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Crystal phase transformation in self-assembled InAs nanowire junctions on patterned Si substrates Torsten Rieger, Daniel Rosenbach, Daniil Vakulov, Sebastian Heedt, Thomas Schaepers, Detlev Grützmacher, and Mihail Ion Lepsa Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b05157 • Publication Date (Web): 16 Feb 2016 Downloaded from http://pubs.acs.org on February 20, 2016
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Crystal phase transformation in self-assembled InAs nanowire junctions on patterned Si substrates Torsten Rieger1,2, Daniel Rosenbach1,2, Daniil Vakulov1,2, Sebastian Heedt1,2, Thomas Schäpers1,2, Detlev Grützmacher1,2, Mihail Ion Lepsa1,2 1
Peter Grünberg Institute (PGI-9), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany 2
Jülich Aachen Research Alliance for Fundamentals of Future Information Technology (JARA-FIT), Germany
E-Mail: [email protected] KEYWORDS: nanowire junctions, InAs nanowire, crystal phase transformation, Shockley partial dislocation, MBE, room temperature transport ABSTRACT We demonstrate the growth and structural characteristics of InAs nanowire junctions evidencing a transformation of the crystalline structure. The junctions are obtained without the use of catalyst particles. Morphological investigations of the junctions reveal three structures having an L-, T- and X-shape. The formation mechanisms of these structures have been identified. The NW junctions reveal large sections of zinc blende crystal structure free of extended defects, despite of the high stacking fault density obtained in individual InAs nanowires. This segment of zinc blende crystal structure in the junction is associated with a crystal phase transformation involving sets of Shockley partial dislocations; the transformation takes place solely in the crystal phase. A model is developed to demonstrate
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that only the zinc blende phase with the same orientation as the substrate can result in monocrystalline junctions. The suitability of the junctions to be used in nanoelectronic devices is confirmed by room temperature electrical experiments.
MANUSCRIPT INTRODUCTION Over the last years, the quasi one-dimensional nature of semiconductor nanowires (NWs) allowed the fabrication of several electrical and optical devices ranging from lasers1 and light emitting diodes2 to device schemes suitable for the investigation of Majorana fermions.3–6 For the latter, multi-terminal structures are required to account for the non-abelian statistics of the Majorana fermions.7 The ability to grow quasi one-dimensional nanowires with multiple terminals offers further compelling new applications. Primary nanowire-based logic applications have already been realized by the implementation of a nanowire processor.8 Such circuits could be improved using merged nanowires with the incentive of creating reconfigurable logic. Hence, each nanowire junction branch could be controlled via local gates. A branched three terminal nanowire can also serve as a T-shaped quantum transistor where a gate placed on top of the branched NW modifies the electron interference pattern.9 Multi-terminal NW structures can be obtained by the formation of “nanotrees”, which are created by growing NWs on the side facets of existing NWs.10–13 This usually involves the deposition of another layer of Au particles on the existing NWs, requiring the removal of the NWs from the growth chamber. An alternative way to obtain such multi-terminal NW structures is to merge two or more NWs during growth.14–17 These junctions are typically produced via Au particles catalyzing the growth of the individual NWs. During the merging process, these Au particles can etch into the existing NW, modifying its crystal structure15 and composition and possibly, Au atoms might remain at the junction18,19. The dynamics of the droplet are of outstanding importance for the quality of the junction and significantly affect the morphology of the obtained structures.14,15 The quality of the junction, i.e. the electrical ACS Paragon Plus Environment
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properties, is mostly defined by the crystal structure and potential defects of the junction. This junction can be rough (due to etching of the Au droplet into the NW)15, contain additional stacking faults20 or only a certain number of the junction has a good crystalline quality16,17. In the case of nanotrees, the branching nanowire often follows a different growth direction, i.e. has different properties and adopts the crystal defects.10,12 Consequently, it is important to understand the formation mechanism of NW junctions and to explore a method to obtain high quality NW junctions independent on the crystal structure and orientation of the individual NWs. The self-assisted growth of NWs, which is used in this report, is a promising option, since no foreign catalyst particle is involved, circumventing any droplet effects in the formation process and also any contamination of the junction is avoided. To obtain NW junctions, two or more NWs have to be grown in such a way that they intersect. Using a plane (111)-oriented substrate, NWs grow perpendicular to the substrates’ surface and thus do not cross. On (100) substrates, NW growth still follows the directions but the NW density is typically too low.21 Additionally, the specific growth directions cannot be controlled. Consequently, NW junctions are possible in principle, but their density would be extremely low. In a previous study, we have demonstrated that InAs and GaAs NWs can be grown on textured Si (100) substrates consisting of pyramids bound by {111} facets. NWs grow perpendicularly on these facets and the NW density is significantly higher than on planar (100) substrates21. The pyramids are positioned randomly and therefore the probability for the formation of NW junctions is still low. Here, we present a method to obtain high density self-assembled NW junctions based on vapor-solid grown InAs NWs. The different shapes, the formation mechanism and the crystal structure of the junctions are analyzed. The quality of the junction is further evaluated by means of room temperature transport measurements.
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Figure 1: Schematic of the substrate preparation for V-grooves on silicon (100) substrates including the NW growth. (a) Si substrate covered by HSQ. (b) electron-beam lithography. (c) Development and hard bake of HSQ. (d,e) KOH etching after subsequently increasing etching times. (f) NW growth.
EXPERIMENTS The self-assisted growth of InAs NW junctions was realized on Si (100) substrates processed with V grooves prepared employing spin coating of hydrogen silsesquioxane (HSQ), electron beam lithography of lines and spaces and wet chemical etching in potassium hydroxide (KOH). Prior to the NW growth, the samples were cleaned wet chemically to remove contaminations from the etching procedure as well as the native silicon oxide layer. Wet chemical reoxidation in hydrogen peroxide was used to rebuild a thin oxide.22 InAs NWs were grown using the self-assisted growth mechanism23,24, i.e. the NWs grow without a droplet (Vapor solid mechanism). The substrate preparation and NW growth are schematically depicted in Fig. 1. Information about the morphology and crystalline structure of the NW junctions was obtained using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. Electronic transport characteristics at room temperature have been determined on processed NW junction devices. Further details are presented in the “Experimental methods” at the end of the manuscript.
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Figure 2: SEM micrographs of the textured substrate (a) prior to and (b-e) after the NW growth. The V-grooves are displayed in (a). (b) shows a top view image of a field of lines and spaces, in (c) the lower left corner of the field is shown in higher magnification. Clearly, high density NWs are grown on the side facets of the V-grooves while NW growth is almost absent on the remaining facets. A tilted view SEM micrograph is depicted in (d), proving that the NWs grow only perpendicular to the 111 side facets of the V-grooves. (e) Cross-sectional view of the V-groove pattern with InAs NWs being grown on the 111 side facets of the grooves.
RESULTS & DISCUSSION The wet chemical etching produces V-grooves being bound by {111} facets, as obvious from the SEM micrograph displayed in Fig. 2a. These {111} planes are caused by the anisotropic nature of the KOH etching, having a significantly lower etch rate of the {111} than the {100} planes. Consequently, the side facets of the V-grooves draw an angle of 54.7° with respect to the (100) substrate.
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Figure 2b-e display typical top view SEM micrographs of a field of lines and spaces, a magnified image of a corner as well as tilted and cross sectional micrographs of the samples after NW growth. The NWs are perfectly aligned, meaning that (in the top view projection) they are grown only along the 011 and 011 directions of the Si substrate. An exception are the edges of the patterned fields where the NWs also align in the 011 and 011 directions due to the presence of additional {111} facets. The tilted view SEM image shown in Fig. 2d as well as the cross sectional image in Fig. 2e demonstrate that the InAs NWs grow perpendicular to the 111 side facets, thus they grow along the 〈111〉 directions of the Si substrate. The InAs NW growth directions draw an angle of 35.3° with respect to the Si 100 surface. Apparently, a high density of NWs is observed on the 111 facets while they are in general not found on remaining 100 facets. On these facets, only crystallites are observed. Similar observations were made when InAs NWs were grown on textured Si substrates and can be assigned to different oxide thicknesses on the (100) and (111) facets as well as a reduced nucleation probability on the (100) facets.21 To obtain NW junctions, the NWs have to be grown on opposing side facets but with (almost) the same lateral position in the V-grooves. A suitable yield of NW junctions is typically achieved when the pitch between two adjacent lines in the pattern is around 2 – 8 µm and the V-grooves are etched almost to the bottom. A detailed analysis of the impact of the distance of the spaces on the yield of NW growth and the formation of NW junctions is presented in the supplementary information (SI). The yield of NW junctions can further be improved by the selective area MBE growth of self-assisted NWs25 in combination with lithography on the V-grooves.
