An Accurate Method for Measuring Activation Energy
  In this letter, we present an accurate method for the measurement of activation energy. This method combined the excitation power dependent photoluminescence and temperature dependent photoluminescence together to obtain activation energy. We found with increasing temperature, there is a step transition from one emission mechanism to another. This step transition gives us an accurate measurement of activation energy. Using this new method we found the activation energy of the free exciton A in a GaN thin film is 24 meV. Our result also gives a reasonable explanation of the debate of the origin of the light emission in GaN at room temperature. 
Xiang-Bai Chen, Jesse Huso, John L. Morrison, and Leah Bergman