Gavin Mitchson

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.

Modifying a charge density wave transition by modulation doping: ferecrystalline compounds ([Sn1−xBixSe]1.15)1(VSe2)1 with 0 ≤ x ≤ 0.66

A series of alloyed ferecrystals ([Sn1−xBixSe]1.15)1(VSe2)1 with 0 ≤ x ≤ 0.66 was synthesized via the modulated elemental reactants technique. X-ray diffraction of the compounds reveals systematic changes of the lattice parameter and the intensities of the Bragg peaks confirming the successful alloying of the compounds corroborated by Rietveld refinements. Interestingly, both constituents of the intergrowth compounds exhibit systematic structural changes as a function of the Bi-content indicating interlayer interaction. The a-axis lattice parameter of the VSe2 layer expands with increasing Bi-content, which signifies changes in the electronic structure of this constituent. Electrical resistivities, Hall and Seebeck coefficients of compounds with a varying Bi-content present a complex scenario. At low Bi-contents an enhancement of the charge density wave transition is observed, whereas further substitution results in a suppression of the effect. At Bi-contents exceeding x = 0.55 minority carriers from the Sn1−xBixSe layer contribute to the transport properties.

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.

Helium ion beam lithography (HIBL) using HafSOx as the resist

Helium ion beam lithography (HIBL) is a novel alternative lithographic technique with the capacity of fabricating highresolution and high-density features. Only limited research has been performed exploring HIBL to date. HafSOx (Hf(OH)4–2x−2y(O2)x(SO4)y·qH2O) is a negative-tone inorganic resist that is one of several candidate resist materials for extreme ultraviolet lithography (EUVL) and e-beam lithography (EBL), and has been demonstrated to show high resolution, moderate sensitivity and low line-edge roughness (LER) in both EUVL and EBL. To date, no ion beam lithography work on HafSOx has been reported. In this study, we tested HafSOx as an HIBL resist and achieved a high sensitivity compared with EBL with a turn-on dose D100 ~ 2-4 μC/cm2. We obtained sub-10 nm line widths with low LER. A simple Monte Carlo simulation suggests that ionizing excitation accounts for most of the incident He ions’ energy loss.

Antiphase Boundaries in the Turbostratically Disordered Misfit Compound (BiSe)(1+δ)NbSe2.

(BiSe)(1+δ)NbSe2 ferecrystals were synthesized in order to determine whether structural modulation in BiSe layers, characterized by periodic antiphase boundaries and Bi-Bi bonding, occurs. Specular X-ray diffraction revealed the formation of the desired compound with a c-axis lattice parameter of 1.21 nm from precursors with a range of initial compositions and initial periodicities. In-plane X-ray diffraction scans could be indexed as hk0 reflections of the constituents, with a rectangular basal BiSe lattice and a trigonal basal NbSe2 lattice. Electron micrographs showed extensive turbostratic disorder in the samples and the presence of periodic antiphase boundaries (approximately 1.5 nm periodicity) in BiSe layers oriented with the [110] direction parallel to the zone axis of the microscope. This indicates that the structural modulation in the BiSe layers is not due to coherency strain resulting from commensurate in-plane lattices. Electrical transport measurements indicate that holes are the dominant charge carrying species, that there is a weak decrease in resistivity as temperature decreases, and that minimal charge transfer occurs from the BiSe to NbSe2 layers. This is consistent with the lack of charge transfer from the BiX to the TX2 layers reported in misfit layer compounds where antiphase boundaries were observed. This suggests that electronic considerations, i.e., localization of electrons in the Bi-Bi pairs at the antiphase boundaries, play a dominant role in stabilizing the structural modulation.

Charge transfer in (PbSe)1+δ(NbSe2)2 and (SnSe)1+δ(NbSe2)2 ferecrystals investigated by photoelectron spectroscopy

Rotationally disordered, layered (PbSe)[Formula: see text](NbSe2)2 and (SnSe)[Formula: see text](NbSe2)2 ferecrystal heterostructures, consisting of stacked two-dimensional bilayers of either PbSe or SnSe alternating with two planes of NbSe2, were synthesized from modulated elemental reactants. The electronic structure of these ternary systems was investigated using x-ray photoelectron spectroscopy and compared to the binary bulk compounds PbSe, SnSe and NbSe2. The Pb and Sn core level spectra show a significant shift towards lower binding energies and the peak shape becomes asymmetric in the ferecrystals, while the electronic structure of the NbSe2 layers does not change compared to the bulk. This is interpreted in terms of an interlayer interaction in the form of a charge transfer of electrons from PbSe or SnSe into the NbSe2 layers, which is supported by valence band spectra and is consistent with prior results from transport measurements.

