Carol Ashby - Academia.edu (original) (raw)

Papers by Carol Ashby

Three characteristic regimes were identified during wet thermal oxidation of AlxGa(1-x)As (x=1 to... more Three characteristic regimes were identified during wet thermal oxidation of AlxGa(1-x)As (x=1 to 0.90) on GaAs: oxidation of Al and Ga in the alloy to form to an amorphous oxide layer, formation and elimination of elemental As and of amorphous As2O3, and crystallization of the oxide film. Residual As can produce up to a 100fold increase in leakage current and a 30% increase in bulk dielectric constant. Very low As levels produce partial Fermi-level pinning at the oxidized AlxGa(1-x)As/GaAs interface. Local Schottky- barrier pinning of the Fermi level at As precipitates at the oxide/GaAs interface may be the source of the apparent high interface state density. The presence of thermodynamically favored interfacial As may impose a fundamental limit on the application of AlGaAs wet oxidation for achieving MIS devices without post-oxidation processing to remove the residual As from the interface.

The Journal of Chemical Physics, 1984

The chemical reactivity of graphite surfaces exposed to atomic hydrogen is enhanced under electro... more The chemical reactivity of graphite surfaces exposed to atomic hydrogen is enhanced under electron bombardment by an enhancement factor &amp;egr;′ which depends on the incident electron energy. A sharp threshold-like increase in &amp;egr;′ is observed for crystalline graphite at an energy Eth which depends on the angle of incidence θ of the electron beam with respect to the basal-plane surface

MRS Proceedings, 1993

ABSTRACTThe growth of high-quality dielectric films from Electron Cyclotron Resonance (ECR) plasm... more ABSTRACTThe growth of high-quality dielectric films from Electron Cyclotron Resonance (ECR) plasmas provides for low-temperature surface passivation of compound semiconductors. Silicon nitride (SiNx) films were grown at temperatures from 30°C to 250°C on GaAs substrates. The stress in the films was measured as a function of bias applied during growth (varied from 0 to 200 V), and as a function of sample annealing treatments. Composition profiles of the samples were measured using ion beam analysis. The GaAs photoluminescence (PL) signal after SiNx growth without an applied bias (ion energy = 30 eV) was twice as large as the PL signal from the cleaned GaAs substrate. The PL signal from samples biased at -50 and -100 V indicated that damage degraded the passivation quality, while atomic force microscopy of these samples showed a three fold increase in rms surface roughness relative to unbiased samples. The sample grown with a bias of-200 V showed the largest reduction in film stress ...

Applied Physics Letters, 1997

U.S. Department of Energy Office of Scientific and Technical Information - OSTI OAI, Jul 20, 2004

U.S. Department of Energy Office of Scientific and Technical Information - OSTI OAI, Jul 29, 2003

Electronics Letters, 1995

ABSTRACT Nonselective reactive ion beam etching (RIBE) using Cl2/Ar mixtures at elevated temperat... more ABSTRACT Nonselective reactive ion beam etching (RIBE) using Cl2/Ar mixtures at elevated temperatures and two isotropic wet chemistries (HCl/HNO3/H2O2 and HCl/H2O2/H2O) have been developed for fabricating 1.3 mμ optical transmission modulators employing periodic multilayers of InGaAs/InAlAs

GaN etching can be affected by a wide variety of parameters including plasma chemistry and plasma... more GaN etching can be affected by a wide variety of parameters including plasma chemistry and plasma density. Chlorine-based plasmas have been the most widely used plasma chemistries to etch GaN due to the high volatility of the GaC1, and NCI etch products. The source of C1 and the addition of secondary gases can dramatically influence the etch characteristics primarily due to their effect on the concentration of reactive C1 generated in the plasma. In addition, high-density plasma etch systems have yielded high quality etching of GaN due to plasma densities which are 2 to 4 orders of magnitude higher than reactive ion etch (RE) plasma systems. The high plasma densities enhance the bond breaking efficiency of the GaN, the formation of volatile etch products, and the sputter desorption of the etch products from the surface. In this study, we report GaN etch results for a high-density inductively coupled plasma (ICP) as a function of BCI,:Cl, flow ratio, dc-bia;, chamber pressure, and ICP source power. GaN etch rates ranging from-100 ii/min to > 8000 A/min were obtained with smooth etch morphology and anisotropic profiles.

