2009. "Response of Strontium Titanate to Ion and Electron Irradiation." Journal of Nuclear Materials 389(2):303-310. doi:10.1016/j.jnucmat.2009.02.014 Abstract Response of strontium titanate (SrTiO3) to ion and electron irradiation is studied at room temperature. For an accurate energy to depth conversion and a better determination of ion-induced disorder profile from Rutherford backscattering spectrometry measurement, a detailed iterative procedure is described and applied to ion channeling spectra to determine the dechanneling yield and the disorder profiles for the Sr and Ti sublattices. The result shows a large underestimation in disorder depth, ~ 40% at the damage peak, which indicates a large overestimation of the electronic stopping power for 1.0 MeV Au ions in SrTiO3 predicted by the SRIM (Stopping and Range of Ions in Matter) code. Overestimation of heavy ion stopping power may lead to an overestimation of the critical dose for amorphization. The current study also demonstrates possible ionization effects in SrTiO3 under ion and electron irradiation. Pre-amorphized SrTiO3 exhibits strong ionization-induced epitaxial recovery at the amorphous/crystalline interface under electron irradiation.
2009. "An Invisible Bend Sensor Based on Porous Crosslinked Polyelectrolyte Film." Sensors and Actuators. A, Physical 151(2):154-158. doi:10.1016/j.sna.2009.02.034 Abstract This paper describes the fabrication of a porous cross-linked polyelectrolyte membrane and the characterization of its humidity sensitivity performance. Electrostatic self-assembly, combined with acid treatment, and post-deposition annealing produced the membrane. The fabrication process offers the ability to control the thickness of the membrane, as well as enabling the engineering of the humidity sensitivity properties. A transparent humidity sensor was fabricated by integrating the membrane into a capacitive structure. In order to improve the moisture absorption and diffusion, both the polyelectrolyte layer and the electrode were made porous. The membrane was cross-linked to enhance the durability in high humid environments. Such a polyelectrolyte membrane showed high sensitivity to relative humidity variation over a range of 25-99%. The see-through property of the structure adds extra features and benefits to the sensor.
2009. "Self-assembled TiO2-Graphene Hybrid Nanostructures for Enhanced Li-ion Insertion ." ACS Nano 3(4):907-914. Abstract We used anionic sulfate surfactants to assist the stabilization of graphene in aqueous solutions and facilitate the self-assembly of in-situ grown nanocrystalline TiO2, rutile and anatase, with graphene. These nanostructured TiO2-graphene hybrid materials were used for investigation of Li-ion insertion properties. The hybrid materials showed significantly enhanced Li-ion insertion/extraction in TiO2. The specific capacity was more than doubled at high charge rates, as compared with the pure TiO2 phase. The improved capacity at high charge-discharge rate may be attributed to increased electrode conductivity in presence of a percolated graphene network embedded into the metal oxide electrodes.
2009. "Microstructure and ionic-conductivity of alternating-multilayer structured Gd-doped ceria and zirconia thin films." Journal of Materials Science 44(8):2021-2026. Abstract Multilayer thin-film of consisting of alternating Gd-doped ceria and zirconia have been grown by sputter-deposition on -Al2O3 (0001) substrates. The films were characterized using x-ray diffraction (XRD), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The Gd-doped ceria and zirconia layers had the fluorite structure and are highly textured such that the (111) plane of the films parallel to the (0001) plane of the -Al2O3. The epitaxial relationship can be written as (111)ZrO2/CeO2//(0001)Al2O3 and [11-2]ZrO2/CeO2//[-2110]Al2O3.. The absence of Ce3+ features in the XPS spectra indicates that the Gd-doped ceria films are completely oxidized. The ionic conductivity of this structure shows great improvement as compared with that of the bulk crystalline material. This research provides insight on designing of material for low-temperature electrolyte applications.
2009. "Crystal and Electronic Structure of Lithiated Nanosized RutileTiO2 by Electron Diffraction and Electron Energy-loss Spectroscopy." Applied Physics Letters 94(23):Art. No.: 233116. doi:10.1063/1.3152783 Abstract The electronic structure of the nanosized rutile TiO2 before and after mechanical lithiation were studied using TEM and EELS. EELS reveals the Li K-edge at the energy-loss position of ~ 61 eV. After lithiation, the separation of the t2g-eg crystal-field splitting on both Ti L2,3-edge and O K-edge decreases, the O K-edge shifts towards a higher energy-loss position and the separation between the pre-edge peak and main peak on the O K-edge decreases. These results suggest that the lithiation of rutile TiO2 was accompanied by the reduction of Ti ion and a charge transfer from Li to Ti.
