Abdelkader Kahouli

Experimental and Theoretical Study of AC Electrical Conduction Mechanisms of Semicrystalline Parylene C Thin Films

The electrical conduction mechanisms of semicrystalline thermoplastic parylene C (-H(2)C-C(6)H(3)Cl-CH(2)-)(n) thin films were studied in large temperature and frequency regions. The alternative current (AC) electrical conduction in parylene C is governed by two processes which can be ascribed to a hopping transport mechanism: correlated barrier hopping (CBH) model at low [77-155 K] and high [473-533 K] temperature and the small polaron tunneling mechanism (SPTM) from 193 to 413 K within the framework of the universal law of dielectric response. The conduction mechanism is explained with the help of Elliots theory, and the Elliots parameters are determined. From frequency- and temperature-conductivity characteristics, the activation energy is found to be 1.27 eV for direct current (DC) conduction interpreted in terms of ionic conduction mechanism. The power law dependence of AC conductivity is interpreted in terms of electron hopping with a density N(E(F)) (~10(18) eV cm(-3)) over a 0.023-0.03 eV high barrier across a distance of 1.46-1.54 Å.

Structural and dielectric study of parylene C thin films

α, β, and γ relaxation mechanisms have been identified in semicrystalline (45% of crystallinity) parylene-C (–H2C–C6H3Cl–CH2–)n films. C–Cl bonds induce the β-relaxation and explain increase in the dielectric constant as the frequency decreases in usual temperatures of operation for devices incorporating parylene-C. At cryogenic temperature (<−20 °C), γ-relaxation is assigned to the local motions of phenyl groups. Both β and γ relaxation processes obey an Arrhenius law with activation energy Ea(β)=91.7 kJ/mole and Ea(γ)=8.68 kJ/mole. α-relaxation associated with cooperative segmental motions of the (–H2C–∅–CH2–)n chains is observed with a peak at 10−2 Hz for T=80 °C and follows a Vogel–Fulcher–Tamman–Hesse law.

Impedance and electric modulus study of amorphous TiTaO thin films: highlight of the interphase effect

The influence of phases and phase’s boundaries of TiO2 and Ta2O5 in the dielectric and electric response of TiTaO (100 nm thick) elaborated by RF magnetron sputtering was highlighted by complex impedance spectroscopy. Dielectric and electric modulus properties were studied over a wide frequency range (0.1–10 5 Hz) and at various temperatures (−160 to 120 ◦ C). The diagram of Argand (e �� versus e � ) shows the contribution of phases, phases’ boundaries and conductivity effect on the electric response of TiTaO thin films. Moreover, the resistance of the material decreases when the temperature increases, thus the material exhibits a negative temperature coefficient of resistance. The electric modulus plot indicates the presence of two peaks of relaxation. The first relaxation process appears at low temperature with activation energy of about 0.22 eV and it is related to the first ionization energy of oxygen vacancies. The second relaxation process appears at high temperature with activation energy of about 0.44 eV. This second peak is attributed to the Maxwell–Wagner–Sillars relaxation. The plots of the complex dielectric modulus and the impedance as a function of frequency allow concluding to a localized relaxation due to the long-range conductivity in the TiTaO film. (Some figures may appear in colour only in the online journal)

Effect of film thickness on structural, morphology, dielectric and electrical properties of parylene C films

Parylene C (C8H7Cl)n thin films were deposited along the Si(100) index on a p-type Si substrate at room temperature by a vapor deposition polymerization method. The effects of film thickness on the crystalline structure, roughness, dielectric and electrical properties of parylene C films were investigated. X-ray diffraction has confirmed an amorphous structure in thinner films (d   40 nm due to surface energy minimization when the film gets thicker. The dielectric constant increases as a power law according to film thickness e′ = dβ with β = 0.042 ± 0.04 for d < 1000 nm and 0.0138 ± 0.02 for...

Effect of O2, Ar/H2 and CF4 plasma treatments on the structural and dielectric properties of parylene-C thin films

Plasma treatment of parylene-C surfaces not only causes structural modification of the surface during the plasma exposure, but also leaves active sites on the surfaces, which decreases the dielectric properties. In this work, the effects of oxygen, argon/hydrogen and fluorine plasma treatment on the surface and dielectric properties of parylene-C thin films were investigated using Fourier transform-IR spectroscopy, energy dispersive x-ray analysis and dielectric spectroscopy measurement. The results showed that the plasma treatment successfully introduced fluorine functional groups and decreased the oxygen content on the parylene-C surfaces. It appears that the replacement of oxygen and hydrogen by fluorine atoms led to a decrease in the local orientational polarizability of parylene-C. Consequently, it was found that the atmospheric fluorine plasma-treated parylene-C possessed lower dielectric characteristics, 16% lower than the untreated parylene-C at industrial frequencies (10?104?Hz). The Ar/H2 plasma treatment is also an experimental means to reduce the dielectric properties and to decrease the oxygen content in parylene-C. In contrast, the oxygen plasma increases the dielectric constant and can cause deterioration of the leakage current associated with carbon depletion showing C?O and C=O formation. CF4 and Ar/H2 plasma treatment does not significantly affect the long molecular motion (?-relaxation). Additional extrinsic oxygen content due to O2 plasma treatment in the parylene-C structure reproduces the increase in the time constant of both the short (?-relaxation) and long molecular motion.

