There has been an urgent need in consumer electronics for infrared optoelectronics including light emitting diodes and photodetectors. To date, however, infrared optoelectronics are served by costly CMOS-incompatible III-V semiconductors. Recently, a new class of semiconductors that addresses the CMOS compatibility issue has emerged based on colloidal quantum dots. When it comes to consumer electronics, the use of RoHS-compliant materials is a prerequisite and therefore there is a strong need for the development of high-performance devices based on environmentally friendly elements — something that in the infrared had remained elusive.
To address this challenge, researchers have discovered that by controlling defects in materials, one can extend the semiconductor’s spectral reach beyond its bandgap, thereby expanding the material availability for the infrared part of the spectrum.
An infrared detector was developed using bismuth sulphide, which has proven to have fast and high photo-response levels in the shortwave infrared range thanks to the formation of defects in the material. Researchers fabricated a photoconductive detector, depositing a very thin layer of Bi2S3 flakes onto a Si/SiO2 substrate. Once built, the team was able to observe that the Bi2S3 flakes possessed sulphur vacancies or defects in the material (sulphur-deficient) that created extended in-gap states, which allowed an increased absorption of light below the bandgap value of Bi2S3. Such features led to a high-gain, low-noise, high-sensitivity photodetector.
To shed insights in the sulphur deficiency mechanism, they built a second photodetector and synthesized the Bi2S3 crystal by performing a sulfurization process (changing the concentration percentages of Bi and S in the crystal) and subsequently refilling the sulphur vacancies. In doing this, they were able to observe that the photodetector had a much faster response time but was limited to the spectral range in the near infrared. To improve the response time without sacrificing its spectral coverage into the infrared, the researchers carried out a mild chemical treatment on the sulphur-deficient-based detector through a surface passivation process of the crystal. Completing the treatment, they observed that the time response had reached a value of approximately 10 ms for the infrared and visible light range 50 times faster than the original sulphur deficient-based detector.
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