Jeffrey F. DeNatale

MILLIMETER-WAVE DIELECTRIC PROPERTIES OF EPITAXIAL VANADIUM DIOXIDE THIN FILMS

The dielectric properties of grain‐oriented thin‐film vanadium dioxide (VO2) on single‐crystal sapphire have been measured as a function of temperature across the phase transformation at 68 °C. The properties of the low‐ and high‐temperature phases were determined by measuring the millimeter‐wave transmittance and phase shift through the samples. The real part of the dielectric constant in the high‐temperature phase increases with increasing thickness from less than 1000 for a 40‐nm‐thick film (the resolvable limit of this measurement technique) to over 90 000 for a 580‐nm‐thick film. The strong thickness dependence in epitaxial VO2 thin films can be attributed to a dielectric mixture phenomenon and can be described by effective‐medium theory. The large real dielectric constant and its thickness dependence is likely due to bonding distortions at the grain boundaries. Analysis of the experimental data shows the VO2 thin films used in this study have no secondary grain‐boundary phases, but do have interfaci...

High‐quality optical and epitaxial Ge films formed by laser evaporation

High‐quality thin films of Ge were deposited by pulsed laser evaporation of molten Ge. Films deposited on 300 °C substrates showed very smooth morphologies and single‐crystal grain structures. Energetic ions in the vapor stream, generated by the laser‐induced plasma, were observed to affect nucleation and bulk‐film growth. Films deposited on ambient temperature substrates by laser evaporation were denser, harder, and exhibited higher values of refractive index and lower levels of intrinsic stress than the films deposited by thermal evaporation.

Fabrication and characterization of diamond moth eye antireflective surfaces on Ge

An antireflective coating on Ge which combines a polycrystalline diamond film with a surface relief (moth eye) structure has been designed and successfully fabricated. This progressive gradation in the effective refractive index between air and the substrate has reduced Fresnel reflection losses to below 1%. This provides a means of overcoming the high refractive index and surface roughness considerations that often limit optical application of polycrystalline diamond thin films.

Microstructure and composition of composite SiO2/TiO2 thin films

The microstructure and composition of the SiO2/TiO2 analog (codeposited) and digital (thin layer pairs) gradient‐index films were examined by transmission electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, x‐ray photoelectron spectroscopy, scatterometry, and in situ ellipsometry. Both analog and digital structures were amorphous as‐grown. The SiO2 was incompletely oxidized at some of the interfaces in the layered structures and in the codeposited films, indicated competition from the TiO2 for available oxygen during growth. Digital structures with thin (65 A) layers remained well defined after annealing at 900 °C, but their order was completely destroyed by 1100 °C. Structures with thick (500 A) layers remained intact up to 1100 °C, with the TiO2 crystallizing throughout the layer width. Both anatase and rutile TiO2 crystallites were present in the layered and codeposited films after a high‐temperature anneal (T≳650 °C), while only the anatase phase was observed for ...

Reactive ion beam deposition of thin films in the bismuth‐calcium‐strontium‐copper oxide ceramic superconductor system

Reactive ion beam deposition has been used to grow c‐axis‐oriented superconducting thin films in the Bi‐Ca‐Sr‐Cu‐O (BCSCO) system around the cation ratio 1:1:1:1 on single‐crystal (001) MgO. The films show a single superconducting transition with an initial onset near 85 K and a critical current of 5×104 A/cm2 at 10 K. Two different BCSCO‐containing phases have been identified in the thin films: one with a tetragonal pseudo‐body‐centered subcell, c=24.4 A, which is not superconducting above 28 K, and a second with c=30.6 A, which is responsible for the superconductivity. Electron diffraction measurements on the 30.6 A phase are consistent with those previously reported for the bulk ceramic.

Intrinsic stress and structural properties of mixed composition thin films

Intrinsic stress and microstructure of mixed composition films were investigated for several binary systems of IR optical materials. These properties were measured for the entire range of compositions and for mixing obtained by codeposition and by layering of alternate pure components. The variation of stress with composition was observed to be significantly different, depending on the method of mixing. Microstructural analysis revealed a corresponding difference in the grain structure of the films. Low compressive stress films were obtained by coevaporation of high tensile stress materials. These can be used to produce thick and mechanically stable gradient-index optical coatings.

Microstructure and orientation effects in diamond thin films

The microstructure and orientation of diamond thin films grown by plasma assisted chemical vapor deposition have been studied as functions of growth temperature, substrate identity, and substrate pre‐treatment. Results indicate that for growth temperatures below 650 °C, competition between film growth and etching can lead to preferential (110) oriented films on a variety of substrate materials. This orientation can be globally sustained during growth by the occurrence of (111) planar defects.

Modal Parameter Tuning of an Axisymmetric Resonator via Mass Perturbation

This paper reports the permanent frequency mismatch reduction of the primary wineglass modes in a planar axisymmetric resonator by strategic mass loading. The resonator consists of a set of concentric rings that are affixed to neighboring rings by a staggered system of spokes. The outer layers of spokes are targets for mass deposition. This paper develops modified ring equations that guide the mass perturbation process, and despite the fact that the deposited mass and deposition locations are quantized, it is possible to systematically reduce the frequency difference of the wineglass modes to effective degeneracy such that two modes cannot be distinguished in a frequency response plot. Results on five resonators are reported with nominal wineglass modes near 14 kHz, quality factors of 50k, and frequency mismatches exceeding 30 Hz in some cases, but with postperturbation mismatches smaller than 80 mHz. Furthermore, it is also shown that the quality factors remain unchanged.

MICROSTRUCTURAL CONTROL OF DIAMOND THIN FILMS BY MICROLITHOGRAPHIC PATTERNING

Microlithographic patterning has been used to elucidate the mechanisms controlling diamond film nucleation and grain growth. The approach is capable of establishing a degree of control over diamond nucleation on the substrate, which can be used to improve film uniformity and enhance fine grained microstructure. The observed microstructures in the patterned films are consistent with an intrinsic growth mechanism based upon defect‐initiated renucleation.

Adhesion improvement in diamond films by microlithographic patterning

The use of microlithographic surface patterning has been investigated as a means of modifying the nucleation and adhesion of diamond films on non-compatible substrates. Significant improvements in film adhesion were achieved using this technique, to the point that interfacial integrity was maintained even at stress levels which induced subsurface fracture in the supporting substrate.