Catherine J. Page

Lanthanum Aluminum Oxide Thin-Film Dielectrics from Aqueous Solution

Amorphous LaAlO3 dielectric thin films were fabricated via solution processing from inorganic nitrate precursors. Precursor solutions contained soluble oligomeric metal-hydroxyl and/or -oxo species as evidenced by dynamic light scattering (DLS) and Raman spectroscopy. Thin-film formation was characterized as a function of annealing temperature using Fourier transform infrared (FTIR), X-ray diffraction (XRD), X-ray reflectivity (XRR), scanning electron microscopy (SEM), and an array of electrical measurements. Annealing temperatures ≥500 °C result in thin films with low leakage-current densities (∼1 × 10(-8) A·cm(-2)) and dielectric constants ranging from 11.0 to 11.5. When incorporated as the gate dielectric layer in a-IGZO thin-film transistors (TFTs), LaAlO3 thin films annealed at 600 °C in air yielded TFTs with relatively low average mobilities (∼4.5 cm(2)·V(-1)·s(-1)) and high turn-on voltages (∼26 V). Interestingly, reannealing the LaAlO3 in 5%H2/95%N2 at 300 °C before deposition of a-IGZO channel layers resulted in TFTs with increased average mobilities (11.1 cm(2)·V(-1)·s(-1)) and lower turn-on voltages (∼6 V).

High-κ Lanthanum Zirconium Oxide Thin Film Dielectrics from Aqueous Solution Precursors

Metal oxide thin films are critical components in modern electronic applications. In particular, high-κ dielectrics are of interest for reducing power consumption in metal-insulator-semiconductor (MIS) field-effect transistors. Although thin-film materials are typically produced via vacuum-based methods, solution deposition offers a scalable and cost-efficient alternative. We report an all-inorganic aqueous solution route to amorphous lanthanum zirconium oxide (La2Zr2O7, LZO) dielectric thin films. LZO films were spin-cast from aqueous solutions of metal nitrates and annealed at temperatures between 300 and 600 °C to produce dense, defect-free, and smooth films with subnanometer roughness. Dielectric constants of 12.2-16.4 and loss tangents <0.6% were obtained for MIS devices utilizing LZO as the dielectric layer (1 kHz). Leakage currents <10-7 A cm-2 at 4 MV cm-1 were measured for samples annealed at 600 °C. The excellent surface morphology, high dielectric constants, and low leakage current densities makes these LZO dielectrics promising candidates for thin-film transistor devices.

A new strontium-copper ethylene glycol complex: synthesis and structure of [Sr(C2H6O2)5][Cu(C2H4O2)2]·C2H6O2

Abstract A new crystalline strontium-copper ethylene glycol complex has been isolated and structurally characterized by single-crystal X-ray diffraction. The solution-phase complex has also been investigated as a possible molecular precursor for use in sol-gel synthesis of SrCuO2. [Sr(C2H6O2)5][Cu(C2H4O2)2]·C2H6O2 has been isolated by crystallization from an ethylene glycol/methylethylketone solution of the strontium-copper salt. Crystallographic data are as follows: orthorhombic, space group Pna21, a = 18.494(3), b = 9.3076(8), c = 16.615(3)A, V = 2860(2) A 3 , Z = 4, R = 0.044 . In this molecule, copper is coordinated by two bidentate ethylene glycolate ligands in roughly a square planar geometry. Strontium is coordinated by three bidentate ethylene glycol molecules and two monodentate ethylene glycol molecules; the eight-fold coordination resembles a highly-distorted square antiprism. An additional ethylene glycol of crystallization is present which does not directly coordinate the strontium ion. Copper and strontium ion complexes are held together by hydrogen bonding. The solution-phase bimetallic complex is hydrolytically unstable, giving rise to a ‘muddy’ brown precipitate upon exposure to water. The apparent decomposition is in direct contrast with hydrolysis of similar barium-copper complexes which gives rise to stable blue gels. This decomposition made the strontium-copper diol complex unsuitable for use in alkoxide sol-gel synthesis of strontium-copper oxides.

