Self-Seeded Axio-Radial InAsâInAs1âxPx Nanowire Heterostructures

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Self-seeded axio-radial InAs-InAs P nanowire heterostructures beyond ''common'' VLS growth bernhard mandl, Mario Keplinger, Maria E Messing, Dominik Kriegner, Lars Reine Wallenberg, Lars Samuelson, Günther Bauer, Julian Stangl, Vaclav Holy, and Knut Deppert Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03668 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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Self-seeded axio-radial InAs-InAs1−xPx nanowire heterostructures beyond ”common” VLS growth Bernhard Mandl,∗,†,k Mario Keplinger,† Maria E. Messing,‡,⊥ Dominik Kriegner,† Reine Wallenberg,¶ Lars Samuelson,‡ Günther Bauer,† Julian Stangl,† Václav Holý,§ and Knut Deppert‡ Semiconductor and Solid State Physics, Johannes Kepler University Linz, A-4040 Linz, Austria, Solid State Physics, Lund University, S-22 100 Lund, Sweden, Polymer and Materials Chemistry, Lund University, S-22 100 Lund, Sweden, and Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic E-mail: [email protected]

Abstract Semiconductors are essential for modern electronic and optoelectronic devices. To further advance the functionality of such devices the ability to fabricate increasingly complex semiconductor nanostructures is of utmost importance. Nanowires offer excellent opportunities for new device concepts - heterostructures have been grown in either the radial or axial direction of the core nanowire, but never along both directions at the same time. This is a consequence of the common use of a foreign metal seed particle with fixed size for nanowire heterostructure ∗ To

whom correspondence should be addressed and Solid State Physics, Johannes Kepler University Linz, A-4040 Linz, Austria ‡ Solid State Physics, Lund University, S-22 100 Lund, Sweden ¶ Polymer and Materials Chemistry, Lund University, S-22 100 Lund, Sweden § Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic k Solid State Physics, Lund University, S-22 100 Lund, Sweden ⊥ Polymer and Materials Chemistry, Lund University, S-22 100 Lund, Sweden † Semiconductor

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growth. In this work we present for the first time a growth method to control heterostructure growth in both axial and radial directions simultaneously, while maintaining an untapered self-seeded growth. This is demonstrated for the the InAs/InAs1−x Px material system. We show how the dimensions and composition of such axio-radial nanowire heterostructures can be designed including the formation of a ’pseudo supperlattice” consisting of five separate InAs1−x Px segments with varying length. The growth of axio-radial nanowire heterostructures offers an exciting platform for novel nanowire structures applicable for fundamental studies as well as nanowire devices. The growth concept for axio-radial nanowire heterostructures is expected to be fully compatible with Si substrates.

Keywords: Nanowires, axio-radial heterostructure, epitaxy, nanowire growth mechanism

Semiconductor nanowires have demonstrated to be highly interesting e.g. for electronic and optoelectronic devices and photovoltaics 1–13 , especially those consiting of III-V semiconductors due to their high electron mobility and direct bandgap. The growth of nanowire heterostructures is a key requirement for such devices and is usually focused on the control of either the axial 14–20 or the radial 21–26 direction. But so far the simultaneous control of axial as well as radial particle assisted heterostructure growth while prohibiting tapering has not been mastered. This would offer control over the dimensions of two connected heterostructures at once and enable interesting new opportunities, e.g.: for fundamental studies a ”superlattice” of connected 0D and 1D nanostructures can be formed. For device fabrication directly connected quantum dots or channels wrapped around a core wire with a direct top contact can be designed. Over the last years tremendous progress on self-seeded VLS growth of nanowires has been made, especially in controlling nanowire nucleation 37,38 and nanowire properties 12,39,39–43 , applying the growth method to different material systems 12,35,36,44,45 as well as in developing the growth theory to describe this type of nanowire growth 46,47 . While the growth of radial or axial heterostructures can be controlled quite well individually the combined homogeneous growth has not been accomplished so far 27,28 . This deficiency results from the use of ”foreign” seed particles

