The new, more sensitive infrared sensor brings benefits to many different technologies. (Image: Aalto University / Xiaolong Liu)

Detecting infrared light is critical in an enormous range of technologies, from remote controls to autofocus systems to self-driving cars and virtual reality headsets. That means there would be major benefits from improving the efficiency of infrared sensors, such as photodiodes.

Researchers at Aalto University have developed a new type of infrared photodiode that is 35 percent more responsive at 1.55 µm, the key wavelength for telecommunications, compared to other germanium-based components. Importantly, this new device can be manufactured using current production techniques, making it highly practical for adoption.

“It took us eight years from the idea to proof-of-concept,” said Hele Savin, Ph.D., Professor, Aalto University.

The basic idea is to make the photodiodes using germanium instead of indium gallium arsenide. Germanium photodiodes are cheaper and already fully compatible with the semiconductor manufacturing process — but so far, germanium photodiodes have performed poorly in terms of capturing infrared light.

Savin’s team managed to make germanium photodiodes that capture nearly all the infrared light that hits them.

“The high performance was made possible by combining several novel approaches: eliminating optical losses using surface nanostructures and minimizing electrical losses in two different ways,” said Hanchen Liu, Doctoral Researcher who built the proof-of-concept device.

The team’s tests showed that their proof-of-concept photodiode outperformed not only existing germanium photodiodes but also commercial indium gallium arsenide photodiodes in responsivity. The new technology captures infrared photons very efficiently and works well across a wide range of wavelengths. The new photodiodes can be readily fabricated by existing manufacturing facilities, and the researchers expect that they can be directly integrated into many technologies.

“The timing couldn’t be better. So many fields nowadays rely on sensing infrared radiation that the technology has become part of our everyday lives,” said Savin.

Savin and the rest of the team are keen to see how their technology will affect existing applications and to discover what new applications become possible with the improved sensitivity.

Here is an exclusive Tech Briefs interview, edited for length and clarity, with Savin.

Tech Briefs: What was the biggest technical challenge you faced over the eight years from idea to proof-of-concept while developing this infrared photodiode?

Savin: There were many challenges that we had to overcome, that is why it took several years to finally make functional devices. Most of them were material-related issues. First, we needed to find a good and stable surface passivation method, then we needed to develop the process for nanostructuring Ge surfaces. The biggest challenge was to combine these two, i.e., to develop a process for surface passivation of Ge nanostructures. After we had succeeded in that, we were sure that the devices would work well, the rest was just normal process development for other parts of the devices.

Challenges to overcome in the future are related to the dark current and speed of the sensors. These are somewhat determined by the intrinsic material properties of Ge, but we have some ideas how to tackle them as well.

Tech Briefs: What was the catalyst for this project?

Savin: We have in the past focused our research a lot on visible and UV radiation detection. This is because our background is in photovoltaics and the highest sunlight intensity is within this wavelength range. The results we obtained with this wavelength range were so good that we established a spinout company ElFys Inc. to commercialize the sensors. After this, I got several requests from the semiconductor industry asking if we could also extend our research to infrared radiation as the application range would be much broader and there are not good solutions available for high sensitivity at the moment (with material that would be CMOS industry compatible). Some even said it would be revolutionary to make high sensitivity IR sensors with silicon or a like material. It sounded like a challenge I was up for.

Tech Briefs: Can you explain in simple terms how it works?

Savin: We are not using conventional p-n-junctions in our sensors to collect the signal, instead, we are creating a signal-collecting electric field using a charged atomic layer deposited (ALD) dielectric layer. Additionally, instead of a conventional antireflection coating we use special nanostructures at the surface of the sensors to eliminate reflectance losses. The latter allows efficient IR photon capture, while the former enables efficient signal collection even at low light conditions.

Tech Briefs: Do you have plans for further research/work/etc.?

Savin: So far, we have made only single-pixel chips. Next, we want to apply the technology to imaging applications, i.e., to make a matrix of the pixels. Eventually we aim for IR CMOS cameras integrated with Si electronics.

Tech Briefs: Is there anything else you’d like to add that I didn’t touch upon?

Savin: So far, we have fabricated only sensor elements, i.e., detector chips. The next logical step is to integrate them into final applications and see what comes out. If the readers of your story have applications where our chips could be used, we would be more than happy to collaborate.

Topics:
Design Optics Photonics Semiconductors & ICs Sensors

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