I–t, J–1/T and J–E characteristics for the understanding of the main mechanism of electric conduction and the determination of the glass transition temperature of parylene C thin films

Measurements under both transient and steady-state conditions on parylene C (?H2C???C6H3Cl???CH2?)n, also called PPX C, were made for different electric fields ranging from 8.33 to 33.33?MV?m?1. The transient current behaviour is hyperbolic in nature up to 125??C. Above, the current is transient free and becomes constant reflecting the presence of the steady state. The decay rate of the transient current increases with increasing temperature and field. The transient current is attributed mainly to the dipolar relaxation due to the polarization of the C?Cl dipole. The J?1/T characteristic reflects the change in the conduction regime occurring at a critical temperature associated with the glass transition temperature of the materials. The J?E measurements show that hopping conduction is the possible mechanism below and above Tg of parylene C. The activation energy is determined to be 0.13?eV, independent of the electric fields below Tg and varies from 0.65 to 0.94?eV above Tg, indicating the presence of more than one type of trapping centres in parylene C. The ionic jump distance ?a? is estimated to be 5.60?6.68?? below Tg and 8.36?26.58?? above Tg.

Electrical characteristics and conduction mechanisms of amorphous subnanometric Al2O3–TiO2 laminate dielectrics deposited by atomic layer deposition

Electric conduction mechanisms of amorphous Al2O3/TiO2 (ATO)-laminates deposited by atomic layer deposition with sub-nanometer individual layer thicknesses were studied in a large temperature range. Two characteristic field regions are identified. In the low field region (E ≤ 0.31 MV/cm), the leakage current is dominated by the trap-assisted tunneling through oxygen vacancies occurring in the TiO2, while in the high electric field region (E > 0.31 MV/cm) the Poole Frenkel (PF) hopping is the appropriate conduction process with energy levels depending on the temperature and the electric field. It is shown that the PF potential levels decrease with the applied ATO field due to the overlapping of the Coulomb potential. Amorphous ATO-laminates show the presence of two intrinsic potential energy levels ϕi, which are 0.18 eV for low temperature region and 0.4 eV at high temperature region. Oxygen vacancies are the main origin of traps, which is consistent with the principal mechanisms for leakage in ATO-laminates.

Dielectric Investigation of Parylene D Thin Films: Relaxation and Conduction Mechanisms

Parylene is a generic name indicating a family of polymers with the basic chemical structure of poly-p-xylylene. Parylene N and Parylene C are the most popular for applications. Curiously, Parylene D (poly( dichloro-p-xylylene), (C8H6Cl2)) was forgotten for applications. This report is the consequence of a later availability of a commercial dimer of Parylene D and also to the recent advent of fluorinated Parylenes allowing extending applications at higher temperatures. In our work, from a dielectric analysis, we present the potentialities of Parylene D for applications particularly interesting for integration in organic field-effect transistors. Dielectric and electrical properties, macromolecular structures, and dynamics interaction with electric field as a function of frequency and temperature are studied in 5.8 μm thick Parylene D grown by chemical vapor deposition. More exactly, the dielectric permittivity, the dissipation factor, the electrical conductivity, and the electric modulus of Parylene D were investigated in a wide temperature and frequency ranges from -140 to +350 °C and from 0.1 Hz to 1 MHz, respectively. According to the temperature dependence of the dielectric permittivity, Parylene D has two different dielectric responses. It is retained as a nonpolar material at very low temperature (like Parylene N) and as a polar material at high temperature (like parylene C). The dissipation factor shows the manifestation of two relaxations mechanisms: γ and β at very low and high temperatures, respectively. The γ relaxation is assigned to the local motions of the C-H end of the chains when the cryogenic temperature range is approached. A broad peak in tan δ is assigned to the β relaxation. It corresponds to rotational motion of some polar C-Cl groups. For temperature above 260 °C a mechanism of Maxwell-Wagner-Sillars polarization at the amorphous/crystalline interfaces was identified with two activation energies of Ea1 = 2.12 eV and Ea2 = 3.8 eV. Moreover, the conductivity and the dielectric permittivity relaxation processes have been discussed in terms of nearly constant loss (NCL) and universal dynamic regime (UDR). Finally, ionic conduction and electrode polarization effects are identified at very high temperatures and their physical origins are discussed.

A comparison of electric and dielectric properties of standard and high-crystallinity polypropylene films

The dielectric properties of two grades of Bi-oriented isotactic polypropylene (BOiPP) are studied, using a variety of techniques: breakdown field measurements, dielectric spectroscopy, thermally stimulated depolarization currents, dc conduction currents. Standard (STPP) and high-crystallinity (HCPP) polypropylene films are investigated. Measurements are carried out over a wide temperature range, up to (-150°C/ +125°C). Breakdown field in both materials show a very small difference. On the other hand, dielectric losses and dc conduction currents are significantly lower in HCPP. Both materials show a decrease of dielectric losses versus temperature in the range (20-90°C), favorable for the application to ac power capacitors. The analysis of dc currents allows to evidence two main conduction mechanisms: (i) below 80°C in both materials, a hopping mechanism due to motion of electrons occurring in the amorphous phase; (ii) above 80°C, an ionic conduction in HCPP and a hopping conduction in STPP.

Dielectric relaxation study of amorphous TiTaO thin films in a large operating temperature range

Two relaxation processes have been identified in amorphous TiTaO thin films deposited by reactive magnetron sputtering. The parallel angle resolved x-ray photoelectron spectroscopy and field emission scanning electron microscopy analyses have shown that this material is composed of an agglomerates mixture of TiO2, Ta2O5, and Ti-Ta bonds. The first relaxation process appears at low temperature with activation energy of about 0.26 eV and is related to the first ionisation of oxygen vacancies and/or the reduction of Ti4+ to Ti3+. The second relaxation process occurs at high temperature with activation energy of 0.95 eV. This last peak is associated to the diffusion of the doubly ionized oxygen vacancies VO. The dispersion phenomena observed at high temperature can be attributed to the development of complex defect such as (VO − 2Ti3+).