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Figure 3: Nanowire junctions with two (a) and three (d) arms. The junction in (a) forms an L-shape and is caused by the coalescence of two NW tips as schematically depicted in (b) and (c). Consequently this junction is a tip-to-tip junction. The junction shown in (d) is created when a NW tip coalesces with the side facet of another NW, thus a tip-to-side junction. This is schematically depicted in (e) and (f). The junction has a T-shape. The inset in (d) shows the onset of the junction formation the formation of the junction. The tip of the left NW influences the growth on the side facet of the right NW.
Once the NWs are grown on the V-grooves, NWs can coalesce during their growth and form NW junctions, similar as in the work performed by Dalacu et al. and Kang et al.14,15 They have related most of their structures to droplet dynamics, involving down crawling of the droplet, the emerging of new branches as well as the etching of the Au droplet into existing NWs. Using NWs grown without a catalyst particle, these droplet dynamics are avoided. Instead, the formation of the junction takes place in the solid and vapor phase. Here, three basic shapes of the junctions can be distinguished. These have an L-, a T- or an X-shape. A SEM micrograph of an L-shaped NW junction is displayed in Fig. 3a. This structure is formed via the coalescence of the tips of two NWs, as schematically depicted in Fig. 3b,c. Both tips vanish during the coalescence, resulting in the absence of further axial growth. Instead radial
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growth takes place which increases the diameters of the NWs. T-shaped NW junctions are displayed in Fig. 3d. Here, the tip of one NW touches the side facet of another NW. During this coalescence, one NW tip vanishes while the other one remains which enables a proceeding of the axial growth. The presence of the NW tip in close vicinity to the {110} side facets of another NW strongly influences the growth. This is evident from the SEM micrograph displayed as an inset in Fig. 3d. The NW tip approaches the side facets, resulting in a slight radial growth on the side facets in the vicinity of the tip, possibly being caused by a change of the local V/III ratio and correspondingly a change of the adatom diffusion. The junction between both NWs is achieved by both the axial extension via the NW tip and the radial growth on the side facets. This may affect the structure of the junction. Since the NW growth is epitaxial on the {111} side facets of the V-grooves, the angle between the two coalesced NWs is 109.47°, which displays the angle between two B directions in zinc blende semiconductors. The third possible junction has the shape of an “X” and is grown in a rather different way without tips playing an active role in the formation of the junction. These junctions are exemplarily displayed in Fig. 4a,b. Around the junction the entire structure is not planar, rather it is three dimensional. This is more evident in the top view SEM micrographs before and after coalescence depicted in Fig. 4c,d, respectively. Before coalescence, two separate NWs slightly displaced with respect to each other are present and their {110} side facets are in contact. This location subsequently acts as a sink for the adatoms resulting in radial growth and coalescence of the two NWs. The formation mechanism of this junction is schematically displayed in Fig. 4e,f from both side and top view. These X-shaped NW junctions are consequently formed in a similar way as observed by Car et al.17.
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Figure 4: Nanowire junction with four branches. The junctions form the shape of an “X” (a,b). The formation of such junctions at different growth stages are shown in (c)-(f). (c) Top view image before coalescence, two independent NWs touch each other with their {110} side facets. (d) Top view image after coalescence clearly evidencing the radial growth at the junction. (e,f) Side and top view schematic illustrations of the formation process of X-shaped nanowire junctions, before (e) and after (f) coalescence.
Due to the arbitrary position of the NWs, their high density and the length fluctuation, also more complex NW junctions are obtained. These complex junctions can always be subdivided into the three basic junctions L, T and X. A collection of such complex NW junctions is depicted in the SI. In two of the three different junctions (L- and T-shapes) NW tips are involved in the formation process while the remaining NW junction (X-shape) is formed by a pure radial growth on the side facets. CRYSTAL STRUCTURE OF InAs NANOWIRE JUNCTIONS The expected crystal structure at the NW junction can be derived from a simple model sketched in Fig. 5. Here, the patterned Si (100) substrate with its {111} lattice planes is
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depicted. NW growth is assumed to take place vertically on the Si {111} facets of the V grooves. Suppose both NWs have the WZ crystal structure, the result is as displayed in Fig. 5a. Here, the {0001} lattice planes of the WZ NWs are displayed. The junction cannot be wurtzite. This is further evident from the atomic model shown in Fig. 5b. Overlapping both WZ NWs at the junction does not create a uniform crystal lattice. Consequently, a single crystal WZ junction without grain boundary between two NWs growing in the {0001} directions is not possible. If both NWs adopt the zinc blende (ZB) crystal structure and the orientation of the substrate (that means no twinning occurs), naturally, the junction between the two NWs is single crystalline and reflects exactly the orientation of the substrate. This is displayed in the schematic in Fig. 5c as well as in the atomic models in Fig. 5d. Kang et al.15 already pointed out that the crystal structure at the merging point of two NWs growing along the (ZB) or (WZ) directions needs to be zinc blende. When one NW contains a twin boundary, e.g. the NW growing on the left facet of the Vgroove in Fig. 5e, again no monocrystalline junction is possible. The atomic model in Fig. 5f clearly illustrates this. In the case that both NWs contain a twin boundary, the crystal lattices at the junction do not agree and a monocrystalline junction is not possible either (see Fig. 5g,h). Thus, if the NWs are twinned, i.e. their crystal structure is rotated with respect to the Si substrate, a monocrystalline junction is not possible. Rather, a “twinned twin” is obtained.17 To be more precise, an even number of twins in both NWs can result in monocrystalline junctions whereas an odd number does not produce a single crystalline junction. Consequently, Car et al. came to the conclusion that 25% of their InSb NW junctions are monocrystalline.17 It is shown in the following, that all L- and T-shaped NW junctions presented here have a monocrystalline ZB junction, although the individual InAs NWs are full of stacking defects.
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In the present study, the NWs do not contain any seed particle, neither Au nor In. Consequently, the junction is formed without any etching into the NW and Au contamination is excluded. The NWs grow along the B growth direction of the zinc blende InAs crystal lattice and adopt the crystallographic orientation of the Si substrate. The crystal structure of self-assisted InAs NWs usually contains a high density of stacking faults and twin planes.26,27 Exemplarily, this is depicted in the SI. Recent studies have demonstrated that the crystal structure of InAs NWs is significantly modified by the addition of Sb, i.e. the formation of InAsSb NWs. This enables to obtain zinc blende InAsSb NWs with twin boundaries spaced by ~ 5 nm27,28 and even pure zinc blende InAsSb NWs29. In the case of the NW junctions, two independent and defect-rich NWs coalesce, making the merging point of high interest. The three different basic junctions, represented by the different types of coalescence (tip-to-tip, tip-to-side and side-to-side), may have different crystallographic properties which are affected by a formation process partly involving NW tips. These are described in the following.
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Figure 5: Sketch of the growth of NWs on the {111} facets of the V-grooves and atomic models displaying the possible atomic arrangements at the junction. (a,b) WZ NWs, (c,d) twin-free ZB NWs, (e,f) ZB NWs, one NW contains a twin, (g,h) ZB NWs, both NWs are twinned. Atomic models were created using VESTA30.
An L-shaped NW junction is displayed in Fig. 6a, a higher resolution image in Fig. 6b. The high density of stacking faults in both arms of the NW is obvious, whereas a uniform ZB crystal structure is observed at the elbow. The ZB structure in the region of the elbow is evidenced by the inset displaying the FFT pattern of ZB structure. The elbow is obtained after the NW tips coalesced, thus the residual triangular corner is filled by the zinc blende segment and the transition to the highly stacking fault rich region is abrupt (see Fig. 6c). Consequently, a monocrystalline junction between both NWs is obtained. A schematic of the coalescence is displayed in the inset of Fig. 6a. An alternative merging process for L-shaped NW junctions is ACS Paragon Plus Environment
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shown in the SI, there the tip of one NW merges with the side facets close to the tip of another NW. Nonetheless, also this results in a ZB structure at the junction.
