Nathan Newman

Synthesis, characterization, and modeling of high quality ferromagnetic Cr-doped AlN thin films

We report a theoretical and experimental investigation of Cr-doped AlN. Density functional calculations predict that the isolated Cr t2 defect level in AlN is 1/3 full, falls approximately at midgap, and broadens into an impurity band for concentrations over 5%. Substitutional Al1−xCrxN random alloys with 0.05⩽x⩽0.15 are predicted to have Curie temperatures over 600 K. Experimentally, we have characterized and optimized the molecular beam epitaxy thin film growth process, and observed room temperature ferromagnetism with a coercive field, Hc, of 120 Oe. The measured magnetic susceptibility indicates that over 33% of the Cr is magnetically active at room temperature and 40% at low temperature.

Spin lifetimes of electrons injected into GaAs and GaN

The spin relaxation times of electrons in GaAs and GaN are determined with a model that includes momentum scattering by phonons and ionized impurities, and spin scattering by the Elliot–Yafet, D’yakonov–Perel, and Bir–Aronov–Pikus mechanisms. Accurate bands generated using a long-range tight-binding Hamiltonian obtained from empirical pseudopotentials are used. The inferred temperature dependence of the spin relaxation lifetime agrees well with measured values in GaAs. We further show that the spin lifetimes decrease rapidly with injected electron energy and reach a local maximum at the longitudinal optical phonon energy. Our calculation predicts that electron spin lifetime in pure GaN is about three orders of magnitude longer than in GaAs at all temperatures, primarily as a result of the lower spin-orbit interaction and higher conduction band density of states.

Thermochemistry of MgB/sub 2/ thin film synthesis

We have investigated the thermodynamic and kinetic barriers involved in the synthesis of MgB/sub 2/ films. This work refines our initial conjectures predicting optimal MgB/sub 2/ thin film growth conditions as a consequence of the unusually large kinetic barrier to MgB/sub 2/ decomposition. The small Mg sticking coefficient at temperatures greater than 300/spl deg/C prevents high temperature synthesis with traditional vacuum growth methods. However, as a result of the large kinetic barrier to MgB/sub 2/ decomposition, in-situ thermal processing can be used to enhance the crystallinity and the superconductivity of MgB/sub 2/ films. We used these methods to produce MgB/sub 2/ thin films with relatively high transition temperatures (/spl sim/37 K) by pulsed laser deposition (PLD).

Switching at small magnetic fields in Josephson junctions fabricated with ferromagnetic barrier layers

Nb-based Josephson junctions have been fabricated, which can select one of two states depending on the relative magnetization of their ferromagnetic barrier layers. To minimize the free-layer switching energy, while maintaining adequate thermal stability at 4.2 K, a dilute Cu-permalloy alloy [Cu0.7(Ni80Fe20)0.3] with a low magnetic saturation (Ms ∼ 80 emu/cm3) is used. The optimal thickness of the permalloy (Ni80Fe20) fixed-layer is shown to be 2.4 nm. Such devices exhibit switching at magnetic fields as low as 5 Oe, demonstrating their potential use in low power non-volatile memory for superconductor digital circuits.

Internally shunted Josephson junctions with barriers tuned near the metal–insulator transition for RSFQ logic applications

The fabrication of self-shunted SNS (superconductor/normal conductor/superconductor) Josephson junctions for rapid single flux quantum (RSFQ) logic could potentially facilitate increased circuit density, as well as reduced parasitic capacitance and inductance over the currently used externally shunted SIS (superconductor/insulator/superconductor) trilayer junction process. We report the deposition, fabrication, and device characterization of Josephson junctions prepared with Nb1?yTiyN electrodes and TaxN barriers tuned near the metal?insulator transition, deposited on practical large-area oxide-buffered silicon wafers. When scaled to practical device dimensions, this type of junction is found to have an IcRn product of over 0.5?mV and a critical current (Ic) and normal resistance (Rn) of magnitudes suitable for single flux quantum digital circuits. A longer than expected normal-metal coherence length (?n) of 5.8?nm is inferred from the thickness dependence of Jc at 4.2?K for junctions fabricated using a barrier resistivity of 13?m??cm. Although not well understood and not quantitatively predicted by conventional theories, this results in a sufficiently high Ic and IcRn to make the junctions suitable for practical applications. Similar observations of unexpectedly large Josephson coupling currents in SNS junctions have been documented in other systems, particularly in cases when the barrier is near the M?I transition, and have become known as the giant proximity effect. The temperature dependence of ?n, IcRn, and Jc are also reported. For this technology to be used in practical applications, significant improvements in our fabrication process are needed as we observe large variations in Ic and Rn values across a 100?mm wafer, presumably as a result of variations in the Ta:N stoichiometry and the resulting changes in the TaxN barrier resistivity.

