Session D37: SPS Undergraduate Research II

Low Temperature Resistive Switching Behavior in a Manganite

The development of new nonvolatile memory devices remains an important field of consumer electronics. A possible candidate is bipolar resistive switching, a method by which the resistance of a material changes when a voltage is applied. Although there is a great deal of research on this topic, not much has been done at low temperatures. In this work, we compare the room temperature and low temperature behaviors of switching in a manganite thin film. The data indicates that the switching is suppressed upon cooling to cryogenic temperatures, and the presence of crystalline charge traps is tied to the physical mechanism.   

Atomic Steps on Thermally Annealed Oxide Substrates

Atomically smooth perovskite oxide substrates are necessary for high quality thin-film growth. We have optimized the annealing conditions for the substrates, LaAlO$_{3}$, SrTiO$_{3}$ and NdGaO$_{3}$, which are commonly used substrates for growing thin-film perovskite oxides. The optimal annealing temperatures were between 950 $^{o}$C$^{ }$and 1050 $^{o}$C and the annealing time for each sample was varied by approximately 20{\%}. Atomic force microscopy was used to compare the surfaces of the annealed substrates with the unannealed ones. These images capture the terrace steps that occur due to the annealing process, and we have confirmed that the heights of the steps are approximately one unit cell (0.4 nm). Currently, we are investigating methods for determining the site termination of the substrates and their effects on thin-film growth.   

Field-Induced Phase Transitions in the S=1/2 Quasi-1D Antiferromagnet BaCo$_{2}$V$_{2}$O$_{8}$

We have performed magnetocaloric effect and specific heat measurements in the S=1/2 quasi-one-dimensional Ising-like antiferromagnet BaCo$_{2}$V$_{2}$O$_{8}$ in magnetic fields up to 24 T at temperatures down to 0.3 K. The magnetocaloric effect measurements were made in order to investigate the field-direction dependence of a magnetic transition near 3.7 T. The specific heat and magnetocaloric effect measurements were employed to determine the H-T phase diagram when the field is applied parallel to the $c$ axis of the sample. We have found a new antiferromagnetically ordered phase between 19.7 T and 23.1 T, most likely a spin-flop phase. At low fields, our results agree with the known N\'{e}el ordered phase boundary.   

Magnetism in Transition Element Doped In$_{2}$O$_{3}$ Dilute Magnetic Semiconductors

There is currently a tremendous research effort in the area of dilute magnetic semiconductors (DMS). It is proposed that a DMS exhibiting ferromagnetic properties at room temperature could be used in a new class of devices termed spintronics. Whereas standard electronics work on the principle of manipulation of charge properties of an electron, spintronics work on controlling electron spin. Indium oxide is a wide band gap semiconductor with unique optical and electrical properties. Defect concentrations such as transition metal doping and oxygen vacancies in In$_{2}$O$_{3}$ can tune the electrical/magnetic behavior from ferromagnetic metal-like to ferromagnetic semiconducting to paramagnetic insulating. Bulk materials of magnetic element (Fe, Co and Cr) doped In$_{2}$O$_{3 }$have been made using a standard solid state reaction method. Structural and magnetic properties have been measured using standard techniques. XRD analysis confirmed single phase In$_{2}$O$_{3 }$with no impurity phases due to addition of magnetic elements. Magnetization as a function of applied magnetic field and temperature were collected on all the samples using a SQUID magnetometer. Detailed structural and magnetic properties will be presented in this talk.   

Magnetic Properties of Ordered Nanoporous Nickel Films

Nanoporous materials have been of great interest for applications such as biosensors and energy storage. Magnetic properties and the magnetization reversal mechanism of nanoporous magnetic materials remain to be fully understood. In this work, we report the fabrication and magnetic properties of ordered nanoporous nickel (Ni) films. The fabrication involved the following steps: self-assembly of monodispersed polystyrene spheres, electrochemical deposition of desired materials, and sphere removal by a dissolution process. Scanning electron microscopy (SEM) images confirmed the highly ordered three-dimensional hexagonal closed pack structures of the Ni films. We characterized magnetic properties of the three-dimensional nanoporous Ni films using vibrating sample magnetometer (VSM). Magnetic hysteresis loops and first-order reversal curves (FORCs) were measured on the nanoporous Ni films of 200 nm pore size with different thicknesses. Analysis on hysteresis loops and FORC distributions shed light on the reversal mechanism of magnetization and magnetostatic interactions of ordered three-dimensional porous structures.   