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Figure 6: (HR)TEM images and diffraction patterns of the three different types of NW junctions. (a-c) L-shaped NW junction with a ZB phase at the elbow (see inset in b and HRTEM image in c). The schematic in the inset of (a) displays the coalescences and the red circle indicates the position of the ZB elbow. (d-e) T-shaped NW junction evidencing a crystal phase transformation of the stacking fault rich crystal structure of the InAs NW into ZB via Shockley partial dislocations.(f) HRTEM image showing the Shockley partial dislocations.(gi) TEM micrographs and diffraction pattern of an X-shaped NW junction with Moiré fringes at the junction. (j-l) TEM micrographs and diffraction pattern of an X-shaped NW junction with a monocrystalline ZB junction.
The growth behavior differs in the case of T-shaped NW junctions. Here, the tip of one NW touches and coalesces with the side facet of another NW. Figure 6d-f display TEM micrographs of this scenario. Interestingly, the crystal structure of the continuous NW at the position of the junction is zinc blende and is uniform over an axial distance of about 100 nm. Such long segments of a uniform crystal structure have not yet been observed in vapor solid grown InAs NWs. At two positions within the zinc blende segment, stacking faults not extending across the entire NW width are observed (see Fig. 6e,f). A close look reveals that the stacking faults change abruptly into a uniform zinc blende structure. This is a typical characteristic of Shockley partial dislocations having Burgers vectors of = /6〈211〉
0〉 (hexagonal notation). A detailed analysis of this (cubic notation) / = /3〈101 region is displayed in Fig. 7, it shows the FFT-filtered image with colored dots denoting the different layer stacks A, B and C. The white lines indicate the stacking sequence. While the crystal structure on the right part of the image is free of any stacking defects, several stacking faults are observed on the left side. The defective structure in the left part is not pure WZ but contains stacking faults and twins. Interestingly, the stacking sequences of the perfect ZB regions in the upper part of the image differ slightly, as obvious by the colored dots and the ACS Paragon Plus Environment
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yellow line. The stacking sequence is shifted by one atomic layer. Such a shift of atoms in a layer can be the result of Shockley partial dislocations.31 Shockley partial dislocations have already been associated with transformations of the crystal structure from pure WZ to ZB in semiconductor NWs31–33: Patriarche et al. observed Shockley partial dislocations and an associated atomic movement by burying WZ GaAs in a ZB GaAs layer31. Zheng et al. obtained a crystal phase transformation in Au-seeded InAs NWs by a remelting of the catalyzing Au droplet at high temperatures, creating a small portion of ZB which then spreads out laterally.32 It is important to mention that the previous studies by Patriarche et al. and Zheng et al. discuss the transformation of large, pure WZ segments into ZB31,32 while here the original crystal structure of the NW is full of stacking faults and twins. Recently, Jacobsson et al. associated a phase transformation in radially merged WZ GaAs NWs to the existence of Shockley partial dislocations, the crystal structure switched to a mixture of ZB and WZ.33 In case of the X-shaped NW junctions, Moiré patterns are observed in the junction (see Fig. 6g,h). These Moiré patterns indicate the presence of two crystal lattices with different lattice constants or a rotation with respect to each other. Here, the lattices are almost identical (InAs, WZ-ZB mixture) but the NWs are rotated by ~110° with respect to each other. A monocrystalline junction is not observed, being also evident from the diffraction pattern displayed in Fig. 6i. These Moiré fringes are observed in almost all of the X-shaped junctions but a small amount of the junctions have another crystallographic configuration. This is depicted in Fig. 6j-l. The TEM micrograph of the central part of the X-shaped junction exhibits a rhombic region having a size of about 170 nm x 200 nm which contains only few stacking faults. This is also clear from the corresponding diffraction pattern displayed in Fig. 6l. It shows a ZB pattern with only light streaks from the few stacking faults.
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Figure 7: FFT-filtered HRTEM image of the set of Shockley partial dislocations displayed in Fig. 6f. Colored dots denote the stacking sequence, which is further indicated by the white lines. The yellow line indicates the stacking sequence of a stacking fault free system. The arrows point at the first and the last stacking faults. Consequently, each type of NW junctions results in a specific crystallographic arrangement at the junction: The coalescence of two tips produces a ZB elbow. A tip merging with the side facet of another NW modifies the existing crystal structure via Shockley partial dislocations and extended segments containing ZB structure occur. A coalescence by radial growth forming a crossing of NW usually creates an overlapping of the two crystals separated by a grain boundary, however, occasionally can also result in a uniform ZB region. The phase transformations from the stacking fault rich structure to the almost pure ZB at the junction can usually be associated with the existence of Shockley partial dislocations, as it has been extensively outlined above. In few cases, single Frank partial dislocations have been identified in the ZB junctions. These also seem to result in a transformation of the crystal phase, as displayed in the SI. Additional TEM micrographs displaying different stages of the crystal phase transformation in tip-to-side NW junctions and a different morphology of a tipto-tip NW junction are depicted in the SI. There, it is observed that the tip of the NW merging with the side facets of the other NW already contains the ZB crystal structure, even though the phase transformation did not start, i.e. no large ZB segment with Shockley partial dislocations is found. Accordingly, the formation of the ZB phase at the NW tip is favored, possibly due ACS Paragon Plus Environment
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the close proximity of the 110 side facets of the other NW modifying the effective V/III ratio. This agrees with the SEM image shown in the inset of Fig. 3d displaying the merging process. Alternatively, a virtual melting at the interface between both NWs may occur taking place below the melting temperature.34,35 This can also lead to a transformation of the crystal phase. In order to gain a deeper understanding of the crystal phase transformation at the NW junction, in situ TEM investigations should be considered for future experiments. These will allow to unravel the details of the transformation process. This can be performed during the NW growth36 or after the growth by annealing at elevated temperatures37 since the phase transformation should be controlled by the dislocation velocity which is temperature dependent.38,39 First results on GaAs NW junctions grown via the vapor liquid solid mechanism with Ga droplets40,41 exhibit a similar crystal phase transformation, giving evidence that the phase transformation is applicable to different materials and different growth methods. Exemplary TEM micrographs demonstrating this are displayed in the supporting information.
ROOM TEMPERATURE TRANSPORT Here, we demonstrate that the merging of two nanowires does not compromise the electrical transport properties and that multi-terminal nanowire junction devices are in fact suitable for nanoelectronic applications. Different kinds of epitaxially merged InAs nanowire devices have been prepared and electrical transport in T-, and X-shaped junctions has been investigated at room temperature (see Fig. 8). The current-voltage ( − ) characteristics measured between any two terminals of the devices shown in Fig. 8 are linear. From the inverse slope of these curves and the device dimensions the resistivity is determined. Owing to diameter variations in the different nanowire branches an effective weighted diameter is calculated. All values of are found to be rather similar. For the T-junction device in Fig. 8a ACS Paragon Plus Environment
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the measurements involving the merged nanowire branch labeled C exhibit somewhat higher resistivities ( = 0.72 Ωcm and = 0.69 Ωcm) than the ordinary nanowire connection ( = 0.41 Ωcm). Taking all measurements into account the average resistivity is & = 0.61 Ωcm. In view of all samples investigated, the variation is not generic for merged branches with single-crystalline junctions but rather reflecting the variation among individual nanowires. Moreover, also the − measurements between different contact pairs of the X-shaped junction shown in Fig. 8b indicate that the resistivity is hardly affected by the presence of the junction region at the nanowire intersection. The resistivities of all branch combinations are very similar with a mean value of ' = 0.54 ± 0.18 Ωcm. Within the measurement accuracy, it seems evident that for the T- and the X-junction devices the presence of the nanowire junction in the current path does not affect the overall resistivity. This indicates that the merging of the nanowires does not result in the formation of a potential barrier, in agreement with the crystalline order of the junction area. A comprehensive transport study of many devices reveals that the resistance-length ratio R/L scales with the cross-section area of the nanowires, which implies that transport is not dominated by a surface electron accumulation layer but instead the entire nanowire cross-section constitutes the channel.42
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Figure 8: Room-temperature electrical transport through a nanowire junction device. (a) Current-voltage characteristics between pairs of the three branches of a T-junction. (b) Corresponding set of measurements for an X-junction. The insets show scanning electron microscope images of the contacted nanowires.
