Observation of birefringence distribution caused by residual strain in bulk c-plane GaN substrates

Strain-induced birefringence has been observed in bulk c-plane GaN substrates. Two-dimensional distribution of index ellipse difference |Δn| revealed a variety of distribution such as gradual U-shape distribution with rotational symmetry and chord- and spot-like patterns with three- and/or six-fold symmetries reflecting the whole manufacturing process of substrates. Although the absolute value of the strain is not obtained yet, because of unknown photoelastic constants, the |Δn| map is useful for qualitative evaluation of strain distribution over the whole substrate.