Vertical stacks of InAs quantum wires in an InP matrix

Journal of Crystal Growth 254 (2003) 1–5

Vertical stacks of InAs quantum wires in an InP matrix H.R. Gutie! rrez*, M.A. Cotta, M.M.G. de Carvalho Instituto de F!ısica Gleb Wataghin, DFA/LPD, UNICAMP, CP 6165, Campinas-SP 13081-970, Brazil Received 23 August 2002; accepted 24 February 2003 Communicated by S. Hiyamizu

Abstract In this work, multilayered films—consisting of layers of self-assembled InAs quantum wires separated by InP spacers—were deposited on (0 0 1) InP substrates. We studied the vertical alignment of the nanostructures by using cross-section transmission electron microscopy (XTEM). A clear relation between the geometry of the wire crosssection and the stacking angles was observed. For asymmetric wires the stacking angle with respect to the growing direction is larger. Moreover, XTEM shows that the strain field generated by two nanowires can induce the nucleation of a unique wider nanowire in the subsequent InAs layer. Similarly to quantum-dot multilayers systems, this mechanism could produce uniform width distribution for the self-assembled nanowires. r 2003 Elsevier Science B.V. All rights reserved. PACS: 81.07.Vb; 81.16.Dn; 68.65.Ac; 68.37.Lp Keywords: A1. Nanostructures; A3. Chemical beam epitaxy; B1. Nanomaterials; B2. Semiconducting III–V materials

Three-dimensional organization of self-assembled quantum dots (QDs) has received a lot of attention by the scientific community [1–8]. The growth of QDs multilayers, consisting of QDs layers separated by thin spacers of a different material, has been found to improve the control over both the size distribution [2,5,8] and the spatial ordering [3,4] of these 3D islands (QDs). The basic mechanisms for three-dimensional ordering are the transmission—through the spacer— of the strain field from one QD layer to the next. The strain field (due to buried islands) creates sites on the spacer surface where the lattice mismatch between spacer and islands has *Corresponding author. Fax: +55-19-37885343. E-mail address: hrguti@ifi.unicamp.br (H.R. Guti!errez).

a local minimum. The probability for island nucleation at these sites is higher than for the rest of the surface [1,2]. The direction of vertical alignment depends strongly on the thickness [1,4] and elastic properties [3,4] of the spacer. Moreover, oblique stacks produced by a complex interplay of surface strain and surface curvature have been observed [6]. However, few works have studied vertical stacks of self-assembled quantum wires (QWr’s) [9–11]. Both correlated (vertically aligned) and anticorrelated arrays of QWr’s have been observed. The last case is commonly observed when InAlAs spacers are used [9–11]. In this case the anticorrelation can be explained by a lateral modulation of the spacer composition [11]. This InAlAs alloy phase separation (or demixing) is strongly

0022-0248/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0248(03)01095-9

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dependent on surface roughness [12] and growth temperature [11]. In this work, we studied the vertical alignment of InAs QWr’s in an InP matrix by using crosssection transmission electron microscopy (XTEM). In this case, the spacer is a binary compound and no lateral modulation of the composition is expected. Mechanisms similar to those observed in quantum dot systems were identified. Moreover, a clear relation between the geometry of the wire cross-section and the stacking angles was observed. The XTEM observations also suggest that the strain field generated by two adjacent nanowires induces the nucleation of a unique wider nanowire in the subsequent InAs layer. In the same way that in QD systems, this mechanism can induce the homogenization of the QWr size distribution from one layer to the next. The InAs/InP multilayers used in this study were grown by chemical beam epitaxy (CBE) using trimethylindium (TMI) diluted with H2 carrier gas as the group III source and thermally decomposed phosphine (PH3) and arsine (AsH3) as group-V sources. InP substrate native oxide was removed by heating the substrate during 10 min at 535 C in the growth chamber under P2 overpressure. Deposition of a 240 nm InP buffer layer was followed by the growth of 6 bilayers, in which 6 ML-thick InAs layers were separated by InP spacers. Different samples with InP spacer thick( were prepared. Both the InP ness of 50 and 100 A buffer and spacer layers were grown in conditions that provide planar surfaces. This can be achieved within a range of growth temperatures and rates [13]. We have also prepared samples with a single InAs buried layer. In this case InGaAs (latticematched to InP) markers periodically spaced with InP layers were grown over the InAs nanostructures. InGaAs markers indicate the growth front shape for different InP spacer thickness. The vertical arrangement of the InAs nanowires was studied using a JEM 3010 URP 300 kV transmission electron microscope (TEM). The atomic force microscope (AFM) study of the surface after the first InAs layer shows that wire-like nanostructures oriented along the [1–10] direction are obtained (Fig. 1). The self-assembled nanowires have width around 20–30 nm, height

Fig. 1. AFM image of self-assembled InAs nanowires grown on an InP buffer layer.

