Space-charge-limited currents in polymethylmethacrylate films

An investigation was undertaken to examine the conduction mechanism in polymethylmethacrylate (PMMA) films. The current was measured in Al/PMMA/Al capacitors in the voltage range 4–300 V.     

Charge carrier trapping in poly(N-vinylcarbazole)-trinitrofluorenone films

An investigation of hole and electron photo-injection in films of the poly(N-vinylcarbazole)-trinitrofluorenone charge transfer complex by the time-of-flight technique revealed the presence of deep carrier traps. The hole trap density was estimated to be (6–13) × 10¹⁹m^-3 in films where poly(N-vinylcarbazole) is mixed with trinitrofluorenone in a 1:0.2 mole ratio with respect to the monomer unit. In 1:1 mole ratio films, the electron trap density was estimated to be (10–29) × 10¹⁹m^-3. In the film samples typically investigated the number of carriers injected per sample normally exceeded the number of deep traps in the sample. The fraction of injected charge that became trapped was small under these circumstances, and the majority of charge carriers were transported through the film.     

Space-charge-limited current analysis in amorphous InSe thin films

The study of space-charge-limited currents (SCLC) in amorphous InSe thin films is presented. The temperature-dependent current–voltage (J–V) measurements were carried out for TO/a-InSe/Au sandwich structures in the range of 200–320 K. For all samples, ohmic behavior was observed up to an electric field strength of about 2×10⁵ V cm^–1. From the temperature dependence of conductivity data, the position of the thermal equilibrium Fermi level E _fo is determined as 250 meV above the valence band E _v. At higher electric field strength values in the SCLC regions, the proportionality constant of voltage changes is between 2 and 2.9 with temperature. The analysis of J–V characteristics using the SCLC method and analytical approach for the determination of density of states (DOS) in the energy range of 190–250 meV shows that DOS changes between 3.8×10¹⁷–1.7×10¹⁸ eV^–1cm^–3 with energy. The energy distribution of DOS is temperature independent indicating that the SCLC in these amorphous films is related to the bulk, not to the surface layer between the contact and the film.     

Dielectric properties of plasma-polymerized ferrocene films

The variations of the dielectric constant ε and of tan δ for Al-polyferrocene-Al sandwich structures were studied over the frequency range 40 kHz–50 MHz and the temperature range 30–400°C. Heating in dry air led to a permanent change in the value of ε by approximately 15% while the change in tan δ was of a temporary nature. From IR studies a possible structure for polyferrocene is suggested.     

Optical spectroscopic characterization of plasma-polymerized thin films

We report optical spectroscopic measurements of plasma-polymerized thin films coated on aluminum (Al) substrates over a spectral range from 0.01 to 6 eV. While the reflectance spectra, R(ω), remain almost unchanged in the infrared range as compared with the bare Al, R(ω) starts to decrease significantly in the higher energy above 2.5 eV. This decrease in R(ω) arises from electronic absorptions characteristic of the polymer films. Moreover, the details of R(ω) in the energy above 3 eV depend on the fabrication conditions of the polymer films. Our results, therefore, demonstrate that this optical measurement can be an excellent method to characterize the quality of polymer films deposited by the plasma polymerization process.     

Dielectric properties of plasma-polymerized diphenyl thin films

Thin polymeric films of diphenyl were prepared by a plasma polymerization technique using a capacitively-coupled reactor. The dielectric relaxation spectroscopy was employed to investigate the behaviour of the as-deposited and heat-treated plasma-polymerized diphenyl (PPDP) thin films in a aluminium/PPDP/aluminium configuration over the frequency range from 10⁻¹ to 10⁶ Hz and temperature range from 223 to 423 K. The ac conductivity is more dependent on temperature in the low frequency region than in the high frequency region. In these materials the conduction may be dominated by hopping of carriers between the localized states at low temperatures and thermally excited carriers from energy levels within the band gap in the vicinity of high temperature. Dielectric constant is dependent on frequency above 303 and 343 K in the as-deposited and heat-treated PPDP, respectively. The as-deposited PPDP shows superposed and β relaxation processes. For different relaxation processes, the activation energies are estimated to be about 0.45 and 0.67 eV for the as-deposited and 1.35 eV for heat-treated PPDP. The dielectric data analyses show the existence of distribution of relaxation time in these materials.     

Synthesis and characterization of plasma-polymerized tert-butylacrylate films

Plasma polymerized tert-butylacrylate (pp-t-BA) film was prepared using tert-butylacrylate monomer under 100 Pa of vapor pressure with varying RF power of 10-250 W and continuous wave RF power of 13.56 MHz. The deposition rates of pp-t-BA films were determined using quartz crystal microbalance (QCM) method. The chemical structure of pp-t-BA films was characterized using FT-IR, contact angle and XPS techniques. The gas sorption properties of pp-t-BA were measured using a QCM sensor array. Results showed that deposition rates of pp-t-BA film were proportional to the polymerization time at 100 Pa of monomer pressure under the same RF power. The deposition amount of pp-t-BA films increased gradually with increasing RF power of 30-150 W. Increasing the RF power on plasma polymerization decreased the amount of ester group in pp-t-BA films. Sensitivity of gas sorption on pp-t-BA films is related to the RF power of polymerization.     

Electron microscopy studies of plasma-polymerized organosilicon thin films

Plasma-polymerized thin films of hexamethylcyclotrisilazane were pyrolysed in vacuum and studied by transmission electron microscopy. Examination of the polymer film after various durations of pyrolysis revealed significant changes in their surface structure and a distinct difference in the thermal stability powder and film phases. The number of powder particles in the surface layer and their size increased with pyrolysis time. The observations showed that powder particles embedded in the film matrix were gradually exposed in the surface layer owing to a decrease in the film thickness during pyrolysis. Good agreement was obtained between the experimental and theoretical distribution curves of particle size. The identical properties of powder particles exposed in the surface of the pyrolysed film and those taken from the interelectrode region indicates that they are primarily formed in the gas phase.     

