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Au-assisted substrate-faceting for inclined nanowire growth Jung-Hyun Kang, Filip Krizek, Magdalena Anna Za#uska-Kotur, Dr. Peter Krogstrup, Perla Kacman, Haim Beidenkopf, and Hadas Shtrikman Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00853 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018
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Nano Letters
Au-assisted substrate-faceting for inclined nanowire growth Jung-Hyun Kang†*, Filip Krizek‡*, Magdalena Zaluska-Kotur§, Peter Krogstrup‡, Perla Kacman§, Haim Beidenkopf† and Hadas Shtrikman† † Dept. of Condensed Matter Physics, Braun Center for Submicron Research, Weizmann Institute of Science, Rehovot 76100, Israel ‡ Center for Quantum Devices and Station Q Copenhagen, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark § Institute of Physics Polish Academy of Science, Al. Lotnikow 32/46, 02-668 Warsaw, Poland
KEYWORDS: InAs, (001) substrate, MBE, Au droplets, craters, {111}B nano-facets, inclining nanowires.
Abstract We study the role of gold droplets in the initial stage of nanowire growth via the vaporliquid-solid method. Apart from serving as a collections center for growth species, the gold droplets carry an additional crucial role that necessarily precedes the nanowire emergence, i.e., they assist the nucleation of nano-craters with strongly faceted {111}B side walls. Only once these facets become sufficiently large and regular, the gold droplets start nucleating and guiding the growth of nanowires. We show that this dual role of the gold droplets, can be detected and monitored by high-energy electron diffraction during growth. Moreover, gold-induced formation of craters and the onset of nanowires growth on the {111}B facets inside the craters are confirmed by the results of Monte Carlo simulations. The detailed insight into the growth mechanism of inclined nanowires will help engineering new and complex nanowire based device architectures.
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The mainspring behind the recent eruption of extremely interesting results, in which semiconductor nanowires (NWs) and their heterostructures play a key role is the development of growth techniques, especially the gold (Au)-assisted vapor-liquid-solid (VLS) process. By this method high quality NWs are usually obtained. Nowadays, the most important application of InAs and InSb NWs and their networks, is in the search for Majorana particles. The pursuit of braiding of such particles drives the quest for formation of NWs intersections and their networks, which are crucial for producing devices for performing non-abelian braiding operations [1, 2]. This is the principle motivation for developing complex NW structures [3-8]. Subsequently, the critical requirement for producing respective devices is a robust side coating of the NW by a superconducting metal [9, 10]. This allows changing the chemical potential in the normal NW. NWs growing perpendicular to the substrate are quite limited in this sense, even though they have been utilized for forming basic intersections by kinking/bending of the NWs [7]. The process requires a high level of expertise and control. Merging of inclining NWs into intersections and more complex networks is quite handy, be it by spreading the Au droplets sporadically or by patterning. Recently, it was shown that besides facilitating the formation of NWs intersections and related networks, the tilt angle assists the side coating of InAs and InSb NWs and their intersections by a superconducting metal, in particular aluminium. Such growth process also allows coverage of more than one arm of the coated intersections [8, 9]. Understanding and controlling the growth of inclining NWs on a (001) surface, where NWs grow mostly tilted to the surface is, therefore, of outmost importance for constructing devices based on such structures. We recently successfully used a three way InAs intersection in a superconductor- semiconductor mesoscopic device to observe quartets states comprised of two Cooper pairs [11]. In another recent work we show, that the well-defined ZB segment formed within the intersections [4, 5, 7], can be potentially used as an intrinsic crystal phase quantum dot [12] providing an interesting platform for in-situ growth of multi-terminal NW structures with embedded quantum dot arrays. However, growth of inclined nanowires is challenging. We note, that semiconducting NWs of both, III–V type (including InAs and GaAs) and II-VI type (including ZnTe and CdTe),
