C. A. Krontiras

High volume fraction carbon nanotube?epoxy composites

A versatile processing technique for fabricating epoxy nanocomposites with a high weight fraction of oxidized multi-walled carbon nanotubes is presented. Thin carbon nanotube based preforms were prepared through an oxidation-filtration protocol and then immersed in a pre-polymerized epoxy/curing agent solution in acetone. By adjusting the conditions for the oxidation of carbon nanotubes and the epoxy concentration in the as-prepared solution, high loading of graphitic nanostructures was obtained. Tensile tests indicated that the elastic modulus and strength of certain composites prepared by in situ polymerization as above were improved by 100% and 60%, respectively, compared to neat epoxy. In addition, the composite sheets showed comparable electrical conductivity values to the neat carbon nanotube paper. These results suggest that targeted chemical modification of the carbon nanotube surface is an effective way to enhance the electrical and mechanical properties of carbon nanotube-polymer composites.

Germanium substrate loss during low temperature annealing and its influence on ion-implanted phosphorous dose loss

In this work we demonstrate germanium loss from uncapped substrates during low temperature drive-in annealing in inert ambient. An Arrhenius law with an activation energy of 2.03 eV describes the measured loss rate of germanium as a function of temperature. Accurate simulations of implanted phosphorous profiles in nonpassivated substrates have been performed considering the extracted germanium loss rate. A capping layer on the germanium surface reduces phosphorous dose loss, with Si3N4 being more efficient than SiO2. The capping layer material also affects the extent of dopant concentration-dependent diffusion for high dose phosphorous implantation.

Some properties of ReSi2

Abstract Thin films of ReSi 2 were prepared by the co-evaporation of rhenium and silicon. The structure of the films is compared with the previously reported structure of this compound. The resistivity and Hall coefficient of the films were measured from 4.2 K to 523 K. Optical transmission measurements were also carried out. The films display semiconducting properties (p-type) characteristic of a small band gap, in agreement with measurements reported for bulk and single-crystal samples. The discrepancy between these results and recent band calculations, indicating a potentially high conductivity, is briefly discussed.

Transient and ac electrical transport under forward and reverse bias conditions in aluminum/porous silicon∕p-cSi structures

Dielectric impedance measurements as well as transient current–voltage (I–V) characteristics under conditions of forward and reverse bias are reported in aluminum/porous silicon (PS)∕p-cSi structures at different temperatures. Under reverse bias conditions, the electrical conduction of the structures can be modeled by a simple equivalent circuit of two parallel RC networks in series combination, representing a bulk and a junction region. The bulk conduction is ohmic. From the detailed analysis of the PS/cSi junction, the carrier concentration NA and the depletion layer width W are evaluated at each temperature. The elapsed time t0 from the onset of the square pulse, at which the transition from the dominant bulk resistance to junction conduction takes place, is a function of temperature. As the temperature decreases, conduction due to bulk resistance dominates over junction conduction. On the other, hand forward conduction is dominated from the bulk. Two conduction mechanisms are present. As the time proc...

The resistivity and Hall coefficient of CoGe and CoGe2 thin films

This is a report of measurements of the electrical resistivity and of the Hall coefficient of CoGe and CoGe2 films between 80 and 520 K. Both germanides behave as metallic conductors with resistivities increasing linearly with increasing T over the whole temperature range. There is a slight deviation from linearity for CoGe2 which is attributed to the fact that the mean free path of the carriers becomes comparable with the lattice dimensions at high temperatures. The Hall coefficient is negative for CoGe, giving an apparent carrier concentration of (0.86+or-0.01)*1028 m-3 and a Hall mobility of (11.0+or-0.1)*10-4 m2V-1s-1 at room temperature. On the other hand, p-type conduction is revealed for CoGe2 with apparent carrier concentration and Hall mobility equal to (1.93+or-0.01)*1028 m-3 and (2.2+or-0.1)*10-4 m2V-1s-1, respectively, at 300 K. For a quantitative explanation of the resistivity and Hall behaviour of germanides, detailed information about the energy band structure near the Fermi level is required. At this information is lacking, the rough approximation of the two-band model is the only way to obtain insight into the electrical properties of germanides.