CONCLUSIONS In conclusion, we have studied the formation, crystal structure and its associated transformation as well as the room temperature transport properties of InAs NW junctions grown via a self-assisted growth mechanism on Si (100) substrates patterned with V-grooves bound by {111} facets. The NW growth on the exposed {111} facets lead to three basic configurations of NW junctions: tip-to-tip, tip-to-side and side-to-side junctions. The latter was found to be caused by a radial merging of two NWs. Despite of the stacking fault rich individual InAs NWs, all the tip-to-tip (L-shape) and tip-to-side (T-shape) junctions exhibited uniform ZB crystal structure, having the same orientation as the Si (100) substrate. Shockley partial dislocations act as the driving force for the solid-phase crystal phase transformation at the junction. In general, the X-shaped junctions were made up of two individual crystals, although also here single crystalline zinc blende junctions have been identified.
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alternative to a modification of the crystal phase using growth parameters. Current-voltage measurements performed at room-temperature further confirmed the potential of the junctions, since their presence did not affect the overall resistivity. The self-assisted formation of NW junctions reveals great potential for NW networks and mesoscopic physics involving T-shaped quantum transistors9 and the braiding of Majorana fermions7. EXPERIMENTAL METHODS SUBSTRATE PREPARATION The Si (100) substrates were covered by a 200 nm thick layer of hydrogen silsesquioxane (HSQ) deposited by spin-coating. The HSQ layer is used as a negative-tone e-beam resist as well as an etch mask for potassium hydroxide (KOH) based wet etching. Electron beam lithography followed by development in MF CD-26 was used to define lines and spaces of various dimensions in the HSQ layer. The obtained pattern was subsequently hard baked for 4h at 450°C. This thermal curing is crucial, since it significantly increases the etch resistance of the HSQ.43,44 Without the hard baking, the KOH etching removed the HSQ layer within a few seconds. The lines and spaces were transferred into the Si substrate via wet chemical etching in a solution containing 33 wt % KOH and 7 vol % isopropyl alcohol. The etching temperature was 40°C, while the etching time was 40 min. Prior to the introduction into the MBE system, the samples were wet chemically cleaned using diluted HCl45, piranha and diluted HF. Subsequently, a thin silicon oxide layer was created by wet chemical reoxidation in hydrogen peroxide22,46. GROWTH DETAILS
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InAs NWs were grown in a self-assisted manner23,24 at a substrate temperature of 490°C with an In rate of 0.05 µm/h and an As4 flux of 10-5 Torr22. Such growth conditions give rise to NWs grown without any droplet, i.e. growth takes place following a vapor-solid mechanism24. Typical growth duration ranged between 3h and 5.5h. CHARACTERIZATION DETAILS The morphological characteristics of the samples were analyzed by scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was used to gather information about the crystalline structure of the NW junctions. Here, a FEI Tecnai G2 F20 operated at 200kV was used. To determine the electrical characteristics of the NW junctions, they were dispersed on Si/SiO2 substrates. Subsequently, individual contacts to the NW branches were prepared by electron beam lithography and deposition of 100 nm thick Ti/Au contact leads. Ar+ sputtering prior to metal deposition was employed to improve the contact resistance.47
ACKNOWLEDGMENT Assistance with the MBE growth by Christoph Krause is gratefully acknowledged. Stefan Trellenkamp is acknowledged for electron beam lithography. The TEM facilities at the Ernst Ruska Centre are gratefully acknowledged. Daniil Vakulov gratefully acknowledges “The scholarship of the President of the Russian Federation for young researchers to conduct a research work abroad”. Supporting Information SEM images of the V-grooves and information on the density of nanowire junctions. SEM images of complex nanowire junctions. The crystal structure of the InAs nanowires. TEM micrographs showing the crystal structure and defects at the tip-to-tip and tip-to-side
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junctions. TEM micrographs demonstrating the crystal phase transformation on vapor liquid solid grown GaAs NW junctions. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Authors *Torsten Rieger, Peter Grünberg Institute (PGI-9), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany, E-Mail: [email protected]
ABBREVIATIONS NW nanowire; MBE molecular beam epitaxy; SEM scanning electron microscopy; KOH potassium hydroxide, HSQ hydrogen silsesquioxane, ZB zinc blende, WZ wurtzite, TEM, transmission electron microscopy, HRTEM high resolution transmission electron microscopy, FFT fast-Fourier transform REFERENCES (1)
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Crystal phase transformation in self-assembled InAs nanowire junctions on patterned Si substrates Torsten Rieger, Daniel Rosenbach, Daniil Vakulov, Sebastian Heedt, Thomas Schäpers, Detlev Grützmacher and Mihail Ion Lepsa
The fabrication of catalyst free InAs nanowire junctions on Si (100) substrates patterned with V-grooves is demonstrated. Morphological investigations of the junctions reveal three structures having an L-, T- and X-shape. The merging process is found to induce a solid phase transformation of the crystalline structure of the junctions from a high density of stacking faults to zinc blende. The results pave the way to complex nanowire structures and a controlled modification of the crystalline structure of existing nanowires.
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Schematic of the substrate preparation for V-grooves on silicon (100) substrates including the NW growth. (a) Si substrate covered by HSQ. (b) electron-beam lithography. (c) Development and hard bake of HSQ. (d,e) KOH etching after subsequently increasing etching times. (f) NW growth. 199x90mm (300 x 300 DPI)
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SEM micrographs of the textured substrate (a) prior to and (b-e) after the NW growth. The V-grooves are displayed in (a). (b) shows a top view image of a field of lines and spaces, in (c) the lower left corner of the field is shown in higher magnification. Clearly, high density NWs are grown on the side facets of the Vgrooves while NW growth is almost absent on the remaining facets. A tilted view SEM micrograph is depicted in (d), proving that the NWs grow only perpendicular to the {111} side facets of the V-grooves. (e) Crosssectional view of the V-groove pattern with InAs NWs being grown on the {111} side facets of the grooves. 80x87mm (300 x 300 DPI)
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Nanowire junctions with two (a) and three (d) arms. The junction in (a) forms an L-shape and is caused by the coalescence of two NW tips as schematically depicted in (b) and (c). Consequently this junction is a tipto-tip junction. The junction shown in (d) is created when a NW tip coalesces with the side facet of another NW, thus a tip-to-side junction. This is schematically depicted in (e) and (f). The junction has a T-shape. The inset in (d) shows the onset of the junction formation the formation of the junction. The tip of the left NW influences the growth on the side facet of the right NW. 80x67mm (300 x 300 DPI)
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Nanowire junction with four branches. The junctions form the shape of an “X” (a,b). The formation of such junctions at different growth stages are shown in (c)-(f). (c) Top view image before coalescence, two independent NWs touch each other with their {110} side facets. (d) Top view image after coalescence clearly evidencing the radial growth at the junction. (e,f) Side and top view schematic illustrations of the formation process of X-shaped nanowire junctions, before (e) and after (f) coalescence. 80x80mm (300 x 300 DPI)
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Sketch of the growth of NWs on the {111} facets of the V-grooves and atomic models displaying the possible atomic arrangements at the junction. (a,b) WZ NWs, (c,d) twin-free ZB NWs, (e,f) ZB NWs, one NW contains a twin, (g,h) ZB NWs, both NWs are twinned. 250x206mm (150 x 150 DPI)
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(HR)TEM images and diffraction patterns of the three different types of NW junctions. (a-c) L-shaped NW junction with a ZB phase at the elbow (see inset in b and HRTEM image in c). The schematic in the inset of (a) displays the coalescences and the red circle indicates the position of the ZB elbow. (d-e) T-shaped NW junction evidencing a crystal phase transformation of the stacking fault rich crystal structure of the InAs NW into ZB via Shockley partial dislocations.(f) HRTEM image showing the Shockley partial dislocations.(g-i) TEM micrographs and diffraction pattern of an X-shaped NW junction with Moiré fringes at the junction. (j-l) TEM micrographs and diffraction pattern of an X-shaped NW junction with a monocrystalline ZB junction. 197x268mm (300 x 300 DPI)
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FFT-filtered HRTEM image of the set of Shockley partial dislocations displayed in Fig. 6f. Colored dots denote the stacking sequence, which is further indicated by the white lines. The yellow line indicates the stacking sequence of a stacking fault free system. The arrows point at the first and the last stacking faults. 119x80mm (300 x 300 DPI)
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Room-temperature electrical transport through a nanowire junction device. (a) Current-voltage characteristics between pairs of the three branches of a T-junction. (b) Corresponding set of measurements for an X-junction. The insets show scanning electron microscope images of the contacted nanowires. 376x183mm (300 x 300 DPI)
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TOC figure 80x40mm (300 x 300 DPI)
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Crystal phase transformation in self-assembled InAs nanowire junctions on patterned Si substrates Torsten Rieger, Daniel Rosenbach, Daniil Vakulov, Sebastian Heedt, Thomas Schaepers, Detlev Grützmacher, and Mihail Ion Lepsa Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b05157 • Publication Date (Web): 16 Feb 2016 Downloaded from http://pubs.acs.org on February 20, 2016
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Crystal phase transformation in self-assembled InAs nanowire junctions on patterned Si substrates Torsten Rieger1,2, Daniel Rosenbach1,2, Daniil Vakulov1,2, Sebastian Heedt1,2, Thomas Schäpers1,2, Detlev Grützmacher1,2, Mihail Ion Lepsa1,2 1
Peter Grünberg Institute (PGI-9), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany 2
Jülich Aachen Research Alliance for Fundamentals of Future Information Technology (JARA-FIT), Germany
E-Mail: [email protected] KEYWORDS: nanowire junctions, InAs nanowire, crystal phase transformation, Shockley partial dislocation, MBE, room temperature transport ABSTRACT We demonstrate the growth and structural characteristics of InAs nanowire junctions evidencing a transformation of the crystalline structure. The junctions are obtained without the use of catalyst particles. Morphological investigations of the junctions reveal three structures having an L-, T- and X-shape. The formation mechanisms of these structures have been identified. The NW junctions reveal large sections of zinc blende crystal structure free of extended defects, despite of the high stacking fault density obtained in individual InAs nanowires. This segment of zinc blende crystal structure in the junction is associated with a crystal phase transformation involving sets of Shockley partial dislocations; the transformation takes place solely in the crystal phase. A model is developed to demonstrate
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that only the zinc blende phase with the same orientation as the substrate can result in monocrystalline junctions. The suitability of the junctions to be used in nanoelectronic devices is confirmed by room temperature electrical experiments.
