Oct 21 2020
At Kaunas University of Technology (KTU) Institute of Materials Science, a research team has developed new silicon and graphene Schottky contact-based infrared radiation sensors with higher efficiency than the infrared sensors available in the market.
Graphene-silicon infrared sensor (view from above). Image Credit: Kaunas University of Technology.
Observations of Earth from space and interplanetary missions to investigate the atmosphere of other planets or to look for life on Mars would not be possible without infrared sensors fitted in different devices.
Such sensors are even used in energy-efficient control systems, night vision devices, motion sensors, optical communication lines, and medicine. At present, airports use these sensors to perform remote measurement of the body temperature of passengers to detect people infected with the Covid-19 virus.
For the past two years, scientists at Kaunas University of Technology (KTU) have been making efforts to optimize these sensors.
Dr. Šarūnas Meškinis, the Principal Investigator of the research group notes that the manufacturing technology of Schottky contact sensors is much simpler than that of other infrared sensors. Multiple arrays of these sensors can be developed on the semiconductor, for instance, silicon, plates. Fast switching speed is the main benefit of Schottky contact sensors.
However, one of the main drawbacks of these sensors is low sensitivity. That is because they can convert only a small fraction of light particles—photons—into photoelectrons. Therefore, we decided to fabricate nanostructured metal plasmonic absorber on the graphene, thus increasing the sensitivity of these sensors.
Dr Šarūnas Meškinis, Study Principal Investigator, Kaunas University of Technology
Application of Sensors—from Meteorological to Space Observations
Schottky contact photosensors find use in artificial earth satellites to track water and land boundaries, lava flows, and forest fires. In addition, they are used in meteorology to evaluate water level in soil and plants, in geology, and in optical communication systems.
These sensors are used to study other planets: mineralogical analysis, atmospheric phenomena in planetary atmospheres, and for finding possible signs of life. They are important to many types of research in space.
Dr Šarūnas Meškinis, Study Principal Investigator, Kaunas University of Technology
Sensors developed by the KTU researchers can be used for multiple applications. Dr. Meškinis stated that the first one is to tweak these sensors for use in optical coding. Optical coders are high-precision optomechanical devices engineered to quantify distances, minimum displacement, the precision of the mechanical devices and their components, as well as their rotational speed.
“In space, optical encoders are used in laser communication terminals, terrestrial optical satellite stations. Also, in laser-based communication between low-orbit satellites and Earth, spacecraft laser locators (LiDARs), satellite-mounted video cameras, space telescopes,” adds the KTU researcher.
Graphene Makes Sensors More Sensitive
In general, the Schottky contact sensor includes the layer of metal developed on a semiconductor. This metal layer generates an electric field in the semiconductor surface layer.
“When photoelectrons—free electrons released from an atom by light—are created in the metal or this semiconductor surface layer, they can be extracted by an electric field and form a so-called photocurrent that can be measured and used to evaluate light intensity (including infrared radiation),” explained Dr. Meškinis.
Traditional sensors can convert only a small part of light particles (or photons) into photoelectrons; therefore, the team resorted to using graphene rather than metal to overcome this problem and thus to boost the sensitivity of such sensors.
Because graphene is ultra-thin, there is no free electron scattering problem. As a result, almost all photoelectrons created in the graphene will reach the graphene and semiconductor junction at an appropriate angle and flow to the semiconductor. The photocurrent created in the sensor will be much bigger than in the metal-semiconductor contact case.
Dr Šarūnas Meškinis, Study Principal Investigator, Kaunas University of Technology
But the ultra-small thickness of the graphene is also a crucial problem to fix. Only 2.3% of the incident photons can be absorbed by a single layer of graphene.
“We solved this problem by forming special metal nanostructures on graphene, known as plasma nanostructured liquids. They increased the sensitivity of the photosensors,” added the researcher.
Despite the fact that metal-semiconductor Schottky contact sensors have been in the market for over three decades, Dr. Meškinis states that semiconductor and graphene Shottky contact photosensors have not yet been produced on a large scale.