Anti-reflective porous Ge by open-circuit and lithography-free metal-assisted chemical etching

Porous Ge (PGe) layer is formed on single-crystalline Ge (c-Ge) as well as in a releasable form (e.g., free-standing PGe) by lithography-free metal-assisted chemical etching (MacEtch) at room temperature under open-circuit. A thin layer of Au is evaporated on the entire surface of c-Ge and Ge on insulator prior to immersion in an etching solution. It is found that an oxide-free interface between the surface and metal catalyst is vital to form uniform PGe layer. PGe layers with different morphologies and thicknesses are produced after various MacEtch times. In order to show the functionality of PGe, reflection spectra of c-Ge (i.e., before etching) and PGe layers are characterized at a wavelength range of 1000–1600 nm. The reflection of PGe is broadly reduced to 10%, which matches well with simulation results based on finite-difference-time-domain method. Among all the modeling factors, thickness of PGe layers is found to be the primary cause of the broadband reduction of the reflection. In addition, transfer-printable free-standing PGe layers are realized. The capability of the simple, clean, and lithography-free MacEtch to achieve PGe on rigid substrates as well as in a free-standing form holds significant potential in photonic and optoelectronic device applications.