Dataset and article "Buried-stressor technology for the epitaxial growth and deviceintegration of site-controlled quantum dots"

Materials for Quantum Technology TOPICAL REVIEW • OPEN ACCESS Buried-stressor technology for the epitaxial growth and device integration of site-controlled quantum dots To cite this article: Kartik Gaur et al 2025 Mater. Quantum. Technol. 5 022002 View the article online for updates and enhancements. You may also like A review of design concerns in superconducting quantum circuits Eli M Levenson-Falk and Sadman Ahmed Shanto - Singly doped colloidal quantum dots as optically addressed nanopositionable qubits Rachel M Barrett and David J Binks - Anisotropy-driven topological quantum phase transition in magnetic impurities G G Blesio, L O Manuel and A A Aligia - This content was downloaded from IP address 147.251.57.228 on 02/10/2025 at 15:36 Mater. Quantum Technol. 5(2025) 022002 https://doi.org/10.1088/2633-4356/add3ad OPEN ACCESS RECEIVED 29 January 2025 REVISED 28 March 2025 ACCEPTED FOR PUBLICATION 2 May 2025 PUBLISHED 15 May 2025 Original Content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. TOPICAL REVIEW Buried-stressor technology for the epitaxial growth and device integration of site-controlled quantum dots Kartik Gaur1, Priyabrata Mudi1, Petr Klenovsky2,3,∗and Stephan Reitzenstein1,∗ 1Institut für Physik und Astronomie, Technische Universität Berlin, Hardenbergstraße 36, D-10623 Berlin, Germany 2Department of Condensed Matter Physics, Masaryk University, Kotlᡠrská 267/2, 611 37 Brno, Czech Republic 3Czech Metrology Institute, Okruˇ zní 31, 63800 Brno, Czech Republic ∗Authors to whom any correspondence should be addressed. E-mail: [email protected] and [email protected] Keywords: quantum dot, site control, positioning, epitaxial growth, buried stressor, quantum light sources, microlaser Abstract Semiconductor quantum dots (QDs) are high-quality nanocrystals that provide three-dimensional carrier confinement on the scale of the de Broglie wavelength. This makes them ideal candidates as light emitters, especially in the emerging field of photonic quantum technologies, where they can act as quantum light sources. However, their self-assembled epitaxial growth leads to randomness in position and emission wavelength, which hinders their scalable integration into photonic quantum devices. This review summarizes and highlights advances in the site-controlled growth of high-quality epitaxial QDs, with a particular focus on the buried stressor concept. Compared to other QD positioning techniques based for instance on nanohole arrays, nanowire arrays, and arrays of inverted pyramids as dot nucleation centers, the buried stressor growth method is distinguished by its ability to achieve not only spatial accuracy and precision, but also control of the local QD density in combination in an industry-compatible process flow. Therefore, the buried stressor growth technique is highly suitable for the development of both QD-based quantum light sources and microlasers. The buried stressor site-controlled QD growth technique involves the sub-surface embedding of a nano-engineered stressor material, which generates localized strain fields at the growth surface that control the nucleation of QDs. We provide an in-depth review of the underlying mechanisms and technological implementations, and discuss the differences and comparative advantages of the buried stressor method over other techniques for site-controlled growth of QDs. We also address persistent challenges, such as scalability and integration with existing semiconductor technologies, and outline potential future research directions. 1. Introduction Quantum dots (QDs) are low-dimensional semiconductor nanostructures that have attracted considerable attention in the broader fields of nanophotonics and photonic quantum information technology due to their unique electronic and optical properties. These properties arise from quantum confinement effects that occur when all dimensions of the QDs are comparable to the de Broglie wavelength of the confined carriers [1]. The resulting discrete energy levels of QDs are analogous to those observed in atoms, which explains why they are often referred to as ‘artificial atoms’. The size-tunable photoluminescence and electronic properties characteristic of these quasi-zero-dimensional nanostructures led to the 2023 Nobel Prize in Chemistry for the development of colloidal QDs [2]. Among the numerous methods developed over the past four decades for the synthesis of QDs, epitaxial growth has emerged as a dominant technique for QDs with applications in optoelectronics and quantum nanophotonics. This prominence is due to the exceptional control over the epitaxial growth process and the ability to produce high-quality QDs. Epitaxial QDs are typically fabricated using sophisticated methods, mainly molecular beam epitaxy and metal–organic chemical vapor deposition (MOCVD) [3]. These techniques allow for the controlled deposition of one material onto a substrate of another with atomically precise interfaces, ensuring a coherent crystal structure while minimizing defects. © 2025 The Author(s). Published by IOP Publishing Ltd Mater. Quantum Technol. 5(2025) 022002 K Gaur et al The self-assembled epitaxial growth process of QDs typically involves the deposition of a few monolayers of a material with a different lattice constant on a host substrate. In this process, the self-organized formation of QDs is driven by strain relaxation mechanisms, and their integration into a crystalline host matrix results in the formation of emitters with superior optical and electronic properties, especially at the single-QD level, compared to those synthesized by alternative methods such as chemical synthesis [4]. Epitaxial QDs have attracted considerable interest in both fundamental research and technological applications. In photonic quantum information technology, such QDs can be used as on-demand sources of flying qubits with long coherence times, excellent single-photon purity and high indistinguishability due to their discrete electronic states and reduced interaction with the solid-state environment [5,6]. In optoelectronics, QDs are used in devices such as lasers [7], and photodetectors [8–10] in which they provide specific advantages such as enhanced optical gain due to their modified electronic density of states, tunable emission wavelengths, and high quantum efficiency [11]. In addition, recent advances show that III/V QDs can be integrated with silicon photonic devices [12] which not only enhances their functionality, but also opens new avenues for innovation in the aforementioned fields, pushing the boundaries of what is possible in next-generation electronic and photonic devices. Despite their promising applications, the practical implementation of QDs faces significant challenges, especially at the single-emitter level. One issue is the spectral inhomogeneity of the QD ensemble emission, limiting their scalable integration into advanced quantum information systems such as quantum repeater networks, which require spatially isolated identical single-photon sources emitting at identical wavelengths on the scale of the homogeneous linewidth [13,14]. Therefore, considerable efforts have been devoted to overcoming the spectral inhomogeneity inherent in self-assembled QDs, resulting in a variety of approaches for spectral control, especially at the single-QD level, tailored to the specific requirements of quantum applications. For example, post-growth spectral tuning exploits the quantum-confined Stark effect [15], temperature tuning [16], and strain tuning using piezoelectric actuators [17] represent a viable solutions for controlling QD emission wavelengths. The random distribution of self-assembled epitaxial QDs in the growth plane poses another challenge to the fabrication of practical quantum photonic devices in a scalable manner. In fact, while single QDs can act as ideal on-demand quantum light emitters, their integration into nanophotonic structures such as resonators for emission enhancement or waveguides for on-chip photon routing leads to process yields in the few percent range using standard lithography techniques due to spatial and spectral mismatch, making any scalable fabrication of single-QD quantum devices practically impossible, see supplementary information of [18]. This led to the invention of deterministic lithography techniques such as in-situ optical lithography [19], in-situ electron beam lithography [20], and marker based lithography [21] in which suitable QDs are first selected and then integrated into the target nanophotonic structure with high spatial and, if necessary, also spectral accuracy. While deterministic lithography techniques are very powerful for fabricating single QD quantum devices in proof-of-principle studies, the scalability of the process is still limited by the need to locate the position of each QD in the pre-selection step before performing lithography at non-predefined positions on the epitaxial wafer. To overcome this scalability issue, techniques for the site-controlled epitaxial growth of QDs have been developed to fabricate highly uniform and ordered arrays of quantum emitters with controlled position and, in the best cases, predefined electrical and optical properties of the emitters. Against this background, this review provides an overview of the current state of epitaxial techniques for SCQDs, with a particular focus on the buried stressor technique, to complement other review articles in this Focus Issue on Site-Controlled QDs. We present the buried stressor approach in a comprehensive manner, including a discussion of the underlying principles and strain calculations, details of the epitaxial growth, and recent advances in device integration. The article covers the potential applications of buried stressor SCQDs in nanophotonics and photonic quantum information technology, highlighting the advantages and challenges associated with each application. Finally, we outline future research directions and potential breakthroughs that could drive the next generation of SCQD technologies. 2. Overview of advanced techniques for the scalable growth of site-controlled QDs The concept of site-controlled growth of QDs was pioneered and developed in the 2000s. An important technique in this regard is based on nanohole arrays, typically prepared by electron beam lithography (EBL) and dry or wet chemical etching, in which the position of the QDs is predefined using the holes as nucleation centers [22–26]. Alternatively, focused ion beam lithography [27] and nanoimprint lithography [28,29] have been used to fabricate nanohole arrays for site-controlled growth of QDs. In the epitaxial growth process, these nanohole arrays effectively locally reduce the critical thickness required for island formation and promote the nucleation of QDs precisely at predefined nanohole sites, first in a (optically inactive) seed layer 2 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al Figure 1. Overview of important techniques for site-controlled growth of epitaxial QDs. (a) SCQD array based on etched nanoholes as nucleation centers for dot formation. Inset: Schematic illustration of the formation of a QD seed layer on the nanohole surface and a top layer of the (later) optically active strain-coupled top layer of SCQDs. (b) Array of inverted pyramids, each containing a SCQD at its apex, as shown in (c). (d) Array of nanowires with a pitch of 4 µm, each with an integrated nanodisk-based QD, as shown in the lower part of the panel. (e) Array of square mesas each containing an oxide aperture acting as a buried stressor for strain-induced formation of SCQDs aligned laterally to the center of the aperture, as schematically shown in (f). (a) Reprinted from [26], with the permission of AIP Publishing. [35] John Wiley & Sons. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b), (c) Reprinted from [36], Copyright (2017), with permission from © 2016 Published by Elsevier B.V. (d) Reprinted (adapted) with permission from [37] Copyright (2013) American Chemical Society. (e) Reprinted from [38], with the permission of AIP Publishing. (f) Reprinted from [39], with the permission of AIP Publishing. followed by one or more layers of optically active QDs whose positions are strain coupled to the dots in the seed layer, see figure 1(a). This approach enables the generation of individual QDs in a perfectly ordered array and guides the growth of QDs with high spatial accuracy on the order of 50nm [26]. However, despite its high QD positioning accuracy and precision, the nanohole technique requires complex processing steps and usually does not achieve the high optical quality characteristic of standard self-assembled QDs, for example in terms of internal quantum efficiency limited to (47 ±14)% [30] and enhanced emission linewidth of 43 µeV [31]. Interestingly, significantly lower values with an average of 13 µeV and a minimum of 7 µeV were reported in [32], but at the cost of a significantly reduced single QD occupancy probability per site of only about 20%. In general, the reduced optical quality of such SCQDs is usually attributed to the unavoidable etched surface near the QD site, which introduces charged defects and non-radiative states at the growth interface [33]. In addition, nanohole techniques have a limitation regarding the maximum distance between nucleation sites on the order of 4µm when optimized for high single QD occupancy probability per site [34]. Exceeding this distance can cause unwanted nucleation, resulting in the formation of additional, unwanted QDs between the target positions defined by the nanoholes. Another relevant technique for site-controlled growth of InGaAs QDs is based on inverted pyramids etched in regular arrays into (111)-oriented GaAs substrate by lithography and wet chemical etching [40,41] as presented in figure 1(b). These pits act as nucleation sites where QDs can preferentially form at the apex of the inverted pyramids, see figure 1(c). This provides spatial control of QD positions on a 10 nm scale combined with pitches as low as 200nm [42] and the ability to integrate such QDs into photonic crystal cavities for enhanced light–matter interaction [43]. Importantly, this method enables the fabrication of highly uniform QDs with inhomogeneous broadening of the ensemble emission smaller than 10meV [40, 44], which are critical for photonic and quantum applications. In addition, growth on (111)-oriented GaAs leads to very small fine structure splitting (FSS) below 4µeV, which is very attractive for the generation of polarization-entangled photon pairs using these QDs [45]. QDs can be incorporated into nanowires grown, for example, by the vapor–liquid–solid (VLS) method of chemical vapor deposition. In this case, the QDs are incorporated into the nanowire with diameters of typically a few tens of nanometers as thin disks of material with a smaller bandgap compared to the surrounding nanowire matrix, as shown in figure 1(d). This offers interesting possibilities to combine materials with (to some extent) different lattice states in the nanowire heterostructure, allowing strain relaxation without the formation of defect states. In addition, the electronic and physical properties of the QDs can be engineered by controlling the diameter and height of the disk during the growth process to cover for instance a wide wavelength range of 880–1550 nm [46]. It is also possible to vertically stack single QDs, and surface passivation can be achieved in core–shell architectures during growth [47]. Site control in this approach is induced by growing the nanowires on pre-patterned substrates, e.g. nanohole arrays in a SiO2are patterned using EBL and reactive ion etching. This results in dense arrays of nanowire-integrated SCQDs, as shown in figure 1(d). Due to the flexibility in material selection and VLS growth techniques, nanowire QDs 3 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al cover a wide wavelength range from the ultraviolet to the telecom wavelength and beyond [48] and exhibit very good optical properties. For example, in an array of GaAs–GaAsP core–shell nanowire QDs with a pitch of 4 µeV, emission was observed with single-QD linewidths as small as 140 µeV, and an ensemble broadening of the emission of >40 NWQDs with 15 meV (FWHM) was reported in [37]. Narrower emission linewidths as low as (9.4±0.7) µeV (under non-resonant excitation) have been reported for single core–shell nanowire QDs in the GaAs material systems [49] and to about twice the Fourier limit in the InP material system [50], reflecting the general potential of this approach to be confirmed also in regular arrays of nanowire QDs. The buried stressor technique [39,51] is another very attractive approach to produce arrays of site-controlled QDs. In this method, stress patterns are created in the GaAs growth plane by a buried oxide aperture to control the nucleation of SCQDs laterally aligned to the center of oxide apertures inside mesa structures, which are patterned in regular arrays as shown in figures 1(e) and (f). The buried stressor technique exploits the interplay between strain and epitaxial growth processes to produce uniform and precisely positioned SCQDs at regions of tensile strain above the oxide aperture. This method not only enhances optical quality but also improves scalability, making it a promising solution for large-scale integration of QDs in advanced quantum devices. The high scalability potential can be a critical factor in the commercialization of QD-based devices, as it makes it possible to integrate these high-performance QDs into real-world quantum information systems. In addition, the buried stressor technique provides self-aligned current injection into the active QD region via the oxide aperture, eliminating the need for complicated alignment between electrical paths and QD sites, further streamlining the fabrication process and enhancing device performance. Furthermore, the technique is unique in that it allows control of not only the position of the QDs, but also of the number of emitters at the predefined position, which is of particular interest for the development of high-βmicrolasers based on a few-QD gain medium. 