[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Adsorption of the atomic layer deposition (ALD) precursor bis(N-tert-butyl-N′-ethylpropionamidinato) cobalt(II) (CoAMD) on SiO2, carbon-doped oxide (CDO), and Cu is reported. Adsorption was performed under ALD cycling conditions with and without H2 coreactant to mimic the first and second ALD half reactions on the substrates. Resultant surface chemistries were evaluated by X-ray photoelectron spectroscopy. Adsorption of CoAMD proved self-limiting and the precursor reduced readily on Cu with and without H2 coreactant to form Co0. Residual C and N signals on Cu suggest that amidinate ligands and decomposition fragments from CoAMD adsorb on the Cu surface. On SiO2 and CDO, CoAMD chemisorbs on O containing moieties, primarily OH, to form Co2+. Accumulation of Co after three ALD cycles was greatest on Cu and least on CDO.
[Show abstract][Hide abstract] ABSTRACT: This work demonstrates the growth of crystalline SrTiO3 (STO) directly on germanium via a chemical method. After thermal deoxidation, the Ge substrate is transferred in vacuo to the deposition chamber where a thin film of STO (2 nm) is deposited by atomic layer deposition (ALD) at 225 °C. Following post-deposition annealing at 650 °C for 5 min, the STO film becomes crystalline with epitaxial registry to the underlying Ge (001) substrate. Thicker STO films (up to 15 nm) are then grown on the crystalline STO seed layer. The crystalline structure and orientation are confirmed via reflection high-energy electron diffraction, X-ray diffraction, and transmission electron microscopy. Electrical measurements of a 15-nm thick epitaxial STO film on Ge show a large dielectric constant (k ≈ 90), but relatively high leakage current of ≈10 A/cm2 for an applied field of 0.7 MV/cm. To suppress the leakage current, an aluminum precursor is cycled during ALD growth to grow crystalline Al-doped STO (SrTi1-xAlxO3-δ) films. With sufficient Al doping (≈13%), the leakage current decreases by two orders of magnitude for an 8-nm thick film. The current work demonstrates the potential of ALD-grown crystalline oxides to be explored for advanced electronic applications, including high-mobility Ge-based transistors.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Low-density sites on planar fused silica surfaces are studied by titration with fluorescent probe molecules in an ultrahigh vacuum environment. Intrinsic sites chemically distinct from either hydroxyl or strained siloxane sites are identified by titration with a perylene derivative containing a vinyl functional group. Evidence is presented that these sites are oxygen vacancy defect (OVD) sites, which have previously been difficult to detect experimentally. The density of intrinsic OVD sites is shown to depend on pretreatment temperature, with an estimated density of approximately 1011 sites/cm2 for fused silica heated to 700 °C in vacuum. The influence of molecular and atomic deuterium exposure on various silica surface sites is also explored.
[Show abstract][Hide abstract] ABSTRACT: Crystalline lanthanum aluminate (LAO) films were grown epitaxially on SrTiO3(001) and on Si(001) with a buffer layer of four unit cells of SrTiO3 by atomic layer deposition. The SrTiO3 buffer layer was grown by molecular beam epitaxy. Tris(N,N'-diisopropylformamidinate)-lanthanum, trimethylaluminum, and water as co-reactants were employed at 250 °C for atomic layer deposition. Films were characterized using ex-situ reflection high-energy electron diffraction, X-ray diffraction and in-situ X-ray photoelectron spectroscopy. The as-deposited LAO films were amorphous. Different annealing conditions were necessary to realize crystalline films because of different degrees of tensile strain between crystalline LAO and the SrTiO3 or the Si(001) substrate. When grown on SrTiO3(001), with a lattice mismatch of 2.9%, annealing temperatures of 750 °C for 2 h were necessary. Crystalline films were realized at 600 °C under vacuum at 2 h for SrTiO3-buffered Si(001), with a lattice mismatch of 1.3%. By keeping the annealing temperature relatively low (2 h at 600 °C under vacuum), the interfacial amorphous layer at the STO/Si interface was minimized to about one monolayer and an abrupt interface between SrTiO3 and LAO was maintained.
