Mark D. Vaudin

Effect of the electrode layer on the polydomain structure of epitaxial PbZr0.2Ti0.8O3 thin films

PbZr0.2Ti0.8O3(PZT) thin films with and without La0.5Sr0.5CoO3(LSCO) electrodes were grown epitaxially on (001) SrTiO3 at 650 °C by pulsed laser deposition. The domain structure of the 400 nm thick PZT films with different electrode layer configurations was investigated by x-ray diffraction and transmission electron microscopy. The c-domain fractions of the PZT films with no electrode layer, with a 50 nm electrode layer between the film and the substrate, and with 50 nm electrode layers on top and bottom of the PZT film were found to be equal. Theoretical estimation of the c-domain fraction based on the minimization of the energy of internal stresses in films is in good agreement with experimental results. This means that depolarizing fields do not affect the polydomain structure of the film. Calculations of the in-plane strains based on the lattice parameters of the LSCO layer in the above configurations led to the conclusion that the bottom electrode layer is coherently strained to match the substrate.

Fabrication of GaN-based nanoscale device structures utilizing focused ion beam induced Pt deposition

In this work we have demonstrated nanoscale GaN device structures made from individual GaN nanowires and electrical contacts utilizing focused ion beam (FIB) induced Pt deposition. These GaN nanowires were grown by direct reaction of Ga vapor with NH3 and had diameters ranging from 100nmto250nm and lengths up to 200μm. As-grown nanowires were dispersed on SiO2 coated p++ Si substrate. A 30keV Ga+ ion beam was used to dissociate (trimethyl)methylcyclopentadienyl-platinum precursor for depositing Pt contacts to GaN nanowires. FIB-deposited Pt contacts to GaN nanowires showed nonlinear I-V characteristics, which turned linear after annealing at 500°C for 30s in argon. Resistivity of a GaN nanowire measured using a four terminal contact geometry fabricated by depositing Pt with a FIB was in the range of 5×10−3Ωcm. Temperature dependent resistivity measurement of the GaN nanowire revealed semiconducting behavior with a weak temperature dependence of the resistivity. In this study both Ohmic and Schottky contac...

Diameter dependent transport properties of gallium nitride nanowire field effect transistors

The authors report transport property measurements of individual GaN nanowire field effect transistors and the correlation of the electron mobilities with the existence of grain boundaries in these nanowires. Room temperature field effect electron mobilities as high as 319cm2V−1s−1 were obtained for the 200nm diameter nanowires. Mobilities calculated from these reliable nanowire field effect transistors indicated that the surface scattering plays a dominant role in smaller diameter nanowires, whereas for intermediate diameter devices transport is dominated by grain boundary scattering. Reduction of the mobility with decreasing diameter of nanowires can be explained using “continuous surface” model.

Horizontal growth and in situ assembly of oriented zinc oxide nanowires

The positioning and directed assembly of semiconductor nanowires (NWs) is of considerable current interest for “bottom-up” approaches to the engineering of intricate structures from nanoscale building blocks. We report a horizontal growth mode for ZnO NWs on the (112¯0) sapphire surface in which NWs grow in the [11¯00]sap direction. This growth mode strictly depends on the size and spacing of the Au nanodroplet catalysts and competes with the vertical growth of the NWs. An approach is presented which promotes the horizontal growth, in situ alignment, and predictable positioning of ZnO NWs. This strategy allows for the large scale assembly of NWs, width control, and production of quantum wires.

Comparison of nanoscale measurements of strain and stress using electron back scattered diffraction and confocal Raman microscopy

Stresses in Si as small as 10 MPa have been measured using electron backscattered diffraction (EBSD) and confocal Raman microscopy (CRM) with spatial resolutions of 10 nm and 100 nm, respectively. In both techniques, data were collected across wedge indentations in (001) Si. EBSD measured the stress and strain tensors and CRM measured the uniaxial stress. The results agreed very well except close to the indentation, where the surface-sensitive EBSD results indicated larger stresses. Results converged when the CRM laser excitation wavelength was reduced, probing smaller depths. The stress profiles are consistent with the inverse-square power law predicted by Eshelby analysis.

