Francisco Santiago

Tunable split-ring resonator for metamaterials using photocapacitance of semi-insulating GaAs

We report a method for tuning a split-ring resonator (SRR) using infrared light. The SRR unit cells are commonly used in arrays to form a metamaterial that exhibits an effective negative permeability and are often used in negative-refractive-index materials. The region of negative permeability is generally limited to a narrow bandwidth at a fixed frequency. In this work, we use photocapacitance as observed in undoped semi-insulating GaAs to modify the gap capacitance of a SRR. We demonstrate a continuously tunable frequency over the range of 0.1–1.5 GHz using a 975 nm laser diode with a power range of 0–189 mW.

Growth and interfacial chemistry of insulating (100) barium fluoride on gallium arsenide

The epitaxial growth of insulating BaF2 films on (100) and (111)‐oriented GaAs substrates has been investigated. (100)‐oriented BaF2 was successfully deposited on (100) GaAs at temperatures as low as room temperature, in contrast to a previous report. This was accomplished by first establishing a chemically reacted template layer a few monolayers thick at the BaF2/GaAs interface. These films consistently exhibited epitaxial reflection high‐energy electron diffraction patterns with three‐dimensional growth modes for a wide range of incident BaF2 flux rates. The epitaxial quality of the (100) films was, however, temperature dependent. A film deposited on a (111) wafer at 600 °C was (111) oriented and showed two‐dimensional growth. X‐ray photoelectron spectroscopy studies of the interface chemistry indicate the existence of a Ba state other than that of the BaF2, authenticating the relevance of the template layer. The (100) BaF2 films are insulating, with a breakdown field of ∼1×106 V/cm.

The Role of Barium In the Heteroepitaxial Growth of Insulator and Semiconductors On Silicon

The epitaxial growth of an insulator, BaF 2 , and semiconductors of the II-VI and the IV-VI families on Si substrates were carried out. In-situ XPS analyses during the growth of the first monolayers were used to study the surface chemical reactions involved. The results point to a common ingredient in these growths: that the Ba atoms are involved in forming interfacial compounds that would facilitate the heteroepitaxies. In the case of BaF 2 /Si, a BaSi 2 compound has been identified previously. In the case of PbTe and CdTe, the heteroepitaxies on Si are made possible with the BaSi 2 buffer. As a result, the impinging semiconductor molecules are broken up, and the metallic elements are ejected from the BaSi 2 surface. A new surface chemical, BaTe, is thereby formed. These surface Ba compounds appear to be the dominant factors as the crystal orientations of the BaF 2 , CdTe, and PbTe layers are independent of those of the Si substrates.

Heteroepitaxial deposition of Group IIa fluorides on gallium arsenide

Abstract The epitaxial deposition of fluoride films has been a subject of research interest because of their potential for integrated electronic and electro-optic device applications. We report here our recent results on the investigation of (100)BaF 2 -GaAs growth. Our approach differs from the conventional method by addressing the problem at the atomic layer level, especially at the interface between the deposited material and the substrate. These investigations have revealed that an interfacial chemical reaction is important in the heteroepitaxy process. As a result of this chemical reaction, an atomic Ba layer is formed on the GaAs surface. It is this Ba-template layer that enables two dimensional, molecular layer-by-layer growth of the BaF 2 film. Films thus grown are of a high epitaxial quality which appears to be limited only by the quality of the GaAs surface. For BaF 2 on GaAs, (100) growth is favored over (111) growth. This is contrary to earlier results by other investigators. It is concluded that the conventional approach to heteroepitaxial growth, relying on lattice match and surface energies is not applicable, at least for the fluoride films. This can be understood from the atomic and molecular structure of Ba and BaF 2 . Implications of these results to the understanding of the heteroepitaxial process, especially involving large lattice mismatches, and to the development of new materials and technologies will be discussed.

Determination of growth mechanisms of MBE grown BaF2 on Si(100) by target angle dependence of RBS yields

Abstract The angular dependence of 2.0 MeV 4He1+ Rutherford backscattering spectroscopy (RBS) yields have been used to determine the growth mechanism of epitaxial BaF2 films grown on Si(100) substrates by molecular beam epitaxy (MBE). RBS yields from uniformly thick layers are characterized by a (cos φ)−1 dependence, where φ is the angle between the incident beam and the target normal. Deviations from this relationship have been attributed to layers which are composed of islands, rather than films of uniform thickness. A series of BaF2 films of increasing deposition time were examined in this way. The results of this analysis show that the BaF2 at first grows in small islands which eventually coalesce into a uniform epitaxial layer.