2009. "Microstructure and Secondary Phase Segregation Correlation in Epitaxial/Oriented ZnO Films with Unfavorable Cr Dopant." Journal of Materials Research 24(2):506-515. doi:DOI: 10.1557/JMR.2009.0054 Abstract We discuss the effect of microstructure on the secondary phase segregation region and mobility of carbon impurities in case of poorly soluble Cr as a dopant in ZnO thin films. Thin films of Cr:ZnO ~50 nm in thickness were grown by metal organic chemical vapor deposition (MOCVD) of Zn(TMHD) and Cr(TMHD) precursors in reactive oxygen partial pressure environment. For an accurate comparison among the differences among the grain-boundary density and degree of orientation on the secondary phase segregation and impurity mobility, simultaneous thin film growths were carried out on single crystals of Si (100), c-plane oriented Al2O3 (c-ALO) and r-plane oriented Al2O3 (r-ALO) substrates. High-resolution transmission electron microscopy (HRTEM) measurements across the film substrate interface indicate that growths on Si(100) and c-ALO resulted in highly oriented Cr:ZnO films whereas a good epitaxial growth was observed on r-ALO. The trace carbon impurity detection, secondary phase formation and their mobility properties were studied by sensitive x-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS). We have observed that secondary phase segregation regions occur near the surface for Cr:ZnO films grown on ALO whereas the region moves near the interface for the growth on Si. Considering the presence of grain boundaries in Cr:ZnO grown on c-ALO and Si, it appears to be a weak relationship between grain boundary density and unfavorable dopant mobility as well as preferred segregation region. A near uniform stress distribution observed at r-ALO/ZnO interface indicates good epitaxial growth by domain matching epitaxy process. We also observe that low carbon impurity distribution in the studied thickness regime remains more or less uniform inside Cr:ZnO. This gives strong evidence that trace amount of carbon is soluble in the Cr:ZnO system as a direct result of oxygen vacancy defects.
2009. "Suppression of conductivity in Mn-Doped ZnO Thin Films." Journal of Applied Physics 105(1):013715. doi:10.1063/1.3063730 Abstract We studied the dopant concentration distribution and conductivity in ZnO:Mn films grown by metalorganic chemical vapor deposition (MOCVD). The ion beam, surface and microstructural properties of undoped ZnO films were compared with Mn-doped ZnO films. Suppression of ZnO conductivity was noticed up to ~ 4.5 atom% Mn doping. The presence of Mn2+, confirmed by X-ray photoelectron spectroscopy (XPS), is correlated with the reduction in conductivity. No major change in the activation energy (~40 meV) and a reduction in the Zn/O ratio as a function of Mn concentration in highly sensitive proton induced X-ray emission (PIXE) technique also support this hypothesis. We discuss our results from a view point of homogeneous Mn distribution, elemental XPS ratio offsets and secondary phase formations in ZnO films.
2009. "Influence of samaria doping on the resistance of ceria thin films and its implications to the planar oxygen sensing devices." Sensors and Actuators. B, Chemical 139(2):380-386. doi:10.1016/j.snb.2009.03.021 Abstract In order to evaluate and analyze the effect of samarium (Sm) doping on the resistance of cerium oxide, we have grown highly oriented samaria doped ceria (SDC) thin films on sapphire, Al2O3 (0001) substrates by using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE). The film growth was monitored using reflection high-energy electron diffraction (RHEED) which shows two-dimensional growth throughout the deposition. Following growth, the thin films were characterized by X-ray photoelectron spectroscopy (XPS), high-resolution X-ray diffraction (HRXRD), and Rutherford backscattering spectrometry (RBS). XPS depth-profile shows Sm atoms are uniformly distributed in ceria lattice throughout the bulk of the film. The valence states of Ce and Sm in doped thin films are found to be Ce4+ and Sm3+, respectively. HRXRD shows the samaria doped ceria films on Al2O3(0001) exhibit (111) preferred orientation. Ion-channeling in RBS measurements confirms high quality of the thin films. The resistance of the samaria doped ceria films, obtained by two probe measurement capability under various oxygen pressure (1mTorr-100Torr) and temperatures (623K to 973K), is significantly lower than that of pure ceria under same conditions. The 6Sm% doped ceria film is the optimum composition for highest conductivity. This is attributed to the increased oxygen vacant sites in fluorite crystal structure of the epitaxial thin films which facilitate faster oxygen diffusion through hopping process.
2009. "New Approaches for Characterizing Sensor and Other Modern Complex Materials." ECS Transactions 19(6):137-148. doi:10.1149/1.3118546 Abstract Advances in understanding of sensor and other modern complex materials are often enabled by new research tools. This paper highlights three capability development themes used to identify new research tools to be provided to users of the U. S. Department of Energy’s Environmental Molecular Sciences Laboratory. These capability development directions address the importance of dynamic measurements in realistic environments, the need for increased resolution in three dimensional analyses as well as the importance of linking theory and experiment. Capability development involves expanding the range of operation for a number of important techniques, developing and applying new capabilities, and advancing methods of data processing. Examples of current developments are provided including those related to magnetic resonance, x-ray diffraction, application of a focused beam capability to fuel cell aging, and near real time analysis of XPS spectra.
2008. "Fabrication of Transparent Capacitive Structure by Self-Assembled Thin Films." Journal of Nanoscience and Nanotechnology 8(6):3008-3012. doi:10.1166/jnn.2008.075 Abstract An approach to fabricating transparent electronic devices by using nanomaterial and nanofabrication is presented in this paper. A see-through capacitor is constructed from selfassembled silica nanoparticle layers that are stacked on the transparent substrate. The electrodes are made of indium tin oxide. Unlike the traditional processes used to fabricate such devices, the self-assembly approach enables one to synthesize the thin film layers at lower temperature and cost, and with a broader availability of nanomaterials. The vertical dimension of the selfassembled thin films can be precisely controlled, as well as the molecular order in the thin film layers. The shape of the capacitor is generated by planar micropatterning. The quartz crystal demonstrates the steady growth of the silica nanoparticle multilayer. In addition, because the nanomaterial synthesis and the device fabrication steps are separate, the device is not affected by the harsh conditions required for the material synthesis. A clear pattern is allowed over a large area on the substrate. The prepared capacitive structure has an optical transparency higher than 92% over the visible spectrum. The capacitive impedance is measured at different frequencies and fit the theoretical results. As one of the fundamental components, this type of capacitive structure can serve in the transparent circuits, interactive media and sensors, as well as being applicable to other transparent devices.
