Evidence for Structural Phase Transitions Induced by the Triple Phase Line Shift in Self-Catalyzed GaAs Nanowires Xuezhe Yu, † Hailong Wang, † Jun Lu, † Jianhua Zhao,* ,† Jennifer Misuraca, ‡ Peng Xiong, ‡ and Stephan von Molna ́ r ‡ † State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China ‡ Department of Physics, Florida State University, Tallahassee, Florida 32306, United States ABSTRACT: Self-catalyzed growth of GaAs nanowires are widely ascribed to the vapor-liquid-solid (VLS) mechanism due to the presence of Ga particles at the nanowire tips. Here we report synthesis of self-catalyzed GaAs nanowires by molecular-beam epitaxy covering a large growth parameter space. By carefully controlling the Ga flux and its ratio with the As flux, GaAs nanowires without Ga particles and exhibiting a flat growth front are produced. Using scanning electron microscopy and high- resolution transmission electron microscopy, we compare the growth rate and structure, especially near the growth front, of the nanowires with and without Ga droplets. We find that regardless of whether Ga droplets are present on top, the nanowires have a short wurtzite section following the zinc-blende bulk structure. The nanowires without Ga droplets are terminated by a thin zinc- blende cap, while the nanowires with Ga droplets do not have such a cap. The bulk zinc-blende phase is attributed to the Ga droplet wetting the sidewall during growth, pinning the triple phase line on the sidewall. The zinc-blend/wurtzite/(zinc-blende) phase transitions at the end of growth are fully consistent with the triple phase line shifting up to the growth front due to the progressive consumption of the Ga in the droplet by crystallization with As. The results imply an identical VLS growth mechanism for both types of GaAs NWs, and their intricate structures provide detailed comparison with and specific experimental verification of the recently proposed growth mechanism for self-catalyzed III-V semiconductor nanowires (Phy. Rev. Lett. 2011, 106, 125505). Using this mechanism as a guideline, we successfully demonstrated controllable fabrication of two distinct types of axial superlattice GaAs NWs consisting of zinc-blende/defect-section and wurtzite/defect-section units. KEYWORDS: Molecular-beam epitaxy, semiconductor nanowires, nanowire growth kinetics, structural transition of nanowires S elf-assembled semiconductor nanowires (NWs) have attracted extensive interest in the past decade due to their potential as building blocks for future semiconductor electronic, photonic, and sensing devices with multifunctional capabilities and higher performance. 1-3 III-V semiconductor NWs are an important class of the vast semiconductor NW family because of the extensive applications of their bulk counterparts in microelectronics and photonics. So far, most of the III-V NWs investigated are produced via a particle-assisted growth mechanism, whose notable characteristic is the existence of nanoparticles on top of the resulting NWs. 4 These particles are in either a liquid or solid state during growth. In the former case, the growth mechanism is the so- called vapor-liquid-solid (VLS) mechanism 5 and for the latter, it is referred to as the vapor-solid-solid mechanism. 6 In principle, for particle-assisted growth mechanisms the primary function of the particle is to collect source material from the vapor phase, which results in a supersaturated state of source material in the particle and facilitates nucleation and precipitation of the source material at the interface between the particle and the solid NWs. 7 In order to generate such particles, catalytic material is normally needed to form an alloy with the source material whose eutectic temperature is below the melting point of the solid phase of the NWs. At present, there are two methods to form particles which can function as catalysts. One is by using foreign material, among which gold is the most common. The other one utilizes the low melting point of one of the source materials (typically a metal) to create liquid droplets and thus is often called self-catalyzed growth. In some of the prominent examples, Ga droplets for GaAs NWs and In droplets for InAs and InP NWs are created. Because of Received: September 6, 2012 Published: September 17, 2012 Letter pubs.acs.org/NanoLett © 2012 American Chemical Society 5436 dx.doi.org/10.1021/nl303323t | Nano Lett. 2012, 12, 5436-5442