Restrictions of Si-based Ge nanodots from porous alumina membranes

This paper reports growth of ordered Ge nanodots (NDs) with uniform sizes on silicon substrates using porous alumina membranes (PAMs) as templates. The relationships between substrate temperatures (400-600 °C) and site distribution of Ge NDs are studied. Ordered arrangements of Ge NDs are realized at 400 °C and 500 °C, respectively. Due to joint effect of substrate temperature and restrictions from PAM, an uncommon size change trend is found. At 400 °C, triangular pyramid-like and short cylindrical Ge NDs are obtained with different nanopore aspect ratios of PAMs. A geometrical optic method is used to analyze the mechanism of Ge NDs with such shapes. Raman characterization is utilized to study the strain in Ge NDs. As a result, almost pure Ge content and 1.5% tensile strain are revealed, which are attributed respectively to the low substrate temperature and thermal mismatch among Si substrate, Ge ND and PAM.