2008. "Growth and structure of epitaxial Ce0.8Sm0.2O1.9 by oxygen-plasma-assisted molecular beam epitaxy." Journal of Crystal Growth 310(2008):2450-2456. Abstract The epitaxial growth of Ce0.8Sm0.2O1.9 films on sapphire (0001) substrate by oxygen-plasma-assisted MBE has been characterized using RHEED, XPS, XRD, AFM, HRTEM and RBS in order to determine their structure and compositions. The composition of the films was determined to be Ce: Sm: O of 0.8:0.2:1.9 by RBS. The film/substrate epitaxial relationship can be written as CeO2 (111)// -Al2O3 (0001) and CeO2 [110]// -Al2O3 . The Ce has only 4+ oxidation state in the films and Sm is fully oxidized in the films with formal oxidation of 3+. CeO2 (111) face is preferred orientation and the thin films are cubic phases.
2008. "Conductivity of Oriented Samaria-Doped Ceria Thin Films Grown by Oxygen-plasma-assisted Molecular Beam Epitaxy." Electrochemical and Solid-State Letters 11(5):B76-B78. doi:10.1149/1.2890122 Abstract We have used oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) to grow highly oriented Ce1-xSmxO2-δ films on single crystal c-Al2O3. The samarium concentration, x, was varied in the range 1-33 atom%. It was observed that dominant (111) orientation in Ce1-xSmxO2-δ films can be maintained up to about 10 samarium atom% concentration. Films higher than 10 atom% Sm concentration started to show polycrystalline features. The highest conductivity of 0.04 S.cm-1, at 600 0C, was observed for films with ~ 5 atom% Sm concentration. A loss of orientation, triggering an enhanced grain boundary scattering, appears to be responsible for the decrease in conductivity at higher dopant concentrations.
2008. "Tilted domain growth of metalorganic chemical vapor (MOCVD)-grown ZnO(0001)on a-Al2O3(0001)." Journal of Materials Research 23(1):13-17. doi:10.1557/JMR.2008.0026 Abstract ZnO grown on -Al2O3 (0001) generally possesses an orientation such that -Al2O3 (0001)//ZnO(0001) and two in-plane domains nucleate such that: -Al2O3 [11-20]//ZnO[11-20] and/or -Al2O3 [11-20]//ZnO[10-10]. In this paper, we report a new growth mode for ZnO grown on -Al2O3 (0001) using MOCVD. We find that -Al2O3 [11-20]//ZnO[10-10] but the (0001) plane of ZnO is tilted relative to the (0001) plane of -Al2O3 such that ZnO(0001) is almost parallel to the -Al2O3 (-1104) plane. This orientation reduces the extent of lattice mismatch. The interface between ZnO and -Al2O3 is abrupt and possesses periodic dislocations.
2008. "Microstructures of ZnO films deposited on (0001) and r-cut α-Al2O3 using metal organic chemical vapor deposition." Thin Solid Films 516(23):8337-8342. doi:10.1016/j.tsf.2008.04.001 Abstract Zinc oxide films were deposited on (0001) and r-cut α-Al2O3 under identical conditions using metal organic chemical vapor deposition. Microstructures of the ZnO films were studied in detail using conventional and high-resolution transmission electron microscopy (HRTEM), electron diffraction, and HRTEM image simulations. The films deposited on these two substrates show distinctive structural differences. The film grown on r-cut α-Al2O3 shows a high quality single crystal with an orientation relationship of α-Al2O3[-101-1]//ZnO[0001] and α-Al2O3(10-1-2)//ZnO(2-1-10). The interface between the film and the substrate was abrupt and decorated with high density of misfit dislocations. Film grown on α-Al2O3 (0001) shows several orientation domains. Typically, one domain correspond to the classic growth model such that α-Al2O3 (0001)//ZnO(0001) and α-Al2O3 [11-20]//ZnO[10-10]. Another domain corresponds to the growth mode such that α-Al2O3 [11-20]//ZnO[10-10] but the (0001) plane of ZnO is tilted relative to the (0001) plane of α-Al2O3 such that ZnO(0001) is almost parallel to the α-Al2O3 (-1104) plane. This orientation reduces the extent of lattice mismatch as compared with the classic growth mode. The interface between ZnO and α-Al2O3 is abrupt and possesses periodic dislocations.
