Donna M. Speckman

Alternatives to arsine: The atmospheric pressure organometallic chemical vapor deposition growth of GaAs using triethylarsenic

Studies on the homoepitaxial growth of unintentionally doped GaAs by atmospheric pressure organometallic chemical vapor deposition using triethylarsenic and trimethylgallium have been carried out, and the effects of growth temperature, V/III ratio, and flow rate on film characteristics are reported. Mirrorlike epitaxial layers of n‐type GaAs were obtained at substrate temperatures of 540–650 °C and at V/III ratios of 6.7–11. The carrier concentrations for these films were approximately 1016–1017 cm−3, and from secondary ion mass spectroscopic analysis, the predominant epilayer impurities were determined to be both carbon and silicon.

Low temperature pulsed laser deposition of titanium carbide on bearing steels

This study focuses on the room temperature pulsed laser deposition (PLD) of titanium carbide (TiC), as a wear-resistant coating on 52100 and 440C bearing steels. The TiC films have been characterized using a variety of analytical techniques. Atomic force and scanning electron microscopy have revealed the deposition of extremely smooth TiC films, having very high uniformity over large areas. Selected area electron diffraction in transmission electron microscopy has shown an excellent correlation between the observed lattice spacings in the deposited films and the reported literature values of TiC. The crystallite size of these films has been established by dark field transmission electron microscopy. The chemical composition and mechanical properties of these films were also investigated. Room-temperature PLD has been successful in depositing high-quality TiC films on bearing steel components, thus avoiding numerous problems that arise during conventional high-temperature deposition of such coatings on steels.

Vapor deposition of high‐purity GaAs epilayers using monoethylarsine

High‐purity GaAs epitaxial layers have been successfully grown using the novel organoarsine reagent source, monoethylarsine (EtAsH2), with trimethylgallium (Me3Ga) as the gallium reagent. Films were found to be n type for all growth parameters examined (V/III=5–30, growth temperature=550–650 °C). Film quality improved as V/III ratio increased, whereas the optimum growth temperature ranged between 575 and 600 °C. The highest purity film produced using EtAsH2 and Me3Ga was grown using a V/III ratio of 30 and a growth temperature of 575 °C. This epilayer exhibited mobilities of 55 300 cm2/V s and 7200 cm2/V s at 77 and 300 K, respectively (as determined by van der Pauw–Hall measurements), and had a net carrier concentration of 6×1014 cm−3. These results closely rival those of the best arsine alternatives studied to date, and indicate that EtAsH2 is an extremely promising reagent to replace arsine for use in vapor deposition applications.

Atomic Oxygen Testing of MgF2 Coatings

Spacecraft orbiting below about 300 nm are exposed to a high flux of atomic oxygen (AO) that can severely degrade surface materials, yet some coatings used on solar cell coverglass have never been qualified in this environment. Combined effects tests comprising atomic oxygen exposure, illumination, and temperature were carried out to qualify magnesium fluoride coated cover-glasses. Unexpectedly severe degradation in the form of optical transmission loss was observed upon exposure to high AO fluences. It was determined that the high intensity vacuum ultraviolet (VUV) light emitted from the AO source used acts in an unexpectedly synergistic fashion with AO to accelerate the optical degradation of these coatings. Since the excess VUV from the ground test AO source is not representative of flight conditions, its unexpected synergistic effect emphasizes the need to define proper ground test conditions when qualifying materials for AO environments

Biodetection using fluorescent quantum dots

Multi-pathogen biosensors that take advantage of sandwich immunoassay detection schemes and utilize conventional fluorescent dye reporter molecules are difficult to make into extremely compact and autonomous packages. The development of a multi-pathogen, immunoassay-based, fiber optic detector that utilizes varying sized fluorescent semiconductor quantum dots (QDs) as the reporter labels has the potential to overcome these problems. In order to develop such a quantum dot-based biosensor, it is essential to demonstrate that QDs can be attached to antibody proteins, such that the specificity of the antibody is maintained. We have been involved in efforts to develop a reproducible method for attaching QDs to antibodies for use in biodetection applications. We have synthesized CdSe/ZnS core-shell QDs of differing size, functionalized their surfaces with several types of organic groups for water solubility, and covalently attached these functionalized QDs to rabbit anti-ovalbumin antibody protein. We also demonstrated that these labeled antibodies exhibit selective binding to ovalbumin antigen. We characterized the QDs at each step in the overall synthesis by UV-VIS absorption spectroscopy and by picosecond (psec) transient photoluminescence (TPL) spectroscopy. TPL spectroscopy measurements indicate that QD lifetime depends on the size of the QD, the intensity of the optical excitation source, and whether or not they are functionalized and conjugated to antibodies. We describe details of these experiments and discuss the impact of our results on our biosensor development program.

