Growth model and structure evolution of Ag layers deposited on Ge films

Abstract

We investigated the crystallinity and optical parameters of silver layers of 10–35 nm thickness as a function 2–10 nm thick Ge wetting films deposited on SiO2 substrates. X-ray reflectometry (XRR) and X-ray diffraction (XRD) measurements proved that segregation of germanium into the surface of the silver film is a result of the gradient growth of silver crystals. The free energy of Ge atoms is reduced by their migration from boundaries of larger grains at the Ag/SiO2 interface to boundaries of smaller grains near the Ag surface. Annealing at different temperatures and various durations allowed for a controlled distribution of crystal dimensions, thus influencing the segregation rate. Furthermore, using ellipsometric and optical transmission measurements we determined the time-dependent evolution of the film structure. If stored under ambient conditions for the first week after deposition, the changes in the transmission spectra are smaller than the measurement accuracy. Over the course of the following three weeks, the segregation-induced effects result in considerably modified transmission spectra. Two months after deposition, the slope of the silver layer density profile derived from the XRR spectra was found to be inverted due to the completed segregation process, and the optical transmission spectra increased uniformly due to the roughened surfaces, corrosion of silver and ongoing recrystallization. The Raman spectra of the Ge wetted Ag films were measured immediately after deposition and ten days later and demonstrated that the Ge atoms at the Ag grain boundaries form clusters of a few atoms where the Ge–Ge bonds are still present.

Inhibiting the Segregation of Germanium in Silver Nanolayers

“…Figure 1a presents survey XPS spectra collected from the very surface of sandwich samples, where a 2-nm-thick layer of germanium is deposited between 20-nm-thick layers of silver and gold. The XPS spectra were collected 30 days after the fabrication of the samples, to allow germanium to fully segregate towards the surface [37]. For comparison, the spectra for sandwich-like structures where the germanium is deposited between two layers of the same metal, measured 10 days after sample fabrication, from works [38,39], are also presented.…”

“…It has previously been demonstrated that Ge atoms segregate to the surface of Ag and Au layers deposited on germanium wetting films, even in case of metal/Ge/metal sandwich structures [37,38,40]. Such behavior is attributed to non-uniform metal grain size distribution within the depth of the film, as evidenced by X-Ray Reflectometry measurements [37,40].…”

“…Physical phenomena at the interfaces of sandwich−like structures composed of an insulating substrate, elemental semiconductor and noble metal have been experimentally studied and considered in thermodynamic models for a few decades [28][29][30][31][32][33][34][35][36][37][38][39][40][41]. Surface segregation and grain boundary segregation of foreign atoms in a solid solvent considerably influence its dielectric functions and are of great importance in metallurgy and materials science in general [42][43][44].…”

“…It has previously been demonstrated that Ge atoms segregate to the surface of Ag and Au layers deposited on germanium wetting films, even in case of metal/Ge/metal sandwich structures [37,38,40]. Such behavior is attributed to non-uniform metal grain size distribution within the depth of the film, as evidenced by X-Ray Reflectometry measurements [37,40]. Ge atoms present in the sub-surface metal grain boundaries separate the metal grains electrically, increasing their ohmic losses [18,19,40] and allowing for localized surface plasmon excitation [7,37,38,40,41].…”

“…Such behavior is attributed to non-uniform metal grain size distribution within the depth of the film, as evidenced by X-Ray Reflectometry measurements [37,40]. Ge atoms present in the sub-surface metal grain boundaries separate the metal grains electrically, increasing their ohmic losses [18,19,40] and allowing for localized surface plasmon excitation [7,37,38,40,41]. Excitation of surface plasmons on Ge-covered metal grains contributes to lower reflectivity in the visible range [7].…”