The increasing use of amorphous semiconducting films in various commercial applications has stimulated considerable interest in the electronic transport properties of such materials. Extensive studies performed over the past decade have revealed that, whilst non-crystalline semiconductors behave in some respects similarly to their crystalline counterparts, a number of new (or previously unnoticed) characteristics may also be manifested. A particular case concerns the diffusion of drifting charge carriers, which is frequently found to be characterised by an anomalous degree of dispersion, when examined in amorphous thin films.In this review, we describe the above phenomenon of 'anomalous dispersion'. Having initially outlined the present view of band structure and carrier transport mechanisms in non-crystalline materials, we continue with a description of the experimental techniques used in the investigation of the drift velocity of excess charge carriers. Details are then provided of the various characteristics associated with both 'conventional' and 'anomalous' dispersion. Models, in which the anomalous behaviour has been associated with both hopping and trap-limited band transport, are reviewed and assessed.Although anomalous dispersion was initially investigated as a phenomenon of interest in its own right, attention has increasingly turned towards the possibility that a detailed examination of the transient behaviour may be of use in determining details of the energy distribution of localised states in disordered films. We outline the various techniques which have been suggested and examine their successes and limitations, concluding with an indication of likely future developments and directions of investigations.

Density of states in tritiated amorphous silicon obtained with the constant photocurrent method J. Appl. Phys. 98, 093705 (2005); 10.1063/1.2123374Density of states in the gap of amorphous semiconductors determined from modulated photocurrent measurements in the recombination regime Electron time-of-flight transient photocurrents have been investigated in stabilized a-Se as a function of electric field, annealing, aging ͑relaxation͒, and alloying with As and doping with Cl. The distribution of localized states ͑DOS͒ in stabilized a-Se has been investigated by comparing the measured and calculated transient photocurrents. The samples were prepared by conventional vacuum deposition techniques. The theoretical analysis of multiple-trapping transport has been done by the discretization of a continuous DOS and the use of Laplace transform formalism. The resulting DOS has distinct features: A first peak at ϳ0.30 eV below E c with an amplitude ϳ10 17 eV −1 cm −3 , a second small peak ͑or shoulder͒ at 0.45-0.50 eV below E c with an amplitude 10 14 -10 15 eV −1 cm −3 , and deep states with an integral concentration 10 11 -10 14 cm −3 lying below 0.65 eV, whose exact distribution could not be resolved over the time scale of present experiments. The influence of doping, aging, annealing, and substrate temperature on the DOS distribution has been investigated. The doping with Cl does not affect the amplitudes of the first and second peaks while the concentration of deep states increases dramatically. The alloying with As reduces the density of deep states and seems to increase the amplitude of first and second peaks. The aging substantially reduces the deep states density and the amplitude of the second peak while the amplitude of the first one remains practically unchanged. The results have been interpreted primarily in terms of thermodynamic and intrinsic structural defects in the chalcogenide glass structure.

We use the post-transit photocurrent in a time-of-Night experiment for spectroscopic purposes. It is shown, within the multiple-trapping framework, that the post-transit current is the Laplace transform of the density of states. A simple inversion procedure is suggested and is shown to be quite adequate provided the gap-state distribution does not vary too strongly with energy. The method has been applied to hydrogenated amorphous silicon. Experimental evidence shows that the posttransit photocurrent truly rejects the release out of deeper-lying traps and is not a consequence of nonuniform, time-dependent fields, contact-related properties, or injection phenomena. The measured po~d are compatible with the deconvoluted density of states. A comparative discussion of our results with other published data obtained with conventional methods like the field-eff'ect, spacecharge-limited current, and deep-level transient spectroscopy techniques is given.