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InGaN/GaN quantum wells (QWs) with sub-nanometer thickness can be employed in short-period superlattices for bandgap engineering of efficient optoelectronic devices, as well as for exploiting topological insulator behavior in III-nitride semiconductors. However, it had been argued that the highest indium content in such ultra-thin QWs is kinetically limited to a maximum of 33%, narrowing down the potential range of applications. Here, it is demonstrated that quasi two-dimensional (quasi-2D) QWs with thickness of one atomic monolayer can be deposited with indium contents far exceeding this limit, under certain growth conditions. Multi-QW heterostructures were grown by plasma-assisted molecular beam epitaxy, and their composition and strain were determined with monolayer-scale spatial resolution using quantitative scanning transmission electron microscopy in combination with atomistic calculations. Key findings such as the self-limited QW thickness and the non-monotonic dependence of the QW composition on the growth temperature under metal-rich growth conditions suggest the existence of a substitutional synthesis mechanism, involving the exchange between indium and gallium atoms at surface sites. The highest indium content in this work approached 50%, in agreement with photoluminescence measurements, surpassing by far the previously regarded compositional limit. The proposed synthesis mechanism can guide growth efforts towards binary InN/GaN quasi-2D QWs.

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Titanium thin films were deposited on silicon nitride (SiN x ) coated Si, NaCl, and sapphire substrates varying the deposition conditions using e-beam evaporation to investigate thin film growth modes. The microstructure and texture evolution in dependence of substrate, deposition rate, film thickness, and substrate temperature were studied using X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. Thin films obtained on SiN x and NaCl substrates were nanocrystalline, while the films deposited on sapphire transformed from nanocrystalline to single crystalline at deposition temperatures above 200 • C. Predominantly, a surface plane orientation of ( 0002) was observed for the single crystalline films due to the minimization of surface energy. The orientation relationship of epitaxial single crystalline films grown on C-plane sapphire substrate is found to be (0002)T i (0006) S apphire , 1120 T i 0330 S apphire . In this orientation relationship, both the total surface and strain energy of the film are minimized. The results were complemented by resistivity measurements using the four-point probe method reporting an increase from ∼60 µ Ω cm to ∼95 µ Ω cm for single crystalline and nanocrystalline films, respectively. 32Apart from the deposition parameters, the lattice matching, 33 symmetry of surface planes, substrate miscut, and the planes 34 forming terraces and steps on the substrate surface [15] are vari-35 ables that can alter the orientation and morphology of thin films, 36 while maintaining substrate chemistry. The adaption of an ori-37 entation relationship (OR) between a substrate and a film is a 38 complex phenomenon [16]. Even with a known substrate sur-39 face symmetry and chemical composition, it is in most cases not 40 possible to predict the preferred OR [17]. Although the (0002) 41 plane of Ti has the lowest surface energy, making it the domi-42 nant surface plane, Ti films have been reported to grow with at 43 least six other surface plane orientations [18].

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