Direct bandgap narrowing in Ge LED's on Si substrates

In this paper we investigate the influence of n-type doping in Ge light emitting diodes on Si substrates on the room temperature emission spectrum. The layer structures are grown with a special low temperature molecular beam epitaxy process resulting in a slight tensile strain of 0.13%. The Ge LED's show a dominant direct bandgap emission with shrinking bandgap at the Gamma point in dependence of n-type doping level. The emission shift (38 meV at 10(20) cm(-3)) is mainly assigned to bandgap narrowing at high doping. The electroluminescence intensity increases with doping concentrations up to 3x10(19) cm(-3) and decreases sharply at higher doping levels. The integrated direct gap emission intensity increases superlinear with electrical current density. Power exponents vary from about 2 at low doping densities up to 3.6 at 10(20) cm(-3) doping density. (C) 2013 Optical Society of America