InAs CoreâShell Nanowires

Article pubs.acs.org/crystal

Crystal Phase Selective Growth in GaAs/InAs Core−Shell Nanowires Torsten Rieger,*,†,‡ Thomas Schap̈ ers,†,‡ Detlev Grützmacher,†,‡ and Mihail Ion Lepsa*,†,‡ †

Peter Grünberg Institute (PGI-9), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany Jülich Aachen Research Alliance for Fundamentals of Future Information Technology (JARA-FIT), 52425 Jülich, Germany

‡

S Supporting Information *

ABSTRACT: We present a novel type of core−shell nanowires in which only certain parts of the core are covered by the shell. This is achieved by the crystal phase selective growth of the InAs shell on zinc blende GaAs nanowires with controlled wurtzite inclusions. The shell grows on the zinc blende phase, but its growth is hindered on the wurtzite crystal phase. Nucleation of InAs occurs exclusively on the zinc blende GaAs regions. The wurtzite segments are placed inside self-catalyzed GaAs nanowires by partially consuming and refilling the Ga droplet. The crystal phase selective growth of InAs on the side facets of the GaAs nanowires is explained by the local environment of each new In atom. Because of unbalanced neighbors on the wurtzite side facets, the growth of a highly lattice mismatched material is hindered. This happens not only on the wurtzite segments, but also on regions being characterized by a high density of twins.

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INTRODUCTION Nanowires (NWs) are promising building blocks for future electronic and optoelectronic devices. In this context, structures with different doping1,2 or materials3,4 are mandatory for most devices. Two kinds of heterostructures in NWs are in principle possible: radial and axial ones. In a radial heterostructure, a NW core is surrounded by one or multiple shells. These can then act as a passivation layer for the core,5 form prismatic quantum heterostructures,6 or behave as tubular conductors.7,8 Such tubular conductors, for example, in the GaAs/InAs core−shell system, show flux periodic magnetoconductance oscillations.7,8 Additionally, InAs quantum dots can be positioned on the side facets of the NWs.9,10 Axial heterostructure NWs contain different materials stacked onto each other, which can result for example in quantum dots.11 The self-catalyzed NWs with axial heterostructures are difficult to grow using the vapor liquid solid (VLS) growth method due to possible memory effects in the droplet, different growth regimes,12 desorption,13 etc. In contrast, radial heterostructures are grown more easily as the catalyzing group III droplet can be consumed completely.1,14 However, there exists a third possibility to form heterostructures: A shell can cover only parts of the core and with that, cores with an axial heterostructure shell can be produced. Such anisotropy of the shell growth might be caused by an already existing axial heterostructure or, more interesting, by different crystal structures in the core. We name the latter mechanism “crystal phase selective growth” (CPSG). Recently, we have demonstrated the growth of GaAs/InAs core−shell NWs,14 where it was observed that a small part on the top of the GaAs core was not covered by the InAs shell. This was explained by the different crystal structure of that part grown during the Ga droplet consumption at the end of the © 2014 American Chemical Society

growth, which is wurtzite (WZ), compared to the rest of the NWs, which has the zinc blende (ZB) crystal structure with an increasing density of twins. On basis of this observation, we have shown afterward that the crystal structure of self-catalyzed GaAs NWs can be tuned by partial consumption and refilling of the Ga droplet, switching from ZB to WZ and backward in a controlled manner.15,16 Similarly, Krogstrup et al.17 and Yu et al.18 were able to control the crystal structure of GaAs NWs by changing the V/III ratio and optimized shutter controls, respectively. In this study, we investigate the CPSG of InAs on GaAs NWs with WZ insertions using molecular beam epitaxy (MBE). The estimation of the growth rate and maximum and minimum lengths of WZ segments in the GaAs NWs are presented. We analyze the transition from ZB to WZ and back to the ZB structure for WZ segments of different length. Morphological and structural characteristics of the CPSG of the InAs shell are described in detail. Finally, we introduce a model of the InAs CPSG.

