Precisely size-controlled fabrication of germanium pyramid array as an effective light-trapping material for photonic devices

Uniform arrays of square-based germanium (Ge) pyramids on the surface of crystalline p-type germanium wafers are presented in this work. Double-cell electrochemical etching method is used to produce different-sized germanium pyramids with high morphological and material homogeneity. The structural and optical characteristics are investigated along with the morphological properties and related to the etching conditions such as current density, etching time and electrolyte solvent concentrations. By patterning the close-packed pyramid arrays, a high structure density is achieved. These arrays can possess strong electronic states coupling, leading to novel optical properties. Moreover, we discuss the possible charge transfer mechanism from perylene to Ge pyramids that quenches the luminescence intensity of dye molecules. The charge transfer properties are characterized by a fluorescence lifetime imaging microscope. According to the obtained two-dimensional fluorescence lifetime images, the charge transfer rates critically depend on pyramid size, occurring for pyramids with base 2.74 and 1.80 mu m, not for smaller ones. The presence of a GeOx layer on the surface of large pyramid provides interfacial trap states and quenches the radiative recombination of perylene dye molecules. A reduction in pyramid size leads to an acceleration of the oxidation rate and GeOx layer oxidized to GeO2. The formation of GeO2 also prevents efficient charge transfer between germanium and absorbed dye molecules at interfaces. Tailoring the nanostructures of dye/oxide/semiconductor heterointerfaces can be an effective method for advanced light trapping and surface engineering in Ge-based photonic devices.