Aluminum-Catalyzed Growth of Silicon Nanowires in High-Energy Growth Directions Mel F. Hainey, Jr.,* ,† Xiaotian Zhang, † Ke Wang, ‡ and Joan M. Redwing* ,†,‡ † Department of Materials Science and Engineering and ‡ Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: Silicon nanowires grown by metal-mediated techniques, such as vapor- liquid-solid growth, typically exhibit a predominant <111> growth direction; however, growth in the <110> and other high-energy directions is also desirable due to their predicted superior transport properties compared to those of <111> wires. In the case of aluminum-catalyzed silicon nanowire growth via chemical vapor deposition (CVD), <110> wire growth has been previously demonstrated; however, the conditions promoting <110> growth over <111> growth are not fully understood. In this report, we demonstrate that variations in precursor partial pressure within the CVD reactor play a significant role in determining the wire growth direction in this process. In the case of growth on Si(110) substrates, the preferential wire growth direction changes from <111> to <110> along the reactor tube length, corresponding to a reduction in the SiH 4 gas-phase concentration due to gas-phase depletion as predicted from computational fluid dynamics simulations. While the change in growth direction occurs without a substantial reduction in the wire growth rate, significant changes occur in the shape of the aluminum-catalyst tip, suggesting a change in growth mechanism arising from possible changes in catalyst supersaturation and/or nanowire sidewall termination. Finally, the identified growth window for <110> wires is used to demonstrate <100> wire growth on Si(100) substrates. KEYWORDS: silicon, nanowire, aluminum, ⟨110⟩ growth direction, CVD, silane, gas-phase chemistry 1. INTRODUCTION Silicon nanowires are attractive materials for nanoscale electronic, 1,2 optoelectronic, 3 sensing, 4 and photovoltaic applications. 5 Considerable efforts have been devoted to understanding the growth of silicon nanowires using the metal-catalyzed vapor-liquid-solid (VLS) process, which commonly produces <111> oriented nanowires. 6-8 Gold is the most common catalyst used but is expensive and acts as a deep-level trap in silicon. 9 In comparison, aluminum is earth- abundant, has a relatively low eutectic temperature with silicon (577 °C), 10 and is a shallow p-type dopant, making it an attractive alternative catalyst material. Initial studies of Al-catalyzed silicon nanowire growth using chemical vapor deposition (CVD) have focused on growth of <111> wires under a variety of conditions, and a growth window of pressure and temperature has been defined under reduced pressure conditions (∼1-700 Torr). 11 Additional research has focused on the effects of hydrogen and precursor partial pressure on nanowire growth and morphology. 12,13 However, silicon nanowire growth in directions such as <110> and <100> is also desirable. Prior theoretical studies have predicted that silicon nanowires grown in the <110> direction will have superior hole mobility relative to <111> or <100> wires 14 and that both <100> and <110> wires are predicted to show increased conductivity relative to <111> wires. 15 Note that increased hole mobility in <110> germanium nanowires relative to <111> wires has been observed experimentally as well. 16 Wires grown in the <100> direction would also allow for direct vertical integration with CMOS electronics. Direct growth of wires in these directions has proven difficult, however, since <111>, the close-packed direction, is the lowest-energy growth direction for silicon. Preferential growth of wires in other directions requires the development of processes that can overcome the energetic differences. We previously demonstrated the Al-catalyzed growth of <110> oriented silicon nanowires on Si(110) substrates using a SiH 4 precursor in H 2 carrier gas, which was found to occur under conditions of high H 2 partial pressures and subeutectic preannealing and growth temperatures. 17 Subeutectic growth conditions were initially believed to be important to limit the Al droplet supersaturation by keeping the droplet in the solid phase. At temperatures at or above the eutectic, the preferential growth direction changed to <111> even for growth from a Si(110) surface. However, this explanation cannot explain several behaviors observed for wire growth under these conditions. The growth rate of the <110> silicon nanowires was ∼200 nm/min, 17 comparable to growth rates for <111> wires at similar SiH 4 partial pressures 11,13 and much faster than any previous reports for growth from a solid catalyst. 18 Additionally, earlier reports on Al-catalyzed nano- wire growth had described that <111> was the preferential growth direction observed at high H 2 partial pressures and 550 °C growth temperature. 11 Thus, further study is needed to Received: June 2, 2018 Accepted: September 28, 2018 Published: September 28, 2018 Article www.acsanm.org Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acsanm.8b00925 ACS Appl. Nano Mater. XXXX, XXX, XXX-XXX Downloaded via 146.185.202.116 on October 13, 2018 at 05:24:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.