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EXPERIMENTAL SECTION

The GaAs core NWs were grown on GaAs (111)B substrates covered by a thin layer of SiOx using a Ga rate of 0.075 μm/h and an As4 beam equivalent pressure (BEP) of ∼10−6 Torr.15 The substrate temperature was set to 590 °C. To obtain the WZ insertions, the growth program shown schematically in Figure 1 was used. The GaAs NWs were first grown continuously for 50 min. Then, different Ga supply interruptions and refillings were applied to partially consume and Received: November 7, 2013 Revised: January 24, 2014 Published: January 31, 2014 1167

dx.doi.org/10.1021/cg401667v | Cryst. Growth Des. 2014, 14, 1167−1174

Crystal Growth & Design

Article

Figure 1. Growth sequence showing the Ga supply interruptions and the temperature curve for core and shell growth. Gray colored areas indicate intentionally placed WZ segment in the GaAs core.

Figure 2. (a) Stitched TEM micrograph of the pure GaAs NW. Red lines denote for WZ segments while black and blue lines indicate long ZB segments and the regions in-between two WZ segments, respectively. (b) TEM micrograph of a WZ segment in a pure GaAs NW grown during 8 min of Ga supply interruption. (c and d) HRTEM micrographs of the corresponding transition regions ZB-WZ and WZ-ZB, respectively. The large arrow in panel b indicates the NW growth direction while the arrows in panels c and d point to WZ segments. dispersive X-ray spectroscopy (EDX). Stitched TEM micrographs were produced using the MosaicJ plugin for ImageJ.19

restore the droplet, respectively. Gray areas in Figure 1 correspond to WZ segments, which are grown during the interruptions. Further details about the growth interruption method can be found in ref 15. Different durations of GaAs growth between two subsequent Ga supply interruptions act as markers for the further analyses. After the core growth, the Ga droplet was consumed completely and the substrate temperature was ramped down to 490 °C. The InAs shell was grown for 10 min using an In rate of 0.1 μm/h and the same As4 BEP as before. At the end, the substrate was cooled down and below 350 °C, the As shutter was closed. A second sample was grown without the InAs shell. This sample differs additionally from the core− shell sample in the sense that the GaAs growth after the fifth interruptions was set to 10 min instead of 20 min as for the core−shell sample. The NWs were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy

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RESULTS AND DISCUSSION

Figure 2a shows a stitched TEM micrograph of an entire GaAs NW. It can be seen that the NW has long regions without defects as well as short ones, in-between the defect density is high. The short defect-free regions are marked with red lines while the long defect-free regions are marked black. High resolution TEM (HRTEM) investigations showed that the long defect-free regions are ZB while the short ones are WZ. The blue lines in Figure 2a mark the regions between two adjacent WZ segments. Correlating the TEM micrograph with the growth sequence of the GaAs NW shown in Figure 1 reveals 1168



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Figure 3. SEM micrographs of a core−shell nanowire with the core having several WZ segments as given by the growth sequence shown in Figure 1. (a) Overview of the nanowire. (b−e) Higher magnifications depicting all visible interruptions from the top to the bottom of the NW. (f) High magnification image of a single interruption; on the right side, the different materials are color-coded (red, InAs; blue, GaAs). The materials and the shape are identified by the contrast of the SEM micrograph.

Figure 4. TEM micrographs demonstrating the CPSG. (a) Bright field TEM micrograph of a complete NW, in which several gaps in the shell can be seen. (b and c) HAADF images and superimposed EDX line scans. (d) Schematic illustration of a shell gap. (e) Low resolution TEM micrograph of a single shell gap, similar to that in panel c. (f and g) HRTEM images demonstrating that the shell growth is also affected by the presence of stacking faults or twins. The red bar in panel f indicates a region with high twinning density, while the green arrows point to WZ segments. The red arrow in panel g points at the twin boundary and the associated reduction of the shell thickness, while the green arrow points at the (inclined) stacking fault in the shell. Growth direction is from the bottom to the top.

