Sci. Adv. 3, e1602783 (2017)

The use of ultrathin layers of semiconductor is potentially attractive for realizing fast, miniaturized optoelectronic devices; however, the absorption of light in such ultrathin layers can be very inefficient. Now, researchers in the USA have made photodetectors from nanomembranes of single-crystal Ge that are only tens of nanometres thick, yet yield good absorption and performance. The key to the advance is using a membrane transfer–printing method to put the thin crystalline Ge films directly onto optical nanocavities that enhance the light–matter interaction. The optical cavities themselves are formed by a dielectric layer of Al2O3 sandwiched between the Ge and a thin silver 'mirror' on silicon. The Ge is thinned down to desired thicknesses between 10 to 60 nm, confirmed by atomic force microscopy. For a 20-nm-thick Ge film photodetector, an absorption of ∼16% of the light at a wavelength of 733 nm would be typical, however, the team achieved a much greater value of 81% in their cavity-enhanced design. The photodetector was fabricated with a 17-nm-thick gallium-doped (p-type) Ge film and delivered a photoresponsivity of up to 4.7 A W−1. The dark current is small due to the low volume of the semiconductor used; with a normalized photocurrent to dark current of ∼105 mW−1. The approach is not limited to Ge and in principle can be applied to other semiconductors.