Ultrafast carrier dynamics in undoped microcrystalline silicon

Abstract

We have studied ultrafast dynamics of photoexcited carriers in mc-Si:H by pump and probe laser spectroscopy. We have found that the dynamics of photoexcited carriers in mc-Si:H depend on the crystallinity of the material: in the samples with low crystalline fraction, the dynamics have a fast decay and resemble those in a-Si:H, while in the samples with high crystallinity the dynamics are slower and similar to those in c-Si. We have identified an intensity dependent bimolecular recombination in the samples with lower crystalline fraction (coefficient B = 2× 10 − 10 cm 3 s − 1 for deposition with silane dilution ratio :5% at a fixed power of 6 W), and no bimolecular recombination in the samples with high crystallinity.

Ultrafast carrier trapping in microcrystalline silicon observed in optical pump–terahertz probe measurements

“…Variation of the wavelength, i.e., absorption length within the sample, indicates that the c-Si:H film exhibits depth dependent electronic properties which are manifest in the measured THz transmission transients. Interestingly, the dynamics that we observe within the first picoseconds after excitation was not observed in earlier pump-probe measurements utilizing visible and near-IR pulses on c-Si:H. 6 While differences in the measured samples cannot be excluded as a reason for the differences, pump-probe measurements with visible/near-IR pulses rely on electronic excitation of carriers from any given state in order to induce an absorption change. Hence, it is not expected a priori that it is possible to distinguish between carriers at the conduction-band minimum from carriers in trap states in the band gap.…”

“…Very few results are found in the literature that treat the dynamics of carriers in c-Si:H on a subpicosecond time scale. 6 c-Si:H is a heterogeneous material consisting of small crystallites with sizes in the range of 10-15 nm. These nanocrystals form columns which have a diameter of, typically, 100-200 nm, and tend to grow perpendicularly to the surface.…”

Carrier dynamics in microcrystalline silicon studied by time-resolved terahertz spectroscopy

“…(i) We observe a significant increase of the amplitude of the slow component for microcrystalline samples as compared to the amorphous one. (ii) In agreement with optical pump-optical probe measurements [7] the amorphous sample exhibits a bimolecular recombination (hyperbolic decay) while in the microcrystalline samples the decay is exponential and significantly slower.…”

Interplay of bimolecular and Auger recombination in photoexcited carrier dynamics in silicon nanocrystal/silicon dioxide superlattices

“…6 we plot recombination coefficients C and B for every sample. These constants are of the same order as those in similar semiconductor systems 22 – 24 and are both increasing with increasing x. The C coefficient grows much faster over two orders of magnitude.…”