Until now, according to the researchers, two major challenges have stood in the way of smaller optoelectronic devices. First, shrinking the size of conventionally used amorphous thin-film materials also reduces their quality. And second, when ultrathin materials become too thin, they lose some ability to gather or absorb light.
To resolve these challenges, the researchers created a device consisting of nano-cavities sandwiched between a top layer of ultrathin single-crystal germanium and a reflecting layer of silver.
"The idea 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," said UW-Madison Professor Zhenqiang Ma.
"Because of the nano-cavities, the photons are 'recycled' so light absorption is substantially increased – even in very thin layers of material.”
However, most germanium thin films begin as germanium in its amorphous form. That also means its quality isn't sufficient for increasingly smaller optoelectronics applications.
To resolve this issue, the researchers used a membrane-transfer technology that allowed them to integrate single crystalline semiconducting materials onto a substrate.
"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," explained UW-Madison Professor Zongfu Yu.
While the researchers demonstrated their advance using a germanium semiconductor, they claim they can apply their method to other semiconductors.
"And importantly, by tuning the nano-cavity, we can control what wavelength we actually absorb," concluded UB Professor Qiaoqiang Gan. "This will open the way to develop lots of different optoelectronic devices."
