Composition and strain of pseudomorphic α-(AlGa)2O3 on sapphire (0001) substrates

Abstract text (incl. figure legends and references)

Recently, it was reported that in initial stages of Ga₂O₃ growth on (0001) sapphire substrates a thin (~3 ML) layer of α-Ga₂O₃ is forming before the formation of β-Ga₂O₃ for three growth techniques [1]. A surprising result, since the most stable polymorph of Ga₂O₃ is the beta phase, whereas the alpha phase is only the third [2]. The authors explain the stabilization of the alpha phase as being a strain effect due to the pseudomorphic character of the layer within an isotropic strain model. In this contribution a similar α-Ga₂O₃ layer was grown on a (0001) sapphire without overgrowth with β-Ga₂O₃ using molecular beam epitaxy (MBE) and investigated using TEM. Fig. 1 shows HAADF STEM image of the layer in a) together with a simulated image of an α-Ga₂O₃ layer on α-Al₂O₃ in b) for the [11-20] zone axis. The respective images for the [11-00] zone axis are depicted in c) and d). The observed contrast pattern qualitatively agrees between experiment and simulation. A close inspection of the contrast pattern for the [1-100] zone axis shows that it does not agree between substrate and layer, which can be explained by the different contributions of Ga and O to the image intensity in the layer compared to Al and O in the substrate. This was already reported in reference [1].

Fig.1: a) and c) HAADF STEM images of the layer in [11-20] and [1-100] zone axis. b) and d) Respective simulated images of Al2O3 with a Ga₂O₃ layer, for respective zone axes.

EDX mappings showed a concentration of 26% Ga. To confirm this concentration composition maps were derived from the HAADF STEM images by a quantitative comparison with simulations according to the method suggested by Rosenauer et al. [3]. A Ga concentration map and a linescan along growth direction are shown in Fig. 2 a) and b). Indeed, the EDX result of 26 % is in good agreement with the HAADF STEM measurement of about 25%. Fig. 2: Ga composition map (a)) and linescan along growth direction (b)). In fact, the presence of the α-Ga₂O₃ in ref. [1] was explained by stabilization of the phase due to strain within an isotropic model. Fig. 3 shows the biaxial stress as a function of the assumed bulk modulus for a Poisson ratio of 0.3 for different compositions (25%, 50%, 75% and 100% Ga). Additionally, single points are added, which correspond to critical stresses, for which the alpha phase is starting to become energetically more favourable than the beta phase. For 100%, 75% and 50 % the stress is still significantly high to be the reason for the stabilization of the alpha phase, which is not given for the measured concentration of 25 %. Nevertheless, the assumption of an isotropic strain model is certainly not correct, especially since the layer seems to be pseudomorphically grown on the substrate and therefore a tetragonal distortion of the material in the layer will take place. We will measure this tetragonal distortion and compare it with density functional theory computations in order to determine, whether α-(AlGa)₂O₃ can be stabilized by a distortion corresponding to a Ga concentration as low as 25 %.

Fig. 3: Biaxial stress as function of bulk modulus in isotropic approximation for different Ga concentrations.

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