Halide Vapor Phase Epitaxy of Ge from an Elemental Source

Halide vapor phase epitaxy shows promise for low-cost photovoltaic device manufacturing because of its high growth rates and lower cost elemental precursors but previously has not been used to deposit epitaxial Ge. Here, we demonstrate Ge deposition by generating GeCl2 in situ from solid Ge and HCl in a N2 ambient. To achieve Ge growth, we inject AsH3 and PH3 as sources of active hydrogen to the growth surface to create a driving force for growth. We do not observe Ge growth unless a supply of hydrogen is added, consistent with thermodynamic calculations. Furthermore, we show the hydrogen source must crack readily on the substrate surface to enable growth; relatively stable sources such as H2 do not cause growth. Unintentional group V doping is one drawback of using AsH3 and PH3 to drive the Ge reaction. We observed As or P concentrations in the Ge films ranging from 4 x 1017 to 1 x 1018 atoms/cm3, concentrations that can drastically influence device characteristics. However, we note there are numerous other "helper molecule" options that can provide active hydrogen without doping or etching the material. This work provides a path forward for Ge deposition for optoelectronic devices from an elemental source.