Electrical characterization of CVD graphene

Graphene is a one atom thick carbon sheet that can be obtained via exfoliation of graphite or via chemical vapor deposition (CVD). By using a very simple shadow masking technique, gold contact pads were evaporated over the graphene thereby eliminating chemical etching that is required when using photolithography and often leads to sample contamination. CVD graphene was electrically characterized in a FET configuration under different experimental conditions that include UV exposure, gas sensing and temperature. Our measurements yielded a carrier mobility of up to 3000 cm$^{2}$/V-s for some devices. Exposure to UV dopes graphene in a controlled manner. The doping level could be maintained indefinitely in vacuum or could be completely reversed by slight heating in air without loss of device performance. The FET's were also tested at different temperatures with little change in the transconductance response. Exposure to ammonia gas $n$-doped graphene while exposure to NO$_{2}$ $p$-doped it.   

Electronic Structure Properties of Graphene/Boron Nitride Layered Systems

We explore the properties of systems composed of two or three layers of graphene and hexagonal boron nitride (hBN) using the Vienna Ab-Initio Simulation Package (VASP), a software package for performing first principles simulations based on density functional theory (DFT). Particular attention is given to the contribution of inter-layer dispersion interactions, which are modeled within VASP by the ``DFT-D2'' method of Grimme. We obtain the binding and van der Waals energies, and inter-layer separations for the most stable stacking configurations of each of the following systems: hBN/graphene, graphene/hBN/graphene, hBN/graphene/hBN, hBN/hBN/graphene, and graphene/graphene/hBN. We observe that the addition of hBN layers to graphene structures induces a band gap, ranging from 0.024 eV, for the graphene/hBN/graphene arrangement, to 0.16 eV, for the hBN/graphene/hBN arrangement. These results, specifically band gaps on the same order as those of silicon and germanium, indicate that graphene/hBN layered structures may have applications in electronics.   

Magnetoresistance of Strained Rare Earth Manganese Oxide Thin Films: Effect of Post-Deposition Thermal Treatment

We have studied the effects of thermal treatment (annealing) on the properties of thin films of manganese oxide materials La$_{0.67}$Ca $_{0.33}$ MnO$_{3 }$and La$_{0.67}$ Ba$_{.0.33}$MnO$_{3}$, which are known to exhibit colossal magnetoresistance (CMR) and an insulator to metal transition . These materials$_{ }$show properties which are desirable for applications such as magnetic sensors, bolometric infrared sensors, or field effect devices. Two properties of interest, magnetoresistance (MR) and insulator metal transition are especially sensitive to strain because of the relationship between the properties and the symmetry of the crystal. The properties are also sensitive to the oxygen stoichiometry of the film due to changes in the valence state of the manganese which determines the charge carrier density. Recent results from our laboratory have shown a large magnetoresistance in strained films grown on lattice mismatched substrates. The MR continues to increase with decreasing temperature in these films. In order to understand the contribution of strain to the observed MR behavior, in we employ `thermal annealing' to allow for relaxation of strain. Annealing also serves to achieve the optimum oxygen stoichiometry. We will present the results of our annealing studies and their implications for understanding the MR in strained films.   

Grain Boundaries In Thin Film Organic Semiconductors

We utilize conductive atomic force microscopy (C-AFM) and tunneling atomic force microscopy (TUNA) to characterize dynamics of electronic transport across fluorinated triethylsilylethynyl anthradithiophene (diF-TES ADT) grain boundaries. The crystallization of diF-TES ADT grown on SiO$_{2}$ will be discussed and related to comparable molecules. The resulting voltage drop between individual crystals as a function of dopants will be discussed in terms of charge transport models and compared to current device work.   

Formation of Interfacial Carbide Layers in Multilayer Ti/DLC Thin Films

Titanium (Ti)/Diamond-like-carbon (DLC) and Chromium (Cr)/Carbon (C) multilayer films were prepared on c-axis oriented single crystal sapphire (Al$_{2}$O$_{3})$ substrates using magnetron sputtering. Interfacial properties of the films were analyzed using x-ray reflectivity and scanning electron microscopy. When DLC is sputtered on a layer of Ti, an interfacial layer of titanium carbide (TiC) forms which is reported for the first time. Energy provided by the substrate bias necessary to facilitate DLC sp3 bond formation is suspected of allowing TiC to synthesize in a thin layer before DLC forms. It was also found that DLC has difficulty forming on Cr. These results are relevant to biomedical applications where DLC is applied as a low friction/wear film that can be formed on the surface of implants composed mainly of titanium. Further investigation into the medical and tribological effects of TiC interfacial layers is suggested.   

