Relationship between defect density and charge carrier transport in amorphous and microcrystalline silicon

Abstract

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 is investigated. The dangling-bond density is varied reproducibly over several orders of magnitude by electron bombardment and subsequent annealing. The defects are investigated by electron-spin-resonance and photoconductivity spectroscopies. Comparing intrinsic amorphous and microcrystalline silicon, it is found that the relationship between defect density and photoconductivity is different in both undoped materials, while a similar strong influence of the position of the Fermi level on photoconductivity via the charge carrier lifetime is found in the doped materials. The latter allows 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 is independent from the spin density which itself can be annealed back to the as-deposited level. For amorphous silicon and material prepared close to the crystalline growth regime, the results for nonequilibrium 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 fails completely. The application of photoconductivity spectroscopy in the constant photocurrent mode ͑CPM͒ is 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 subgap absorption is confirmed for defect densities below 10 18 cm −3 . Beyond this defect level, a sublinear relation is found i.e., not all spin-detected defects are also visible in the CPM spectra. Finally, the evaluation of CPM spectra in defect-rich microcrystalline silicon shows complete absence of any correlation between spindetected defects and subband gap absorption determined from CPM: a result which casts considerable doubt on the usefulness of this technique for the determination of defect densities in microcrystalline silicon. The result can be related to the inhomogeneous structure of microcrystalline silicon with its consequences on transport and recombination processes.

Variation of the Fermi level in n‐type microcrystalline silicon by electron bombardment and successive annealing: ESR and conductivity studies

“…In the samples of low doping (≤10 ppm) the electron bombardment was sufficient to reduce the dark conductivity close to the intrinsic level [15]. Clear correlation between N S CE and σ dark is observed over 4 orders of magnitude of conductivity in individual samples and 6 orders of magnitude for the total σ dark range covered.…”

“…The defect density in µc-Si:H is varied by 2 orders of magnitude [12][13][14][15] which leads to a correspondent change in the donor states occupation and thus to the shift of the Fermi level.…”

“…The samples were stepwise annealed in vacuum (10 -5 hPa or better) at temperatures 50, 80, 120 and 160 °C for 30 minutes each and measured after each annealing step. For more detailed description of experiment see [15].…”

Thermopower and Hall‐effect investigations of microcrystalline silicon films

“…Gas mixtures of hydrogen and silane and phosphine were used and the silane concentration (SC) was varied from 2% to 8% [6,7]. The samples presented here were all deposited with 10 ppm phosphine doping but for 1 ppm phosphine doping, quite similar results were obtained.…”

Variation of the defect density in a-Si:H and μc-Si:H based solar cells with 2 MeV electron bombardment

“…Irradiation with high energy charged particles is a more convenient way to vary the defect density in a given device, and application of protons or electrons with energy between hundreds of keV to several MeV has been demonstrated [7][8][9][10][11][12][13]. However previous studies were mostly focused either on the feasibility of space application of solar cells or on the properties of the absorber material itself.…”