Gary S. Tompa

Challenges and Opportunities for Multi-functional Oxide Thin Films for Voltage Tunable Radio Frequency/Microwave Components

There has been significant progress on the fundamental science and technological applications of complex oxides and multiferroics. Among complex oxide thin films, barium strontium titanate (BST) has become the material of choice for room-temperature-based voltage-tunable dielectric thin films, due to its large dielectric tunability and low microwave loss at room temperature. BST thin film varactor technology based reconfigurable radio frequency (RF)/microwave components have been demonstrated with the potential to lower the size, weight, and power needs of a future generation of communication and radar systems. Low-power multiferroic devices have also been recently demonstrated. Strong magneto-electric coupling has also been demonstrated in different multiferroic heterostructures, which show giant voltage control of the ferromagnetic resonance frequency of more than two octaves. This manuscript reviews recent advances in the processing, and application development for the complex oxides and multiferroics, with the focus on voltage tunable RF/microwave components. The over-arching goal of this review is to provide a synopsis of the current state-of the-art of complex oxide and multiferroic thin film materials and devices, identify technical issues and technical challenges that need to be overcome for successful insertion of the technology for both military and commercial applications, and provide mitigation strategies to address these technical challenges.

Strain engineered barium strontium titanate for tunable thin film resonators

Piezoelectric properties of epitaxial (001) barium strontium titanate (BST) films are computed as functions of composition, misfit strain, and temperature using a non-linear thermodynamic model. Results show that through adjusting in-plane strains, a highly adaptive rhombohedral ferroelectric phase can be stabilized at room temperature with outstanding piezoelectric response exceeding those of lead based piezoceramics. Furthermore, by adjusting the composition and the in-plane misfit, an electrically tunable piezoelectric response can be obtained in the paraelectric state. These findings indicate that strain engineered BST films can be utilized in the development of electrically tunable and switchable surface and bulk acoustic wave resonators.

Laser annealing of laser assisted molecular beam deposited ZnO thin films with application to metal-semiconductor-metal photodetectors

We report on the effect of postdeposition laser annealing of undoped zinc oxide (ZnO) thin films grown by laser assisted molecular beam deposition. Hall-effect measurements show that some undoped ZnO films change from n type with mobility values in the range of 200cm2V−1s−1 to p-type material with mobility value of 73cm2V−1s−1, after laser annealing. The photoconductive behavior was clearly seen on the laser-annealed samples, with values of 0.28mΩ−1. The structural and optical properties of the films were improved with laser annealing as shown by scanning electron microscopy, x-ray photoelectron spectroscopy analysis, and photoluminescence measurement. All of the nonlaser and laser annealed samples showed near-band emission at ∼3.3eV. Metal-semiconductor-metal photodetectors were fabricated from the films.

Correlated conductivity and work function changes in epitaxial graphene

Correlation between conductance and surface work function (SWF) changes caused by molecular adsorption on epitaxial graphene on both faces of 6 H-SiC has been investigated. The SWF and conductance changes, explained on the basis of graphene band diagram, indicate C-face multilayer and Si-face few layer graphene behave as p and n-type sensing layers, respectively. A quantitative model correlating conductance and SWF changes has been proposed within the framework of Boltzmann transport theory. Our results further indicate that for epitaxial graphene, the charge interaction by the adsorbed molecules and related work function changes can be strongly influenced by the SiC substrate.

Thermal and flow issues in the design of metalorganic chemical vapor deposition reactors

Abstract The influence of the equipment design and deposition process parameters on the flow and temperature uniformity across the substrate for a vertical high speed metalorganic chemical vapor deposition (MOCVD) rotating disk reactor (RDR) was investigated. The substrate temperature unifirmity was found to strongly depend upon process temperature and pressure, reactant flow, and wafer carrier rotation speed. With a single-zone heater element, the temperature uniformity could be optimized only for a limited field of process parameters. We have demonstrated a significant improvement to the substrate temperature uniformity over a wide range of process parameters by utilizing a multi-zone heater.

