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. 2018 Sep 13;9:3718. doi: 10.1038/s41467-018-05917-8

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© The Author(s) 2018

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 3 — Doping dependence of the PL emission of neutral and charged biexcitons. a Gate dependence map of the PL intensity above the onset for observing both biexciton species (I₀ = 4 × 10³ W cm⁻²). The intrinsic regime is characterized by the strong emission from X^D. In the n-doped and p-doped regimes, trions and other species emerge. The XX peak at 1.703 eV is only observed in the intrinsic regime, indicating a neutral biexciton composed of a neutral X⁰ and X^D. Another peak with superlinear power dependence is observed at 1.671 eV, 52 meV below X⁰. It is assigned to a charged biexciton state, as it appears only in the low n-doped regime. b Normalized PL intensity of the exciton species relevant for the formation of biexcitons as a function of doping level. The neutral biexciton (green) emission is limited to the intrinsic regime, in which X^D (dark blue) is observed, showing the important role of X^D in the formation of XX. The XX⁻ emission (dark red) is strongest in the low n-doped regime, where X⁰ (light blue), X^D and the two X⁻ species coexist (light red, only lower X⁻ shown for clarity)