Reactive DC Magnetron Sputtering of (GeOx-SiO2) Superlattices for Ge Nanocrystal Formation

The underlying motivation of this work is the tailored growth of Ge nanocrystals (NC) for photovoltaic applications. The use of superlattices (SL) delivers a reliable method to control the Ge NC size after phase separation. In this contribution we report the deposition of (GeOx-SiO2) SL via reactive dc magnetron sputtering and the self-ordered Ge NC formation during subsequent annealing. Main attention is directed to define proper deposition conditions for tuning the GeOx composition between elemental Ge (x = 0) and GeO2 (x = 2) by the variation of the deposition temperature and the oxygen partial pressure. A convenient process window has been found which allows sequential GeOx-SiO2 deposition without changing the oxygen partial pressure during deposition. The phase separation and Ge NC formation after subsequent annealing were investigated with in−situ X-ray scattering, Raman spectroscopy and electron microscopy. By these methods the existence of 2-5 nm Ge NC at annealing temperatures of 600-750°C has been confirmed which is within the SL stability range. The used technique allows to produce SL stacks with very smooth interfaces (roughness <1 nm), thus the Ge NC layers could be separated by very thin SiO2 films (d > 3 nm) which offers interesting possibilities for charge transport via direct tunneling.