The influence of dangling-bond defects and the position of the Fermi level on the charge carrier transport properties in undoped and phosphorous doped thin-film silicon with structure compositions all the way from highly crystalline to amorphous was investigated. The dangling-bond density was varied reproducibly over several orders of magnitude by electron bombardment and subsequent annealing. The defects were investigated by electron spin resonance and photoconductivity spectroscopy. Comparing intrinsic amorphous and microcrystalline silicon, it was found that the relationship between defect density and photoconductivity was different in both undoped materials, while a similar strong influence of the position of the Fermi level on photoconductivity via the charge carrier lifetime was found in the doped materials. The latter allowed a quantitative determination of the value of the transport gap energy in microcrystalline silicon. The photoconductivity in intrinsic microcrystalline silicon is, on one hand, considerably less affected by the bombardment but, on the other hand, does not generally recover with annealing of the defects and was independent from the spin density which itself could be annealed back to the as-deposited level. For amorphous silicon and material prepared close to the crystalline growth regime, the results for non-equilibrium transport fit perfectly to a recombination model based on direct capture into neutral dangling bonds over a wide range of defect densities. For the heterogeneous microcrystalline silicon, this model failed completely. The application of photoconductivity spectroscopy in the constant photocurrent mode was explored for the entire structure composition range over a wide variation in defect densities. For amorphous silicon previously reported linear correlation between the spin density and the sub-gap absorption was confirmed for defect densities below 1018/cm3. Beyond this defect level, a sub-linear relation was found i.e., not all spin-detected defects were also visible in the constant photocurrent mode spectra. Finally, the evaluation of constant photocurrent mode spectra in defect-rich microcrystalline silicon showed complete absence of any correlation between spin-detected defects and sub-bandgap absorption determined from constant photocurrent mode: a result which casts considerable doubt on the usefulness of this technique for the determination of defect densities in microcrystalline silicon. The result could be related to the inhomogeneous structure of microcrystalline silicon with its consequences on transport and recombination processes.

Relationship between Defect Density and Charge Carrier Transport in Amorphous and Microcrystalline Silicon. O.Astakhov, R.Carius, F.Finger, Y.Petrusenko, V.Borysenko, D.Barankov: Physical Review B, 2009, 79[10], 104205