Facet-dependent growth of InAsP quantum wells in InP nanowire and nanomembrane arrays

Abstract

Selective area epitaxy is a powerful growth technique that has been used to produce III–V semiconductor nanowire and nanomembrane arrays for photonic and electronic applications. The incorporation of a heterostructure such as quantum wells (QWs) brings new functionality and further broadens their applications. Using InP nanowires and nanomembranes as templates, we investigate the growth of InAsP QWs on these pure wurtzite nanostructures. InAsP QWs grow both axially and laterally on the nanowires and nanomembranes, forming a zinc blende phase axially and wurtzite phase on the sidewalls. On the non-polar {11̄00} sidewalls, the radial QW selectively grows on one sidewall which is located at the semi-polar 〈112̄〉 A side of the axial QW, causing the shape evolution of the nanowires from hexagonal to triangular cross section. For nanomembranes with {11̄00} sidewalls, the radial QW grows asymmetrically on the {11̄00} facet, destroying their symmetry. In comparison, nanomembranes with {112̄0} sidewalls are shown to be an ideal template for the growth of InAsP QWs, thanks to the uniform QW formation. These QWs emit strongly in the near IR region at room temperature and their emission can be tuned by changing their thickness or composition. These findings enrich our understanding of the QW growth, which provides new insights for heterostructure design in other III–V nanostructures.

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New concepts

In this work, using selective area grown pure wurtzite InP nanowire and nanomembrane arrays as a template, we revealed the growth behavior of InAsP quantum wells (QWs) for the first time, including their crystal structure, morphology evolution, polarity preference, facet-dependent composition fluctuation, and related QW emission behavior. Specifically, the semi-polar ZB InAsP axial QW drives the formation of asymmetrical radial InAsP QWs on the non-polar sidewalls, ruining the morphology symmetry of the 〈110〉-oriented nanomembrane. In comparison, uniform InAsP QWs can be formed in the 〈112〉-oriented InP nanomembrane even after 5 QW growth, showing a bright and uniform emission in the telecommunication wavelength range at room temperature. Previously, no such information on growth has been revealed. Nearly all the non-symmetric radial growth is driven by the polarity of the sidewalls and no facet-dependent QW growth behavior on the wurtzite nanomembrane has been studied. Consequently, the revealed growth fundamentally enriches our understanding of the QW growth and provides a good example for nanoshape and network heterostructure fabrication using the SAE technique.

Introduction

Bottom-up epitaxy of III–V semiconductor nanostructures has produced a diverse range of “building-blocks” for applications in photonic and optoelectronic devices.^1,2 Among all the synthetic methods, selective area epitaxy (SAE) is of great interest due to its unbeatable advantages in producing uniform nanostructure arrays with controllable geometric patterns, such as diameter and site position.^3,4 Indeed SAE has been used to grow nanowires for solar cells^5,6 and light emitting diodes.⁷ In addition to nanowire arrays, several research groups have successfully fabricated other III–V semiconductor nanoshape arrays, such as nanomembranes, nanorings and in-plane nanowire arrays,^8–12 which may provide new functionalities. Furthermore, SAE has been used to fabricate InAs and InSb nano-networks that are required for realizing topologically protected majorana-based qubits.^13–16 Based on the principle of SAE, IBM researchers and Borg et al. developed a template-assisted method to realize the integration of III–V nanostructures on silicon substrates, and demonstrated state-of-the-art field effect transistors and GaAs microdisk lasers.^17–19 These advances show the potential to monolithically integrate III–V nanostructures with the matured silicon CMOS processes to achieve devices with better optical and electronic properties. Moreover, researchers have also combined metal-catalysed growth with SAE by incorporating the catalyst into the pattern openings to achieve more flexibility in growth. Using this so-called SA vapor–liquid–solid approach, growth conditions can be easily tuned to form axial^20–22 and lateral^23,24 nanowire heterojunctions and even nanowire networks.^25,26 These achievements demonstrate that SAE is of interest and can be used to produce nanostructure and network arrays for future photonic, electronic and quantum science applications.

