Search Results (13 total)

Amorphous hydrogenated silicon suboxides (a-SiO_x:H) deposited by plasma enhanced chemical vapor deposition have a band gap which can be tuned from 1.9 to 3.0 eV by varying the oxygen content [O] from 0 to 50 at. %. n- and p-type doping is realized by adding PH₃ and B₂H₆, respectively, to the source gases SiH₄, H₂, and CO₂. Alloying with increasing amounts of oxygen reduces the average coordination number 〈r〉 from a value close to 4 (a-Si:H) to ≈2.7, which gradually approaches the ideal value of 〈r〉=2.4 for network glasses. This goes along with a softening of the amorphous SiO_x network, i.e., a reduction of the mechanical hardness of the material, which is also predicted by rigidity percolation theory. Also the incorporation of dopant atoms into electrically active, fourfold coordinated sites becomes more unlikely with increasing [O]. As a consequence, n- and p-type doped SiO_x shows increasingly intrinsic character for higher oxygen concentrations. Doping fails for values of 〈r〉<3 and the doping efficiency tends towards zero. Thus, an overall fourfold coordination was found to be a crucial requirement for efficient doping in amorphous semiconductors.

A cubic silicon carbide (β-SiC) buried layer was synthesized in Si(111) using a combination of multienergy carbon ion implantation at room temperature and post-thermal annealing. The crystal structure and the crystalline quality of the β-SiC layer was identified by x-ray diffraction in the θ–2θ mode and was examined by pole figure measurement of x-ray diffraction. Interestingly, by using the multienergy implantation technique, the β-SiC buried layer showed epitaxial growth at annealing temperatures as low as 400 °C. At an annealing temperature of 800 °C, the x-ray pole figures show that the β-SiC buried layer has a near-perfect epitaxial relationship with the silicon substrate.

For the heteroepitaxial deposition of diamond on silicon using the bias-enhanced nucleation procedure, several different processes contributing to the final misalignment of the layers can be identified: (i) The interface of Si/diamond or Si/SiC and SiC/diamond, respectively. (ii) The growth of individual grains during the biasing process. (iii) The growth competition between differently oriented grains and their coalescence during the growth of thick films. X-ray-diffraction texture studies revealed that the azimuthal alignment is essentially determined by the nucleation step. Oriented nucleation is only possible within a defined time window. Within this time window the azimuthal misalignment shows a characteristic variation depending on the absolute value of the bias voltage. The alignment of the SiC interlayer as measured by synchrotron radiation cannot explain the observed variation. In contrast, texture measurements of thick oriented films after exposure to the bias conditions suggest that the limitation of the process time window for oriented nucleation as well as the variation of misorientation with biasing time can be traced back to the detrimental effect of bias-assisted growth. Based on this mechanism, a model is proposed which allows one (a) to describe the temporal development of the azimuthal misorientation within the process time window, and (b) to estimate the contribution of bias-assisted growth on the misorientation. Finally, some epitaxial diamond films have been deposited on high-quality β-SiC layers. A minimum value of 2.9° for the width of the azimuthal distribution has been found.

We report a determination of the irreversibility line of epitaxial c-axis-oriented YBa₂Cu₃O₇ thin films with different thicknesses obtained by irreversible magnetization measurements. While close to T_c the irreversibility line is obtained by direct measurements, a universal relation is found at lower temperatures which allows a determination of the irreversibility line down to 4.2 K. This line is found to shift towards larger values when the film thickness d is increased and saturates for d larger than the penetration depth λ_ab(0). The results are discussed in the framework of flux creep in quasi-two-dimensional flux-line-lattice and vortex-lattice melting models.

c-axis-oriented YBa₂Cu₃O₇ (1:2:3) and YBa₂Cu₄O₈ (1:2:4) thin films were charged with hydrogen at 463 K and a H₂ pressure of 100 mbar. The hydrogen concentrations and depth profiles were measured with the ¹⁵N nuclear reaction method. In contrast to earlier studies on bulk samples where a constant T_c,onset is reported, we find in our 1:2:3 films that the superconducting transition temperature decreases continuously with increasing hydrogen concentration and that the transitions remain relatively sharp, e.g., T_c,onset=38 K and ΔT_c=6.2 K for [H]/cell≃0.6. The electrical resistivity and Hall-effect measurements in the normal state indicate that the main effect of hydrogen doping is a reduction of the charge carrier concentration, whereas the carrier mobility is not changed significantly. At an average hydrogen concentration of [H]/cell≃2 in both systems, a new hydride phase is observed, which for 1:2:3 is characterized by a c-axis expansion of 16% and for 1:2:4 by a c-axis expansion of 1.5%.

