Researchers Develop a Thin Photodetector for the Future of Optoelectronics

Posted Jul 17, 2017 by

Tiny materials have become a must in today's increasingly powerful electronics, 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. For example, 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 to use a very thin material to realize the same function of devices which need to use a very thick material. 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. Nano-cavities are made up of an orderly series of tiny, interconnected molecules that essentially reflect, or circulate, light. Qiaoqiang 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 Zhenqiang 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 to look at a wide variety of optoelectronics that can go to even smaller footprints, smaller sizes. While the researchers demonstrated their advance using a germanium semiconductor, also can apply their method to other semiconductors. And importantly, by tuning the nano-cavity, they can control what wavelength they actually absorb. This will open the way to develop lots of different optoelectronic devices.

The researchers are applying jointly for a patent on the technology through the Wisconsin Alumni Research Foundation.