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Defect-Free Axial GaAs/GaAsP Nanowire Quantum Dots with Strong Carrier Confinement

Defect-free axial quantum dots (QDs) in self-catalysed nanowires (NWs) offer superior compatibility with Si technology and a high degree of structural controllability. However, there has been relatively little optimisation of their structural properties for operation at high temperatures and with long-term stability, particularly in terms of defect reduction, attainment of deep carrier confinement and incorporation of efficient surface passivation. We report a detailed study of the growth, and structural and optical characterisation of self-catalyzed, defect-free axial GaAsP NWs with deep confinement GaAs QDs. The QDs have very sharp interfaces (1.8~3.6 nm) and dots with very similar structural properties can be closely stacked. High structural quality is maintained when up to 50 QDs are placed in a single NW. The addition of surface passivation layers is shown to be critical in achieving good optical properties, particularly at elevated temperatures. Emission at room temperature, an emission linewidth of<10 meV at 140 K (comparable to the best reported values for all III-V QDs, including nitride ones) and stability of structures stored in an ambient atmosphere for over 6 months are reported. The QDs have the deepest reported carrier confinement (~90 meV) and largest exciton-biexciton splitting (~11 meV) for non-nitride-based III-V NWQDs. These properties indicate the suitability of the structures for devices operating well above liquid-nitrogen temperatures, significantly reducing the operating costs of non-nitride-based III-V NWQD devices.


Introduction
Semiconductor nanowires (NWs) have a unique one-dimensional morphology, which has attracted significant interest, with many potential applications for novel optical emitters. 1-3 By changing the axial composition during NW growth, it is possible to form hetero-structures and introduce one or more quantum structure, for example, quantum dots (QDs), quantum wells (QWs) or superlattices (SLs). QDs provide a unique semiconductor structure with fully quantised electronic states, permitting the fabrication of both high-efficiency classical and non-classical photon emitters.
Examples include low threshold current density lasers with reduced sensitivity to temperature variation, trains of regularly spaced single photons, and entangled photon pairs. 4,5,6 To date, the vast majority of physics and device studies on QDs have utilised self-assembled QDs within planar epitaxial structures, whose formation is strain driven via the Stranski-Krastanov growth mode. 7 These QDs have a number of significant disadvantages, including: formation at random positions, large inhomogeneous size distributions, limited shape and size control, difficulty in forming stacks of uniform QDs and restrictions on which semiconductors can be combined in a single structure.
Formation of dots at pre-determined positions is an essential prerequisite for applications that require either a single, or small, number of QDs.
The growth of QDs within a NW, i.e., a NWQD, is a promising approach for future nano-optoelectronic devices. Unlike self-assembled QDs, QD formation in a NW is not generally strain driven, enabling a greater range of semiconductor material combinations to be used. In addition, the small NW diameter provides high strain tolerance, which permits the combination of materials with large lattice and thermal expansion coefficient mismatch. 8 The small NW diameter also results in dislocations being unstable, allowing the growth of dislocation-free structures. 9 The position of the QDs in a NW is fully controlled by the epitaxial growth sequence. Control of both their shape and size is possible, where the QD diameter is determined by the diameter of the NWs and the height by the epitaxial thickness. Nominally identical QDs can be closely stacked, allowing the formation of molecular systems. 10 Other advantages include growth along the [111]B direction, which should minimise or eliminate exciton splitting 11,12 , the absence of a wetting layer, and that the NW forms an optical cavity, giving enhanced photon extraction efficiency and directionality. 11 NW axial hetero-structures, including QDs, have been studied in a number of systems, e.g. InAsP-InP 13 , GaAs/GaAsP 14 and AlGaAs-GaAs 15 . Very narrow spectral linewidths have been reported at low temperatures. A line width of 10 µeV, which is comparable to the best values for self-assembled QDs, has been reported for a GaAs QD in an AlGaAs NW. [15][16][17][18] NW QDs have been used to generate both, single photons with extremely low multiphoton probability (g 2 (0) < 0.005), 19 and polarization entangled photon pairs (via the biexciton−exciton cascading) with fidelities exceeding 80%. 11,20,21 The majority of previous relevant studies have used NWs fabricated using the Au-catalyzed growth method, 19,22,23 possibly related to structural control, including sharp QD interfaces and the crystal phases. 24,25,26 However, NWs catalysed by Au are incompatible with Si-based electronics, and it has been found that Au can be incorporated into GaAs and InAs NWs at levels of the order of 10 17 -10 18 cm −3 . 27,28 More recently, the non-foreign-metal-catalysed NW growth mode has been developed and applied to produce axial hetero-structures with enhanced properties. 29 For example, GaAsSb-based multiple axial superlattices 30 and axially stacked InGaAs QDs 29 have demonstrated reduced thresholds for single NW lasers. However, the majority of reports of QD growth still exhibit high-densities of stacking faults, with a mixture of zinc blende (ZB) and wurtzite (WZ) crystal structures, 31 especially for the widely-used selfcatalyzed vapour-liquid-solid growth mode. 32 As the ZB and WZ crystal structures have different band gaps, the presence of both polytypes may have a significant impact on the electrical and optical properties of the NW QDs. 33,34,35 In addition, simulations of GaAs NWs suggest that even well separated twin defects may influence both the optical and electronic properties, 36 and calculations for bulk Si has demonstrated carrier scattering by a single twin. 37 The self-catalysed growth method utilizes nano-sized group-III metal catalytic droplets that are highly sensitive to the growth environment. Defect-free growth has been reported but for NWs with a diameter in the micrometre range. 38 However, full 3D confinement requires the NW diameters to be less than ~100 nm. The sensitivity of the catalytic droplets to their environment increases with reduced droplet size. For example, the Gibbs−Thomson effect, 39 makes it challenging to grow structures that are sufficiently small to exhibit true QD behaviour.
In a previous paper, we reported a preliminary demonstration of the growth of a single defect-free axial GaAs inclusion inside self-catalyzed GaAsP nanowires (NWs), with diameters ~50 nm. 14 However, the barrier material had a low P composition (up to 20%) and the emission from the GaAs region did not show features consistent with a QD. In addition, the emission quenched rapidly at low temperatures (~50K); a result of weak carrier confinement and the absence of any surface passivation. Hence, the suitability of the structures for practical device applications, in particular, stable operation at high temperatures, was not demonstrated. Previously, only nitride NWQDs have consistently demonstrated operation at high temperatures (for example, above liquid nitrogen temperature of 77K). However, nitride materials have a number of limitations.
For example, their high growth temperature (commonly >800 °C) can cause damage to other structures and is thus not compatible with many applications. In contrast, nonnitride NWQDs have a much lower growth temperature. However, to date, there have been no detailed reports of stable optical properties above 77K, in particular radiative efficiency and emission linewidth. To improve the operating temperature and carrier confinement, a higher barrier phosphorus composition is required. Efficient surface passivation is also necessary to reduce carrier loss at the NW surface and provide longterm protection of the QDs. In addition, the demonstration of closely spaced, identical QDs, forming molecular-like structures, or arrays of uniform QDs for laser applications, is highly desirable. To date, there has been a lack of reports of detailed studies of defectfree axial QD structures reproducibly grown by self-catalyzed methods.
In this work, we report a investigation of deep carrier confinement, self-catalyzed defect-free GaAs/GaAsP single and multiple axially stacked NWQDs. With robust surface passivation, the QDs exhibit very good optical properties, including narrow emission line widths well above liquid nitrogen temperatures, strong carrier confinement, and a large exciton-biexciton splitting, all beneficial for devices operating at high temperatures.