MANUSCRIPT INTRODUCTION Over the last years, the quasi one-dimensional nature of semiconductor nanowires (NWs) allowed the fabrication of several electrical and optical devices ranging from lasers1 and light emitting diodes2 to device schemes suitable for the investigation of Majorana fermions.3–6 For the latter, multi-terminal structures are required to account for the non-abelian statistics of the Majorana fermions.7 The ability to grow quasi one-dimensional nanowires with multiple terminals offers further compelling new applications. Primary nanowire-based logic applications have already been realized by the implementation of a nanowire processor.8 Such circuits could be improved using merged nanowires with the incentive of creating reconfigurable logic. Hence, each nanowire junction branch could be controlled via local gates. A branched three terminal nanowire can also serve as a T-shaped quantum transistor where a gate placed on top of the branched NW modifies the electron interference pattern.9 Multi-terminal NW structures can be obtained by the formation of “nanotrees”, which are created by growing NWs on the side facets of existing NWs.10–13 This usually involves the deposition of another layer of Au particles on the existing NWs, requiring the removal of the NWs from the growth chamber. An alternative way to obtain such multi-terminal NW structures is to merge two or more NWs during growth.14–17 These junctions are typically produced via Au particles catalyzing the growth of the individual NWs. During the merging process, these Au particles can etch into the existing NW, modifying its crystal structure15 and composition and possibly, Au atoms might remain at the junction18,19. The dynamics of the droplet are of outstanding importance for the quality of the junction and significantly affect the morphology of the obtained structures.14,15 The quality of the junction, i.e. the electrical ACS Paragon Plus Environment
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properties, is mostly defined by the crystal structure and potential defects of the junction. This junction can be rough (due to etching of the Au droplet into the NW)15, contain additional stacking faults20 or only a certain number of the junction has a good crystalline quality16,17. In the case of nanotrees, the branching nanowire often follows a different growth direction, i.e. has different properties and adopts the crystal defects.10,12 Consequently, it is important to understand the formation mechanism of NW junctions and to explore a method to obtain high quality NW junctions independent on the crystal structure and orientation of the individual NWs. The self-assisted growth of NWs, which is used in this report, is a promising option, since no foreign catalyst particle is involved, circumventing any droplet effects in the formation process and also any contamination of the junction is avoided. To obtain NW junctions, two or more NWs have to be grown in such a way that they intersect. Using a plane (111)-oriented substrate, NWs grow perpendicular to the substrates’ surface and thus do not cross. On (100) substrates, NW growth still follows the directions but the NW density is typically too low.21 Additionally, the specific growth directions cannot be controlled. Consequently, NW junctions are possible in principle, but their density would be extremely low. In a previous study, we have demonstrated that InAs and GaAs NWs can be grown on textured Si (100) substrates consisting of pyramids bound by {111} facets. NWs grow perpendicularly on these facets and the NW density is significantly higher than on planar (100) substrates21. The pyramids are positioned randomly and therefore the probability for the formation of NW junctions is still low. Here, we present a method to obtain high density self-assembled NW junctions based on vapor-solid grown InAs NWs. The different shapes, the formation mechanism and the crystal structure of the junctions are analyzed. The quality of the junction is further evaluated by means of room temperature transport measurements.
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Figure 1: Schematic of the substrate preparation for V-grooves on silicon (100) substrates including the NW growth. (a) Si substrate covered by HSQ. (b) electron-beam lithography. (c) Development and hard bake of HSQ. (d,e) KOH etching after subsequently increasing etching times. (f) NW growth.
EXPERIMENTS The self-assisted growth of InAs NW junctions was realized on Si (100) substrates processed with V grooves prepared employing spin coating of hydrogen silsesquioxane (HSQ), electron beam lithography of lines and spaces and wet chemical etching in potassium hydroxide (KOH). Prior to the NW growth, the samples were cleaned wet chemically to remove contaminations from the etching procedure as well as the native silicon oxide layer. Wet chemical reoxidation in hydrogen peroxide was used to rebuild a thin oxide.22 InAs NWs were grown using the self-assisted growth mechanism23,24, i.e. the NWs grow without a droplet (Vapor solid mechanism). The substrate preparation and NW growth are schematically depicted in Fig. 1. Information about the morphology and crystalline structure of the NW junctions was obtained using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. Electronic transport characteristics at room temperature have been determined on processed NW junction devices. Further details are presented in the “Experimental methods” at the end of the manuscript.
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Figure 2: SEM micrographs of the textured substrate (a) prior to and (b-e) after the NW growth. The V-grooves are displayed in (a). (b) shows a top view image of a field of lines and spaces, in (c) the lower left corner of the field is shown in higher magnification. Clearly, high density NWs are grown on the side facets of the V-grooves while NW growth is almost absent on the remaining facets. A tilted view SEM micrograph is depicted in (d), proving that the NWs grow only perpendicular to the 111 side facets of the V-grooves. (e) Cross-sectional view of the V-groove pattern with InAs NWs being grown on the 111 side facets of the grooves.
RESULTS & DISCUSSION The wet chemical etching produces V-grooves being bound by {111} facets, as obvious from the SEM micrograph displayed in Fig. 2a. These {111} planes are caused by the anisotropic nature of the KOH etching, having a significantly lower etch rate of the {111} than the {100} planes. Consequently, the side facets of the V-grooves draw an angle of 54.7° with respect to the (100) substrate.
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Figure 2b-e display typical top view SEM micrographs of a field of lines and spaces, a magnified image of a corner as well as tilted and cross sectional micrographs of the samples after NW growth. The NWs are perfectly aligned, meaning that (in the top view projection) they are grown only along the 011 and 011 directions of the Si substrate. An exception are the edges of the patterned fields where the NWs also align in the 011 and 011 directions due to the presence of additional {111} facets. The tilted view SEM image shown in Fig. 2d as well as the cross sectional image in Fig. 2e demonstrate that the InAs NWs grow perpendicular to the 111 side facets, thus they grow along the 〈111〉 directions of the Si substrate. The InAs NW growth directions draw an angle of 35.3° with respect to the Si 100 surface. Apparently, a high density of NWs is observed on the 111 facets while they are in general not found on remaining 100 facets. On these facets, only crystallites are observed. Similar observations were made when InAs NWs were grown on textured Si substrates and can be assigned to different oxide thicknesses on the (100) and (111) facets as well as a reduced nucleation probability on the (100) facets.21 To obtain NW junctions, the NWs have to be grown on opposing side facets but with (almost) the same lateral position in the V-grooves. A suitable yield of NW junctions is typically achieved when the pitch between two adjacent lines in the pattern is around 2 – 8 µm and the V-grooves are etched almost to the bottom. A detailed analysis of the impact of the distance of the spaces on the yield of NW growth and the formation of NW junctions is presented in the supplementary information (SI). The yield of NW junctions can further be improved by the selective area MBE growth of self-assisted NWs25 in combination with lithography on the V-grooves.
