At the Institute of Materials Science of Kaunas University of Technology (KTU), a research team has developed new infrared radiation sensors based on silicon and graphene Schottky contacts, which are more efficient than infrared sensors currently on the market.

Graphene-silicon infrared sensor (top view). Image courtesy of Kaunas University of Technology.

Without installing infrared sensors in various devices, it would be impossible to observe Earth from space and interplanetary missions to investigate the atmospheres of other planets or search for life on Mars.

These sensors are even used in energy-saving control systems, night vision equipment, motion sensors, optical communication lines, and pharmaceuticals. Currently, airports use these sensors to remotely measure passengers' body temperature to detect individuals infected with the Covid-19 virus.

Over the past two years, scientists at Kaunas University of Technology (KTU) have been working to optimize these sensors.

Dr. ŠarūnasMeškinis, the principal investigator of the research group, pointed out that the manufacturing technology for Schottky contact sensors is much simpler than that for other infrared sensors. Arrays of these sensors can be developed on semiconductors, such as silicon wafers. Fast switching speed is the main advantage of Schottky contact sensors.

However, one of the main disadvantages of these sensors is low sensitivity. This is because they can only convert a small fraction of light particles (photons) into photoelectrons. Therefore, we decided to fabricate nanostructured metal plasmonic absorbers on graphene, thereby improving the sensitivity of these sensors.

Dr. ŠarūnasMeškinis, Research Director at Kaunas University of Technology

Sensor applications—from meteorology to space observation

Schottky contact photosensors can be used in artificial Earth satellites to track water and land boundaries, lava flows, and forest fires. Furthermore, they are used in meteorology to assess soil and vegetation, in geology, and for water level monitoring in optical communication systems.

These sensors are used to study other planets: mineralogical analysis, atmospheric phenomena in planetary atmospheres, and the search for possible signs of life. They are important for many types of space research.

Dr. ŠarūnasMeškinis, Research Director at Kaunas University of Technology.

Sensors developed by KTU researchers can be used in a variety of applications. Dr. Meškinis stated that the first application is adapting these sensors for optical encoding. An optical encoder is a high-precision optomechanical device designed to quantify distance, minimal displacement, the precision of mechanical equipment and its components, and rotational speed.

"In space, optical encoders are used in laser communication terminals and ground-based optical satellite stations. Additionally, in laser-based communication between low-Earth-orbit satellites and Earth, spacecraft laser pointers (LiDAR), satellite-mounted cameras, and space telescopes," added KTU researchers.

Graphene makes sensors more sensitive

Typically, Schottky contact sensors consist of a metal layer formed on a semiconductor. This metal layer generates an electric field in the surface layer of the semiconductor.

"When photoelectrons (free electrons released from atoms by light) are generated in the metal or the semiconductor surface layer, they can be extracted by the electric field and form a so-called photocurrent, which can be measured and used to evaluate light intensity (including infrared radiation)," explained Dr. Meškinis.

Conventional sensors can only convert a small fraction of light particles (or photons) into photoelectrons. Therefore, the research team used graphene instead of metal to address this issue, thereby improving the sensitivity of such sensors.

Because graphene is ultra-thin, there is no problem of free electron scattering. As a result, nearly all photoelectrons generated in graphene will reach the graphene-semiconductor junction at an appropriate angle and flow into the semiconductor. The photocurrent generated in the sensor will be significantly larger than in the case of metal-semiconductor contacts.

Dr. ŠarūnasMeškinis, Research Director at Kaunas University of Technology

However, the ultra-small thickness of graphene is also a key issue to address. A single layer of graphene can absorb only 2.3% of incident photons.

"We solved this problem by forming special metal nanostructures on graphene, known as plasmonic nanostructures. They improve the sensitivity of the photosensor," the researchers added.

Although metal-semiconductor Schottky contact sensors have been on the market for more than thirty years, Dr. Meškinis noted that semiconductor and graphene Shottky contact photosensors are not yet mass-produced.