Shock-induced brittle cracking in HVPE-GaN processed by laser lift-off techniques

A study on brittle cracking in GaN films processed by laser lift-off is presented. Two kinds of cracks were found in the N-polar face of GaN after the laser lift-off process, namely perpendicular cracks along the {1 −1 0 0} planes and lateral cracks along the (0 0 0 −1) plane, respectively. Single-shot laser damage is studied to understand the cracking mechanism. The damage morphology indicates that the GaN material on the edge of the laser ablation area experiences three loading modes: shear stress PS, longitudinal compressive stress PL and transverse tensile stress PT. Under shock PL, lateral cracks likely appear and extend from the illuminated region along the interface in mode I. Furthermore, two different kinds of perpendicular cracks were found, namely shear cracks (PC I) and deflection cracks (PC II). A strong PS gives rise to PC I while a cooperative action of PL and PT results in PC II. In addition, there exist a critical effective spot size dPth and a critical ratio of the laser spot size dL to the effective spot size dP, when cracks occur over them.