NO2 gas sensor with excellent performance based on thermally modified nitrogen-hyperdoped silicon

NO ₂ gas sensing properties of the nitrogen-hyperdoped black silicon (N-Si) modified at different annealing temperatures are studied. Owing to the abundant defects in the material and their changes with the annealing, the thermal modification brings a series of novel sensing behaviors and characteristics. Working as the sensitive material in a conductometric gas sensor, the pristine N-Si exhibits an undesirable n- to p-type response transition for higher NO ₂ concentration, which severely reduces its upper limit of detection (< 5 ppm). However, for the thermally modified N-Si after annealing at higher temperature (≥ 673 K), the abnormal response transition induced by higher concentration disappears. These modified N-Si show consistent p-type response to all tested NO ₂ concentrations, successfully breaking the detection limit. More interestingly, there is an optimal annealing temperature ~ 873 K, at which the sensor demonstrates outstanding sensing performances, including wide dynamic range spanning 5 orders of magnitude, rapid adsorption and desorption ability, high response and good selectivity, etc. Results indicate that through the thermal modification a novel N-Si gas-sensitive material is obtained. The mechanism for the thermally-induced response type conversion is discussed, in which the activation of acceptor energy levels provided by the complexes associated with substitutional nitrogen are considered.