Guy Adriaenssens

Charge carrier mobility in doped semiconducting polymers

An analytic model of the equilibrium hopping conductivity in a disordered organic semiconductor at large charge carrier densities is formulated. Calculated dependences of the equilibrium hopping mobility upon the carrier density are compared with recent experimental data obtained on doped poly(3-hexylthiophene) films. Doping is shown to create additional energy disorder due to potential fluctuations caused by the Coulomb field of randomly distributed dopant ions.

Structure and defects of detonation synthesis nanodiamond

Abstract Characterization of the structure and defects in detonation synthesis, ultradisperse diamond (UDD) is reported. X-ray and proton nuclear magnetic resonance results on UDD powders are interpreted in terms of the different structure of the shell of UDD particles, produced under different conditions. In spite of the comparable contents of carbon and nitrogen atoms in the precursor, no (

Hydrogenated amorphous silicon deposited at very high growth rates by an expanding Ar–H2–SiH4 plasma

The properties of hydrogenated amorphous silicon (a-Si:H) deposited at very high growth rates (6–80 nm/s) by means of a remote Ar–H2–SiH4 plasma have been investigated as a function of the H2 flow in the Ar–H2 operated plasma source. Both the structural and optoelectronic properties of the films improve with increasing H2 flow, and a-Si:H suitable for the application in solar cells has been obtained at deposition rates of 10 nm/s for high H2 flows and a substrate temperature of 400 °C. The “optimized” material has a hole drift mobility which is about a factor of 10 higher than for standard a-Si:H. The electron drift mobility, however, is slightly lower than for standard a-Si:H. Furthermore, preliminary results on solar cells with intrinsic a-Si:H deposited at 7 nm/s are presented. Relating the film properties to the SiH4 dissociation reactions reveals that optimum film quality is obtained for conditions where H from the plasma source governs SiH4 dissociation and where SiH3 contributes dominantly to film ...

Photochromism of vacancy-related centres in diamond

Optical experiments, involving photoluminescence (PL), the PL excitation and quenching spectra, as well as transmission and its quenching, were used to analyse the photochromic behaviour of some vacancy-related complexes in diamond. The 2.156 eV, 1.945 eV and 1.68 eV optical centres in CVD diamond are attributed to the neutral nitrogen-vacancy, negative nitrogen-vacancy and neutral silicon-vacancy ([Si-V]0 ) centres, respectively. Oscillatory behaviour in the excitation spectrum of the 1.68 eV luminescence is observed and from the threshold of the oscillations a position of EC - 2.05 eV is suggested for the ground state of the [Si-V]0 centre.

Weak-field carrier hopping in disordered organic semiconductors: the effects of deep traps and partly filled density-of-states distribution

An analytic model of the weak-field carrier transport in an energetically disordered and positionally random hopping system is formulated. Within the framework of this model, the carrier mobility can be calculated by either direct averaging of carrier hopping rates or by the use of the effective transport energy concept. It is shown that multiple carrier jumps within pairs of occasionally close hopping sites affect the position of the effective transport level on the energy scale. In good quantitative agreement with experimental data and results of Monte Carlo simulation, the temperature and concentration dependences of the mobility can be almost perfectly factorized, i.e. represented as a product of two functions one of which depends solely upon the temperature while the other governs the dependence upon the density of localized states. The model is also used for the calculation of trap-controlled hopping mobility and for the analysis of hopping transport at high charge-carrier densities.

Space-charge-limited currents in materials with Gaussian energy distributions of localized states

A model is formulated describing trap-controlled space-charge-limited currents (SCLCs) in an organic material with a Gaussian density-of-states (DOS) distribution. It is shown that SCLC is not always controlled by carrier release from localized states around the Fermi level and, therefore, a Gaussian DOS can serve as either shallow or deep distribution of localized states depending upon the carrier and/or current density.

Optical characterization of natural Argyle diamonds

Optical centers in natural diamonds from the Argyle mine have been characterized by absorption and photoluminescence techniques.Many of those diamonds show characteristic PL spectra with a large number of slowly decaying centers, most of which can be attributed to the Ni impurity.A linear correlation was observed between the intensity of the 3107 cm hydrogeny1 related peak and the concentration of IR-active nitrogen.It may be accounted for by the presence of NH , NH and N gases as 34 2 sources of nitrogen during diamond growth.It is shown that the unique blue-gray color of some Argyle diamonds is determined not only by the high hydrogen content, but also a small concentration of tri-nitrogen complexes in combination with a large concentration of two- and four-nitrogen defects. 2002 Elsevier Science B.V. All rights reserved.

Non-Langevin recombination in disordered materials with random potential distributions

A concept of random spatial fluctuations in the potential landscape of disordered semiconductors is employed to explain the effect of suppressed charge carrier recombination in these materials. The rate of bimolecular recombination is shown to be anomalously low due to spatial separation of electrons and holes in the fluctuating potential landscape. Photoexcited carriers can avoid geminate recombination due to the effect of a sufficiently strong local electric field coupled to the potential fluctuations.

Dominant defect levels in diamond thin films: A photocurrent and electron paramagnetic resonance study

Characteristic features in photocurrent (PC) and electron paramagnetic resonance (EPR) spectra are discussed and attributed to main defects in the gap of optical-quality chemical vapor deposited diamond. A shoulder in the PC spectra with an onset at about 2.2 eV is attributed to the single-substitutional nitrogen defect (EPR P1 resonance at g=2.0024). A second feature in the PC spectra with an onset of about 1.3 eV is observed on “as-grown” samples with a hydrogen terminated surface. The defect level associated with this feature is hydrogen related, and this defect disappears after oxidation of the diamond sample surface. The EPR g=2.0028, which was also suggested to be H-related, is discussed.

Influence of various distributions of localized states upon transit pulse dispersion in amorphous semiconductors

Abstract Monte Carlo simulation techniques have been employed in the examination of anomalously-dispersive transport characteristics of specimens with various energy distributions of localized states. For each of the four distributions studied, the ‘initial slope’ dispersion parameter, α1, varies similarly with temperature, and exhibits an approximately linear variation in the low-temperature regime. This demonstrates that such behaviour does not constitute evidence for the presence of a particular (e.g. exponential) energy distribution of traps. The ‘final slope’ dispersion parameter, α2 is found to be appreciably more sensitive than α1 to changes in the energy distribution of localized states. A comparison of experimental behaviour with the simulation data suggests that various non-crystalline semiconductors possess a comparatively structured distribution of trapping centres, rather than the featureless tail of localized states which has been inferred from some recent studies.