E. Snow

Study of the electronic structure of amorphous silicon using reverse‐recovery techniques

We have carried out a series of reverse‐recovery experiments on pin diodes of amorphous‐Si:F:H of thicknesses up to 3.5 μm under pulsed high‐level injection conditions. No evidence for charge storage was obtained. Our results indicate that the room‐temperature band mobility for electrons moving near the mobility edge in a‐Si:F:H is larger than 100 cm2/Vs. In addition, our data suggest that either an intrinsic or induced gap exists in the density of localized states below the conduction‐band mobility edge, in surprising contrast with conclusions deduced from a variety of recent experiments.

Extended-state mobility in hydrogenated amorphous silicon

Abstract We use the results of time-of-flight experiments in conjunction with recent conclusions about the behavior of the density of localized states below the conduction-band mobility edge to calculate the mobility of electrons moving in extended states in a-Si:H. We find that the extended-state mobility is considerably larger than previous estimates, which were based on the assumption that the exponential behavior responsible for dispersive transport extends all the way to the mobility edge. Using a recent estimate for the density of localized states, we find that the extended state mobility in a-Si:H is about 500 cm2/V-s, a value consistent with the results deduced from high-level injection experiments on p-i-n structures.

Transient space charge limited currents in a-Si:H

Abstract Transient space charge limited currents in a-Si:H films have been calculated including recent estimates of the density of localized states. The multiple trapping model for an exponential density of states predicts a decay of the current before a transit time proportional to tα−1 × [1 − K tα]−2 where K is a constant depending upon the escape frequency from a trap, the free carrier transit time and α = T/T0 The space charge limited transit time is (.78)1/α times less than the space charge free transit time. After a few transit times, the current decays as tα−1 until the final steady state space charge limited current is reached. For a pulse excitation of CV the current is proportional to tα−1 × exp(βtα) where β depends upon α and the free carrier transit time. After a space charge transit time the current decays as t−α and then as t−(α+1).

Transient photodecay measurements as a probe of the density of localized states in amorphous semiconductors

Abstract We show that structure in the density of localized states of an amorphous semiconductor beyond the apparently ubiquitous exponential band tails yield deviations from the usual power-law decay of the photocurrent, i(t) ∝ tα−1, which can be described analytically. With our expression, dispersive-transport results can be deconvoluted to provide a spectroscopy of the localized-state distribution when well-defined defect centers and band tails are simultaneously present.

Comparison of fast transient response between crystal and amorphous silicon pin photodiodes

Abstract We report on a comparison between the fast transient response of crystalline and amorphous silicon pin photodiodes. We studied time of flight, forward bias transients and reverse recovery and find that the response of the crystalline and amorphous devices are qualitatively the same except that charge storage after forward bias is more pronounced in the crystal than in the amorphous material.

Calculation of the extended-state electron mobility in hydrogenated amorphous silicon

Abstract In a recent paper, Marshall and his collaborators conclude that the extended-state electron mobility in hydrogenated amorphous silicon is 10–20 cm2/V-s. This deduction is based on experimental results indicating the demarcation level does not exceed 0.14 eV, in disagreement with conventional multiple-trapping theory. In this paper, we show that the data can be understood without modification of the existing theory, and reiterate that the observed drift mobility and density of localized states require an extended-state electron mobility in excess of 100 cm2/V-s.

Transient single-carrier injection currents in a-Si:H

Abstract Single-carrier transient injection currents are analyzed and determined to be electrode limited. Consequently, the information on the bulk properties, available with space-charge-limited currents, is lost. A method is presented which recovers this information in the form of time-dependent drift mobility data.

Transient electronic response in hydrogenated amorphous silicon

We analyze the problem of the transient response after a photogeneration pulse in a time‐of‐flight geometry in a material in which the density of localized states is characterized by a weakly decaying exponential just below the mobility edge and a more rapidly decaying exponential at lower energies. We find that it is then possible to observe essentially nondispersive transport over wide ranges of time and temperature, due to the finite trapping times to some of the deeper states. With this approach, we are able to understand some recent nondispersive time‐of‐flight data on films of hydrogenated amorphous silicon, provided that the band mobility of electrons is large (≊100 cm2/V s), ballistic transport predominates, and the trapping cross section is small (∼1016 cm2).

Geminate recombination and mobility in a-Si alloys

Abstract Geminate recombination in a-Si:H has not been observed at room temperature. This may result from a very slow final recombination step, large random fields or a large extended state mobility. In this paper, we report on the results and implications of Monte-Carlo simulations considering all three possibilities. We found: (1) a slow final step recombination predicts an exponential decay of the luminescence at intermediate temperatures which is not observed experimentally and (2) random short range fields do not affect the geminate yield but only alter the time dependence. We conclude that in the absence of other results, that a high extended state mobility is the most feasible explanation for the room temperature results on geminate recombination.

What is the majority carrier drift mobility in a-Si alloys?

Abstract In this work we have tested the accepted value of 1 cm 2 /V s for the majority carrier mobility in a-Si:F:H by studying the transient response of a p-i-n junction to forward bias and by the reverse recovery technique. The transit time of the carriers in a 3.5 μm cell under 2 V bias was less than 10 −9 s. We therefore estimate that the previous values are incorrect and the majority carrier mobility is in fact significantly greater than 60 cm 2 /V s.