Klaus Hermann Dahmen

Organometallic chemical vapor deposition of high Tc superconducting films using a volatile, fluorocarbon-based precursor

Uniform films of the high Tc superconductor YBa2Cu3O7−δ have been prepared by organometallic chemical vapor deposition using the volatile metalorganic precursors Cu(acetylacetonate)2, Y(dipivaloylmethanate)3, and Ba(heptafluorodimethyloctanedionate)2. With argon as a carrier gas and water vapor as a reactant, film growth rates of 10–30 nm/min are achieved. After annealing under oxygen, energy dispersive x‐ray analysis, profilometry, and x‐ray diffraction data reveal that such YBa2Cu3O7−δ films on [100] single‐crystal MgO have good compositional and dimensional uniformity as well as preferential orientation of crystallite c axes perpendicular to the substrate surface. Four‐probe resistivity measurements reveal the onset of superconductivity at ∼90 K and zero resistance by 66.2 K.

Large positive magnetoresistance in thin films of silver telluride

Thin films of Ag2Te were obtained by vapor transport and electron beam evaporation methods. The film phase and chemical compositions were analyzed by x-ray diffraction and x-ray photoelectron spectroscopy analyses, respectively. Film composition was close to stoichiometric with a slight excess of tellurium. Strong (121) texture was found in the films obtained by the electron beam evaporation. Magnetoresistance (MR) measurements show different temperature dependencies for oriented and nonoriented films. MR of the oriented films has a strong peak up to 390% at 90 K and 5 T, whereas MR of the nonoriented films is almost temperature independent in the 10–100 K range. Some analogies can be found between these materials and colossal MR and nonmagnetic multilayer MR materials.

Growth of iridium films by metal organic chemical vapour deposition

Abstract Thin films of metallic iridium were grown by thermal metal organic chemical vapour deposition in a horizontal hot-wall reactor. The new solid compound Ir(2,2,6,6-tetramethyl-3,5-heptadione)(1,5-cyclooctadiene) was used as the iridium source. Evaporation rate experiments showed a rather poor volatility of this compound compared with Ir(acetylacetone)(1,5-cyclooctadiene). However, high growth rates of 0.25 μ h −1 could be realized at substrate temperatures of 350 °C using a graphite susceptor coated with iridium andisopropanol as a component of the reaction gas. The films were pure metallic, as shown by emission spectroscopy for chemical analysis, and had typical sheet resistances of 50 μω □ −1 .

Low pressure organometallic chemical vapor deposition of high-Tc superconducting YBa2Cu3O7−δ films

Abstract High-Tc superconducting YBa2Cu3O7−δ films have been prepared by low-pressure organometallic chemical vapor deposition using Cu(acetylacetonate)2, Y(dipivaloylmethanate)3, and Ba(dipivaloylmethanate)2 as volatile metal-organic precursors. An argon carrier gas is employed and water vapor serves as the reactant. Film growth rates of 10–30 nm min-1 are achieved at a system pressure of 5 torr. After annealing under oxygen, the YBa2Cu3O7−δ films deposited on SrTiO3 exhibit excellent compositional and structural uniformity in addition to preferential orientation of crystalline c axes perpendicular to the SrTiO3 surface. The onset of superconductivity is at ca. 90 K and zero resistance is achieved by 47 K.

Crystallinity and magnetoresistance in La1−xCaxMnO3 thin films

The crystallinity of La1−xCaxMnO3 (LCMO) thin films can be controlled by preparing the films on substrates with different lattice parameters. Closely lattice matched LCMO thin films on LaAlO3 (LAO) substrates exhibit a high degree of crystallinity and have magnetoresistance (MR) and MR ratio [R(H=0 T)-R(H=6 T)]/R(H=6 T) that is sharply peaked in the vicinity of the metal-insulator transition temperature (TMI). Notably, films grown on LAO(011) have TMI shifted up in temperature ∼20 K higher than the same films on LAO(001) substrates. LCMO films on Al2O3 and Y-ZrO2, that are not as closely lattice matched, have a lower degree of crystallinity, less sharply peaked MR and a nearly constant MR ratio below TMI over a broad temperature range.

