Direct band gap electroluminescence from bulk germanium at room temperature using an asymmetric metal/germanium/metal structure

As a promising material for fabricating on-chip optoelectronic devices, germanium (Ge) has a direct band gap of 0.8 eV, which matches with the wavelength for optical communication. The energy difference is only 134 meV between direct and indirect band gaps, implying the possibility of a direct band gap light emission. In general, a p-i-n diode structure is used for a Ge photo emitter, of which fabrication process is relatively complicated and high-quality n-type doping is still an issue. Recently we achieved high Schottky barrier heights for electrons Φ_BN = 0.60 eV (HfGe/n-Ge) and holes Φ_BP = 0.57 eV (TiN/p-Ge) [1,2]. Based on this technology, we demonstrate direct band gap room temperature electroluminescence (EL) from bulk Ge using a fin-type asymmetric metel/Ge/metal (HfGe/Ge/TiN) structure.