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We studied the effect of light ion and heavy ion irradiations on pnp Si BJTs. A mismatch in DLTS deep peak amplitude for devices with same final gain but irradiated with different ion species was observed. Also, different ions cause different gain degradation when the DLTS spectra are matched. Pre-dosed ion-irradiated samples show that ion induced ionization does not account for the differences in DLTS peak height but isochronal annealing studies suggest that light ions produce more VP defects than heavy ions to compensate for the lack of clusters that heavy ions produce. The creation of defect clusters by heavy ions is evident by the higher content of E4 and ${\mathrm {V}}_{2}^{\ast}$ defects compared to light ions.

The purpose of this work is to demonstrate equivalent bipolar transistor damage response to neutrons and silicon ions. We report on irradiation tests performed at the White Sands Missile Range Fast Burst Reactor, the Sandia National Laboratories (SNL) Annular Core Research Reactor, the SNL SPHINX accelerator, and the SNL Ion Beam Laboratory using commercial silicon npn bipolar junction transistors (BJTs) and III–V Npn heterojunction bipolar transistors (HBTs). Late time and early time gain metrics as well as defect spectra measurements are reported.

Electrical performance and defect characterization of vertical GaN P-i-N diodes before and after irradiation with 2.5 MeV protons and neutrons is investigated. Devices exhibit increase in specific on-resistance following irradiation with protons and neutrons, indicating displacement damage introduces defects into the p-GaN and n- drift regions of the device that impact on-state device performance. The breakdown voltage of these devices, initially above 1700 V, is observed to decrease only slightly for particle fluence

We present a series of metrics for comparison between displacement damage due to heavy ion and neutron irradiation in silicon bipolar junction transistors. We have compared ion and fast neutron irradiations to determine an ion-to-neutron damage equivalence. We find that a combination of metrics (damage factor, deep level transient spectroscopy (DLTS) and annealing factor) are needed to ensure a comprehensive understanding of the physics involved in the ion-to-neutron conversion. The linearity of the damage factor (primarily probing the base-emitter junction) is not enough to ensure a valid comparison; rather, we must also use additional techniques (DLTS and capacitance measurements) to ensure that collector compensation is not occurring. As a result, care must be taken in choosing the irradiation beam for ion exposures. The displacement damage should peak in the sensitive region of the device to both ensure maximum gain degradation and to minimize collector compensation.

Following irradiation, the gain of silicon bipolar transistors can be improved by annealing at 350 K. We show that both the number of defects measured by deep level transient spectroscopy (DLTS) and the gain can be restored to the post irradiation state by injection of minority carriers. One can cycle between the post irradiation state and the 350 K annealed state by alternating minority carrier injection at 300 K with zero- or reverse-bias anneals at 350 K. The structure of the bistable defects is not known, but we observe that they affect capture kinetics into the shallow charge state of the silicon divacancy defect, V 2 (=/−). This suggests that the bistable defects are located within the neutron or ion damage cluster.

Ta2O5 is a candidate for use in metal–oxide–metal (MOM) capacitors in several areas of silicon device technology. Understanding and controlling leakage current is critical for successful implementation of this material. We have studied thermal and photoconductive charge transport processes in Ta2O5 MOM capacitors fabricated by anodization, reactive sputtering, and chemical vapor deposition. We find that the results from each of these three methods are similar if one compares films that have the same thickness and electrodes. Two types of leakage current are identified: (a) a transient current that charges the bulk states of the films and (b) a steady state activated process involving electron transport via a defect band. The transient process involves either tunneling conductivity into states near the Fermi energy or ion motion. The steady state process, seen most commonly in films

Pb(Zr0.53Ti0.47)O3 (PZT) on diamond is a potentially robust structure for surface acoustic wave (SAW) device applications. We have studied the growth and physical characteristics of PZT on diamond and other substrates by pulsed laser deposition. Under a broad range of processing conditions we explored, PZT deposited directly on diamond is almost exclusively pyrochlore-type, which is nonferroelectric. Growth of ferroelectric perovskite PZT is promoted via the use of a PbTiO3 buffer layer within a narrow window of processing parameters [i.e., P(O2)=100–200 mTorr, T=550–650 °C, 1–2 J/cm2]. Similar results were also obtained for deposition of PZT on Si, Pt-coated Si, and Pt-coated diamond substrates. The dielectric constants of the perovskite PZT films are 500–650 at 1 V and 100 kHz. The piezoelectric coefficients of these films are in the range of 50×10−12–350×10−12 m/V. The SAW velocity of perovskite PZT films is similar to that of highly oriented sputter deposited ZnO films. The acoustic attenuation in per...