The effect of deposition conditions on the atomic oxygen induced degradation of MgF2 anti-reflective coatings

Spacecraft orbiting below about 800 km are exposed to a high flux of atomic oxygen (AO) that can severely degrade surface materials. Of particular concern are the anti-reflective (AR) coating on solar array surfaces that not only protect the solar panel from the environment but help to boost the transmission of incoming light to the solar cells. Degradation of this AR coating can have serious effects on the performance of the solar panels, leading to insufficient power being available to perform any useful mission. A common space borne solar cell cover consists of a Ceria (CeO2) doped borosilicate glass (CMG) substrate coated by a 90nm MgF2 AR coating. Our previous work has shown that the transmission of commercially purchased MgF2 anti-reflective coatings degrades significantly when exposed to an AO flux. In view of this MgF2 films were deposited in-house where the deposition conditions were controlled. MgF2 films were deposited on ceria micro sheet substrates via electron beam evaporation. The system base pressure 2.3×10−6 torr without the use of the liquid nitrogen (LN2) trap and <5×102212;7 torr when the LN2 trap is cooled. The thickness of the film was monitored by a quartz crystal microbalance and the substrate was mounted on a temperature controlled heater block. The source material was commercially purchased MgF2 granules. The as-deposited and AO exposed films were analyzed via spectroscopic ellipsometry, x-ray diffractometry and auger electron spectroscopy. We will show that the transmission degradation can be reduced by an order of magnitude, without significantly changing the optical properties of the film, by controlling the oxygen content of the as-deposited film.

Atomic oxygen induced degradation of MGF 2 anti-reflective coatings

Spacecraft orbiting below about 1200 km are exposed to a high flux of atomic oxygen (AO) that can severely degrade surface materials. An Electron Cyclotron Resonance (ECR) based system was used to simulate LEO conditions. The ECR source was characterized by Kapton dosimetry and in-situ four point probe silver film resistively measurements. The VUV output was measured by a calibrated VUV spectrometer. Ceria (CeO2) doped quartz (CMG) were used to compare and contrast the degradation of a thin MgF2 AR coating on CMG. The effect of temperature and VUV radiation in enhancing/reducing the degradation was also studied. Unexpectedly severe degradation in the form of optical transmission loss was observed upon exposure to high AO fluences with and without VUV exposure. We will show that oxygen penetrates the MgF2 films to depths in excess of 20nm and there is clear oxygen bonding to the structure.

B-Site Doped Lanthanum Strontium Manganites by the DAAS Technique

Magnetic perovskites of the general form La 0.7 Sr 0.3 Mn 1−y Fe y O 3 (y = 0, 0.05, 0.10, and 0.15) have successfully been synthesized using deposition by aqueous acetate solution (DAAS). Crystalline, iron-doped, lanthanum strontium manganite (Fe-doped LSMO) powders are obtained by preparing an aqueous solution of metal acetate precursors in the proper stoichiometry, drying the solution to generate a glassy gel, consolidating the gel, and then firing the gel for short periods of time ( 0.7 Sr 0.3 Mn 1−y Fe y O 3 prepared by DAAS and annealed for 100 minutes at 1200°C are of high purity, are single phase, and exhibit excellent electrical and magnetic characteristics. Powders annealed at 1200°C or greater exhibit sharp metalinsulator transitions. Increasing the iron dopant concentration in these powders from 0% to 15% decreases the metal-insulator transition temperature of these samples from ∼360K to about 140K. The resistivity of these powders also increases with increasing substitution of the lattice B-site with iron, as does the unit cell volume of the lattice. Preparation of an iron doped LSMO powder that exhibits a maximum magnetoresistance at 305K was successfully carried out via a careful selection of iron content and anneal temperature. This compound, La 0.7 Sr 0.3 Mn 0.93 Fe 0.07 O 3 , exhibits a magnetoresistance of 40% at 305K and an applied field of 5 Tesla.