Transient Photocurrents in Microcrystalline Silicon Films

Abstract

Transient photoconductivity in microcrystalline silicon thin films prepared by VHF PECVD over a range of [silane]: [silane + hydrogen] concentrations γ from 3 to 6.3% has been measured as a function of temperature (210 K - 390 K) and laser pulse density (1013 – 1016 cm3). The behavior of highly crystalline films (γ ∼ 3%) bears certain similarities to n-type amorphous silicon, where the power law decay at long times is controlled by hole rather than electron kinetics. As γ approaches 6% the decay begins to resemble that of intrinsic amorphous silicon, consistent with structural studies which indicate a transition from crystalline to amorphous character. Transient photocurrent density of states spectroscopy on transitional films suggests a similar deep defect density to high-quality amorphous silicon, but a larger conduction band tail energy of some 30 meV and a tenfold increase in localised state density at a depth of about 0.4 eV.

Aging effects in microcrystalline silicon films studied by transient photoconductivity

“…The effects of the 10-day aging cycle are still reversible after 6 months and as before may be accounted for by electrostatic effects. DOS curves showing similar behaviour have previously been reported for films spanning a range of crystallinities and dark conductivities [10,14].…”

Hole Drift-Mobility Measurements and Multiple-Trapping in Microcrystalline Silicon

“…The mobility-edge has been widely applied to amorphous semiconductors [7,15], and has recently been applied to microcrystalline samples with a larger fraction of amorphous "tissue" [16]. Here we are suggesting that it be applied to samples that are predominantly crystalline.…”

“…The conduction bandtail in amorphous silicon has a width around 22 meV [7,15]. Electron properties in samples quite similar to the present ones have been studied using post-transit time-of-flight [16]; bandtail multiple-trapping did not apply for these transients. An interesting possibility is therefore that electron transport may be governed by effective masses in exactly the same material for which holes require a mobility-edge approach.…”

Numerical modelling of transient photoconductivity for density‐of‐states determination in microcrystalline silicon

“…Main et al [12] and Reynolds et al [6] suggested that band-tail states only act as traps for free carriers and that capture into the band-tail attached to the opposite band is prohibited or very unlikely. I.e., free electrons can be captured by the conduction band tail (and dangling bonds) but not or with small probability by the trapped holes in the valence-band tail.…”

“…Transient and modulated photoconductivity have become an informative tool for determining the density-of-states profile in hydrogenated amorphous silicon, and more recently, in microcrystalline silicon [1][2][3][4][5][6]. For this purpose these experiments are usually performed in co-planar electrode configuration.…”