Parallel synthesis together with high-throughput screening was used to identify candidate materials for integrated circuit applications that demand a superior high permittivity dielectric thin film. Specifically, we developed a continuous-composition spread (CCS) technique to synthesize much of a pseudoternary oxide system in a single deposition and used this in conjunction with a high-throughput measurement protocol, thereby allowing each chemical system to be deposited and evaluated in about 24 h. This approach led to the identification of compositions in the Zr-Sn-Ti-O system with promising properties. The same technique was used to determine the optimum compositions as a function of processing parameters. Films with the composition Zr .2 Sn .2 Ti .6 O 2 were then prepared using a conventional synthetic technique (on-axis sputtering) and were verified to have excellent properties. Thus, the CCS technique has demonstrated utility in rapidly identifying and developing a useful new material.

α-hexathienyl (α–6T) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported.1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult.2-5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of α–6T (α–6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Hückel theory (EHT) band structure calculations show that α–6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Γ. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as κ−(ET)2 Cu(NCS)2.

We report the synthesis of a new strontium ruthenium oxycarbonate, Sr2RuO4 · 0.25 CO2, and its use to synthesize Sr3Ru2O7, the n = 2 member of the (SrO)n+1 (RuO2)n Ruddlesden-Popper series. Like layered K2NiF4-type Sr2RuO4, and three-dimensional perovskite-type SrRuO3, Sr3Ru2O7 is a metallic conductor. In Sr3Ru2O7, the magnetic moments (d4, low spin) interact weakly with each other and develop strong antiferromagnetic correlations below 15 K. Small substitutions of Ir for Ru in Sr3Ru2O7 result in a metal-insulator transition.

C 60 films up to 1000 A in thickness have been grown by room-temperature sublimation onto Si(100) and Si(111) substrates that have been prepared with surfaces passivated against dangling bonds. X-ray diffraction measurements reveal that films deposited on such substrates have a high degree of (111)-textured crystallinity which is absent in films deposited on untreated Si. Examination of surface topographies with tapping mode atomic force microscopy shows a thickness-dependent increase in roughness which is greater for the treated samples. Our results are placed in context with respect to what is presently known about the growth of C 60 films on a variety of substrates. Implications for possible device applications will also be discussed.

We report superconductivity in Cs3C60 at 40K using ac susceptibility measurements under hydrostatic conditions up to 15 kbar. Cs3C60 was prepared by reaction of C60 with Cs in liquid ammonia, followed by heating at 150°C. This route circumvents formation of the energetically more stable Cs1C60 and Cs4C60 phases. We have studied the synthesis and phase formation by measuring the symmetric Ag pentagonal pinch mode of C60 using Raman spectroscopy. Whereas Raman spectroscopy indicates homogeneous charge transfer of three electrons, x-ray diffraction indicates two phases other than the commonly observed fcc structure. This superconducting transition temperature is considerably higher than for known doped C60 compounds and intermetallic compounds.

Thin-film multilayer structures with up to 20 repeat layers have been grown in a high-vacuum chamber by sequential deposition of aluminum (Al) and fullerene (${\mathrm{C}}_{60}$) onto room-temperature substrates. The periodicity of the layers is confirmed by x-ray-diffraction and in situ resistance measurements. The presence of underlying layers of ${\mathrm{C}}_{60}$ reduces the critical thickness at which Al becomes conducting from \ensuremath{\sim}35 to \ensuremath{\sim}20 \AA{}. In addition, there is a sudden increase in resistance that occurs when each Al layer is covered by a monolayer of ${\mathrm{C}}_{60}$. These observations, together with the measurement of a downward shift in frequency of a considerably broadened Raman-active ${\mathit{A}}_{\mathit{g}}$(2) pentagonal-pinch mode, imply that up to six electrons per ${\mathrm{C}}_{60}$ are transferred from the Al to the ${\mathrm{C}}_{60}$ layer. This demonstration of charge transfer across planar metal-${\mathrm{C}}_{60}$ interfaces suggests that multilayers may be a useful vehicle for forming fullerene interface compounds in two-dimensional structures.