part with a 8 min Ga supply interruption are shown. For shorter interruptions, similar information is presented in the Supporting Information, section 1b. One can see that the crystal structure changes from ZB to WZ and back. The transition from ZB to WZ is narrower than vice versa. For the 8 min Ga supply interruption, the transition from ZB to WZ occurs within ∼100 nm, while it takes about ∼200 nm to switch back to the ZB phase. However, the transition lengths strongly

that the short WZ segments are grown during the Ga supply interruptions. During the Ga supply interruptions, As consumes Ga from the Ga droplet, reducing the contact angle and giving rise to nucleation at the triple phase line.18 In each case, the lengths of the blue marked regions correspond to a growth rate of about 1.8−2 μm/h, being in agreement with the previously determined GaAs NW growth rate using similar parameters.16 In Figure 2b−d, TEM and HRTEM micrographs of the NW 1169



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lengths in the shell, which can be observed by changes in the contrast as well as in the NW diameter. In order to verify the suppression of InAs growth on certain parts, EDX line scans were acquired. Four such EDX line scans on a shell gap are superimposed over a high angle annular dark field (HAADF) image in Figure 4b. The green and red dots denote Ga and In, respectively. As seen from the horizontal scans, a GaAs/InAs core−shell structure is observed in the upper and lower profile, while no In was found in the middle profile. This has been confirmed by an additional scan in the vertical direction, where In is detected only in the upper and lower part of the gap while the middle is free of In. The gap in the shell has a length of about 100 nm. Because of the limited sensitivity of EDX, the presence of an InAs wetting layer on the WZ GaAs crystal phase can neither be excluded nor confirmed. A shorter gap of only ∼50 nm length is shown in Figure 4c. Again, EDX was used to confirm the absence of In. The blue triangles represent the In signal multiplied by 5 in order to see the shape of the In profile more clearly. The slope of the line is steeper at the end of the gap, demonstrating that the shell profile is more abrupt when changing from WZ to ZB than vice versa. Interestingly, this is inverse than the switching of the crystal structure in the core, where we have already shown that the change from pure ZB to pure WZ is more abrupt than from WZ to ZB (see Figure 2 and Supporting Information, section 1b). However, important for the transition regions presented here is the exact crystal structure, especially the presence of any WZ segment. This will be discussed later. The information about the shell abruptness at the gap obtained using the EDX profiles is in qualitative agreement with the results from the SEM micrographs shown in Figure 3. There, clearly different shapes are seen at the beginning and the end of the gap (see Figure 3f). A simple sketch of the shell gap based on the results obtained from TEM, EDX and SEM is drawn in Figure 4d. Figure 4e depicts the gap shown in c by a low resolution TEM micrograph. The presence of the gap is obviously demonstrated by the absence of Moiré fringes and a narrower diameter. Above and below the gap, stacking faults and twins can be seen, these being correlated with a similar phase structure present in the core. As already observed in Figure 3f and shown schematically in Figure 4d, the InAs originating from the top covers less the WZ GaAs core than that one starting from the bottom. The growth dynamics of the InAs shell are indicated by a time-dependent growth series in the Supporting Information, section 2. There, it is found that nucleation of the InAs takes place only on the ZB phase, and a continuous shell covering the ZB regions is formed after few minutes of shell growth. The growth on the WZ phase starts later, in each case originating from the ZB phase. The small, tongue-like growth front on the WZ phase is seen until coalescence of the InAs layer occurs. Additionally, the discontinuity in the diameter is maintained even after 30 min of shell growth, demonstrating the hindered growth on the WZ phase. Thus, growth on the WZ phase is significantly slower than on the ZB phase and maintains the tongue-like profile until they coalesce. For the entire investigated shell growth durations, no direct nucleation of InAs on WZ GaAs is observed. Only lateral growth of the InAs tongues originating from the ZB parts. The results presented so far show that the growth of InAs on the WZ phase of the GaAs core is hindered, but not suppressed completely. In fact, InAs does not grow directly on the WZ GaAs core but it advances from the ZB regions. This grows