Nonuniform Composition Profiles in Amorphous Multimetal Oxide Thin Films Deposited from Aqueous Solution

Metal oxide thin films are ubiquitous in technological applications. Often, multiple metal components are used to achieve desired film properties for specific functions. Solution deposition offers an attractive route for producing these multimetal oxides because it allows for careful control of film composition through the manipulation of precursor stoichiometry. Although it has been generally assumed that homogeneous precursor solutions yield homogeneous thin films, we recently reported evidence of nonuniform electron density profiles in aqueous-deposited films. Herein, we show that nonuniform electron densities in lanthanum zirconium oxide (LZO) thin films are the result of inhomogeneous distributions of metal components. Specifically, La aggregates at the film surface, whereas Zr is relatively evenly distributed throughout single-layer films. This inhomogeneous metal distribution persists in stacked multilayer films, resulting in La-rich interfaces between the sequentially deposited layers. Testing of metal-insulator-semiconductor devices fabricated from single and multilayer LZO films shows that multilayer films have higher dielectric constants, indicating that La-rich interfaces in multilayer films do not detrimentally impact film properties. We attribute the enhanced dielectric properties of multilayer films to greater condensation and densification relative to single-layer films, and these results suggest that multilayer films may be preferred for device applications despite the presence of layering artifacts.

Structural Changes in 2D BiSe Bilayers as n Increases in (BiSe)1+δ(NbSe2)n (n = 1–4) Heterostructures

(BiSe)1+δ(NbSe2)n heterostructures with n = 1-4 were synthesized using modulated elemental reactants. The BiSe bilayer structure changed from a rectangular basal plane with n = 1 to a square basal plane for n = 2-4. The BiSe in-plane structure was also influenced by small changes in the structure of the precursor, without significantly changing the out-of-plane diffraction pattern or value of the misfit parameter, δ. Density functional theory calculations on isolated BiSe bilayers showed that its lattice is very flexible, which may explain its readiness to adjust shape and size depending on the environment. Correlated with the changes in the BiSe basal plane structure, analysis of scanning transmission electron microscope images revealed that the occurrence of antiphase boundaries, found throughout the n = 1 compound, is dramatically reduced for the n = 2-4 compounds. X-ray photoelectron spectroscopy measurements showed that the Bi 5d3/2, 5d5/2 doublet peaks narrowed toward higher binding energies as n increased from 1 to 2, also consistent with a reduction in the number of antiphase boundaries. Temperature-dependent electrical resistivity and Hall coefficient measurements of nominally stoichiometric samples in conjunction with structural refinements and XPS data suggest a constant amount of interlayer charge transfer independent of n. Constant interlayer charge transfer is surprising given the changes in the BiSe in-plane structure. The structural flexibility of the BiSe layer may be useful in designing multiple constituent heterostructures as an interlayer between structurally dissimilar constituents.

Long-Range Order in [(SnSe)1.2]1[TiSe2]1 Prepared from Designed Precursors

Self-assembly of designed precursors has enabled the synthesis of novel heterostructures that exhibit extensive rotational disorder between constituents. In (SnSe)1.2TiSe2 nanoscale regions of long-range order were observed in scanning transmission electron microscopy (STEM) cross sectional images. Here a combination of techniques are used to determine the structure of this compound, and the information is used to infer the origin of the order. In-plane X-ray diffraction indicates that the SnSe basal plane distorts to match TiSe2. This results in a rectangular unit cell that deviates from both the bulk structure and the square in-plane unit cell previously observed in heterostructures containing SnSe bilayers separated by layers of dichalcogenides. The distortion results from lattice matching of the two constituents, which occurs along the <100> SnSe and the <110> TiSe2 directions as √3 × aTiSe2 equals aSnSe. Fast Fourier transform analysis of the STEM images exhibits sharp maxima in hkl families where h,k ≠ 0. The period is the same as that observed for 00l reflections, indicating regions of long-range superlattice order in all directions. X-ray reciprocal space maps contain broad maxima in hkl families of TiSe2 and SnSe based reflections consistent with the superlattice period, indicating that long-range order is present throughout a significant fraction of the film. The STEM images show that <110> planes of TiSe2 are adjacent to <100> planes of SnSe. Density functional theory suggests the preferred orientation is due to favored directions of nucleation with significant energy differences between islands of SnSe with different orientation relative to TiSe2. The calculations suggest that the long-range order in (SnSe)1.2TiSe2 results from an accidental coincidence in the lattice parameters of SnSe and TiSe2. These findings support a layer by layer nucleation process for the self-assembly of heterostructures from designed precursors, which rationalizes how designed precursors enable compounds with different constituents, defined thicknesses, and specific layer sequences to be prepared.

Interface-Driven Structural Distortions and Composition Segregation in Two-Dimensional Heterostructures

The discovery of emergent phenomena in 2D materials has sparked substantial research efforts in the materials community. A significant experimental challenge for this field is exerting atomistic control over the structure and composition of the constituent 2D layers and understanding how the interactions between layers drive both structure and properties. While no segregation for single bilayers was observed, segregation of Pb to the surface of three bilayer thick PbSe-SnSe alloy layers was discovered within [(Pbx Sn1-x Se)1+δ ]n (TiSe2 )1 heterostructures using electron microscopy. This segregation is thermodynamically favored to occur when Pbx Sn1-x Se layers are interdigitated with TiSe2 monolayers. DFT calculations indicate that the observed segregation depends on what is adjacent to the Pbx Sn1-x Se layers. The interplay between interface- and volume-free energies controls both the structure and composition of the constituent layers, which can be tuned using layer thickness.