Metal-Organic Chemical Vapor Deposition (MOCVD) of GeSbTe-based Chalcogenide Thin Films

Chalcogenide Random Access Memory (C-RAM) has shown significant promise in combining the desired attributes of an ideal memory, including: nonvolatility, fast read/write/erase speed, low read/write/erase voltage/power, high endurance, and radiation hardness. Current C-RAM production technology relies on sputtering to deposit the active chalcogenide layer. The sputtering process leads to difficulties in meeting requirements for device conformality (in particular – filling vias), film adherence, compositional control, wafer yield, and surface damage. Ultimately, a viable CVD manufacturing process is needed for high-density products to realize the full potential of C-RAM. In this work, we discuss the Metal-Organic Chemical Vapor Deposition (MOCVD) tool technology used to produce the films and report the materials properties of GeSbTe-based chalcogenide thin films grown in small research scale and in large production scale MOCVD reactors. Films were grown at low pressures at temperatures ranging from 350 C to 600 C. X-Ray Fluorescence (XRF) and Auger Electron Spectroscopy (AES) were performed and determined that the film composition is controllable and uniform.

Voltage induced acoustic resonance in metal organic chemical vapor deposition SrTiO3 thin film

A solidly mounted acoustic wave resonator was fabricated using a 150 nm thick SrTiO3 film deposited by metal organic chemical vapor deposition and platinum electrodes deposited by sputtering. The substrate was (0001) sapphire with a multilayer SiO2/Ta2O5 acoustic Bragg reflector. Dielectric characterization of the SrTiO3 film showed low leakage current and the characteristic capacitance–voltage behavior of a paraelectric film. Measurement of the radio frequency transmission characteristics showed no resonance with zero bias voltage across the SrTiO3 film. At 1.0 V applied DC bias, a well defined resonance peak was observed near 5.6 GHz. With increasing voltage across the SrTiO3 film, the resonance increased in intensity and shifted to lower frequency. The calculated electromechanical coupling coefficient for the device was 1.3% in the range of 3–5 V applied bias. The maximum observed quality factor was approximately 10.

LaAlO 3 /SrTiO 3 Epitaxial Heterostructures by Atomic Layer Deposition

Thin films of LaAlO3 were deposited on TiO2-terminated (100) SrTiO3 crystals by atomic layer deposition (ALD), using tris(iso-propylcyclopentadienyl)lanthanum and trimethyl aluminum precursors. Water was used as the oxidizer. The film composition was shown to be controlled by the ratio of La/Al precursor pulses during ALD, with near-stoichiometric LaAlO3 resulting at precursor pulse ratios of 4/1 to 5/1. Films near the stoichiometric LaAlO3 composition were shown to crystallize on subsequent annealing to form epitaxial LaAlO3/SrTiO3 heterostructures. Electrical characterization of these structures was done by two-terminal direct-current (DC) current–voltage scans at room temperature and under high-vacuum conditions. The results show electrical conductivity for the ALD-deposited epitaxial LaAlO3/SrTiO3 heterostructures, which turns on for thickness above four unit cells for the LaAlO3 film.

Metalorganic chemical vapor deposition of superconducting YBa2Cu3O7−x in a high‐speed rotating disk reactor

Metalorganic chemical vapor deposition (MOCVD) of YBa2Cu3O7−x films on (100) SrTiO3 was performed in a high‐speed rotating disk reactor. Metal β‐diketonates were used as source materials. Energy dispersive spectroscopy analysis showed as‐grown and post‐annealed samples to have a composition matching that of superconducting thin‐film standards. X‐ray data showed the annealed films to be highly oriented with the c axis perpendicular to the substrate. The best films exhibited an onset temperature of 90 K and zero resistance at 88 K. The width of the superconductive transition (<2 K) is the best value thus far reported for 123 films grown in a commercially available MOCVD system.

Investigation Of Efficiency Improvement on Silicon Solar Cells Due to Porous Layers

Observation of light emission from porous Si has demonstrated that the optical properties of Si can be drastically altered by the quantum size effects. We have investigated the improvement of absorption properties of Si material by forming a porous Si layer. Shallow-junction commercial crystalline as well as polycrystalline Si solar cells without anti-reflective coatings have been processed into porous Si solar cells by a wet chemical etching technique. Our best results have demonstrated more than 15% improvement in short-circuit current with no change in open-circuit voltage. The performance of the porous Si solar cells has been found to be sensitive to the porous layer thickness. The efficiency can be reduced when the porous layer is relatively deep, presumably due to the penetration of pores through the shallow junction. We believe porous Si can be optimized for photovoltaic applications by properly controlling its porosity and thickness.