The influence of defects and postdeposition treatments on the free carrier density in lightly phosphorus-doped large-grained polycrystalline silicon films

The influence of postdeposition treatments (rapid thermal annealing and hydrogenation) on the doping of large-grained polycrystalline silicon p+nn+ thin-film diodes on glass substrates is investigated using resistivity and impedance analysis measurements. Whereas in the lightly phosphorus-doped base region both treatments are found to cause an increase in the active doping concentration, hydrogenation decreases the active doping concentration of both heavily doped layers (Al and P). The different behavior is attributed to acceptorlike defects which are present in the nonhydrogenated base region in a similar concentration as the atomic phosphorus concentration and which are well passivated by hydrogenation. From posthydrogenation annealing experiments and temperature-dependent impedance analysis measurements, different temperature dependences and activation energies (depending on the posthydrogenation annealing temperature) are found for the lightly doped base region. The temperature dependences are quantitatively described using a simple model comprising three energy levels within the silicon band gap: the phosphorus doping level, a shallow donorlike defect level about 0.2 eV below the conduction-band edge, and a midgap acceptorlike defect level. From this model it can be concluded that (i) the density of the acceptorlike defect in our n-type base region is at least 1×1017cm−3 after the rapid thermal anneal, and (ii) that hydrogenation reduces this defect density by more than a factor of 10. The results of this work demonstrate that accurate control of the doping in large-grained polycrystalline Si films is possible, provided the effects of defects and the necessary postdeposition treatments (such as rapid thermal annealing and hydrogenation) are carefully accounted for.