3. Physical and technological principles of buried stressor SCQD epitaxy The buried stressor growth technique has become a highly effective approach for the scalable fabrication of strain-controlled SCQDs with very high optical quality. The distinctive feature of this technique is its capacity to facilitate the formation of a defect-free growth interface, an area where conventional site-controlled techniques often encounter limitations due to the introduction of structural defects that impair the optical quality of QDs. In contrast, the buried-stressor technique ensures a high-quality growth surface, yielding QDs with emission linewidths comparable to those of standard non-positioned Stranski–Krastanow (SK) QDs. First introduced in 2012 by Strittmatter et al [39,51], the method leverages strain fields generated by a stressor layer buried beneath the epitaxial surface to create precise conditions for position-controlled QD growth. This eliminates the need for complex surface nanopatterning techniques and is compatible with industrial-scale vertical-cavity surface-emitting laser (VCSEL) fabrication processes. As such, this approach simplifies the fabrication process and supports large-scale production, which is particularly valuable for the development of single-photon emitter arrays, where precise control over QD positioning is essential to optimize device efficiency. In fact, unlike traditional methods that require costly and time-intensive EBL for pattern definition, the buried stressor approach employs standard high-throughput UV lithography, making it both efficient and economical. While VCSELs use aluminum oxide (AlOx) layers primarily to form insulating apertures for current flow regulation and lateral mode confinement, the buried stressor SCQD technique employs these oxide layers to induce a custom strain field at the epitaxial growth surface. This tailored strain facilitates QD formation precisely aligned with the oxide aperture locations. Since aperture-induced strain engineering plays a central role in the buried stressor SCQD concept, in the following we first discuss the numerical modeling of the corresponding strain effects before moving on to the technological implementation of this advanced epitaxial growth concept in section 3.2. 3.1. Numerical modeling of the buried stressor induced strain In order to better understand the underlying physics of the buried stressor method and to reduce the time and cost of the structure fabrication, numerical theory tools are indispensable. The key parameter to focus on in this work is the characterization of the elastic strain distribution in the sample structure caused by the presence of the buried stressor. In that respect, a number of numerical physics theory tools have been developed in the past. Those include atomistic methods, such as the density functional theory or the valence force field model, the latter used in conjunction with the empirical tight binding method for calculations of the electronic structure of nanostructures from different materials [52,53]. Since the task of calculating the strain in the whole mesa structure, the size of which is on the order of several µm in lateral dimensions, would necessarily involve 4 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al taking account all atoms present there, the aforementioned methods become numerically impossible to employ, even using current supercomputers. To overcome that obstacle, one needs to utilize techniques that are perhaps less precise but are, on the other hand, numerically considerably more feasible. One of these methods is, e.g. the continuum elasticity theory applied within the rectilinear simulation grid. In that method, the strain in the structure is found by minimizing the total strain energy in the structure. 3.1.1. Mathematical formalism For the mathematical formulation [54] of that method, we first write down the elastic displacement vector u(r) = r′(r)−r,(1) where rand r′are position vectors in the unchanged and distorted crystal lattices, respectively. The elastic displacement is connected with the crystal lattice distortion tensor by eij =∂ui ∂rj ,(2) where i,j∈ {x,y,z}. From the symmetric part of equation (2) one obtains the elastic tensor as εij =1 2(eij +eji).(3) The elastic strain energy is then given by E=ˆV 1 2CijklεijεkldV,(4) where Cijkl is the elasticity tensor and Vrefers to the volume of the whole simulation space. Since Cijkl and ε are different for different materials and spatial positions rin the considered structure, minimization of the energy in equation (4) leads to the desired strain distribution in the structure. We note that while the method lacks atomistic resolution, by appropriate choice of simulation grid one can adapt the computational demands of the method while keeping reasonable precision of the obtained results. Further advantage of the continuum elasticity method is the fact that, at least in the case of nanostructures, it provides a neat connection with eigenvalue solvers using the envelope function method based on k·papproximation [52, 55]. Furthermore, the single-particle electronic states obtained by the k·pmethod can then be utilized for multi-particle configuration interaction calculations [56–58]. 3.1.2. Simulation results Of central interest in the epitaxial growth of positioned QDs using the buried stressor technique is the magnitude of the εxx +εyy biaxial strain at the sample surface. Here, the continuum elasticity introduced earlier provides very good results, at least in qualitative and often in quantitative agreement with experimental results [59,60]. In an exemplary calculation, we first model the vertical evolution of εxx +εyy through the whole realistic structure of the mesa. The results obtained are shown in figure 2(a) for an oxide aperture diameter of 1000 nm. The numerical results shown in figure 2(a), where the calculated biaxial strain is color-coded, clearly demonstrate the role of the oxidized AlAs layer (denoted by ‘AlO’). As the oxidation proceeds from the sides of the mesa, the oxidized AlO layer compresses the GaAs above and below it. The resulting compressive strain in the above GaAs layer is large, with magnitudes up to −4%. At the same time, the AlO layer is also significantly strained in tension (up to 4%). These effects result from the different lattice constants of GaAs of 5.653 25 Å and Al203of 4.785 Å. Furthermore, since AlAs has a lattice constant of 5.660 Å, i.e. considerably larger than Al203, the center of the buried stressor layer is pulled toward the mesa side edges, creating a tensile strain on the center AlAs. The material that makes up the entire mesa structure then attempts to relax the strain created by the buried stressor, thereby reducing the overall strain energy in the structure. Thus, the magnitude of the compressive strain is progressively reduced toward the top surface of the mesa. At about 170 nm above the buried stressor, the strain in GaAs even changes sign, creating a layer of tensile strain. This tensile strain, however, is not laterally uniform across the entire sample surface, but is modulated in the area just above the AlAs region of the buried stressor layer [59], where in the present example two tensile strain maxima occur, corresponding in 3D to