2008. "Surface & Interface Properties of 10-12 Unit Cells Thick Sputter Deposited Epitaxial CeO2 Films." Research Letters in Materials Science 2008:Article ID 206019. Abstract Ultra-thin and continuous epitaxial films with relaxed lattice strain can potentially maintain more of its bulk physical and chemical properties and are useful as buffer layers. We study surface, interface and micro-structural properties of ultra-thin (~10-12 unit cells thick) epitaxial ceria films grown on single crystal YSZ substrates. The out-of -plane and in-plane lattice parameters indicate relaxation in the continuous film due to misfit dislocations seen by high-resolution transmission electron microscopy (HRTEM) and substrate roughness of ~1-2 unit cells, confirmed by atomic force microscopy and HRTEM. A combination of secondary sputtering, substrate roughness and surface reduction creating secondary phase were likely causes of surface roughness which should be reduced to a minimum level for effective use of it as buffer layers.
2008. "Growth and characterization of highly oriented gadolinia-doped ceria (111) thin films on zirconia (111)/sapphire (0001) substrates." Thin Solid Films 516(18):6088-6094. doi:10.1016/j.tsf.2007.11.007 Abstract Highly-oriented pure and gadolinia-doped ceria thin films have been grown on pure and ZrO2 (111)-buffered Al2O3 (0001) substrates using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) to understand the oxygen ionic transport processes in ceria based oxide thin films. Gadolinia-doped ceria films grown on pure Al2O3(0001) substrate show polycrystalline features due to structural deformations resulting from the large lattice mismatch between the Al2O3(0001) substrate and the films. However, the films, grown on a thin layer of ZrO2(111) buffered Al2O3 (0001) substrate, appears to be highly oriented. These films were characterized using high resolution transmission electron microscopy (HRTEM) and x-ray photoelectron spectroscopy (XPS) depth profiling. Oxygen ionic conductivity in gadolinia-doped ceria films was measured as a function of Gd concentration and these results were compared with the ion conductance data of the polycrystalline and single crystalline yttria-stabilized zirconia (YSZ).
2007. "Transparent thin-film transistor with self-assembled nanocrystals." Nanotechnology 18(19):Art. No. 195204. doi:10.1088/0957-4484/18/19/195204 Abstract The paper presents the fabrication of transparent devices using nanofabrication and nanomaterials. Clear and functional structures can be constructed over a large area with highly transparent nanocrystals via layer-by-layer self-assembly. The 12 nm SnO2 and SiO2 nanocrystals can form ultra-thin and uniform multilayers, serving as the active and insulating layers, respectively. Transistors fabricated by this method have a high optical transparency and reliable electrical performance. The averaged optical transmittance is higher than 85%. The usage of nanofabrication and nanomaterials solves some of the current fabricating problems for transparent devices. This technique is applicable to other transistor devices and circuits.
2007. "Synthesis and Characterization of Compositionally Graded Si1-xGex Layers on Si substrate." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 261(1-2):723-726. Abstract Thin film of silicon germanium (Si1-xGex) with tailored composition was grown on Si (100) substrate at 650oC in an ultrahigh vacuum molecular beam epitaxy system. The nominal x-value is ranged from 0 to 0.14. The quality of the film was investigated by Rutherford backscattering spectrometry (RBS) in random and channeling geometries, glancing angle x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDX), and atomic force microscopy (AFM). RBS/Channeling measurements indicate that the strain associated with lattice mismatch is compressive in the film. Both RBS and EDX analyses indicate the compositional graded incorporation of Ge in the film with x ranging from 0 to 0.14. The film shows island growth with each island centering around an interface dislocation.
2007. "Nucleation and Growth of MOCVD Grown (Cr, Zn)O Films – Uniform Doping vs. Secondary Phase Formation." Journal of the Electrochemical Society 154(3):D134-D138. doi:10.1149/1.2424422 Abstract We report a detailed study of chromium solubility and secondary phase formation in MOCVD grown (Cr, Zn)O-based films on silicon (100). Simultaneous deposition of 0.15M Cr(TMHD) and 0.025M Zn(TMHD) based precursors in an oxidizing environment with a flow ratio of 1:10 resulted in secondary phase formation rather than uniform Cr doping. Based on several surface and micro-structural techniques, we have identified nano-crystalline ZnCr2O4 and disordered Cr2O3 as the secondary Cr-containing phases that nucleate. Analysis suggests that ZnCr2O4 crystallites are dispersed throughout the film and that disordered Cr2O3 layer may form at the interface. These results reveal a strong tendency for Cr to exist in octahedral, rather than tetrahedral coordination.
2007. "Metalorganic chemical vapor deposition of carbon-free ZnO using the bis(2,2,6,6 tetramethyl-3,5-heptanedionato)zinc precursor." Journal of Materials Research 22(5):1230-1234. doi:10.1557/JMR.2007.0146 Abstract We report the growth of c-axis oriented ZnO films on silicon (100) single crystal substrates by MOCVD. A relatively uninvestigated precursor, Zn(TMHD)2, was used in a cold-wall MOCVD reactor. XPS and RBS analysis yielded a zinc-to-oxygen atom percent ratio of 0.98 and 1.00 at the surface and in the overall film, respectively indicative of stoichometric ZnO. Due to the presence of carbon at the surface, the excess oxygen at the surface was in the form of C-O bonding. The c-axis orientation was confirmed by HRTEM and XRD. We look at these results from a viewpoint of an ongoing effort to ensure cleaner decompositions using Zn(TMHD)2.
