DOI: 10.1021/cg1006814
Faceting of InAs-InSb Heterostructured Nanowires
2010, Vol. 10 4038–4042
Lorenzo Lugani,*,† Daniele Ercolani,†,‡ Francesca Rossi,# Giancarlo Salviati,# Fabio Beltram,†,‡ and Lucia Sorba‡,† †
NEST, Scuola Normale Superiore, Piazza S. Silvestro 12, I-56127 Pisa, Italy, ‡NEST, Istituto Nanoscienze-CNR, Piazza S. Silvestro 12, I-56127 Pisa, Italy, and #IMEM-CNR, Parco Area delle Scienze 37/A, I43010 Parma, Italy Received May 21, 2010; Revised Manuscript Received July 12, 2010
ABSTRACT: We report on the morphology of InAs-InSb heterostructured nanowires grown by Au-assisted chemical beam epitaxy. Using scanning and transmission electron microscopy, along with high angle annular dark field image analysis, we show that the hexagons defining the cross section of the two segments of the nanowires are rotated one with respect to the other by 30 around the growth direction and that the corners of these hexagons are rounded off by six small facets. Six additional facets that are not parallel to the growth direction are found in the InSb segment at the InAs-InSb interface and are indexed. Finally, the relation between the dimensions of the two segments composing the nanowires is discussed quantitatively.
Introduction III-V semiconductor nanowires (NWs) are attracting increasing interest as potential building blocks for electronic and optoelectronic devices1,2 due to their nanoscale dimension. Vertical wrap-gated NW transistors,3 resonant tunneling diodes,4 lasers,5 light emitting diodes,6 and gas-sensors7 were realized in the past years, and a large effort is currently under way in order to gain better control of their morphology and to assemble them in complex architectures. What makes III-V semiconductor NWs most attractive is the possibility of growing heterostructures with materials with very different lattice constants.8 High quality heterostructured nanowires of highly mismatched semiconductors were realized,9 and the epitaxial growth of III-V semiconductor NWs on Si substrates was demonstrated6,10,11 opening the way to the direct integration of III-V materials with Si-based technologies. Among III-V materials, InSb has the smallest band gap, the smallest effective mass, the largest bulk electron mobility, the largest Lande g-factor, and the largest thermopower figure of merit.12,13 This makes InSb an ideal candidate for a wide range of applications, such as infrared detectors,14 high speed devices,15 and spin-related applications.16 Despite its attractive properties, in the past years InSb received little attention owing to its large lattice mismatch with common substrates such as Si or GaAs, which hinders the realization of high quality two-dimensional (2D) heteroepitaxial layers comprising this material.17 However, recent works demonstrated the possibility of growing heterostructured nanowires containing high quality, defect-free, InSb segments by metal organic chemical vapor deposition (MOCVD)13,18 and chemical beam epitaxy (CBE),19 paving the way to the realization of InSbbased NW devices. In order to precisely control the properties of these nanostructures, a detailed knowledge of their morphology is required, but such a study was not performed for InSb-containing heterostructured NWs. In this work, we report on the faceting of InAs-InSb heterostructured nanowires grown by CBE. By means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM), *To whom correspondence should be addressed. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 07/28/2010
we find that the hexagon defining the cross section of both segments rotates by 30 along the growth direction when passing from InAs to InSb. Six additional facets were found and identified at the InAs-InSb interface, and the relation between the dimensions of the InAs and InSb cross sections was accurately quantified. Experimental Section InAs-InSb heterostructured NWs were grown on InAs(111)B substrates by CBE in a Riber Compact-21 system by Au-assisted growth19 using trimethylindium (TMIn), tertiarybutylarsine (TBAs), and trimethylantimony (TMSb) as metal-organic (MO) precursors. Because of their high decomposition temperature, TBAs and TMSb were precracked in the injector at 1000 C. A 0.5 nm thick Au film was first deposited by thermal evaporation on the InAs wafer in a separate evaporator chamber and then transferred to the CBE system. The wafer was then annealed at 520 C in order to remove the surface oxide and generate the Au nanoparticles by thermal dewetting. The InAs segment was grown for 45 min at a temperature of (435 ( 10) C, with MO line pressures of 0.3 and 1.0 Torr for TMIn and TBAs, respectively. Then the group-V precursor was abruptly switched from TBAs to TMSb, with a pressure of 1.0 Torr, with no interruption or variation in the TMIn flux or substrate temperature. The growth proceeded for 45 more minutes and was terminated by simultaneously stopping the TMIn and TMSb fluxes and cooling the substrate to room temperature. NWs were characterized using a Zeiss Ultra Plus field emission gun scanning electron microscope (SEM). Some NWs were mechanically transferred on carbon-coated copper grids by gently rubbing the grids on the substrate and then analyzed by means of transmission electron microscopy (TEM) in a JEOL 2200FS microscope operating at 200 keV.