a ring-like tensile strain region aligned with the center of the mesa. It is that ring-like tensile strain region which favors InAs QD growth using the SK method. The reason is that the dots in that 5 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al Figure 2. Numerical simulations on the buried stressor induced strain and its effect on the emission wavelength of SCQDs. (a) Vertical evolution of the biaxial strain εxx +εyy, computed by the continuum elasticity theory [55] for a stressor aperture diameter of 1000nm. (b) A sketch with dimensions of a typical In(Ga)As QD cross-section used in our calculations. (c) Effect of biaxial strain εxx +εyy on the emission wavelength of a QD with a geometry as defined in (b). Figure 3. Vertical evolution of the biaxial strain εxx +εyy for AlO stressor layer thicknesses of (a) 60nm and (b) 300nm. The width of the AlAs stressor was kept at 500 nm in both panels, and the thickness of the top GaAs layer was kept the same as in figure 2(a). The white horizontal line on top of the panels marks the interface between the mesa top surface and air. region do not need to relax the lattice offset to GaAs that much compared to other parts of the sample surface, which in turn considerably reduces the QD built-in strain and associated elastic energy. Because of the aforementioned, the positioning of QDs above the aperture region is a robust property and one of the main advantages of the buried stressor method. Interestingly, the buried stressor induced tensile surface strain and its lateral modulation are not only the basis for site-controlled growth of the InGaAs QDs [59,60], it also reduces the strain within the body of the grown QDs, red-shifting their emission wavelength. To quantify this effect, we have calculated a typical truncated cone-shaped In0.45Ga0.55As/GaAs QD with a base diameter of 20nm and a height of 3 nm, positioned on a 1.5 ML thick In0.3Ga0.7As wetting layer (WL), using the Nextnano++ simulation suite [55]. When calculating the QD electronic structure, we also determined the internal QD strain and added the values of the externally applied εxx +εyy biaxial strain, following the procedure reported in [57]. As a result, we obtained the dependence of the change in QD emission wavelength as a function of εxx +εyy induced by the buried stressor. As shown in figure 2(c), with εxx +εyy increasing from negative to positive values, the QD emission wavelength increases by a significant amount of more than 125nm for the strain range considered. This is a very interesting result as it offers the possibility of combining site control with spectral redshift of the QD emission, potentially towards the telecom wavelength range of Oand C-band relevant for fiber based quantum communication networks without the need of complex strain reducing layers as usually applied for this purpose [61]. The magnitude of the surface strain εxx +εyy induced by the buried stressor can be tuned by the epitaxial layer design, and depends strongly on the relative thicknesses of the layers in the mesa. This is demonstrated in figure 3, where the bottom and top GaAs layers in the structure were kept the same as in figure 2(a), but the thickness of the stressor layer was set to 60nm and 300 nm in (a) and (b), respectively. It can be seen that 6 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al Figure 4. Simulated values of the lateral strain profile and effect of the AlO aperture height on the surface strain and the QD emission wavelength. Panels (a) and (b) show the mesa top surface line cuts of biaxial strain εxx +εyy for aperture diameters of 500 nm and 1000 nm, respectively. In panel (c) we show mesa top surface values of εxx +εyy on the left vertical axis as a function of the AlO aperture height for an aperture width of 500 nm. The right vertical axis in (c) gives the values of the emission wavelength of the QD defined in figure 2(b) strained by εxx +εyy given on the left vertical axis in (c). The calculations in (c) are done for an aperture diameter of 500nm. The data in all panels are given for top GaAs layer thicknesses of 50 nm, 100 nm, 150 nm, and 200 nm. for the thicker stressor layer, the tensile strain is already partially relaxed in it, which leads to less compressive strain in the GaAs layer above for the thinner stressor layer. As a result, the magnitude of the biaxial tensile strain εxx +εyy at the surface is almost negligible. This example highlights the importance of performing strain calculations to optimize the layer design prior to epitaxial growth for optimal QD positioning. Equally interesting and important is the case where the thickness of the stressor layer is kept constant, as in figure 2(a), and the height of the top GaAs layer is varied. The results of corresponding strain calculations are plotted in figures 4(a) and (b) for aperture widths of 500 nm and 1000 nm, respectively. In figures 4(a) and (b) we focus on the line sections of the surface strain εxx +εyy. We see an expected trend of the overall increase in the magnitude of εxx +εyy toward tensile values as the thickness of the top GaAs layer increases. This is simply because for a thicker top GaAs layer, there is proportionally more material to relax the substantial compressive stress caused by the aperture stressor layer. However, the strain relaxation is not uniform throughout the surface layer and is less pronounced in the area just above the aperture. In figures 4(a) and (b) we see increased εxx +εyy above the edges of the aperture and relatively reduced values just above the center of the aperture for thinner upper GaAs layers. On the other hand, for thicker GaAs layers, εxx +εyy is reduced towards compressive strain with respect to the rest of the mesa surface. The aforementioned behavior occurs for both considered aperture thicknesses of 500nm and 1000 nm, respectively, and points to the necessity of a careful choice of growth conditions for the growth of buried stressor samples, demonstrating at the same time possibilities of intentional experimental variability of the buried stressor method. The main purpose of the buried stressor method is to grow QDs at a predefined lateral position on the surface of the etched GaAs mesa structure, preferably directly above the oxide aperture. Another goal is to reduce the strain in the InGaAs QD body to red-shift its emission wavelength towards the telecom range. To provide some theoretical guidance in this regard, we show in figure 4(c) the surface strain εxx +εyy and the associated QD emission wavelength as a function of the top GaAs and AlO aperture layers, respectively, and for QDs positioned directly over the center of the 500nm wide stressor aperture. We see a wide range of possibilities. Clearly, a combination of a thinner top GaAs layer and a thicker stressor layer leads to a compressive stress on the surface and a resulting blue shift of the QD emission. Conversely, the reverse possibility, i.e. thicker GaAs layer and thinner stressor layer, leads to the desired red-shift of the QD emission, 7 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al Figure 5. Schematic illustration of the buried stressor SCQD growth process. (a) Epitaxial growth of the oxidation template with a buried AlAs layer. (b) Mesa structures patterned with dimensions of approximately 20 µm to expose the AlAs layer. (c) Selective oxidation of the AlAs layer performed in an oxidation furnace. (d) In-situ microscope image capturing the oxidation progress, where the dark region within the mesa represents the oxide aperture opening. (e) Overgrowth step, where QD nucleation is confined to the tensile-strained surface within the oxide aperture at the mesa’s center. Reproduced from [62]. CC BY 4.0. and for a 200 nm thick top GaAs layer, up to an almost 1160 nm QD emission is predicted, close to the telecom O-band which is around 1300nm. However, figure 4(c) also suggests that regardless of the thickness of the top GaAs layer, when the stressor layer becomes very thick, it can compensate for the oxidation-induced tensile stress on its own and thus exerts less and less stress on the top GaAs layer. This results in reduced εxx +εyy tensile strain on the top surface of the mesa for very thick stressor layers. Overall, the results presented show that theoretical modeling provides important insights into the strain fields induced by the buried stressor, making it an indispensable tool for layer design in this growth technique. Moreover, they highlight that due to the huge strain exerted in the structure, the buried stressor method can be used not only to position the QDs but also to significantly shift their emission wavelength towards desired values. 