epitaxial-growth-perovskite-strontium-titanate-on-germanium-via

Epitaxial Growth of Perovskite Strontium Titanate on Germanium via Atomic Layer Deposition

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and ...

The University of Texas at Austin

Thin films ( approximately 30 nm) of amorphous RuP alloys (P approximately 15-20%) can be grown by CVD from the single source precursor cis-H2Ru(PMe3)4 at 250-300 degrees C and 200 mTorr pressure on native SiO2.

Growth of ultrathin films of amorphous ruthenium-phosphorus alloys using a single source CVD precursor.

The hydrolysis of β--O--4 bonds in two lignin model compounds was studied in an acidic ionic liquid, 1-H-3-methylimidazolium chloride. The β--O--4 bonds of both guaiacylglycerol-β-guaiacyl ether and veratrylglycerol-β-guaiacyl ether underwent catalytic hydrolysis to produce guaiacol as the primary product with more than 70 % yield at 150 °C. Up to 32 wt % substrate concentration could be treated in the system without a decrease in guaiacol production. The ionic liquid could be reused without loss of activity in guaiacol production from both guaiacylglycerol-β-guaiacyl ether and veratrylglycerol-β-guaiacyl ether. A possible mechanism accounting for the guaiacol production is presented.

Cleaving the β--O--4 bonds of lignin model compounds in an acidic ionic liquid, 1-H-3-methylimidazolium chloride: an optional strategy for the degradation of lignin.

Oak wood lignin, which was separated from the wood using dissolution in the ionic liquid 1-methyl-3-ethylimidazolium acetate and subsequent precipitation, was successfully depolymerized in the acidic ionic liquid 1-H-3-methylimidazolium chloride under mild conditions (110-150 °C). Based on gel permeation chromatography results, an increase in temperature from 110 to 150 °C increased the rate of reaction, but did not significantly change the final size of the lignin fragments. Nuclear magnetic resonance and infrared spectroscopy were utilized to demonstrate that the depolymerization proceeded via a hydrolysis reaction that cleaved the alkyl-aryl ether linkages. Coupling of the lignin fragments was also shown to occur in the reaction mixture. These hydrolysis results are consistent with the literature on acid catalyzed depolymerization of lignin in conventional solvents and with recent model compound studies involving guaiacylglycerol-β-guaiacyl ether and veratrylglycerol-β-guaiacyl ether done in acidic ionic liquids.

Depolymerization of oak wood lignin under mild conditions using the acidic ionic liquid 1-H-3-methylimidazolium chloride as both solvent and catalyst.

The acidic ionic liquid 1-H-3-methylimidazolium chloride can effectively pretreat yellow pine wood chips under mild conditions for enzymatic saccharification. Wood samples were treated at temperatures between 110 and 150°C for up to 5 h in the ionic liquid and three fractions collected; a cellulose rich fraction, lignin, and an aqueous fraction. This treatment caused the hemicellulose and the lignin to be degraded and dissolved from the cell walls of the pine wood. The lignin was depolymerized and subsequently dissolved in the ionic liquid. This process occurred more quickly at higher temperatures, although at the highest temperatures tested, significant cellulose degradation also occurred. The cellulose rich fraction was saccharified using cellulase from Trichoderma viride, with longer pretreatment times at 130°C resulting in higher glucose yields.

Pretreatment of yellow pine in an acidic ionic liquid: extraction of hemicellulose and lignin to facilitate enzymatic digestion.

Niobium-modified TiO2 hierarchical spherical micrometer-size particles, which consist of many nanowires, are synthesized by solvothermal synthesis and studied as photoelectrodes for water photo-oxidation and dye-sensitized solar cell (DSSC) applications. Incorporation of Nb leads to a rutile-to-anatase TiO2 phase transition in the TiO2 hierarchical spheres (HSs), with the anatase percentage increasing from 0% for the pristine TiO2 HSs to 47.6% for the 1.82 at.% Nb-incorporated TiO2 sample. Incorporation of Nb leads to significant improvements in water photo-oxidation with the photocurrents reaching 70.5 μA cm(-2) at 1.23 V versus the reversible hydrogen electrode, compared with 28.3 μA cm(-2) for the pristine TiO2 sample. The photoconversion efficiency of Nb:TiO2 HS-based DSSCs reaches 6.09±0.15% at 0.25 at.% Nb, significantly higher than that for the pristine TiO2 HS cells (3.99±0.02%). In addition, the incident-photon-to-current efficiency spectra for DSSCs show that employing TiO2 and Nb:TiO2 HSs provides better light harvesting, especially of long-wavelength photons, than anatase TiO2 nanoparticle-based DSSCs.

Improvement of solar energy conversion with Nb-incorporated TiO2 hierarchical microspheres.