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Figure 3: Nanowire junctions with two (a) and three (d) arms. The junction in (a) forms an L-shape and is caused by the coalescence of two NW tips as schematically depicted in (b) and (c). Consequently this junction is a tip-to-tip junction. The junction shown in (d) is created when a NW tip coalesces with the side facet of another NW, thus a tip-to-side junction. This is schematically depicted in (e) and (f). The junction has a T-shape. The inset in (d) shows the onset of the junction formation the formation of the junction. The tip of the left NW influences the growth on the side facet of the right NW.
Once the NWs are grown on the V-grooves, NWs can coalesce during their growth and form NW junctions, similar as in the work performed by Dalacu et al. and Kang et al.14,15 They have related most of their structures to droplet dynamics, involving down crawling of the droplet, the emerging of new branches as well as the etching of the Au droplet into existing NWs. Using NWs grown without a catalyst particle, these droplet dynamics are avoided. Instead, the formation of the junction takes place in the solid and vapor phase. Here, three basic shapes of the junctions can be distinguished. These have an L-, a T- or an X-shape. A SEM micrograph of an L-shaped NW junction is displayed in Fig. 3a. This structure is formed via the coalescence of the tips of two NWs, as schematically depicted in Fig. 3b,c. Both tips vanish during the coalescence, resulting in the absence of further axial growth. Instead radial
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growth takes place which increases the diameters of the NWs. T-shaped NW junctions are displayed in Fig. 3d. Here, the tip of one NW touches the side facet of another NW. During this coalescence, one NW tip vanishes while the other one remains which enables a proceeding of the axial growth. The presence of the NW tip in close vicinity to the {110} side facets of another NW strongly influences the growth. This is evident from the SEM micrograph displayed as an inset in Fig. 3d. The NW tip approaches the side facets, resulting in a slight radial growth on the side facets in the vicinity of the tip, possibly being caused by a change of the local V/III ratio and correspondingly a change of the adatom diffusion. The junction between both NWs is achieved by both the axial extension via the NW tip and the radial growth on the side facets. This may affect the structure of the junction. Since the NW growth is epitaxial on the {111} side facets of the V-grooves, the angle between the two coalesced NWs is 109.47°, which displays the angle between two B directions in zinc blende semiconductors. The third possible junction has the shape of an “X” and is grown in a rather different way without tips playing an active role in the formation of the junction. These junctions are exemplarily displayed in Fig. 4a,b. Around the junction the entire structure is not planar, rather it is three dimensional. This is more evident in the top view SEM micrographs before and after coalescence depicted in Fig. 4c,d, respectively. Before coalescence, two separate NWs slightly displaced with respect to each other are present and their {110} side facets are in contact. This location subsequently acts as a sink for the adatoms resulting in radial growth and coalescence of the two NWs. The formation mechanism of this junction is schematically displayed in Fig. 4e,f from both side and top view. These X-shaped NW junctions are consequently formed in a similar way as observed by Car et al.17.
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Figure 4: Nanowire junction with four branches. The junctions form the shape of an “X” (a,b). The formation of such junctions at different growth stages are shown in (c)-(f). (c) Top view image before coalescence, two independent NWs touch each other with their {110} side facets. (d) Top view image after coalescence clearly evidencing the radial growth at the junction. (e,f) Side and top view schematic illustrations of the formation process of X-shaped nanowire junctions, before (e) and after (f) coalescence.
Due to the arbitrary position of the NWs, their high density and the length fluctuation, also more complex NW junctions are obtained. These complex junctions can always be subdivided into the three basic junctions L, T and X. A collection of such complex NW junctions is depicted in the SI. In two of the three different junctions (L- and T-shapes) NW tips are involved in the formation process while the remaining NW junction (X-shape) is formed by a pure radial growth on the side facets. CRYSTAL STRUCTURE OF InAs NANOWIRE JUNCTIONS The expected crystal structure at the NW junction can be derived from a simple model sketched in Fig. 5. Here, the patterned Si (100) substrate with its {111} lattice planes is
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depicted. NW growth is assumed to take place vertically on the Si {111} facets of the V grooves. Suppose both NWs have the WZ crystal structure, the result is as displayed in Fig. 5a. Here, the {0001} lattice planes of the WZ NWs are displayed. The junction cannot be wurtzite. This is further evident from the atomic model shown in Fig. 5b. Overlapping both WZ NWs at the junction does not create a uniform crystal lattice. Consequently, a single crystal WZ junction without grain boundary between two NWs growing in the {0001} directions is not possible. If both NWs adopt the zinc blende (ZB) crystal structure and the orientation of the substrate (that means no twinning occurs), naturally, the junction between the two NWs is single crystalline and reflects exactly the orientation of the substrate. This is displayed in the schematic in Fig. 5c as well as in the atomic models in Fig. 5d. Kang et al.15 already pointed out that the crystal structure at the merging point of two NWs growing along the (ZB) or (WZ) directions needs to be zinc blende. When one NW contains a twin boundary, e.g. the NW growing on the left facet of the Vgroove in Fig. 5e, again no monocrystalline junction is possible. The atomic model in Fig. 5f clearly illustrates this. In the case that both NWs contain a twin boundary, the crystal lattices at the junction do not agree and a monocrystalline junction is not possible either (see Fig. 5g,h). Thus, if the NWs are twinned, i.e. their crystal structure is rotated with respect to the Si substrate, a monocrystalline junction is not possible. Rather, a “twinned twin” is obtained.17 To be more precise, an even number of twins in both NWs can result in monocrystalline junctions whereas an odd number does not produce a single crystalline junction. Consequently, Car et al. came to the conclusion that 25% of their InSb NW junctions are monocrystalline.17 It is shown in the following, that all L- and T-shaped NW junctions presented here have a monocrystalline ZB junction, although the individual InAs NWs are full of stacking defects.
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In the present study, the NWs do not contain any seed particle, neither Au nor In. Consequently, the junction is formed without any etching into the NW and Au contamination is excluded. The NWs grow along the B growth direction of the zinc blende InAs crystal lattice and adopt the crystallographic orientation of the Si substrate. The crystal structure of self-assisted InAs NWs usually contains a high density of stacking faults and twin planes.26,27 Exemplarily, this is depicted in the SI. Recent studies have demonstrated that the crystal structure of InAs NWs is significantly modified by the addition of Sb, i.e. the formation of InAsSb NWs. This enables to obtain zinc blende InAsSb NWs with twin boundaries spaced by ~ 5 nm27,28 and even pure zinc blende InAsSb NWs29. In the case of the NW junctions, two independent and defect-rich NWs coalesce, making the merging point of high interest. The three different basic junctions, represented by the different types of coalescence (tip-to-tip, tip-to-side and side-to-side), may have different crystallographic properties which are affected by a formation process partly involving NW tips. These are described in the following.
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Figure 5: Sketch of the growth of NWs on the {111} facets of the V-grooves and atomic models displaying the possible atomic arrangements at the junction. (a,b) WZ NWs, (c,d) twin-free ZB NWs, (e,f) ZB NWs, one NW contains a twin, (g,h) ZB NWs, both NWs are twinned. Atomic models were created using VESTA30.
An L-shaped NW junction is displayed in Fig. 6a, a higher resolution image in Fig. 6b. The high density of stacking faults in both arms of the NW is obvious, whereas a uniform ZB crystal structure is observed at the elbow. The ZB structure in the region of the elbow is evidenced by the inset displaying the FFT pattern of ZB structure. The elbow is obtained after the NW tips coalesced, thus the residual triangular corner is filled by the zinc blende segment and the transition to the highly stacking fault rich region is abrupt (see Fig. 6c). Consequently, a monocrystalline junction between both NWs is obtained. A schematic of the coalescence is displayed in the inset of Fig. 6a. An alternative merging process for L-shaped NW junctions is ACS Paragon Plus Environment
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shown in the SI, there the tip of one NW merges with the side facets close to the tip of another NW. Nonetheless, also this results in a ZB structure at the junction.