Engineering issues in high-frequency RSFQ circuits

Abstract This paper reports progress on several projects that contribute to advancing the state of the art of rapid single flux quantum (RSFQ) logic. The first project is aimed to demonstrate, with true digital testing, the performance of RSFQ circuits of significant size and importance at a frequency that challenges the best semiconductor circuits, with only a miniscule fraction of their power dissipation. The second is a demonstration of an internally shunted SNS junction that has a high I c R n product and is intended as a drop-in replacement for the now-common resistively shunted tunnel junction; the advantage of this device is reduction of size, minimization of parasitic inductances, as well as high I c R n product for higher frequency operation. In the third project, we are trying to break the memory bottleneck that has long plagued superconductor digital electronics by using a hybrid of Josephson and CMOS technologies.

The dominance of paramagnetic loss in microwave dielectric ceramics at cryogenic temperatures

Commercial manufacturers add transition metals and other impurities to high performance microwave ceramic compounds to improve their manufacturability. Measurements of microwave loss tangent were performed on Ba(Zn1/3Ta2/3)O3, ZrTiO4-ZnNb2O6, Ba(Zn1/3Nb2/3)O3, and BaTi4O9-BaZn2Ti4O11 ceramics. A marked increase in the loss at low temperatures is found in materials containing transition metal with unpaired d-electrons as a result of resonant spin excitations in isolated atoms (light doping) or exchange coupled clusters (moderate to high doping). The loss tangent can be drastically reduced by applying static magnetic fields. Our measurements also show that this mechanism significantly contributes to room temperature loss, but does not dominate.

Effect of stoichiometry on oxygen incorporation in MgB2 thin films

The amount of oxygen incorporated into MgB2 thin films upon exposure to atmospheric gasses is found to depend strongly on the materials stoichiometry. Rutherford backscattering spectrometry was used to monitor changes in oxygen incorporation resulting from exposure to: (a) ambient atmosphere, (b) humid atmospheres, (c) anneals in air and (d) anneals in oxygen. The study investigated thin-film samples with compositions that were systematically varied from Mg0.9B2 to Mg1.1B2. A significant surface oxygen contamination was observed in all of these films. The oxygen content in the bulk of the film, on the other hand, increased significantly only in Mg-rich films and in films exposed to humid atmospheres.

Nanoscale disorder in pure and doped MgB2 thin films

MgB2 thin films have superior superconducting properties compared to bulk MgB2 and demonstrate the potential for further improving the performances of MgB2 wires and tapes. Using transmission electron microscopy, we have characterized the microstructure of pure and C-doped MgB2 using various carbon sources grown by hybrid physical?chemical vapor deposition (HPCVD), and cold-grown?annealed film deposited by molecular beam epitaxy (MBE). The MgB2 HPCVD films increase in crystal quality in the order (MeCp)2Mg-sourced films, CH4-sourced films, B(CH3)3-sourced films, pure films, while the Hc2 values of these films follow the opposite order. The cold-grown?annealed MgB2 MBE film contains non-epitaxial ? 10?nm MgB2 grains and MgO nanoparticles. The microstructural origins of electron scattering and flux pinning in both films are discussed. We also show the structure and chemistry of the degraded phase in HPCVD films and its effects on superconducting properties.

Nanoporous Delafossite CuAlO2 from Inorganic/Polymer Double Gels: A Desirable High-Surface-Area p-Type Transparent Electrode Material

Nanoporous structures of a p-type semiconductor, delafossite CuAlO(2), with a high crystallinity have been fabricated through an inorganic/polymer double-gel process and characterized for the first time via Mott-Schottky measurements. The effect of the precursor concentration, calcination temperature, and atmosphere were examined to achieve high crystallinity and photoelectrochemical properties while maximizing the porosity. The optical properties of the nanoporous CuAlO(2) are in good agreement with the literature with an optical band gap of 3.9 eV, and the observed high electrical conductivity and hole concentrations conform to highly crystalline and well-sintered nanoparticles observed in the product. The Mott-Schottky plot from the electrochemical impedance spectroscopy studies indicates a flat-band potential of 0.49 V versus Ag/AgCl. It is concluded that CuAlO(2) exhibits band energies very close to those of NiO but with electrical properties very desirable in the fabrication of photoelectrochemical devices including dye-sensitized solar cells.