3.2. Epitaxial growth of buried stressor SCQDs The process flow of the buried-stressor growth approach starts with the growth of an oxidation template by depositing an Al0.9Ga0.1As/AlAs/Al0.9Ga0.1As heterostructure on a (001) GaAs substrate, followed by capping with approximately 100 nm of GaAs, as depicted in figure 5(a). Typically, the AlAs layer has a thickness of 30 nm, while each Al0.9Ga0.1As layer in the stack measures around 50 nm. While this simple layer design is typically used for growth optimization and fundamental studies of SCQD properties, the template structure often includes a distributed Bragg reflector (DBR) as a backside mirror in more device relevant epitaxial designs aiming for instance on the fabrication of micropillar lasers. Subsequently, the oxidation template is removed from the reactor and patterned using conventional UV lithography to define square mesas approximately 20 µm in size, laterally exposing the AlAs matrix, as illustrated in figure 5(b). The patterned template is then processed in a VCSEL-compatible workflow by transferring it to an oxidation furnace. The furnace operates at 420 ◦C under vacuum conditions, with water vapor carried by N2gas. Here, water vapor acts as the oxidizing agent, which chemically replaces arsenic anions with oxygen, described by 2AlAs +6H2O→Al2O3+As2O3+6H2.(5) While this exothermic reaction has a Gibbs free energy change of −473 kJ mol−1, the oxidation of GaAs is endothermic, resulting in a high selectivity for oxidizing AlAs over GaAs layers. Notably, oxide formation occurs without generating defects in the surrounding semiconductor layers. Experimentally, oxidation reduces the thickness of AlAs layers by approximately 12%–13% as a consequence of the tensile strain in this layer. This creates the desired surface strain, whose profile can be controlled by the size of the oxide aperture, as discussed in section 3.1. To enable a precise strain control, an in-situ microscope integrated into the oxidation furnace, monitors the oxidation progress through image contrast changes caused by refractive index variations. Figure 5(d) presents an in-situ microscope image of a mesa during oxidation, highlighting the oxide aperture as a dark area. The oxidation is stopped before the complete conversion of AlAs to Al2O3, forming an oxide aperture with an opening ranging from a few hundred nanometers to about 2 µm, depending on the number of SCQDs to be grown (see section 4.1). In practice, to achieve systematic tuning 8 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al Figure 10. (a) Schematic illustration of a 4 ×4 section from a 28 ×28 mesa array. Darker gray shades in the layer structure represent regions with higher aluminum content in the AlGaAs. Superimposed on the mesa surface are AFM images of oxidized mesas, where surface morphology changes reflect the strain field in the mesa center induced by the oxide aperture. (b) Optical microscopy images of a portion of the mesa array captured after the second MOCVD growth phase. (c) Representative g(2)(τ) histogram measured using a Hanbury Brown and Twiss setup from an SCQD under continuous-wave p-shell excitation (dots), along with the corresponding deconvoluted fit (solid line), yielding g(2) deconv(0) = 0.026 ±0.026. The inset provides results from a statistical analysis of SCQDs in a 4 ×4 section under off-resonant excitation, showing their deconvoluted g(2) deconv(0)values and corresponding decay times τdecay from the fits. (d) Corresponding g(2)(τ)measurement for the same QD as in panel (c), obtained under p-shell excitation using a Hong–Ou–Mandel setup. The results are shown for co-polarized (black dots) and cross-polarized (blue dots) configurations, along with convoluted fits (solid lines). These measurements reveal a HOM visibility of Vdeconv = (87.1±9.7)% and a coherence time of τc= (194 ±7)ps. The inset shows a magnified view around τ=0, with the deconvoluted fits (solid lines). Reprinted from [38], with the permission of AIP Publishing. (X) and biexcitonic (XX) lines are included in the analysis. Notably, this broadening is significantly smaller than the inhomogeneous broadening of 17.8 meV observed for non-positioned InGaAs QDs in our mesa structures, highlighting the positive impact of strain-engineering on the ensemble properties of the SCQDs. The determined inhomogeneous broadening of the SCQDs is comparable to the values reported for highly homogeneous SCQDs on GaAs (111), which range from 7.6 meV to 20 meV [36,40,94]. In addition to investigating the optical properties of the SCQDs, their quantum optical characteristics were analyzed by measuring the photon autocorrelation function for a 4 ×4 section of the 10 ×10 subarray under off-resonant CW excitation at 785 nm using a HBT configuration. An exemplary HBT histogram recorded for the XX line of an SCQD at saturation pump power is shown in figure 10(c). The measurement yields a raw value of g(2) raw(0) = 0.08 and a deconvoluted value of g(2) deconv(0) = 0.026 ±0.026, accounting for the time resolution of the HBT setup and demonstrating excellent multi-photon suppression for these quantum emitters. This finding is further supported by a statistical evaluation of multi-photon suppression at saturation across all SCQDs in the 4 ×4 section. The results, presented in figure 10(d) along with the corresponding decay times τdecay, reveal that 15 of the studied SCQDs have g(2)(0)values below 5%, with only one XX line showing a value slightly above 10%. These results underscore the high quantum optical quality of the buried-stressor SCQDs. Another important feature, for applications in photonic quantum technologies, is the indistinguishability of photons. This deeply quantum optical aspect was examined for the same emitter as in figure 10(c) using Hong–Ou–Mandel (HOM)-type two-photon interference (TPI) of consecutively emitted photons from the quantum emitter. Figure 10(d) presents the results along with the corresponding convolved and deconvolved fits for the co-polarized and cross-polarized HOM configurations. From these fits, a raw HOM visibility of Vraw =65.4%, a deconvoluted HOM visibility of Vdeconv = (87.1±9.7)%, and a coherence time of τc= (194 ±7)ps were determined. The observed TPI visibility for the site-controlled emitter exceeds the previously reported value of up to 73% for an SCQD fabricated using etched nanohole arrays [32], which is the only other report on the photon indistinguishability of SCQDs. However, it should be noted that in [32], pulsed excitation-which is more relevant for practical applications was used, making a direct comparison of the achieved HOM visibility difficult. 15 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al Figure 11. Integration of SCQDs microlenses for PEE enhancement. In the workflow, first the epitaxial structure is grown using the buried stressor method (a), followed by the microlens processing through in-situ electron beam lithography and subsequent ICP-RIE dry etching (b). (c) SEM image of mesa structure which includes a SCQD-microlens in its center (upper part), along with a 2D cathodoluminescence (CL) map of the same structure, highlighting the distinct luminescence from the SCQD (lower part). The positions of the SCQD and aperture are indicated with dashed circles and squares, respectively. The corresponding spectrum of the SCQD, taken during CL-mapping in the in-situ EBL process, is shown in the inset. (d) SEM image of the fully processed microlens structure. The mesa structure and the CL mapping region are highlighted in (c) and (d) with dashed red and yellow rectangles, respectively. Reprinted from [95], Copyright (2017), with permission from © 2017 Elsevier B.V. All rights reserved. 