Single photon sources are critical for a variety of applications such as quantum communication and quantum computing^[1−7]. Semiconductor quantum dots are promising candidates for preparing single photon sources due to their excellent properties such as narrow-linewidth and tunable wavelength^{[8, 9]}. To achieve such applications, much work has been done to study the controlled growth of the self-assembled high-quality quantum dots^[10−19]. It has also been demonstrated that self-assembled quantum dots exhibit good quantum optical properties^[9−15]. Nevertheless, there are still some unsolved problems to further improve the performance of single photon sources. On the one hand, the size control of quantum dots is urgently required to obtain a higher photon indistinguishability^[20−24]. To overcome this problem, many schemes have been proposed^[24−30]. Among them, the growth of embedded quantum dot (growth of narrow bandgap quantum dot in a wide bandgap nanowire) is one of the most promising technologies to control the size of quantum dots. In this scheme, the size of quantum dots can be easily controlled by tuning the growth parameters such as the growth time. More importantly, the wide bandgap nanowires can act as photonic structures to enhance light extraction^[31−38]. On the other hand, we know that semiconductor quantum dots are easily oxidized. Thus, a large number of surface states and high surface recombination velocity of quantum dots would degenerate their optical properties^[39−42]. To solve this problem, traditionally, passivation layers are always grown on the surface of the quantum dots after the quantum dot growth^{[41, 42]}. If semiconductor quantum dots can be embedded and passivated in free standing nanowires during the quantum dot growth, it is expected to make above issues to be eased in a convenient way.

As one of the most important semiconductors, GaAs_1−xSb_x ternary alloys have tunable direct bandgap, which makes them promising candidates for applications in high-performance optical and optoelectronic devices^[43−46]. In particular, GaAs_1−xSb_x quantum dots are promising to be used to fabricate stable single photon sources in telecommunication bands. In recent years, high-quality GaAs_1−xSb_x(0 ≤ x ≤ 1) nanowires with controllable sizes have been successfully grown using the Ga self-catalyzed growth manner^[47−57]. However, up to now, to the best of our knowledge, the growth of high-quality embedded GaAs_1−xSb_x quantum dots has been rarely reported.

In this work, we report the growth of embedded GaAs_1−xSb_x quantum dots in self-catalyzed GaAs nanowires on Si (111) substrates by molecular-beam epitaxy (MBE). We find that the size of the GaAs_1−xSb_x quantum dot can be well-defined by the GaAs nanowire. By tuning the growth temperature, the large-composition-range embedded GaAs_1−xSb_x (0 ≤ x ≤ 0.36) quantum dots can be obtained. Structural studies confirm that GaAs_1−xSb_x quantum dots have a pure zinc-blende phase. On this basis, we have developed a new technology to grow GaAs passivation layers on the sidewalls of the GaAs_1−xSb_x quantum dots. Different from the traditional growth process of the passivation layer, GaAs passivation layers can be grown simultaneously with the growth of the embedded GaAs_1−xSb_x quantum dots. The ability to prepare embedded high-quality GaAs_1−xSb_x quantum dots in GaAs nanowires provides opportunities for preparing GaAs_1−xSb_x-based single photon sources.

All samples were grown in a solid source MBE system (VG V80H). Commercial p-type Si (111) wafers were used as the substrates. Before loading them into the MBE chamber, the Si substrates were pretreated by chemical etching as follows. Firstly, the native oxidized layer was removed using HF solutions. After that, the substrates were coated with new oxidized layers by dipping the Si substrates in a solution of H₂SO₄ and H₂O₂ (volume ratio = 4 : 1)^[55]. To obtain embedded GaAs_1−xSb_x quantum dots, the bottom GaAs nanowires were first grown at 590 °C. The growth time, Ga flux and As flux were 30 min, 7 × 10⁻⁸ and 1.9 × 10⁻⁶ mbar, respectively. Then, the GaAs_1−xSb_x quantum dots were grown on the top of GaAs nanowires. In this step, the corresponding Sb flux was 6.0 × 10⁻⁷ mbar. To control the quantum dot size, the Sb supply time was set to 12−20 s. Finally, the Sb shutter was closed to grow the upper GaAs nanowires for 30 min. During the growth of embedded GaAs_1−xSb_x quantum dots, the growth temperature, Ga flux and As flux remained constant. Similarly, to obtain embedded GaAs_1−xSb_x quantum dots with spontaneous GaAs passivation layers, the bottom GaAs nanowires were first grown at 590 °C. The growth time, Ga flux and As flux were 30 min, 7 × 10⁻⁸ and 9 × 10⁻⁷ mbar, respectively. Then, the growth temperatures was decreased to 510 and 530 °C to grow two batches of GaAs_1−xSb_x quantum dots. In this step, to obtain a higher Sb content, the As shutter was closed and the As flux was supplied by the As background. The growth time, Ga flux and Sb flux were 12 s, 7 × 10⁻⁸ and 6.0 × 10⁻⁷ mbar, respectively. Finally, the upper GaAs nanowires and GaAs passivation layers were grown by closing the Sb shutter and opening the As shutter. The growth time was 30 min. The scanning electron microscope (SEM, FEI NanoSEM 650) operated at 10 kV was used to characterize the morphology of samples. The transmission electron microscope (TEM, JEOL JEM-F200) operated at 200 kV was used to characterize the crystal structure of samples. The chemical composition of samples was confirmed by the energy dispersive spectrum (EDS) operated in the high-angle annular dark field scanning TEM (HAADF-STEM) using the JEM-F200 TEM system.