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(e.g. gold) in vapour liquid solid (VLS) growth. These seed particles provide a method to initiate and control nanowire growth. One of the key properties is that the diameter of the nanowire directly at the interface to the seed particle is determined by the seed particle diameter and can not change over the entire nanowire growth process 29,30 except at the interface of an axial heterostructure 31–33 . If nucleation at the entire nanowire side walls occurs when using e.g. gold as seed particle it leads to a non-uniform diameter along the nanowire length as the diameter below the seed particle can not change while the diameter along the rest of the wire continuously increases with time resulting in the largest diameter at the wire base. This effect is commonly refered to as tapering 13,28,30,34 . This tapering is the reason for the incompatibility of uniform axial and radial heterostructures when using ”foreign” seed particles for nanowire growth. In the case of side wall growth during nanowire growth a tapered radial growth (shell) is formed around the nanowire instead of the required uniform shell 28 , this situation is illustrated by the red X in Fig. 1 (a). To overcome this limitation as required for homogeneous radial heterostructure growth (commonly referred to as core-shell heterostructures) the growth parameters for shell growth are chosen in a way that layer growth is dominant, turning the seed particle ineffective and consequently terminating nanowire growth 21 , illustrated by the red X in Fig. 1 (c). These opposing growth concepts are necessary for heterostructure growth as long as ”foreign” metal seed particles are used and make it impossible to combine uniform axial and radial heterostructures in one growth process to form one or several axio-radial heterostructures within a nanowire. In order to combine these growth processes in addition to diameter growth for which the growth rate can be controlled a seed particle is required which is able to change its size during growth and thus compensating tapering due to diameter growth. Those conditions can be provided by selfseeded nanowire growth. As the seed particle consists of material supplied for nanowire growth (e.g. In for InAs nanowires) the seed particle can increase its size by absorbing some of the supplied material 35,36 . This offers the possibility to maintain VLS growth conditions for nanowire

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growth while continuously growing a homogeneous shell around the entire wire, as is illustrated in Fig. 1 (b).

As outlined above the challenge axio-radial nanowire heterostructure growth poses is the rather large number of properties that have to be controlled simultaneously. In this work we show how all key aspects of such axio-radial heterostructure growth can be controlled. These are: 1. Growth of a material B (here InAs1−x Px ) on top and side of the initial wire of material A (here InAs) while maintaining particle assisted growth. 2. Uniform diameter over the entire wire length, i. e. no tapering. 3. Growth parameters to control top segment length (ltop ) and shell thickness (t) independently of each other. 4. Control over material composition (here the concentration x of InAs1−x Px ) By combining the results of this work with other growth methods a broad variety of combinations of axial and radial heterostructures can be fabricated. Furthermore the combination of this type of heterostructure with silicon technology is expected to be feasible as, as the formation of an axioradial heterostructure is based on the continuation of self-seeded nanowire growth which we have demonstrated for growth of InAs on Si 38,53 and has been studied over the recent years by many groups 37,39,48–50 . How the above mentioned four requirements can be achieved simultaneously is demonstrated in this work for the self-seeded growth of axio-radial InAs / InAs1−x Px heterostructures.

1. Growth of InAs1−x Px on top and side of an initial InAs nanowire while maintaining particle assisted growth In a previous work the existence of self-seeded nanowire growth using metal organic vapor phase epitaxy (MOVPE) was proven by comparing nanowires after a growth containing an interruption

