IRGs

In collaboration with Marks and Prof. Alex K. -Y. Jen group (U. Washington), the Ho group has, for the first time, successfully demonstrated a novel organic EO modulator using ZnO and In2O3 as TCO electrodes and device geometry designs. This helped achieve 2.8V switching voltage for a 8mm-long device at wavelength of 1.31μm, which corresponding to a Vπ=1.1V switching voltage for a 1cm long device in a push-pull configuration. The push-pull VπL figure of merit normalized for the EO coefficient is thus 1.1V-cm (35pm/V), which is 3-4 times lower than conventional modulator. This has potential for important impact on enabling RF photonics (transmitting high frequency electrical RF signal using optical fiber). The Ho group has also reported a general computational model of complex material media for electrodynamics simulation using Finite-Difference Time-Domain (FDTD) method.
Wessels is developing epitaxial multifunctional oxide thin films, such as Fe3O4, deposited by MBE on MgO and SrTiO3 substrates. The magneto-optical properties are measured using the magneto-optical Kerr effect (MOKE). Spectrally resolved MOKE was also measured over the spectral range of 1.5 to 3.5 eV. X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) of the epitaxial iron oxide films was measured at the Advanced Photon Source at Argonne National Lab. The magnetic properties seem quite sensitive to the deposition conditions. Dravid is examining these films with advanced TEM, especially across the substrate-film interfaces for possible Fe valence changes.

Marks focused on the design, synthesis, and utilization of new precursors for efficient MOCVD growth of TCOs, including ligands which encapsulate metal ions of interest in protective, volatility-enhancing, non-polar ligation–especially challenging for ions having large radius:charge ratios. Marks demonstrated successful epitaxial growth of CdO, and MxCd1-xO (M = Sc+3, Y+3, Ga+3, Sn+4, Mo+6). A combined microstructural, charge transport, and theoretical study with Dravid, Kannewurf, and Freeman led to new guidelines for the design of high-performance TCOs. With Hersam (IRG#3), Marks applied conductive scanning probe techniques for characterization of TCO surfaces as a function of growth and cleaning procedure. This work shows now evidence for “hot spots” and “dead spots” on the surface, and that scanning probe techniques can be used to “write” nanoscale patterns on TCO surfaces. Marks also developed new families of volatile copper precursors for MOCVD growth of Cu2S thin films for photovoltaic applications, which are the most conductive to date.

Mason showed (in collaboration with Klein- Darmstadt) that indium oxide and indium-tin oxide (ITO) exhibit a propensity for chemical depletion at their surfaces, leading to Fermi levels inside the band gap at the surface. This is attributable to the bixbyite crystal structure (with readily available oxygen interstitial sites) and resulting bulk point defect structure (including donor-interstitial associates in ITO), leading to chemical depletion by surface oxidation. These findings have major ramifications for TCO applications (e.g., in OLEDs and in organic solar cells), and may explain why ITO tends to be so variable and unstable in such devices. In contrast, conventionally-doped TCOs without available oxygen interstitial sites (e.g., Al-doped ZnO and Sb-doped SnO2) seem to be relatively unaffected by such chemical depletion effects. Mason model accounts for the difference in band gaps measured by diffuse reflectance in bulk specimens vs. those measured by transmission in thin films. Stimulated by Mason results, Freeman developed and applied thin film FLAPW method and code to calculate work functions for ZnO films having both nonpolar and polar surfaces.

Freeman performed first principles FLAPW calculations on CdO and MxCd1-xO. A combined theoretical/experimental study with Dravid, Kannewurf and Marks led to new guidelines for the design of high temperature TCO’s. A unique feature of this work is the use of the self-consistent screened-exchange – local density approximation (sX-LDA), which provides a considerably improved description of the optical properties obtained with the LDA. Freeman predicted Cu doped ZnO to be a strong half-metallic ferromagnet and estimated its Curie temperature, by mean field theory, to be around 380 K, and confirmed by Chang and Ketterson.

Barnett focused on measurements of the nanometer-scale layers in the ZnO/ZnMgO structures, utilizing a combination of x-ray diffraction and computer simulations. Fitting to the measured x-ray scans using the simulation allowed detailed information about the nano-layers, such as the exact layer thicknesses, the roughness of the interfaces between layers, as well as the extent of intermixing between layers. Such structural information will be important for interpreting results on electrical properties, i.e. how fast can electrons move along these interfaces. In regards to improvement in the quality of films on sapphire substrates, Barnett has grown ZnO buffer layers and annealed in air at elevated temperatures. Results indicate that annealing temperatures of 800-900oC yield substantial improvements in crystal quality. These annealed ZnO surface can then be used for nano-layer growth, and should yield substantially improved crystalline perfection.

Dravid developed a facile route to pattern solid-state inorganic materials, termed Soft Electron Beam Lithography (Soft-eBL). Soft-eBL synergistically combines the advantages of high resolution e-beam lithography with versatile wet chemistry (sol-gel method, in particular). It expands the scope of nanopatterning capabilities to fabricate multifunctional and multidimensional nanostructures on virtually any substrate. Dravid successfully employed the soft-eBL to fabricate diverse functional oxide nanopatterns (e.g., ZnO, BiFeO3, PZT and CoFe2O4), and 3D hierarchical multifunctional nanostructures, such as radially-stacked heterostructures with ferroelectric PZT nano-shell and magnetic CoFe2O4 core. Wessels is assisting in FE measurements of these structures and MOKE studies.
Ellis has developed algorithms to simulate growth and curing (cross-linking) of semi-disordered films, grown on crystalline substrates. Key parameters of deposition density, defect density, and cross-linking frequency are controlled to produce interface structures suitable for both classical and quantum modeling. Density Functional theory has been applied in both periodic supercell and embedded cluster. Ellis has begun the extension of DFT and transport theory to model effects of multilayer and interfacial structures important for spintronic applications. These approaches complement first-principles studies by Freeman.