Pulse time (s) The Youngs's modulus and Poisson's ratio of GaN have been determined from previous studies to be 325.3 GPa and 0.25, respectively. This gives a tensile stress of about 1.95 GPa for the GaN thin film on a Si substrate. Tensile strain and stress in the GaN film is expected due to the large difference in lattice constant and crystal structure of cubic Si to hexagonal GaN.