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Figure 6: (HR)TEM images and diffraction patterns of the three different types of NW junctions. (a-c) L-shaped NW junction with a ZB phase at the elbow (see inset in b and HRTEM image in c). The schematic in the inset of (a) displays the coalescences and the red circle indicates the position of the ZB elbow. (d-e) T-shaped NW junction evidencing a crystal phase transformation of the stacking fault rich crystal structure of the InAs NW into ZB via Shockley partial dislocations.(f) HRTEM image showing the Shockley partial dislocations.(gi) TEM micrographs and diffraction pattern of an X-shaped NW junction with Moiré fringes at the junction. (j-l) TEM micrographs and diffraction pattern of an X-shaped NW junction with a monocrystalline ZB junction.
The growth behavior differs in the case of T-shaped NW junctions. Here, the tip of one NW touches and coalesces with the side facet of another NW. Figure 6d-f display TEM micrographs of this scenario. Interestingly, the crystal structure of the continuous NW at the position of the junction is zinc blende and is uniform over an axial distance of about 100 nm. Such long segments of a uniform crystal structure have not yet been observed in vapor solid grown InAs NWs. At two positions within the zinc blende segment, stacking faults not extending across the entire NW width are observed (see Fig. 6e,f). A close look reveals that the stacking faults change abruptly into a uniform zinc blende structure. This is a typical characteristic of Shockley partial dislocations having Burgers vectors of = /6〈211〉
0〉 (hexagonal notation). A detailed analysis of this (cubic notation) / = /3〈101 region is displayed in Fig. 7, it shows the FFT-filtered image with colored dots denoting the different layer stacks A, B and C. The white lines indicate the stacking sequence. While the crystal structure on the right part of the image is free of any stacking defects, several stacking faults are observed on the left side. The defective structure in the left part is not pure WZ but contains stacking faults and twins. Interestingly, the stacking sequences of the perfect ZB regions in the upper part of the image differ slightly, as obvious by the colored dots and the ACS Paragon Plus Environment
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yellow line. The stacking sequence is shifted by one atomic layer. Such a shift of atoms in a layer can be the result of Shockley partial dislocations.31 Shockley partial dislocations have already been associated with transformations of the crystal structure from pure WZ to ZB in semiconductor NWs31–33: Patriarche et al. observed Shockley partial dislocations and an associated atomic movement by burying WZ GaAs in a ZB GaAs layer31. Zheng et al. obtained a crystal phase transformation in Au-seeded InAs NWs by a remelting of the catalyzing Au droplet at high temperatures, creating a small portion of ZB which then spreads out laterally.32 It is important to mention that the previous studies by Patriarche et al. and Zheng et al. discuss the transformation of large, pure WZ segments into ZB31,32 while here the original crystal structure of the NW is full of stacking faults and twins. Recently, Jacobsson et al. associated a phase transformation in radially merged WZ GaAs NWs to the existence of Shockley partial dislocations, the crystal structure switched to a mixture of ZB and WZ.33 In case of the X-shaped NW junctions, Moiré patterns are observed in the junction (see Fig. 6g,h). These Moiré patterns indicate the presence of two crystal lattices with different lattice constants or a rotation with respect to each other. Here, the lattices are almost identical (InAs, WZ-ZB mixture) but the NWs are rotated by ~110° with respect to each other. A monocrystalline junction is not observed, being also evident from the diffraction pattern displayed in Fig. 6i. These Moiré fringes are observed in almost all of the X-shaped junctions but a small amount of the junctions have another crystallographic configuration. This is depicted in Fig. 6j-l. The TEM micrograph of the central part of the X-shaped junction exhibits a rhombic region having a size of about 170 nm x 200 nm which contains only few stacking faults. This is also clear from the corresponding diffraction pattern displayed in Fig. 6l. It shows a ZB pattern with only light streaks from the few stacking faults.
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Figure 7: FFT-filtered HRTEM image of the set of Shockley partial dislocations displayed in Fig. 6f. Colored dots denote the stacking sequence, which is further indicated by the white lines. The yellow line indicates the stacking sequence of a stacking fault free system. The arrows point at the first and the last stacking faults. Consequently, each type of NW junctions results in a specific crystallographic arrangement at the junction: The coalescence of two tips produces a ZB elbow. A tip merging with the side facet of another NW modifies the existing crystal structure via Shockley partial dislocations and extended segments containing ZB structure occur. A coalescence by radial growth forming a crossing of NW usually creates an overlapping of the two crystals separated by a grain boundary, however, occasionally can also result in a uniform ZB region. The phase transformations from the stacking fault rich structure to the almost pure ZB at the junction can usually be associated with the existence of Shockley partial dislocations, as it has been extensively outlined above. In few cases, single Frank partial dislocations have been identified in the ZB junctions. These also seem to result in a transformation of the crystal phase, as displayed in the SI. Additional TEM micrographs displaying different stages of the crystal phase transformation in tip-to-side NW junctions and a different morphology of a tipto-tip NW junction are depicted in the SI. There, it is observed that the tip of the NW merging with the side facets of the other NW already contains the ZB crystal structure, even though the phase transformation did not start, i.e. no large ZB segment with Shockley partial dislocations is found. Accordingly, the formation of the ZB phase at the NW tip is favored, possibly due ACS Paragon Plus Environment
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the close proximity of the 110 side facets of the other NW modifying the effective V/III ratio. This agrees with the SEM image shown in the inset of Fig. 3d displaying the merging process. Alternatively, a virtual melting at the interface between both NWs may occur taking place below the melting temperature.34,35 This can also lead to a transformation of the crystal phase. In order to gain a deeper understanding of the crystal phase transformation at the NW junction, in situ TEM investigations should be considered for future experiments. These will allow to unravel the details of the transformation process. This can be performed during the NW growth36 or after the growth by annealing at elevated temperatures37 since the phase transformation should be controlled by the dislocation velocity which is temperature dependent.38,39 First results on GaAs NW junctions grown via the vapor liquid solid mechanism with Ga droplets40,41 exhibit a similar crystal phase transformation, giving evidence that the phase transformation is applicable to different materials and different growth methods. Exemplary TEM micrographs demonstrating this are displayed in the supporting information.
ROOM TEMPERATURE TRANSPORT Here, we demonstrate that the merging of two nanowires does not compromise the electrical transport properties and that multi-terminal nanowire junction devices are in fact suitable for nanoelectronic applications. Different kinds of epitaxially merged InAs nanowire devices have been prepared and electrical transport in T-, and X-shaped junctions has been investigated at room temperature (see Fig. 8). The current-voltage ( − ) characteristics measured between any two terminals of the devices shown in Fig. 8 are linear. From the inverse slope of these curves and the device dimensions the resistivity is determined. Owing to diameter variations in the different nanowire branches an effective weighted diameter is calculated. All values of are found to be rather similar. For the T-junction device in Fig. 8a ACS Paragon Plus Environment
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the measurements involving the merged nanowire branch labeled C exhibit somewhat higher resistivities ( = 0.72 Ωcm and = 0.69 Ωcm) than the ordinary nanowire connection ( = 0.41 Ωcm). Taking all measurements into account the average resistivity is & = 0.61 Ωcm. In view of all samples investigated, the variation is not generic for merged branches with single-crystalline junctions but rather reflecting the variation among individual nanowires. Moreover, also the − measurements between different contact pairs of the X-shaped junction shown in Fig. 8b indicate that the resistivity is hardly affected by the presence of the junction region at the nanowire intersection. The resistivities of all branch combinations are very similar with a mean value of ' = 0.54 ± 0.18 Ωcm. Within the measurement accuracy, it seems evident that for the T- and the X-junction devices the presence of the nanowire junction in the current path does not affect the overall resistivity. This indicates that the merging of the nanowires does not result in the formation of a potential barrier, in agreement with the crystalline order of the junction area. A comprehensive transport study of many devices reveals that the resistance-length ratio R/L scales with the cross-section area of the nanowires, which implies that transport is not dominated by a surface electron accumulation layer but instead the entire nanowire cross-section constitutes the channel.42
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Figure 8: Room-temperature electrical transport through a nanowire junction device. (a) Current-voltage characteristics between pairs of the three branches of a T-junction. (b) Corresponding set of measurements for an X-junction. The insets show scanning electron microscope images of the contacted nanowires.
