S. Reynolds

Determination of gap-state distributions in amorphous semiconductors from transient photocurrents using a fourier transform technique

Abstract We present a novel spectroscopic technique for the computation of the distribution of gap-states (DOS) in amorphous semiconductors from transient photocurrent decay exhibiting either anomalous or conventional dispersion. A numerical Fourier Transform procedure is used to convert the impulse response time decay data to phase and a]plitude frequency spectra. The DOS is then computed using a procedure developed for analysis of modulated photocurrent (MPC) data in a multiple-trapping context. The method avoids many of the distortions and practical difficulties associated with other TPC analytical techniques. Computer generated TPC and MPC data are used to evaluate the transform technique in comparison with a ‘direct’ interpretive method for model systems of discrete and distributed states.

An experimental evaluation of transient and modulated photocurrent density-of-states spectroscopies

Abstract An evaluation of transient photocurrent (TPC) and modulated photocurrent (MPC) spectroscopies as a means of studying the density and capture properties of localized states in amorphous semiconductors is presented. Freauency-domain analysis of TPC data via the discrete Fourier transform (TPC FT) permits a direct comparison with MPC data obtained using conventional lock-in techniaues to be made. Results obtained from undoped hydrogenated amorphous silicon over a wide range of temperatures and optical excitations are used to explore the limits of resolution and applicability, and to highlight the relative merits, of each approach. It is shown that TPC spectroscopy offers significant practical advantages over MPC spectroscopy from the viewpoint of signal-to-noise performance. Discrepancies between TPC FT and MPC data obtained from the same sample under equivalent conditions suggest that the Fourier transform pairing of these methods is not exact, even when the reauirements of low excitation are met. Capture coefficients of defect states calculated from the temperature dependence of the TPC data and the optical excitation dependence of the MPC data disagree, having values of 7 × 10−9 and 5 × 10−7 cm3 s−1 respectively. Density-of-states profiles agree well for tail states (measured at low temperatures) but are less satisfactory at deeper energies.

Modulated and transient photoconductivity in a-As2 Se3 *

Modulated photocurrent phase shift measurements (MPC) have been used to probe the valence band tail density of states (DOS) in a-As2 Se3. In the absence of obvious structure, use of optical bias to define a ‘thermalisation energy limit’ allows MPC to be used to determine the attempt-to-escape frequency. Computer modelling of steady state and transient photoconductivity reveals inconsistencies in a charged defect interpretation.

Time and frequency domain studies of photoconductivity in amorphous semiconductors

Abstract We present a general spectroscopic technique for the computation of the distribution of gap-states (DOS) in amorphous semiconductors from transient photocurrent decay (TPC). The technique assumes trap-limited and is otherwise model-independent. It is valid whether the TPC exhibits anomalous or conventional dispersion, and also works without modification for pre- and post-recombination regions of the decay. A numerical Fourier integral procedure is used to convert the TPC i ( t ) data to frequency domain spectra I ( ω ). The DOS is then computed using a procedure developed by the authors [1] for analysis of modulated photocurrent (MPC) data. The method avoids distortions and computational difficulties associated with other TPC analytical techniques. We report on the application of the method to experimental data on a-Si:H, demonstrating the wide energy range of states accessed, and highlighting the observation that the observed long-time power-law TPC decay, normally associated with a featureless exponential state distribution is consistent with structure in the DOS.

Sublinear photoconductivity in n-type a-Si:H — analysis and computer modelling

Abstract In this paper an approach is suggested to examine photoconductivity in n-type a-Si:H, in which band tails are required to play an important role as reservoirs of trapped charge but do not provide a significant recombination path. The model arises from observations of steady state photoconductivity in n-type a-Si:H which reveal sublinear behaviour of photocurrent δI extending several orders of magnitude below the ‘dark’ current IO. This is unexpected behaviour, since most models predict linearity for small-signal conditions. Computer simulations using the suggested model reproduce this behaviour and also fit the observed temperature dependence of the index, γ.

A comparative study of photoconductivity and carrier transport in oligomeric films

Abstract DC conductivity and transient photoconductivity (TPC) measurements on 1 μm films of pentacene and thienylene-vinylene oligomers deposited by thermal evaporation (TE) and by laser ablation are reported. Conduction properties are found to be relatively insensitive to both chemical composition and deposition method. DC conductivity prefactor magnitudes of order 0.1 S cm−1, and the observation of field-dependent conduction, are indicative of hopping transport. In all formulations we observed a two-power-law photocurrent decay, consistent with transport initially by hopping down in energy in an exponential distribution of localised states followed by thermal excitation and multiple-trapping (MT). The temperature-dependence of photocurrent decay has been studied for pentacene and yields an exponential tail slope of 650 K (56 meV). A maximum mobility of 10−5 cm2 V−1 s−1 is estimated from the TPC data, 2–3 orders less than field-effect mobilities reported previously. This discrepancy may be due to differences between surface and interior conduction processes.

Investigation of collection efficiencies much larger than unity in a-Si:H p-i-n structures

An apparent quantum efficiency much larger than unity is observed under reverse bias voltage conditions, in hydrogenated amorphous silicon p-i-n structures. High collection efficiencies are measured for low-level probe illumination of the device n side with red light, with simultaneous bias illumination from the device p side with strongly absorbed blue light. The photogating effect responsible varies experimentally with reverse bias voltage, and collection efficiencies for the probe excitation of up to 50 are obtained. Detailed computer simulations corroborate such high values of quantum efficiency and the underlying mechanisms for the effect are revealed. We present the influence on quantum efficiency of bias light wavelength and photon flux, probe light photon flux, applied voltage, and defect density.

Optical sensing of polarization using conical diffraction phenomenon

A KGd(WO4)2 crystal has been used as the central piece of a prototype novel optical sensor to identify the plane of polarization of a beam of light. The device identified the plane of polarization of the incident beam of light and was found to be accurate to within ±4°, comparable to the common polarizer-analyzer based setup in use in many labs. In order to test a practical application, a polarimetry test was carried out to identify the angle of rotation caused by optically active liquids, showing excellent agreement between the expected values and those measured using the conical diffraction phenomenon. The prototype provides an easy to use tool for fast and effective analysis of linear polarization for practical applications.

Carrier mobility, band tails and defects in microcrystalline silicon

The development of microcrystalline silicon thin films and devices is briefly reviewed. Transport mechanisms, and the attendant key parameters of carrier mobility, band-tail width and defect density, are linked to film structure and composition. In particular we discuss the wide (but systematic) variations in time-of-flight mobility and its unusual field-dependence. While microcrystalline silicon remains an inferior semiconductor to single-crystal silicon, we propose, and support by means of a computer model, that present device-grade material may be of sufficient quality to justify re-examining whether useful thin-film bipolar devices might be developed. These could find application as more sensitive photo-detectors, and as current drivers in organic LED displays and logic circuits.

Transient photoconductivity, density of tail states and doping effect in amorphous silicon

The Transient Photoconductivity (TPC) is studied in the pre-recombination time range on the basis of a Multiple Trapping (MT) model which uses a transient occupation function to account for simultaneous interactions of all the tail states with the conduction band. A direct inversion method for the extraction of the Density Of States (DOS) from the TPC data in amorphous semiconductors is derived. Application of this transient spectroscopy to n-type a-Si:H results in a very narrow band tail (tail width of 14 meV) starting around 0.16 eV below the mobility edge and leading to low DOS. This finding is in favour of the defect pool concept, and involves some doping effects at the donor level.