Semiconductor sensors with high UV responsivity

The semiconductor sensors that are available on the market have only a very limited UV responsivity. Even the best UV photodiodes have a quantum efficiency of less than 80 % in the spectral range between 200 nm and 300 nm. In this context, the external quantum efficiency is always considered, i.e. the number of charge carrier pairs detected per photon. Scientists from Finland, Spain and Germany have now succeeded in developing a novel silicon photodiode which achieves a quantum efficiency of over 130 % in the UV range. PTB was able to validate these high responsivities metrologically with its capabilities in UV detector radiometry for precise and traceable radiometric measurements.

The UV responsivity of a photodiode is limited by two fundamental technological hurdles: on the one hand, the high reflection losses of the incident radiation directly at the surface, and on the other hand, the near-surface recombination of the generated charge carriers. The first hurdle was negotiated by means of a nanostructured surface with a columnar and conical morphology which displays very low reflectivity. The surface which, in the case of conventional silicon, usually shimmers in a blueish way, now appears black. Due to the nanostructuring, the surface recombination rate could have been expected to have increased. However, the rate of recombination could be reduced by passivating the surface by means of Al₂O₃. And thus, the second hurdle was also negotiated. Furthermore, the surface charge remaining in the Al₂O₃ layer induces a pn junction in the silicon. Due to this, an addition of dopants to create a pn junction, which is necessary for a photodiode, can be avoided. This also contributes to the high quantum efficiency.

The experiences and findings gained in this way suggest that the quantum efficiency of photodiodes of black silicon (b-Si) could even be further enhanced. The combination of surface nanostructure and of the pn junction induced by surface passivation can, in the long run, also be applied to other semiconducting materials.