Modeling of Interface and Internal Disorder Applied to XRD Analysis of Ag-Based Nano-Multilayers

Abstract

Multilayered structures are a promising route to tailor electronic, magnetic, optical, and/or mechanical properties and durability of functional materials. Sputter deposition at room temperature, being an out-of-equilibrium process, introduces structural defects and confers to these nanosystems an intrinsic thermodynamical instability. As-deposited materials exhibit a large amount of internal atomic displacements within each constituent block as well as severe interface roughness between different layers. To access and characterize the internal multilayer disorder and its thermal evolution, X-ray diffraction investigation and analysis are performed systematically at differently grown Ag-Ge/aluminum nitride (AlN) multilayers (co-deposited, sequentially deposited with and without radio frequency (RF) bias) samples and after high-temperature annealing treatment. We report here on model calculations based on a kinematic formalism describing the displacement disorder both within the multilayer blocks and at the interfaces to reproduce the experimental X-ray diffraction intensities. Mixing and displacements at the interface are found to be considerably reduced after thermal treatment for co- and sequentially deposited Ag-Ge/AlN samples. The application of a RF bias during the deposition causes the highest interface mixing and introduces random intercalates in the AlN layers. X-ray analysis is contrasted to transmission electron microscopy pictures to validate the approach.

The Effect of Interfacial Ge and RF-Bias on the Microstructure and Stress Evolution upon Annealing of Ag/AlN Multilayers

“…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.…”

“…Intense modulated reflections (commonly referred as satellite peaks) which originate from the periodicity of NML structure [22] arise for the non-bias sample after annealing in Ar (indicated by arrows in panel a). The appearance of intense satellite modulations upon annealing have been correlated with a decrease of the interface roughness [15]. The respective pole figures also show sharper Ag(111) and AlN(0002) rings after Ar annealing (Figure 3c), confirming a higher degree of crystallinity of the non-bias samples after annealing in Ar at T > 200 • C. The change in the NML microstructure of the bias samples after annealing in Ar is different: only very weak satellites appear at 400 • C, which again vanish at 700 • C, while, at the same time, new Ag reflections appear ( Figure 4b).…”

“…Furthermore, the increase of ohmic losses in Ag/Ge/fused silica sandwiches was observed due to Ge segregation towards the Ag film surface and probably also the grain boundaries [14]. The effect of co-alloying of Ag nanolayers with Ge in Ag/AlN NMLs on the interfacial roughness before and after thermal treatment was investigated by XRD combined with TEM analysis [15]. The addition of Ge to the nano-confined Ag layers was found to induce structural disorder at the Ag[Ge]/AlN interfaces; subsequent annealing led to an improved interface coherency.…”

Inhibiting the Segregation of Germanium in Silver Nanolayers

“…However, in the nanoscale, poor adhesion of silver to commonly used substrates such as ultra-smooth fused silica or sapphire results in a rough metal-free surface, which increases the scattering losses of the metal film. To tackle this problem, a number of approaches have been proposed, the most prominent being the use of wetting materials exhibiting good adhesion to both silver and the substrate [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. The greatest reduction of surface roughness-up to an order of magnitude-is observed when the substrate is wetted with a~2-nm-thick layer of germanium prior to the deposition of silver [2,11].…”

“…Instead of covering the silver layer with a material which exhibits grain boundary diffusion within silver, we introduce another layer below germanium, which is made of gold. Germanium has been proven to segregate within the nanolayers of both silver and gold [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], as Ag and Au exhibit many crucial similarities, like their atomic radius and lattice constant. However, the free enthalpy of the segregation process is influenced by many quantities in which silver and gold differ-e.g., bulk modulus, shear modulus and surface energies [28,42].…”