Strain states and relaxation for α-(AlxGa1-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

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 >= 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 e5</mml:msubsup> 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 e5</mml:msubsup> are modeled as function of x employing elasticity theory, confirming theoretical values of the elastic stiffness tensor for alpha -Ga2O<mml:msub>3, especially the non-zero value of the <mml:msub>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.Graphic abstract<fig id="Figa" position="anchor"><graphic position="anchor" specific-use="HTML" mime-subtype="PNG" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="MediaObjects/43578_2021_375_Figa_HTML.png" id="MO100"></graphic