Thermally activated pop-in and indentation size effects in GaN films

Indentation tests on epitaxial GaN thin films, grown by metalorganic chemical vapour deposition on Si (1 1 1) substrate, were conducted at temperatures of 23, 100 and 180 degrees C. The first pop-in events were statistically analysed with Schuh's model and resulted in thermal activation energy of 850 +/- 36 meV, activation volume of 10.8 +/- 1.6 angstrom(3) and frequency factor of 4.23 x 10(21) m(-3) s(-1). In addition, the experimental results show that the critical force at the first pop-in event and the measured shear strength depend on the indenter tip radius.