Multiple radial phosphorus segregations in GaAsP core-shell nanowires
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partment of Physics, University of Warwick, Coventry CV4 7AL, UK Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, UK
© The Author(s) 2020 Received: 9 July 2020 / Revised: 13 August 2020 / Accepted: 14 August 2020
ABSTRACT Highly faceted geometries such as nanowires are prone to form self-formed features, especially those that are driven by segregation. Understanding these features is important in preventing their formation, understanding their effects on nanowire properties, or engineering them for applications. Single elemental segregation lines that run along the radii of the hexagonal cross-section have been a common observation in alloy semiconductor nanowires. Here, in GaAsP nanowires, two additional P rich bands are formed on either side of the primary band, resulting in a total of three segregation bands in the vicinity of three of the alternating radii. These bands are less intense than the primary band and their formation can be attributed to the inclined nanofacets that form in the vicinity of the vertices. The formation of the secondary bands requires a higher composition of P in the shell, and to be grown under conditions that increase the diffusivity difference between As and P. Furthermore, it is observed that the primary band can split into two narrow and parallel bands. This can take place in all six radii, making the cross sections to have up to a maximum of 18 radial segregation bands. With controlled growth, these features could be exploited to assemble multiple different quantum structures in a new dimension (circumferential direction) within nanowires.
KEYWORDS compound semiconductor alloys, radial segregations, three-fold symmetry, surface chemical potential
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Introduction
Epitaxial growth may deviate from its simple form due to growth rate anisotropy, strain, entropy of mixing in alloys and capillarity, forming unintended self-formed features [1, 2]. Nanowires, which themselves are a result of growth rate anisotropy, are particularly prone to develop such self-formed features due to coexistence of multiple growth facets. Different types of such nanowire features have been reported, including unintentional core–shell structures [3, 4], radial elemental segregations [5–12], alloy fluctuations [13, 14], longitudinal wires that form along the vertical edges [15–17] and pyramidical elemental segregations [18, 19]. Most of these exhibit optical properties consistent with self-formed passivation layers [3] or quantum structures such as quantum dots [13, 18, 19], wires [15, 16], and rings [20, 21]. While some of these could be detrimental by giving rise to unintentional emission or acting as carrier traps, others such as the pyramidical elemental segregations have been reported to show superior optical properties, far exceeding that of those that are intentionally grown [18, 19]. On the other hand, self-assembly has long been used as a means of assembling structures in the nanoscale with a level of precision and ease that may not otherwise have been
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