Infrared response of charge-coupled devices

With a band gap of silicon of 1.1eV, the largest wavelength that can excite electrons from the valence to the conduction band is roughly 1100nm. As a consequence, in, for instance, a charge-coupled device, the quantum efficiency (QE) for wavelengths larger than 1100nm is assumed to be zero. We found that there is a response at those longer wavelengths and that the response decreases with increasing wavelength. The QE increases with increasing chip temperature which suggests a thermally activated process. Impurities in the silicon provide the energy levels in the band gap, from which electrons can be excited either thermally or by absorption of a photon. It is these impurities that contribute to the infrared response. We characterized the response at chip temperatures of 248 K to 293 K for wavelengths from 1200 nm to 1600 nm and calculated the activation energies at these wavelengths. We found that hot pixels, i.e., pixels with extraordinary high counts in a dark frame, tend to respond stronger to infrared light than normal pixels. This correlation gets stronger for longer wavelengths. It is argued that this response can be used for probing the impurities present in the silicon bulk of the sensors.