[Show abstract][Hide abstract] ABSTRACT: Photophysical processes occurring in two-dimensional systems of perylene derivatives covalently attached to planar silica surfaces are studied using a combination of steady-state and time-resolved measurements. Monomeric emission is observed for low fluorophore densities (below 0.15 nm–2). A broad emission feature centered around 530 nm is observed for high fluorophore densities (above 0.44 nm–2) and is attributed to excimer-like emission from partially overlapping perylene moieties. No distinct emission feature from the fully relaxed excimer state is observed, although the presence of a small fraction of such sites is inferred from both steady-state and time-resolved measurements. A one-carbon chain is used to anchor the fluorophore to the surface by the reaction of perylene-3-methanol with free hydroxyl and strained siloxane sites, likely hindering the ability of neighboring molecules to form a fully relaxed excimer state. Analysis of fluorescence decay profiles indicates energy migration between sites occurs even for low fluorophore densities.
The Journal of Physical Chemistry C. 01/2014; 118(4):2104–2114.
[Show abstract][Hide abstract] ABSTRACT: Atomic layer deposition (ALD) of epitaxial c-axis oriented BaTiO3 (BTO) on Si(001) using a thin (1.6 nm) buffer layer of SrTiO3 (STO) grown by molecular beam epitaxy is reported. The ALD growth of crystalline BTO films at 225 °C used barium bis(triisopropylcyclopentadienyl), titanium tetraisopropoxide, and water as co-reactants. X-ray diffraction (XRD) reveals a high degree of crystallinity and c-axis orientation of as-deposited BTO films. Crystallinity is improved after vacuum annealing at 600 °C. Two-dimensional XRD confirms the tetragonal structure and orientation of 7–20-nm thick films. The effect of the annealing process on the BTO structure is discussed. A clean STO/Si interface is found using in-situ X-ray photoelectron spectroscopy and confirmed by cross-sectional scanning transmission electron microscopy. The capacitance-voltage characteristics of 7–20 nm-thick BTO films are examined and show an effective dielectric constant of ∼660 for the heterostructure.
[Show abstract][Hide abstract] ABSTRACT: The use of PH3 as the P source in the growth of amorphous ruthenium–phosphorus (Ru(P)) alloy films by dual-source chemical vapor deposition (CVD) with Ru3(CO)12 to produce thin (~ 3 nm) Cu diffusion barriers is examined. Comparisons are made to films grown using P(CH3)3. Carbon contamination of 10 at.% carbon or less was observed in PH3-produced Ru(P) films, compared to greater than 30 atomic % carbon in films using P(CH3)3, and lower resistivity was also observed. PH3 was found to be much more reactive than previously-used P precursors, requiring the use of very low PH3 partial pressures (~ 0.13 mPa) and a sequenced addition process that allowed accumulated P to diffuse into the Ru(P) film during growth. X-ray reflectivity and atomic force microscopy indicate that films of good continuity and smoothness can be grown by CVD in the 3 nm thickness range. X-ray diffraction shows the amorphous phase to be stable for annealing at 400 °C for 3 h. Electric field stress tests to failure for Cu/Ru(P)/SiO2/Si stacks indicate that low-carbon Ru(P) barrier films function at least as well as their higher-carbon counterparts as Cu barriers and better than Ta/TaN stacks of similar thickness grown for comparison purposes.
Thin Solid Films 01/2014; 558:160–164. · 1.87 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Strontium titanate, SrTiO3 (STO), thin films incorporated with lanthanum are grown on Si (001) substrates at a thickness range of 5–25 nm. Atomic layer deposition (ALD) is used to grow the LaxSr1−xTiO3 (La:STO) films after buffering the Si (001) substrate with four-unit-cells of STO deposited by molecular beam epitaxy. The crystalline structure and orientation of the La:STO films are confirmed via reflection high-energy electron diffraction, X-ray diffraction, and cross-sectional transmission electron microscopy. The low temperature ALD growth (∼225 °C) and post-deposition annealing at 550 °C for 5 min maintains an abrupt interface between Si (001) and the crystalline oxide. Higher annealing temperatures (650 °C) show more complete La activation with film resistivities of ∼2.0 × 10−2 Ω cm for 20-nm-thick La:STO (x ∼ 0.15); however, the STO-Si interface is slightly degraded due to the increased annealing temperature. To demonstrate the selective incorporation of lanthanum by ALD, a layered heterostructure is grown with an undoped STO layer sandwiched between two conductive La:STO layers. Based on this work, an epitaxial oxide stack centered on La:STO and BaTiO3 integrated with Si is envisioned as a material candidate for a ferroelectric field-effect transistor.