The incorporation of QWs and quantum dots (QDs) is vital for improving the properties and functionality of III–V semiconductors. For instance, the axially embedded QDs in the nanowires have high emission efficiency, thanks to the natural Fabry–Pérot cavity present in the nanowires, and are widely applied in lasing,²⁷ single photon emission^23,28 and infrared photodetection applications.²⁹ However, despite the great success of SAE in homogeneous III–V nanostructure arrays, heterostructure formation is less studied. Here, InP is chosen for heterostructure growth in consideration of its unique properties among the III–V semiconductors and the well-accumulated knowledge of growth in both nanowire and nanomembrane forms. SAE of InP nanostructures was pioneered by Fukui et al.⁴ with the focus on nanowires, and has been further developed by Wang et al.⁹ and Staudinger et al.^30,31 to achieve pure WZ InP nanomembrane, nanoring and microdisk arrays^9,32 for applications in various fields.^5,33–36 In comparison, the SAE of the InP-based heterojunction is less investigated. Yang et al. observed the formation of QW and shape evolution during SAE of InP/InGaAs nanowires.³⁷ Han et al. demonstrated the in-plane InP/InGaAs QW nanowire array lasers grown on a silicon substrate.³⁸ Mohan et al. successfully grew InP/InAs QW nanowire arrays by changing the growth conditions to promote lateral growth.³⁹ Compared with the well-studied metal-catalysed InP/InAsP and InP/InGaAs QD nanowires,^{21,29,40–43} the development of SAE based heterostructures is slower due to the more challenging growth conditions, as it is harder to selectively control the axial and lateral growth rates by only tuning the growth conditions without the help of a catalyst. This situation leads to the insufficient understanding of heterostructure incorporation. Recently, with the increasing interest in nanoshape and network structures, the growth understanding gained from nanowire research should be enhanced.

In this work, using our previously optimized pure WZ InP nanowire and nanomembrane arrays as templates, we investigate the growth behaviour of InAsP QWs, including the crystal structure, morphology, polarity and composition. We reveal the asymmetric growth of InAsP on non-polar {11̄00} sidewalls. In comparison, InAsP growth on InP {112̄0} sidewalls is quite uniform whilst maintaining the morphological symmetry of the 〈112̄〉-oriented nanomembranes. Bright emission is observed from the nanomembrane QW heterostructures.

Results and discussion

Typical morphology and structure of InP/InAs_0.15P_0.85 QW nanowires are shown in Fig. 1. These QW nanowire arrays can be formed with a broad range of diameters. The diameter can be reduced to as small as ∼40 nm, which is a requirement for single photon emitters.²⁸ The position of the InAs_0.15P_0.85 QW can be easily spotted due to the discontinuity of the side facets, as indicated using the red arrow in Fig. 1b. For nanowires with a larger diameter, scanning electron microscopy (SEM) images show that most of them have a uniform triangle-like cross-section. No facet rotation induced by QW growth is observed.³⁷ The triangular morphology of the InP/InAsP QW nanowire is also true for other investigated QW growth conditions (see Fig. S1, ESI†). Normally, the non-polar nature of the WZ nanowire sidewalls leads to a hexagonal shape for InP nanowires, as demonstrated previously⁹ and in Fig. S2 (ESI†). Therefore, it is suggested that InAsP QW growth causes the morphological transformation from hexagonal to triangular shape, as also observed in the InP/InAs core/shell nanowire array.⁴⁴ Thickness resolved high-angle annular dark-field (HAADF) image analysis demonstrates that the InP/InAsP QW nanowire sidewalls remain non-polar {11̄00} (see analysis in Fig. S1f, ESI†). After InAsP QW growth, the diameter of the top InP segments becomes smaller. This is caused by the shrinkage of the {111}A facet area due to the formation of inclined side facets during InAsP QW growth (see Fig. 1c). The InAsP QW growth alters the following InP growth. The axial growth rate of InP is reduced by a factor of 2.2 ± 0.2 after the QW growth, which could be due to an increased competition of lateral growth.

Fig. 1 InP/InAsP QW nanowire array. 45° tilted SEM images of InP/InAs_0.15P_0.85 QW nanowire arrays with different diameters: (a) ∼55 nm and (b) ∼200 nm. (c) Low magnification HAADF image of a QW nanowire with a smaller diameter (48 nm) along the [112̄0] zone axis. The scale bars are 1 μm in (a and b).