4.2.2. Increase of photon extraction efficiency by photonic device integration of single SCQDs A very application relevant aspect of QD quantum light sources is the increase of the photon extraction efficiency PEE using photonic nanostructures such as micro/nano cavities, photonic wires and microlenses [5]. These structures allow one to increase the PEE, i.e. the probability of transferring a photon generated by the quantum emitter by an optical or electrical pulse into the target optical element such as an optical fiber, in the case of epitaxial QDs from about 1% in a simple as-grown planar structure to values above 70%. While PEE enhancement by integration into nanophotonic structures is now a standard procedure for conventional non-positioned QDs [5], only a few works have reported the same for SCQDs. These include the integration of nanohole-based SCQDs into micropillar cavities [34,96] and photonic crystal nanocavities [97]. The latter have also been used for emission enhancement and to study the cQED effects of inverted pyramid-based SCQDs [98,99]. In the case of buried stressor SCQDs, both the micropillar and microlens approaches have been used to enhance their emission properties. While the micropillar approach, which allowed for the demonstration of Purcell-enhanced single-SCQD emission without studying the single-photon emission characteristics at that time [70], was already discussed in the previous section, there is another relevant work by Kagansky et al in which such emitters were deterministically integrated into microlens structures for PEE enhancement using in-situ EBL [95]. The sample fabrication started with the buried stressor growth of SCQDs as described in section 3. In this case, the heterostructure includes a bottom DBR made of 27 pairs of λ/4-thick Al0.90Ga0.10As/GaAs layers, followed by the oxide aperture stack, InGaAs QDs, and a GaAs capping layer with a thickness hof 418 nm, see figure 11. This height corresponds to the numerically optimized microlens geometry with a height of 418 and a radius of r=1040 nm, see 11(b), predicting a photon extraction efficiency (PEE) of 27% for a numerical aperture (NA) of 0.4 [95]. 16 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al After epitaxial growth, the sample was spin-coated with a 110 nm thick layer of the electron-beam resist CSAR 62, followed by deterministic SCQD integration using the in-situ EBL technology platform [20,100]. The lower part of figure 11(c) shows a 2D CL map of the mesa surface, taken at 10K with a low exposure dose as the first step of in-situ EBL. This map, with a spectral range of 925.21–925.49 nm, reveals localized luminescence from the charged trion state of the SCQD, whose spectrum is shown in the inset. The SCQD position is marked by a dashed red circle in figure 11(c), while the SEM image highlights the oxide aperture’s position. A comparison of both maps shows a moderate misalignment of about 800nm between the center of the aperture and the SCQD. This misalignment precluded direct patterning of microlenses at the mesa center, as achieving the desired PEE increase requires an emitter-microlens alignment accuracy better than 50 nm. Thus, in-situ EBL was crucial to ensure precise microlens positioning over the SCQD. In the second in-situ EBL step, lenses with the optimized height h=418 nm and radius r=1040 nm) (see figure 11(b)) were patterned into the resist at the SCQD positions at 10K using a high exposure dose to invert the resist locally. During the subsequent development process, the resist was removed from the scanned area except at the lens positions. The fabrication was finalized by inductively coupled plasma reactive ion dry etching (ICP-RIE) etching of the SCQD-microlens (see figure 11(c)). Figure 11(d) shows an SEM image of the processed device, with a zoomed-in view of the microlens in the inset. Notably, this approach achieves a high yield for SCQD realization, with 17 out of 27 mesa structures containing SCQDs, corresponding to a yield of 63%. The average displacement of SCQDs from the center of the oxide aperture was measured to be 640 nm. The optical performance of the fabricated SCQD microlenses was evaluated by µPL spectroscopy and photon autocorrelation measurement and showed a PEE of up to (21 ±2)% in a collection optics with an NA of 0.4 as well as a very high suppression of multi-photon emission events with g(2)(0) = 0.03 ±0.01 after background subtraction. These results highlight the possibilities of integrating SCQDs into nanophotonic structures for emission enhancement. Future studies could focus on the integration of SCQDs into circular Bragg grating resonators, promising significantly higher (and still broadband) PEE at the cost of a more complex device design. In addition, it would be attractive to improve the positional accuracy towards 50nm, to avoid the need for elaborate deterministic lithography and SCQD resonator alignment, simply by placing the photonic structure in the center of the mesa structure in a simple and scalable fabrication process. 4.2.3. Electrically driven single-photon sources based on single SCQDs Electrical operation of QD SPSs is also of high application relevance. In fact, while optical excitation using bulky external lasers is usually the method of choice for proof-of-principle experiments in basic science, electrical excitation of QD-based quantum light sources (and microlasers) is highly desirable in practical real-world systems. In the case of semiconductor QDs, electrical operation is typically enabled by carrier injection in pin-doped heterostructures. Significant progress has been made in the development of electrically driven SPS based on standard non-positioned QDs. These include simple diode-like structures without cavity enhancement of emission [102], and microcavity structures such as micropillar cavities exploiting cQED effects to increase the PEE up to the record value of 61% reported in [103]. In the context of this article, it is interesting to note that Schneider et al succeeded in presenting single-photon emission with g(2)(0) = 0.42 from nanohole array-based SCQDs integrated into electrically contacted micropillar cavities [34]. Electrical operation of a buried stressor SCQD single-photon source was demonstrated by W. Unrau et al in 2012 [101]. In the device design, based on a pin-doped SCQD heterostructure with backside DBR mirror, the embedded oxide aperture has two functionalities: first, it acts as buried stressor for this positioned growth of QDs and second, it defines and controls the current path through the device in analogy to the current control in VCSELs. This injection scheme which features a self-aligned current path through the target QD minimizes the required injection current because parallel paths are effectively suppressed by the oxide layer, and it ensures selective excitation of the SCQD in the center of the structure as illustrated in figure 12(a). The micro-electroluminescence emission spectra of such a quantum device are plotted in figure 12(b) for different injection currents in the range of 0.01–0.6 µA. Very clean single-QD emission is observed for all injection currents and different excitonic transitions are identified by their characteristic excitation power and polarization dependence. It is noteworthy that even under electrical carrier injection resolution-limited emission linewidths as low as 25 µeV are achieved in this work [101]. Single-photon emission was tested in an HBT autocorrelation experiment under CW electrical excitation, and the recorded correlation histogram shown in figure 12(b) yields strong multiphoton suppression, with g(2)(0) = 0.05, which is in the range of the best values obtained under optical pumping for buried stressor SCQDs. These results highlight the potential of the developed device concept for electrically driven SPSs based on buried stressor SCQDs, which in turn is VCSEL process compatible and thus, in principle, can be implemented on an industrial scale. Future developments of this device concept could focus on combining it with efficient light extraction strategies compatible with electrical current injection. Obvious choices are micropillar or photonic defect 17 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al Figure 12. Electrically driven single-photon source (SPS) based on buried stressor SCQD. (a) Schematic cross-section of the device, showing the SCQD position, aperture size, and the current flow (represented by green lines). (b) Micro-electroluminescence spectra as a function of injection current, highlighting emission lines corresponding to neutral exciton (X), biexciton (XX), and charged exciton (X+) transitions. (c) Photon auto-correlation measurement histogram for the X+emission. The red line represents a fit of the second-order auto-correlation function to the raw data, yielding g(2)(0) = 0.05. Reprinted from [101], with the permission of AIP Publishing. based cavity solutions as discussed in section 4.1, but with doped DBR mirrors and electrical contacts. Another attractive direction would be to combine buried stressor SCQDs with electrically contacted CBGs in a recently realized device concept using non-positioned QDs and their deterministic device integration by marker-based EBL [104]. 