The elastic properties of GaN were evaluated by Schwarz et al.,1 Yu et al.2 and finally confirmed by Nowak's et al.3 nanoindentation testing that agreed with the earlier first principles calculations.4-5 Our previous results run contrary a common expectation that GaN is particularly brittle material.3 Despite its high Young's modulus (295 GPa) and hardness (20 GPa), the nanoindentation stress-strain curves suggest elasto-plastic behaviour of the crystal. Consequently, the present research aims to resolve the surface deformation of GaN by using both microscopic observation of the surface features and structure, as well as finite element simulation of the nanoindentation deformation.
The HRTEM of the GaN structure revealed perfect atom arrangement in the crystals obtained by the self-seeding process under the pressure of 15x108 Pa and supplied by High Pressure Research Centre. The AFM examination of residual impressions, led to registration of the profile of the deformed surface that was compared with the FEM-simulated surface pile up around indenter. This resulted in FEM-based correction of the yield point of GaN. The lack of indentation cracks and the pile-up served as a qualitative experimental proof of low-brittleness of GaN - predicted by numerical simulation.
The FEM-simulation of nanoindentation deformation of Al2O3 crystal was a proper comparison for a less-examined GaN and proved effectiveness of the analysis.6
1. R. Schwarz et al., Appl. Phys. Lett. 70, 1122 (1997)
2. G. Yu et al., J. Cryst. Growth 189, 701 (1998)
3. R. Nowak, M. Pessa, M. Suganuma, M. Leszczynski, I. Grzegory, S. Porowski and F. Yoshida, Appl. Phys. Lett. 75, 2070 (1999)
4. V. Fiorentini et al., Phys. Rev. B 47, 13353 (1993)
5. K. Shimada et al., Jap. J. Appl. Phys, 37, L1421 (1998).
6. R. Nowak, T. Manninen, K. Heiskanen, T. Sekino, A. Hikasa, K. Niihara and T. Takagi, Appl. Phys. Lett. 83, 5214 (2003) |