Properties of Cr$_{2}$AlC MAX phase thin films prepared by reactive magnetron sputtering

M$_{n+ 1}$AX$_{n}$ (MAX) phases, where $n$ is 1, 2, and 3, M is an early transition metal, A is an A-group element, and X is either C or N, are ternary carbides with unique properties such as low density, easy machinability, and good oxidation resistance. The MAX phase Cr$_{2}$AlC is of particular interest for industrial applications to its excellent high-temperature oxidation resistance and relatively low synthesis temperature. We prepared Cr$_{2}$AlC thin films on c-axis oriented single crystal Al$_{2}$O$_{3}$, glassy carbon and Si thermal oxide substrates using reactive magnetron sputtering as precursor materials for carbide-derived carbon (CDC) films for ``on-chip'' supercapacitors. Film deposition was optimized using elemental composition data obtained by WDXRF. Optimized films were characterized using XRD and scanning electron microscopy. It was found that textured Cr$_{2}$AlC films only form when the composition was Al-rich allowing the formation of a Cr$_{5}$Al$_{8}$ interfacial layer. As film composition was optimized, the interfacial layer did not form but the XRD peaks associated with the Cr$_{2}$AlC also decreased in magnitude. Extremely high-textured films were grown when a thin buffer layer of CrAl$_{2}$ was deposited on the substrate before depositing the Cr$_{2}$AlC films. This result suggests that Cr$_{2}$AlC films may not be ideal for CDC applications since the films may ``lift-off'' during conversion due to the existence of the naturally occurring buffer-layer.   

Fluid-like conducting regions in solid manganite thin films

Hole-doped manganese oxides (manganites) exhibit a fluid-phase separated state in which the conducting and ferromagnetic phase (FMM) behaves like a fluid embedded in an insulating background phase [1]. While the manganite remains a solid, its local electrical properties behave as a fluid. The fluid behavior of the conducting regions can be tested by observing the effect of an electric field on the shape of the FMM regions. This change in shape is expected to result in anisotropic resistivity. This property was tested by designing microstructures of the manganite film, which simplify the experiment by localizing the electric field to a region the size of the FMM regions. We are testing for possible magnetic anisotropy since the conducting regions are also ferromagnetic. We will also discuss the size effect on the magnetic properties. ~ [1] Dhakal et. al., Phys. Rev. B 75, 092404 (2007)   

The intrinsic coupling of polarization with terahertz pulses in a ferroelectric nanowire

We use first-principles-based molecular dynamics simulations to study the interaction of a terahertz (THz) radiation with polarization in a ferroelectric ultrathin nanowire made of a lead zirconate titanate alloy. In our computational experiment, a 12 nm thick nanowire is first annealed down to temperature of 300 K and then subjected to a wide variety of THz pulses which differ in width, strength and frequency. Such nanowire develops an electrical polarization along the nanowire axial direction which couples strongly with incoming THz radiation. The atomistic resolution of our computational experiments allows us to trace the intrinsic polarization response and energy propagation/dissipation mechanisms that occur at the scale of femtoseconds. Our simulations were carried out under MVE and MVT conditions and the results did not vary significantly between the two ensembles. We further explore how the unique features of such response could be utilized in an ultrafast THz nanoswitches.   

Magnetic Element Doped Transparent Conducting In$_{2}$O$_{3 }$Thin Films for Spintronic Applications

Tremendous research efforts are underway to exploit the property of electron spin in spintronics. Spintronic devices critically depend on the availability of a specific materials system for spin injection, manipulation and detection. Transition metal (Cr, Fe, or Co) doped wide band gap oxide semiconductors possess these properties. ~Indium oxide (In$_{2}$O$_{3})$ is a wide band gap semiconductor with unique optical and electrical properties. Here, we investigate the effect of Cr, Fe, or Co doping on electrical and optical properties of In$_{2}$O$_{3}$ thin films. Thin films have been grown on sapphire and quartz substrates using pulse laser deposition method. Electrical and optical characteristics have been measured using UV-VIS spectroscopy and magneto-transport techniques. Optical transmittance and electrical parameters such as carrier concentration and carrier mobility vary with growth parameters such as growth temperature of the substrate and oxygen pressure of the chamber. These details will be discussed during this presentation.