Light detector records photons from UV through IR

16 Nov 2016

Researchers at Aalto University reach near-unity response; with applications in medicine and safety.

A potentially market-disruptive light detector, developed by a team led by Professor Hele Savin at Finland’s University of Aalto, captures more than 96 percent of available photons, across the entire range of visible, ultraviolet and infrared wavelengths.

“Present-day light detectors suffer from severe reflection losses as the currently used antireflection coatings are limited to specific wavelengths and a fixed angle of incidence,” said Prof. Savin. “Our detector captures light without such limitations by taking advantage of a nanostructured surface. Low incident angle is useful especially in scintillating X-ray sensors.”

No doping required

The new detector does not require the addition any dopants to collect the wide range of light frequencies; instead it uses an inversion layer, which is generated by atomic layer deposited thin film. The team’s work is described in the latest edition of Nature Photonics.

Prof Savin said, “We have also addressed electrical losses occurring in traditional sensors that utilize semiconductor pn-junctions for light collection. Our detector does not need any dopants to collect light; instead we use an inversion layer generated by atomic layer deposited thin film.”

The new concept for light detection was developed from the team’s earlier research on nanostructured solar cells. Indeed, the nanostructure used in the new light detector is similar to that used by the team a couple of years ago in their record-high efficiency black silicon solar cells.

The team filed a patent application for the new light detector. Prototype detectors are currently being tested in imaging applications related to medicine and safety. The team is seeking new applications for the invention, especially among the ultraviolet and infrared ranges that would benefit from the superior spectral response.

Article abstract

The Nature Photonics article says that so-called “ideal photodiodes” can detect all incoming photons independently of the wavelength, angle or intensity of the incident light. It states: “Present-day, non-ideal photodiodes often suffer from optical losses and generated charge carriers are often lost via recombination. The Aalto team has demonstrated a light sensitive device with an external quantum efficiency above 96% over the wavelength range 250–950nm.

“Instead of a conventional p–n junction, the new detector uses negatively-charged alumina to form an inversion layer that generates a collecting junction extending to a depth of 30µm in n-type silicon with bulk resistivity larger than 10 kΩ cm.

“Collection efficiency is enhanced further by nanostructuring the photodiode surface, which results in higher effective charge density and increased charge-carrier concentration in the inversion layer. Additionally, nanostructuring and efficient surface passivation allow for a reliable device response with incident angles up to 70 degrees.” Savin’s team expects that further developments in the device will improve data quality, reduce the area of photodiodes as well as decrease the cost per pixel.