White Light Emission from Black Germanium

We demonstrate the nearly perfect absorption and quantum dots-mediated enhanced visible light emissions from defect engineered Ge nanopyramids or black germanium. High-resolution 3D photoluminescence (PL) imaging of the pyramid structure elucidated the position dependency of defects and their emission: Stronger photoluminescence yield was observed at the nanopyramid tips, which is correlated to the efficient light nanofocusing at the tips where increased defect density and roughness at the interface between Ge and surface oxide (GeO2) also takes place. Furthermore, the visible light emissions from this GeO2/Ge interface can be enhanced,similar to 15-fold when CdTe quantum dots (QDs) are adsorbed on the GeO2/Ge system. The enhanced luminescence of our structure can be attributed to the extraordinary light harvesting property of pyramid structure; strong antireflection effect, pronounced defect formation at the nanopyramid tips, and anomalous resonant energy transfer between GeO2 defects and CdTe QDs. The proposed methodology can be applied to other nanostructured wide bandgap materials to turn them into solar light harvesters and bright white light-emitting phosphors.