Journal of Applied Physics 01/2014; 115(22):224108-224108-8. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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 non-linear 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 is 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 nm(-2) - 0.02 nm(-2) for pretreatment temperatures of 300 °C - 700 °C, demonstrating the sensitivity of this technique.
[Show abstract][Hide abstract] ABSTRACT: Ruthenium was deposited on SiO2/Si(001) substrates at 473 K by chemical vapor deposition (CVD) using triruthenium dodecacarbonyl with and without an overpressure of CO. Carbon monoxide was employed to inhibit the growth of previously nucleated islands to allow the formation of additional nuclei. Carbon monoxide also competed with the precursor for free hydroxyl sites on SiO2 sites where precursor adsorption and decomposition is favored. Total pressure was maintained at 84 mTorr, and CO was introduced at partial pressures of 2.5 and 8.4 mTorr at various intervals during 15 min growth runs. The nucleation density decreases with increasing CO overpressure when CO and precursor are injected simultaneously from the beginning; in this case, CO blocks the free hydroxyls where the Ru precursor dissociates. When 8.4 mTorr CO is introduced for 5 min to the CVD chamber after a 10 min period of deposition without CO, the maximum nucleation density was achieved (16.4 × 1011/cm2), which is twice as much as the Ru particle density found for 15 min deposition without added CO. After 10 min of growth, hydroxyl groups have mostly reacted and the injected CO adsorbs on Ru nanoparticles, inhibiting growth and forcing additional Ru nucleation on the SiO2 substrate. Growth was extended to 2 h to explore the influence of CO on ultrathin Ru film characteristics. The film grown without CO for 10 min and then with 8.4 mTorr CO for 1 h 50 min was thinner and smoother than the film grown without CO for 2 h because CO adsorption on the Ru surface slows the Ru islands/film growth rate.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Cobalt oxide (CoO) films were grown epitaxially on Si(001) by atomic
layer deposition (ALD) using a thin (1.6 nm) buffer layer of strontium
titanate (STO) grown by molecular beam epitaxy (MBE). Reflection
high-energy electron diffraction, X-ray diffraction, and cross sectional
scanning transmission electron microscopy were performed to characterize
the crystalline structure of the films. The CoO films were found to be
crystalline as-deposited even at the low growth temperature with no
evidence of Co diffusion into Si. In-situ X-ray photoelectron
spectroscopy (XPS) was used to measure the band alignment of the two
heterojunctions, CoO/STO and CoO/TiO2. The experimental band
alignment is compared to electronic structure calculations using density
[Show abstract][Hide abstract] ABSTRACT: 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 5h 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.
[Show abstract][Hide abstract] ABSTRACT: Cobalt oxide (CoO) films are grown epitaxially on Si(001) by atomic layer deposition (ALD) using a thin (1.6 nm) buffer layer of strontium titanate (STO) grown by molecular beam epitaxy. The ALD growth of CoO films is done at low temperature (170–180 °C), using cobalt bis(diisopropylacetamidinate) and water as co-reactants. Reflection high-energy electron diffraction, X-ray diffraction, and cross-sectional scanning transmission electron microscopy are performed to characterize the crystalline structure of the films. The CoO films are found to be crystalline as-deposited even at the low growth temperature with no evidence of Co diffusion into Si. The STO-buffered Si (001) is used as a template for ALD growth of relatively thicker epitaxial STO and TiO2 films. Epitaxial and polycrystalline CoO films are then grown by ALD on the STO and TiO2 layers, respectively, creating thin-film heterostructures for photoelectrochemical testing. Both types of heterostructures, CoO/STO/Si and CoO/TiO2/STO/Si, demonstrate water photooxidation activity under visible light illumination. In-situ X-ray photoelectron spectroscopy is used to measure the band alignment of the two heterojunctions, CoO/STO and CoO/TiO2. The experimental band alignment is compared to electronic structure calculations using density functional theory.