The Z-contrast HAADF image in Fig. 1c shows the structure of the QW nanowire. The InAsP regions look brighter than the InP matrix due to the heavier As element. Despite the optimized growth conditions for axial growth, both axial and lateral InAsP QWs are observed, which are confirmed to be ZB and WZ phase using fast Fourier transform (FFT) analysis in the insets, respectively. The formation of both axial and radial QWs in InP nanowires grown using SAE has been observed in InP/InGaAs^37,45 and InP/AlInP systems.⁴⁶ The simultaneous incorporation of both axial and radial QWs makes the SAE of purely axial or radial nanowires challenging, and therefore a thorough understanding of the growth behaviour would be highly valuable.

Atomically resolved HAADF images in Fig. 2a and b show the details of the InAs_0.15P_0.85 QW structure. Axially, InAsP QW forms a ZB phase with a (111)A polarity and the crystal phase of the top InP segment switches back to WZ phase immediately after InAsP QW growth, thus forming an atomically sharp interface between the ZB InAsP QW and WZ InP. The axial InAsP QW forms inclined {111}B side facets, suggesting that the stable polarity of the InAsP (111) facet is B-polar. The growth of InAsP along the 〈111〉 A direction is not stable and cannot be maintained after a long growth time (see Fig. S3, ESI†), since InAsP nanowires prefer to grow along the 〈111〉 B-polar direction.²⁹ These inclined side facets lead to a morphological discontinuity between the two InP segments and hence the axial QW position can be easily identified as observed in Fig. 1b. The other side facets of the InAsP QW consist of vertical {112̄}A (as determined by FFT of ZB QW⁴⁷) and a small inclined facet. The atomic arrangement between the InAsP QW and InP is schematically illustrated in Fig. 2c. Below the axial QW in Fig. 2b, the lateral QW growth inherits the WZ stacking sequence of the InP nanowire. The three-dimensional distribution of InAsP is revealed using energy dispersive X-ray (EDX) mapping as shown in Fig. 2d and e. The intensity of the P element is slightly stronger on the left-hand side of the nanowire due to a thicker region, confirming the triangle-like cross-section. A strong As element signal is detected at both the axial QW position and near the right-hand edge of the nanowire, demonstrating the formation of radial InAsP QWs along the nanowire. Moreover, a weaker As signal can also be seen on the left-hand side (thicker region) of the nanowire. Considering the results of crystal symmetry, HAADF, EDX and SEM analyses, it is suggested that three InAsP radial QWs are formed on the {11̄00} sidewalls, as illustrated by the 3D schematic images in Fig. 2f.

Fig. 2 Detailed structural analysis of an InP/InAsP QW nanowire: (a and b) atomically resolved HAADF images of the QW interface region as highlighted by the colored boxes in Fig. 1. (c) Corresponding atomic model of the InP/InAsP QW nanowire. EDX mapping of (d) P and (e) As element. (f) Schematic illustration of the InP/InAsP QW nanowire with cross-sections showing the distribution of the axial and radial QWs. Scale bars are 2 nm in (a and b).

Facet-selective growth has been widely observed during heterostructure formation due to the differences in the surface energies and polarity.^48,49 Thus, the asymmetric growth for the radial InAsP QW observed above is unexpected since the {11̄00} side facets of the InP nanowire are non-polar. The only polar sidewalls in this InP/InAsP QW nanowire system are the {112̄} sidewalls of the ZB axial InAsP QW. Taking this polarity into consideration, radial QW growth always locates on the {112̄}A polar side (for more structure analyses, see Fig. S4, ESI†), suggesting a common validity for this conclusion. The relation between the polarity of axial InAsP QW and asymmetric lateral QW growth is further discussed in the InP/InAsP QW nanomembrane. The difference in surface property is one explanation for the shape evolution.⁴⁷ However, it only explains the side facet transformation from {11̄00} to {112̄0}, which is not the case here. Yang et al. recently reported that the growth of ZB InGaAs QW could alter the growth of the subsequent InP layer.³⁷ They observed a similar triangle-like radial InGaAs QW formation and attributed it to the fast nucleation rate in the ZB axial InGaAs QW, which drove the lateral growth and shape evolution. The preference for {11̄0} sidewalls in axial InGaAs QW finally led to facet rotation of subsequent InP growth, forming a hexagonal InP top with {112̄0} facets.³⁷ However, in our case, the ZB axial InAsP QW growth seems to play a major role in affecting the lateral growth of the InP segment below the axial QW. The sidewalls for the whole nanowire remain the same as {11̄00} but the cross-section transforms from hexagonal to truncated-triangle after the growth of InAsP and the overlying InP layer.

