Ge substrates made by Ge-condensation technique: Challenges and current understanding

Ultra-thin Si1−xGex films on insulator (SGOI) with high Ge fraction were fabricated through Ge condensation. To achieve that strained Si0.75Ge0.25 layers were grown epitaxially on a silicon-on-insulator (SOI) wafers and subsequently oxidized in O2 at 1150 °C for various times. Raman Spectroscopy as well as Cross-sectional Transmission Electron Microscopy (X-TEM) revealed that purity of oxidation ambient among other factors is very important for the oxidation process. Traces of moisture in the oxygen flux accelerated the process causing a switch from dry to wet oxidation mechanism, which resulted in complete oxidation of the starting layer in only 1 h. The crystal quality and overall uniformity of such a layer was poor. The use of 100% dry oxygen on the other hand provides the necessary control over the oxidation process. This allowed relaxed (strain 1% and below) Si1−xGex layers with concentration of Ge up to 47% and uniform Ge distribution to be obtained. X-TEM analysis indicated that the crystal quality of the layers, is good and no threading dislocations are introduced for strain relaxation during the oxidation process. Uniformity of the layers was good as revealed by Raman Spectroscopy and Cross-sectional Scanning Electron Microscopy (X-SEM).