GeSn \textit{pin} diodes: from pure Ge to direct-gap materials

Complete $n-i-p$ Ge$_{\mathrm{1-y}}$Sn$_{y}$ diode structures (y$=$0-0.09) were fabricated on Si substrates with Sn concentrations covering the entire range between pure Ge and direct-gap materials. The structures typically consist of a thick ( \textgreater 1 $\mu $m) $n++$ Ge buffer layer grown by Gas Source Molecular Epitaxy using Ge$_{4}$H$_{10}$ and either P(SiH$_{3})_{3}$ or P(GeH$_{3})_{3}$, followed by a GeSn intrinsic layer ($\sim$ 500 nm), grown by Chemical Vapor Deposition (CVD) using Ge$_{3}$H$_{8}$ and SnD$_{4}$, and a GeSn $p$-type top layer ($\sim$ 200 nm) grown by CVD using Ge$_{3}$H$_{8}$, SnD$_{4}$ and B$_{2}$H$_{6}$. Temperature-dependence of the $I-V$ characteristics of these diodes as well as the forward-bias dependence of their electroluminescence (EL) signal were investigated, making it possible for the first time to extract the compositional dependence of parameters such as band gaps, activation energies, and dark currents. The EL spectra are dominated by direct-gap emission, which shifts from 1590 nm to 2300 nm, in agreement with photoluminescence results.