Results and Discussion Figure 1 is a tilted SEM image of the InAs-InSb heterostructured NWs. Consistent with previous works,13,18,19 the NWs grow in the vertical [111]B direction and are composed of two parts: a lower, smaller diameter InAs stem and an upper InSb segment of larger diameter. It was also found that while InAs grows in the hexagonal wurtzite phase with the Æ0001æ direction along the NW axis, InSb grows in the cubic zincblende phase with the Æ111æ direction along the NW r 2010 American Chemical Society
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Figure 3. (a) HAADF image of an InAs/InSb NW aligned along the InSb Æ110æ direction. The signal intensity along the InSb (b) and InAs (c) sections reveal a 30 rotation around the growth direction of the hexagonal section. The flat surface seen in the InSb part indicates that the InSb crystal is terminated by {110} facets.
Figure 1. 45 tilted SEM micrograph of the as-grown sample. The inset is the top view SEM image of the base of a broken NW. The arrows indicate the crystallographic directions of the substrate.
Figure 2. (a) SEM image of a NW viewed along the Æ0110æ axis of the InAs stem. (d) SEM image of a NW viewed along the Æ2110æ axis of the InAs stem. The number of facets seen in the two projections suggest that the cross sections of the two segments are both hexagonal but rotated around the growth direction by 30 one with respect to the other, as schematically depicted in (c). Panel (b) is a schematic drawing highlighting relevant lengths measurable from micrographs.
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axis.13,19 The relative orientation of the two crystals was determined by electron diffraction experiments and the following crystallographic relations were found: Æ110æInSb Æ2110æInAs and Æ112æInSb Æ0110æInAs.19 A high resolution TEM (HRTEM) micrograph and the corresponding FFT of a typical NW can be found in the Supporting Information as Figure S2. The catalyst particle at the tip of the NWs consists of AuIn2 (see Supporting Information, Figure S1), confirming previous findings.13,19 It should be noted that the In content is higher than during InAs growth where the In/Au ratio has been reported to be between 0.29 and 0.69.20,21 The inset of Figure 1 is a top view SEM image of the base of a broken NW along with the crystallographic directions of the substrate. The image shows that the InAs stem has an hexagonal cross section with facets parallel to the {112} planes of the substrate, which means, consistently with earlier reports,22,23 that the InAs crystal is terminated by six {0110} facets. The SEM micrographs of Figure 2 show two wires viewed from different directions. In Figure 2a, a NW is viewed along the Æ0110æ axis of the InAs stem, and we see three and two
facets in the InAs and InSb segments, respectively. Conversely, in Figure 2d a NW is viewed along the Æ2110æ axis of the InAs stem, and we see two and three facets in the InAs and InSb segments, respectively. This suggests that also the InSb segment has an hexagonal cross section, but rotated by 30 around the growth direction, as depicted schematically in Figure 2c. This rotation can be investigated more quantitatively by means of TEM, in particular, by high angle annular dark field (HAADF) imaging. Exploiting the dependence of HAADF intensity on sample thickness,24 projected thickness maps can be obtained,25 and the faceted NW shape along the beam direction can be reconstructed. Figure 3a is a HAADF image of a NW aligned along the InSb Æ110æ zone axis. The intensity profiles of the HAADF signal along a InSb and a InAs section of the NW are presented in Figure 3, panels b and c, respectively. While for the InAs stem we find a sudden decrease in thickness if we move away from the center, in the InSb segment there is a flat central region, consistent with the 30 rotation of the hexagonal cross section. Furthermore, the presence of the flat central region in the InSb segment clearly indicates that the InSb crystal is terminated by six {110} facets. Although supported theoretically,26 the termination by {110} side facets is rare for zincblende NWs, the presence of {112} facets being far more common.23 Considering that both an expansion and a rotation of the cross section take place at the initial stage of InSb growth, additional facets, not parallel to the NW axis, must be present at the base of the InSb segment. We therefore developed a model for the InSb base consisting of low index facets. Given the lower energy of low Miller index surfaces in other III-V materials,27 we considered only {100}, {110}, and {111} facets and excluded all the higher-index ones. Considering that the zincblende belongs to the F43m space group, there are four equivalent Æ111æB directions. In order to avoid ambiguities, we set [111]B