Self-Formed Quantum Wires and Dots in GaAsP–GaAsP Core–Shell Nanowires

Quantum structures designed using nanowires as a basis are excellent candidates to achieve novel design architectures. Here, triplets of quantum wires (QWRs) that form at the core–shell interface of GaAsP–GaAsP nanowires are reported. Their formation, on only three of the six vertices of the hexagonal nanowire, is governed by the three-fold symmetry of the cubic crystal on the (111) plane. In twinned nanowires, the QWRs are segmented, to alternating vertices, forming quantum dots (QDs). Simulations confirm the possibility of QWR and QD-like behavior from the respective regions. Optical measurements confirm the presence of two different types of quantum emitters in the twinned individual nanowires. The possibility to control the relative formation of QWRs or QDs, and resulting emission wavelengths of the QDs, by controlling the twinning of the nanowire core, opens up new possibilities for designing nanowire devices.


Section S2: Polarity determination of the cross-sections
III-V nanowires commonly grow in the [111] B direction on Si (111) substrates. This has been shown to be true for GaAsP nanowires grown on Si (111) 2 Figure S1, the microtomed nanowire slices float-off in an orientation where the tip side, i.e. the [111] B direction is downwards.

. As indicated in
When collected on the TEM grid, the [111] B direction of the nanowires is directed upwards. The grids are inserted into the TEM so that the electron beam is incident on the sample side. This means that the [111] B direction is pointing upwards when inside the TEM.
Hence, the method elaborated in Jiang et al. 3 was used for the determination of the polarity of the <112> directions. This was further verified by atomic resolution images, as described in Zheng et al. 4      compared to the constrained material in the simulation (e) and scanner distortions. However, it could be seen that the general strain distribution of the region is comparable, while some of the subtle features seen in the simulation are indistinguishable or lacking in the experimentally obtained map, possibly due to the same reasons as above.

GaAsP nanowire cores
Considering the interplay between the composition, strain and size (quantum confinement), the axial wires can be classified into four types according to their expected optical behaviour.
Type 1: P composition in the wire is lower than that of the core and the combination of compressive strain of the wire and quantum confinement is not sufficient to push the lowest confined state above the bandgap of the core. The electrons and holes are confined in the wire region, as this gives the lowest energy states of the system, allowing them to behave like QWRs.
Type 2: P composition of the wire is lower than that of the core, but the combination of the compressive strain of the wire (from the P rich band and GaAsP shell) and the quantum confinement is sufficient to increase the energy of the lowest confined states in the wire above those of the core.
The example nanowire discussed in Figures 2 (c) and 3 (b) in the main manuscript falls into this category. Here, even though the wire region initially has the lowest P composition the compressive strain exerted on the wire increases the band gap of the wire region almost to the same value as that of the strained core, as shown in Figure 3 (b) in the main manuscript. The effects of quantum confinement further elevate the energy of the confined states above the band gap of the core. As a result, the nanowire core encompasses the lowest energy states. However, as shown in Figure 3  and by probability densities for the lowest and highest electrons and hole states pertaining to the wire region in Figure 3 (c) and (d) in the main manuscript, the wire is still able to localise higher energy electron and hole states. The lowest electron and highest hole levels confined within the QWR lie 28 and 10 meV above and below that of the respective core states. This is similar to the behaviour seen for self-assembled QDs in the GaAs-AlGaAs material system, where the lowest energy continuum states are associated with the GaAs layer 6 .

Type 3:
The P composition of the wire is higher than that of the core and its band gap is further increased by the compressive strain exerted by the GaAsP shell and P rich band. The confinement will further elevate the energy of the confined QWR states. Yet, similar to type 2, it is still able to localise electron and hole states.

Type 4:
This is similar to type 3, but no confinement of higher energy QWR-like states is observed within the number of electron and hole levels computed. This occurs when the effect of the P rich band becomes weak, either due to its effective band gap decreasing due to the tensile strain exerted by the core and the sufficiently large wire, and/or when the difference in composition between the wire and the P rich band becomes too low. Therefore, in summary, it could be seen that three types of wires can behave as optically active QWRs with most having confined states that are higher in energy than that of the nanowire core. Figure S5. (a) A schematic of the simulated QD structure, formed by sandwiching a thin segment with similar cross-section to that in Figure 3 (a) between the nanowire core and shell slices that do not contain QWRs. (b) and (c) Probability densities of the lowest electron and highest hole states that are confined to an 8 nm high QD, with a parameter set similar to that of the QWR simulated in Figure 3 of the main manuscript, respectively. The white frame indicates the QD region for clarity.

Section S7: Proposed growth mechanism of the QWRs
Although a hexagonal cross-sectional shape is more common in wurtzite (WZ) and most zincblende (ZB) nanowire cores, truncated triangular, nonagonal and Reuleaux triangular cross sectional shapes have also been observed in the cores of ZB nanowires due to the asymmetry of the {112} facets (or their constituent {111} and {113}) facets), arising from the two polarities 3,7,8 . A similar observation is made here, with the nanowire core marked in Figure S6 (a) exhibiting a shape that is closer to that of a nonagon with partly formed {110} facets. The P rich band appears to have been formed during the initial stages of the Ga particle consumption step, when the Ga source is completely cut off and the P and As fluxes reduced. Similar bands have been found to form around the core in other nanowire growths, where particle consumption step was performed as shown in Figure S6 (b). The growth is presumed to have taken place using the residual Ga left in the chamber, incorporating P that is preferentially segregated on the ZB surfaces. The preferential segregation of P could be a result of the different segregation energies of P and As on the ZB surfaces under the growth conditions. 9 The P rich band seen in the current growth, which is in contrast to the P rich ring in the {110} faceted core of a hexagonal shape, is a result of the much higher growth rate on the {112} B facets compared to the {110} facets 10 . In some nanowires, this P rich growth has been sufficient to complete the {110} faceted hexagonal shape, resulting in P rich triangles rather than bands, as shown in Figure S6 (c).
Also, in some other nanowires, a combination of QWRs and P rich triangles are seen as a result of the unequal sizes of the initial {112} B facets. The continued particle consumption step could have an annealing effect on the parasitic island growth 11 that has taken place on the substrate during nanowire core growth, decomposing them. This decomposed material can redeposit on the preferential {112}B facets in a self-limiting manner 10,12 forming the triangular regions and hence completing the {110} faceted hexagonal shape. The subsequent intentional GaAsP shell has a higher P composition than the triangular regions, making the latter relatively As rich QWRs that are surrounded by P rich regions.
The relatively large size and composition variation observed in these self-formed QWRs can be attributed to the random positioning of the nanowires on the substrate and variation in nanowire core sizes that result from variations in self-formed Ga droplet sizes 11,[13][14][15] . Controlling the uniformity of these structures is paramount for their usability in future applications. Growth on patterned substrates could greatly equalise composition variations arising from the presence of randomly positioned neighbouring nanowires and variation in size in the initial droplets 13,15,16 . Figure S6. ADF images of (a) a nanowire cross-section from the discussed sample, showing the nonagon shaped (six {110} facets and three {112} B facets) GaAsP core within the P rich bands. (b) a nanowire cross-section from a different GaAsP-GaAsP core shell sample which also included a Ga particle consumption step. Here, the GaAsP core is hexagonal. However, a P rich band is visible at the core-shell interface in dark contrast. The dark spot visible closer to the centre of the core is due to beam damage caused during analysis. (c) a nanowire cross-section from the main discussed sample, where P rich triangular regions, instead of bands have formed.