Journal of Applied Physics 01/2013; 114(8):084901-084901-8. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Lanthanum aluminate (LAO) films were grown epitaxially on Si (001) by atomic layer deposition (ALD) using a buffer layer of strontium titanate (STO) grown by molecular beam epitaxy. The ALD growth of LAO was done at 250 °C by using tris(N,N′-diisopropylformamidinate)–lanthanum, trimethylaluminum, and water as co-reactants. Reflection high-energy electron diffraction, X-ray diffraction and transmission electron microscopy were used to determine film crystallinity. The as-deposited LAO films were amorphous and became crystalline after vacuum annealing at 600 °C for 2 h. In-situ X-ray photoelectron spectroscopy (XPS) was used to characterize the LAO/STO/Si interfaces at various stages throughout the growth and annealing process. XPS analysis showed minimal SiO bonding at the STO/Si interface after the ALD process and after post-deposition annealing at 600 °C for 2 h. The results demonstrate a method to integrate epitaxial LAO films on Si (001) substrates by ALD.
Journal of Crystal Growth 01/2013; 363:150–157. · 1.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Epitaxial strontium titanate (STO) films have been grown by atomic layer deposition (ALD) on Si(001) substrates with a thin STO buffer layer grown by molecular beam epitaxy (MBE). Four unit cells of STO grown by MBE serve as the surface template for ALD growth. The STO films grown by ALD are crystalline as-deposited with minimal, if any, amorphous SiOx layer at the STO-Si interface. The growth of STO was achieved using bis(triisopropylcyclopentadienyl)-strontium, titanium tetraisopropoxide, and water as the coreactants at a substrate temperature of 250 °C. In situ x-ray photoelectron spectroscopy (XPS) analysis revealed that the ALD process did not induce additional Si–O bonding at the STO-Si interface. Postdeposition XPS analysis also revealed sporadic carbon incorporation in the as-deposited films. However, annealing at a temperature of 250 °C for 30 min in moderate to high vacuum (10-6–10-9 Torr) removed the carbon species. Higher annealing temperatures (>275 °C) gave rise to a small increase in Si–O bonding, as indicated by XPS, but no reduced Ti species were observed. X-ray diffraction revealed that the as-deposited STO films were c-axis oriented and fully crystalline. A rocking curve around the STO(002) reflection gave a full width at half maximum of 0.30° ± 0.06° for film thicknesses ranging from 5 to 25 nm. Cross-sectional transmission electron microscopy revealed that the STO films were continuous with conformal growth to the substrate and smooth interfaces between the ALD- and MBE-grown STO. Overall, the results indicate that thick, crystalline STO can be grown on Si(001) substrates by ALD with minimal formation of an amorphous SiOx layer using a four-unit-cell STO buffer layer grown by MBE to serve as the surface template.
Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/2013; 31(1):01A136-01A136-9. · 1.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Epitaxial anatase titanium dioxide (TiO2) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer grown by molecular beam epitaxy (MBE) to serve as a surface template. The growth of TiO2 was achieved using titanium isopropoxide and water as the co-reactants at a substrate temperature of 225–250 °C. To preserve the quality of the MBE-grown STO, the samples were transferred in-situ from the MBE chamber to the ALD chamber. After ALD growth, the samples were annealed in-situ at 600 °C in vacuum (10− 7 Pa) for 1–2 h. Reflection high-energy electron diffraction was performed during the MBE growth of STO on Si(001), as well as after deposition of TiO2 by ALD. The ALD films were shown to be highly ordered with the substrate. At least four unit cells of STO must be present to create a stable template on the Si(001) substrate for epitaxial anatase TiO2 growth. X-ray diffraction revealed that the TiO2 films were anatase with only the (004) reflection present at 2θ = 38.2°, indicating that the c-axis is slightly reduced from that of anatase powder (2θ = 37.9°). Anatase TiO2 films up to 100 nm thick have been grown that remain highly ordered in the (001) direction on STO-buffered Si(001) substrates.
Thin Solid Films 08/2012; 520(21):6525–6530. · 1.87 Impact Factor