We report synthesis, structures and properties of strontium intercalated fullerite, SrxC60 compounds. This is identified as the only fulleride system where fcc and bcc phases compete in the same compositional range. In the vicinity of x=3, a bcc A15 phase coexists with a fcc phase with lattice constants 11.140 and 14.144 A, respectively. For Sr6C60, an Im 3 bcc phase with ao=10.975 A is observed. This phase becomes superconducting at T=4 K. These structural and electronic properties, being intermediate to grossly different calcium and barium intercalated fullerides, provide a unifying picture of all alkaline-earth intercalated fullerides.

The quaternary intermetallic compounds LnPt 2 B 2 C (Ln=La, Pr and Y) were recently shown to be superconducting at temperatures up to 10.5 K. Annealing conditions were not found, however, which yielded single-phase material. Here it is reported that the partial substitution of Au for Pt in LnPt 2 B 2 C yields essentially single-phase polycrystalline material under annealing conditions readily accessible by conventional techniques. This is accomplished through arcmelting LnPt 1.5 Au 0.6 B 2 C, a composition slightly superstoichiometric in transition metal, and annealing at 1100°C in Ta foil: a small amount of melt phase formed in the anneal is wicked away from the sample leaving good-quality material behind.

Previous high-resolution electron microscopy (HREM) observations of the carbon nanotubes have led to a "Russian doll" structural model that is based on hollow concentric cylinders capped at both ends. The structures of the carbon nanotubes and particles were characterized here by bulk physical and chemical property measurements. The individual nanostructure is as compressible as graphite in the c axis, and such nanostructures can be intercalated with potassium and rubidium, leading to a saturation composition of "MC 8 ." These results are counter to expectations that are based on a Russian doll structure. HREM after intercalation with potassium and deintercalation indicates that individual nanoparticles are a "paper-mache" of smaller graphite layers. Direct current magnetization and electron spin resonance measurements indicate that the electronic properties of the nanostructures are distinctly different from those of graphite. Although the nanostructures have distinct morphologies and electronic properties, they are highly defective and have a local structure similar to turbostratic graphite.

The irradiation of sublimed fullerene (C60 and C70) thin films with ultraviolet light in an oxygen-rich ambient has been found to lead to a substantially increased cohesive energy in the fullerene solid. The decreased solubility and lower vapor pressure of the phototransformed material enables wet (organic solvents) or dry (thermal or photon-induced sublimation) development of photo-defined negative images. One micrometer wide lines with good edge definition are demonstrated. X-ray, infrared, optical absorption, and high performance liquid chromatography reveal that photo-oxygenated C60 retains its fcc crystal structure but with a substantial fraction of the C60 molecules modified with carbonyl (C=O) bonds.

The dielectric loss in amorphous, thin-film oxide insulators produces a real part of the ac conductivity σ′(ω) that scales as ωs with s∼1. Conventional models explain this frequency dependence by hopping or tunneling of charge between neighboring defect sites. These models fail at low temperatures since they predict that σ′ should vanish at T=0. We observe that the ac conductivity of Ta2O5, ZnO, and SiO2 has a nonzero extrapolated value at T=0. We propose that this behavior is consistent with the predictions of a Coulomb glass, an insulator with a random distribution of charged defects.The dielectric loss in amorphous, thin-film oxide insulators produces a real part of the ac conductivity σ′(ω) that scales as ωs with s∼1. Conventional models explain this frequency dependence by hopping or tunneling of charge between neighboring defect sites. These models fail at low temperatures since they predict that σ′ should vanish at T=0. We observe that the ac conductivity of Ta2O5, ZnO, and SiO2 has a nonzero extrapolated value at T=0. We propose that this behavior is consistent with the predictions of a Coulomb glass, an insulator with a random distribution of charged defects.