Since InP nanostructures grow along a different polarity in comparison to the III-As and III-Sb compounds, a thorough understanding of InP/InAsP QW growth behavior is highly valuable. WZ InP nanowires grown by SAE lack the commonly observed {112̄0} facets. However, to fully understand the InAsP QW growth behavior, it is vital to investigate the growth of InAsP on different InP surfaces. Luckily, the recently reported pure WZ InP nanomembranes with mainly either {112̄0} or {11̄00} sidewalls⁹ provide an ideal platform for investigating this. Fig. 3 shows the growth of 〈11̄0〉-oriented InP/InAsP QW nanomembranes. As reported in ref. 9, the InP nanomembrane array is very uniform and they have very smooth {11̄00} sidewalls. However, after the growth of InAsP QWs, the nanomembrane array uniformity is reduced and the sidewalls appear not to be smooth. Cross-sectional lamellae were prepared by focused ion beam (FIB) milling along the white dotted line as indicated in Fig. 3a to expose the heterostructure interface. HAADF images along the [112̄0] zone axis (see Fig. 3b–f) show that the QW formation is similar to that of the nanowires. Axially, InAsP is grown as a ZB phase while radially it grows as a WZ phase only on one {11̄00} sidewall. The radial QW grows along the 〈112̄〉 A-polar side of the axial QW. The radial QW thickness reduces from the top to the bottom of the nanomembrane (see more examples in Fig. S5, ESI†). Besides, the larger dimension of the nanomembranes leads to a new growth phenomenon. First, WZ InAsP is found to grow on (11̄02) facets of the InP nanomembrane (see Fig. 3c), which is similar to that of the InP/InGaAs nanowire heterostructure with larger diameters.³⁷ This is a result of the top {0001} facet not joining the {11̄00} sidewall at a right angle, thus forming inclined {11̄02} facets. Subsequent growth of InP after the InAsP QW results in the formation of a thin ZB phase (∼6.4 nm) before switching back to the preferred WZ phase (see Fig. 3c). One possible explanation is the larger surface energy of the InAsP {111}A facet than the {111}B facet leading to ZB nucleation instead of the normally observed WZ phase.³² The lateral InP capping layer growth rate is also larger at the 〈112̄〉 A side of the axial InAsP QW (see more examples in Fig. S5, ESI†) in comparison to nearly zero growth rate along the 〈112̄〉 B direction. This polarity driven growth rate difference is quite common in III–V nanowire heterostructure growth.⁵⁰ The WZ InP capping layer also prefers to grow faster along the 〈112̄〉 A direction. Moreover, this growth rate is larger when it is closer to the ZB section of the InP capping layer, leading to a thickness variation on the {11̄00} sidewall and the formation of inclined {11̄01} sidewalls. The above lateral WZ and ZB InP capping layer growth behavior in nanowires and nanomembranes suggests that the asymmetric lateral growth rate on non-polar {11̄00} facets could be related to the faster ZB InP growth on the semi-polar {112̄}A facets, which is similar to the InP/InGaAs QW nanowires.³⁷ Further increasing the number of InAsP QWs deteriorates the uniformity of the nanomembrane, forming a “protruding head” on one side of the nanomembrane (see Fig. S6b, ESI†). HAADF image of an InP/InAsP 〈11̄0〉-orientated nanomembrane heterostructure with 5 QWs shown in Fig. 3g shows the InAsP QW formation on the {0001}, {11̄00} and {11̄01} facets. The radial QWs are mostly formed on the 〈112̄〉 A sidewall but with a significant thickness nonuniformity. Moreover, the inclined {11̄01} facets consistently grow larger with the number of QWs, forming an irregular head as observed in the SEM image shown in Fig. S6b (ESI†). An interesting point to note is that the axial growth rate is reduced to nearly zero after the first QW growth, most likely as a result of increased competition from lateral growth.