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either with or without In flow 36 . This growth process relies on a liquid seed particle (here Indium) on top of the nanowire which leads to preferential uniaxial growth forming a nanowire. The result reported in Ref. 36 demonstrated that nanowire growth can be interrupted and lateron continued by maintaining a continuous In flow and thereby the In seed particle. Furthermore wires grown under such conditions did not only grow in length but also in diameter and offered a perfect starting point for axio-radial heterostructure growth. This growth scheme is now applied for axio-radial heterostructure growth. To grow nanowires the substrate is coated with a thin SiOx layer and nanowire growth is performed using MOVPE. To demonstrate the ability to rapidly change between the two material compositions, a nanowire with several InAs/InAs1−x Px heterostructures is grown. The starting point is the growth of an InAs core wire. Then material change is initiated by stopping the group V precursor flow and pausing growth under maintained In flow. Following that, nanowire growth is resumed by reactivating the group V flow. The composition of the material is defined by either activating only AsH3 for InAs growth or AsH3 and PH3 for InAs1−x Px growth using the growth parameters listed in the following. Moreover, the influence of growth duration on the segment length is studied by varying InAs1−x Px growth time from 1 min to 10 s for different segments within one and the same nanowire.

In detail the growth is performed using the following parameters. InP (111)B substrates are covered by a ≈ 13 Å thin SiOx layer (x ≈ 1), deposited by thermal sublimation. The layer thickness is controlled by an oscillating quartz system mounted in the deposition chamber. The coated substrates are loaded into the growth system (Aixtron AIX 200/4 MOVPE) and heated to a growth temperature of 580 ◦ C if not stated otherwise in Tab. 1. After the growth temperature is reached, the source flows are activated. As carrier gas H2 at a flow of 13000 mL/min is used, and the precursors for In, As, and P are trimethylindium (TMI), arsine, and phosphine, respectively. The TMI molar fraction (mf) is always kept at 1.1×10−5 . For self-seeded particle-assisted growth of InAs nanowires a steady supply of In is necessary to maintain nanowire growth, which was shown in de-

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tail in a previous work 36 . To maintain axial nanowire growth under conditions where the supply of As or P is altered, we adapt this growth sequence in the following scheme: First an InAs nanowire is grown for 2 min at an arsine mf of 3.8×10−4 . To create the heterostructure the AsH3 flow is terminated and the sample is kept for 10 s in a TMI + H2 atmosphere. In the following AsH3 and PH3 sources are activated to initiate the growth of an InAs1−x Px segment and thus to create a heterostructure. The according arsine and phosphine molar fractions are found in Tab. 1 for the samples in this work. For a single heterostructure segment, the nanowire growth is stopped after the InAs1−x Px segment by deactivating the TMI supply. After growth, the samples are cooled to 300 ◦ C under constant arsine and phosphine flow. At a temperature lower than 200 ◦ C the samples are taken out of the reactor. For the detailed growth parameters of the temperature and PH3 /AsH3 flow ratio series see Table 1.

A sample with multiple InAs/InAs1−x Px segments is grown by repeating the growth interruption step under maintained In flow (for 10 s) by switching from AsH3 to AsH3 and PH3 or from the latter two to the first for either InAs or InAs1−x Px growth, respectively. In detail the growth program for the growth of the wire in Fig. 2 is: InAs basis nanowire, growth time 2 min; the grown heterostructures are: 1. InAs1−x Px segment, 1 min; 2. InAs1−x Px segment, 30 s; 3. InAs1−x Px segment, 20 s; 4. InAs1−x Px segment, 15 s; 5. InAs1−x Px segment, 10 s. To separate the individual InAs1−x Px segments an InAs segment was grown after each InAs1−x Px structure for 10 s. The mf’s for the core wire and InAs segments are 1.1×10−5 and 3.7×10−4 for TMI and AsH3 , respectively. For the InAs1−x Px segment molar fractions of 1.1×10−5 , 1.9×10−4 and 6.0×10−3 for TMI, AsH3 and PH3 are used, respectively.

As a first step it is necessary to verify the growth of a heterostructure. The structural properties of the samples are characterized using a scanning electron microscope (SEM) in either a JEOL 6400F or a LEO Supra 35 system, as well as in a transmission electron microscope (TEM) using a JEOL 3000F microscope, equipped with a field emission gun operated at 300 kV. X-ray energy

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dispersive spectroscopy (XEDS) analysis is carried out in the TEM setup using scanning mode to obtain information on the nanowire composition along (axial direction) and perpendicular (radial direction) to the growth axis. TEM samples are prepared by mechanical transfer of the nanowires onto lacey carbon Cu TEM-grids.