Alkali metal fullerides (A x C 60 ) are a subject of considerable current interest because of the occurrence of superconductivity for A x C 60 at temperatures surpassed only by the high T c copper oxides. The preparation and characterization of A x C 60 (A = alkali metal, x = 2,3,4,6) by powder X-ray diffraction, NMR ( 13 C, 23 Na and 87 Rb), and d.c. magnetization are reported. The structures are described as intercalation compounds of the FCC structure of pristine c 60 or of hypothetical BCC or BCT structures. The structures and phase diagrams can be rationalized on the basis of ion size and electrostatic considerations. Only the A 3 C 60 compounds are metallic (and superconducting). The superconducting T c increases nearly linearly with unit cell size. EHT (Extended Huckel Theory) calculations and 13 C NMR relaxation measurements indicate higher densities of states for the higher T c compositions.

INTERCALATING solid C60 with dopant atoms yields materials with a remarkable range of properties1. Alkali metal atoms, for example, readily form charge-transfer compounds2,3, AxC60 (where A is an alkali metal), which can be metallic, superconducting or insulating depending on the dopant concentration4–9. In all cases, the superconducting phase has a face-centred cubic (f.c.c.) structure and stoichiometry A3C60, which suggests a common mechanism for superconductivity dependent, at least in part, on the external coordination number of the C60 molecules. More recently, it has been shown10 that the alkaline earth metal calcium can also be intercalated with fulleride to form a superconducting phase, again with a f.c.c.-derived structure, near a Ca:C60 ratio of 5:1. Here we report the intercalation of fulleride with barium, in which a pure body-centred cubic phase with a lattice constant of 11.171 A is realized near a stoichiometry of Ba6C60. This phase is also superconducting (with a transition temperature of 7 K), suggesting that the mechanism of superconductivity is related to an intrinsic property of the C60 molecules, rather than the external coordination number.

Single-crystal epitaxial thin films of the isotropic metallic oxides Sr1–xCaxRuO3 (0 ≤ x ≤ 1) were grown on miscut SrTiO3(100) substrates in situ by 90° off-axis sputtering. These thin films exhibit low isotropic resistivities, excellent chemical and thermal stability, good surface smoothness, and high crystalline quality. Furthermore, the lattice parameters and magnetic properties can be varied by simply changing the strontium/calcium ratio. These epitaxial thin films, and their multilayer structures with other oxide materials, can be used for the fabrication of superconducting, ferroelectric, magneto-optic, and electro-optic devices.

Alkali metal fullerides (A x C 60 ) are a subject of considerable current interest because of the occurrence of superconductivity for A x C 60 at temperatures surpassed only by the high T c copper oxides. The preparation and characterization of A x C 60 (A = alkali metal, x = 2,3,4,6) by powder X-ray diffraction, NMR ( 13 C, 23 Na and 87 Rb), and d.c. magnetization are reported. The structures are described as intercalation compounds of the FCC structure of pristine c 60 or of hypothetical BCC or BCT structures. The structures and phase diagrams can be rationalized on the basis of ion size and electrostatic considerations. Only the A 3 C 60 compounds are metallic (and superconducting). The superconducting T c increases nearly linearly with unit cell size. EHT (Extended Huckel Theory) calculations and 13 C NMR relaxation measurements indicate higher densities of states for the higher T c compositions.

Bulk-sensitive, polarization-dependent oxygen K- and copper L-edge absorption spectra of single-domain ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ with ${\mathrm{CuO}}_{2}$ planes tilted off the sample surface were measured. A novel experimental configuration was utilized to achieve complete polarization geometry and eliminate optical path variations. Contrary to previous measurements, our data show for both intrinsic and doped holes a significant amount of O2${\mathit{p}}_{\mathit{z}}$ character, likely associated with apical O, but a very weak Cu3${\mathit{d}}_{3\mathit{z}}^{2}$-${\mathit{r}}^{2}$ character. The quantitative results impose strong contraints on superconductivity models invoking out-of-plane orbitals, and suggest that the apical O 2${\mathit{p}}_{\mathit{z}}$ orbital may play an important role in high-${\mathit{T}}_{\mathit{c}}$ phenomena.

IT has been demonstrated1–6 that solid C60 can be readily intercalated with group IA alkali metals to give metallic or insulating compounds, depending on the dopant concentration. The metal atoms diffuse into tetrahedral and octahedral interstitial sites of the C60 lattice with little disturbance to the face-centred cubic (f.c.c.) packing7. The A3C60f.c.c. phases (A is K, Rb, Cs and mixtures of these) exhibit superconductivity with a transition temperature that increases with lattice constant8. At higher dopant concentration a body-centred tetragonal A4C60 phase9 and an insulating, body-centred cubic A4C60 phase10 are found. Here we report that the divalent group IIA intercalant calcium can be intercalated into the f.c.c. sites of C60 to form a solid solution, and that, near a Ca:C60 ratio of 5:1, a phase transformation occurs to a simple cubic phase. Measurements of microwave loss, magnetic susceptibility and Meissner effect show that the simple cubic phase becomes superconducting below 8.4 K.