Characterization of films of La1−xSrxMnO3−δ grown by means of metal organic chemical vapor deposition

An effort to develop La manganites for possible sensor applications has led to the successful growth of La1−xBxMnO3−δ (B=Sr, Ca) thin films on single crystal MgO, Al2O3, and LaAlO3 substrates. Here we concentrate on Sr-doped material on LaAlO3 with Curie temperature Tc=262±2 K. Tc was determined from magnetization measurements using Arrott plots, and agrees well with the observed resistivity peak in zero applied magnetic field (H). This Tc corresponds closely to a Sr content x=0.17 reported in bulk materials. At H=60 kOe the maximum magnetoresistance Δρ/ρ≈0.95. No discernible hysteresis, as might be expected for structural phase transitions, was observed. When plotted as a function of reduced temperature T/Tc, the magnetization extrapolated to zero H is best fit by a Brillouin function with an effective spin value 〈S〉 much larger than the 〈S〉=1.9 indicated by the composition. This may be indicative of magnetic polaron formation, especially near the magnetic and transport transition temperatures. At temper...

Morphology analysis of nickel thin films grown by MOCVD

Abstract Nickel films on quartz substrates were prepared by metal-organic chemical vapor deposition (MOCVD), from the precursor bis(dimethylglyoximato)Ni(II), [Ni(dmg) 2 ]. The deposition was carried out at reduced pressure in a horizontal quartz glass reactor. The samples were analysed by profilometry, X-ray diffraction, electron spectroscopy for chemical analysis, atomic force microscopy (AFM) and scanning electron microscopy. Resistances were determined by four-point resistivity measurements. The film microstructure was described by a simplified two-layer model using the data obtained from AFM, profilometry and resistivity measurements. In this model, the films consist of a compact layer (density ≈ 100%) between the substrate and the surface layer and a porous surface layer (density

An improvement of surface smoothness and lattice match of CeO2 buffer layers on R-sapphire processed by MOCVD

Abstract Epitaxial thin films of CeO 2 and Ce 1− x R x O 2− y (R=Y, Nd; x =0–0.32) on R-sapphire were prepared by band flash evaporation MOCVD. It was demonstrated that the smoothness of the films can be improved by a decrease of deposition rate and by post deposition annealing at 1000 °C. The lattice match of buffer layers with R-Al 2 O 3 and YBa 2 Cu 3 O 7−δ was improved by doping of ceria yttrium and neodymium correspondingly. A bilayer structure Ce 0.7 Nd 0.3 O 2− y /Ce 0.68 Y 0.32 O 2 − y /R-Al 2 O 3 is proposed as potential substrate material for YBa 2 Cu 3 O 7 − 6 deposition.

Dry Etch Patterning of LaCaMnO3 and SmCo Thin Films

A number of different plasma chemistries have been employed for patterning of LaCaMnO 3 and SmCo thin films for application in magnetic-field-biased structures based on the colossal magnetoresistive effect. For LaCaMnO 3 there was no chemical enhancement in etch rate over simple Ar sputtering for Cl 2 , SF 6 , and CH 4 /H 2 plasmas under high ion density conditions. This is expected based on the vapor pressures of the prospective etch products. For SmCo, however, etch rates up to 7000 A min 1 were obtained in Cl 2 /Ar plasmas, which is an order of magnitude faster than Ar sputtering under the same experimental conditions. Smooth etched surface morphologies and anistropic sidewalls were obtained for both materials over a wide range of plasma source and chuck powers.

Development of chemically assisted dry etching methods for magnetic device structures

There is a strong need for advanced pattern transfer methods for magnetic devices such as magnetic random access memories, sensors for avionics and mine detection, and read/write heads for high density information storage. As the critical dimensions in these devices are decreased, the use of ion milling for pattern transfer presents major obstacles, including sidewall redeposition (which degrades magnetic performance) and poor mask selectivity. Most magnetic materials do not form volatile etch products in conventional reactive ion etching. We have recently found that high density plasmas provide efficient ion-assisted desorption of metal chloride etch products, provided that the etch production formation and removal are balanced by correct choice of ion/neutral ratio. We have completed the survey of plasma chemistries for etching of giant magnetoresistance (GMR) (NiFe, NiMnSb) and collossal magnetoresistance (CMR) (LaCaMnO3,LaSrMnO3,PrBaCaMnO3) materials. The optimum choices are Cl2/Ar for CMR oxides, SF6...