Well-crystallized Y-type hexagonal ferrite (Me2-Y, Me=Zn and Co) films with highly c-axis orientation were successfully prepared on Ag substrates by dip coating technique. The procedure consisted of preparation of homogeneous metal-organic solution by polymerized complex method, dip coating and subsequent heat-treatment. The films deposited on α-Al2O3 easily reacted with the substrate, while the films deposited on Ag formed hexagonal planar structures. The formation temperature of Me2-Y structures on Ag was 800 °C, which was much lower than that of Me2-Y bulk crystals. The Me2-Y films on Ag had a large in-plane magnetic anisotropy. The observed anisotropy field of the Zn2-Y film was about 13 kOe and that of Co2-Y was too large beyond our experimental limits.

Composite thin films of 60 wt% Ba0.6Sr0.4TiO3 and 40 wt% MgO were produced by Pulsed Laser Deposition. The biaxial texture of the BST component on the MgO substrate has been established with XRD. All as-deposited films had an enlarged BST out-of-plane lattice parameter. A more relaxed lattice constant as well as higher degree of texture has been obtained in the films deposited at higher temperature and lower deposition rate. Post-deposition annealing in flowing oxygen results in a further relaxation and alignment of the BST lattice. The as-deposited films were not tunable at room temperature. The greatest dielectric tuning was achieved in films annealed at 1200 °C. The observed difference in tunability for the films annealed at different temperatures may result from a spatial redistribution of BST material on the substrate surface during annealing.

The application of ferroelectric thin films for the development of frequency and phase agile microwave components has been the reason behind very encouraging demonstrations of tunable microwave devices in recent years. Thus, one could conclude with basically a general consensus, that the question of the suitability of thin film ferroelectric technology for the fabrication of superior tunable microwave components has been already answered in a favorable way. However, what is still pending in regards to the validation of this technology is the development of evaluation methodologies to set forth the standards for the material quality and subsequent performance criteria of specific components and devices based on this technology. In this paper we discuss the evaluation methodology under implementation at NASA Glenn Research Center aimed at identifying and optimizing the most relevant parameters of BaxSr1−xTiO3 (BSTO) ferroelectric thin films (i.e., tunability, losses, thickness, crystalline quality, etc.) as defined by a specific microwave application (in our case, phase shifters for reflectarray antennas). Results of our material analysis based on X-ray diffraction and ellipsometry, and how these properties correlate with RF performance for a targeted circuit, will be presented.

In order to develop wide-band microwave absorbers we tried to prepare composite absorbers between the magnetic loss (Ni-Zn-ferrite) and the dielectric loss (SiC) materials. The dry or wet mixtures of Ni-Zn-ferrite and polycarbosilane (PCS) were cured in air and were then subjected to pyrolysis forming of the ferrite composites. The composites produced by the wet mixing had granular structure: the PCS surrounding the ferrite particles suppressed the sintering and the grain growth of the ferrite composites during pyrolysis. The matching frequency due to the magnetic loss linearly increased with increasing the PCS content. Moreover the ferrite/SiC hemi-crystallized composite cured at 473 K for 24 hours showed wide-band absorption spectrum having the both magnetic and dielectric losses.

A series of crystalline isomorphic solid solutions of Yttrium-Lutetium Aluminum Garnets (Y1−cLuc)3Al5O12 (c = 0 – 1) has been studied. The measurements of dielectric loss (DL) tangent tgδ has been performed at electromagnetic wavelengths 1 – 0.6 mm and temperature T = 300 K. The results obtained has been compared with longitudinal acoustic waves (AW) attenuation data for frequencies f = 1 – 9.4 GHz, T = 4.2 – 300 K and with theory. The correlation between the DL and the AW attenuation is observed. The dependencies of DL (at T = 300 K) and AW attenuation (at T > 77 K) on concentration c are qualitatively identical in the whole interval c = 0 – 1. From comparison with theory it follows that the observed DL is due to the two-phonon intrinsic lattice loss. It is caused by the lattice anharmonicity as well as the AW attenuation at not too low temperatures. Also the prediction for concentration dependence of DL at very low temperatures is discussed that follows from comparison with the results of “heat pulse” propagation experiments for the same samples at T = 2 K and theory.

Single phase Na0.5K0.5NbO3 (NKN) thin films have been pulsed laser deposited on Al2O3(0112), LaAlO3(001), and MgO(001) single crystal substrates as well as on the SiO2/Si(001) wafers to demonstrate films feasibility for voltage tunable microwave device applications. NKN film texture has been found to be quite different on three different single crystals: highly c-axis oriented on Al2O3, “cube-on-cube” epitaxial quality on LaAlO3, bi-axial textures on MgO, while NKN films grown on Si substrate with various thickness of SiO2 buffer layer possess highly c-axis oriented quadrupled structure. NKN film interdigital capacitors fabricated onto single crystal oxide substrates showed tunability of 30-40 % and dissipation factor of 0.01-0.02 at 1 MHz and applied electric field of 100kV/cm. Microwave frequency measurements for NKN/Si varactors yield 13 % tunability and dielectric loss tanδ as low as 0.012 at 40 GHz under 200 kV/cm applied bias.