as the growth direction. Therefore, among all the {111} facets, only the three (111), (111), and (111) facets are useful for our model. These are indeed the only {111} facets able to produce an expansion of the NW section along the growth direction. For the same reasons, only the three (110), (101), and (011) and the three (100), (010), and (001) facets are considered among the 12 {110} facets and the six {100} facets. A stereographic projection of the outward directions corresponding to the selected facets is reported in Figure S3 of the Supporting Information. A model for the InSb base can be built using six facets, one for each side facet of the InAs stem. These facets should intersect the {0110} facets of the InAs crystal, corresponding
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Figure 5. TEM images of the InAs-InSb interface of two NWs aligned along the (a) Æ110æ and (b) Æ112æ zone axis.
Figure 4. Different views of the two model structures A and B. (a-c): top, Æ110æ and Æ112æ views of the model A. (d-f): top, Æ110æ and Æ112æ views of the model B.
to {112} planes in the zincblende reference system, in a line lying on the (111) plane. The stereographic projection of Figure S3 reveals that the {100}, the {110}, and the {111} facets we selected indeed satisfy this condition. We therefore end up with two possible models, both consisting of six facets: model A consisting of three {100} and three {111} facets, and model B consisting of three {100} and three {110} facets. Figure 4 shows the morphology of the interface region viewed along the growth direction (a,d), the Æ110æ (b,e) and the Æ112æ direction (c,f) for both models. It is possible to establish experimentally the correct model by measuring the angles formed by the base facets and the growth direction. If the NW is aligned along the Æ110æ direction, these angles can be measured directly because in this projection two base facets are vertical (see Figure 4b,e). When viewed along the Æ110æ direction, a NW should appear asymmetric, with the base facets forming angles of 145 and 161 (145 and 125) for model A (B). Viewed along the Æ112æ direction, a NW should instead appear symmetric, with the base facets forming angles of 148 (129) in the case of model A (B) (see Figure 4c,f). Figure 5a,b shows high resolution TEM (HRTEM) images of NWs aligned along the Æ110æ and Æ112æ zone axis. Angles of
145 ( 2 and 160 ( 2 are measured in Figure 5a, while from Figure 5b we obtain an angle of 146 ( 5. Furthermore, the SEM micrographs of Figure 6 show additional characteristic features. In the Æ112æ view of Figure 6a, a large triangular facet is indicated by the arrow, while in the Æ110æ view of Figure 6b two smaller triangles can be seen at the base of the InSb segment. Therefore, both the TEM and the SEM observations support model A and are incompatible with model B. We conclude that the InAs-InSb interface is composed of three {100} and three {111} facets. We also investigated quantitatively the relation between the dimensions of the InAs and InSb segments. The parameter which best describes the expansion of the cross section is the ratio r between the length of the InSb and InAs hexagon sides: r = sInSb/sInAs (see Figure 2b for clarity). Considering the geometry of the NWs, it is clear that r cannot be obtained directly by measuring the ratio between the width of the InSb and the InAs segments seen in a particular projection. LabelÆhklæ ing δÆhklæ InAs (δInSb) the width of the InAs (InSb) segment seen in the Æhklæ projection (see Figure 2b) and defining the two directly measurable, dimensionless quantities FÆ110æ = δÆ110æ InSb / Æ112æ Æ112æ and F = δ /δ , r is given by the following δÆ110æ InAs Æ112æ InSb InAs simple formulas: pffiffiffi pffiffiffi r ¼ 3FÆ110æ =2 r ¼ 2FÆ112æ = 3 Measurements over 20 NWs yielded FÆ110æ = 1.30 ( 0.03, from which we calculate r = 1.13 ( 0.03, while for FÆ112æ we obtained a value of 1.07 ( 0.02, which gives r = 1.24 ( 0.02. The values obtained from the two projections should coincide within the experimental error, but the difference between the two results is actually rather large. Moreover, the value of r obtained from FÆ110æ implies that in the Æ112æ projection the InAs stem should appear larger than the InSb segment, but
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Considering that the expansion of the cross section is most likely due to the change in particle composition that takes place during growth when passing from InAs to InSb,13,19 different growth conditions can lead to different expansions. Conclusions Figure 6. SEM micrographs of two NWs aligned approximately along the (a) Æ112æ and (b) Æ110æ zone axis. The triangular features that can be recognized at the InAs-InSb interface are indicated by the arrows. In both images the scale bar is 100 nm long.