The recent synthesis of macroscopic quantities of spherical molecular carbon compounds, commonly called fullerenes, has stimulated a wide variety of studies of the chemical and physical properties of this novel class of compounds. We discovered that the smallest of the known fullerenes, C60, could be made conducting and superconducting by reaction with alkali metals. In this paper, an overview of the motivation for these discoveries and some recent results are presented.

We characterize the molecular reorientational dynamics in two phases of solid ${\mathrm{C}}_{60}$ with $^{13}\mathrm{C}$ NMR measurements. A change in the nature of the dynamics, indicated by a change in kinetic parameters extracted from spin-lattice relaxation data, occurs at the phase transition at 260 K. Above the transition, the molecules appear to execute continuous rotational diffusion; below the transition, they appear to jump between symmetry-equivalent orientations. This interpretation is consistent with the x-ray-diffraction results of Heiney et al. as well as our NMR relaxation and spectral data.

Crystals of C60 or C70 cocrystallized with n-pentane grow as elongated, ten-sided columns. X-ray diffraction shows ordering of C60 or C70 molecules along the column and a remarkable tenfold symmetry normal to the column. The ratio of the x-ray-diffraction vectors of the two lowest-order diffraction spots is nearly equal to the “golden ratio,” τ. Despite these similarities with decagonal, quasicrystalline order, the diffraction indicates crystalline order with a twinned unit cell.

The preparation and solid-state characterization of the 1,3-phenylene-bridged bis(dithiadiazolyl) and bis(diselenadiazolyl) diradicals 1,3-[(E2N2C)C6H4(CN2E2)] (E = S, Se) are reported. The isomorphous crystals of 1,3-[(E2N2C)C6H4(CN2E2)] so obtained are tetragonal, space group I41/a. Stacks of diradical molecules, linked vertically in a zigzag fashion through alternaie ends by long E- - -E contacts (mean 3.140/3.284 Å, for E = S/Se), are arranged in pinwheellike clusters about the 41 and ¯4 axes, producing complex patterns of interstack E- - -E contacts. Both compounds show the presence of spin defects in the lattice, and there is a very large enhancement in the paramagnetism of the sulfur compound at high temperatures. The selenium compound is a semiconductor, with a room temperature conductivity of 2 × 10-4 S cm-1. Solid-state NMR experiments find enhanced relaxation times, which have their origin in the presence of a mobile paramagnetic defect. Extended Hückel band structure calculations show the materials to be semiconductors, with band gaps of about 1.0/0.8 eV for E = S/Se. Although the compounds adopt a columnar structure, the calculations indicate significant interactions between the stacks and the materials exhibit well-developed three dimensionality. The enhanced paramagnetism in the sulfur compound is attributed to the presence of thermally generated phase kinks in the lattice, whereas the selenium compound is classified as an intrinsic semiconductor.

Electrolysis of fused Li{sub 2}MoO{sub 4} at 850C resulted in the growth of large crystals of H{sub 0.23}Li{sub 1.68}Mo{sub 1.05}O{sub 3} at the anode. The composition of the crystals was determined by chemical analysis, thermogravimetry, and X-ray diffraction. Single crystal X-ray structure analysis was carried out and confirmed that the crystals have the trigonal {alpha}-NaFeO{sub 2} structure type previously reported for Li{sub 2}MoO{sub 3}, but with evidence for a displacive superstructure. Magnetic susceptibility measurements down to 4.2 K show weak Curie-Weiss paramagnetism with {mu}{sub eff} = 0.35 {mu}{sub B}. This contrasts with the almost total absence of paramagnetism in stoichiometric Li{sub 2}MoO{sub 3} prepared by hydrogen reduction of Li{sub 2}MoO{sub 4}.