2007. "Fabrication of SiO2 Microdisk Arrays for Optics and Light Trapping Experiments." Microelectronic Engineering 84(12):2799-2803. doi:10.1016/j.mee.2007.02.001 Abstract We present a simple silicon based microfabrication process that produces an array of SiO2 microdisks using UV lithography. High-resolution SEM images of these structures indicate a smooth outer microdisk cavity surface. Photoemission measurements were performed at different spots on the microdisk and compared with measurements inside the cavity. A silicon to oxygen atomic concentration ratio of 1:2 obtained during depth profiling confirms that the entire microdisk is made up of stoichometric SiO2. In contrast, the inner cavity is mostly silicon with native oxide on top. We discuss the usefulness of SiO2 microdisks in optics for light trapping experiments.
2007. "Differential etching of chalcogenides for infrared photonic waveguide structures." Journal of Non-crystalline Solids 354(10-11):813-816. doi:10.1016/j.jnoncrysol.2007.08.061 Abstract Chemical etching rates for two different chalcogenide glass compositions (As2S3 and As24S38Se38) were studied using sodium hydroxide based etchant solutions. Etching was performed using a variation of standard photolithographic masking and wet-etching techniques. Variations in etch rate with NaOH concentration and glass composition were observed. The depth of etch was characterized using an optical profilometer. Etch rate differences as large as three orders of magnitude between these two glasses were observed at low NaOH concentration (0.053 M). We present a single variable etch rate curve of etch depth per time (nm/s) versus NaOH overall solution concentration (in M) for these two different chalcogenide glasses (As2S3 and As24S38Se38). This technology shows promise for fabricating highly asymmetrical photonic structures and has potential applications in fabricating novel photonic bandgap (PBG) structures that will function in the long-wave infrared (LWIR) regime.
2007. "Adsorption of iso-/n-butane on an Anatase Thin Film: A Molecular Beam Scattering and TDS Study." Catalysis Letters 116(1-2):9-14. doi:10.1007/s10562-007-9121-x Abstract Binding energies and adsorption probabilities have been determined for n/iso-butane adsorption on an anatase thin film grown on SrTiO3(001) by means of thermal desorption spectroscopy (TDS) and molecular beam scattering. The sample has been characterized by x-ray diffraction (XRD) and Auger electrons spectroscopy (AES).
2006. "Study of Hydrogen Stability in Low-k Dielectric Films by Ion Beam Techniques." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 249(1-2):335–338. doi:10.1016/j.nimb.2006.04.022 Abstract With shrinking device geometries into the 0.13 µm technology node, a transition to low-k dielectrics becomes increasingly attractive. Negative bias temperature instability, which is associated with hydrogen migration at elevated temperatures, becomes the main degradation mechanism of concern for conductivity breakdown in semiconductor devices. The possibility of hydrogen release during each of the fabrication process is, therefore, of great interest to the understanding of device reliability. In the current study, various low-k dielectric films were subjected to thermal annealing in N2 ambient at temperatures that are generally used for device fabrication. Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA) were used to investigate composition change and hydrogen redistribution of the dielectric films. The results indicate that organosilicate glass, silicon nitride and silicon oxynitride films were stable at temperatures up to 500 °C. In phosphorus doped silicon glass and plasma-enhanced tetraethylorthosilicate films, significant hydrogen release from the surface region was evident after heat treatment in N2 purged environment at 300 °C for 30 min, further hydrogen release is observed as temperature increases.
2006. "Distribution of Oxygen Vacancies and Gadolinium Dopants in ZrO2-CeO2 Multi-Layer Films Grown on α-Al2O3." Solid State Ionics 177(15-16):1299-1306. doi:10.1016/j.ssi.2006.05.036 Abstract Gd-doped ZrO2 and CeO2 multi-layer films were deposited on α-Al2O3 (0001) using oxygen plasma assisted molecular beam epitaxy. Oxygen vacancies and Gd dopant distributions were investigated in these multi-layer films using x-ray diffraction (XRD), conventional and high-resolution transmission electron microscopy (HRTEM), annular dark-filed imaging in scanning transmission electron microscopy (STEM), energy dispersive x-ray spectroscopy (EDS) elemental mapping and x-ray photoelectron spectroscopy (XPS) depth profiling. EDS and XPS reveal that Gd concentration in the ZrO2 layer is lower than that in the CeO2 layer. As a result, higher oxygen vacancy concentration exists in CeO2 layers compared to that in ZrO2 layers. In addition, Gd is found to segregate only at the interfaces formed during the deposition of CeO2 layers on ZrO2 layers. On the other hand, the interfaces formed during the deposition of ZrO2 layers on CeO2 layers did not show any Gd segregation. The Gd segregation behavior at the every other interface is believed to be associated with the low solubility of Gd in ZrO2.
2006. "Correlation among Channeling, Morphological and Micro-structural Properties in Epitaxial CeO2 Films." Electrochemical and Solid-State Letters 9(5):J17-J20. doi:10.1149/1.2186029 Abstract We report an evidence of a critical thickness at ~ 64 nm in epitaxial CeO2 films grown at 750 0C on YSZ substrates by dc magnetron sputtering where optimum ion channeling can be correlated with overall strain relaxation and film surface roughness. The occurrence of saturation in ion channeling yield, enhancement in the average surface roughness and relaxation in c-axis strain is clearly evident in thicker films beyond the critical thickness. Despite excellent surface smoothness and overall epitaxial growth, CeO2 films grown at 650 0C did not show optimum ion channeling properties due to high misfit dislocation and defect density. These results are discussed from a viewpoint of the need for such an optimum thickness to develop multilayers with smooth interfaces with relative overall lattice relaxation.
