Direct band gap (DBG) electroluminescence (EL) and photo detection were studied at room temperature for n-type bulk germanium (Ge) diodes with a fin type lateral HfGe/Ge/TiN structure. DBG EL spectra peaked at 1.55 mu m were clearly observed due to small hole and electron barrier heights of HfGe/Ge and TiN/Ge contacts. DBG EL peak intensity increased with increasing doping level of Ge substrate due to increased electron population in direct conduction band. The integrated intensity of DBG EL spectrum is proportional to the area of active region, implying a good surface-uniformity of EL efficiency. Small dark current intensity was measured as 2.4 x 10(-7) A under a reverse bias voltage of -1 V, corresponding to dark current densities of 5.3 x 10(-10) A/mu m or 3.2 x 10(-1)0 A/mu m(2). At the wavelength of 1.55 mu m, a linear dependence of photo current intensity on laser power was observed with a responsivity of 0.44 A/W at a reverse bias voltage of -1 V. (C) 2015 Elsevier B.V. All rights reserved.