To determine whether the material change was successful an XEDS measurement in a scanning transmission electron microscope (STEM) setup is performed. Fig. 2 (a) illustrates a line scan along the growth direction of a heterostructure wire, showing several distinct changes in P concentration. This confirms the change from InAs to InAs1−x Px and back. The growth duration determines the InAs1−x Px segment length, see Fig. 3 (g). When comparing the length of the longest InAs1−x Px segment to the nanowire diameter, it can easily be seen in Fig. 2 (a) that the segment length is larger than the nanowire diameter, confirming preferential growth in axial direction. This verifies the continuation of self-seeded particle assisted nanowire growth after the material transition. In a second XEDS measurement the P concentration across the wire is determined and compared to expected profiles, see Fig. 2 (f). This is done for two regions one with low (scan (1)) and one with high (scan (2)) P concentration, see Fig. 2 (a). Comparing these two regions a distinct difference in the height and shape of the P signals is found (Fig. 2 (b)). This difference is not found for the As and In signals, see Fig. 3 (e). The region with high P concentration (scan (2)) shows a signal typical for a homogeneous P distribution over the wire diameter. Scan (1) in the region of low P concentration shows the signal typical for an InAs1−x Px shell around an InAs wire, confirming the growth of an InAs1−x Px shell.

To determine the average arsenic and phosphorous content of the InAs1−x Px hetero-segments as well as the nanowires geometrical parameters, x-ray diffraction (XRD) measurements and calculations of the diffracted intensities based on input from finite element (FE) simulations are performed. For XRD measurements we use a diffractometer equipped with a Copper rotating anode

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source and a parabolic double bent multilayer mirror with a Ge (220) channel cut monochromator to select the Kα1 wavelength. The diffracted intensity distributions depending on the scattering angles are recorded using a 1D detector. For the FE simulations the commercial FE program COMSOL Multiphysics with the structural mechanics add-on is used. First the local crystal structure of a wire is determined using high resolution TEM (HRTEM). This analysis shows a mixture of ZB and WZ segments along the length of the nanowire with twinned ZB segments, see Fig. 3 (a). No difference in the density of these polytypes can be found along the wire regardless if the material is InAs or InAs1−x Px . Due to the different lattice spacings of the polytypes 40,51 it is necessary to include them in the following analysis. To deduce the average composition of the nanowire ensembles of different samples the intensity distributions around the ¯ wurtzite (WZ) cubic zincblende (ZB) Bragg reflections (111), (224), (331) twin (TW), and (101.5) segments are recorded, to which we refer as reciprocal space maps (RSMs), see Fig. 3 (b). The Miller indices notation for cubic lattice and Miller-Bravais indices for hexagonal lattice is used ¯ Bragg reflections to point out the different crystal polytypes. The (224), (331) TW, and (101.5) provide a distinction between the crystal phases within the samples, i.e. zinc-blende phase only contributes scattering intensity to the (224), and (331) TW Bragg reflection, whereas the WZ phase ¯ Bragg reflection. The visibility of all those Bragg peaks in phase only contributes to the (101.5) our samples confirms the structure found locally for a nanowire with HRTEM and makes it necessary to include a sequence of ZB and WZ segments in the finite element model. The (111) Bragg reflection provides information about the entire nanowire ensemble and is the signal to which the calculated diffraction signal will be fitted.