An insulating polymorph with a composition close to Bi2Sr2CuO6 complicates the synthesis of the single copper layer superconductor Bi2+xSr2−yCuO6±δ. We concentrate on the region where the material is single phase in air synthesis (2.18 < x < 2.29, 1.44 < y < 1.7). The physical properties from insulating to marginally conducting as the composition moves toward x = y = 0. Like to two-layer compound, the material has a superlattice due to a buckling of the planar structure, however the superlattice coherence is degraded by defects at finite values of x and y. Density measurements indicate that the nonstoichiometry of Bi2+xSr2−y CuO6±δ is not accompanied by the formation of a large number of vacancies. The insulating polymorph, Bi2Sr2CuO6, can be suppressed by synthesis in modest oxygen pressures (above 5–10 atm). The result is a bulk superconductor (Tc ≈ 10 K).

Using x-ray photoelectron spectroscopy and Rutherford backscattering spectrometry, we have studied structures used in metal–oxide–metal capacitors including Ta2O5/TiN/Ti, Ta2O5/Ti, Ta2O5/TaN/Ti, Ta2O5/WN/Ti, and Ta2O5/M, where M=Ta, Pt, W, Al, and Si. We find that Ti and Al are able to reduce the Ta2O5 to Ta, forming oxides of Ti and Al, respectively. The diffusion barriers TiN, TaN, and WN hamper the diffusion of oxygen and therefore postpone the reduction of Ta2O5 to higher temperatures. As judged by the temperatures at which the reduction of Ta2O5 occurs, TaN and WN are more effective oxygen-diffusion barriers than TiN. We observe no oxygen remaining in the diffusion barrier when a Ti layer is present underneath. We observe no reduction of Ta2O5 when M=Pt, W, or Si.

The crystal structure of the La 1.6 Sr 0.4 CaCu 2 O 6± δ superconductor has been determined by neutron diffraction powder profile refinement at room temperature on a slightly oxygen deficient sample (δ=-0.06, T c =55 K). The crystal structure is very similar to that of the nonsuperconducting host compound La 1.9 Ca 1.1 Cu 2 O 6−δ . Sr substitutes on the nine coordinate La sites, and there may be some La/Ca mixing on the eight coordinate metal site between the CuO 2 planes. Oxygen deficiency occurs in the apical oxygen sites. The compound has the tetragonal symmetry characteristic of a square CuO 2 plane at room temperature and 10 K, making it differ from the single-layer analog (La, Sr) 2 CuO 4 which has a buckled, distorted CuO 2 plane in the superconducting state.

The charge-density-wave (CDW) phase transition has been studied for a range of depth of 20\char21{}10000 \AA{} below the (201\ifmmode\bar\else\textasciimacron\fi{}) surface of ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$ by use of grazing-incidence x-ray diffraction. It is found that the CDW wave vector is the same as in the bulk. The transition temperature as well as the temperature dependence of the order parameter (down to 125 K) appear to be independent of the depth probed by the x-ray beam. The fact that the surface does not appear to modify the characteristics of the CDW in this particular case is probably related to the weak coupling between the slabs of ${\mathrm{MoO}}_{6}$ octahedra, perpendicular to the (201\ifmmode\bar\else\textasciimacron\fi{}) surface.

Mesures de transport, de magnetoconductivite, et d'effet tunnel dans ces cristaux qui presentent une localisation bidimensionnelle dans les plans CuO 2 . L'affaiblissement du desordre dans les plans est lui-meme insuffisant pour la supraconductivite. Observation d'une magnetoconductivite negative et d'un appariement de type supraconducteur lorsqu'un phenomene de transport perpendiculaire aux plans indique la presence d'etats etendus tridimensionnels

Transport anisotropies of ${\ensuremath{\rho}}_{c}$.${\ensuremath{\rho}}_{\mathrm{ab}}$\ensuremath{\simeq}${10}^{4}$ to ${10}^{5}$ were measured for superconducting and nonsuperconducting ${\mathrm{Bi}}_{2+\mathrm{x}}$${\mathrm{Sr}}_{2\mathrm{\ensuremath{-}}\mathrm{y}}$${\mathrm{CuO}}_{6\ifmmode\pm\else\textpm\fi{}\mathrm{\ensuremath{\delta}}}$ crystals. In superconducting samples ${\ensuremath{\rho}}_{\mathrm{ab}}$ increases linearly with temperature from just above ${T}_{c}$\ensuremath{\simeq}7 to 700 K. The implication of the ${\ensuremath{\rho}}_{\mathrm{ab}}$ results is that classical electron-phonon scattering mechanisms are inadequate. The anisotropy and ${T}_{c}$ for various layered superconductor systems are compared. In all crystals studied ${\ensuremath{\rho}}_{c}$ is nonmetallic, varying as a power law ${T}^{\mathrm{\ensuremath{-}}\ensuremath{\alpha}}$, \ensuremath{\alpha}\ensuremath{\simeq}0.5--1.

