## 2 Results
### Microstructure of the Annealed Ag[Gel/AIN NMLs
#### 2.2.2 Annealed Microstructure by SEM  
The XRD analysis indicates that the NML structure is affected by both the annealing temperature and atmosphere. SEM micrographs of the non-bias and bias samples after annealing at 400 \({}^{\circ}\)C in air are shown in Figure 5a,b, respectively. Corresponding SEM cross-sections of the air-annealed NMLs are presented in Figure 5c,d. The air-annealed bias sample manifests a buckling pattern (Figure 5b), which originates from the accumulation of Ag at the interface between the substrate and the NML (Figure 5d). The EDX (Energy Dispersive X-ray) inset in Figure 5b shows that the Ag signal is indeed  
Figure 4: XRD scans of samples annealed at different temperatures for bias and non-bias samples, annealed in Ar (**a**,**c**) and in air (**b**,**d**), respectively. The arrows in (**c**) indicate the satellite appeareance pointing out a smoother interface upon annealing.  
buried (when compared to the Ag EDX signal in Figure 6e). The delaminated part of the NML adopts a sinusoidal shape, similar to the telephone cords structures commonly observed in a vast variety of compressively-stressed thin-film systems [23]. The non-bias NMLs exhibit a strikingly different morphology after the air-annealing at 400 \({}^{\circ}\)C: i.e., the NML surface is decorated with an interconnected network of cracks (buckling is not observed; see Figure 5a). Occasionally, some Ag outflow along the cracks is observed. Figure 5c shows a cross-sectional Focus Ion Beam cut through such a surface crack with Ag outflow, which indicates that the crack extends down about two thirds through the NML stack. Notably, annealing in Ar up to 400 \({}^{\circ}\)C (results not shown here) did not noticeably affect the microstructure of the bias samples, whereas the non-bias samples showed similar surface cracking behavior (as observed upon air-annealing up to 400 \({}^{\circ}\)C, albeit with tentatively less pronounced Ag outflow along the surface cracks). The effect of a further increase of the annealing temperature up to 700 \({}^{\circ}\)C on the resulting NML microstructure is shown in Figure 6. For the non-bias samples, the network of surface cracks, as initiated at 400 \({}^{\circ}\)C, has extended (Figure 6a). Ag can migrate along these cracks and form Ag polycrystalline particles on the outer surface. Strikingly, the amount of outflow of Ag is much less for the annealing of the non-bias NML in Ar than in air (compare Figure 6a,c). The nanolaminated structure of the bias samples is completed destroyed after annealing at 700 \({}^{\circ}\)C. As indicated in the cross-section of Figure 6b), the destroyed NML structure of the Ar-annealed bias sample is largely depleted of Ag, which is mainly allocated on the surface.  
Figure 5: SEM planar images of samples annealed at 400 \({}^{\circ}\)C in air (**a**) without bias and (**b**) with bias; (**c**) cross section SEM of (**a**) through a crack with Ag outflow; (**d**) SEM cross section of (**b**) through a wrinkle. The inset in (**b**) is the elemental EDX analysis.  
Annealing at 700 \({}^{\circ}\)C in air causes massive Ag outflow to the surface (see Figure 6c,d). For non-bias samples, the Ag outflow mainly occurs along previously formed cracks (Figure 6c), whereas, for bias samples, Ag outflow occurs more homogeneously on the surface. Some Ge accumulation at the surface is also made evident at 700 \({}^{\circ}\)C in correspondence with the Ag particles (see EDX analysis in the inset in Figure 6e).  
Figure 6: SEM micrographs in planar (**top**) and cross-sectional view (**bottom**) of the Ag[Ge]/AlN NMLs, as deposited without bias, after annealing at 700 \({}^{\circ}\)C (**a**) in Ar and (**c**) in air. Corresponding SEM micrographs of the Ag[Ge]/AlN NMLs, as deposited with bias, after annealing at 700 \({}^{\circ}\)C (**b**) in Ar and (**d**) in air; (**e**) EDX mapping of (**d**) which shows that the surface particles are pure Ag and close to them there are Germanium-enriched surface regions (indicated by arrows).