H. Michiel

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.

Determination of localized state distributions from anomalously-dispersive transport data

Abstract A general relation is deduced between the density of localized states N(E) and the transient photocurrent I(t) for an amorphous semiconductor exhibiting trapcontrolled electronic transport. It is demonstrated that the waiting-time distribution functionψ(t) is related to I(t) through a Volterra integral equation, which may be solved numerically. The relative density of states N(E) may be deduced in a straightforward manner from ψ(t), using an approximation which should not introduce more than a small degree of distortion. The computational method is applied to the case of an exponential tail of localized states, and to a system with three discrete sets of trapping centres. Finally, the consequences of an incomplete knowledge of some of the system parameters are considered.

On the density of localized states obtainable from transient photodecay measurements

Abstract The transient photodecay as seen for a structured density of localized states in an amorphous semiconductor has been calculated through general linear algebra methods. The results are compared to recent calculations for the same distributions by Marshall and Street, who used Monte Carlo simulation, and by Silver Snow and Adler, who based their analytical solution on the thermalization approximation. Our calculations are in good agreement with the simulation data and only qualitatively support the thermalization results. We show that the thermalization approximation does not give a proper description of the actual carrier thermalization in a structured distribution.

Analysis of time-of-flight transit times based on the multiple-trapping model of charge-carrier transport

Abstract Within the context of the multiple-trapping model, we discuss different theoretical ways of defining a transit time in the time-of-flight experiment. A new definition based on the first moment with respect to space of the free-carrier distribution is proposed. At the same time various experimentally used definitions of transit time are identified and the corresponding operational values are extracted from Monte Carlo simulated time-of-flight current traces for different field strengths and temperatures, with either an exponential or a linear density of tail states. A comparison between the theoretical and Monte Carlo results for the different approaches is made. Our calculations show that definitions of transit time that emphasize the fastest of the drifting carriers exhibit a larger electric field dependence and a smaller apparent activation energy than definitions which are based on the totality of excess charge carriers. Consequently we conclude that, if one analyses drift-mobility data in the...

Spectroscopic analysis of transient photocurrents in a-As2Se3

A recently developed spectroscopic method, based on trap-controlled band-transport (TCBT) in semiconductors, is applied to the transient photocurrent in glassy As2Se3. The analysis points to the presence of a local maximum in the density of localised states at some 0.45 eV inside the gap; beyond this maximum, the density decreases exponentially over a range of approximately 0.15 eV. Difficulties concerning the spectroscopic interpretation of transport data are further examined.

Spectral distribution of steady-state photoconductivity in amorphous As2Se3

Abstract The spectral distribution of steady-state photoconductivity has been examined for a series of bulk and evaporated samples of amorphous As 2 Se 3 . All bulk samples, irrespective of preparation technique or sample treatment, show a distinct shoulder in the photocurrent spectral distribution at energies near 1.4 eV. This feature is interpreted as spectroscopic evidence for the existence of a well-defined defect level in the gap of amorphous As 2 Se 3 . Evaporated As 2 Se 3 films do not show any structure in the spectral response.

Preparation of hydrogenated amorphous-silicon with tunable gap by homogeneous chemical vapor-deposition

This paper reports on the properties of doped and undoped amorphous silicon films deposited by the homogeneous chemical vapor deposition (HOMOCVD) technique. It is shown that good quality films can be grown at reasonable deposition rates of 100–150 A/min. It is also shown that in this growth regime, the main precursor is Si2H4, shifting to SiH2 at higher H2 dilution. The real limit for the growth rate is set by the phenomena of homogeneous and local nucleation. The optical band gap of undoped films deposited at these high growth rates, changes from 2.6 eV for a substrate temperature of 20 °C down to 1.6 eV at 280 °C. Very conductive B‐doped HOMOCVD amorphous silicon films with tunable band gap can be obtained. This is very important for the use of such films for window layers in photovoltaic applications as an alternative to siliconcarbide. At a substrate temperature of 40 °C films were obtained with an optical gap of 2.34 eV and a room‐temperature dark conductivity of 1.6×10−5/Ω cm. Down to a thickness o...

Influences of trace metal impurities on the thermal quenching of photoluminescence in hydrogenated amorphous silicon by homogeneous chemical vapor deposition

The influence of trace metal impurities of low‐temperature undoped amorphous silicon by homogeneous chemical vapor deposited a‐Si:H has been explored for the first time. The metal impurities Ni, Cr, and Fe cause a shift of the transition temperature for the double‐activated regime to a relatively low value. However, Ga and B impurities quench the photoluminescence intensity at a low temperature. Both of them cause weak photoemission of the films at room temperature. The shift of the transition temperature can be explained by the presence of non‐radiative deep recombination centers. The quenching of the photoluminescence intensity is caused by the presence of nonradiative recombination centers.

Tail-state distribution and extended-state mobility in a-Si:H

Abstract Analyses of drift mobilities in a-Si:H have been carried out for a number of model density of states distributions by means of an algebraic discretization formalism for multiple trapping transport. The extended state mobility υo is found to be not larger than 50 cm 2 V −1 s −1 . None of the distributions proposed so far in the literature give satisfactory agreement with experimental data.

Photoconductivity self-recovery effect and self-annealing behaviour of undoped A-Si:H by homogeneous chemical vapour deposition

Abstract A self-annealing effect during illumination and photoconductivity room temperature recovery effect after light soaking in undoped HOMOCVD α-Si:H films are described. The activation energy of the thermal relaxation time τR is as low as 0.28 eV for HOMOCVD α-Si:H in contrast with 1.5 eV for glow discharge α-Si:H.