depend on the NW growth history. It is found that for an earlier Ga supply interruption, the transitions are more abrupt than for another one which occurs later in the growth sequence, for example, transition lengths from ZB to WZ of ∼20 and ∼40 nm and from WZ to ZB of ∼40 and ∼60 nm were measured for Ga supply interruptions of 3 and 4 min, respectively. The interface region contains both crystal structures, but in general, a larger amount of the ZB phase. The amount of WZ phase in the transition region is higher (or equal) when switching from WZ to ZB than vice versa (see Figures 2c and d and Figure S3 in the Supporting Information). In this context, the transition regions from ZB to WZ have 3, 4, and 7 short WZ segments for Ga supply interruptions of 3, 4, and 8 min, respectively. For the same interruptions, 3, 6, and 13 short WZ segments were found within the transition from WZ to ZB. The length of the WZ segments in the GaAs NW core not only depends on the consumption time but also on the initial size and contact angle of the droplet. This is demonstrated by simulation results presented in detail in the Supporting Information, section 1. Because of the reduction of the droplet contact angle, the growth rate of the WZ phase decreases in time, with a steeper slope for thinner NWs. The maximum length of the WZ segment is proportional to the NW radius, the proportionality factor depending on the initial droplet contact angle. Thus, theoretically, starting with the contact angle observed experimentally of about 137°,15 a WZ length corresponding to 10 times the NW radius can be grown during the Ga droplet consumption. As expected from the simulations, using the program from Figure 1, we could insert WZ segments with the length varying from few tens to several hundreds of nm as can be seen by the red lines in Figure 2a. Further details about the formation of the WZ segments and their limitations can be found in ref 15 and the Supporting Information. After the growth of the InAs shell, the NWs look like in Figure 3. As seen in the overview image in Figure 3a, on the GaAs core there are regions where the diameter is reduced. At these constrictions, it seems that the shell did not grow or grows only with a significant lower rate. Using the abovementioned marker technique (different durations of GaAs growth between two Ga supply interruptions), a clear correlation between the constrictions/shell gaps and the growth sequence is given. The distance between two adjacent shell gaps reflects the GaAs NW growth rate, that is, again 1.8−2 μm/h. Additionally, the length of the shell gaps increases along the NW axis. This is once again in agreement with the growth sequence and the previously shown investigation of the pure GaAs NW. Overall, it can be concluded that the shell gaps have the WZ crystal structure while InAs shells are grown over the GaAs core having the ZB phase. In the SEM images in Figure 3b−e, it can be seen that the lengths of the gaps vary from few nanometers to several hundreds. As seen in the SEM micrographs with higher magnification (Figure 3b−f), the InAs coverage does not end abruptly but forms tongue-like shapes. Thus, the shell thickness slowly decreases and also its lateral dimensions on the side facets decrease. These tongues are more pronounced in the NW growth direction than in the reverse direction. For further investigation of the correlation between the growth and the shell gaps, the NWs were examined by TEM. The results clearly show that the core crystal structure corresponding to the shell gaps is WZ, being embedded by stacking fault-rich parts. Exemplary TEM micrographs are presented in Figure 4. The complete NW from Figure 4a has several gaps of different 1170



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Figure 5. (a) HRTEM image of the InAs shell grown on the transition region from ZB to WZ of the GaAs core. Insets are FFTs taken from the framed areas. (b) HRTEM image of the InAs shell grown on the opposite transition region from WZ to ZB. Again, FFTs shown in the insets correspond to the framed areas.

