Showing papers by "Douglas H. Lowndes published in 1995"

Method for adhesion of metal films to ceramics

Abstract: Methods for making strongly bonded metal-ceramic materials. The methods include irradiating a portion of the surface of the ceramic material with a pulsed ultraviolet laser having an energy density sufficient to effect activation of the irradiated surface of the ceramic material so that adhesion of metals subsequently deposited onto the irradiated surface is substantially increased. Advantages of the invention include (i) the need for only a small number of laser pulses at relatively low focused energy density, (ii) a smoother substrate surface, (iii) activation of the laser-treated surface which provides a chemical bond between the surface and a metal deposited thereon, (iv) only low temperature annealing is required to produce the strong metal-ceramic bond; (v) the ability to obtain strong adhesion between ceramic materials and oxidation resistant metals; (vi) ability to store the laser treated ceramic materials for later deposition of metals thereon.

The adhesion of metallic films to laser‐irradiated alumina

Abstract: The adhesion strength of copper and nickel films deposited onto polycrystalline alumina (Al2O3) substrates is greatly increased by pulsed ultraviolet excimer (XeCl, 308 nm) laser irradiation of the alumina prior to metal‐film deposition. Adhesion enhancement occurs for pulsed‐laser energy densities, Ed, both below and above alumina’s melting threshold (Edth∼0.7 J/cm2). Cross‐section transmission electron micrographs of alumina irradiated at Ed≳ Edth reveal an outer amorphous layer 40–60 nm thick that is formed during the rapid solidification process that follows pulsed‐laser melting. Our results for gold, copper, and nickel films demonstrate that several factors contribute to metal‐alumina bonding: (1) the chemical nature of the metal, as indicated by quite different adhesion strengths of sputter‐deposited metals on unirradiated alumina substrates: 0.1 MPa for Au, 13 MPa for Cu, and 32 MPa for Ni; (2) the type and extent of laser‐generated disorder/damage, since for gold significant bonding enhancement is...

Greatly improved adhesion of gold films sputter-deposited on laser-treated and thermally annealed alumina

Abstract: The effects on adhesion strength of pulsed-laser irradiation prior to film deposition, and of thermal annealing following film deposition, have been studied for gold films deposited by ion beam sputtering on polished polycrystalline alumina (Al2O3) substrates. Three different atmospheres were employed for the laser irradiations: Ar-4% H2, air, and oxygen, all at 1 atmosphere pressure. A similar functional dependence of adhesion strength on laser energy density, Ed, was found for all of the gaseous environments: No change of adhesion strength for Ed⩽0.5 J/cm2, maximum adhesion for Ed=1.0–1.5 J/cm2, and decreased adhesion for higher Ed. A pronounced increase in gold-alumina adhesion strength was achieved when the alumina substrates were irradiated in an oxygen-rich atmosphere. A post-deposition 300 °C anneal doubled the adhesion strength to ∼50 MPa, approximately 500 times the strength of untreated gold-alumina couples. The adhesion strength initially increased rapidly with annealing time but saturated for ...

Pulsed laser ablation growth and doping of epitaxial compound semiconductor films

Abstract: Pulsed laser ablation (PLA) has several characteristics that are potentially attractive for the growth and doping of chemically complex compound semiconductors including (1) stoichiometric (congruent) transfer of composition from target to film, (2) the use of reactive gases to control film composition and/or doping via energetic-beam-induced reactions, and (3) low-temperature nonequilibrium phase formation in the laser-generated plasma ``plume.`` However, the electrical properties of compound semiconductors are far more sensitive to low concentrations of defects than are the oxide metals/ceramics for which PLA has been so successful. Only recently have doped epitaxial compound semiconductor films been grown by PLA. Fundamental studies are being carried out to relate film electrical and microstructural properties to the energy distribution of ablated species, to the temporal evolution of the ablation pulse in ambient gases, and to beam assisted surface and/or gas-phase reactions. In this paper the authors describe results of ex situ Hall effect, high-resolution x-ray diffraction, transmission electron microscopy, and Rutherford backscattering measurements that are being used in combination with in situ RHEED and time-resolved ion probe measurements to evaluate PLA for growth of doped epitaxial compound semiconductor films and heterostructures. Examples are presented and results analyzed for doped II-VI, I-III-VI, and column-III nitride materials grown recently in this and other laboratories.

Phonon-drag thermopower correlations to Tc in superconducting SrxNd1-xCuO2- delta : Evidence for phonon-mediated pairing in the high-Tc parent compounds.

Abstract: The {ital A}CuO{sub 2} compounds---containing only CuO{sub 2} planes separated by {ital A} ions---are the basic ``building blocks`` of the more complex high-{ital T}{sub {ital c}} superconductors. Trivalently doped Sr{sub {ital x}}Nd{sub 1{minus}{ital x}}CuO{sub 2{minus}{delta}} thin films are shown to superconduct with the appearance of a phonon-drag contribution to the thermopower at the superconducting doping threshold. A monotonic increase in this phonon-drag contribution with {ital T}{sub {ital c}} suggests that electron-phonon interactions play an important role in the pairing mechanism. A correlation between the deduced BCS coupling constants and {ital T}{sub {ital c}} is consistent with strong-coupling theory.

Growth of Highly Doped P-Type Znte Films by Pulsed Laser ablation in Molecular Nitrogen

Abstract: Highly p-doped ZnTe films have been grown on semi-insulating GaAs (001) substrates by pulsed-laser ablation (PLA) of a stoichiometric ZnTe target in a high-purity N{sub 2} ambient without the use of any assisting (DC or AC) plasma source. Free hole concentrations in the mid-10{sup 19} cm{sup {minus}3} to > 10{sup 20} cm{sup {minus}3} range were obtained for a range of nitrogen pressures The maximum hole concentration equals the highest hole doping reported to date for any wide band gap II-VI compound. The highest hole mobilities were attained for nitrogen pressures of 50--100 mTorr ({approximately}6.5-13 Pa). Unlike recent experiments in which atomic nitrogen beams, extracted from RF and DC plasma sources, were used to produce p-type doping during molecular beam epitaxy deposition, spectroscopic measurements carried out during PLA of ZnTe in N{sub 2} do not reveal the presence of atomic nitrogen. This suggests that the high hole concentrations in laser ablated ZnTe are produced by a new and different mechanism, possibly energetic beam-induced reactions with excited molecular nitrogen adsorbed on the growing film surface, or transient formation of Zn-N complexes in the energetic ablation plume. This appears to be the first time that any wide band gap (Eg > 2 eV) II-VI compound (or other) semiconductor has been impurity-doped from the gas phase by laser ablation. In combination with the recent discovery that epitaxial ZnSe{sub l-x}S{sub x} films and heterostructures with continuously variable composition can be grown by ablation from a single target of fixed composition, these results appear to open the way to explore PLA growth and doping of compound semiconductors as a possible alternative to molecular beam epitaxy.