Elena Cimpoiasu

Electrical characterization of single GaN nanowires

In this paper a statistically significant study of 1096 individual GaN nanowire (NW) devices is presented. We have correlated the effects of changing growth parameters for hot-wall chemically-vapour-deposited (HW-CVD) NW sf abricated via the vapour–liquid–solid mechanism. We first describe an optical lithographic method for creating Ohmic contacts to NW field effect transistors with both top and bottom electrostatic gates to characterize carrier density and mobility. Multiprobe measurements show that carrier modulation occurs in the channel and is not a contact effect. We then show that NW fabrication runs with nominally identical growth parameters yield similar electrical results across sample populations of >50 devices. By systematically altering th eg rowth parameters we were able to decrease the average carrier concentration for these as-grown GaN NWs ∼10-fold, from 2.29 × 10 20 to 2.45 × 10 19 cm −3 ,a nd successfully elucidate the parameters that exert the strongest influence on wire quality. Furthermore, this study shows that nitrogen vacancies, and not oxygen impurities, are the dominant intrinsic dopant in HW-CVD GaN NWs.

Aluminum Oxide Layers as Possible Components for Layered Tunnel Barriers

We have studied transport properties of Nb/Al/AlOx/Nb tunnel junctions with ultrathin aluminum oxide layers formed by (i) thermal oxidation and (ii) plasma oxidation, before and after rapid thermal postannealing of the completed structures at temperatures up to 550 °C. Postannealing at temperatures above 300 °C results in a significant decrease of the tunneling conductance of thermally grown barriers, while plasma-grown barriers start to change only at annealing temperatures above 450 °C. Fitting the experimental I-V curves of the junctions using the results of the microscopic theory of direct tunneling shows that the annealing of thermally grown oxides at temperatures above 300 °C results in a substantial increase of their average tunnel barriers height, from ∼1.8 eV to ∼2.45 eV, versus the practically unchanged height of ∼2.0 eV for plasma-grown layers. This difference, together with high endurance of annealed barriers under electric stress (breakdown field above 10 MV/cm) may enable all-AlOx and SiO2/A...

The effect of Mg doping on GaN nanowires

We present a comparison between the structural, chemical, and electrical properties of Mg-doped GaN nanowires grown by hot-wall chemical vapour deposition using two different Mg sources, namely, metallo-organic bis(methylcyclopentadienyl) magnesium and magnesium nitride powder. We find that Mg from the solid nitride source is more effectively incorporated into the nanowires while better maintaining the nanowire integrity. After Mg activation, the nanowires are partially or fully compensated. In comparison, vapour phase doping results in an obvious degradation of the nanowire morphology in spite of lower Mg incorporation levels.

Plastic vortex creep above the second magnetization peak in Bi2Sr2CaCu2O8+δ single crystals

Abstract The analysis of the activation energy in the magnetization relaxation of Bi 2 Sr 2 CaCu 2 O 8+ δ (BSCCO) single crystals reveals plastic vortex creep above the second magnetization peak (SMP). Above the peak field, the magnetic field dependence of the activation energy U ( B ) is very close to the form U ( B )∝ B −1/2 and the intrinsic variation of U with the current density is weak. The existence of a crossover from elastic to plastic vortex creep across the SMP of high-temperature superconductors (HTSC) seems to be a general behavior, and may have important consequences on the nature of the thermally induced vortex solid–vortex fluid transition at high magnetic fields.

Tunneling properties of barriers in Nb/Al/AlO/sub x//Nb junctions

We have measured DC I-V curves of niobium-trilayer (Nb/Al/AlO/sub x//Nb) junctions with barriers thermally grown within a broad range of oxygen exposure E=Pt, from 2/spl times/10/sup 5/ to 2/spl times/10/sup 9/ Pa-s, and for applied electric fields ranging from zero all the way up to the breakdown - typically, above 10 MV/cm. The data can be reasonably well fitted by the direct theory assuming trapezoidal barrier profile and using the numerical solution of the Schrodinger equation. (The traditional WKB approximation gives considerable errors for barriers so thin and sharp.) The fitting has shown that with the increase of oxygen exposure, the effective oxide thickness d/sub ef//spl equiv/(m/m/sub 0/)/sup /spl alpha//d where m is the effective mass of the tunneling electron and (/spl alpha//spl ap/0.51) grows from 0.83 to 1.08 nm, while the average barrier height grows from 1.7 to 1.9 eV, and the zero-voltage conductance G/sub 0/ continues to drop as E/sup -1/2/ through all the studied exposure range.

