A team of engineers from the University of Wisconsin–Madison and the University at Buffalo have developed a nanoscale photodetector that combines a unique fabrication method and light-trapping structures. The researchers — electrical engineering professors Zhenqiang (Jack) Ma and Zongfu Yu at UW–Madison and Qiaoqiang Gan at UB — described their device, a single-crystalline germanium nano-membrane photodetector on a nano-cavity substrate, in the journal Science Advances.
“The idea, basically, is you want to use a very thin material to realise the same function of devices in which you need to use a very thick material,” says Ma. The device consists of nano-cavities sandwiched between a top layer of ultrathin single-crystal germanium and a reflecting layer of silver.
“Because of the nano-cavities, the photons are ‘recycled’ so light absorption is substantially increased — even in very thin layers of material,” says Ma. Nano-cavities are made up of an orderly series of tiny, interconnected molecules that essentially reflect, or circulate, light. Gan already has shown that his nano-cavity structures increase the amount of light that thin semiconducting materials like germanium can absorb.
However, most germanium thin films begin as germanium in its amorphous form — meaning the material’s atomic arrangement lacks the regular, repeating order of a crystal. That also means its quality isn’t sufficient for increasingly smaller optoelectronics applications.
That’s where Ma’s expertise comes into play. A world expert in semiconductor nano-membrane devices, Ma used a revolutionary membrane-transfer technology that allows him to easily integrate single crystalline semiconducting materials onto a substrate.
The result is a very thin, yet very effective, light-absorbing photodetector — a building block for the future of optoelectronics. “It is an enabling technology that allows you to look at a wide variety of optoelectronics that can go to even smaller footprints, smaller sizes,” says Yu, who conducted computational analysis of the detectors.
While the researchers demonstrated their advance using a germanium semiconductor, they also can apply their method to other semiconductors. “And importantly, by tuning the nano-cavity, we can control what wavelength we actually absorb,” says Gan. “This will open the way to develop lots of different optoelectronic devices.”