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Short period superlattices comprising ultra-thin InGaN/GaN quantum wells (QWs) with thickness of a few (0002) monolayers (MLs) have gained significant attention in the field of advanced optoelectronics. These nano-heterostructures offer the ability to tune the band gap by precisely controlling the thickness of both the QW and the GaN barrier [1]. Moreover, their potential applications in quantum computing and spintronics have sparked interest due to their possible topological insulator behaviour [2,3]. These properties are intricately linked to the indium content as well as the thicknesses of the QWs and GaN barriers. Growth efforts aim at high quality heterostructures with ML-thick QWs and high indium content. Previous theoretical and experimental studies have indicated that the highest indium content in such ultra-thin QWs is kinetically limited to a maximum of ~33% for growth under nitrogen-rich conditions. Meanwhile, strained substrates offer a solution to surpass the limits of indium incorporation in InGaN QWs [4]. The aim of this work was to determine the impact of growth temperature on the incorporation of indium atoms in GaN, to develop a growth model of such ultra-thin QWs and investigate the influence of strained GaN barriers on band-gap variation in this material system. An integrated methodology combining quantitative high resolution scanning transmission electron microscopy (HRSTEM), empirical potential and density functional theory (DFT) calculations, image simulations, and strain analysis was developed and employed [5,6,7]. A series of multi-QW heterostructures were fabricated by plasma-assisted molecular beam epitaxy (PAMBE) under metal-rich conditions, varying the growth temperatures of the QWs and GaN barriers. Our STEM-based analysis verified the formation of ultra-thin QWs with self-limited thickness up to 2 MLs along with the formation of ML-thick QWs with the highest known composition (~45%) (see Fig. 1(a)) under specific growth conditions [6].

Elastic accommodation of the high lattice mismatch in GaAs/In(Al,Ga)As core-shell nanowires holds great potential for applications in high-frequency electronics and optoelectronics. The unique core-shell geometry offers several advantages over planar systems. The reduced dimensionality of the core enables strain relaxation along the lateral dimension and strain partitioning, ultimately reducing the overall strain energy and expanding the limits of coherency [1]. Consequently, the GaAs core can undergo extreme elastic stretching, particularly along the nanowire axis, without the onset of plastic relaxation. Growing thick Inx(Al,Ga)1-xAs shells with high In-contents on narrow [111]-oriented GaAs cores leads to extreme elastic dilatation of the core that promotes a 40% bandgap reduction [2] as well as a 30-50% boost in electron mobility [3]. In order to elucidate the intricate 3D strain fields of such nanowires, we have employed transmission and scanning-transmission electron microscopy ((S)TEM) methods to study a series of nanowires featuring narrow cores (25 nm diameter) and thick shells (80 nm thickness) with composition x ranging from 0.20 up to 1. To enable the nanoscale investigation of all strain components, we developed elaborate sample preparation methods to facilitate observation along three zone axes, i.e. [111], <1-10> and <11-2>, as shown in Fig.1. For (S)TEM and high-resolution TEM (HRTEM) observations, a 200 kV JEOL JEM F200 CFEG microscope was used. High resolution STEM (HRSTEM) was performed in a 300 kV probe-corrected Thermo Fisher Scientific Titan Themis 60/300 microscope. In an integrated approach, experimental strain fields were compared with finite element (FE) calculations, performed using thermal expansivity to model the lattice mismatch, and with energetic calculations by molecular dynamics (MD) using the Tersoff interatomic potential [4]. (S)TEM observations revealed that for indium contents up to x≈50%, the shells were coherent and dislocatio