As is shown in Figs. 4 (a) and (c) in between the nanowires also occasionally some crystallites grow. To exclude them from the XRD analysis RSM’s are recorded in the as-grown state of the sample (Fig. 3 (b)) and in addition after the complete removal of the nanowires using an ultrasonic bath in deionized water, Fig. 3 (c). The removal of the wires is confirmed by investigations of the surface morphology using optical microscopy and SEM. The difference of the two x-ray diffrac-

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tion data sets corresponds to the diffracted intensity distribution from the nanowires alone, see Fig. 3 (d) for the (111) peak. For the quantitative analysis below we used only the difference signal. Since axio-radial heterostructures exhibit a rather complicated strain state 52 and the existence of zincblende, wurtzite and twin segments in the nanowire increases the complexity of this strain distribution a FE model accounting for these properties is required, see Fig. 3 (e). The displacement field obtained by this model is used for the calculation of the diffracted signal. To account for inhomogeneities during growth a variation of shell thickness and P concentration of the InAs1−x Px segments is introduced by calculating a set of wires with a variation in these parameters and combining their calculated signals. As criteria for the required number of simulations a relative rootmean square deviation σI /I below 45% 52 is chosen. This is determined as sufficient to maintain the main features of the simulated signal by comparing it to a simulation with σI /I below 10% performed for a selected sample. As starting point for the parameter variations the length and diameter of the as grown axio-radial wire ensemble as well as the expected dimensions of the core wire, obtained from growth of pure InAs wires, are used. The so calculated signal is fitted to the obtained (111) wire signal. By varying the dimensions and chemical composition we obtained the fit to the nanowire signal shown in Fig. 3 (d). The resulting parameters of the entire axio-radial heterostructure wire are used later on when discussing heterostructure dimensions and composition with the minimum and maximum values used in the FE models as error-bars.

2. Maintaining nanowire diameter (uniform shape over the entire wire length, i. e. no tapering)

The formation of a segment covering the entire wire during axial heterostructure growth can be found in various earlier publications, e.g. Ref. 32 . Under such conditions the radial growth is, however, not homogeneous. In previous works on metal-organic vapor phase epitaxy of InAs nanowires based on self-seeded growth it was observed that no tapering can be found in these InAs

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nanowires even though there is diameter growth 36,53 . To form a homogeneous radial heterostructure, a growth avoiding diameter variation is required. In Figs. 2 (c) and 5 the shape of a single nanowire in high resolution as well as a nanowire ensembles can be seen, respectively. In both cases no change of nanowire diameter over the entire length can be measured showing that as in the case of pure InAs growth also for axio-radial heterostructure growth untapered growth is maintained.

3. Growth parameters to control top segment length and shell thickness independent of each other

In the following the focus is to control the dimensions and composition of axio-radial heterostructures. First we concentrate on the dimensions. As has been shown above a variation in growth time gives control over InAs1−x Px segment length or diameter, see Fig. 2 (a) and (g). Below we show that the ratio between length and diameter growth is controlled by the chosen growth temperature.

To study the influence of different growth temperatures a sample design is chosen in which a single InAs1−x Px heterostructure is grown on top of an InAs core wire, see Fig. 5 (a). For comparability, the growth durations of core wire and heterostructure are kept constant. Detailed growth parameters for this study on the influence of the growth temperature can be found in Tab. 1 (samples TF1, T0, T2 and T3). The sample surface after growth is shown in Fig. 4 (a) and (c) for samples F4 and T3, respectively. The lengths and diameters of the final nanowires are obtained from ensembles measured using SEM (see Fig. 4). The evaluation of core wire and the InAs1−x Px heterostructure dimensions is based on the XRD and finite element method (FEM) analysis as described above.

Using the lengths (lwire , lcore and ltop ) and diameters (dwire , dcore and t) of the nanowires 10


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it is possible to calculate their respective ratios, see Fig. 5 (b), which in turn yields the respective growth rate ratio. For all three growth ratios (wire, core, shell) the values are comparable and represent a linear behavior in an Arrhenius plot. Most important for our work is the ratio t/ltop . This aspect ratio varies from ≈0.02 at 560◦ C to ≈0.3 at 620◦ C, thus illustrating how the growth temperature does control the ratio of length to diameter growth rate.