Fig. 3 InAsP QWs grown on 〈11̄0〉-orientated InP nanomembranes. (a) 30° tilted SEM image of the InP/InAsP QW nanomembrane array. The white dotted line indicates the ion milling direction for the TEM lamella preparation. (b) Cross-sectional HAADF image of the nanomembrane with one QW. (c–f) Atomically resolved HAADF images at different regions indicated by the colored boxes in (b) showing the crystal structure at the InP/InAsP interface. Inset in (e) is the FFT of the ZB InAsP axial QW. (g) Cross-sectional HAADF image of the InP nanomembrane with 5 InAsP QWs.

On the other hand, InAsP QW grown on the 〈112̄〉-oriented InP nanomembranes is more uniform. No surface roughening is observed (see Fig. 4a). The HAADF image of the cross-section along the [11̄00] zone axis in Fig. 4b shows that the radial QW grow on both {112̄0} sidewalls, forming a sharp interface with the InP matrix without any dislocation (see Fig. 4d). The radial QW has a uniform thickness from the top to the bottom of the sidewalls with a thickness of ∼2 nm. In comparison, the radial QW shown in Fig. 3 has a thickness of 6.4 and 3.7 nm at the top and middle of the nanomembrane, respectively. Even after the growth of 5 InAsP QWs, the nanomembranes still maintain a good shape (see Fig. 4c, e and Fig. S6c, S7a, ESI†). In the 〈112̄〉-oriented nanomembranes, the axial growth rate of the QW is reduced significantly compared to the radial growth rate. Indeed, it may be even suppressed to nearly zero (see Fig. S7b, ESI†). These results show that {112̄0} facets are more desirable for multi-QW growth. The comparison of the InAsP QW composition in the two nanomembranes with different orientations is shown in Fig. 4f and g. The As-element intensity maps show a relatively more uniform QW in the 〈112̄〉-oriented membrane compared to that of the 〈11̄0〉-oriented ones. The EDX spectra at different positions on the nanomembranes are extracted for quantitative compositional analysis and are shown in Fig. 4g. By comparing the intensity of the P-element with that of the InP reference sample, the InAsP composition in the QWs at locations 1–3 is determined to be InAs_0.63P_0.37, InAs_0.79P_0.21 and InAs_0.72P_0.28, respectively. The extracted composition difference between the axial and radial QWs for the 〈11̄0〉-orientated nanomembrane is 16%, suggesting that the QW composition is highly facet-dependent. The variation in composition could lead to different emission peaks and a complex emission behavior. On the contrary, for the 〈112̄〉-orientated InP nanomembrane, only the radial QW is formed with uniform distribution.

Fig. 4 InAsP QWs grown on the 〈112̄〉-oriented InP nanomembranes. (a) 30° tilted SEM image of the nanomembrane array with one InAsP QW. Low magnification HAADF images of the nanomembrane with a single (b) and five (c) InAsP QWs. The magnified HAADF images in (d) and (e) are taken from the boxed region in (b) and (c), respectively. (f) EDX maps of As and P from the 〈11̄0〉-oriented nanomembrane with one QW and 〈112̄〉-orientated nanomembrane with one and five QWs. (g) EDX spectra taken at points 1–4 as indicated in (f). Scale bars are 2 μm in (a), 100 nm in (b) and 20 nm in (e).

The spatial-resolved optical properties of InP/InAsP QW nanowires and nanomembranes are studied using cathodoluminescence (CL), as shown in Fig. 5. The InP/InAsP QW nanowire (see Fig. 5a) shows the strongest emission from the two ends of the nanowire due to a Fabry–Perot cavity.³² Emission from the WZ InP matrix is observed along the nanowire while the QW position shows a strong but broad emission peak from 900 to 1000 nm. The broad emission peak width from our QW nanowire is unexpected since the interface between InAsP and InP is quite sharp. One possible explanation is the substantial composition fluctuation in the QW. Emission from the radial InAsP QW is not observed at room temperature most likely due to the very thin layer of the QW which results in poor carrier confinement. The emission wavelength from the axial QW can be easily tuned by changing the QW growth time or composition (see Fig. S8, ESI†).