Amorphous Mixed-Metal Oxide Thin Films from Aqueous Solution Precursors with Near-Atomic Smoothness

Thin films with tunable and homogeneous composition are required for many applications. We report the synthesis and characterization of a new class of compositionally homogeneous thin films that are amorphous solid solutions of Al2O3 and transition metal oxides (TMOx) including VOx, CrOx, MnOx, Fe2O3, CoOx, NiO, CuOx, and ZnO. The synthesis is enabled by the rapid decomposition of molecular transition-metal nitrates TM(NO3)x at low temperature along with precondensed oligomeric Al(OH)x(NO3)3-x cluster species, both of which can be processed from aq solution. The films are dense, ultrasmooth (Rrms < 1 nm, near 0.1 nm in many cases), and atomically mixed amorphous metal-oxide alloys over a large composition range. We assess the chemical principles that favor the formation of amorphous homogeneous films over rougher phase-segregated nanocrystalline films. The synthesis is easily extended to other compositions of transition and main-group metal oxides. To demonstrate versatility, we synthesized amorphous V0.1Cr0.1Mn0.1Fe0.1Zn0.1Al0.5Ox and V0.2Cr0.2Fe0.2Al0.4Ox with Rrms ≈ 0.1 nm and uniform composition. The combination of ideal physical properties (dense, smooth, uniform) and broad composition tunability provides a platform for film synthesis that can be used to study fundamental phenomena when the effects of transition metal cation identity, solid-state concentration of d-electrons or d-states, and/or crystallinity need to be controlled. The new platform has broad potential use in controlling interfacial phenomena such as electron transfer in solar-cell contacts or surface reactivity in heterogeneous catalysis.

Diffusion of dissolved ions from wet silica sol–gel monoliths: Implications for biological encapsulation

Divalent nickel (Ni(2+)), Cu(II)EDTA, methyl orange, and dichromate were used to investigate diffusion from hydrated silica sol-gel monoliths. The objective was to examine diffusion of compounds on a size regime relevant to supporting biological components encapsulated within silica gel prepared in a biologically compatible process space with no post-gelation treatments. With an initial sample set, gels prepared from tetraethoxysilane were explored in a factorial design with Ni(2+) as the tracer, varying water content during hydrolysis, acid catalyst present during hydrolysis, and the final concentration of silica. A second sample set explored diffusion of all four tracers in gels prepared with aqueous silica precursors and a variety of organically modified siloxanes. Excluding six outliers which displayed significant syneresis, the mean diffusion constant (D(gel)) across the entire process space of sample set 1 was 2.42×10(-10) m(2) s(-1); approximately 24% of the diffusion coefficient of Ni(2+) in unconfined aqueous solution. In sample set 2, the tracer size and not gel hydrophobicity was the primary determinant of changes in diffusion rates. A strong linear inverse correlation was found between tracer size and the magnitude of D(gel). Based on correlation with the tracers used in this investigation, the characteristic 1-h diffusion distance for carbonate species relevant to supporting active phototrophic organisms was approximately 1.5mm. These results support the notion that silica sol-gel formulations may be optimized for a given biological entity of interest with manageable impact to the diffusion of small ions and molecules.

Self-assembly of thin film superstructures based on alternating metal-bisphosphonate and cobalt-diisocyanide layers

Abstract A new chemical strategy for the synthesis of self-assembled (SA) multilayer superstructures is described which allows facile incorporation of aligned nonlinear optical (NLO) chromophores. This strategy involves alternating layers of the well-established metal-bisphosphonate system with layers of the recently reported cobalt-diisocyanide SA system. Previously, building SA multilayers with oriented NLO chromophores has required extra activation or deprotection steps that are time consuming and which often involve harsh chemical treatments. Several results showing the feasiblity of the new approach are presented. A film incorporating the desired chemistries was synthesized and characterized by ellipsometry. Control experiments demonstrating the necessary metal-ligand binding specificity (cobalt with isocyanide versus hafnium with phosphonate ligands) are reported. Cobalt-diisocyanohexane films exhibiting alkyl-isocyanide-cobalt linkages are reported for the first time, including characterization by ellipsometry and grazing angle X-ray diffraction. Thus, both alkyl- and aryl-isocyanide functionalities in cobalt-isocyanide multilayers have now been shown to be feasible components of the proposed supramolecular structures. Finally, preliminary second harmonic generation data are presented, demonstrating the potential utility of the materials.