The role of radiation damage structure and fine scale precipitation in the pinning improvement of thermal neutron irradiated lithium fluoride-doped YBa2Cu3O7−x

Abstract The improvement of intragranular critical current density, j c , and flux pinning obtained by thermal neutron irradiation (fluences between 10 16 −10 18 neutrons/cm 2 ) in YBa 2 Cu 3 O 7− x doped with 8 mol% lithium fluoride (LiF) (YBCO–LiF) is explained by the occurrence of various irradiation defects (point defect clusters, dislocation loops, stacking faults) observed by transmission electron microscopy (TEM). A twin fading process is also revealed accompanied by a lowering of the orthorhombicity index. At higher irradiation fluences (∼5×10 17 ÷10 18 neutrons/cm 2 ), a new mechanism related to the dispersion of fine Cu 2 O precipitates in the superconducting YBCO–LiF matrix could explain the enhancement of up to eight times obtained for j c values. A good agreement between the dependence of the measured j c values on the applied magnetic field and the calculated curves, j c = f ( B ), derived from a theoretical model, previously proposed for the pinning contribution of fine particles dispersed in YBCO matrix, is obtained. The role of thermal neutron irradiation for the flux pinning improvement by introducing of strong pinning centers (irradiation defects and Cu 2 O microdispersoids) is also discussed.

Electron mobility study of hot-wall CVD GaN and InN nanowires

The group III-nitrides and their alloys represent a promising system for semiconducting device applications, especially for photonic devices, because they are direct band gap semiconductors with potential light emission from ultraviolet to infrared. Growth of GaN and InN nanowires was reported by a number of groups employing mainly catalyzed vaporliquid-solid (VLS) or catalyst-free vapor-solid (VS) growths ([1], [2] and References therein). One simple, economical and very successful method to grow both types of nanowires is using hot-wall chemical vapor deposition. Understanding the electronic properties of the as-grown nanowires is a crucial step towards their implementation in useful devices. Here we summarize the electric properties of a large number of devices (field-effect transistors) that we built with nanowires grown using this simple process. Both indium nitride and gallium nitride nanowires exhibit high carrier concentrations, with mobilities limited by impurity scattering. In particular, the gallium nitride nanowires appear to grow heavily compensated, as inferred from our theoretical estimates and annealing experiments.

Relationship between Conductivity and Phase Coherence Length in Cuprates

The large (102–105) and strongly temperature dependent resistive anisotropy η=(σab/σc)1/2 of cuprates perhaps holds the key to understanding their normal state in-plane σab and out-of-plane σc conductivities. It can be shown that η is determined by the ratio of the phase coherence lenghts li in the respective directions: . In layered crystals in which the out-of-plane transport is incoherent, lc is fixed, equal to the interlayer spacing. As a result, the T-dependence of η is determined by that of lab, and vice versa, the in-plane phase coherence lenght can be obtained directly by measuring the resistive anisotropy. We present data for hole-doped YBa2Cu3Oy (6.3<y<6.9) and Y1-xPrxBa2O7-δ(0<x≤0.55) and show that σcb of crystals with different doping levels can be well described by a two parameter universal function of the in-plane phase coherence length. In the electron-doped Nd2-xCexCuO4-y, the dependence σab(η) indicates a crossover from incoherent transport in the c-direction.

Nonmonotonous Variation of the Superconducting Parameters of Neutron Irradiated Li-Doped YBa2Cu3O7−x

The effect of thermal and epithermal neutron irradiation on the superconducting critical temperature and critical current density of some Li-doped YBa2Cu3O7−x samples was studied. The critical temperature exhibits a peak and the critical current density a valley in their dependence on neutron fluence, for moderate dose. A simple model, based on the Van Hove scenario and the kinetics of the defect production, is used to describe both phenomena.

Charge Transport in Spin-Textured YBa2Cu3O6.25

The electric transport of the charged particles in a spin texture was investigated in a strongly underdoped YBa2Cu3O6.25 single crystal in order to identify the characteristic electrical transport mechanism. The in-plane resistivity revealed three different regimes of charge transport: a chiral 2D VRH regime up to 55 K with a characteristic temperature Td ≈ 12,400 K, an impurity band conduction regime above 55 K, and a metallic-like regime beyond 170 K. The out-of-plane resistivity has only one crossover at 115 K, but the conduction mechanisms controlling the two regimes are not clear.