Understanding temporal evolution of microstructures on metal-assisted chemically etched Ge surface and its applications

We report on metal-assisted chemical etching (MaCE) of single crystalline p-Ge(1 0 0) substrates at an elevated temperature, using dilute H2O2 solution, for various etching times in the range of 30–1200 s. We carry out atomic force microscopic and scanning electron microscopic studies to investigate the temporal evolution of microstructures on Ge substrates. It is observed that at lower etching times rough surfaces evolve, whereas higher etching times lead to the formation of pyramidally textured surfaces. In order to understand the present observations numerical simulation studies, based on continuum theory of stress-induced morphological instability, are carried out. Further, in an attempt to demonstrate the tunable multifunctional properties of pyramidally textured-Ge surfaces, we study light trapping and cold cathode electron emission properties of the same as a function of etching time. Interestingly, the optical reflectance of pyramidally textured surfaces reduces systematically with increasing etching time and the one evolved under an etching time of 1200 s can go down to an unprecedented low value of 0.23% over a broad spectral range (600–3000 nm). In addition, the turn-on potential for cold cathode electron emission from the etched-Ge surfaces also reduces steadily with increasing etching time with the lowest one being 1.7 V µm−1 (with the field-enhancement factor of 15,171) for the highest etching time of 1200 s. These are by far the best known results for any chemically etched-Ge surface and offer tremendous potential to fabricate various Ge-based optoelectronic devices.