Electric-Field-Assisted Phase Switching in GaAs Nanowires

The vapor-liquid-solid (VLS) method for nanowires growth was introduced by Wagner and Ellis in the 1960s¹. Since then, intensive researches focus on engineering the morphology, composition and crystal phase of nanowires due to their promising applications in nanotechnology². While III-V bulk semiconductors generally exist in only one crystal phase, in nanowires, multiple crystal phases can coexist (polytypism). On the one hand, this phenomenon can be considered as a crystal defect; on the other hand, it enables the formation of crystal phase quantum dots (CPQDs), i.e. insertion of segments of one phase within a nanowire of a different phase. Due to the band alignment of two crystal phases and confinement of nanowire size, CPQDs feature sharp and intense spectral lines, single-photon emission and sub-nanosecond exciton lifetime³. Moreover, in contrast to compositional heterojunctions, CPQDs have intrinsically abrupt interfaces and do not suffer from alloy intermixing, which hampers precise control of the electronic properties in compositional heterostructures. However, despite over 15-year of research, technological applications of CPQDs remain severely limited by the poor understanding of the phase switching mechanisms and the difficulty of controlling their formation. In this study, beyond commonly used methods of flux and temperature modulation, we solve this issue by introducing an external electric field (E-field)⁴ to instantaneously switch the crystal phase and create GaAs CPQDs with monolayer precision. This process is monitored in real-time using in-situ transmission electron microscopy (TEM). Thanks to custom-made substrates, GaAs nanowires are grown epitaxially on Si (111) by chemical vapor deposition inside TEM. The substrate is shaped as a micro capacitor which allows us to apply an E-filed up to several V/nm in the direction parallel to the nanowire growth, and this strong E-field allows to achieve phase switching faster than monolayer formation. We will present high-resolution videos showing the controlled phase switching induced by the E-field in GaAs nanowires, and the formation of single and multiple CPQDs. Finally, we will discuss the E-field-induced phase switching mechanisms and propose a model to explain the experimental results based on theoretical calculations and finite element simulations.<br/><br/><b>References:</b><br/>(1) Wagner, R. S.; Ellis, W. C. Vapor-liquid-solid Mechanism of Single Crystal Growth. <i>Appl. Phys. Lett.</i> <b>1964</b>, <i>4</i> (5), 89–90. https://doi.org/10.1063/1.1753975.<br/>(2) Bouwes Bavinck, M.; Jöns, K. D.; Zielinski, M.; Patriarche, G.; Harmand, J.-C.; Akopian, N.; Zwiller, V. Photon Cascade from a Single Crystal Phase Nanowire Quantum Dot. <i>Nano Lett.</i> <b>2016</b>, <i>16</i> (2), 1081–1085. https://doi.org/10.1021/acs.nanolett.5b04217.<br/>(3) Akopian, N.; Patriarche, G.; Liu, L.; Harmand, J.-C.; Zwiller, V. Crystal Phase Quantum Dots. <i>Nano Lett.</i> <b>2010</b>, <i>10</i> (4), 1198–1201. https://doi.org/10.1021/nl903534n.<br/>(4) Panciera, F.; Norton, M. M.; Alam, S. B.; Hofmann, S.; Mølhave, K.; Ross, F. M. Controlling Nanowire Growth through Electric Field-Induced Deformation of the Catalyst Droplet. <i>Nat. Commun.</i> <b>2016</b>, <i>7</i> (1), 12271. https://doi.org/10.1038/ncomms12271.