Nanoindentation, microcompression, X-ray diffraction, EBSD, Raman spectroscopy and cathodoluminescence spectroscopy tests are conducted on wuirtzite semiconductors. Based on the results, the following conclusions are drawn:
1. The quality of GaN thin films and ZnO wafers are identified by X-ray diffraction and Raman spectrometer. The narrow FWHM of rocking curve and E2 peak of Raman spectrometer implies the fine quality of samples.
2. The theoretical values of GaN epilayers and ZnO wafers are calculated in this study. For ZnO, the theoretical Poisson ratio are th, c-axis = 0.33 and th, a-axis = 0.25, the theoretical Young’s modulus are Eth, c-plane and Eth, a-plane are 144 and 148 GPa respectively. The theoretical shear modulus and stress are Gth = 40 and th = 4 GPa. For GaN, the theoretical Poisson ratio are th, c-axis = 0.25 and th, a-axis = 0.4, the theoretical Young’s modulus are Eth, c-plane and Eth, a-plane are 281 and 341 GPa respectively. The theoretical shear modulus and stress are Gth = 99 and th = 10 GPa. The estimated values are coincided with the literature value and experimental results, but the theoretical Poisson ratio of c-plane GaN (0.25) is different with the literature value (0.35). In this study, we choice 0.35 as the Poisson ratio of c-plane GaN thin film. The experimental values of ZnO/GaN coincide to the literature values. Table 13 and Table 15 list the comparison of theoretical and experimental results.
3. Taking all considerations for the higher resulting Schmid factor and lower Burgers’ vector. The most possible slip system for c-plane hexagonal structure under [0001] stress should be
(101 1)
[1 011], and the Schmid factor is calculated to be 0.41.
4. Polar plane c-ZnO wafers are tested by nanoindentation system with CSM mode at strain rate from 1x10-2 s-1 to 1x10-4 s-1. No significant mechanical properties changing are found within this range. The E of as-grown and annealed ZnO is Eas-grown = 140 ± 7 GPa and Eannealed = 140 ± 9 GPa. To address the near surface analysis, the c-plane ZnO wafers are tested with LRC mode by nanoindentation system. The mean pop-in depth hcrit for as-grown and as-annealed c-plane ZnO was both around 20 nm, the corresponding maximum normal stress and a maximum shear stress are max = 12 ± 1.0 GPa and max = 3.6 ± 0.3 GPa. The maximum shear stress is equivalent to /10. The experimental max is close to the theoretical shear stress level. Annealing process did not affect the young’s modulus and maximum shear stress. This is expected since the near surface region can be seen as defect free zone for high quality single crystal. The Young’s modulus and theoretical shear stress is an intrinsic property which will not be affected
5. For nanoindentation testing, the H of as-grown and annealed c-ZnO is Hannealed = 6.3 ± 0.3 GPa and Has-grown = 7.1 ± 0.4 GPa. For microcompression testing, the normal yield stress ys of as-grown and annealed c-ZnO micropillar are 3 ± 0.5 GPa and 2 ± 0.2 GPa respectively. Both results reveal the annealed c-ZnO wafer appeared to be softer. Since the dislocation densities in both the as-grown and annealed samples are low, the reduction is probably not caused by
dislocation-related mechanisms. From the fact that the CL signal/noise ratio was enhanced by annealing, the decrease in yield strength might also be attributable to the reduction of defect density after thermal treatment.
6. 1 m GaN and ZnO micropillars are fabricated successfully from the GaN thin film and ZnO wafer by using the focus ion beam. Samples are measured by microcompression testing at the strain rate range from 1x10-2 s-1 to 1x10-4 s-1. No significant influence from strain rate in this range is observed. The E of GaN and ZnO pillar are 226 ± 17 GPa and 123 ± 17 GPa respectively. Compared with the evaluated yield stress value from nanoindentation data, the higher value of elastic modulus and yield stress is due to the small volume constrain effect. The semiconductors appeared to have higher mechanical properties in the nanoscale than microscale measurements.
7. The E2 peak of Raman spectrometer exhibits high residual compression stress (~1.5 GPa) constrain in the c-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 yielding point, nil residual stress remains in the c-GaN pillar. No residual stress is detected in ZnO wafer.
8. The Raman spectra indicate a slightly distortion after microcompression testing.
The A1(LO) and E1(LO) peaks combined into QLO peak which locate between them.
Due to the back scattering incident laser beam deviate from the ideal [0001]
direction. The XTEM result implies the same results as Raman spectrometer. The massive dislocations leave the trace on pyramidal plane and are seen on bright field images. There is few degree of slightly distortion on the top of the pillar.
9. The FIB induced Ga ion implanted can be recovered by thermal treatment. The direct implantation reduced the intensity of CL spectrum. After annealing at 900oC for 1 hour, the S/N ratio of the BL peak increased ~1.5 times. Results indicate the refinement of crystal quality and decreasing of defects density after the thermal treatment, consistent with previous results.
10. The XCL specimen is fabricated successfully from the c-ZnO wafer by using the focus ion beam. The planar and cross section CL analysis of micropillar reveal the BL peak red shift after FIB milling process.