As mentioned above, achieving the controlled growth of high-quality embedded quantum dots is of great importance for the realization of high-performance single photon sources. Based on the previous reports, the high-quality GaAs and GaAs_1−xSb_x nanowires can be grown by MBE using a self-catalyzed growth manner^[51−57]. Hence, we attempt to perform the self-catalyzed growth of embedded GaAs_1−xSb_x quantum dots by MBE. Figs. 1(a)−1(d) show the schematic illustration of the growth process of embedded GaAs_1−xSb_x quantum dots on Si (111) substrates. Firstly, it is necessary to grow Ga droplets on the Si (111) substrates (see Fig. 1(a)). Then, the bottom GaAs nanowires are grown by the Ga self-catalyzed growth manner (see Fig. 1(b)). After that, the Sb shutter is opened to grow GaAs_1−xSb_x quantum dots on the top of GaAs nanowires (see Fig. 1(c)). Finally, the upper GaAs nanowires are further grown to obtain the so-called embedded GaAs_1−xSb_x quantum dots (see Fig. 1(d)). In this way, the size of quantum dots (including the diameter and the axial length) can be well-defined by controlling the nanowire diameter and the growth time^[31]. For example, the axial length of quantum dots can be controlled from 24 to 44 nm by changing the growth time from 12 to 20 s (see Fig. S1 and Fig. S2).

To obtain high-quality embedded GaAs_1−xSb_x quantum dots, it is first necessary to grow high-quality GaAs nanowires. As shown in Fig. 1(e), self-catalyzed GaAs nanowires with high aspect ratios can be successfully grown on the Si (111) substrate by MBE. The diameter of GaAs nanowires can be determined to be 44 ± 4 nm. Detailed structural studies confirm that the obtained GaAs nanowires show a pure zinc-blende phase (see Fig. S3), and only a few twin defects are observed in the nanowires (the twin defect density is 2 μm⁻¹). On the basis of high-quality GaAs nanowires, we further perform the growth of embedded GaAs_1−xSb_x quantum dots. As shown in Fig. 1(f), it can be determined that the diameter of nanowires is 46 ± 6 nm, which is the same as the results of GaAs nanowires. It indicates that adding the Sb to the nanowires has no obvious effect on nanowire growth due to the short Sb supply time. Furthermore, we can see that the axial length of these nanowires is longer than that of the GaAs nanowires, and there is no obvious overgrowth on the sidewalls of nanowires, which suggests that GaAs_1−xSb_x quantum dots may have been successfully embedded in the GaAs nanowires.

To confirm the position and crystal structure of GaAs_1−xSb_x quantum dots, EDS analyses and TEM observations are performed. It is found that the GaAs_1−xSb_x quantum dots are well embedded into GaAs nanowires, and they are pure zinc-blende single crystals. Fig. 2 shows the EDS analyses and TEM results of a typical embedded GaAs_1−xSb_x quantum dot. As shown in Fig. 2(a), there is no obvious change in the nanowire diameter after opening the Sb shutter, which is consistent with the SEM results. The EDS line scan and maps confirm that the GaAs_1−xSb_x quantum dot with a diameter of 49 nm is well-defined by the GaAs nanowire (see Figs. 2(b)−2(e)), and the axial length of this quantum dot is 24 nm. The quantitative EDS data prove that the Sb content x of the GaAs_1−xSb_x quantum dot is 0.2. It is worth mentioning that when the growth temperature is reduced to 570 °C, the Sb content x of the GaAs_1−xSb_x quantum dot can be increased to 0.36, which means that the Sb content of the quantum dots can be controlled by adjusting the growth temperature (see Fig. S4). Figs. 2(f)−2(l) show the high-resolution TEM (HRTEM) and fast Fourier transform (FFT) images of this sample. As expected, the bottom GaAs nanowire shows a pure zinc-blende phase (see Figs. 2(f) and 2(g)). Above the GaAs nanowire, the GaAs_1−xSb_x quantum dot is a pure zinc-blende single crystal, and no planar defects are observed in the whole dot region (see Figs. 2(h)−2(j)). In addition, a large number of stacking faults are found in the upper GaAs nanowire, and the GaAs nanowire shows a mixture of zinc-blende and wurtzite phases. This result is similar to previous reports, which is related to the nucleation mechanism of GaAs_1−xSb_x nanowires^[58−60]. Above the stacking faults region, the GaAs nanowire has a pure wurtzite phase (see Figs. 2(k) and 2(l)). Above results prove that the high-quality GaAs_1−xSb_x quantum dot with the well-defined size can be obtained by the self-catalyzed growth manner.