4. Composition control of the InAs1−x Px segment

As final step we want to be able to vary the composition x of the InAs1−x Px segment. To prove this a further sample series is grown. The growth follows the same process as for the temperature series but this time the temperature is kept constant at 580◦ C and the PH3 /AsH3 ratio for the InAs1−x Px segment is varied from 1 to 32. Detailed growth parameters can be found in Tab. 1, samples TF1 and F2 - F4. The results from XRD characterization of this sample series is shown in Fig. 5 (c). First, we want to determine that there is no cross influence of the change in AsH3 to PH3 flow on the dimensions of the grown heterostructure wires. When looking at the ratios of dwire /lwire , dcore /lcore and t/ltop we find that the influence on any of these ratios is negligible. Next we investigate the composition of the InAs1−x Px segment using the same finite element based XRD method as before. We find that the P concentration of the InAs1−x Px axio-radial segment depends linearly on the PH3 /AsH3 ratio during growth and can be controlled between x=0 and 0.3 without influence on growth rates. To verify the XRD results the InAs1−x Px segment of samples TF1 and F2 are measured using XEDS in TEM. These results are shown in Fig. 5 (c) and agree well with the results from XRD. As a last point we have to check if the growth temperature significantly changes the incorporation of As to P in the InAs1−x Px segment. To do so the composition of the InAs1−x Px segments of the series of samples for which the growth temperature was varied is characterized. In Fig. 5 b the results are plotted showing a slight influence of growth temperature on the P concentration. This influence, as can be expected, is small in comparison to the change in As to P concentration

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reached by a change in the ratio of the AsH3 to PH3 flow. This enables us to design the composition of the InAs1−x Px axio-radial heterostructure without influencing its dimensions.

In summary we prove the ability to simultaneously control the growth of axial and core-shell nanowire heterostructures within the same untapered nanowire using self-seeded VLS growth. Further we discussed why this is a type of heterostructure growth which can not be formed when using ”common” gold seeded nanowire growth. By fulfilling all four of the above mentioned key requirements we accomplished not only the simultaneous growth of two types of heterostructures, but, also showed that one has full design freedom over their dimensions and material composition. Furthermore the growth of several axio-radial heterostructures within one wire proves that this process is applicable for both directions of heterointerfaces, i.e. switching from InAs to InAs1−x Px and vice versa and thus greatly enhances the design freedom for complex nanowire heterostructures. The growth of self-seeded InAs nanowires with constant wire diameter on Si substrates covered with patterned SiO2 layers has been demonstrated both by us and other groups as well in the past and a control of the nanowire density was achieved. Thus we see no obstacle to extend our presented results to the growth of two-dimensionally ordered axio-radial nanowire heterostructures with adjustable wire distances based on self-seeded In particles in patterned holes of SiO2 layers on Si substrat. Consequently our ability to grow single and multiple axio-radial heterostructures offers indeed new avenues for completely novel nanowire device designs.

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Acknowledgements This work was carried out within the Nanometer Structure Consortium at Lund University (nmC@LU) and was supported by the Swedish Foundation for Strategic Research (SSF), the Swedish Research Council (VR), and the Knut and Alice Wallenberg Foundation, the EU Projects NODE (cont. no. 015783), SANDiE (cont. no. NMP4-CT-2004-500101), AMON-RA (cont. no. 214814) and by the FWF Vienna by the SFB project IRON (F2507-N08). The authors want to thank the beamline staffs at the ESRF and Hasylab.

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Figure 1: Sketch of the three primary particle assisted nanowire heterostructure growth scenarios with maintained diameter growth. (a) Heterostructure growth on tapered wires. Here a variation of the nanowire and shell diameter along the nanowire length is mainly caused by by the combination of a fixed seed particle size and inhomogeneous growth on the side facets. (b) Homogeneous heterostructure growth of a shell and top segment over a homogeneous core wire. During the whole growth process the seed particle is maintained and can change in size to accommodate the diameter growth of the nanowire. (c) Growth of a shell around a nanowire by abandoning the seed particle. Such a growth enables a homogeneous shell around a core wire (so called core shell heterostructure) but ultimately terminates particle assisted growth.

