Progress in a-Si:H based multispectral sensor technology and material recognition

In this paper we describe the development of four different amorphous silicon based pixn multispectral photodetectors and discuss their optical characteristics as a result of extensive bandgap engineering. Upcoming from a sensor structure providing narrow band absorption peaks between 450 nm and 540 nm with a maximum applied bias of −12 V, we developed bias optimized detectors with increased bandwidth by changing the composition and thickness of the absorbing material. By applying just −2.5 V, one sensor obtains almost a Gaussian spectral response with peaks ranging from 420 nm to 580 nm. We present a specific algorithm to simulate color recognition probabilities for 20 different whitish powders by using two similar detectors. For the sensor providing sensitivity maxima reaching from 450 nm to 600 nm with sampling peaks in the range between 400 nm and 670 nm, the simulation discloses enhanced recognition probabilities of more than 70.2%, requiring a readout time of at least 15.5 ms. As assumed, the competetive sensor structure providing just a sampling bandwidth between 420 nm and 630 nm achieves recognition probabilities of 62.5% with a reduced readout time of only 6.1 ms. Possible sensor applications may exist in fields of fluorescence and spectrophotometric measurements, in chemical analysis, medical diagnostics or in colorimetric and multispectral imagery.