Fig. 5 Spatially resolved CL emission of the InAsP/InP QW nanowire and nanomembranes. (a) SEM image of a nanowire together with the CL map (b); (e) and (i) are the 〈11̄0〉- and 〈112̄〉-oriented InP/InAsP QW nanomembranes, respectively. (f–h) and (i–l) are the corresponding normalized intensity maps of the nanomembranes at different wavelength windows. The corresponding CL spectra from each position indicated by points P1–P4 are shown in (c) and (d). The scale bar in (a) is 1 μm.

CL mapping was also performed on nanomembranes that have been knocked down from their growth substrate (InP) to evaluate the emission from the QW. Based on the fracture at the bottom (111) plane, the growth direction of the 〈11̄0〉- and 〈112̄〉-oriented InP/InAsP QW nanomembranes is represented using green arrows in Fig. 5e and i, respectively. For the 〈11̄0〉-oriented nanomembrane, the CL emission spectra (see Fig. 5c) contain emissions from WZ InP (860–880 nm), radial QW (940–960 nm) and axial QW (1550–1570 nm). The emission maps for these wavelength regions are shown in Fig. 5f–h. The emission from WZ InP and radial QW has similar intensity distribution and is strongest at the lower half of the nanomembrane. At the top half section, the emission is dominated by the axial QW emission and the spectral range extends beyond 1.6 μm, which is the detection limit of our InGaAs detector. The observed emission wavelength and intensity of the axial and radial QWs vary from one nanomembrane to another due to the inhomogeneous thickness distribution and composition variation in the QW (see Fig. S9a–e, ESI† for another example). In comparison, QW emission from the 〈112̄〉-oriented nanomembrane is more uniform (see Fig. 5j–l and Fig. S9f–j, ESI†). Fully avoiding composition inhomogeneity in the ternary InAsP QW could be challenging. Instead, using InAs to form an InP/InAs QW structure could be an appropriate approach to obtain a high-quality QW structure with both sharp interface and no composition difference. The signal from the WZ InP nanomembrane is fairly uniform across the whole nanomembrane. The radial QW emission spectrum can be deconvoluted into two peaks at 1280 and 1340 nm (Fig. 5d). The intensity distributions of these two peaks are presented in Fig. 5k and l, showing a similar spatial distribution trend. Except for the slightly stronger emission intensity at the right-hand side, QW emission is observed for the whole {112̄0} facet. These results suggest a slight composition or thickness variation of the radial QW on the two {112̄0} sidewalls. Even for different nanomembranes, QW emission distributes uniformly in the array, showing a strong and broad emission peak around 1.3 μm (see Fig. S10, ESI†). No axial QW related emission is observed from the 〈112̄〉-oriented nanomembranes. These growth studies also suggest that uniform radial InAsP QW formation is possible if the conditions are tuned to favor radial growth and suppress the axial growth rate. For instance, a lower temperature and higher V/III ratio is suggested based on the previous growth studies of InP nanowire and nanoshape arrays.^9,32

