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Absence of redshift in the direct bandgap of silicon nanocrystals with reduced size

Nature Nanotechnology volume 12, pages 930–932 (2017)Cite this article

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To the Editor — Silicon is an indirect-bandgap semiconductor and thus an inefficient light emitter, a fact that has posed a serious impediment to the long-standing dream of integrating Si electronics with Si photonics into a combined dual-functional monolithic platform1,2. An encouraging experiment that held promise of creating a breakthrough for large-scale integrated complementary metal-oxide–semiconductor-based optoelectronics was published in this journal by de Boer et al.3, who observed, in an ensemble of Si nanocrystals, a high-energy direct transition that rapidly lowered its energy (redshifted) with decreasing nanocrystal size, projected to lead at sufficiently small sizes to a Si nanocrystal with a truly direct gap. The authors observed a hot photoluminescence band and wrote, “...we assign this band to no-phonon hot carrier radiative recombination at levels in the vicinity of the Г-point in the Brillouin zone, with a red spectral shift that can be regarded as a reduction of the direct bandgap energy due to quantum confinement.” They added, in discussing the hot photoluminescence, “This will induce enhancement of PL [photoluminescence] intensity due to radiative recombination via the direct channel.” Kovalev, writing in the News and Views of the same issue4, shared the opinion that this was excellent news for the Si optoelectronic community: “Can silicon ever be a true direct-bandgap semiconductor? The first observation of a new, short-lived photoluminescence band from silicon nanocrystals offers fresh hope.”

Because of the great difficulty of performing a single-dot spectroscopy experiment in an indirect-gap material, at that time de Boer et al. had to perform an ensemble experiment in which they observed many different sizes3. Also, the theoretical calculations had to be done at the time by deducing the nanocrystal energy levels from the simplified effective mass approach5 rather than by the more accurate (but demanding) method of considering a finite nanocrystal as a giant molecule in its own right6,7. We have recently overcome both difficulties and were able to combine our well-tested6,8,9,10 atomistic pseudopotential theory of single passivated Si nanocrystals7,11, including excitonic effects, with our newly developed single-dot absorption spectroscopy to reveal the origin of the redshifted transition in Si nanocrystals. These advanced theoretical and experimental methods enabled us to question the main points of the original paper3, as we found no significant redshifted direct-bandgap transitions in Si nanocrystals that could be attributed to the effects of decreasing size at the nanoscale.

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Figure 1: Photoluminescence of bulk and nanocrystalline silicon.

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Acknowledgements

J.-W.L. was supported by the National Young 1000 Talents Plan and the National Science Foundation of China (NSFC grant 61474116). A.Z. was supported by the US Department of Energy grant DE-FG02-13ER46959 to the University of Colorado, Boulder. I.S., F.P. and J.L. acknowledge support from the Swedish Research Council (VR) through an individual contract and through a Linné grant (ADOPT), and from the Göran Gustafssons Foundation.

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Authors and Affiliations

  1. State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, PO Box 912, Beijing, 100083, China

    Jun-Wei Luo & Shu-Shen Li

  2. Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, 230026, Anhui, China

    Jun-Wei Luo & Shu-Shen Li

  3. Materials and Nano Physics Department, KTH — Royal Institute of Technology, Kista, 16440, Stockholm, Sweden

    Ilya Sychugov, Federico Pevere & Jan Linnros

  4. Renewable and Sustainable Energy Institute, University of Colorado, Boulder, 80309, Colorado, USA

    Alex Zunger

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  1. Jun-Wei Luo
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Correspondence to Jun-Wei Luo or Alex Zunger.

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Luo, JW., Li, SS., Sychugov, I. et al. Absence of redshift in the direct bandgap of silicon nanocrystals with reduced size. Nature Nanotech 12, 930–932 (2017). https://doi.org/10.1038/nnano.2017.190

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