Nanophotodetectors with nanocavities to improve the performance of optoelectronic devices.

In today's increasingly powerful electronics, tiny materials are a must as manufacturers seek to increase performance without adding bulk.

Smaller also is better for optoelectronic devices -- like camera sensors or solar cells -- which collect light and convert it to electrical energy. Think, for example, about reducing the size and weight of a series of solar panels, producing a higher-quality photo in low lighting conditions, or even transmitting data more quickly.

However, two major challenges have stood in the way: First, shrinking the size of conventionally used "amorphous" thin-film materials also reduces their quality. And second, when ultrathin materials become too thin, they become almost transparent and actually lose some ability to gather or absorb light.

Now, in a nanoscale photodetector that combines a unique fabrication method and light-trapping structures, a team of engineers from the University of Wisconsin-Madison and the University at Buffalo has overcome both of those obstacles.

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 realize 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."

Journal Reference:

1. Zhenyang Xia, Haomin Song, Munho Kim, Ming Zhou, Tzu-Hsuan Chang, Dong Liu, Xin Yin, Kanglin Xiong, Hongyi Mi, Xudong Wang, Fengnian Xia, Zongfu Yu, Zhenqiang (Jack) Ma, Qiaoqiang Gan. Single-crystalline germanium nanomembrane photodetectors on foreign nanocavities. Science Advances, 2017; 3 (7): e1602783 DOI: 10.1126/sciadv.1602783