Ultrafast carrier dynamics and the role of grain boundaries in polycrystalline silicon thin films grown by molecular beam epitaxy

From National Research Council Canada

DOIResolve DOI: https://doi.org/10.1088/0268-1242/31/10/105017
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Name affiliation
  1. National Research Council of Canada. Information and Communication Technologies
  2. National Research Council of Canada. Measurement Science and Standards
FormatText, Article
Journal titleSemiconductor Science and Technology
ISSN0268-1242
1361-6641
Volume31
Issue10
Pages105017-1–105017-8
Subjectsilicon thin films; terahertz spectroscopy; grain boundaries; low-temperature MBE; carrier dynamics
Abstract
We have used time-resolved terahertz spectroscopy to study microscopic photoconductivity and ultrafast photoexcited carrier dynamics in thin, pure, non-hydrogenated silicon films grown by molecular beam epitaxy on quartz substrates at temperatures ranging from 335 °C to 572 °C. By controlling the growth temperature, thin silicon films ranging from completely amorphous to polycrystalline with minimal amorphous phase can be achieved. Film morphology, in turn, determines its photoconductive properties: in the amorphous phase, carriers are trapped in bandtail states on sub-picosecond time scales, while the carriers excited in crystalline grains remain free for tens of picoseconds. We also find that in polycrystalline silicon the photoexcited carrier mobility is carrier-density-dependent, with higher carrier densities mitigating the effects of grain boundaries on inter-grain transport. In a film grown at the highest temperature of 572 °C, the morphology changes along the growth direction from polycrystalline with needles of single crystals in the bulk of the film to small crystallites interspersed with amorphous silicon at the top of the film. Depth profiling using different excitation wavelengths shows corresponding differences in the photoconductivity: the photoexcited carrier lifetime and mobility are higher in the first 100–150 nm from the substrate, suggesting that thinner, low-temperature grown polycrystalline silicon films are preferable for photovoltaic applications.
Publication date
PublisherIOP : Institute of Physics
LanguageEnglish
Peer reviewedYes
NPARC number23000872
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