Dielectric response and ac conductivity analysis of hafnium oxide nanopowder

The dielectric response of hafnium oxide nanopowder was studied in the frequency range of 10−2–106 MHz and in the temperature range of 20–180 °C. Broadband dielectric spectroscopy was applied and the experimental results were analyzed and discussed using the electric modulus (M*) and alternating current (ac) conductivity formalisms. The analyses of the dc conductivity and electric modulus data revealed the presence of mechanisms which are thermally activated, both with almost the same activation energy of 1.01 eV. A fitting procedure involving the superposition of the thermally activated dc conductivity, the universal dielectric responce and the near constant loss terms has been used to describe the frequency evolution of the real part of the specific electrical conductivity. The conductivity master curve was obtained, suggesting that the time–temperature superposition principle applies for the studied system, thus implying that the conductivity mechanisms are temperature independent.

Probing the properties of atomic layer deposited ZrO2 films on p-Germanium substrates

Zirconium oxide (ZrO2) thin films of 5 and 25 nm thickness were deposited by atomic layer deposition at 250 °C on p-type Ge substrates. The stoichiometry, thickness, and valence band electronic structure of the ZrO2 films were investigated by x-ray and ultraviolet photoelectron spectroscopies. For the electrical characterization, metal-oxide-semiconductor (MOS) capacitive structures (Pt/ZrO2/p-Ge) have been fabricated. Capacitance–voltage and conductance–voltage (C–V, G–V) measurements performed by ac impedance spectroscopy in the temperature range from 153 to 313 K reveal a typical MOS behaviour with moderate frequency dispersion at the accumulation region attributed to leakage currents. For the determination of the leakage currents conduction mechanisms, current density–voltage (J–V) measurements were carried out in the whole temperature range.

CuO/Ta2O5 core/shell nanoparticles synthesized in immersed arc-discharge: production conditions and dielectric response

We reported recently on a novel nanostructured material produced by the arc-discharge in water method, and extended studies were realized to identify the nature of this material, i.e., CuO/Ta2O5 core/shell crystalline nanoparticles (NPs). As a continuation of this investigation on the possibility of complex NP synthesis using immersed arc-discharge, the production conditions of the CuO/Ta2O5 NPs are herein presented in detail and the electrical properties of the nanopowder are examined comprehensively. The discharge is thus probed in situ by electrical measurements, optical emission spectroscopy and high speed imaging, and the electrical behavior of the NPs is considered by means of broadband dielectric spectroscopy. This combined study provides an integrated characterization of this new material, unveils its potential applications, and makes available suggestions on the process control.

Electrical transport properties of NbSi2 thin films

Films of NbSi2 have been prepared by the reaction of Nb films with silicon. Transport studies have been performed in the temperature range 170-640 K. The resistivity versus temperature exhibits a deviation from the ideal metallic behaviour, approaching a saturation value as temperature increases. This value is estimated to be 446 mu Omega cm. The calculated mean free paths are 20 AA at room temperature and 3.3 AA for the saturation resistivity value. The latter is consistent with a mean free path approaching the interatomic distance, where Blochs theory breaks down. The Hall coefficient remains negative in the whole measured temperature region, suggesting that electrons are the predominant carriers, giving an apparent electron concentration of 2.5*1022 cm-3 at room temperature.

ZrO2 and Al2O3 Thin Films on Ge(100) Grown by ALD: An XPS Investigation

Thin films of aluminium oxide (Al2O3) and zirconium oxide (ZrO2) were prepared by Atomic Layer Deposition (ALD) on p-type (100) germanium substrates. In the present work a detailed analysis of the films by X-ray photoelectron spectroscopy (XPS) in order to investigate their chemical composition is presented. This study is dedicated to an XPS investigation of the principal core levels (Al, Zr, O) of Al2O3 and ZrO2 thin films. In particular, wide scan spectra, detailed scans for the Zr 3d, Al 2p, O 1s, and C 1s regions and related data for zirconia and alumina films are presented and discussed. The results point out the formation of Al2O3 and ZrO2 films with the presence of OH groups and carbon contamination on the surface.