Stacked Ge nanocrystals with ultrathin SiO_2 separation layers

摘要

The aim of this work is the tailored growth of Ge nanocrystals (NCs) in (GeO_x/SiO_2) multilayers(ML) for photovoltaic applications. For this purpose the fabrication of regularly stacked Ge NCs separated by ultrathin SiO_2 layers is essential to enable charge carrier transport by direct tunnelling. In this paper we report on the fabrication of (GeO _x/SiO_2)_(50) multilayer stacks via reactive dc magnetron sputtering and Ge NCs formation after subsequent annealing. It is shown that magnetron sputtering allows us to deposit very regular ML stacks with a total thickness of about 300nm, characterized by ultrathin (down to 1nm) and very smooth (roughness ～ 0.6nm) SiO_2 separation layers. A main challenge is to keep these properties for a thermal budget necessary to form Ge NCs. For this reason, the temperature dependence of phase separation. Ge crystallization and ML morphology was investigated by Rutherford backscattering, x-ray scattering, Raman spectroscopy and electron microscopy. The formation of size confined Ge NCs of about 5nm after annealing of only 550 °C is confirmed. This low thermal budget ensures the suppression of GeO emanation and multilayer stability. Spectroscopic ellipsometry was applied to determine the optical Ge NC bandgap to (1.65 0.5)eV.