Reduction of the energy gap pressure coefficient of GaN due to the constraining presence of the sapphire substrate

We have performed a detailed investigation of the photoluminescence pressure dependence of heteroepitaxial GaN thin films on sapphire substrates. A comparison between as grown GaN on sapphire and free-standing GaN membranes, created using a laser assisted substrate liftoff process, revealed that the presence of the sapphire substrate leads to an energy gap pressure coefficient reduction of approximately 5%. This result agrees with the numerical simulations presented in this article. We established that the linear pressure coefficient of free-standing GaN is 41.4 +/- 0.2 meV/GPa, and that the deformation potential of the energy gap is -9.36 +/- 0.04 eV. Our results also suggest a new, lower value of the pressure derivative for the bulk modulus of GaN (B' = 3.5). (C) 1999 American Institute of Physics. [S0021-8979(99)06803- 6].