Fig. 2. Cross-section (g=(0 0 2) dark-field) TEM image. The InGaAs markers (bright) show the evolution of the InP spacer growth front for different thicknesses.

between 1 and 3 nm and length of approximately 1 mm. The formation of these highly anisotropic nanostructures has been discussed in previous works [14–16]. A XTEM image (Fig. 2) of a sample with ( spaced) shows that the InGaAs markers (10 A growth front of InP spacers becomes flat after deposition of an InP amount equivalent to a 5 nmthick layer. For thinner layers, the presence of InAs nanostructures produces undulations on the spacer surface. In Fig. 3 two InAs stacks with 10 nm InP spacers are shown. One sample (Fig. 3a) was ( for InAs grown at a slower growth rate—0.3 A/s ( wires and 0.6 A/s for InP spacers. The second sample (Fig. 3b) was deposited with twice the

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Fig. 3. (a) and (b) bright-field XTEM images of InAs (dark)/ ( (InAs wires) and InP (bright) multilayers grown at: (a) 0.3 A/s ( (InP spacers), (b) 0.5 A/s ( (InAs wires) and 1.2 A/s ( (InP 0.6 A/s spacers). (c) and (d) XTEM (g=(0 0 2) dark-field) of InGaAs (bright) markers spaced by 5 nm of InP (dark). The first layer with bright contrast corresponds to an InAs wire layer. (c) and (d) were deposited in the same growth conditions as (a) and (b), respectively.

value of these rates. For the first sample the angle of nanostructure vertical alignment (with respect to the growth direction [0 0 1]) is higher than 20 C. Samples deposited at the higher rate (Fig. 3b) present a vertical alignment very close to the growth direction. We obtained the same result reducing the spacer thickness to 5 nm. Variations of the stacking angle could be linked to changes in the InP surface curvature [6]. Xie et al. [17] demonstrated that, depending on the growth conditions, the strain field generated by buried nanostructures avoids the formation of planar surfaces for very thin spacers. However, in our case, samples with InGaAs markers (5 nm spaced) have shown that the growth front after 5 nm is flat for both growth conditions used in the experiment (Figs. 3c and d). No surface curvature like that reported in Ref. [6] was observed. A detailed analysis of the wires cross-section (Fig. 4) suggests a relation between the wire geometry and the stacking angle. Fig. 4b shows that the stacking angle of symmetric wires is very close to the growth direction. On the other hand, asymmetric wires originate stacking angles higher than 20 C (Fig. 4a). Schematics in Fig. 4 illustrate this behavior. Wire width increases from one InAs layer to the next due to the stress relaxation of the InP in between nanostructures of consecutive

Fig. 4. Bright-field XTEM image showing the relation between wire cross-section and stacking angle. (a) Asymmetric wires, (b) symmetric wires. Bottom schematics show the angle of vertical alignment for the two cases.

layers. For this reason the estimation of the stacking angle must take in to account both borders of the wires (as shown in Fig. 4). Although the origin of asymmetric wires is not clear, it may be formed from the collapse of two neighboring wires with different heights. This process could be enhanced for InAs grown at the slow growth rate, for which the As/P exchange modify the actual amount of InAs deposited [18]. We have observed that higher and wider nanowires are obtained when slow growth rates are used. The nanostructure vertical self-organization can be explained by the transmission—through the spacer—of the stress field provided by buried nanostructures [1]. This creates preferential sites for the nucleation of nanostructures in the subsequent InAs layer. The position of these nucleation sites depends on the elastic properties of the spacer and the growth direction [3]. In our case, the maximum stress of the InP spacer surface seems to be just above the maximum height position of the buried nanostructure (see Fig. 5). This point represents a minimum in lattice mismatch between the InP surface and the

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Fig. 5. Schematic representation of the strain field produced by buried wires with different cross-sections. The angular position of the most probable nucleation site for the next InAs nanostructure is also indicated.

Fig. 6. Bright-field XTEM image showing the mechanism of wire merging. The dashed lines indicate that wider and more separated wires are obtained with increasing InAs layer number.

subsequent InAs nanostructure. For symmetric wires the preferential nucleation site coincide with the center of the buried nanostructure, originating a vertical alignment very close to the growth direction. In the case of asymmetric wires, the minimum misfit is slightly shifted from the center of the buried wire and the nucleation of a new InAs wire will occur at a higher angle with respect to the growth direction (Fig. 5). On the other hand, the XTEM image in Fig. 6 shows an interesting mechanism of wire size filtering. At the first InAs layer the wires have widths of about 25 nm. Increasing the number of InAs layers, the overlap of strain fields, produced by two wires that are very close, leads to the nucleation of a unique wider nanostructure over them. The new wire has a base size almost twice larger than the underlying ones. Following the dashed lines in Fig. 6 we can notice that this mechanism increases not only the wire width but also the lateral spacing in between them, leading to a reduction of the surface wire density. A similar

mechanism has been identified to produce uniform size distributions in three-dimensional islands [2,5,8]. It could produce a similar effect in the wire width distribution for the self-assembled nanowires [19]. In conclusion, we have studied the mechanisms of three-dimensional organization in vertical stacks of self-assembled InAs nanowires in an InP matrix. A remarkable correlation between stacking angle and wire cross-section was observed. Asymmetric wires produce sloped stacks instead the usual on-top vertical alignment. We have observed mechanisms of size filtering similar to those reported for self-assembled quantum dots. The overlap of the strain fields produced by narrow neighboring wires induces the formation of a wider nanostructure on the next InAs layer. In this way, two-dimensional arrays with a lower wire density but better size uniformity can be obtained. One of the authors (H.R. Gutie! rrez) acknowledges financial support from FAPESP. This work was supported by FAPESP, CNPq and FINEP. HRTEM measurements were made at the LME of the National Synchrotron Light Laboratory (LNLS)(Brazil). A carbon nanotube tip for AFM images was provided by Dr. D. Ugarte (LME/ LNLS).

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