Layer transfer of bulk gallium nitride by controlled spalling

Successful wafer-scale layer transfer from high-quality 2-in. diameter bulk gallium nitride substrates was demonstrated using the Controlled Spalling technique. The crystal quality of both the as-fractured bulk substrate and the spalled GaN film was assessed using transmission electron microscopy analysis, and the defect density was below the detection limit (mid 107 cm-2) for both samples. By using the experimentally determined critical conditions for tensile stress and thickness of the Ni stressor layer, an effective fracture toughness KIC of 1.7 MPa m could be calculated for [0001] fracture using the Suo and Hutchinson mechanical model. The resulting in-plane contraction of the GaN film after spalling permitted a novel method for measuring film strain without knowledge of the elastic properties of the material. This was used to measure the Raman E2(high) peak shift coefficient of Δω(cm-1) = 1411ϵ which, when converted to a stress coefficient (2.95 cm-1/GPa), was in agreement with only one other literature value.