Record Pure Zincblende Phase in GaAs Nanowires down to 5 nm in Radius Evelyne Gil, †,‡, * Vladimir G. Dubrovskii, §,∥,⊥ Geoffrey Avit, †,‡ Yamina Andre ́ , †,‡ Christine Leroux, #,¶ Kaddour Lekhal, †,‡ Jurij Grecenkov, § Agne ̀ s Trassoudaine, †,‡,□ Dominique Castelluci, †,‡ Guillaume Monier, †,‡ Reda M. Ramdani, †,‡ Christine Robert-Goumet, †,‡ Luc Bideux, †,‡ Jean Christophe Harmand, ■ and Frank Glas ■ † Clermont Universite ́ , Universite ́ Blaise Pascal, Institut Pascal, BP 10448, F-63000 Clermont-Ferrand, France ‡ CNRS, UMR 6602, IP, F-63177 Aubie ̀ re, France § St. Petersburg Academic University, Khlopina 8/3, 194021 St. Petersburg, Russia ∥ Ioffe Physical Technical Institute of the Russian Academy of Sciences, Polytekhnicheskaya 26, 194021 St. Petersburg, Russia ⊥ St. Petersburg State University (Physical Faculty), Ulianovskaya Street 3, Petrodvorets, 198504 St. Petersburg, Russia # Universite ́ de Toulon, IM2NP, Bâ t.R, B.P.20132, 83957 La Garde Cedex, France ¶ CNRS, UMR 7334, 83957 La Garde Cedex, France □ Clermont Universite ́ , Universite ́ d’Auvergne, Institut Pascal, BP 10448, F-63000 Clermont-Ferrand, France ■ CNRS-LPN, Route de Nozay, 91460 Marcoussis, France ABSTRACT: We report the Au catalyst-assisted synthesis of 20 μm long GaAs nanowires by the vapor−liquid−solid hydride vapor phase epitaxy (HVPE) exhibiting a polytypism- free zincblende phase for record radii lower than 15 nm down to 5 nm. HVPE makes use of GaCl gaseous growth precursors at high mass input of which fast dechlorination at the usual process temperature of 715 °C results in high planar growth rate (standard 30−40 μm/h). When it comes to the vapor− liquid−solid growth of nanowires, fast solidification at a rate higher than 100 μm/h is observed. Nanowire growth by HVPE only proceeds by introduction of precursors in the catalyst droplets from the vapor phase. This promotes almost pure axial growth leading to nanowires with a constant cylinder shape over unusual length. The question of the cubic zincblende structure observed in HVPE-grown GaAs nanowires regardless of their radius is at the heart of the paper. We demonstrate that the vapor− liquid−solid growth in our conditions takes place at high liquid chemical potential that originates from very high influxes of both As and Ga. This yields a Ga concentration systematically higher than 0.62 in the Au−Ga−As droplets. The high Ga concentration decreases the surface energy of the droplets, which disables nucleation at the triple phase line thus preventing the formation of wurtzite structure whatever the nanowire radius is. KEYWORDS: Nanowire, GaAs, HVPE, VLS, crystal structure, chemical potential T he III−V high carrier mobility and direct bandgap semiconductor nanowires (NWs) have been extensively studied for fundamental physics and nanoscale electronic, photonic, and sensing device applications. 1−7 First micro- and nanosized wires were synthesized for Si and III−V materials by the vapor−liquid−solid (VLS) growth. 8,9 Continuous efforts have been put forth into catalyst-assisted VLS growth of III-V NWs and most commonly used GaAs NWs for almost 15 years. The processes involved are molecular beam epitaxy (MBE) and metal−organic vapor phase epitaxy (MOVPE), as the most widespread growth techniques for III−V compounds since the 1980s. VLS growth is particularly well mastered nowadays for both of these epitaxial tools and enables growth of complex NW structures. 10−12 Whatever the structure device is, high material crystal quality is required. The control of the crystal phase of III−V NWs has been a challenging task for a while. 13−16 Indeed, GaAs NWs often feature spontaneous zincblende (ZB)-wurtzite (WZ) polytypism and stacking faults can form between alternating WZ and ZB layers along the ⟨111⟩ axis of the NWs. 6,17−20 It is now admitted that the swapping between WZ and ZB sequences is related to surface energy values and crystal-growth conditions. In the Au- catalyzed MOVPE procedures, high V/III ratio and low temperature (T) have been demonstrated to favor predom- inantly ZB NWs, while low V/III and high T are suitable for Received: April 3, 2014 Revised: May 22, 2014 Published: May 29, 2014 Letter pubs.acs.org/NanoLett © 2014 American Chemical Society 3938 dx.doi.org/10.1021/nl501239h | Nano Lett. 2014, 14, 3938−3944