An improved method for measuring the cross sections for carrier trapping at defects in semiconductors is described. This method, a variation of deep level transient spectroscopy (DLTS) used with bipolar transistors, is applied to hot carrier trapping at vacancy-oxygen, carbon-oxygen, and three charge states of divacancy centers (V2) in n- and p-type silicon. Unlike standard DLTS, we fill traps by injecting carriers into the depletion region of a bipolar transistor diode using a pulse of forward bias current applied to the adjacent diode. We show that this technique is capable of accurately measuring a wide range of capture cross sections at varying electric fields due to the control of the carrier density it provides. Because this technique can be applied to a variety of carrier energy distributions, it should be valuable in modeling the effect of radiation-induced generation-recombination currents in bipolar devices.

A low temperature oxygen/nitrogen plasma process is reported that substantially reduces leakage currents in chemical vapor deposited (CVD) and physical vapor deposited (PVD) films of tantalum oxide. We show that a combination of nitrogen and oxygen in a remote downstream microwave plasma source reduces leakage currents in CVD films of tantalum oxide and also reduces trap densities as measured by charge pumping. The as deposited CVD films show a high level of photoluminescence that is substantially lowered by the plasma anneal due to a reduction in the density of midgap states. For films deposited by PVD in the thickness range of 100 nm we find low leakage currents with a substantial improvement from the introduction of nitrogen into the plasma. However, PVD films in the thickness range of 20 nm show larger relative leakage currents and less of an improvement from the addition of nitrogen. The role of nitrogen in lowering leakage currents and charge trapping is thought to occur from a reduction in the density of bulk trap states in the oxide due to partial incorporation of nitrogen in the oxide. Both of these low temperature deposition and annealing processes are compatible with integration into the upper levels of metallization for high density circuits.

N‐channel field effect transistors with excellent device characteristics have been fabricated by utilizing C60 as the active element. Measurements on C60 thin films in ultrahigh vacuum show on‐off ratios as high as 106 and field effect mobilities up to 0.08 cm2/V s.

New classes of high dielectric constant materials based on binary and ternary amorphous metal oxides will be discussed. These films are compatible with low temperature processing within the metal layers of a circuit and can therefore be used with conventional metal electrodes like TiN. Different classes of mixed metal oxides will be discussed that are based on alloys of tantalum oxide and titanium oxide.

The fullerene C(60) can be converted into two different structures by high pressure and temperature. They are metastable and revert to pristine C(60) on reheating to 300 degrees C at ambient pressure. For synthesis temperatures between 300 degrees and 400 degrees C and pressures of 5 gigapascals, a nominal face-centered-cubic structure is produced with a lattice parameter a(o) = 13.6 angstroms. When treated at 500 degrees to 800 degrees C at the same pressure, C(60) transforms into a rhombohedral structure with hexagonal lattice parameters of a(o) = 9.22 angstroms and c(o) = 24.6 angstroms. The intermolecular distance is small enough that a chemical bond can form, in accord with the reduced solubility of the pressure-induced phases. Infrared, Raman, and nuclear magnetic resonance studies show a drastic reduction of icosahedral symmetry, as might occur if the C(60) molecules are linked.

Colossal magnetoresistance with more than a thousandfold change in resistivity (ΔR/RH=127 000% at 77 K, H=6 T) has been obtained in epitaxially grown La‐Ca‐Mn‐O thin films. This magnetoresistance value is about three orders of magnitude higher than is typically seen in the giant‐magnetoresistance‐type metallic, superlattice films. The temperature of peak magnetoresistance is located in the region of metallic resistivity behavior. As the magnetoresistance peak occurs not at the temperature of magnetic transition but at a temperature where the magnetization is still substantial, the spin‐disorder scattering is not likely to be the main mechanism in these highly magnetoresistive films. The peak can be shifted to near room temperature by adjusting processing parameters. Near‐room‐temperature ΔR/RH values of ∼1300% at 260 K and ∼400% at 280 K have been observed. The presence of grain boundaries appears to be very detrimental to achieving large magnetoresistance in the lanthanum manganite compounds. The fact th...