2006. "Patterned Regions of Porous Silicon through Trapped Hydrogen Bubbles." Applied Physics Letters 88(14):143113 (3 pages). doi:10.1063/1.2188039 Abstract Hydrogen bubbles trapped around lithographically formed etch pits on silicon single crystal surfaces were used to guide the formation of patterned regions of porous silicon. Optical lithography and wet (electro) chemical etching were utilized in a series of steps to form the etched surface and guide this growth. It was observed that the pits were surrounded by round regions of porous Si suggesting that hydrogen bubbles defined the areas of porous silicon formation. Consistent with earlier results where porosity seemed to form primarily at the bubble edges, a lower overall etch rate was observed near the bubble center. This is consistent with a variable rate of bubble expansion as etching occurred and an ability of the hydrogen to mask or slow the reaction in the bubble center. Contrary to our initial expectations, the surface profile suggested that the region away from the bubbles was also etched, but very uniformly.
2006. "Synthesis and Characterization of Cobalt Silicide Films on Silicon." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 249(1-2):532–535. doi:10.1016/j.nimb.2006.03.046 Abstract Cobalt silicide has emerged as a leading contact material in silicon technology due to its low resistivity, high stability and small lattice mismatch. In this study, 0.2-0.4 micron thick Co films were deposited on Si(100) wafers by RF magnetron sputtering at room temperature, and annealed at temperatures from 600 °C to 900°C in vacuum. The as-deposited and annealed samples were characterized by Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), x-ray diffraction (XRD) and scanning electron microscopy (SEM). Although the Si substrates were sputter cleaned before the deposition, all the samples showed a thin oxide layer at the Si/Co interfaces. Annealing up to 700 °C did not alter the composition at the interface except small amount Co diffusion into Si. Annealing at 800 °C promotes the evaporation of the oxides from the interface and, as a result, clean CoSi2 films were formed. Although the interface appeared to be sharp within the RBS resolution, the surface topography was relatively rough with varying size of the crystal grains after high temperature annealing.
2005. "Microstructure of ZrO2-CeO2 Hetero-Multi-Layer Films Grown on YSZ Substrate." Acta materialia 53(7):1921-1929. Abstract Multi-layer films of pure ZrO2 and CeO2 were grown using oxygen plasma assisted molecular beam epitaxy on yttria stabilized zirconia (YSZ) substrates. The microstructure of the film was analyzed using x-ray diffraction (XRD), conventional and high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), energy dispersive x-ray (EDX) elemental mapping, selected area electron diffraction, and dynamical electron diffraction calculations. The deposited pure CeO2 layers exist in the cubic fluorite structure, and the ZrO2 layers show a good epitaxial orientation with respect to the CeO2 layers. However, distinctive forbidden diffraction spots of (odd, odd, even) type were observed on the selected area electron diffraction patterns of the film. Dark-field imaging clearly reveals that these forbidden diffraction spots were contributed solely by the ZrO2 layers. Dynamical electron diffraction calculation based on the tetragonal phase of unity tetragonality (space group P42/nmc) with oxygen displaced along the c-axis does not match with the experimental pattern. Instead, a diffraction pattern calculated based on a cubic structure (space group P43m) for which the oxygen sub-lattice was displaced along the <111> matches with the experimental data. It is further suggested that the displacement of the oxygen from the ideal (¼,¼,¼) position was introduced during the film growth process.
2005. "Oxygen Transport Studies in Nanocrystalline Ceria Films." Journal of Materials Research 20(5):1295-1299. Abstract Oxygen uptake and conductivity were measured by nuclear reaction analysis and ac impedance technique at the intermediate temperature range on sol-gel grown nanocrystalline ceria films with average grain-sizes 7 nm and 38 nm synthesized at 723 K and 1173 K, respectively. Higher oxygen uptake and lower ionic conductivity are observed in ceria films with ~7 nm grain-size. High permeation-assisted oxygen diffusion in nanocrystallites combined with oxygen trapping in the disordered region contributes in higher oxygen uptake. However the lower ionic conductivity in the film results from absence of long-range lattice ordering. Relationship between oxygen uptake and conductivity in ceria is discussed in details by considering grain-size dependent defect density, related surface area and enhanced oxygen mobility.
2005. "Hydrogen Bubbles and Formation of Nanoporous Silicon during Electrochemical Etching." Surface and Interface Analysis 37(6):555-561. Abstract Many nanoporous Si structures, including those formed by common electrochemical etching procedures, produce a uniformly etch nanoporous surface. If the electrochemical etch rate is slowed down, details of the etch process can be explored and process parameters may be varied to test hypotheses and obtain controlled nanoporous and defect structures. For example, after electrochemical etching of a heavily n-doped (R = 0.05-0.5 Ω -cm) <100> silicon at a current density of 10 mA/cm² in buffer oxide etch (BOE) electrolyte solution defect craters, containing textured nanopores, were observed to occur in ring shaped patterns of rings. The defect craters apparently originate at the hydrogen-BOE bubble interface, which forms during hydrogen evolution in the reaction. The slower hydrogen evolution due to low current density allows sufficient bubble residence time so that a high defect density appears at the bubble edges where local reaction rates are highest. Current carrying Si-OH species are most likely responsible for the widening in the craters. Reducing the defect/doping density in silicon lowers the defect concentration and thereby the density of nanopores. Measurements of photoluminescence lifetime and intensity show a distinct feature when the low density of nanopores formed at ring edges are isolated from each other. Overall features observed in photoluminescence (PL), X-ray photoelectron spectroscopy (XPS) intensity strongly emphasize the role of surface oxide that influences these properties.