CONCLUSIONS In conclusion, we have studied the formation, crystal structure and its associated transformation as well as the room temperature transport properties of InAs NW junctions grown via a self-assisted growth mechanism on Si (100) substrates patterned with V-grooves bound by {111} facets. The NW growth on the exposed {111} facets lead to three basic configurations of NW junctions: tip-to-tip, tip-to-side and side-to-side junctions. The latter was found to be caused by a radial merging of two NWs. Despite of the stacking fault rich individual InAs NWs, all the tip-to-tip (L-shape) and tip-to-side (T-shape) junctions exhibited uniform ZB crystal structure, having the same orientation as the Si (100) substrate. Shockley partial dislocations act as the driving force for the solid-phase crystal phase transformation at the junction. In general, the X-shaped junctions were made up of two individual crystals, although also here single crystalline zinc blende junctions have been identified.
The
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alternative to a modification of the crystal phase using growth parameters. Current-voltage measurements performed at room-temperature further confirmed the potential of the junctions, since their presence did not affect the overall resistivity. The self-assisted formation of NW junctions reveals great potential for NW networks and mesoscopic physics involving T-shaped quantum transistors9 and the braiding of Majorana fermions7. EXPERIMENTAL METHODS SUBSTRATE PREPARATION The Si (100) substrates were covered by a 200 nm thick layer of hydrogen silsesquioxane (HSQ) deposited by spin-coating. The HSQ layer is used as a negative-tone e-beam resist as well as an etch mask for potassium hydroxide (KOH) based wet etching. Electron beam lithography followed by development in MF CD-26 was used to define lines and spaces of various dimensions in the HSQ layer. The obtained pattern was subsequently hard baked for 4h at 450°C. This thermal curing is crucial, since it significantly increases the etch resistance of the HSQ.43,44 Without the hard baking, the KOH etching removed the HSQ layer within a few seconds. The lines and spaces were transferred into the Si substrate via wet chemical etching in a solution containing 33 wt % KOH and 7 vol % isopropyl alcohol. The etching temperature was 40°C, while the etching time was 40 min. Prior to the introduction into the MBE system, the samples were wet chemically cleaned using diluted HCl45, piranha and diluted HF. Subsequently, a thin silicon oxide layer was created by wet chemical reoxidation in hydrogen peroxide22,46. GROWTH DETAILS
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InAs NWs were grown in a self-assisted manner23,24 at a substrate temperature of 490°C with an In rate of 0.05 µm/h and an As4 flux of 10-5 Torr22. Such growth conditions give rise to NWs grown without any droplet, i.e. growth takes place following a vapor-solid mechanism24. Typical growth duration ranged between 3h and 5.5h. CHARACTERIZATION DETAILS The morphological characteristics of the samples were analyzed by scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was used to gather information about the crystalline structure of the NW junctions. Here, a FEI Tecnai G2 F20 operated at 200kV was used. To determine the electrical characteristics of the NW junctions, they were dispersed on Si/SiO2 substrates. Subsequently, individual contacts to the NW branches were prepared by electron beam lithography and deposition of 100 nm thick Ti/Au contact leads. Ar+ sputtering prior to metal deposition was employed to improve the contact resistance.47
ACKNOWLEDGMENT Assistance with the MBE growth by Christoph Krause is gratefully acknowledged. Stefan Trellenkamp is acknowledged for electron beam lithography. The TEM facilities at the Ernst Ruska Centre are gratefully acknowledged. Daniil Vakulov gratefully acknowledges “The scholarship of the President of the Russian Federation for young researchers to conduct a research work abroad”. Supporting Information SEM images of the V-grooves and information on the density of nanowire junctions. SEM images of complex nanowire junctions. The crystal structure of the InAs nanowires. TEM micrographs showing the crystal structure and defects at the tip-to-tip and tip-to-side
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junctions. TEM micrographs demonstrating the crystal phase transformation on vapor liquid solid grown GaAs NW junctions. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Authors *Torsten Rieger, Peter Grünberg Institute (PGI-9), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany, E-Mail: [email protected]
ABBREVIATIONS NW nanowire; MBE molecular beam epitaxy; SEM scanning electron microscopy; KOH potassium hydroxide, HSQ hydrogen silsesquioxane, ZB zinc blende, WZ wurtzite, TEM, transmission electron microscopy, HRTEM high resolution transmission electron microscopy, FFT fast-Fourier transform REFERENCES (1)
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For Table of Contents Use Only:
Crystal phase transformation in self-assembled InAs nanowire junctions on patterned Si substrates Torsten Rieger, Daniel Rosenbach, Daniil Vakulov, Sebastian Heedt, Thomas Schäpers, Detlev Grützmacher and Mihail Ion Lepsa
The fabrication of catalyst free InAs nanowire junctions on Si (100) substrates patterned with V-grooves is demonstrated. Morphological investigations of the junctions reveal three structures having an L-, T- and X-shape. The merging process is found to induce a solid phase transformation of the crystalline structure of the junctions from a high density of stacking faults to zinc blende. The results pave the way to complex nanowire structures and a controlled modification of the crystalline structure of existing nanowires.
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Schematic of the substrate preparation for V-grooves on silicon (100) substrates including the NW growth. (a) Si substrate covered by HSQ. (b) electron-beam lithography. (c) Development and hard bake of HSQ. (d,e) KOH etching after subsequently increasing etching times. (f) NW growth. 199x90mm (300 x 300 DPI)
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SEM micrographs of the textured substrate (a) prior to and (b-e) after the NW growth. The V-grooves are displayed in (a). (b) shows a top view image of a field of lines and spaces, in (c) the lower left corner of the field is shown in higher magnification. Clearly, high density NWs are grown on the side facets of the Vgrooves while NW growth is almost absent on the remaining facets. A tilted view SEM micrograph is depicted in (d), proving that the NWs grow only perpendicular to the {111} side facets of the V-grooves. (e) Crosssectional view of the V-groove pattern with InAs NWs being grown on the {111} side facets of the grooves. 80x87mm (300 x 300 DPI)
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Nanowire junctions with two (a) and three (d) arms. The junction in (a) forms an L-shape and is caused by the coalescence of two NW tips as schematically depicted in (b) and (c). Consequently this junction is a tipto-tip junction. The junction shown in (d) is created when a NW tip coalesces with the side facet of another NW, thus a tip-to-side junction. This is schematically depicted in (e) and (f). The junction has a T-shape. The inset in (d) shows the onset of the junction formation the formation of the junction. The tip of the left NW influences the growth on the side facet of the right NW. 80x67mm (300 x 300 DPI)
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Nanowire junction with four branches. The junctions form the shape of an “X” (a,b). The formation of such junctions at different growth stages are shown in (c)-(f). (c) Top view image before coalescence, two independent NWs touch each other with their {110} side facets. (d) Top view image after coalescence clearly evidencing the radial growth at the junction. (e,f) Side and top view schematic illustrations of the formation process of X-shaped nanowire junctions, before (e) and after (f) coalescence. 80x80mm (300 x 300 DPI)
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Sketch of the growth of NWs on the {111} facets of the V-grooves and atomic models displaying the possible atomic arrangements at the junction. (a,b) WZ NWs, (c,d) twin-free ZB NWs, (e,f) ZB NWs, one NW contains a twin, (g,h) ZB NWs, both NWs are twinned. 250x206mm (150 x 150 DPI)
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(HR)TEM images and diffraction patterns of the three different types of NW junctions. (a-c) L-shaped NW junction with a ZB phase at the elbow (see inset in b and HRTEM image in c). The schematic in the inset of (a) displays the coalescences and the red circle indicates the position of the ZB elbow. (d-e) T-shaped NW junction evidencing a crystal phase transformation of the stacking fault rich crystal structure of the InAs NW into ZB via Shockley partial dislocations.(f) HRTEM image showing the Shockley partial dislocations.(g-i) TEM micrographs and diffraction pattern of an X-shaped NW junction with Moiré fringes at the junction. (j-l) TEM micrographs and diffraction pattern of an X-shaped NW junction with a monocrystalline ZB junction. 197x268mm (300 x 300 DPI)
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FFT-filtered HRTEM image of the set of Shockley partial dislocations displayed in Fig. 6f. Colored dots denote the stacking sequence, which is further indicated by the white lines. The yellow line indicates the stacking sequence of a stacking fault free system. The arrows point at the first and the last stacking faults. 119x80mm (300 x 300 DPI)
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Room-temperature electrical transport through a nanowire junction device. (a) Current-voltage characteristics between pairs of the three branches of a T-junction. (b) Corresponding set of measurements for an X-junction. The insets show scanning electron microscope images of the contacted nanowires. 376x183mm (300 x 300 DPI)
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TOC figure 80x40mm (300 x 300 DPI)
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