Anisotropic Fracture Toughness of Bulk GaN

The wurtzite structure of GaN determines its anisotropic mechanical properties, which is significant in the processes of material preparation and application. By using nanoindentation method, the fracture toughnesses along the c ‐, a ‐, and m ‐planes of GaN were measured. Experimental results reveal that the fracture toughnesses along nonpolar m ‐ and a ‐planes are similar, 1.55 ± 0.13 MPa · m 1/2 and 1.59 ± 0.12 MPa · m 1/2 , respectively, but obviously smaller than the value of c ‐plane, about 2.12 ± 0.21 MPa · m 1/2 . This could be understood by the differences of GaN bond densities and energies among the corresponding planes. Using first‐principles calculations, the fracture toughness of GaN under different stress states was investigated within the framework of density functional theory. The anisotropic property that the fracture toughness for c ‐plane GaN is higher than that of a ‐ or m ‐plane GaN for all cases of stress states, should be the fundamental reason for the great difficulty in complete separation of thick GaN films, which is the key process in manufacturing GaN substrates.