Barium strontium titanate (BSTO) films were synthesized by the pulsed laser deposition technique (PLD) on silicon substrates at room temperature. The thin films were synthesized at ambient temperature and 30 mT oxygen partial pressure, with 300, 400 and 500 mJ/cm2 laser fluence at 5, 10 and 20 pulses per second on silicon wafer substrates. All films were subsequently post-annealed at 750°C in a continuous oxygen stream. The microstructure, crystallinity and lattice constant of the BSTO films were studied with the aid of atomic force microscopy (FEM) and Glancing Angle X-ray Diffraction analysis (GAXRD). The hardness and modulus of elasticity of the films were studied with the aid of a nanohardness indenter. The film stoichiometry was determined with the aid of Rutherford Backscattering Spectrometry (RBS). The results of this research will be combined with the results of our previous work [1, 2] on the effect of substrate temperature and oxygen partial pressure on the microstructure and properties of the BSTO films in order to construct a structural zone model (SZM) of the BSTO films synthesized by PLD.

Barium hexaferrite films having thickness from 3 to 30 μm were deposited onto 0.5 mm Magnesium Oxide (111) substrates, and were examined by vibrating sample magnetometry, torque magnetometry, x-ray diffraction, and scanning electron microscope measurements. Ferrimagnetic resonance measurements were taken with the magnetic field applied perpendicular to the film plane. The FMR linewidth values for as-produced 3 - 30 μm films was 0.45 - 0.70 kOe, and was reduced to 0.06 - 0.45 kOe after annealing at 1000 °C for 2 hours. Further annealing increased the linewidth. The FMR linewidth for the 30 μm thick film was also found to decrease upon mechanically removing 55 ∼ 77 % of the substrate thickness. A 6-fold in-plane anistropy symmetry in the anisotropy energy was observed in torque magnetometry measurements for films thicker than 30 μm. These results can be interpreted as due to film inhomogeneity, stress, and the growth of small regions of non c-axis textured material.

Pulsed laser deposition has been used to fabricate thin-film capacitor structures in which the dielectric layer is composed of a superlattice of Ba0.8Sr0.2TiO3 and Ba0.2Sr0.8TiO3. The properties of the capacitors were investigated as a function of superlattice periodicity. The dielectric constant was significantly enhanced, and temperature migration of the peak in dielectric constant as a function of frequency was observed, at stacking periodicities of a few unit cells. However, such “relaxor-like’ features were found to be associated with high dielectric loss. Analysis of the imaginary permittivity as a function of frequency showed that fine-scale superlattices conform to Maxwell-Wagner behaviour, indicating that the observed features may be an artefact of increased carrier mobility. Modelling showed that both dielectric enhancement and frequency relaxation could readily be reproduced by Maxwell-Wagner formalism.

The composition 40 wt% Ba0.55Sr0.45TiO3 (BST) – 60 wt% MgO bulk material was recently found to be a promising candidate as a low loss, tunable material for electronically scanning antennas (ESA). However, due to the high permittivity of the BST-MgO composite impedance mismatch within the antenna could be a source of loss. Electrophoretic deposition (EPD) could be used as a processing tool to develop functionally graded structures tailored to reduce the impedance mismatch without a reduction in the tunability. In addition, EPD could be used in thin or thick films to decrease the required biasing fields. In this study, EPD was used as a method for deposit thick films of BST functionally graded composites. The concentration of the suspension, solvent type, mixed solvent concentrations, bias field, time, particle size of powders and distance between electrodes were studied to control the deposition yield and thickness. SEM or calipers were used to measure deposit thickness. The relative weight ratio of BST to MgO in the suspension was compared to the final structure as measured by EDS.

Ferroelectrics are presently of interest for phase shifters, filters, and true time delay devices. Voltage tunable paraelectrics have the potential to lower device cost and reduce power consumption compared with presently available devices. In order to improve device performance to acceptable levels, materials must have high tunability, low dielectric constant, low loss tangent, and low leakage current. Using existing predictive techniques, compositions of Ba0.6Sr0.4TiO3-based ferroelectrics with charge-compensated substitutions for Ti4+ were synthesized. Results of capacitance measurements are used to obtain dielectric constant and tunability in the paraelectric (T > TC) regime. The relevance to device requirements is discussed. Results for substituted samples are compared to those for (unsubstituted) Ba0.6Sr0.4TiO3. Discussion of the impact of the results on predictive techniques for tunability is addressed.

A ZnO/Zn0.8Mg0.2O double barrier resonant tunneling device (DBRTD) is reported here for the first time. The structure consists of 6 nm ZnO quantum wells and 7 nm Zn0.8Mg0.2O barriers grown by pulsed laser deposition (PLD) on c-cut sapphire substrates. Negative differential resistance (NDR) peaks were obtained at room temperature. The structure is developed by using an indium-tin oxide (ITO) layer both as the back contact electrode and as an etch-stopping layer. The PLD growth quality, wet etching processing for developing the mesa structure, and the I-V characteristics of the device are reported.

Ba1−xSrxTiO3 thin films are being developed for high-density DRAM devices. The nonlinearity of its dielectric properties with respect to applied dc voltage makes it attractive for tunable microwave devices. For successful integration into microwave devices, extremely reliable Ba1−xSrxTiO3 thin films with enhanced dielectric and insulating properties are desired. Properties of Ba1−xSrxTiO3are typically varied by changing the Ba/Sr ratio and/or doping. In this paper, we reports on the effects of acceptor and donor doping on the microstructural and electrical properties of Ba0.6Sr0.4TiO3 (BST) thin films deposited by metalorganic solution deposition technique on platinum coated silicon substrates. The effects of doping on structure, dielectric permittivity, dielectric loss tangent, and leakage current have been analyzed. The structure of the films was analyzed by x-ray diffraction (XRD). The surface morphology of the films was examined by field emission scanning electron microscope (FESEM) and atomic force microscope (AFM). The electrical measurements were conducted on MIM capacitors using Pt as the top and bottom electrode. It was possible to significantly improve the dielectric loss and leakage current characteristics, and control the dielectric tunability by doping the BST thin films.