Ge–Si–O phase separation and Ge nanocrystal growth in Ge:SiO_x/SiO₂multilayers—a new dc magnetron approach

Abstract

Ge:SiO(x)/SiO(2) multilayers are fabricated using a new reactive dc magnetron sputtering approach. The influence of the multilayer stoichiometry on the ternary Ge-Si-O phase separation and the subsequent size-controlled Ge nanocrystal formation is explored by means of x-ray absorption spectroscopy, x-ray diffraction, electron microscopy and Raman spectroscopy. The ternary system Ge-Si-O reveals complete Ge-O phase separation at 400 °C which does not differ significantly to the binary Ge-O system. Ge nanocrystals of < 5 nm size are generated after subsequent annealing below 700 °C. It is shown that Ge oxides contained in the as-deposited multilayers are reduced by a surrounding unsaturated silica matrix. A stoichiometric regime was found where almost no GeO(2) is present after annealing. Thus, the Ge nanocrystals become completely embedded in a stoichiometric silica matrix favouring the use for photovoltaic applications.

A Review on Ge Nanocrystals Embedded in SiO2 and High‐k Dielectrics

“…[89] Also superlattice structures were used to synthesize size and position controlled Ge nanocrystals. [26,32,52,66,90,91] However, since SiO 2 is not a sufficient diffusion barrier for the Ge, the synthesis of controlled Ge nanocrystals is challenging. [52,92] The stability against the Ge diffusion can be increased by using SiO 2 þ GeO 2 separation layers.…”

“…Also superlattice structures were used to synthesize size and position controlled Ge nanocrystals . However, since SiO 2 is not a sufficient diffusion barrier for the Ge, the synthesis of controlled Ge nanocrystals is challenging .…”

Ge Quantum Dots Coated with Metal Shells (Al, Ta, and Ti) Embedded in Alumina Thin Films for Solar Energy Conversion

“…The samples richer in pure Ge will clearly show the presence of the Ge(0) peak, while the samples with mixed contributions of pure and oxide phases will be shown as superposition of the two reference spectra. A detailed discussion on the XANES analysis of Ge oxides in a similar kind of material can be found in ref. − …”

Structural changes in amorphous GexSiOyon the way to nanocrystal formation

“…Because the difference in intensity between the shoulder structure and the maximum of the spectrum is small, the occurrence of predominantly Ge 0 with only a small amount of Ge 4+ configuration can be assumed. Oxygen bonds preferentially to silicon due to a lower configuration energy ( H SiO 2 = −902 kcal mol −1 ; H GeO 2 = −538 kcal mol −1 ) [38]. Because the shape of the spectrum cannot be explained by a simple superposition of Ge (or SiGe) and GeO 2 reference spectra, a contribution of sub-oxide configurations can also be assumed for sample A.…”

“…using chemical vapor deposition [26,27], the sol-gel method [28][29][30], rf [31][32][33][34] or dc magnetron sputtering [35][36][37]. Here, the NC formation is strongly correlated to Ge-Si-O phase separation via GeO x reduction and SiO x oxidation during temperature treatment [38]. The temperature induced phase separation in the ternary system Ge-Si-O via reduction of Ge sub-oxides by the surrounding Si oxide matrix was studied recently for Ge:SiO x /SiO 2 multilayer systems [39].…”