Structural and magnetic properties of η-phase manganese nitride films grown by molecular-beam epitaxy

Face-centered tetragonal (fct) η-phase manganese nitride films have been grown on magnesium oxide (001) substrates by molecular-beam epitaxy. For growth conditions described here, reflection high energy electron diffraction and neutron scattering show primarily two types of domains rotated by 90° to each other with their c axes in the surface plane. Scanning tunneling microscopy images reveal surface domains consisting of row structures which correspond directly to the bulk domains. Neutron diffraction data confirm that the Mn moments are aligned in a layered antiferromagnetic structure. The data are consistent with the fct model of G. Kreiner and H. Jacobs for bulk Mn3N2 [J. Alloys Compd. 183, 345 (1992)].

Epitaxial growth of BaTiO3 thin films at 600 °C by metalorganic chemical vapor deposition

BaTiO3 thin films were grown epitaxially on (100) MgO substrates by metalorganic chemical vapor deposition (MOCVD) at a temperature of 600 °C. This substrate temperature is the lowest reported temperature for the growth of epitaxial BaTiO3 films by an MOCVD process. The films had a cube–cube orientation relationship with the substrate and were oriented with an a‐axis perpendicular to the substrate plane. Nanoscale energy dispersive x‐ray spectrometry measurements showed no evidence of interdiffusion between the film and substrate.

Brittle intergranular failure in 2D microstructures: Experiments and computer simulations

Brittle intergranular fracture (BIF) is a common mode of failure for monolithic ceramics and intermetallics, as well as for some refractory metals and metals exposed to environmental corrosion, stress corrosion cracking or high temperature creep. As interest in applications for these materials grows, research programs have been developed to characterize and predict their fracture behavior. In order to experimentally quantify the effects of microstructure on local BIF, systems which have a minimum number of variables which influence fracture must be used. Evaluation of materials with two dimensional (2D) microstructures can considerably reduce the complexity of the system. In addition, providing a biaxial stress state in the 2D microstructure ensures that all boundaries experience exclusively Mode I loading prior to failure. Biaxial elastic loading of this simplified microstructure allows the calculation of (a) local stress and strain fields (and their concentrations) prior to failure, as well as (b) prediction of grain boundary strength criteria, and (c) prediction of intergranular crack paths. This can be achieved by conducting computer simulations of the experimentally observed fracture phenomena in polycrystalline specimens having a given texture and microgeometry. These simulations use high resolution finite-difference grids below the crystal scale, and involve the derivation of a spring-network model for arbitrary in-plane crystal anisotropy. Since the grain boundary strength criterion is easily controllable in such simulations, it can be inferred by a comparison with actual experimental results. The latter is complemented by results on fracture of materials with very weak grain boundaries, thus providing a clear perspective on evolution of the failure process for varying degrees of embrittlement.

Moire fringe images of twin boundaries in chemical vapor deposited diamond

Features in lattice image micrographs of chemical vapor deposited diamond can be interpreted as Moire fringes that occur when viewing twin boundaries that are inclined to the electron beam. The periodicities in images of inclined twin boundaries with Σ=3 and Σ=9 misorientations have been modeled by computer graphic simulation.

A method for crystallographic texture investigations using standard x-ray equipment

A fast and accurate method has been developed for measuring crystalline texture in homogeneous materials. The method uses a conventional powder x-ray diffractometer capable of {theta} scans. Two scans are recorded from the sample: first, a high resolution {theta}-2{theta} scan is obtained of a Bragg peak whose diffracting planes are normal to the preferred orientation direction; second, a {theta} scan is obtained using this peak. The {theta} scan contains the required texture information, but the intensities must be corrected for defocusing and absorption to obtain the texture profile. The {theta}-2{theta} scan of the Bragg peak is used to make the defocusing correction, and first principles calculations are used to correct for absorption. The theory behind these corrections is presented here. The validity of the technique has been verified by making measurements on untextured alumina. Data obtained from Bi{sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10} superconducting tape specimens with this technique are compared with texture data obtained with a four-circle diffractometer. {copyright} {ital 1998 Materials Research Society.}