Paul C. McIntyre

Atomic layer-deposited tunnel oxide stabilizes silicon photoanodes for water oxidation

A leading approach for large-scale electrochemical energy production with minimal global-warming gas emission is to use a renewable source of electricity, such as solar energy, to oxidize water, providing the abundant source of electrons needed in fuel synthesis. We report corrosion-resistant, nanocomposite anodes for the oxidation of water required to produce renewable fuels. Silicon, an earth-abundant element and an efficient photovoltaic material, is protected by atomic layer deposition (ALD) of a highly uniform, 2 nm thick layer of titanium dioxide (TiO(2)) and then coated with an optically transmitting layer of a known catalyst (3 nm iridium). Photoelectrochemical water oxidation was observed to occur below the reversible potential whereas dark electrochemical water oxidation was found to have low-to-moderate overpotentials at all pH values, resulting in an inferred photovoltage of ~550 mV. Water oxidation is sustained at these anodes for many hours in harsh pH and oxidative environments whereas comparable silicon anodes without the TiO(2) coating quickly fail. The desirable electrochemical efficiency and corrosion resistance of these anodes is made possible by the low electron-tunnelling resistance (<0.006 Ω cm(2) for p(+)-Si) and uniform thickness of atomic-layer deposited TiO(2).

Germanium nanowire field-effect transistors with SiO2 and high-κ HfO2 gate dielectrics

Single-crystal Ge nanowires are synthesized by a low-temperature (275 °C) chemical vapor deposition (CVD) method. Boron doped p-type GeNW field-effect transistors (FETs) with back-gates and thin SiO2 (10 nm) gate insulators are constructed. Hole mobility higher than 600 cm2/V s is observed in these devices, suggesting high quality and excellent electrical properties of as-grown Ge wires. In addition, integration of high-κ HfO2 (12 nm) gate dielectric into nanowire FETs with top-gates is accomplished with promising device characteristics obtained. The nanowire synthesis and device fabrication steps are all performed below 400 °C, opening a possibility of building three-dimensional electronics with CVD-derived Ge nanowires.

Germanium MOS capacitors incorporating ultrathin high-/spl kappa/ gate dielectric

For the first time, we have successfully demonstrated the feasibility of integrating a high-permittivity (/spl kappa/) gate dielectric material zirconium oxide into the MOS capacitors fabricated on pure germanium substrates. The entire fabrication process was essentially performed at room temperature with the exception of a 410/spl deg/C forming gas anneal. After processing steps intended to remove the germanium native oxide interlayer between the zirconium oxide dielectric and germanium substrate, an excellent capacitance-based equivalent SiO/sub 2/ thickness (EOT) on the order of 5-8 /spl Aring/ and capacitance-voltage (C-V) characteristics with hysteresis of 16 mV have been achieved. Additionally, excellent device yield and uniformity were possible using this low thermal budget process.

Effect of growth conditions on the properties and morphology of chemically derived epitaxial thin films of Ba2YCu3O7−x on (001) LaAlO3

Epitaxial thin films of Ba2YCu3O7−x (BYC) were prepared on (001) LaAlO3 single‐crystal substrates by metalorganic deposition of metal trifluoroacetate precursors. This is an ex situ process that requires high‐temperature annealing in a humid atmosphere to produce stoichiometric BYC thin films. The chemically derived superconducting films were found to have high critical temperatures and high current densities when crystallized under low‐oxygen partial pressures. Superconducting films of 70 nm thickness with zero‐field critical current densities greater than 5×106 A/cm2 at 77 K and zero resistance at 92 K were prepared by annealing at 780 and 830 °C in 2.5 × 10−4–1 × 10−3 atm oxygen furnace atmospheres. As the film thickness was increased, the superconducting properties and surface smoothness of the films tended to degrade. This behavior was consistent with a microstructural model in which the films are composed of a dense slab of c‐axis normal BYC near the film/substrate interface with the overlying mater...

Metalorganic deposition of high‐Jc Ba2YCu3O7−x thin films from trifluoroacetate precursors onto (100) SrTiO3

Superconducting thin films of Ba2YCu3O7‐x were prepared on (100) SrTiO3 substrates by metalorganic deposition (MOD) of trifluoroacetate precursors. The best electrical transport properties were measured in films annealed at 750 °C in a humid, low PO2 gas mixture followed by slow cooling in oxygen. These specimens had sharp resistive transitions with Tc above 90 K and zero‐field critical current densities in excess of 106 A/cm2 at 77 K. Critical current densities of this magnitude have not previously been reported in films produced by MOD. The highest Jc obtained in films fired only in humid oxygen was 3×105 A/cm2. Annealing at high temperature in the low PO2 atmosphere also resulted in a smoother surface morphology than was observed in the oxygen‐fired films. Use of the low PO2 furnace gas appeared to suppress the formation of b‐axis normal oriented grains in the superconducting films and to strengthen c‐axis normal texture. X‐ray powder diffraction indicated the presence of a‐axis normal textured material in the films, although it was not present as separate microstructural features which could be identified by scanning electron microscopy.Superconducting thin films of Ba2YCu3O7‐x were prepared on (100) SrTiO3 substrates by metalorganic deposition (MOD) of trifluoroacetate precursors. The best electrical transport properties were measured in films annealed at 750 °C in a humid, low PO2 gas mixture followed by slow cooling in oxygen. These specimens had sharp resistive transitions with Tc above 90 K and zero‐field critical current densities in excess of 106 A/cm2 at 77 K. Critical current densities of this magnitude have not previously been reported in films produced by MOD. The highest Jc obtained in films fired only in humid oxygen was 3×105 A/cm2. Annealing at high temperature in the low PO2 atmosphere also resulted in a smoother surface morphology than was observed in the oxygen‐fired films. Use of the low PO2 furnace gas appeared to suppress the formation of b‐axis normal oriented grains in the superconducting films and to strengthen c‐axis normal texture. X‐ray powder diffraction indicated the presence of a‐axis normal textured materia...

