||Wide band gap semiconductors ZnO/GaN attracted a great deal of interests for decade, due to their wide direct band, high electron binding energy, excellent chemical and thermal stability, good heat conductivity and capability, high electron mobility and transparent properties at room temperature. They have many potential applications such as laser, biosensor, piezoelectric power generator, nano-electromechanical systems and flat panel field emission displays. However, unexpected contact loading during processing or packaging may induce residual stresses and/or an increase in defect concentration in ZnO/GaN wafer or thin film, causing possible degenerated reliability and efficient operation of the piezoelectric and photonic device. To ensure and improve the performance of devices based on ZnO/GaN, a better understanding of the mechanical/optoelectronic response under different processing and loading conditions and even the measuring methods are necessary. |
In this thesis, our aim is to reveal a comprehensive investigation of the mechanical responses on polar/non-polar GaN/ZnO single crystal under low dimensional stress. We try to provide the fundamental theoretical and experimental studies for further application and researches, such as tension testing, residual stress, low temperature cathodoluminescence and Raman spectroscopy analysis.
In this study, the theoretical Young’s modulus and Poisson ratio of ZnO/GaN are extracted from elastic constants for comparison and further estimation. The nano-scaled mechanical properties, such as Young’s modulus, hardness and yield stress, are identified by using the nanoindentation system. The experimental values were fitting by the Hertzian contact theory. The results are in good agreement with the theoretical predictions. No significant strain rate influence is observed over the strain rate from 1x10-2 s-1 to 1x10-4 s-1. The comparisons of mechanical properties between the polar and non-polar planes of ZnO are firstly examined. The results reveal that the non-polar planes are softer than the polar plane. Both a-plane and m-plane ZnO have lower hardness and yield stress than c-plane ZnO. The microstructure and deformation mechanism are analyzed by using X-TEM and SEM. No pop-out or slope changing was found in their load-displacement curves, suggesting no phase transformation, twining or crack domain deformation occurred under microcompression and nanoindentation testing. Taking all considerations for the higher resulting Schmid factor and lower Burgers’ vector, the most possible slip system for c-plane hexagonal structures is the pyramidal plane. The a-plane has shorter burger’s vector on the slip plane which leads the lower yield stress than c-plane.
To erase the effect of FIB induced Ga ion implantation, the c-plane ZnO was annealed at 900oC for 1 hour. We found that the yield stress under microcompression decreases and the intensity of the cathodoluminescence spectrum increases after the annealing process. This result indicates that the thermal treatment is a good way to refine the crystal quality and decrease the defects density. The E2 peak of Raman spectrometer exhibits high residual compression stress constrain in the c-plane GaN thin film. Due to the high surface/volume ratio of pillar, nil residual stress remains in the GaN pillar after the FIB milling process. Even after the yield point, nil residual stress remains in the c-GaN pillar. Results indicate that the one dimensional geography is a good way to erase residual stress.