Results and discussion
GaAsP NWs with phosphorous compositions of 20 or 40% and containing GaAs QDs of varying number and sizes were grown using a flux compensation technique (Supporting information S1). All nanowires had a morphology similar to that shown in the scanning electron microscope (SEM) image of Fig. 1a. It can be seen that the NWs have a highly uniform diameter of 50-60 nm along their entire length. For the single and two QD structures, the QDs are located around the NW mid-point (an example can be seen in the low magnification TEM image and higher magnification inset of the QD in Fig.1a). TEM analysis was carried out for QDs with different sizes. Fig. 1b shows a ~10 nm thick single GaAs QD in a GaAs 0.8 P 0.2 NW. The QD has a pure-ZB structure without the presence of any twins. Fig. 1c shows the composition profile along the NW axis for a structure containing a taller, single QD than is shown in Fig.1b. The QD is composed of almost pure GaAs (Although EDX showed that some P was present in the QD, the amount was too low to be accurately quantified). To provide stronger carrier confinement, QDs with higher potential barriers are required. This is achieved by increasing the P content of the GaAsP NW. GaAs QDs with heights ranging from 5-30 nm were grown in GaAs 0.6 P 0.4 NWs, all of which were found to exhibit a pure-ZB structure. Figs. 1d-f showing annular dark-field (ADF) images and respective average intensity profiles for ~5, ~10 and ~30 (image only) nm high QDs. The Matthews and Blakeslee limit for relaxation of a planar GaAs layer grown on GaAs 0.6 P 0.4 is estimated to be ~15 nm, which is less than the current maximum QD height of 30 nm. 40 This suggests that the current structures are able to accommodate a relatively high degree of strain, a consequence of the small cross section of the NWs.
An important requirement for QDs is clearly defined boundaries with sharp interfaces.
The interface abruptness should be maximised, especially, for the growth of small height QDs, otherwise the effective height of the QD is increased. The composition profiles shown in Figs. 1d and e demonstrate sharp QD interfaces. The lower (GaAsPto-GaAs) interface is only ~6 monolayers (º 1.8 nm) wide and the upper (GaAs-to-GaAsP) interface is ~13 monolayers (º 3.6 nm). Both interfaces are significantly more abrupt than those observed in droplet-catalysed NWs where the group-III elements are switched (15-70 nm). 41,42 To the best of our knowledge, the current interface widths represent the smallest values for defect-free NW QDs where the group-V elements are switched. One reason for the sharp interfaces in the present structures is the low solubility of the group-V elements within the Ga droplet at the high growth temperature of 640 °C (Supporting Information 2). 26 The lower interface (growth of GaAs on GaAsP) is slightly sharper than the upper one due to the As/P exchange effect (Supporting