Electrical and materials properties of ZrO2 gate dielectrics grown by atomic layer chemical vapor deposition

Structural and electrical properties of gate stack structures containing ZrO2 dielectrics were investigated. The ZrO2 films were deposited by atomic layer chemical vapor deposition (ALCVD) after different substrate preparations. The structure, composition, and interfacial characteristics of these gate stacks were examined using cross-sectional transmission electron microscopy and x-ray photoelectron spectroscopy. The ZrO2 films were polycrystalline with either a cubic or tetragonal crystal structure. An amorphous interfacial layer with a moderate dielectric constant formed between the ZrO2 layer and the substrate during ALCVD growth on chemical oxide-terminated silicon. Gate stacks with a measured equivalent oxide thickness (EOT) of 1.3 nm showed leakage values of 10−5 A/cm2 at a bias of −1 V from flatband, which is significantly less than that seen with SiO2 dielectrics of similar EOT. A hysteresis of 8–10 mV was seen for ±2 V sweeps while a midgap interface state density (Dit) of ∼3×1011 states/cm eV wa...

Effects of crystallization on the electrical properties of ultrathin HfO2 dielectrics grown by atomic layer deposition

Microstructural evolution and resulting changes in electrical properties of atomic-layerdeposition-grown HfO2 on SiO2/Si substrates were studied as a function of annealing temperature in a N2 ambient. As deposited ∼30-A-thick HfO2 on 15 and 25 A thermal SiO2 were almost entirely amorphous, although a low density of crystalline seeds were observed and crystallization occurred from these nuclei during furnace anneals at temperatures >∼500 °C. The major crystalline phase thus formed was monoclinic, and some fraction of tetragonal phase was observed during crystallization according to transmission electron microscopy electron diffraction analysis. Complete crystallization occurred around 700 °C and, at higher temperatures, significant interfacial silicon dioxide growth was observed due to the presence of a small partial pressure of oxygen in the annealing ambient. No significant increase of leakage current in the trap-assisted tunneling conduction regime was observed during the intermediate and final stage of...

Border traps in Al2O3/In0.53Ga0.47As (100) gate stacks and their passivation by hydrogen anneals

Charge-trapping defects in Pt/Al2O3/In0.53Ga0.47As metal-oxide-semiconductor capacitors and their passivation by hydrogen are investigated in samples with abrupt oxide/III-V interfaces. Tunneling of electrons into defect states (border traps) in the atomic layer deposited Al2O3 near the oxide/semiconductor interface is found to control the frequency dispersion of the capacitance in accumulation. Hydrogen anneals effectively passivate border traps in the oxide, in addition to some of the midgap states that control carrier generation in the channel. This is evident in the reduced frequency dispersion in accumulation, reduced capacitance-voltage stretch-out through depletion, and suppression of the inversion carrier response in capacitance-voltage measurements.

Ge-Interface Engineering With Ozone Oxidation for Low Interface-State Density

Passivation of Ge has been a critical issue for Ge MOS applications in future technology nodes. In this letter, we introduce ozone oxidation to engineer Ge/insulator interface. Density of interface states (D_it) across the bandgap and close to the conduction band edge was extracted using conductance technique at low temperatures. D_it dependence on growth conditions was studied. Minimum D_it of 3 times 10¹¹ cm^-2V^-1 was demonstrated. Physical quality of the interface was investigated through Ge 3d spectra measurements. We found that the interface and D_it are strongly affected by the distribution of oxidation states and the quality of the suboxide.

Interfacial characteristics of HfO2 grown on nitrided Ge (100) substrates by atomic-layer deposition

The microstructural and electrical properties of Ge-based metal–oxide–semiconductor capacitors containing high-k gate dielectric layers were investigated with and without the presence of a GeOxNy interface layer. The effect of this nitrided layer on thermal stability of the metal oxide/Ge structures was probed by medium energy ion energy spectroscopy (MEIS). Atomic-layer deposited HfO2 on a chemical oxide-terminated Ge (100) surface exhibited poor capacitance–voltage behavior; however, direct substrate surface nitridation at 600°C in NH3 ambient before HfO2 deposition improved the carrier trapping characteristics. Diffusion of metal impurities (including Hf) into the interfacial oxide/Ge-substrate may be an important source of the measured degradation of electrical properties. MEIS results suggested that the GeOxNy interface layer may inhibit Hf diffusion into the underlying semiconductor at the temperatures investigated.