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Figure 2: TEM measurement of a wire with multiple InAs/InAs1−x Px transitions. (a) XEDS linescan along the nanowire axis showing 5 distinct InAs1−x Px segments introduced into an InAs nanowire. (b) P signal of XEDS line scans along lines (1) and (2) indicated in panel (a). The P signal for scan (1) shows a minimum in the center of the wire which is not observed for scan (2). (c) Transmission electron microscope image of the entire wire after transfer to a lacey carbon grid. (d) As and In signal of the XEDS line scans along (1) and (2) in (a). (e) Sketch of the axio-radial heterostructure and the expected XEDS signals of cross-section scans for the top and core-shell part of the wire. (f) Plot of the segment length measured in (a).

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Figure 3: (a) High resolution TEM image of the crystal structure of a nanowire with the same ”quasi superlattice” as shown in Fig. 2 (a). (b, c) Reciprocal space maps (RSM’s) of the diffracted x-ray intensity distribution and simulation results for sample F2. (b) is the RSM of the (224), (331), ¯ Bragg reflections of the as grown sample (before removing the nanowires), and (c) after and (101.5) the nanowires are removed. (d) shows the subtracted scattering intensity of the (111) Bragg peak along with the simulated scattering intensity. left) Integrated intensities along the Q[1¯ 12] ¯ direction, where the color plot is the measurement and the line plot the simulation. right) blue line represents the measurement and the green line the simulation. The simulation was made using a P content of 9-12 % and a WZ content of 70-80 %. (e) FE simulation result of a nanowire with an axio-radial heterostructure. The result of a simulation with a P content of 29 at. % and a wurtzite content of 75% is shown. Here εyy is the strain state in the lateral direction, and εzz the strain state in the growth direction.

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Figure 4: Scanning electron microscopy investigation of sample surfaces after axio-radial heterostructure nanowire growth. (a) SEM image of sample F4 showing InAs/InAs1−x Px axio-radial heterostructure nanowires as well as crystallites in between those wires. The sample is tilted by 45◦ . (b) SEM image of the cross-section of a single InAs/InAs1−x Px nanowire under a tilt of 0◦ . (c) SEM image of sample T3 after growth. The sample is tilted by 45◦ .

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Figure 5: Temperature and PH3 /AsH3 flow study. (a) illustration of the parameters determined for the axio-radial InAs/InAs1−x Px heterostructure. (b) Arrhenius plot of the ratios dwire /lwire , dcore /lcore and t/ltop . The P concentration of the InAs1−x Px segment is plotted with respect to the right hand y axis. (c) Plot of the ratio between axial and radial dimensions of the final and core nanowire as well as the InAs1−x Px heterostructure and its P concentration over PH3 /AsH3 ratio during growth of the InAs1−x Px heterostructure segment. The error bars in (b), (c) are the minimum and maximum values of the FEM simulations with exception of the XEDS data, there the error bars represent the standard deviation.

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Table 1: Growth parameters of the samples used in this work with one InAs1−x Px segment. The TMI flow is active during the entire growth time. sample

Tg

TMI

TF1 F2 F3 F4 T0 T2 T3

(◦ C) 580 580 580 580 560 600 620

(mf 10−5 ) 1.1 1.1 1.1 1.1 1.1 1.1 1.1

AsH3 flow core (mf 10−4 ) 3.7 3.7 3.7 3.7 3.7 3.7 3.7

AsH3 flow PH3 flow InAs1−x Px InAs1−x Px (mf 10−4 ) (mf 10−3 ) 1.9 6.0 3.7 6.0 3.7 3.0 3.7 0.4 1.9 6.0 1.9 6.0 1.9 6.0

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growth time annealing time growth time core InAs1−x Px (s) (s) (s) 120 10 120 120 10 120 120 10 120 120 10 120 120 10 120 120 10 120 120 10 120


Self-Seeded Axio-Radial InAsâInAs1âxPx Nanowire Heterostructures

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