Crystal phase transformation in self-assembled. - Supporting Information -

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1 Crystal phase transformation in self-assembled InAs nanowire junctions on patterned Si substrates - Supporting Information - Torsten Rieger 1,2, Daniel Rosenbach 1,2, Daniil Vakulov 1,2, Sebastian Heedt 1,2, Thomas Schäpers 1,2, Detlev Grützmacher 1,2, Mihail Ion Lepsa 1,2 1 Peter Grünberg Institute (PGI-9), Forschungszentrum Jülich GmbH, Jülich, Germany 2 Jülich Aachen Research Alliance for Fundamentals of Future Information Technology (JARA-FIT), Germany t.rieger@fz-juelich.de

2 1. V-grooves and nanowire junction density A variety of SEM-images of V-grooves prior to the NW growth is displayed in Fig. S1. The distances between the {111} facets of the V-grooves are varied over a wide range, from about 200 nm to 10 µm. Figure S1: SEM micrographs of KOH-etched V-grooves: (a) tilted view and (b-f) top view images. In b-f the distances between the etched {111} facets of the V-grooves are increased. In certain fields of the chip, the V-grooves are arranged in such a way that their size varies within the field, i.e. the distance is varied. SEM micrographs of nanowires (NWs) grown in these fields are displayed in Fig. S2 from the top (a-e) and in tilted view (f-j). For small {111} facets of the grooves (Fig. S2b,g; either due to small dimensions of the spaces or due to a short etching time), the number of nucleated NWs is low and consequently, also the number of NW junctions is small. Consequently, the probability to form NW junctions is small as well. The larger the {111} facets, the more NWs are nucleated on the facets and the probability to form junctions increases. If the distance between the {111} facets increases, the NW length required to form NW junctions is enhanced. Consequently, it exists an optimal size of the pattern which depends on the length of the NWs, while the etching should be as long as possible to obtain complete V-grooves.

3 Figure S2: SEM micrographs of InAs NWs grown on a field where the size of the V-grooves is varied. The red areas indicate the dimensions of the V-grooves. Although the junctions are formed purely self-assembled and in a statistical manner, the number density of NW junctions is relatively high. Figure S3a,b display SEM micrographs of the pattern from two different orientations, NW junctions are colored. As obvious from both images, several NW junctions with very different geometries have formed. Figure S3: SEM micrographs of NWs grown on the V-grooves, the junctions are colored in order to indicate the amount of NW junctions. 2. Complex nanowire junctions A selection of complex NW junctions based on a combination of three or more NWs are displayed in Fig. S4. As obvious many different junctions are possible and are also observed experimentally.

4 Figure S4: Collection of complex NW junctions The density of these complex NW junctions can be rather high as seen from the cross sectional SEM images in Fig. S5.

5 Figure S5: Cross sectional SEM images showing the high density of complex InAs NW junctions.

6 3. Crystal structure of InAs nanowires InAs NWs grown via the self-assisted growth method usually contain a density of stacking faults 1,2. This is exemplarily displayed in Fig. S6 in a low and a high resolution TEM micrograph. These clearly demonstrate the large number of stacking faults and an almost arbitrary switching between the zinc blende and wurtzite crystal phases. Figure S6: TEM micrographs of self-assisted InAs NWs. Red areas in the right image indicate zinc blende segments, light blue areas wurtzite segments.

7 4. Crystal structure of the tip-to-tip junction In the main part of publication, the crystal structure of a tip-to-tip NW has been described. There, both NWs coalesced in such a way that exactly the tips merged and a triangular gap remains, which is filled by a ZB section. The merging process leading to L-shaped NW junctions can, however, also be slightly different as depicted schematically in Fig. S7d where the NW tip coalesces with the side facets very close to the tip of the second NW. Also in the case, both NW tips vanish and axial growth is stopped. The crystal structure of this junction differs, as illustrated in Fig. S7a-c. The ZB section is not found at the elbow of the junction but just at the merging point, similar as for the tip-to-side junctions. Figure S7: TEM images of an L-shaped NW junction. (a) Low resolution image. (b) High resolution image of the central part of the junction having ZB crystal structure (red overlay). The inset shows the FFT of the red marked region. (c) High resolution image of the outer part of the junction between both NWs. (d) Schematic illustration of the formation of the junction. 5. Crystal structure of the tip-to-side junction It was shown that the crystal structure at the NW junction is transferred via a solid phase mechanism from the stacking fault rich structure to the ZB one. In general, only rather large ZB segments have been observed. Figure S8 displays a significantly shorter ZB segment, most probably reflecting an early stage of the phase transformation. The ZB segment is already visible but did not yet spread out laterally. Figure S8: T-shaped NW junction at the beginning of the phase transformation into ZB. (a) Low resolution TEM image. (b) HRTEM micrograph. The red area in (b) shows the ZB segment.