Second Harmonic Generation from Multilayers of Oriented Metal Bisphosphonates

Second order nonlinear optical properties (NLO) require the presence of a polarizable moiety situated in an anharmonic potential. The approach to incorporating such properties into self-assembled multilayers involves use of asymmetric {alpha},{omega} bisphosphonates which meet this requirement by virtue of their chemical structure and binding properties. The authors have developed and optimized protection and deprotection schemes to allow for oriented layering of these molecules. Characterization by optical ellipsometry and grazing angle X-ray diffraction provides insight on average layer thicknesses and bulk film densities. Second harmonic generation (SHG) intensity from the bulk film is measured to verify NLO activity.

Same Precursor, Two Different Products: Comparing the Structural Evolution of In–Ga–O “Gel-Derived” Powders and Solution-Cast Films Using Pair Distribution Function Analysis

Amorphous metal oxides are central to a variety of technological applications. In particular, indium gallium oxide has garnered attention as a thin-film transistor channel layer material. In this work we examine the structural evolution of indium gallium oxide gel-derived powders and thin films using infrared vibrational spectroscopy, X-ray diffraction, and pair distribution function (PDF) analysis of X-ray total scattering from standard and normal incidence thin-film geometries (tfPDF). We find that the gel-derived powders and films from the same aqueous precursor evolve differently with temperature, forming mixtures of Ga-substituted In2O3 and In-substituted β-Ga2O3 with different degrees of substitution. X-ray total scattering and PDF analysis indicate that the majority phase for both the powders and films is an amorphous/nanocrystalline β-Ga2O3 phase, with a minor constituent of In2O3 with significantly larger coherence lengths. This amorphous β-Ga2O3 phase could not be identified using the conventional Bragg diffraction techniques traditionally used to study crystalline metal oxide thin films. The combination of Bragg diffraction and tfPDF provides a much more complete description of film composition and structure, which can be used to detail the effect of processing conditions and structure-property relationships. This study also demonstrates how structural features of amorphous materials, traditionally difficult to characterize by standard diffraction, can be elucidated using tfPDF.

Application of HAADF STEM image analysis to structure determination in rotationally disordered and amorphous multilayered films

We report results from high angle annular dark field scanning transmission electron microscopy (HAADF STEM) image analysis of complex semi-crystalline and amorphous materials, and apply the insights gained from local structure information towards global structure determination. Variations in HAADF STEM intensities for a rotationally disordered heterostructure and an amorphous oxide film are statistically analyzed to extract information regarding the inhomogeneity of the films perpendicular to the substrate. By assuming chemical homogeneity in the film axis parallel to the substrate, the signal intensity variation parallel to the substrate is used to estimate the signal noise level, allowing evaluation of the significance of intensity differences in the substrate normal direction. The positions of HAADF STEM intensity peaks in the perpendicular direction, averaged from multiple images, provide a valuable initial model for a Rietveld refinement of the global c-axis structure of the heterostructure. For an amorphous multi-coat solution-cast oxide sample, the analysis reveals statistically significant variations in the HAADF STEM intensity profile perpendicular to the substrate. These variations indicate an inhomogeneous density profile, presumably related to the spin-casting of individual layers and have implications for understanding the chemical interactions that occur between layers when preparing multilayer amorphous oxide films from solution.

Low-temperature fabrication of lithium aluminum oxide phosphate solid electrolyte thin films from aqueous precursors

Low-temperature routes to solid electrolytes are important for construction of solid-state batteries, electrochromic devices, electrolyte-gated transistors, high-energy capacitors and sensors. Here we report an environmentally friendly aqueous solution route to amorphous thin films of the solid electrolyte lithium aluminum oxide phosphate (LiAlPO). This route allows production of high quality films at very low temperatures (275 °C), 600 °C lower than traditional melt quenching routes to LiAlPO. Films of thicknesses ranging from 20–150 nm produced by this route are extremely smooth and fully dense, with temperature dependent conductivities similar to those reported for samples made by melt quenching techniques. The average room temperature conductivity of LiAlPO films was measured to be σDC = 2.6 × 10−8 S cm−1. Film evolution was monitored by TGA-DSC and FTIR, and resulting films were characterized using FTIR, XPS, SEM, and XRD. These techniques indicate that water and nitrate removal is complete by 250 °C, and the films remain amorphous to 400 °C. The approach developed demonstrates a simple, inexpensive and environmentally benign deposition route for the fabrication of inorganic solid electrolyte thin films, using LiAlPO as a model system.