Epitaxial ferroelectric SrRuO3/Pb(Zr0.52Ti0.48)O3/SrRuO3 heterostructures have been fabricated employing isotropic metallic oxide electrodes on (100) SrTiO3 and (100) Si with an yttria stabilized zirconia buffer layer. The structures have been grown in situ by 90° off‐axis sputtering, which allows the growth of uniform stoichiometric films over large areas with excellent step coverage. X‐ray diffraction, Rutherford backscattering spectroscopy, and cross‐sectional transmission electron microscopy reveal high crystalline quality and coherent interfaces. They exhibit superior fatigue characteristics over those made with metal electrodes, showing little degradation over 1010 cycles, with a large remnant polarization.

A WIDE variety of alkali-metal fullerides (AxC60) have been prepared, with stoichiometries varying between 1⩽=x⩽=11 (refs 1,2). Most of the compounds with x = 3 are superconductors with unusually high transition temperatures (Tc) which increase with the size of the face-centred cubic unit cell3. The increase in Tc on lattice expansion is attributed to a decrease in the conduction band width, and therefore one ultimately expects a saturation or downturn of Tc followed by a metal–insulator transition. We have sought to explore this regime by expanding the A3C60 structure through intercalation of neutral molecules capable of solvating the A+ ions. Here we report that the reaction of ammonia with Na2CsC60 (Tc = 10.5K; lattice constant α = 14.132 A) produces the compound (NH3)4Na2CsC60, which has an expanded unit cell (a = 14.473 A) and an increased Tc of 29.6 K. The structure contains the Na(NH3)+4 cation on the octahedral site.

We have measured the temperature and field dependence of emission rates from five traps in electron damaged GaAs. Four of the traps have previously been identified as radiation defects. One of the traps, seen in higher doped diodes, has not been previously identified. We have fit the data to a multiphonon emission theory that allows recombination in GaAs to be characterized over a broad range of temperature and electric field. These results demonstrate an efficient method to calculate field-dependent emission rates in GaAs.

INTERCALATING solid C60 with dopant atoms yields materials with a remarkable range of properties1. Alkali metal atoms, for example, readily form charge-transfer compounds2,3, AxC60 (where A is an alkali metal), which can be metallic, superconducting or insulating depending on the dopant concentration4–9. In all cases, the superconducting phase has a face-centred cubic (f.c.c.) structure and stoichiometry A3C60, which suggests a common mechanism for superconductivity dependent, at least in part, on the external coordination number of the C60 molecules. More recently, it has been shown10 that the alkaline earth metal calcium can also be intercalated with fulleride to form a superconducting phase, again with a f.c.c.-derived structure, near a Ca:C60 ratio of 5:1. Here we report the intercalation of fulleride with barium, in which a pure body-centred cubic phase with a lattice constant of 11.171 A is realized near a stoichiometry of Ba6C60. This phase is also superconducting (with a transition temperature of 7 K), suggesting that the mechanism of superconductivity is related to an intrinsic property of the C60 molecules, rather than the external coordination number.

The two fold degeneracy inherent to epitaxial growth of high Tc films on (101) perovskite substrates has been removed successfully. This is demonstrated by single domain, (103) oriented La2−xSrxCuO4 films of 0.04≤x≤0.34 on vicinal (101) SrTiO3 substrates using 90° off‐axis sputtering. (101) substrates that have the surface normal rotated about [010] by 0.5°–3.5° produced essentially single crystal films with the c‐axis direction determined by the sense of the miscut. Misoriented antidomains (103)’ have been eliminated effectively to a percentage less than 1 part in 104. The mechanism for symmetry breaking is understood on the basis of a surface step model in which the energetics promoting single domain growth is derived from nucleation and epitaxy on the (001) face found at surface steps of vicinal substrates. Furthermore, the incommensuration of the c axis interplanar spacings with the a, b, plane lattice parameters in the La2−xSrxCuO4 structure provides a natural selection of (103) domains over (103)’ d...