MRS Proceedings, 1995

ABSTRACTElectron cyclotron resonance (ECR) etching of GaN in Cl2/H2/Ar, Cl2/SF6/Ar, BCl3/H2Ar and... more ABSTRACTElectron cyclotron resonance (ECR) etching of GaN in Cl2/H2/Ar, Cl2/SF6/Ar, BCl3/H2Ar and BCl3/SF6/Ar plasmas is reported as a function of percent H2 and SF6. GaN etch rates were found to be 2 to 3 times greater in Cl2/H2/Ar discharges than in BCl3/H2/Ar discharges independent of the H2 concentration. In both discharges, the etch rates decreased as the H2 concentration increased above 10%. When SF6 was substituted for H2, the GaN etch rates in BCl3-based plasmas were greater than those for the Cl2-based discharges as the SF6 concentration increased. GaN etch rates were greater in Cl2/H2/Ar discharges as compared to Cl2/SF6/Ar discharges whereas the opposite trend was observed for BCl3,-based discharges. Variations in surface morphology and near-surface stoichiometry due to plasma chemistries were also investigated using atomic force microscopy and Auger spectroscopy, respectively.

MRS Proceedings, 1983

ABSTRACTUltraviolet irradiation of the surface of graphite leads to the enhancement of the reacti... more ABSTRACTUltraviolet irradiation of the surface of graphite leads to the enhancement of the reaction of graphite with hydrogen to form methane under conditions where photo-induced thermal effects are negligible. Wavelength dependence of the photoenhancement correlates with excitation of the π-valence to π-conduction transition of graphite centered at 260 nm. Subsequent formation of some reactive excited state species leads to enhanced reaction rates. Likely candidates for such reactive species have been identified by comparative kinetic studies of the thermal and the photoenhanced reactions. For example, at low temperatures (< 500 K), the rate-limiting step of the thermal reaction is addition of H to surface CH3 groups, and the observed photoenhancement can be explained by activation of these CH3 groups.

MRS Proceedings, 2000

ABSTRACTAmorphous diamond is a new material for surface-micromachined microelectromechanical syst... more ABSTRACTAmorphous diamond is a new material for surface-micromachined microelectromechanical systems (MEMS) that offers promise for reducing wear and stiction of MEMS components. The material is an amorphous mixture of 4-fold and 3-fold coordinated carbon with mechanical properties close to that of crystalline diamond. A unique form of structural relaxation permits the residual stress in the material to be reduced from an as-deposited value of 8 GPa compressive down to zero stress or even to slightly tensile values. Irreversible plastic deformation, achieved by heat treating elastically strained structures, is also possible in this material. Several types of amorphous diamond MEMS devices have been fabricated, including electrostatically-actuated comb drives, micro-tensile test structures, and cantilever beams. Measurements using these structures indicate the material has an elastic modulus close to 800 GPa, fracture toughness of 8 MPa.m½, an advancing H2O contact angle of 84° to 94...

Journal of Materials Research, 1994

The development of passive and active electronics from high-temperature superconducting thin film... more The development of passive and active electronics from high-temperature superconducting thin films depends on the development of process technology capable of producing appropriate feature sizes without degrading the key superconducting properties. We present a new class of chelating etches based on di- and tri-carboxylic acids that are compatible with positive photoresists and can produce submicron feature sizes while typically producing increases in the microwave surface resistance at 94 GHz by less than 10%. This simple etching process works well for both the Y-Ba-Cu-O and Tl-Ba-Ca-Cu-O systems. In addition, we demonstrate that the use of chelating etches with an activator such as HF allows the etching of related oxides such as LaAlO3, which is a key substrate material, and Pb(Zr0.53Ti0.47)O3 (PZT) which is a key ferroelectric material for HTS and other applications such as nonvolatile memories.

Journal of Applied Physics, 1989

The production of a tailored implant‐damage profile by sequential implantation of ions at several... more The production of a tailored implant‐damage profile by sequential implantation of ions at several different energies rather than at a single energy can increase the depth and improve the wall smoothness of features produced by ion‐bombardment‐enhanced etching of a solid. This has been demonstrated in the enhanced etching of LiNbO3 by HF following He+ implantation at several energies and fluences selected to produce a relatively flat damage depth profile. The use of lighter ions permits the etching of deeper features than is possible with heavier ions of the same energy. Buried microcracks are observed in heavily implanted Z‐cut samples. Similar microcracks may play an important role in determining propagation losses in ion‐implanted waveguides.

Journal of the American Chemical Society, 1978

ABSTRACT