slowly on the WZ {21̅ 10} facets in [0001]/[0001̅] direction (see Supporting Information, section 2). The growth on the WZ {2̅110} facets in ⟨11̅00⟩ direction is even slower explaining finally the already mentioned tongue like profiles of the InAs shell at the gaps. This situation is additionally illustrated in the Supporting Information, sections 2 and 3 by SEM micrographs for the time dependent growth series and small shell gaps. When the WZ segment in the GaAs core is small, the lower and upper fronts of the InAs shell almost meet in the center of the NW side facets or there is no gap due to coalescence (see Figure S6 in the Supporting Information, section 3). The extending of the InAs shell on the WZ segments of the GaAs core in the way described above can be correlated with the crystal phase structure of the transition regions between the pure ZB and WZ phases in the core. The most dominant impact comes from the WZ phase but stacking faults and twins from these regions have also an influence. All these were investigated by HRTEM. Figure 4f and g show HRTEM micrographs from positions where the shell is not even. In any situation, an uneven shell can be associated with defects in the core. In Figure 4f, a reduction of the shell thickness is observed at two positions. In the first case (indicated by the red bar), this is due to a local high density of twins. The second reduction of the shell thickness (green arrows) is due to the occurrence of WZ segments in the core. However, not only high defect densities or longer WZ segments can affect the shell growth, but even a single twin, as shown in Figure 4g, can do that. There, the thickness reduction is correlated with a single twin boundary separating two ZB regions (red arrow). It should be mentioned that the core NW always has smooth side facets, independent of twin boundaries or stacking faults (see Figure 2 and Supporting Information Figure S3). No twin-induced roughening of the GaAs side facets is observed. Thus, the thickness reduction is not due to morphological changes in the core but due to growth dynamics induced by crystallographic defects. Apart from the thickness reduction, an inclined stacking fault in the shell is seen (green arrow). This stacking fault is only present in the shell and not in the core. Most likely, this is due to Shockley partial dislocations at the GaAs/InAs interface. As it was shown in ref 14, the strain is relaxed by the

inclusion of different dislocations: perfect dislocations as well as Frank and Shockley partial dislocations. The latter two introduce stacking faults in the shell. Although even single or few crystal defects such as twins can influence the growth of the InAs shell, WZ segments have the most dominant impact. In this context, the growth of the InAs shell in the vicinity of a long WZ segment in the GaAs core similar with that discussed above (see Figure 2 and related text) is analyzed in Figure 5. Figure 5a refers to the InAs growth on the transition region from ZB to WZ. Fast Fourier transforms (FFTs) were performed on different parts along the transition as indicated by the colored frames. The FFTs are plotted as insets. It is evident that the crystal structure on the left-hand side is twinned ZB (inset I) and a core−shell structure is formed. Contrary, on the right side the crystal structure is WZ (inset VI) without a grown shell. In the transition region, both crystal structures can be observed (insets II−V), but it is mostly ZB with a high density of twins. The shell thickness decreases stepwise, every step being finished with a WZ segment (insets III and V). The WZ stripes within the transition region are pointed out by arrows. In the core, when switching from ZB to WZ, the number of WZ segments is small (see Figure 2a); therefore, the growth is hindered only in few positions. The growth front reaches the long WZ segment relatively fast and once it does, it continues to grow there slowly (inset V). The shell growth when changing from WZ to ZB, is shown in Figure 5b. Again, several FFTs were calculated and are marked by colored frames. As seen, the structure changes from a pure WZ core (inset I) to a ZB core−shell structure (inset VI). In between, both phases are found (insets II−V). Similar as before, the transition is characterized by several steps, each ending (in the direction toward the long WZ segment) with a thin WZ region (inset IV). The number of WZ segments within the transition region is higher when changing from WZ to ZB than vice versa, identical as in the GaAs core (see Figure 2). Since each WZ segment acts as a barrier for the growth of InAs, this means that the shell grows slower toward the NW bottom. Additionally, this transition region in the shell is more abrupt than that from ZB to WZ, having a smaller extension of the InAs shell on the WZ GaAs. The same was observed also in the 1171



