R.C. Dynes

Mesotaxy: Single‐crystal growth of buried CoSi2 layers

Buried single‐crystal CoSi2 layers in silicon have been formed by high dose implantation of cobalt followed by annealing. These layers grow in both the (100) and (111) orientations—those in (111) have better crystallinity, but those in (100) are of higher electrical quality. Electrical transport measurements on the layers give values for the resistance ratios and superconducting critical temperatures that are better than the best films grown by conventional techniques and comparable to bulk CoSi2.

Controllable reduction of critical currents in YBa2Cu3O7−δ films

The critical currents in high quality thin films of the high Tc superconductor, YBa2Cu3O7−δ, can be controllably reduced by orders of magnitude using ion irradiation. This reduction in critical current occurs without substantial decrease in Tc or increase in room‐temperature resistivity. Using this technique, we have fabricated weak links that exhibit an ac Josephson effect at 77 K.The critical currents in high quality thin films of the high Tc superconductor, YBa2Cu3O7−δ, can be controllably reduced by orders of magnitude using ion irradiation. This reduction in critical current occurs without substantial decrease in Tc or increase in room‐temperature resistivity. Using this technique, we have fabricated weak links that exhibit an ac Josephson effect at 77 K.

Mesotaxy: Synthesis of buried single-crystal silicide layers by implantation

Abstract High dose implantation of metal ions in silicon followed by annealing can be used to form buried single-crystal disilicide layers with atomically abrupt interfaces. This technique, which we call mesotaxy, works particularly well for cubic silicides with a small lattice mismatch with Si, such as CoSi 2 and NiSi 2 . However, oriented layers of hexagonal disilicides, such as CrSi 2 and YSi 2 , that are difficult to grow by conventional techniques have also been fabricated in (111) Si. Recent results on oriented growth of a silicide with a completely different lattice structure, CaSi 2 , will also be presented.

Comparison of conductivity produced in polymers and carbon films by pyrolysis and high energy ion irradiation

We compare the effects of pyrolysis and 2 MeV Ar+ ion irradiation in modifying the conductivity of polymeric and carbon films. Chemical degradation (in the polymer films) and structural rearrangement (in both polymer and carbon films) are introduced by either process. Metallic carrier densities (1022 − 1023cm−3) have been observed in these films by Hall measurements. Transmission electron microscope (TEM) images show the formation of graphitic ordering with correlation lengths ∼ 20 A in the highly conductive films, with planes oriented parallel to the film surface. Crystalline graphite has a small overlap of the conduction and valence bands (≲ 30 meV), and consequently a low carrier density (∼ 1018 cm−3). We believe the modified carbon films to have larger overlap of the bands (∼1 eV) at the Fermi level, due to significant smearing of the carrier energy associated with the short scattering (correlation) lengths. While pyrolysis and ion irradiation produce similar end results, ion irradiation can yield ultimately higher carrier densities in these films as the ion-annealing process is self-limiting in terms of the extent of crystalline order produced in the film.

Implantation, damage, and regrowth of high Tc superconductors

Abstract We have observed superconductivity in thin films of (La1−xSrx)2CuOy which are fabricated by implanting evaporated La/Cu multilayer films with Sr and annealing in oxygen. The films are insulating and partially transparent as-implanted, but they darken and become conducting at annealing temperatures as low as 500°C. The resistive behavior is very sensitive to the annealing conditions and the superconducting layer appears to be buried beneath the surface. Similarly, implantation of F, O, and Ne into single crystals and high quality (χmin

Quantum transport in narrow MOSFET channels

Abstract Narrow MOSFET channels are investigated as quasi-one-dimensional electron systems containing a small number of electrons. New multiterminal devices allow resistance measurements of adjacent channel segments as small as 30 nm wide and 100 nm long. Even at high electron densities, the quantum transmission (conductance) through small segments depends strongly on electron energy (gate voltage) and magnetic field, and the behavior of adjacent segments is uncorrelated. This conductance structure saturates at low temperatures as inelastic scattering becomes negligible, and the strongest transmission “resonances” remain temperature independent as high as 20 K. The “universal conductance fluctuations” recently calculated by Lee and Stone may provide a framework for understanding these results. In larger devices, the change of scattering due to the trapping of a single electron at a particular interface trap can be isolated, and the surrounding perturbation can be spatially resolved.

Planar tunnel junctions on 90 K and 60 K YBCO single crystals: Superconducting and normal state properties

Abstract We have studied the tunneling characteristics of planar junctions made on YBCO single crystals with T c =90 K and T c =60 K . Natural barriers of good quality have been obtained on both compounds. In comparison with the 90 K phase, the superconducting conductance curves of the 60 K phase show gap-like structures reduced in amplitude and shifted towards higher energies. For the higher T c material, a gap opening, at about 90 K, is observed. Normal state conductances, measured at T > T c , are highly linear in voltage on both compounds and have steeper slopes on the 60 K phase. The slopes are quite insensitive to temperature variations, and do not depend on the junction resistances nor on the materials used as counterelectrodes.