2005. "Ceria Incorporation into YSZ Columnar Nanostructures." Electrochemical and Solid-State Letters 8(10):A525-A527. Abstract We report the growth of porous yttria-stabilized zirconia (YSZ) columnar nanostructures by glancing angle deposition (GLAD) technique. CeO₂ sol solution was incorporated into YSZ creating high interface density columnar nanostructures. Initial experiments suggest higher conductivity in CeO₂/YSZ columnar nanostructures than polycrystalline CeO₂ and lower conductivity than single crystal YSZ in the intermediate temperature range of 600-825 K. GLAD/sol-gel process combination to create high density columnar nanostructures is discussed in the context of solid oxide fuel cells operating at intermediate temperatures.
2005. "Simple Method for Estimating and Comparing of X-Ray Damage Rates." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 23(6):1740-1744. doi:10.1116/1.2073387 Abstract This note describes an approach for estimating and comparing rates or thresholds for x-ray induced specimen damage during surface analysis. The method uses a common reference material to compare x-ray damage rates reported in the literature and in publications to the rates of damage that may occur on a specific instrument. Although the method makes several assumptions that are only partially valid, results from a few damage data-sets appear acceptably consistent when comparing estimates to a time for 10% damage or signal change.
2005. "Nanoscale Effects on Ion Conductance of Layer-by-Layer Structures of Gadolinia-doped Ceria and Zirconia." Applied Physics Letters 86(13):131906-131909. Abstract Layer-by-layer structures of gadolinia-doped ceria and zirconia have been synthesized on Al2O3(0001) using oxygen plasma-assisted molecular beam epitaxy. Oxygen ion conductivity greatly increased with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia doped ceria electrolytes. The conductivity enhancement in this layered electrolyte is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.
2004. "Room-Temperature Ferromagnetism in Ion-Implanted Co-Doped TiO₂(110) Rutile." Applied Physics Letters 84(22):4466-4468. Abstract Interest in diluted magnetic semiconductros (DMS) is growing rapidly within the emerging field of spintronics. For example, the ability to efficiently inject spin-polarized carriers into nonmagnetic semiconductor heterostructures creates new and exciting possibilities for utilizing DMS materials in spin-based devices. Several III-V and II-VI semiconductor materials doped with magnetic transition metal elements have been explored. Although these materials show promising behavior in some cases, most exhibit Curie temperatures of ~170 K or less. It has recently been shown that certain oxide semiconductors doped with magnetic transition elements show room-temperature ferromagnetism.
2004. "Room Temperature Ferromagnetism in Ion-implanted Co-doped TiO₂(110) Rutile." Applied Physics Letters 84(22):4466-4468. Abstract Ferromagnetic Co-doped rutile TiO₂ singel crystals were successfully synthesized by high temperature ion implantation and charaterized by a variety of techniques. Co is uniformly distrubuted to the depth of ~300nm with an average concentration of ~2 at. %, except in the near-surface region, where the concentration is ~3 at. %. Magnetic measurements reval ferromagnetic behavior at room temperature with an effective saturation magnetization of ~0.6 ub/Co atom. The Co formal oxidation state is found to be +2 throughouth the implanted region, and no Co(0) is detected.
2004. "Lateral Ordering of Microfabricated SiO₂ Nanotips." Electrochemical and Solid-State Letters 7(1):C7-C9. Abstract Micro -fabricated SiO₂ nano - tips are potentially useful as scanning tips in near field optical microscopy and sensor related functions. We report a process in which the rounding nature of isotropic etching is effectively used to micro -fabricate laterally ordered SiO₂ nano -tips. Combination of excessive isotropic wet etching of thermally grown SiO₂ with anisotropic etching of n type silicon along <100> planes leads to the formation of nano -tips with sharpness ~ 15 nm. Uniform periodic array of nano -tips form due to coalescence of excessively etched SiO₂ resulting in nano -tips length the separation between the original photolithographic features. Finally, the overall process of nano -tip formation is discussed by considering the roles of rapid isotropic etching of SiO₂ in buffered oxide etch solution, anisotropic etching of Si <100> in KOH solution, and slow SiO₂ etching in KOH solution.
2004. "Distinguishibility of Oxygen Desorption From the Surface Region with Mobility Dominant Effects in Nanocrystalline Ceria Films." Journal of Applied Physics 96(10):5756-5760 . Abstract We present an investigation of oxygen (18O) uptake measurements in 1 m thick nanocrystalline ceria films grown on single crystal Al₂O₃ (0001) by nuclear reaction analysis (NRA). Oxygen uptake measurements were carried out in the temperature range of 200 0C-600 0C at a background 18O pressure of 4.0 x 10-6 Torr. Average grain-size in the as-grown films, synthesized by sol-gel process was ~ 3 nm confirmed by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) measurements. From the diffusion depth profiles, changes in intensity and slopes in surface and interface regions indicate dominating oxygen mobility effects. Oxygen desorption is clearly distinguishable in the film surface region as a result of shift in the oxygen peak intensity. It is argued that high defect density in nanocrystalline ceria which is associated with nano-grain surface combined with intermediate temperature reducing environment triggers multiple processes like diffusion, desorption and exchange interaction. The promising nature of NRA is realized as an effective tool to acquire the depth-dependent information regarding such complex reactions that exists in nanocrystalline environment.