X-ray photoelectron spectroscopy indication of decomposition species in the residue of shocked polytetrafluoroethylene powder

Forty-eight percent porous polytetrafluoroethylene (PTFE) powder placed inside a steel closed container has been impact shock loaded with a gas gun and soft recovered. The initial stress in the powder is calculated to be 7.2 kbar. The residue in the container showed dark regions where the originally white powder had decomposed to form black soot. X-ray photoelectron spectroscopy (XPS) was used to analyze in situ a portion of a dark region. The resulting spectrum showed a large amorphous carbon peak that was not observed in the unshocked powder spectrum. In addition, the shocked material showed several peaks containing hydrogen and/or oxygen, suggesting reactions of dissociation products with ambient air and/or water vapor in the polymer pores or possibly with residual water in the polymer. (The residual gas analyzer in the XPS system detected water vapor in the unshocked specimen.) Both spectra showed peaks corresponding to the PTFE linear polymer chain F–C–F.

Circuit Analysis of Photosensitive Capacitance in Semi-Insulating GaAs

We describe a circuit model for photosensitive capacitance in bulk semi-insulating GaAs toward tunable resonant applications. Capacitances from two separate regions are considered to interpret experimental results. A smaller valued capacitance exists between the depletion edges within the bulk material. Photodoping in this region progressively shorts out the bulk capacitance, leaving only the higher valued depletion capacitance. The depletion capacitance also increases with illumination, and numerical simulation is used to aid interpretation. Thus, the series combination of capacitance can be optically varied over orders of magnitude. Our results indicate that capacitance is nearly independent of applied voltage over a usable parameter space, making the concept attractive for linear application.

Nanomaterials for sensor applications

There is a lot of interest by society at large for reliable and affordable sensor and detection systems. Recent threats regarding public exposure to both biological and chemical agents have helped focus attention to the development of new sensor and detector technologies. Unfortunately, there is not even consensus in the scientific community on a uniform terminology of the subject. A large body of knowledge in sensor materials is available but most existing sensor materials are very limited in their ability to sense and discriminate small amounts of harmful agents embedded in large amounts of chemically inert but complex background. New materials are needed to meet the challenges ahead. Nanomaterials may be a solution to this problem. In this work we review the current state of the sensor lexicon. This is followed by a proposition of a consistent definition of terms used in the sensor and detection technology. Finally, we review a promising IR sensor system being developed using nanomaterials and several detector systems also developed using nanomaterials for nano solar cells and nano smart materials.

Heavy ion backscattering spectroscopy (HIBS) analysis of gallium arsenide diffusion into barium fluoride layers grown by molecular beam epitaxy

Abstract Heavy Ion Backscattering Spectroscopy (HIBS) using 12 MeV 12C ions was used to examine GaAs (1 0 0) and GaAs (1 1 1) layers grown by Molecular Beam Epitaxy (MBE) on BaF2 (1 0 0) and BaF2 (1 1 1) layers, respectively. HIBS was the only available technique able to verify the Ga As ratio in these samples because X-ray Photoelectron Spectroscopy (XPS) is too surface sensitive for this task and the lighter ions used in Rutherford Backscattering Spectroscopy (RBS) cannot resolve the Ga and As signals. A modified RBS analysis program was used to analyze the HIBS spectra. Using the stopping power data of Ziegler et al., the HIBS analysis overestimated the thickness of a BaF2 layer by about 14% when compared to RBS analysis of the same layer. HIBS was able to resolve the Ga and As peaks, as well as the two isotopes of Ga. This enhanced mass resolution made the determination of the GaAs layer stoichiometry possible, and introduced spectral features that aided in fitting HIBS simulation spectra to the data points. HIBS analysis of GaAs/BaF2/Si heteroepitaxies also showed diffusion of Ga and As into BaF2. This diffusion is temperature dependent and therefore subject to control. The information gathered from the HIBS analysis was used to confirm information obtained by in situ MBE diagnostic techniques, and in the case of the diffusion studies, provided information that was not available by those surface sensitive techniques.

Optically Controllable Composite Dielectric Based on Photo-Conductive Particulates

We report a method to optically control the effective permittivity of a composite dielectric in which the active inclusions are fabricated from pulverized semi-insulating GaAs. The electric dipoles from the inclusions are controlled by photo-generation of charge carriers using infrared light. An infrared transparent binder provides the matrix material. Our primary purpose is to develop a low-cost pigment-based ink, or paint, for optically tuning electromagnetic devices including metamaterials, frequency-selective surfaces, filters, phase delays, and antennas. In bulk form, applications of the optically controllable dielectric are imagined including a gradient index lens in which inhomogeneous light intensity can provide a dynamic response for focusing or steering of beams. An important feature is that the effective permittivity is linear with electric field intensity over a usable parameter space. We present experimental results as well as a simple model to describe the behavior qualitatively.