Microcompression testing

indentations made on each specimen, the mean pop-in depth h_crit for the non-polar m-plane and a-plane ZnO wafer are around 33.0 and 21.5 nm. The corresponding maximum normal stress y of the m-plane and a-plane ZnO wafers are 15.1 and 12.5 GPa, respectively.

4.2.2. GaN

The basic data on the GaN thin films were also extracted from nanoindentation with the Berkovich tip under the XP CSM constant displacement rate mode. The displacement rates were set up to be 0.005 ~ 1 nm/s. No notable strain rate effect of hardness and modulus is found within this range. For c-plane GaN, the displacement rate is set to be 1 nm/s. Data are collected within the steady region (200 - 400 nm) to avoid substrate effect (film thickness ~ 5 m). The goal depth is set up to be 1000 nm.

The Poison ratio is set up to be 0.35. The average E and H of c-plane GaN are 272 ± 4 GPa and 15 ± 0.1 GPa, respectively. Figure 56 shows the load-displacement curve of c-GaN with the CSM constant displacement rate. The first pop-in is observed at depth

~30.8 ± 3.5 nm (Figure 57). The corresponding maximum normal stress and a maximum shear stress are max = 29.8 GPa and max = 12.2 GPa.

The aspect ratio is about 2. The constant strain rate of this microcompression testing is set to be 2 nm/s, using the MTS nanoindentation system. The XP CSM H and E constant displacement rate mode are applied with the flat Berkovich with a triangle flat surface 14 m on its edge. The Figure 58 represents the SEM images of the 1 m as-grown pillar preset to 300 nm, the annealed pillar preset to 300 nm, as well as their associated stress-strain curves of microcompression testing.

The Young’s modulus and flow stress of the tapered micropillars are corrected by a method suggested previously. To follow the mechanical response of the pillars, tests were interrupted at displacements of 300 nm, representing the fully plastic conditions.

Since the micropillars are tapered at a small angle (specifically, 1.4^o and 1.7^o in the as-grown and annealed pillars, respectively). Taking h₀ = 2 m d0 = 1 m and  = 0.0262 under consideration, the elastic modulus formula can be roughly corrected as:



 



 

 ^

h E_pillar P



3  10 9 .

2 , (4-21)

where (P/h) is the unloading part slope of load-displacement curve. Consequently, the corrected moduli of the as-grown and annealed micropillars are computed to be 123 ± 17 and 120 ± 15 GPa, respectively.

Particularly noted in Figure 58 is that there is an apparent strain burst occurring at load P. Taking pillar diameter = 1 m under consideration, the corresponding normal yield stress of microcompression testing in this study is:

ys 1.27P

 . (4-22)

The yield stress of as-grown c-plane ZnO pillar is 3 ± 0.5 GPa. With the calculated Schmidt factor of 0.41, the corresponding shear stress of pillar (p) for the first strain burst in the deformed micropillar machined from the as-grown wafer to be

~1.2 ± 0.2 GPa (p = 0.41ys). In parallel, the microcompression on the annealed sample shows ys and p of 2 ± 0.2 GPa and 0.8 ± 0.1 GPa, respectively.

The normal yield stress ys of a-plane and m-plane ZnO wafer are 0.8 ± 0.04 GPa and 0.5 ± 0.02 GPa respectively. Comparing to c-plane, a-plane and m-plane have lower yield stresses, suggesting that the slip systems of both planes have lower Burgers’ vector ([1010] or ^[2110]) than c-plane



[1 011].

