C. Main

An evaluation of phase-shift analysis of modulated photocurrents

Abstract The accuracy and limitations of phase-shift analysis of modulated photocurrents as a means of determining the energy profile of localized states in amorphous semiconductors have been investigated. The basis of the existing ‘recursion procedure’ is reviewed and its deficiencies are revealed by applying the method to computer-generated phase-shift spectra for model trap systems. Realistic conditions which can produce severe distortions in the trap profiles reconstructed in this way have been identified and examined. We propose a new method, based on the same experiment, which is equally simple in application and avoids most of the deficiencies inherent in the recursive method. The new method is demonstrated for both computer-generated data and preliminary experimental data on samples of arsenic triselenide.

Calculation of localized-state energy distributions from transient-photoresponse data

Abstract Time-of-flight studies of carrier transport through films of an amorphous semiconductor may be used to explore the energy distribution of shallow localized states in such materials. In this paper, we describe a new and computationally straightforward procedure for this purpose. The effectiveness of both this and previous techniques is evaluated using computer-generated time-of-flight data for model semiconductor films having known energy distributions of localized states. The procedures are also assessed in relation to recent experimental data for electron transport in amorphous silicon.

Transient photoconductivity in n-type a-Si: H

Abstract The long-time transient photocurrent decay in annealed and light-exposed P-doped a-Si: H is examined experimentally and by numerical modelling. The decay is a dispersive power law with sublinear index, extending to times longer than 1s, and the decay rate increases with temperature. Light exposure dramatically decreases the decay amplitude but does not affect the rate of decay. The phenomenon is discussed in terms of a comprehensive multiple-trapping model in which transport of thermalized electrons is essentially non-dispersive, and recombination of free carriers via defects is dispersive, owing to continued thermalization of excess holes. The slower recombination step is free-hole capture by D− states, while the decay of the total excess ensemble is controlled by hole release from valence-band tail traps. The index of the excess photoelectron decay provides information on the valence-band tail states, which are exponentially distributed, with a characteristic energy estimated as 0·06 eV.

Electrical properties of nanocrystalline CdSe thin films prepared by thermal vacuum evaporation

Thin films of CdSe with thicknesses of 50, 75 and 100 nm were prepared by physical vapour deposition using a one-step or a step-by-step deposition approach. The influence of the deposition conditions and film thickness on the microstructure and electrical properties has been explored by means of atomic force microscopy (AFM), dark conductivity, photoconductivity and thermally stimulated conductivity measurements. The AFM results have shown that smaller grains are formed in thinner films and that the size decrease is enhanced when the step-by-step deposition approach is used. A significant increase of the dark current activation energy with decreasing layer thickness has been registered and identified with a Fermi-level shift. A decrease of dark conductivity and photoconductivity has been observed with decreasing layer thickness which has been connected with both a size-induced increase of the interface defect density and appearance of new faster recombination centres. The observed decrease is stronger in step-by step layers, in which the density of incorporated oxygen is higher. The oxygen creates new acceptor defects and causes band bending at grain boundaries. From the thermally stimulated conductivity data two kinds of defect states have been resolved situated at about 0.55 eV and 0.67 eV below the conduction band. A simultaneous fitting method has been applied to determine the main trap parameters.

A new procedure for calculating the density and energy distribution of localized hopping sites in disordered semiconductors, using low-temperature electrical conductivity data

Quantum mechanical tunneling between localized sites will dominate carrier transport inxa0disordered solids, at sufficiently low temperatures, predicated by the localized state concentration and energy distribution. We previously advanced a simple procedure for interpreting carrier mobility data, for an energy-independent density of states (DOS). Here, wexa0show that it can easily be extended to interpret electrical conductivity data, for both energy-independent and energy-dependent DOS distributions. We also show that the concept of transport energy is of considerable value in understanding the factors that underlie the experimental behavior. Furthermore, the new procedure yields credible and entirely self-consistent results when applied to published conductivity data. Finally, we contrast its success with the major inconsistencies that arise when results obtained using the Mott T−1/4 model are examined in more than superficial detail.

Charge trapping effects in amorphous silicon/silicon nitride thin film transistors

Abstract We report simultaneous measurements of the threshold voltage shift under positive bias stress and transient discharge following such stressing, on amorphous silicon - silicon nitride thin film transistors (α-Si : H TFTs). The discharge transients exhibit two components which are both due to emission from deep states within the α-Si : H.

The electronic and optical properties of nickel doped arsenic triselenide

Abstract An extensive study of the electronic properties of nickel doped arsenic triselenide revealed enormous changes from those of the undoped material. The d.c. conducvitity increases by 11 orders of magnitude and the position of the Fermi-level is shifted by ΔEE approximately 0.6 eV. In addition, we present conductivity (σ) versus 103/T data in which the activation energy decreases with increasing temperature. Such behaviour has never been observed before in arsenic triselenide.

Transient photoconductivity, trap saturation and optical bias in n-type a-Si : H

Abstract Comprehensive measurements have been made of the short-time transient photoconductivity in lightly As-doped a-Si:H as a function of pulse excitation density, temperature, and optical bias. These studies have revealed a number of detailed features of trapping dynamics consistent with multi-trapping electron thermalisation and trap saturation effects in an exponentially distributed band-tail with characteristic temperature Tc of 460 K for the particular material studied. It is demonstrated that such methods can provide up to three or four independent means of measuring parameters such as Tc, and can also be applied in a model-independent way, without prior assumption regarding the distribution of trapping states.

Evidence for metastable defects in amorphous silicon thin film transistors

Abstract When a positive gate voltage is applied to an amorphous silicon thin film transistor, electrons become trapped in states close to the silicon/dielectric interface. The density of these states rises continuously during the gate field application period. Various characteristics of the phenomenon are identified and a model is proposed in which metastable silicon dangling bonds are generated as a consequence of field effect induced band bending.

Some aspects of the role of holes in the transient response of a-Si:H pin-diodes

Abstract We model the transient response of a-Si:H pin-diodes and focus on the behaviour of the photocurrent after switch-on or off of steady illumination. The distinct experimental features such as an overshoot of the photocurrent after switch-on are reproduced in the simulation. The reason for the overshoot is identified as a change over from recombination at the contacts to recombination with the opposite carrier via dangling bonds. This occurs with some delay so that the electron density reaches a quasi-steady state value which is then reduced at later times and in the steady state. The response of holes for both illumination through the n- or p-side of the diode is shown to determine the response time of the total current for low defect diodes.