4.3. Comparison of the buried stressor technique with other concepts for positioned QD growth Before concluding this review, we provide a comparison of the main techniques for site-controlled epitaxial growth of epitaxial QDs. We compare the properties of the four techniques for the growth of SCQDs, discussed in section 2, and for completeness, we also include information on substrate-encoded size-reducing epitaxy (SESRE) growth technique. SESRE is a method for the positioned growth of QDs that utilizes patterned substrates with features larger than the final desired structures. During epitaxial growth, material is deposited preferentially within the patterned areas, and surface diffusion combined with atomic-scale processes leads to a self-limiting reduction in size as the structure evolves. This technique allows precise control over the size and position of SCQDS, making it interesting for applications requiring uniform and well-aligned QDs, for more details of this technique we refer to [105]. A summary of a comparative analysis of the mentioned growth techniques is presented in table 1for QDs emitting in the near infrared and telecom wavelengths, with key technological and optical parameters of the SCQD systems, such as information on cavity integration, photon extraction efficiency, positioning accuracy, wavelength range, average pattern pitch for each technique, emission linewidths. A key parameter in this area is the positioning accuracy, for which the SESRE and inverted pyramid growth techniques achieve the best values in the 10 nm range, while the buried stressor approach falls back with an accuracy in the 100–500 nm range. The inverted pyramid growth technique also leads to very homogeneous QD arrays with an inhomogeneous ensemble broadening of only 2nm, which is the lowest value achieved among the SCQD techniques considered. With regard to the possibility of controlling the number of SCQDs per nucleation site, the buried stressor technique stands out as the only one that allows this parameter to be controlled by the size of the buried stressor. With respect to the SCQD emission wavelength range, nanowire SCQDs benefit from the flexibility of material combinations and the possibility to influence the spectral properties by the nanowire diameter during growth, resulting in a wide emission range spanning from 880 to 1550nm. With respect to the minimum achievable pitch of the SCQD arrays, values as low as about 200nm can be realized for nanohole, nanowire, and inverted pyramid-based dots. Except for nanowire SCQDs, all positioned growth schemes allow cavity integration for enhanced light–matter interaction and lasing in the case of buried stressor SCQDs, and current injection has been demonstrated for nanohole SCQDs and buried stressor SCQDs. In terms of single-emitter properties, emission linewidths down to 10 µeV were achieved for nanohole, nanowire, and buried stressor SCQDs, where narrow emission was accompanied by low single-emitter site occupancy for the nanohole system and strict resonant excitation was applied for the buried stressor approach. The photon extraction efficiency was experimentally evaluated for SERSE, nanowire, and buried stressor SCQDs, and values up to 42% were reported for bottom-up tailored nanowires [106], and 21% were reported for the deterministically fabricated buried stressor-SCQD microlens structures [95]. It is noteworthy that the nanowires with integrated QDs in [106] were grown using randomly dispersed Au particles rather than a regular patterned array, as in [107]. All considered SCQDs exhibit single-photon emission with high multi-photon suppression and g(2)(0)typically in the few percent range, and photon indistinguishability up to 82% was observed for the SERSE technique. 18 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al Table 1. Comparison of performance parameters of different techniques for the positioned growth of epitaxial QDs. Abbreviations: PA: positioning accuracy, IB: inhomogeneous ensemble broadening, MQP: multi QD positioning, WR: wavelength range, MP: minimum pitch, MI: microcavity integration, EE: electrical excitation, LW: emission linewidth, NIR: near infrared. Technique PA (nm) IB (nm) MQP WR (nm) MP (µm) MI EE LW (µeV) PEE g(2)(0)VHOM Reference Nanohole arrays 50 0.11 No NIR 1 Yes Yes 13 — 0.02 0.73 [26,32,34] SESRE 5–10 2.8 Yes NIR 5 No No 45 12% 0.015 0.82 [105] Inverted pyramids <10 2–6 Yes NIR 0.4 Yes Yes 18–250 — 0.028 — [40,42,44,108–112] Nanowires 200 15 Yes NIR-Telecom C 0.2 No No 10 42%a0.02 — [37,49,106,107,113–115] Buried stressors 100–500 14 Yes NIR, Telecom O 20 Yes Yes 10 21% 0.026 0.87 [38,51,59,64,92,95] aValue is from [106], where the nanowires with integrated QDs were grown using randomly dispersed Au particles instead of regularly patterned arrays of nanowires as in [107]. 19 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al Overall, the compared SCQD growth techniques offer many opportunities for scalable emitter integration into devices that can act as quantum light sources and microlasers with enhanced light–matter interaction over a wide range of different wavelengths. Depending on the application scenario, one or the other SCQD growth technique can enable the best possible performance of such devices. 5. Open questions and future perspectives Despite the tremendous progress that has been made in the development of buried stressor growth and device integration of SCQDs since the pioneering work of Strittmatter et al [51] in 2012, there are several open questions that need to be addressed in future research activities. In addition, the full potential of this technique has not yet been realized, and there are interesting perspectives that can lead to exciting future developments. The buried-stressor concept could, in principle, also be extended to InPand GaN-based QD systems. However, its implementation requires careful selection of stressor materials and tailored oxidation or strain-engineering strategies. This is due to differences in oxidation behavior, lattice mismatch, and strain distribution compared to the GaAs-based material system. The advancements in this field could potentially expand SCQDs beyond the GaAs system, rendering site-controlled quantum emitters viable for visible, UV, and telecom applications while preserving the high spatial precision and scalability characteristic of buried-stressor technology. However, a comprehensive and precise discussion of the technology for these material systems necessitates critical details regarding the systems in question, which fall outside the scope of this review article. Instead, in the following, we focus on some of the most important open questions and provide ideas for the future development of buried stressor growth techniques in GaAs material and their application to optoelectronic and quantum nanophotonic devices. 5.1. Process control and scalability of the buried stressor SCQD technology The reported progress in the epitaxial growth and device integration of SCQDs buried stressors underscores the high potential of this technique for the fabrication of both classical and quantum photonic devices. However, there are several aspects that could be explored and improved in future optimizations of buried stressor growth, particularly with respect to process control and scalability. In terms of process control, the buried stressor SCQD technique could be advanced in the following aspects. First, as mentioned above, the positioning accuracy is on the order of a few hundred nanometers, which is acceptable for integration into micropillar cavities and photonic defect microcavities, but sets limits for scalable integration into circular Bragg grating resonators, which require alignment accuracy better than 50 nm for optimal performance [116]. Thus, further optimizations should explore the current limitations of QD positioning accuracy and aim to overcome these limitations through design improvements (via numerical simulations), leading to narrower strain profiles at the growth surface. Second, it would be interesting to evaluate the extent to which strain engineering can reduce the inhomogeneous ensemble broadening of SCQDs. Addressing this question and potentially reducing inhomogeneous broadening is important for both quantum device and microlaser applications of buried stressor SCQDs, since both benefit from spectral matching between the emitters and the optical modes of the devices, for example to maximize the Purcell factor and the spontaneous emission coupling factor. Third, it would be interesting to reduce the pitch of SCQD arrays from several tens of microns to a few microns by mesa design modifications to enable dense arrays of quantum emitters, which could be applied in photonic quantum computing [117], and dense arrays of microlasers, with applications in photonic neuromorphic computing [118,119]. Wafer-size scalability of buried stressor growth would require a high control of the oxide aperture sizes across the wafer with an accuracy on the order of 100 nm to enable the formation of SCQDs with small inhomogeneity in terms of the number of nucleating emitters and the emission wavelength. Achieving such wafer-scale homogeneity is challenging because of process deviations, especially in the water-vapor supported oxidation step using standard oxidation ovens, which can lead to deviations of the oxide aperture size of more than 500nm (for mesas with constant size) across a quarter 2” wafer [120]. Technical innovations are needed to overcome such oxidation inhomogeneities and to improve the scalability of the SCQD growth process, for instance by improving the homogeneity of water-vapor flow in the furnace across the wafer. 