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directions for both crystal structures, Δai = aInAs,i − aGaAs,i/ aGaAs,i and Δci = cInAs,i − cGaAs,i/cGaAs,i, where i stands for ZB or WZ, it comes out that the lattice mismatch along the a direction is slightly smaller in the ZB lattice than in the WZ one while along the c direction this is larger in the ZB structure than in the WZ structure (see Table 1). Although there is a large uncertainty in the exact lattice mismatch, the general tendency is clear: in the cubic lattice the mismatch is identical in all directions while in the WZ lattice this differs being smaller in the c- than in the a-direction. As shown before in the SEM and TEM micrographs, the growth of InAs on WZ GaAs occurs, but being much slower than on ZB GaAs and developing from the ZB phases. The growth proceeds via the formation of “tongues”, that is, having a small growth front and being laterally confined, demonstrating that the growth rate along the c-direction is higher than along the a-direction. In this sense, additional information is presented in the Supporting Information, section 3, where SEM micrographs show almost closed shells on core WZ segments, only the edges being still opened. This means that the growth along the a-direction is the restricting factor, finally hindering the growth on the WZ phase. The conclusion is supported by the fact that the lattice mismatch along the a-direction is larger in the WZ phase than in the ZB one (see Table 1). The impact of the lattice mismatch in the a-direction is even enhanced when the local environment for each new incorporated atom is taken into account. Figure 6a and b show nonreconstructed {110} and {2̅110} GaAs surfaces corresponding to the ZB and WZ phases, respectively. After reconstruction of the {110} surface, the positions of As and Ga in the upper layer change only slightly: As moves upward while Ga atoms move downward.26,27 However, the general shape of the surface (viewed along the [11̅0] direction) is not modified. For the WZ GaAs {2̅110} facets, no information about the reconstruction is reported. In this sense, for simplicity we assume a nonreconstructed surface for both cases. In the bulk, each group III atom is surrounded by 4 group V atoms forming a tetrahedron. The second nearest neighbors are again of group III having a coordination number of 12. They form a cuboctahedron and anticuboctahedron in zinc blende and wurtzite, respectively. The cuboctahedron (see Figure 6c) is characterized by the inversion symmetry which does not apply for the anticuboctahedron (see Figure 6d). These different symmetries become important when lattice mismatched materials are combined. The blue circles in Figure 6 a and b denote a new In atom, which is incorporated on the side facets in order to form a nucleus for a new layer. This new In atom will see a repulsive force of each surrounding Ga atom. On ZB GaAs, these forces will cancel out each other along the [1̅1̅1̅] and the [1̅2̅1] directions, regardless of the placement of the In atom on the stacking sequence (see Figure 6a). In the WZ arrangement however, the distribution of existing Ga atoms around the new In atom is not homogeneous. In the drawing shown in Figure 6b, one can see that when the In atom is placed on an A-layer of the stack, three neighboring Ga atoms are located on the left side while only one is on the right side of the In atom. This arrangement will then shift the new In atom slightly to the right in order to minimize its energy. On the Blayer the situation is inverted and the new In atom will be shifted to the left. Thus, when growing on the WZ GaAs, the In atoms move closer together being contrary to the normally larger lattice constant of InAs and making this growth nonpreferred. This does not only happen for the first InAs

EDX line scans from Figure 4c. This is attributed to the different amounts of WZ segments in the transition region. Comparing the results shown in Figure 5 and Figure 2, we can assert that the crystal structure of the shell repeats the one of the core, including all stacking faults, twins and short WZ segments. It should be mentioned here, that we have tried a variety of growth conditions for the InAs shell and never observed a direct growth of the InAs on the WZ GaAs. Even decreasing the substrate temperature for the shell growth by 100 °C and therefore decreasing the adatom mobility did not change the growth behavior. However, in many situations, especially when the As4 BEP is increased, even the growth of InAs on ZB GaAs was limited to only few spots without forming a continuous layer (see ref 14). A definitive explanation of the observed CPSG is difficult to give. However, in the following, we attempt to discuss several possibilities. First of all, the wurtzite side facets ({2̅110}) have a lower surface energy than the cubic side facets ({110})20,21 and the facet energies of InAs are in general lower than those of GaAs. Thus, from this point of view, the growth of InAs on both facets ({110} and {2̅110}) would be expected since it means a gain in energy compared to GaAs side facets. However, not only the side facet energy is important but also the bulk energy, which is lower for the cubic phase.22 In this sense, the lower bulk energy of the cubic phase could be an explanation for the observed CPSG: the growth of InAs occurs on the crystal phase where its total energy will be the lowest. Because of the relatively large NW diameters, the total energy will be dominated by the bulk energy and the energy associated with dislocations formed at the interface. The energy of the side facets will play a minor role. However, the difference in bulk energy between ZB and WZ InAs is very small,22 making the observed strong discrepancy in the growth hard to understand. Additionally, it does not explain why existing stacking faults and twins in the core have an impact on the shell growth. A second possibility is related to different lattice mismatches in the zinc blende and wurtzite crystal structures. The ZB cubic lattice, having the lattice constant ac, can be converted into an equivalent hexagonal structure with the lattice constants aZB = ac/√2 and cZB = ac√3 along the cubic [110] and [111] directions, respectively. In Table 1, experimentally determined Table 1. Experimental ZB and WZ Lattice Parameters for GaAs23,24 and InAs23,25 and the Corresponding Lattice Mismatcha GaAs InAs lattice mismatch Δai, Δci