Conclusions

In conclusion, we demonstrated high-quality InP/InAsP QW growth in both nanowires and nanomembranes. The growth behavior of InAsP QWs was thoroughly studied, including crystal structure, composition, and spatial distribution together with the impact on the shape evolution. InAsP QWs grow both axially and laterally, forming ZB and WZ QWs, respectively. Structurally, QW growth along the 〈111〉 direction can form a different phase (for instance, ZB phase) with respect to the WZ InP matrix, while laterally formed QWs along all other directions only inherit the crystal stacking sequence of InP. Both axial and lateral QWs present an atomically sharp interface. In terms of composition, growth direction dependent composition difference is observed, reaching as high as 16%. Moreover, there may exist composition fluctuations in the QW, which lead to a broad emission peak in the CL and photoluminescence (PL) spectra. The axial QW growth rate is reduced due to competitive lateral growth. In 〈112̄〉-oriented nanomembranes, the axial growth rate can be suppressed to nearly zero. Lateral QW distribution varies on different sidewalls. QW formation on the {11̄00} side facets is non-symmetric and non-uniform, growing only on the 〈112̄〉 A polar side of the axial ZB QW. This asymmetric QW growth is probably driven by the faster lateral growth rate of ZB QW along the 〈112̄〉 A polar direction. In comparison, QW grows uniformly on the {112̄0} sidewalls even after 5 QW growth. Emission property from QW in nanowires and 〈11̄0〉-oriented nanomembranes varies from one to another. In comparison, strong and uniform emission at telecommunication wavelengths is observed for the 〈112̄〉-oriented QW nanomembrane. The growth of a subsequent InP layer is strongly affected after InAsP QW growth. First, the axial growth rate of InP is reduced due to the increased lateral growth rate. Second, the asymmetric growth deteriorates the symmetry of the nanowires and nanomembranes. The cross-section of the InAsP/InP QW nanowire transforms from a hexagonal to a triangular shape. For 〈11̄0〉-orientated nanomembranes, inclined {11̄01} sidewalls are formed after QW growth, forming a “protruding head” at the top.

Our results show that careful growth optimization is required for the growth of InAsP/InP QW nanowires and nanomembranes in order to preserve the symmetry and shapes of the nanostructures and to obtain high selectivity between the axial and radial QW growth rate. The revealed growth fundamental of the InAsP/InP QW nanowire and nanomembrane provides a good example for the fabrication of other III–V heterostructures using the SAE technique.

Methods

The details on the preparation process of the patterned substrate using electron beam lithography and reactive ion etching can be found in our previous work.⁹ InP/InAsP nanowire arrays were grown using a horizontal Metalorganic Chemical Vapor Deposition (MOCVD, Aixtron 200/4) reactor. Trimethylindium (TMIn), arsine (AsH₃), and phosphine (PH₃) were used as precursors for In, As and P elements, respectively. For growth, the reactor was heated to 750 °C under the flow of PH₃ and kept for 10 min to desorb possible contaminants. InP nanowires were grown at 730 °C with a TMIn flow rate of 3.373 × 10⁻⁶ mol min⁻¹ and V/III ratio of 172. After InP growth for 16 min, InAs_1−xP_x QW was grown for 3 seconds by turning on AsH₃ with an appropriate flow rate to obtain a nominal composition of InAs_0.15P_0.85. Finally, InP was grown again for another 9 min as the top barrier. Other conditions for QW growth were also investigated and the results are provided in the ESI.†

InP/InAsP QW nanomembranes were grown using a close coupled showerhead (CCS) MOCVD reactor. The InP nanomembrane templates were grown at a surface temperature of 683 °C for 3 min with PH₃ and TMIn at a flow rate of 1.25 × 10⁻³ and 4.20 × 10⁻⁶ mol min⁻¹, respectively.⁹ A QW with nominal composition of InAs_0.3P_0.7 was then grown under the same conditions with AsH₃ flow rate of 5.44 × 10⁻⁴ mol min⁻¹ for 3 seconds, followed by the growth of an InP capping layer for another 1.5 min. For multiple QWs, the InP barrier layer growth time is reduced to 30 seconds.

The morphology of the nanostructures was characterized using an FEI Verios 460 SEM. A Gatan MonoCL4 Elite CL spectroscope equipped with the SEM instrument and a commercial micro-Raman system (Renishaw inVia) was used to perform optical characterization. Structural and compositional analyses were performed using an aberration-corrected scanning transmission electron microscope (Cs-STEM) (JEOL JEM-ARM200f) equipped with an energy dispersive X-ray spectroscopy system.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

National Natural Science Foundation of China (No. 61974166, 51702368 and 61874141); Hunan Provincial Natural Science Foundation of China (2018JJ3684); Open Project of the State Key Laboratory of Luminescence and Applications (SKLA-2018-07); and The Australian Research Council (ARC) are acknowledged for financial support. The Australian National Fabrication Facility, ACT Node and the Australian Microscopy and Microanalysis Research Facility are acknowledged for providing access to facilities used in this work.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nh00410c

‡ These authors contributed equally to this work.

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