Magnetic properties of epitaxial thin films of YBa₂Cu₃O₇ have been studied in the superconducting state in fields perpendicular to the sample, along the c crystallographic axis. In the low-temperature state, susceptibility, hysteresis loop, and remanent magnetization (trapped flux) studies show that a critical state is established in very low applied fields. The flux penetration process and the related thickness properties are well accounted for by simple electromagnetic models for the critical current. It is shown that the evaluation of the critical current J_c by the classical Bean formula is roughly valid, in spite of the curvature of the vortices. A decrease in the low-temperature value of J_c with increasing thickness is found above 100 nm. In the temperature range close to T_c, the irreversibility line moves towards higher field with increasing thickness according to a d^1/2 law. This is discussed in relation with the theories of vortex-lattice and vortex-glass melting.

Reflection high-energy electron diffraction oscillations during pulsed laser ablation of YBa₂Cu₃O_7-x on SrTiO₃(100) substrates were measured. We have found for the first time that the oscillations are modulated by the laser-pulse repetition frequency. Each laser pulse deposits 0.14 nm of material. This causes the specularly reflected intensity to decrease instantaneously, but then it increases exponentially due to crystallization. An activation energy of 0.7±0.1 eV and a diffusion constant of 1.4×10^-12 cm²/sec at 720°C for the formation of c-axis oriented films was determined.

We report on the low-temperature hydrogen implantation and subsequent investigation of superconductivity for dilute alloys of Mn in palladium. A simple Abrikosov-Gorkov-type pair breaking is observed. Our resistivity determination of T_c is compared with an existing study in which hydrogen charging was carried out electrolytically and partial superconductivity, as resolved by susceptibility measurements, persisted to larger manganese concentrations.

Through measurements of the low-temperature (0.06-2.0 K) thermal conductivity of dielectric crystals in the boundary-scattering regime we are searching for causes of diffuse phonon scattering by crystal surfaces. Thin (≤ 100 Å) gold films on polished sapphire surfaces cause diffuse phonon scattering above 0.3 K. Below that temperature, specular reflection again begins to dominate. The same diffuse scattering can be caused by implantation of Xe ions which damage a layer ≃500-Å thick underneath the sapphire surface. For gold films, the effect diminishes with increasing thickness, i.e., greater perfection of the film. Because of the very small thickness of the layers in which the diffuse scattering occurs the phonon-scattering strength must be very much stronger than any phonon scattering previously observed in the bulk of dielectric crystals. Several potential explanations are discussed. However, none appear to be sufficiently general to account for both kinds of surface treatments.

The ac susceptibility of thin PdFe films has been measured before and after low-temperature irradiation with inert noble gases by using a newly developed susceptometer. The disorder introduced by the irradiation causes a reduction in both the Curie temperature by a factor of 2 and the Curie constant by a factor of about 7. From this result it is calculated that the low-temperature irradiation reduces the susceptibility of the Pd host by more than an order of magnitude, thus explaining the recently observed superconductivity in irradiated Pd.

A classical time-of-flight method was used to determine the temperature of evaporated Si atoms and thus the lattice temperature of Si during pulsed laser annealing. The resulting temperatures between 1200 and 3000 K for energy densities between 1.0 and 2.5 J/cm² clearly support a strictly thermal annealing model including melting of the surface region of Si.

Pure palladium films, evaporated between 4.2 and 300 K, can be transformed into superconductors by means of irradiation at low temperatures with He⁺ ions. The maximum transition temperature obtained is 3.2 K. It is shown that the presence of a special kind of defects produced during the irradiation is a necessary precondition for the occurrence of superconductivity.