Eric Garfunkel

Ultrathin (<4 nm) SiO2 and Si–O–N gate dielectric layers for silicon microelectronics: Understanding the processing, structure, and physical and electrical limits

The outstanding properties of SiO2, which include high resistivity, excellent dielectric strength, a large band gap, a high melting point, and a native, low defect density interface with Si, are in large part responsible for enabling the microelectronics revolution. The Si/SiO2 interface, which forms the heart of the modern metal–oxide–semiconductor field effect transistor, the building block of the integrated circuit, is arguably the worlds most economically and technologically important materials interface. This article summarizes recent progress and current scientific understanding of ultrathin (<4 nm) SiO2 and Si–O–N (silicon oxynitride) gate dielectrics on Si based devices. We will emphasize an understanding of the limits of these gate dielectrics, i.e., how their continuously shrinking thickness, dictated by integrated circuit device scaling, results in physical and electrical property changes that impose limits on their usefulness. We observe, in conclusion, that although Si microelectronic devices...

Photochemical Water Oxidation by Crystalline Polymorphs of Manganese Oxides: Structural Requirements for Catalysis

Manganese oxides occur naturally as minerals in at least 30 different crystal structures, providing a rigorous test system to explore the significance of atomic positions on the catalytic efficiency of water oxidation. In this study, we chose to systematically compare eight synthetic oxide structures containing Mn(III) and Mn(IV) only, with particular emphasis on the five known structural polymorphs of MnO2. We have adapted literature synthesis methods to obtain pure polymorphs and validated their homogeneity and crystallinity by powder X-ray diffraction and both transmission and scanning electron microscopies. Measurement of water oxidation rate by oxygen evolution in aqueous solution was conducted with dispersed nanoparticulate manganese oxides and a standard ruthenium dye photo-oxidant system. No Ru was absorbed on the catalyst surface as observed by XPS and EDX. The post reaction atomic structure was completely preserved with no amorphization, as observed by HRTEM. Catalytic activities, normalized to surface area (BET), decrease in the series Mn2O3 > Mn3O4 ≫ λ-MnO2, where the latter is derived from spinel LiMn2O4 following partial Li(+) removal. No catalytic activity is observed from LiMn2O4 and four of the MnO2 polymorphs, in contrast to some literature reports with polydispersed manganese oxides and electro-deposited films. Catalytic activity within the eight examined Mn oxides was found exclusively for (distorted) cubic phases, Mn2O3 (bixbyite), Mn3O4 (hausmannite), and λ-MnO2 (spinel), all containing Mn(III) possessing longer Mn-O bonds between edge-sharing MnO6 octahedra. Electronically degenerate Mn(III) has antibonding electronic configuration e(g)(1) which imparts lattice distortions due to the Jahn-Teller effect that are hypothesized to contribute to structural flexibility important for catalytic turnover in water oxidation at the surface.

Intermixing at the tantalum oxide/silicon interface in gate dielectric structures

Metal oxides with high dielectric constants have the potential to extend scaling of transistor gate capacitance beyond that of ultrathin silicon dioxide. However, during deposition of most metal oxides on silicon, an interfacial region of SiOx can form that limits the specific capacitance of the gate structure. We have examined the composition of this layer using high-resolution depth profiling of medium ion energy scattering combined with infrared spectroscopy and transmission electron microscopy. We find that the interfacial region is not pure SiO2, but is a complex depth-dependent ternary oxide of Si–Tax–Oy with a dielectric constant at least twice that of pure SiO2 as inferred from electrical measurements. High-temperature annealing crystallizes the Ta2O5 film and converts the composite oxide to a more pure SiO2 layer with a lower capacitance density. Using low postanneal temperatures, a stable composite oxide structure can be obtained with good electrical properties and an effective SiO2 thickness of...

HfO2 and Al2O3 gate dielectrics on GaAs grown by atomic layer deposition

High-performance metal-oxide-semiconductor field effect transistors (MOSFETs) on III–V semiconductors have long proven elusive. High-permittivity (high-κ) gate dielectrics may enable their fabrication. We have studied hafnium oxide and aluminum oxide grown on gallium arsenide by atomic layer deposition. As-deposited films are continuous and predominantly amorphous. A native oxide remains intact underneath HfO2 during growth, while thinning occurs during Al2O3 deposition. Hydrofluoric acid etching prior to growth minimizes the final interlayer thickness. Thermal treatments at ∼600°C decompose arsenic oxides and remove interfacial oxygen. These observations explain the improved electrical quality and increased gate stack capacitance after thermal treatments.