8 A significantly longer ZB segment is displayed in Fig. S9. No Shockley partial dislocations within the ZB segment are present, confirming a complete phase transformation at the junction. Figure S9: TEM images of a T-shaped NW junction with pure ZB phase at the junction. One arm of the junction was broken off during NW transfer to the TEM grid. Red overlays denote the ZB phase at the junction. In some cases, not only Shockley partial dislocations are found but also others. This is exemplarily depicted in Fig. S10 showing a tip-to-side junction. A large ZB segment is present with defects located at the upper (c) and lower (d) end of the segment. The stacking defect at the lower end (d) can be associated to Shockley partial dislocations, frequently observed in the junctions (see main text). However, the defect at the upper end (c) is not a Shockley partial dislocation but a Frank partial dislocation. The associated strain field is clearly visible in the TEM images (b,c). The Frank partial dislocation is further confirmed by the HRTEM images shown in Fig. S11a as well as the FFT-filtered image (displaying only the (111) lattice planes) in Fig. S11b. The presence of the Frank partial rather than the Shockley partial dislocation might be associated to slight tilt of the NW growth directions away from the <111> directions of the substrate. This creates a small mismatch at the junction. Nonetheless, also the Frank partial dislocation seems to be involved in the crystal phase transformation. Figure S10: TEM micrographs of a T-shaped NW junction with a long ZB segment at the junction and one defect at both sides of the pure crystal phase in the junction. (a,b) Low resolution images showing the shape of the junction and the dimensions of the pure phase (red overlay) in the junction. (c) Defect at the upper part of the pure phase. (d) Defect at the lower part of the junction.

9 Figure S11: HRTEM (a) and FFT-filtered (b) images of the defect shown in Fig. S11c. Colored circles in (a) denote stacking sequence. The FFT-filtered image clearly demonstrates a dislocation. 6. GaAs nanowire junctions The crystal phase transformation does not only take place in InAs nanowires grown via the vapor solid mechanism, but also takes place in vapor liquid solid grown nanowires with group III droplets. Exemplary, this is shown for GaAs nanowires. The GaAs nanowires are grown on textured Si (100) substrates 3 instead of Si (100) substrates with V-grooves. This does not affect the overall result of the crystal phase transformation. However, two different scenarios of nanowire coalescences can be observed: (1) The <111>B nanowire growth directions meet under an angle of ~ 110, that is the angle between the <111>B directions in a zinc blende crystal. This is identical to the growth on V- grooves. In this case, a crystal phase transformation is observed, as displayed in Figs. S12 and S13. Figure S12: TEM and HRTEM images of GaAs nanowire junctions, the individual GaAs nanowires are grown via the vapor liquid solid mechanism on textured Si (100) substrates.

10 Figure S13: TEM and HRTEM images of GaAs nanowire junctions, the individual GaAs nanowires are grown via the vapor liquid solid mechanism on textured Si (100) substrates. (2) The <111>B nanowire growth directions meet under an angle of ~ 70, which does not correspond to the angle between the <111>B directions in a zinc blende crystal. Rather, this agrees with the angle between a <111>B and a <111>A direction in a zinc blende crystal. However, the nanowire growth still proceeds in the <111>B direction and consequently, a monocrystalline junction is not possible due to polarity mismatch. The Ga droplet is squeezed out of the structure and can even nucleate a new branch. This is displayed in the SEM and TEM images as well as EDX maps in Figs, S14-S16. Figure S14: SEM images of GaAs nanowire junctions with coalescence under an angle of ~ 70. Figure S15: TEM images (a,b), HAADF (c) image and EDX maps (d: As, e: Ga) of a GaAs nanowire junction merged under an angle of ~ 70. The Ga droplet is squeezed out of the structure.

11 Figure S16: TEM images GaAs nanowires junction merged under an angle of ~ 70. The Ga droplets are squeezed out of the structure. References (1) Morkötter, S.; Funk, S.; Liang, M.; Döblinger, M.; Hertenberger, S.; Treu, J.; Rudolph, D.; Yadav, A.; Becker, J.; Bichler, M.; Scarpa, G.; Lugli, P.; Zardo, I.; Finley, J. J.; Abstreiter, G.; Koblmüller, G. Phys. Rev. B 2013, 87, (2) Sourribes, M. J. L.; Isakov, I.; Panfilova, M.; Liu, H.; Warburton, P. A. Nano Lett. 2014, 14, (3) Rieger, T.; Rosenbach, D.; Mussler, G.; Schäpers, T.; Grützmacher, D.; Lepsa, M. I. Nano Lett. 2015, 15,