## 3 Discussion  
The Ag[Ge]/AlN NMLs, as deposited without bias, have more uniform nanolayer thicknesses, a lower interface roughness and also possess less structural disorder than the ones deposited with bias (see Figures 2 and 3 and Ref. [15] where the interface roughness was quantitatively derived by XRD analysis for both samples: bias sample having an interface displacement disorder 60 times higher than non-bias sample). The thickness uniformity and interface roughness of the non-bias Ag[Ge]/AlN NMLs is also better as compared to the Ag/AlN NML without Ge in Ref. [8]. Step-like corrugation and non-laterally closed Ag layers observed in Ref. [8] are not observed in the presence of Ge where the layers appear smoother and more laterally homogeneus. This can be attributed to the improved wetting of Ag on the Ge-covered AlN surface (as compared to the clean AlN surface) [24], which results in faster impingement and coalescence of nucleated Ag islands into a continuous Ag layer during each successive deposition step. However, the presence of Ge at the Ag/AlN interface hinders the formation of an (semi-)coherent interface between Ag and AlN, as found for Ag/AlN NMLs [8]. This can be due to the amorphous nature of the deposited Ge layer, which hinders the formation of a coherent interface between Ag and AlN (i.e., AlN imposes its hexagonal structure on Ag) [8]. Another possible scenario is that interfacial Ge atoms (re)segregate to the Ag surface during the Ag deposition step, thereby reducing the surface mobility of Ag, which leads to the random nucleation of new Ag grains [11]. Indeed, the Ag nanograins in the Ag[Ge]/AlN NMLs possess a more-or-less random in-plane orientation, which results in a fiber-texture in the measured Ag(111) pole (note: no distinct in-plane texture is found for the non-bias samples: see Figure 3a,b). The different thermal stabilities and mass transport behaviors of the bias and non-bias NMLs as function of the temperature and the gas atmosphere can be related to their different microstructures and intrinsic stress states in the as-deposited state, as described below. The Effect of the Initial Stress State and Interfacial Ge on the Mechanism of Stress Relaxation during Annealing  
The growth (or so-called) intrinsic residual stresses in the as-deposited Ag[Ge]/AlN NMLs, as generated during the deposition process (with or without bias), will be superimposed by thermal (so-called extrinsic) stress contributions upon heating (due to the different thermal expansion coefficients between Ag, AlN and Al\({}_{2}\)O\({}_{3}\)). The average residual stresses in the confined Ag nanolayers before and after annealing (at 400 \({}^{\circ}\)C and 700 \({}^{\circ}\)C) were determined by XRD (after cooling down to room temperature; see Methods for details on the applied method for residual stress analysis): see Figure 7. It follows that the as-deposited bias and non-bias NMLs exhibit a strikingly different intrinsic stress state. The Ag nanolayers deposited with bias have an average compressive stress of \(-\)120 MPa, whereas the non-bias samples possess a very high tensile stress of about 500 MPa at room temperature (see Figure 7). Notably, Ag/AlN multilayers (i.e., without interfacial Ge and without bias) generally exhibit a compressive intrinsic stress state [8]. Evidently, the presence of Ge at the Ag/AlN interface affects the stress evolution during Ag thin-film growth differently for the bias and non-bias samples. It has been reported that the addition of a small amount of Ge during the Ag deposition process strongly retards the surface diffusivity of the Ag adatoms, which promotes the continuous nucleation and coalescence of randomly-oriented Ag grains, resulting in a rather texture-less microstructure with fine equiaxed nanograins [11]. Consequently, the compressive-tensile-compressive stress evolution, which is typical for Volmer-Weber type of film growth, shifts to lower thicknesses [25].