Interfacial Strain and Inversion Domains in Coherent AlBScN/GaN Epitaxial Interfaces

Nitride ferroelectrics are among the emerging materials with promising potential for a wide range of electronic, optoelectronic, and memory applications because of their high remanent polarization, low growth temperature, and complementary metal-oxide-semiconductor (CMOS) compatibility. Growth of wurtzite-structured nitride ferroelectrics on bottom electrodes or Si substrates has resulted in defective and amorphous interfaces. Epitaxial growth on wide bandgap nitride substrates, such as gallium nitride (GaN), is preferred to achieve sharp interfaces and improved integration in semiconductor devices. However, the formation of inversion domains (IDs) on these substrates degrades the electrical properties. Here, an epitaxial aluminum boron scandium nitride (Al0.78(BSc)0.22N) ferroelectric thin film grown on an n-type GaN substrate is characterized using scanning transmission electron microscopy (STEM) to determine the formation mechanism of IDs. Integrated differential phase contrast (iDPC) imaging directly exhibits the Al-N dumbbell orientation across an inversion domain boundary (IDB) and orientation discontinuity close to the Al0.78(BSc)0.22N/GaN interface. Four-dimensional scanning transmission electron microscopy (4D-STEM) strain mapping reveals the role of substrate-induced pinning in the formation of the IDs. This demonstrates local shear strain variations along IDBs. This study highlights the importance of STEM-based techniques in directly correlating the local lattice variations with the polarization and switching behavior of this class of wurtzite ferroelectric films.