Information 3). This is similar to the observation by Priante et al. in their
heterojunctions grown in self-catalyzed AlGaAs NWs. 43 However, the heterojunctions in [43] contain a high-density of stacking faults, which could be a result of uncompensated growth during switching of source fluxes. In contrast, the current interfaces are defect-free.
To investigate if the QD separation affects their structural properties, QD pairs were grown with a small separation of ~10 nm. As seen in Figs. 2a and b these QD pairs are stacking-fault free, with very similar composition profiles and flat interfaces, indicating that the formation of closely spaced QDs with uniform properties is possible. This provides the potential to form QD molecules or large arrays of closely stacked QDs for laser applications.
To further investigate the suitability of the flux compensation method for the growth of axially-stacked QDs, a sequence of 50 nominally identical GaAs QDs were grown in a GaAs 0.6 P 0.4 NW. These QDs appear as white segments in Fig. 2c and d. The NW has a highly uniform diameter of 50 nm along its length and the round Ga droplet is retained at the tip, despite the long NW length of ~12 µm. Both of these observations indicate a highly stable growth environment during the long growth of 1.5 hours. From structural studies on a single representative NW, 65% of the QDs are found to be defect-free ( Fig.   1e) and 31% contained a low density of twinning planes (1-4 / dot, as shown in Fig. 2f).
Only 4% of the QDs dots (2 dots) were found to have very thin WZ inserts, as shown in Fig. 2g. This demonstrates the growth of a large number of axially stacked QDs with a large percentage (96%) having high structural quality. The twinning typically occurs just above the GaAsP-to-GaAs (first) interface, indicating that the observed defects are formed at the beginning of the QD growth (red arrows in Fig. 2f). This suggests a small droplet size fluctuation at the start of the QD growth due to the flux switching, which should be eliminated by further optimisation of the flux compensation technique.
Neighbouring QDs have very similar separations, heights and composition profiles, as shown by composition scans along the NW axis (Figs. 2h and 2i). There is a gradual increase in the QD height and separation from the base to tip of the NW (see Supporting The emission properties of the QDs were studied by performing microphotoluminescence (µ-PL) measurements on NWs that were transferred to a Si/SiO 2 substrate. The NW surface forms a thin oxidized layer, which can result in efficient non-radiative carrier recombination. 44 For GaAs/GaAs 0.6 P 0.4 QDs with a bare surface (no passivation layer) only weak QD emission is observed, shown as the black spectrum in Fig. 3. Following surface cleaning in a dilute ammonia solution (NH 4 OH:H 2 O=1: 19) for one minute, stronger but very broad QD emission is observed, blue spectrum in Fig.   3. To improve the optical properties ~6 nm GaAs 0.6 P 0.4 (to form 3D QD confinement), ~18 nm Al 0.5 Ga 0.5 As 0.6 P 0.4 , and ~9 nm GaAs 0.6 P 0.4 shell layers were grown radially around the GaAsP core. These layers form a potential barrier to confine carriers within the core region and inhibit them from reaching the NW surface. 45 The sample with the additional passivation layers, exhibits emission consisting of only a single QD peak at low laser powers, with a narrow linewidth of ~500 µeV. This behaviour is preserved after over 6 months storage in ambient atmosphere (the red spectrum in Fig. 3), demonstrating the importance of the passivation layers for long-term stability. Unless otherwise stated, the measurements described below are for QDs passivated by this method.
The detailed optical emission properties of the QDs were studied by performing µ-PL measurements on surface passivated GaAs 0.6 P 0.4 NWs with a core diameter of 50 nm and containing a single GaAs QD of nominal height 25 nm. The QD emission at 6K consists of a single peak at low laser powers (Fig. 4a). Spectra (Fig. 4a) recorded for different positions along the NW axis show that the emission is highly spatially localized, with an intensity/position profile (inset) determined by the focussed laser spot size (~1 µm) and consistent with a small spatially emitting region below the diffraction limit. The majority of NWs exhibit a single, sharp QD emission line at low laser powers although some demonstrate a more complex, slightly red-shifted form, which indicates the presence of defects that are able to perturb the local electronic structure.
With increasing laser power, additional emission lines appear, as shown in Fig. 4b. The inset plots the intensities of the two most spectrally resolved lines as a function of laser power. The different gradients are consistent with exciton and biexciton recombination and the lines demonstrate the expected high power saturation. Their separation is ~11 meV which is larger than previously reported values for non-nitride III-V QDs in a NW (for example 6 meV for InAs QDs in GaAs NWs 46 and 3 meV for GaAsP QDs in GaP NWs). 47 A large exciton-biexciton separation is beneficial for single photon emission at elevated temperatures.
where G 0 is the linewidth at low temperatures (1 meV for the current QD) and G a and For single-photon emission, the exciton and biexciton lines must be spectrally resolvable; this requires their linewidths to be comparable to, or less than, their separation. Thus, the observed separation of 11 meV indicates that single photon emission up to at least 140 K should be possible.
The temperature behaviour of the integrated intensity of a single QD emission exhibits a complex behaviour (Fig. 5c) Fig. 5d.