Electrochemical evaluation of the reliability of plasma-polymerized methylcyclohexane films

Plasma-polymerized thin films are developed for electronic devices to satisfy the important requirement of a low dielectric constant in the interlayer dielectrics. Three types of methylcyclohexane coatings are deposited on copper as interlayer dielectrics by plasma-enhanced chemical vapor deposition at three different deposition temperatures. The coating performance is evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization testing in a 3.5 wt.% NaCl solution. The coatings are also analyzed by surface analyses, including atomic force microscopy, Fourier transform infrared spectroscopy, and contact angle measurements. The electrochemical behavior of the coatings is improved by increasing the deposition temperature. The methylcyclohexane films on the copper substrate show high protective efficiency, charge transfer resistance and low porosity, which indicate that the coating performance increased with increasing deposition temperature. Atomic force microscopy, Fourier transform infrared spectroscopy and contact angle measurements confirm the enhanced formation of C-H, C-C, and CC stretching configurations, improved surface roughness and wettability with increasing deposition temperature.     

Space-charge-limited current measurement of traps in p-type electrochemically deposited CdTe thin films

Space-charge-limited current measurements were performed on high resistivity p-type electrodeposited CdTe thin films. The observed dominant trap is a hole trap located at 0.54 eV above the valence band with a density of 1 × 10¹⁴ cm^?3     

The adhesion of thin plasma-polymerized organic films to metal substrates

The direct pull method was used to study the adhesion of thin (<1 μm) plasma-polymerized films on metal substrates. Application of this method provides information on the cohesive, as well as the adhesive, properties of these films. After breakage of the films by the pull method, the interface was analyzed by scanning electron microscopy to determine the nature of the breakage at the interface. Electron spectroscopy for chemical analysis was used to determine whether any chemically adsorbed layers of organic films remained after the films had been pulled from the metal substrate. The adhesive or cohesive strengths of plasma-polymerized tetrafluoroethylene and chlorotrifluoroethylene films on 304 stainless steel, aluminum and silver were measured by the pull method. Adhesive strength was found to be higher than cohesive strength for plasma-polymerized tetrafluoroethylene films on the various substrates. The result for plasma-polymerized chlorotrifluoroethylene films was not as expected, indicating that considerable care is required in the interpretation of tensile tests of laminates such as those conducted in this study.     

On the origin of the dielectric breakdown mechanism in plasma-polymerized hexamethyldisiloxane films

Dielectric breakdown phenomena of plasma-deposited hexamethyldisiloxane are discussed. We show that the deposition conditions and post-treatment can change the dielectric strength. A lowering of the breakdown field was attributed to reactions with oxygen species during or after deposition.     

An investigation of the optical properties of plasma-polymerized diphenyl thin films

Highly adhering plasma-polymerized diphenyl (PPDP) thin films were grown by a dry and flawless process in glow discharge. The structural analyses of as-deposited and heat treated PPDP samples (unaged and aged) were performed by infrared (IR) spectroscopy. The PPDP thin films were transparent and light yellow in colour. Optical properties were evaluated from ultraviolet–visible (UV–Vis) spectroscopic measurements of as-deposited, heat treated and aged (as-deposited and heat treated) PPDP thin films. From the optical absorption data band gaps, allowed direct and indirect transition energy gaps were determined. The results are discussed in the light of possible structural modification in PPDP thin films on heat treatment and aging.     

Polymer light-emitting devices based on plasma-polymerized benzene and plasma-polymerized naphthalene

Polymer light-emitting devices (PLED) were fabricated utilizing plasma-polymerized benzene (PPB) and plasma-polymerized naphthalene (PPN) as an electroluminescent (EL) emitting layer. The plasma polymerization is well suited for forming the transparent, sturdy thin film for EL polymer layers. For the ITO/PPB/Al and ITO/PPN/Al devices, the turn-on voltage of the device was at 12V and 6V, respectively. The luminance of the PPB device reached 6 cdm ^-2 at 10 V, whereas the PPN device reached 11 cd m ^-2 at 14 V. The external quantum efficency was 0.0035% for the PPB device and 0.0056% for the PPN device. The dense crosslinked structure formed by the plasma polymerization makes the EL device relatively stable during operation. © 2001 Kluwer Academic Publishers     

X-ray photoelectron spectroscopic studies on pyrolysis of thin films of plasma-polymerized acrylonitrile

X-ray photoelectron spectroscopy is used to elucidate the chemical environments of the atoms in as-deposited plasma-polymerized acrylonitrile (PPAN) thin film and PPAN films pyrolysed at the temperatures 573 and 773 K. The photoelectron spectra are collected at the electron take-off angles of 20° and 70°. The C 1s, N 1s, and O 1s spectra of all the samples demonstrate clearly that the structural modifications take place owing to pyrolysis in the PPAN structure. Apart from the structural modifications, a decrease of nitrogen and an uptake of oxygen are evidenced. A few chemical species other than the usual ones are also detected.     

Space charge limited currents in graded films

The equilibrium current density is calculated for films in which the conductivity is a function of depth. Three specific cases are considered: (a) conductivity falling monotically with distance away from the injecting electrode; (b) conductivity rising monotonically; and (c) conductivity sharply peaked in the middle of the specimen. In all cases the current density is ohmic at low voltage and obeys the Mott-Gurney law at high applied voltage. There is a smooth transition between the limits, with no negative resistance region, and switching is unlikely to occur for this combination of ohmic and space charge limited transport mechanisms.