Black Germanium Photodetector Exceeds External Quantum Efficiency of 160%

In this work, a viable method is demonstrated to realize high-performance germanium (Ge) photodetectors (PDs) on the nanostructured Ge surface, namely black Ge, formed by chlorine (Cl-2) gas-based reactive ion etching at room temperature. Black Ge surface has spike-like pyramidal structures with a width and height up to 150 and 570 nm, respectively. Average reflection of black Ge is reduced to 2% at a wavelength range from 1 to 2 mu m, while that of planar Ge is approximate to 37%. Light absorption is strongly enhanced by the significantly reduced reflection, thereby leading to an increase in responsivity of black Ge PDs. Moreover, external quantum efficiency (EQE) exceeds 160% at 1550 nm, indicating the existence of internal gain resulted from multiple carrier generation in Ge nanostructures. Therefore, this work provides an effective and reliable approach to significantly enhance photodetection performance of Ge-based optoelectronic devices.