Figure 7. (a) Model of the NW morphology including six {112} and six {2110} thin facets. (b) Higly magnified SEM image of a NW showing the presence of thin facets at the edges of the InAs and InSb crystals.
this is not the case. These facts could be explained if the InSb and the InAs cross sections were hexagons with the corners rounded off by thin {112} and {2110} facets of width wInAs and wInSb, respectively (see Figure 7a). Facets of this kind are indeed present in our NWs, as can be seen in Figure 7b. In order to have an estimate of wInAs and wInSb, we assume that they are a fixed fraction ε of the length of the edge of the corresponding hexagon, that is, wInAs/sInAs = wInSb/sInSb = ε. It can be shown that, under this assumption, the values of r and ε are given by the following expressions: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r ¼ FÆ110æ FÆ112æ ε ¼ 2 3 - 3 FÆ110æ =FÆ112æ from which we calculate r = 1.18 ( 0.02 and ε = 0.16 ( 0.06. Figures S4 and S5 of the Supporting Information show HAADF images and the corresponding projected thickness maps of some NWs where these thin facets can be recognized and their width can be measured. The values of ε obtained are in good agreement with the calculated one. We therefore conclude that the dimensions of the NW increase by (18 ( 2)% when passing from InAs to InSb. This result is much lower than the value of 40% reported by Caroff et al.,13 but this is not surprising because in this work the rotation of the hexagon defining the cross section of the NW was not taken into account and the NWs might were always imaged along the Æ110æ direction, which leads to an overestimation of the expansion. Furthermore, the use of MOCVD as the growth technique can lead to very different growth conditions and this in turn can affect the supersaturation of the particle, giving a different concentration of indium during growth.
We investigated by means of TEM and SEM the morphology of InAs-InSb heterostructured nanowires grown by Auassisted CBE. We found that the hexagons defining the cross section of the two segments of the nanowires are rotated one with respect to the other by 30 around the growth direction and that the corners of these hexagons are rounded off by six small facets. Six additional facets, not parallel to the growth direction, were found in the InSb segment at the InAs-InSb interface and were identified as belonging to the {111} and {100} family. Finally, we showed that the dimensions of the nanowire cross section increase by (18 ( 2)% when passing from the InAs to the InSb segment. We believe that the detailed knowledge of the morphology of these nanostructures will be helpful for precisely tailoring the properties of InSb-based NW devices. Acknowledgment. We acknowledge financial support from Monte dei Paschi di Siena with the project “Implementazione del laboratorio di crescita dedicato alla sintesi di nanofili a semiconduttore”, the bilateral project of Ministero degli Affari Esteri “Nanocharacterization of nanowires, nanomagnets and laser diodes for sensors, optoelectronics and data storage (N3)”, the EU program NODE 015783 and the FIRB project prot. RBIN067A39_002. Supporting Information Available: HRTEM images, stereographic projection, and HAADF images in PDF format. This material is available free of charge via the Internet at http:// pubs.acs.org/.
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