2004. "Synthesis and Characterization of Pure and Doped Ceria Films by Sol-gel and Sputtering." Journal of Undergraduate Research Volume IV, 2004:84-90. Abstract Synthesis and Characterization of Pure and Doped Ceria Films by Sol-gel and Sputtering. KURT T. KOCH (University of Missouri, Rolla, MO, 65409) LAXMIKANT SARAF (Environmental and Molecular Science Laboratory (Part of Pacific Northwest National Laboratory), Richland, Washington 99352). Pure and doped Ceria are known for their ability to gain or lose Oxygen, which is of interest to the Solid Oxide Fuel Cell (SOFC) and catalyst community. Current efforts are focused in SOFCs to reduce the operating temperature of the cell while maintaining ionic conduction. Ceria is known for its high ionic conductivity in the intermediate temperature region. (600-800° C) We have prepared pure and doped Ceria films by Sol-gel and magnetron sputtering methods. These films were characterized by X-ray diffraction (XRD), nuclear reaction analysis (NRA), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Oxygen conduction measurements. We have observed greater volume diffusion in nanocrystalline Ceria compared to bulk polycrystalline films as a result of low density. Near surface diffusion properties with increasing temperature indicate a decrease in the volume diffusion as a result of grain growth. However, a linear increase in O2 content at ~600nm depth was observed and can be correlated to the redistribution of O2 in the samples. Surface roughness of <111> and <200> oriented Ceria films on Al2O3 and YSZ was observed to be 0.13nm and 0.397nm, respectively. In the case of Ceria grown on YSZ, structural properties from XRD results showed a highly oriented structure with cube on cube growth. XRD results from Ceria grown on Al2O3 showed an oriented state near the surface. structure whose degree of orientation appeared to be partially dependent on substrate temperature. Preliminary XPS results indicate reduction in Ceria from the Ce4+ to Ce3+ state near the surface.
2004. "Persistent Photoconductivity in Chemically-Modified Single Wall Carbon Nanotubes ." Journal of Physical Chemistry B 108(52):19976-19981. Abstract Control of the conductivity of single wall carbon nanotubes (SWNTs) is crucial for the use of carbon nanotubes in molecular electronics. We report a new fundamental characteristic of semiconducting SWNTs – the persistent photoconductivity of chemically-modified carbon nanotube films. Covalent attachment of Ruthenium(II)-tris(2,2’-bipyridine) Ru(bpy) ²⁺/₃ to SWNT makes carbon nanotubes sensitive to light absorbed by the ruthenium complex and persistently photoconductive, thus opening opportunities for the selective light control of the conductivity. Illumination of carboxylated SWNTs with ultraviolet or visible light causes a persistent decrease in the conductivity of semiconducting films. Infrared illumination restores conductivity of SWNT films.
2004. "Direct Determination of Volume Changes in Ion-Beam-Irradiated SiC." Journal of Applied Physics 95(9):4687-4690. Abstract A single crystal 6H-SiC wafer was sequentially implanted in two areas at 873 and 295 K using 2.0 MeV Au₂⁺ ions under off-axis conditions. Identical Au profiles, as a function of atomic areal density, were produced at 873 and 295 K. The linear expansion in the amorphous state produced at 295 K was measured relative to the slightly damaged state produced at 873 K, using the Au profiles as references. The red-shift of the plasmon-loss peak was also used to directly measure the local density changes. Based on these measurements, the volume expansion of the amorphous state in 6H-SiC at 295 K is 11.51.9%, while that in the slightly damaged state at 873 is 0.9%.
2003. "Temperature-Induced Phase Separation in Chromium Films." Applied Physics Letters 82(14):2230-2232. Abstract Study of phase seperton at Cr/Si interface. Vacuum evaporation of chromium on Si(100) results in an interfacial nanophase layer followed by a mixed grain region. In this work, the mixed grain region in chromium is nearly transformed into a single-phase region, which resulted in a maximum phase separation between nano -A15 phase and bulk -bcc chromium. It is achieved by creating a temperature gradient across chromium-silicon interface by using rapid evaporation at a growth rate of ~3 m/min at a source-tosubstrate (SS) distance of 2 cm. Separately-grown nanophase chromium films at SS distance of 32 cm show predominant -A15 phase. Short-SS-distance rapid evaporation has a potential to produce selective phase separation by combining temperature gradient with the interfacial stress.
2002. "Atomic disorder in Heusler Co2MnGe measured by anomalous x-ray diffraction." Applied Physics Letters 81(15):2812-2814. Abstract In "spintronic" devices, the spin of the conduction electron is used to control trasport properties. A ferromagnet, which has an imbalance in the nuber of spin-up and spin-down electrons near the Fermi level, plays the role of spin injector or spin filter.