A technique enabling the detection and quantification of low density sites on planar SiO2 surfaces is demonstrated. Fluorescent probes are used to titrate free hydroxyl and strained siloxane sites on the surface of amorphous SiO2 substrates in vacuum. The titration of free hydroxyl sites was performed to validate the method and to provide a reference for the measurement of the strained siloxane site density. Perylene derivatives with different functional groups are chemisorbed onto the surface sites, enabling in situ photoluminescence (PL) measurements of the bound fluorophores. An amine functional group is used to selectively titrate strained siloxane sites, while an alcohol group is used for the titration of free hydroxyl sites. Emission intensity was found to be nonlinear with coverage for bound fluorophore densities greater than 0.1 nm(-2), necessitating the removal of molecules from the surface into a solution to obtain accurate density measurements. For lower densities, the coverage of bound fluorophores can be estimated directly from in situ PL measurements. The measured areal densities of bound fluorophores after titrating free hydroxyl sites are in good agreement with literature values for the densities of such sites on high surface area silica. PL measurements of SiO2 surfaces titrated with an amine derivative of perylene indicate that strained siloxane sites exist for vacuum pretreatment temperatures of 300 °C and increase with increasing pretreatment temperature. Densities of strained siloxane sites on the silica surface are estimated at 0.004-0.02 nm(-2) for pretreatment temperatures of 300-700 °C, demonstrating the sensitivity of this technique.

Titration of free hydroxyl and strained siloxane sites on silicon dioxide with fluorescent probes.

TiO2 is being widely explored as an active resistive switching (RS) material for resistive random access memory. We report a detailed analysis of the RS characteristics of single-crystal anatase-TiO2 thin films epitaxially grown on silicon by atomic layer deposition. We demonstrate that although the valence change mechanism is responsible for the observed RS, single-crystal anatase-TiO2 thin films show electrical characteristics that are very different from the usual switching behaviors observed for polycrystalline or amorphous TiO2 and instead very similar to those found in electrochemical metallization memory. In addition, we demonstrate highly stable and reproducible quantized conductance that is well controlled by application of a compliance current and that suggests the localized formation of conducting Magnéli-like nanophases. The quantized conductance observed results in multiple well-defined resistance states suitable for implementation of multilevel memory cells.

Highly controllable and stable quantized conductance and resistive switching mechanism in single-crystal TiO2 resistive memory on silicon.

The rapidly increasing global demand for energy combined with the environmental impact of fossil fuels has spurred the search for alternative sources of clean energy. One promising approach is to convert solar energy into hydrogen fuel using photoelectrochemical cells. However, the semiconducting photoelectrodes used in these cells typically have low efficiencies and/or stabilities. Here we show that a silicon-based photocathode with a capping epitaxial oxide layer can provide efficient and stable hydrogen production from water. In particular, a thin epitaxial layer of strontium titanate (SrTiO3) was grown directly on Si(001) by molecular beam epitaxy. Photogenerated electrons can be transported easily through this layer because of the conduction-band alignment and lattice match between single-crystalline SrTiO3 and silicon. The approach was used to create a metal-insulator-semiconductor photocathode that, under a broad-spectrum illumination at 100 mW cm(-2), exhibits a maximum photocurrent density of 35 mA cm(-2) and an open circuit potential of 450 mV; there was no observable decrease in performance after 35 hours of operation in 0.5 M H2SO4. The performance of the photocathode was also found to be highly dependent on the size and spacing of the structured metal catalyst. Therefore, mesh-like Ti/Pt nanostructured catalysts were created using a nanosphere lithography lift-off process and an applied-bias photon-to-current efficiency of 4.9% was achieved.

A silicon-based photocathode for water reduction with an epitaxial SrTiO3 protection layer and a nanostructured catalyst.

Electromigration of copper in integrated circuits leads to device failure. Potential solutions involve capping the copper with ultrathin cobalt films. We report the properties of cobalt films after deposition on polycrystalline Cu at 265 °C by atomic layer deposition from H2 and bis(N-tert-butyl-N'-ethylpropionamidinato) cobalt(II) (CoAMD). We find intermixing of Co and Cu producing a transition layer on the Cu nearly as thick as the Co-rich overlayer. X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry depth profiling reveal that a finite amount of Cu continuously segregates to the progressing Co surface, minimizing the free surface energy, throughout deposition up to at least 16 nm. The Cu-stabilized Co film initially follows 2D growth and strain-relieving 3D crystal formation is apparent beyond 2 nm of film growth. Depth profiling indicates that Cu likely diffuses within the Co film and along the polycrystalline Co grain boundaries.

Atomic Interdiffusion and Diffusive Stabilization of Cobalt by Copper During Atomic Layer Deposition from Bis(N-tert-butyl-N'-ethylpropionamidinato) Cobalt(II).

Germanium (Ge)-based metal-oxide-semiconductor field-effect transistors are a promising candidate for high performance, low power electronics at the 7 nm technology node and beyond. However, the availability of high quality gate oxide/Ge interfaces that provide low leakage current density and equivalent oxide thickness (EOT), robust scalability, and acceptable interface state density (D(it)) has emerged as one of the most challenging hurdles in the development of such devices. Here we demonstrate and present detailed electrical characterization of a high-κ epitaxial oxide gate stack based on crystalline SrHfO3 grown on Ge (001) by atomic layer deposition. Metal-oxide-Ge capacitor structures show extremely low gate leakage, small and scalable EOT, and good and reducible D(it). Detailed growth strategies and postgrowth annealing schemes are demonstrated to reduce Dit. The physical mechanisms behind these phenomena are studied and suggest approaches for further reduction of D(it).

John G. Ekerdt

McKetta Department of Chemical Engineering

The University of Texas at Austin

Epitaxial Growth of Perovskite Strontium Titanate on Germanium via Atomic Layer Deposition