High temperature stability in lanthanum and zirconia-based gate dielectrics

Gate dielectrics composed primarily of lanthana and zirconia were prepared by reactive evaporation. The stability of the layers during high temperature anneals was investigated. By controlling the oxygen partial pressure during heat treatment, lanthana and zirconia films could be protected against reaction with the underlying Si substrate and against the growth of low-e interface layers. The electrical thickness of the dielectrics could be maintained after a 900 °C exposure. The critical oxygen pressure at 900 °C for low-e interface formation beneath ZrO2 and La2O3 dielectrics was ∼2e−4 Torr. The interfaces that formed beneath the ZrO2 and La2O3 layers are distinctly different. The sub-ZrO2 interface, influenced primarily by phase separation, tends towards pure SiO2, while the sub-La2O3 interface, influenced primarily by silicate formation, tends towards a La–Si–O alloy. For both materials, reducing the oxygen pressure to values below 10−7 Torr resulted in rapid degradation of the metal oxide. This dielec...

Growth and characterization of ultrathin nitrided silicon oxide films

This paper reviews recent progress in understanding microstructural and growth-mechanistic aspects of ultrathin (<4 nm) oxynitride films for gate dielectric applications. Different techniques for characterizing these films are summarized. We discuss several nitridation methods, including thermal (oxy)nitridation in NO, N2O, and N2 as well as a variety of deposition methods. We show that a basic understanding of the gas-phase and thin-film oxygen and nitrogen incorporation chemistries facilitates the processing of layered oxynitride nanostructures with desirable electrical properties.

Titanium and reduced titania overlayers on titanium dioxide(110)

Abstract The adsorption of titanium on titanium dioxide TiO 2 (110) has been studied by X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS). The XPS data for Ti overlayers are interpreted using peak fitting based on experimental standard spectra. 4 A of Ti deposited at 150 K reacts with the substrate to produce ≈ 12 A of intermediate oxidation state Ti. Adsorption of neutral metal begins on top of this interface oxide film, but 20 A of deposited Ti are needed to cover the oxide completely. LEIS data indicate a tendency for clustering of Ti on top of the interface oxide. Ar + sputtering of stoichiometric TiO 2 leads to preferential loss of O from the near surface region. This reduction of the clean, annealed oxide surface by Ar + ion bombardment starts immediately and does not reach a steady state until 3 × 10 17 ions cm −2 , at which point the reduced overlayer is 17 A thick.

Production of Graphene Sheets by Direct Dispersion with Aromatic Healing Agents

Graphene exhibits remarkable properties for various novel applications. One of many appealing applications of graphene would be to fabricate transparent conductive films to replace indium tinoxide (ITO).Theuseof graphene is promisingdue to its high optical transmittance, low resistance, high chemical stability, and high mechanical strength. This, as well as other applications, requires a large quantity of high-quality graphene as the basic component. Among the reported methods to prepare graphene, liquid-phase methods have drawn tremendous attention due to their scalability and ease of functionalization. Compared to chemical vapor deposition (CVD) approaches, which produce graphene films with the highest conductivity yet obtained, one advantage of liquid-phase methods is that the produced graphene can be conveniently deposited on any substratewith simple processing, such as spincoating or inkjet-printing on plastic substrates. Therefore, liquid-based techniqueshave thepotential to realize large-scale organic devices including photovoltaic cells.

Soft x-ray photoemission studies of the HfO2/SiO2/Si system

Soft x-ray photoelectron spectroscopy with synchrotron radiation was employed to study the valence-band offsets for the HfO2/SiO2/Si and HfO2/SiOxNy/Si systems. We obtained a valence-band offset difference of −1.05±0.1 eV between HfO2 (in HfO2/15 A SiO2/Si) and SiO2 (in 15 A SiO2/Si). There is no measurable difference between the HfO2 valence-band maximum positions of the HfO2/10 A SiOxNy/Si and HfO2/15 A SiO2/Si systems.

Ga-doped ZnO single-crystal nanotips grown on fused silica by metalorganic chemical vapor deposition

In situ Ga-doped ZnO nanotips were grown on amorphous fused silica substrates using metalorganic chemical vapor deposition. Structural, optical, and electrical properties of as-grown ZnO nanotips are investigated. Despite the amorphous nature of fused silica substrates, Ga-doped ZnO nanotips are found to be single crystalline and oriented along the c-axis. Photoluminescence (PL) spectra of Ga-doped ZnO nanotips are dominated by near-band-edge emission with negligible deep-level emission. The increase in PL intensity from Ga doping has been attributed to the increase of Ga donor-related impurity emission. Current–voltage characteristics of the ZnO nanotips are measured by conductive-tip atomic force microscopy, which shows the conductivity enhancement due to Ga doping.