A thin film technology compatible with multilayer device fabrication is critical for exploring the potential of the 39-K superconductor magnesium diboride for superconducting electronics. Using a Hybrid Physical-Chemical Vapor Deposition (HPCVD) process, it is shown that the high Mg vapor pressure necessary to keep the MgB 2 phase thermodynamically stable can be achieved for the in situ growth of MgB 2 thin films. The films grow epitaxially on (0001) sapphire and (0001) 4H-SiC substrates and show a bulk-like T c of 39 K, a J c (4.2K) of 1.2 × 10 7 A/cm 2 in zero field, and a H c2 (0) of 29.2 T in parallel magnetic field. The surface is smooth with a root-mean-square roughness of 2.5 nm for MgB 2 films on SiC. This deposition method opens tremendous opportunities for superconducting electronics using MgB 2 .

We have used two polytypes of silicon carbide single crystals, 4H-SiC and 6H-SiC, as the substrates for MgB 2 thin films grown by hybrid physical-chemical vapor deposition ͑HPCVD͒. The c-cut surface of both polytypes has a hexagonal lattice that matches closely with that of MgB 2. Thermodynamic calculations indicate that SiC is chemically stable under the in situ deposition conditions for MgB 2 using HPCVD. The MgB 2 films on both polytypes show high-quality epitaxy with a Rutherford backscattering channeling yield of 12%. They have T c above 40 K, low resistivities, high residual resistivity ratios, and high critical current densities. The results demonstrate that SiC is an ideal substrate for MgB 2 thin films.

We have studied the effect of deposition rate and layer thickness on the properties of epitaxial MgB 2 thin films grown by hybrid physical-chemical vapor deposition on 4H-SiC substrates. The MgB 2 film deposition rate depends linearly on the concentration of B 2 H 6 in the inlet gas mixture. We found that the superconducting and normal-state properties of the MgB 2 films are determined by the film thickness, not by the deposition rate. When the film thickness was increased, the transition temperature, T c , increased and the residual resistivity, ρ 0 , decreased. Above 300 nm, a T c of 41.8 K, a ρ 0 of 0.28 µΩ·cm, and a residual resistance ratio RRR of over 30 were obtained. These values represent the best MgB 2 properties reported thus far.

We have studied the effect of damage induced by 2 MeV alpha particles on the critical temperature, T c , and resistivity, , of MgB 2 thin films. This technique allows defects to be controllably introduced into MgB 2 in small successive steps. T c decreases linearly as the intragrain resistivity at 40 K increases, while the intergrain connectivity is not changed. T c is ultimately reduced to less than 7 K and we see no evidence for a saturation of T c at about 20 K, contrary to the predictions of the T c of MgB 2 in the dirty limit of interband scattering.

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The high resistivity of many bulk and film samples of MgB 2 is most readily explained by the suggestion that only a fraction of the cross-sectional area of the samples is effectively carrying current. Hence the supercurrent (J c ) in such samples will be limited by the same area factor, arising for example from porosity or from insulating oxides present at the grain boundaries. We suggest that a correlation should exist, J c ~ 1/∆ρ 300-50K , where ∆ρ 300-50K is the change in the apparent resistivity from 300 K to 50 K. We report measurements of ρ(T) and J c for a number of films made by hybrid physical-chemical vapor deposition which demonstrate this correlation, although the "reduced effective area" argument alone is not sufficient. We suggest that this argument can also apply to many polycrystalline bulk and wire samples of MgB 2 .

We have studied structural and superconducting properties of MgB 2 thin films doped with carbon during the hybrid physical-chemical vapor deposition process. A carbon-containing metalorganic precursor bis(methylcyclopentadienyl)magnesium was added to the carrier gas to achieve carbon doping. As the amount of carbon in the film increases, the resistivity increases, T c decreases, and the upper critical field increases dramatically as compared to clean films. The selffield J c in the carbon doped film is lower than that in the clean film, but J c remains relatively high to much higher magnetic fields, indicating stronger pinning. Structurally, the doped films are textured with columnar nano-grains and highly resistive amorphous areas at the grain boundaries. The carbon doping approach can be used to produce MgB 2 materials for high magnetic-field applications. a)Electronic address: avp11@psu.edu

A novel photothermal process to spatially modulate the concentration of sub-wavelength, high-index nanocrystals in a multicomponent Ge-As-Pb-Se chalcogenide glass thin film resulting in an optically functional infrared grating is demonstrated. The process results in the formation of an optical nanocomposite possessing ultralow dispersion over unprecedented bandwidth. The spatially tailored index and dispersion modification enables creation of arbitrary refractive index gradients. Sub-bandgap laser exposure generates a Pb-rich amorphous phase transforming on heat treatment to high-index crystal phases. Spatially varying nanocrystal density is controlled by laser dose and is correlated to index change, yielding local index modification to ≈+0.1 in the mid-infrared.