Experimental determination of critical thickness limitations of (010) -(Al_xGa_1-x)₂O₃ heteroepitaxial films

The effect of heteroepitaxial beta-(AlxGa(1-x))(2)O-3 film thickness and Al content on surface morphology was characterized to experimentally determine the critical thickness limitations of the (010) beta-(AlxGa1-x)(2)O-3/Ga2O3 heterostructure. High-resolution x-ray diffraction was used to assess the strain state of the films; reciprocal space mapping (RSM) revealed that even cracked films were still fully strained. In cracked films, diffuse scattering was observed in RSMs, indicating lattice tilting. Cracking of the films was investigated using atomic force microscopy (AFM), x-ray topography (XRT), bright-field scanning transmission electron microscopy (BF-STEM), and high-resolution transmission electron microscopy. Using both AFM and XRT, the [001] direction was observed to be the most prevalent crack direction; however, cracks were also observed in the [100] direction. In uncracked regions of the films, XRT revealed the alignment of threading dislocations along the [001] direction. Cross-sectional imaging of the crack geometry and propagation was performed using BF-STEM, and it was observed that the cracks in the [001] direction extended through the thickness of the beta-(AlxGa1-x)(2)O-3 film (similar to 205 nm) and a further similar to 100-200 nm into the beta-Ga2O3 substrate. Experimental data for critical film thickness showed good agreement with previous theoretical calculations based on the Griffith criterion for crack propagation.