4.3.2. GaN

The GaN micropillars were milled by FIB system. Cu surface is coated on GaN by the Psur-100HB multi-target sputter with the 500 nm thickness for the indent tip alignment. The height and diameter of pillars are about 2 and 1 m respectively. The aspect ratio is about 2. The constant strain rate is set to be 2 nm/s. The microcompression testing on GaN pillar are examined by MTS nanoindentation system. The XP CSM H and E constant displacement rate mode are also applied with the 14 m flat Berkovich. Figure 59 represents the SEM images of the 1 m as-FIB-milled pillar and the first strain burst micropillar, along with the load-displacement curves of microcompression. Figure 59 depicts the load-displacement and transformed engineering stress-strain curves of the compressed 1 m micropillars at a strain rate of ~2 nm/s. There is an apparent strain burst occurring at 7.4 ± 1 mN in load. By using previous results, when the taper angle is small and

sin, the extracted elastic modulus E of 1 m micropillars is ~226 ± 17 GPa. And the engineering yield stress for the first strain burst is calculated to be ~10 ± 1 GPa.

Parallel compression tests were also conducted at 2~0.02 nm/s, there seems no significant influence from the strain rate over this range. Figure 60 and Figure 61 show the load displacement curve of non-polar plane a-plane and m-plane, respectively.

4.3.3. Raman spectrometer analyses

Figure 62 exhibits the back scattering Raman spectrum of the c-plane ZnO wafer.

The shifting of E2 peak can be indexed for characterizing the residual stress. Table 16 lists the main Raman spectra peaks position and FWHM of c-ZnO wafer. It is corresponding with the literature results (Table 17). No A1(LO) peak is shown in ZnO film Raman spectrum. The appearance of A_1(LO) peak indicates the native point defects increase after FIB milling. Figure 63 shows the Raman spectra of the same ZnO film under four FIB processing condition. The spectra exhibit the increasing intensity of A1(LO) peak, suggesting that the increasing of A1(LO) peak intensity is caused by directly Ga ion implanted as well. Raman results show the E₂ peak center position of the as-grown pillar (438.6 cm^-1, Figure 64), preset 40 nm pillar (438.0 cm^-1, Figure 65) and the deformed pillar to a preset 300 nm displacement (438.2 cm^-1, Figure 66), essentially very similar to that of the free-standing ZnO wafer (438.6 cm^-1) with nil residual stress. This suggests that no apparent residual stress remains in the as-grown and as annealed wafers as well as the as-FIB-machined and deformed pillars.

The quality of GaN and ZnO single crystal can be identified by Raman spectrometer. For GaN, Figure 67 reveals the E2 peak of GaN film located at 570.6 cm^-1 with FWHM ~2 cm^-1. It is 3.6 cm^-1 blue shifting compared with the literature

value of free standing GaN (568.0 cm^-1). After FIB milling, the E₂ peak of as-milled GaN pillar shifts to 567.6 cm^-1. It implies that most of the biaxial residual stress is released by the surface due to the high surface/volume ratio. The E₂ peak of preset 100 nm pillar (before strain burst) and preset 200 nm pillar (after strain burst) are 567.5 cm^-1 and 567 cm^-1 which is very similar to as-grown micropillars (567.6 cm^-1) or free standing GaN. Table 18 reveals the measurement of Raman E2 peak position, FWHM and estimating residual stress of GaN films and micropillars.

The FWHM of E₂ peak and intensity of A_1(LO) reveal the quality of GaN film and micropillars. The narrow FWHM indicates the perfect crystalline quality, and the broadening implies the accumulated of crystal defects. As for the as-grown GaN film and as-milled GaN pillars, the FWHM is around 2.0 to 2.5 cm^-1, reflecting the low defects content in these two cases. After microcompression testing, the FWHM increases significantly to 5.7 cm^-1 and 6.2 cm^-1. It means that even though GaN pillar has high surface/volume ratio to release most of the residual stress, the large defects accumulated still after compression even before the strain burst. In addition, the quasi-longitudinal optic phonon peak of pillars (QLO) at 732 cm^-1 is seen to be the consequence of combination of A1(LO) (727 cm^-1) and E1(LO) (735 cm^-1) of c-plane GaN film. Due to the distortion of crystalline structure, the incident laser beam slightly deviates from the [0001] back scattering direction. The degradation of crystal quality and distortion of structure observations are consistent with the results of XTEM results.