Onset of uncontrolled polytypism during the Au-catalyzed growth of wurtzite GaAs nanowires

Letter

Onset of uncontrolled polytypism during the Au-catalyzed growth of wurtzite GaAs nanowires

Wouter H. J. Peeters, Marco Vettori, Elham M. T. Fadaly, Alexandre Danescu, Chenyang Mao, Marcel A. Verheijen, and Erik P. A. M. Bakkers

Phys. Rev. Materials 8, L020401 – Published 27 February 2024

Abstract

The optoelectronic properties of a semiconductor are determined by the combination of its elemental composition and the crystal structure. The vapor-liquid-solid nanowire growth mechanism offers the controlled epitaxy of the zinc-blende and wurtzite polytype for a number of semiconductors. Long, thin, and phase-pure wurtzite GaAs nanowires are desirable as epitaxial templates for the growth of hexagonal SiGe shells, but the growth of such nanowires remains a challenge. Here, we study the growth of wurtzite GaAs nanowires and find a diameter dependent critical length beyond which the crystal phase becomes mixed. The onset of uncontrolled polytypism is modeled with a small contribution of As diffusion during growth. Due to this increased supply of As during prolonged growth, Ga is depleted from the liquid catalyst, thereby decreasing the contact angle. We investigate possible pathways of As through diffusion on the facets and edges of the nanowire, and from the scaling of the critical length we deduce that edge diffusion has an important contribution. This study offers new insights for realizing long, phase-pure wurtzite GaAs nanowires with high aspect ratio.

Received 7 November 2023
Accepted 19 January 2024

DOI:https://doi.org/10.1103/PhysRevMaterials.8.L020401

Physics Subject Headings (PhySH)

Crystal defects Crystal growth

Nanowires

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Wouter H. J. Peeters ^1,*, Marco Vettori^1,*, Elham M. T. Fadaly ^1,*, Alexandre Danescu ^2,*, Chenyang Mao¹, Marcel A. Verheijen ^1,3, and Erik P. A. M. Bakkers ^1,†

¹Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
²University of Lyon, Lyon Institute of Nanotechnologies, UMR CNRS-5270, Ecole Centrale de Lyon, 69130 Ecully, France
³Eurofins Materials Science Netherlands BV, 5656 AE Eindhoven, The Netherlands

^*These authors contributed equally to this work.
^†e.p.a.m.bakkers@tue.nl

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Issue

Vol. 8, Iss. 2 — February 2024

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Images

Figure 1
(a), (b) SEM images of NW arrays with the same diameter ( $D = 80$ nm), grown for one hour or five hours, respectively. The right panel shows an enlarged view of a single NW from both panels. The colored boxes correspond to regions studied in TEM. (c), (d) Bright field TEM images in the $[11 \bar{2} 0]$ zone axis. Short WZ NWs show no tapering, a WZ crystal phase, and a contact angle of approximately $90^{\circ}$ . In contrast, long NWs show a mixed phase at the top. The contact angle is larger than $90^{\circ}$ related to to a shrinking NW diameter at the top.
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Figure 2
(a) Schematic representation of the approach to determine the critical length. First, the Au droplet is chemically etched from the GaAs, after which a Hex ${Si}_{0.2} {Ge}_{0.8}$ shell is deposited. The shell is thicker on the mixed-phase part due to an increased growth rate. (b) Bright field TEM images around the transition from WZ to mixed-phase core. The NW has the hexagonal crystal structure with I3 defects in the shell below the critical length (yellow). Both core and shell are mixed-phase above the critical length (red). Narrow bands of hexagonal stacking are visible where the diameter increases (blue). (c) SEM images of NWs after deposition of a Hex ${Si}_{0.2} {Ge}_{0.8}$ shell; the measured critical length based on the shell thickness is indicated.
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Figure 3
Length of the wurtzite segment in the GaAs NWs, measured by SEM. The wurtzite length increases with the diameter of the GaAs NWs (closed circles). No transition toward mixed phase was observed above a diameter of 120 nm for the longest growth time probed (open circles). Vertical error bars represent the standard deviation of length between different NWs. The data is fitted with a model containing only facet diffusion (green line), only edge diffusion (orange line), or a combination of facet and edge diffusion (purple line). Combinations of diameter and length where the NWs are WZ or mixed phase are highlighted in blue and red, respectively. The dashed region highlights the disagreement between the models.
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Figure 4
Proposed time evolution of the NW growth until the critical length. (a) Au-Ga eutectic with initial contact angle $θ_{0}$ on the GaAs substrate. Axial growth is due to direct impingement of Ga and As precursors on the droplet (blue arrows). (b) Supply of As through facet and edge diffusion decreases the catalyst volume (black dashed lines) compared to the initial catalyst (red dashed lines). Thus, $θ < θ_{0}$ . (c) The NW has reached the the critical length $L_{cr}$ . Arsenic diffusion resulted in a total decrease of the catalyst volume $Δ V$ due to a consumption of Ga. Consequently, the contact angle decreased to $θ_{cr}$ . (d) Growth beyond the critical length. The contact angle is too large, leading to nucleation of mixed-phase GaAs.
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