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grow preferentially along the -direction, following the substrate dangling bonds. The main reason behind this preferential direction is that the free energy of the NWs is the lowest along the -direction [13, 14]. Semiconductor NWs are typically grown on a (111)B (anion terminated) substrate surface, where they emerge vertically [15-17]. Nevertheless, growth along the -axis commonly takes place regardless of the orientation of the substrate used [18-20]. The foundation of growing inclined NWs on surfaces other than (111)B, where the impinging atoms lack sufficiently large migration length due to the highly stepped plane, is a facetted structure comprised of micro {111} facets [20]. Such flat micro-facets provide regions where the migration length is tremendously enhanced. This leads to the possibility that in the VLS process NWs growth guided by Au droplets can win over the bulk growth. Therefore, faceting and craters formation are critical to the success of such growth; even more so since it is affected by practically all growth parameters, including, droplets size and distribution, growth temperature, growth rate, As/III flux ratio, super saturation of the Au droplets. The detailed mechanism of substrate faceting that allows the emergence of inclined NWs is quite intriguing and to the best of our knowledge it had never been comprehended in detail before. Deeper understanding will provide better control over the growth process and thus more diverse and complex NW architectures. Tilted NWs often possess superior structural properties to -oriented NWs grown vertical to a (111)B substrate, such as increased length, the lack of stacking faults [21]. Due to the high impinging angle, which facilitates the surface kinetics, they support as well higher doping levels [17]. The built-in tilted orientation of inclined NWs is also advantageous for enhancing optical processes, such as an increased photoluminescence efficiency [22-24]. Cathodoluminescence measurements proved that the luminescence intensity from inclined GaN NWs is much higher than that from their vertical neighbours [22, 23]. Tilted GaAs NWs grown on a (001) substrate were recently used to provide a larger cross section for terahertz measurements [24]. Moreover, inclined NWs have been used for solar cell devices [25, 26] as well as for biological sensing [27]. Given its unique advantages, associated with the larger impinging
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angle of the source materials on the growth surface and NWs sidewalls, tilted growth is of general interest and importance for making NWs growth by the VLS method more versatile. In Figure 1 the emergence of InAs NWs on four different InAs substrates oriented in all three singular [(001), (011), (111)] crystal planes is compared. First we note that the nucleation of NWs on the (111) and (011) surfaces (Figures 1a and 1b, respectively) starts from extruded nano-piramid structures, while NWs growth on (001) and its related vicinal surface (311)B requires formation of craters with {111}B nano-facets, as seen at Figures 1c and 1d, respectively. In the latter case the Au-assisted nucleation of inclined NWs must be, therefore, preceded by {111}-faceting of the substrate surface, which is also Auassisted. The main cause for this distinction is the difference in migration length of the impinging atoms over the various surfaces [28]. As shown in Figure 1, on a (111)B surface NWs grow normal to the substrate (Figure 1a), while on the (011) they are tilted, all at the same inclining angle between the (011) and (111)B surfaces of about 55° (Figure 1b). We note that in both cases the NWs strictly follow the predetermined growth direction appearing parallel to each other. In Figure 1c InAs NWs can be seen emerging from a (311)B substrate, as well at a tilt of 55° to the surface. This results from the fact that craters formed on the (311)B surface are strongly affected by the tilt of this surface with respect to the (001) plane, what eliminates one set of {111}B of their side walls. These facets cannot attain a large enough collection area to promote NWs nucleation and consequently NWs can emerge only from the other set of {111}B facets. Thus, again all NWs grow parallel to each other following the same direction. Finally, on the (001) surface, shown in Figure 1d, the Au-induced faceting results in formation of craters with two mirror-symmetric opposite {111}B side facets. These are tilted at 35.3° to the (001) substrate surface and lye parallel to the direction. In this case inclined InAs NWs can emerge in two different -directions. These results show, therefore, that the formation of NW intersections [3-8] is possible only on -oriented substrates. Thus, while we have studied different surface orientations, as shown in Figure 1, and other materials (see Figure S1 in Supplementary Information), in the following we concentrate mostly on the growth of the InAs NWs on the (001) InAs
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surface which are the key players in current mesoscopic physics experiments, which requires NW networks.
Figure 1: (a)-(d) SEM images of NWs grown on four different surfaces: (111)B, (110), (311)B and (001), respectively. In (a) and (b) the NWs are seen to emerge from faceted nano-pyramids whereas in (c-d), i.e., on the (001)-type surfaces, they emerge from craters with {111}B facets (as shown in the insets).
The NWs are grown in a high purity molecular beam epitaxy (MBE, RIBER-32) system. The InAs substrate, regardless of orientation, undergoes an oxide blow-off process with no As overpressure in a separate chamber attached to the MBE system. For the oxide blowoff the temperature is ramped up to ~550 - 600 °C at a rate of 10°C/min, remaining for 5 min at the peak temperature and ramped down at a rate of 20 °C/min to ~200 °C. At this temperature, Au-evaporation subsequently takes place in the same chamber. A thin (