ac

aZB

cZB

aWZ

cWZ

5.653623 6.058323

3.9977 4.2839 0.0715

9.7923 10.4933 0.0715

3.98824 4.274225 0.0718

6.56224 7.025225 0.0705

a

The ZB lattice parameters are converted into a hexagonal system using aZB = ac/√2 and cZB = ac√3. All values are in Å.

values of the hexagonal ZB and WZ lattice parameters for GaAs and InAs are listed. As seen, cZB/pZB < cWZ/pWZ (with pZB = 3 and pWZ = 2, the number of bilayers along the c-axis within the unit cell) and aZB > aWZ. Thus, the neighboring atoms along the c-axis are more close in the ZB structure while along the a-axis, they are more close in the WZ structure. After calculating the lattice mismatch between GaAs and InAs along the a and c 1172



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Figure 6. (a and b) Models of the nonreconstructed NW side facets for ZB and WZ GaAs phases, respectively. Large circles denote atoms at the most outer position, while small circles are for the layer behind. The blue circle shows the next incorporated In atom. Blue lines around the In atom indicate the second nearest neighbor atoms. (c and d) NW surface with the complete coordination polyhedron (second nearest neighbors) for ZB and WZ structures, respectively. A cut along the gray plan gives the same nearest neighbors as in panels a and b. Labels next to the coordination polyhedron refer to the layers within it. The central atom (in layer 2) corresponds to the blue In atom in panels a and b. Panels c and d were created using VESTA.32

different ZB directions were observed. This has been explained by a higher nucleation probability on the ZB phase compared to the WZ one. However, in both cases direct growth on both crystal structures was observed, being different than in the situation discussed here. Although the nucleation probability on the ZB and WZ phase might also be strongly different in our case (due to the above-mentioned reasons of the local environment and strain), a model only based on the nucleation probability and growth rate cannot explain the observed impact of twins and WZ segments.

monolayer, but also for the next ones. We have shown previously using Moiré fringes14 and X-ray diffraction28 that the relaxation in the InAs shell increases with the shell thickness. Thus, the first layer of InAs on GaAs will have a lattice constant close to that one of GaAs. Each further layer goes nearer to the lattice constant of InAs. So, even in the next layers of InAs on strained WZ InAs, newly attached In atoms see the force mentioned above. Figures 6c and d show the full coordination polyhedron on ZB and WZ side facets. As can be seen in the ZB case (Figure 6c), the forces in the ⟨1̅1̅1̅⟩ and the ⟨2̅1̅1⟩ directions on the central In atom are balanced within each of the side facet layers (layers 1, 2, and 3).29 Contrary, in the WZ case, the forces in ⟨0001̅⟩ and ⟨11̅00⟩ directions acting on the central atom are balanced only when also the layer labeled 3 is completed. Adding atoms to layer 2 will shift the central atom in the reverse direction than it was done by atoms from layer 1. Layer 3 will provide again a force in the same direction as layer 1, finally both of them compensating layer 2. Here, it has to be mentioned that an In atom on the A-layer will also add a force on surrounding In atoms on the B-layers. Because of this force, that one induced by the Ga atoms underneath can be partially compensated. However, on edges the forces will always be unbalanced, hindering the lateral growth of a nucleus. This model does not only explain the suppressed direct growth of InAs on WZ GaAs, but also gives an explanation on the influence of stacking faults and twins on the growth of InAs. So, each stacking fault or twin can be seen as a single layer of WZ, making any growth there unfavorable. Finally it should be mentioned that differences in the growth of core−shell structures depending on the crystal structure have been observed previously, for example when growing InP shells around InAs NWs30 or GaAsSb shells around GaAs NWs.31 The growth rate on the WZ phase was found to be lower than on the ZB phase and even differences in the growth rate along

