Epitaxial GeSn: impact of process conditions on material quality

The electrical and optical material properties of epitaxial Ge1-xSnx and SiyGe1-x-ySnx are of high interest for novel device applications. However, the limited Sn solubility in Ge makes the epitaxial growth of Ge1-xSnx and SiyGe1-x-ySnx challenging. Most of the literature describing the epitaxial growth is for Ge2H6 and SnCl4 as Ge and Sn precursors, respectively. A more recent publication deals with the epitaxial growth of high-quality Ge1-xSnx with the more conventional GeH4. In this manuscript, we compare the structural and optical material quality of Ge1-xSnx, epitaxially grown on Ge virtual substrates as a function of growth pressure, growth temperature, the choice of the carrier gas (H-2 or N-2) and the choice of the Ge precursor (GeH4 versus Ge2H6). The best material quality in terms of surface morphology and photoluminescence characteristics is obtained if GeH4 is used as a Ge precursor. For Ge1-xSnx grown with Ge2H6 and at atmospheric pressure, pyramidical defects can be seen and there is a risk for uncontrolled local Sn agglomeration. The pyramidical defects are not observed on Ge1-xSnx layers grown at reduced pressure, but the highest achievable substitutional Sn concentration is lower. No pyramidical defects are found for Ge1-xSnx layers grown with GeH4 and the issue of uncontrolled local Sn agglomeration does not appear.