the residual strains in the epilayers was found to be primarily due to thermal misfit and defect/impurity strains.

K29: Variable Range Hopping Conduction and Magnetic Properties of Single Crystal Semiconducting and Topological Insulator Sr3SnO: Yi-Fang Lee1; Jagdish Narayan1; Justin Schwartz1; 1North Carolina State University
    We report epitaxial growth of topological insulator candidate Sr3SnO (SSO) integrated with Si (001) using cubic yttria-stabilized zirconia (c-YSZ) as buffer. X-ray φ-scans demonstrated the heteroepitaxial relationship as (001)[100]SSO║(001)[100]c-YSZ║(001)[100]Si which was further confirmed by electron diffraction patterns. The electrical resistivities have been measured over a wide range of temperature (5-285K), suggesting semiconductor behavior. The charge transport mechanism was well described by the Variable-Range-Hopping (VRH) law. In particular, two distinct temperature behaviors of resistivity, i.e., the Mott VRH hopping law lnρ ∝ (1/T) 1/4 and the ES VRH hopping law lnρ ∝ (1/T) 1/2 are observed sequentially in appropriate temperature ranges. The hopping conduction parameters such as the characteristic temperature (T0), hopping distance (R), hopping energy (E) and density of states at Fermi level (N(EF) have been calculated. We found room temperature ferromagnetism (RTFM) in SSO, which possess a high saturated magnetization, and a finite non-zero coercivity persisting up to RT.