It is well known that a large number of surface states and high surface recombination velocity of GaAs_1−xSb_x will degenerate their optical properties^{[39, 40]}. To solve this problem, a passivation layer is necessary to improve the optical properties. Here, we develop a new technology for growing spontaneous GaAs passivation layers on the sidewalls of GaAs_1−xSb_x quantum dots. Figs. 3(a)−3(d) show the schematic illustration of the growth process of embedded GaAs_1−xSb_x quantum dots covered with spontaneous GaAs passivation layers. Firstly, the Ga self-catalyzed GaAs nanowires are grown at a higher temperature (T₁ ~590 °C) (see Figs. 3(a) and 3(b)). Then, the substrate temperature is decreased to the lower value (T₂) to grow GaAs_1−xSb_x quantum dots (see Fig. 3(c)). Finally, the upper GaAs nanowires and GaAs passivation layers are simultaneously grown by closing the Sb shutter and opening the As shutter (see Fig. 3(d)). In our work, GaAs nanowires are grown with a vapor−liquid−solid growth mechanism and the GaAs passivation layer grow on the sidewall of the bottom GaAs, GaAs_1−xSb_x quantum dot and top GaAs simultaneously with a vapor−solid growth mechanism. Figs. 3(e) and 3(f) show the side-view SEM images of nanowires with GaAs_1−xSb_x quantum dots grown at different temperatures (T₂). We can see that as the growth temperature (T₂) decreases, the diameter of the upper GaAs nanowires become thicker. This phenomenon is attributed to the fact that the lateral growth of nanowires at lower growth temperatures is enhanced^{[61, 62]}. The lateral growth makes it possible to obtain spontaneous GaAs passivation layers on the sidewalls of GaAs_1−xSb_x quantum dots.

It is difficult to determine from SEM images whether the GaAs_1−xSb_x quantum dots are covered with spontaneous GaAs passivation layers. Hence, we further perform the EDS analyses and TEM observations. As shown in Figs. 4(a)−4(e), it is easily found that the GaAs_1−xSb_x quantum dot is embedded in the GaAs nanowire and the sidewalls are covered by the GaAs passivation layer. It can also be determined that the diameter and the axial length of GaAs_1−xSb_x quantum dot are 55 and 26 nm, respectively. And the spontaneous GaAs passivation layer thickness is 11 nm. Additionally, it can be observed from Fig. 4(a) that the nanowire embedded the GaAs_1−xSb_x quantum dot has the largest diameter. According to the TEM measurements, the radial growth rate of the GaAs passivation layer on the sidewall of the GaAs_1−xSb_x quantum dot is larger than that on the sidewall of the GaAs nanowire. Thus, the nanowire has the largest diameter around the quantum dot. The quantitative EDS data prove that the Sb content x in the quantum dot region is 0.42. Figs. 4(f)−4(k) show the HRTEM images and FFT images of this sample. It is confirmed that the bottom GaAs nanowire and the GaAs_1−xSb_x quantum dot show a pure zinc-blende phase (see Figs. 4(f)−4(h)). It is worth noting that a few stacking faults can also be found in the GaAs_1−xSb_x quantum dot grown at the lower temperature (see Fig. S5)^{[63, 64]}. On the sidewalls of GaAs_1−xSb_x quantum dot, the spontaneous GaAs passivation layer shows a pure zinc-blende phase due to the strict epitaxial relationship between the quantum dot and the passivation layer (see Fig. S6). Above the GaAs_1−xSb_x quantum dot, the crystal structure of the GaAs nanowire is the same as that of embedded GaAs_1−xSb_x quantum dots without spontaneous GaAs passivation layers (see Figs. 4(i)−4(k)). These results demonstrate that the pure-phase embedded GaAs_1−xSb_x quantum dot covered with spontaneous GaAs passivation layer can be successfully achieved by tuning growth kinetics.

In summary, we have demonstrated the self-catalyzed growth of embedded GaAs_1−xSb_x quantum dots in GaAs nanowires by MBE. By tuning growth temperatures, the well-defined GaAs_1−xSb_x quantum dots have been obtained, and the antimony content x can be up to 0.36. All GaAs_1−xSb_x quantum dots are pure zinc-blende single crystals. To improve the optical properties, a new technology has been developed to grow the spontaneous GaAs passivation layers on the sidewalls of the embedded GaAs_1−xSb_x quantum dots. The GaAs passivation layer also has a pure zinc-blende phase due to the strict epitaxial relationship between the quantum dot and the passivation layer. The successful fabrication of spontaneous GaAs passivation layers on high-quality embedded GaAs_1−xSb_x quantum dots is beneficial to following optical properties measurements for these quantum dots. Our work also lays a foundation for the controlled growth of high-quality full-composition-range GaAs_1−xSb_x (0 ≤ x ≤ 1) quantum dots in the next step, and as well as opens up band-engineering opportunities for quantum optical devices.