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  Kneiß, M.; Splith, D.; von Wenckstern, H.; Lorenz, M.; Schultz, T.; Koch, N.; Grundmann, M.: Strain states and relaxation for α-(Al_xGa_1-x)₂O₃ thin films on prismatic planes of α-Al₂O₃ in the full composition range: Fundamental difference of a- and m-epitaxial planes in the manifestation of shear strain and lattice tilt. Journal of Materials Research 36 (2021), p. 4816–4831

10.1557/s43578-021-00375-3
Open Access Version

Abstract:
Pseudomorphic and relaxed Alpha-(AlxGa1-x)2O3 thin films are grown by combinatorial pulsed laser deposition in the entire composition range on prismatic a- and m-plane alpha-Al2O3 substrates. Pseudomorphic growth on m-plane sapphire has been achieved for x bigger equal 0.45. A distinct difference between the a- and m-epitaxial plane is observed in reciprocal space map measurements being in agreement with continuum elasticity theory for rhombohedral heterostructures. While pseudomorphic layers on m-plane sapphire show a pronounced shear strain e'5 along the c-axis direction, relaxed layers exhibit a global lattice tilt in the same direction. Both effects are not present on the a-epitaxial plane. Out-of-plane lattice constants as well as e'5 are modeled as function of x employing elasticity theory, confirming theoretical values of the elastic stiffness tensor for alpha-Ga2O3, especially the non-zero value of the C14 component. Possible pyramidal slip systems for strain relaxation in c-axis direction are examined to explain and numerically model the difference in lattice tilt for the two substrate orientations.