Conflict of interests
The authors declare no competing interests.

Additional information
Supplementary information is available for this paper at https://doi.org/ Correspondence and requests for materials should be addressed to Y.Y.Z.       To grow high quality axial hetero-structures with sharp interfaces, rapid switching of the growth fluxes is required. However, these flux switches have to be performed very carefully to avoid fluctuations of the droplet size, and hence the formation of stacking faults, and the potential formation of WZ segments. 1,2 ,3 To avoid these fluctuations when growing axial hetero-structures, it is crucial to maintain the droplet super-saturation, which is achieved using a "flux compensation" method. When the P flux is turned off/on, the As flux has to increase/decrease accordingly, with the change in As flux volume proportional to that of the P flux volume. Our previous studies have shown that the P nucleation is stronger than that of As, 4 so the compensating As flux should be larger than that of the P flux change. By varying the ratio (As flux change) : (P flux change) for the growth of a number of samples, with P compositions of either 20 or 40%, we have determined the optimum value to be between 1.48 and 1.80, with the precise value depending on the NW density, inter-NW parasitic growth density, growth temperature and III-V ratio. QDs grown outside this optimum compensation range tend to have a high-density of stacking faults. Fig. S2 shows TEM images obtained from GaAs QDs grown in GaAs 0.8 P 0.2 NWs using a compensation ratio of 2.2. As this compensation ratio is outside the optimum compensation range, the QDs of different heights (10~50 nm) contain a high-density of stacking faults; in contrast to the high quality, defect-free QDs grown within the optimum compensation range. This comparison clearly demonstrates the potential of the flux compensation technique and that its use is critical in obtaining high quality QDs.

S2. Reduced reservoir effect for As and P in Ga metal droplets
The high solubility of group-III metals in the metal droplets used by the VLS method results in a significant reservoir effect; this can prevent the fabrication of sharp hetero-interfaces, especially interfaces that rely on a significant depletion of one element. 5 In contrast, group-V elements have a much lower solubility in the liquid metal compared to group-III metals, especially at high growth temperatures. This allows for a very fast material depletion and switching. 6 The current GaAsP NW growth is performed at a relatively high temperature of 640 °C. This minimises the reservoir effect for As and P, resulting in the formation of sharp hetero-interfaces and the growth of almost pure GaAs QDs (Fig. 1c, main paper).

S3. Asymmetrical GaAs/GaAsP hetero interfaces
During compositional switching, As/P inter-diffusion occurs, reducing the sharpness of the interface. P atoms are more strongly bonded to Ga atoms, hence it is more difficult to replace P atoms with As atoms. 7 As a result, inter-diffusion at the GaAsP-to-GaAs interface is weaker than at the GaAs-to-GaAsP interface, leading to the former interface being sharper. There is an increase in both the QD height and QD separation from the base to tip of the NW (Fig. S3), which suggests a gradual environment change in source material collection. By gradually reducing the QD growth time during the NW growth it should be possible to achieve uniform QDs along the entire NW, as required for laser applications. line is the fit to our temperature dependent data (Fig. 5b). "SA" indicates selective-area growth. "Au"

S4. Variation in QD size and separation along the NW axis
indicates Au catalysed growth and "GaN" indicates gallium nitride-based NWs.  Fig. S4 and Table S1 summarize previous reports of emission linewidths for different III-V QDNW systems. The current work represents the first report of narrow emission linewidths for non-nitride based NWQDs above 20K. High-temperature emission from QDs in a NW is typically observed for systems with a wide bandgap and large exciton binding energy, e.g. GaN.
Despite a much smaller band gap and exciton binding energy, we observe emission at 140K with a linewidth of 9.8 meV. This value is comparable with the best-reported values for nitride