5.2. Telecom-wavelength buried stressor SCQDs In terms of real-world applications of QD devices, sources emitting at telecom wavelengths in the O-band at 1.3 µm and in the C-band at 1.55 µm, which are important for fiber-based data communication, are of great interest. Recently, enormous research activities have been devoted to the development of O-band and C-band single-photon sources using the conventional SK growth technique of non-positioned In(Ga)As QDs [121, 122]. However, the growth is more complex than for mature In(Ga)As QDs emitting in the 900–980 nm range, because either a SRL [84,85,123] or a MMB [86–89] must be included in the layer design to 20 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al accommodate the increasing lattice constant mismatch of high In content In(Ga)As QDs required to reach the target wavelength in the GaAs material system. Alternatively, telecom wavelength QDs, especially for C-band emission, can be grown on InP substrates using suitable buffer layers [124–127]. While pure single-photon emission has been observed in both cases, the demonstration of high photon indistinguishability above 50% is still pending [128–130]. At least in the case of GaAs-based telecom QDs, the limited photon indistinguishability can be attributed to defect states introduced by the SRL or MMB layer. Buried stressor SCQDs could provide an attractive solution for developing high quality quantum emitters at telecom wavelengths, with the added benefit of positioned growth within a scalable technology platform. In fact, as discussed above, the buried stressor not only defines the growth position, but can also be used to strain the QD emission wavelength. First results reported by Limame et al in [59] are promising and demonstrate the high potential of using the buried stressor to realize long wavelength SCQDs. Future developments, based on the close interaction between numerical modeling and epitaxial growth, could focus on optimizing the epitaxial layer design and the geometry of the buried stressor to maximize the red shift of the emission of SCQDs towards the O-band and perhaps even towards the C-band. In the best case, the target wavelengths can be achieved without the need for an SRL of the MMB layer, which would be particularly beneficial for increasing photon indistinguishability. In addition, such advances in the growth of telecom-emitting SCQDs for quantum photonic devices could also be used to fabricate telecom-wavelength QD microlasers based on stacked layers of site-controlled QDs and using larger aperture sizes to increase the number of positioned QDs interacting with the mode field of the laser cavity. 5.3. Buried stressor SCQD devices with electrical control Another important aspect of application relevance is the electrical control and current injection of SCQD-based devices. As discussed in section 4.2.3, first promising results have also been achieved for buried stressor SCQDs in the absence of cavity effects [101], and there are several prospects for future developments in this area. Most importantly, the integration of such SCQDs into electrically controlled microcavity structures can lead to very attractive application scenarios. These include bright electrically controlled single-photon sources based on SCQDs embedded in circular Bragg grating resonators in a recently established design concept using ridges for contacting QDs in doped heterostructures. Transferring this scheme to buried stressor SCQDs could pave the way for large-scale arrays of bright electrically driven or electrically injected quantum light sources, where electrical control could be used to spectrally tune the emission wavelength of the sources via the quantum confined Stark effect. Such source modules could provide multiple single-photon states for photonic quantum computing in a compact, scalable device concept. In addition, nanophotonics applications of SCQDs can benefit from the additional possibilities of electrical control. In this context, further developments of microlasers and photonic defect microlasers based on buried stressor SCQDs should focus on incorporating doped layers and electrical contacts to enable current injection in electrically driven high-βmicrolasers with low threshold currents. Such an approach could greatly advance the development of neuromorphic computing concepts that rely on dense arrays of coherent light emitters with high energy efficiency as nanophotonic hardware components [75–77,119]. 6. Conclusion In conclusion, in this review article, we have discussed the current state of site-controlled QD growth approaches with a special focus on buried stressor technology. Site-controlled growth technologies in general play an important role in advancing the field of quantum photonics, effectively bridging the gap between materials engineering, nanotechnology, and practical applications of QD optoelectronics and photonic quantum information technologies. The buried stressor SCQDs technology is characterized by a robust and industry-compatible process flow without the need for complex e-beam lithography, as well as the unique ability among SCQD approaches to control not only the position but also the number of nucleating QDs, the latter making it particularly interesting for microlaser applications. Moreover, a high degree of scalability is provided by the self-aligned integration of buried stressor SCQDs into photonic defect microcavity structures, which can act as efficient microlasers and single-photon sources in a robust quasi-planar cavity design compatible with electric current injection. Remarkably, the buried stressor technology allows the use of well-known and mastered techniques in solid state quantum theory (continuum elasticity, k.p) to predict the physical behavior of the devices with surprisingly high precision for this system, driving the development of device structures for fabrication using well-mastered growth techniques historically perfected by the semiconductor industry. Such a combination provides a clear path to the fabrication of practical devices that will help translate concepts from fundamental materials science and nanotechnology into real-world applications, such as neuromorphic computing and photonic quantum technologies. 21 Mater. Quantum Technol. 5(2025) 022002 K Gaur et al Data availability statement This topical review includes figures from previously published articles. We do not have access to the original data. The data that support the findings of this study are available upon reasonable request from the authors. Acknowledgment S R acknowledges funding from the German Federal Ministry of Education and Research (BMBF) via the project MultiCoreSPS (Grant No. 16KIS1819K) and from the Berlin Quantum initiative. P K acknowledges funding from the European Innovation Council Pathfinder program under Grant Agreement No. 101185617 (QCEED), support by the project Quantum materials for applications in sustainable technologies, CZ.02.01.01/00/22 008/0004572, and partly funding by Institutional Subsidy for Long-Term Conceptual Development of a Research Organization granted to the Czech Metrology Institute by the Ministry of Industry and Trade of the Czech Republic. 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