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CONCLUSIONS

In conclusion, we have demonstrated the CPSG of InAs on ZB GaAs NWs with WZ insertions. The controlled insertion of WZ segments in ZB GaAs NWs is realized by Ga droplet consumption and refilling. Using SEM and TEM (EDX), the CPSG of InAs shell is carefully investigated. We find that the growth of InAs on the WZ phase of the GaAs core is hindered, but not suppressed completely. The shell does not grow directly on the WZ GaAs core but it advances slowly from the neighboring ZB region. Because of the different growth rate on the WZ {2̅110} facets in [0001]/[0001̅] and ⟨11̅00⟩ directions, the InAs shell has a tongue-like profile at the gaps. (HR)TEM structural analyses of the interface between ZB and WZ phases in the core and the gap region in the shell are correlated and show the major effect of the WZ phase but also crystal defects, such as twins, influence the growth of the shell. The observed selective growth of the InAs is qualitatively explained in terms of different lattice mismatches in the ZB and WZ crystal structures and the local structural environment of In atoms. Because of unbalanced neighbors on the WZ side facets, the growth of a highly lattice mismatched material is hindered. This novel growth phenomenon is specific only to NWs because of 1173



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their polytypism between ZB and WZ and not observed in layers which in general have ZB crystal structure. The presented growth mechanism gives new insights in understanding the differences between zinc blende and wurtzite crystal structures and can represent the base for new devices. For example, the position of InAs quantum dots on GaAs NWs9,10 might be controlled by controlling the crystal structure of the core and InAs shell segments forming constrictions or having point contacts to neighboring segments can be fabricated. For the latter, Supporting Information Figure S5 shows a SEM micrograph with two almost coalesced “tongues”, which can be considered as a specific point contact. Their dimensions can then be controlled by the NW diameter and the InAs shell growth time.

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ASSOCIATED CONTENT

* Supporting Information S

Description of the droplet consumption, achievable WZ segment lengths, HRTEM images of the growth interruptions, SEM micrographs of a shell growth time sequence, SEM micrographs showing the coalescence of InAs tongues on WZ GaAs, and SEM micrograph demonstrating the possibility to form point contacts between the InAs layers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-Mail: [email protected] *E-Mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The TEM facilities at the Ernst-Ruska-Centre are gratefully acknowledged. Assistance with the TEM and the MBE growth by Martina Luysberg and Christoph Krause, respectively, are gratefully acknowledged. Andreas Biermanns is acknowledged for discussions about the lattice parameters.

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ABBREVIATIONS NW nanowire; MBE molecular beam epitaxy; BEP beam equivalent pressure; SEM scanning electron microscopy; TEM transmission electron microscopy; HRTEM high resolution transmission electron microscopy; HAADF high angular annular dark field, EDX energy dispersive X-ray spectroscopy; CPSG crystal phase selective growth; ZB zinc blende; WZ wurtzite; FFT fast Fourier transform

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REFERENCES

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NOTE ADDED IN PROOF Two recent publications (Hjort et al., Nano Lett. 2013, 13, 4492; Capiod et al., Appl. Phys. Lett. 2013, 103, 122104) indicate that the assumption of nonreconstructed GaAs nanowire side facets seems to be valid. The authors thank Martin Hjort for providing this information.

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