Electronic Transport Transition of Hydrogenated Amorphous Silicon Irradiated With Self Ions

Electric conductivity variations of hydrogenated amorphous silicon due to self-ion irradiation, i.e. irradiation with Si or H ions, are comprehensively investigated. The anomalous enhancement of dark conductivity (DC) and photoconductivity (PC) are firstly observed due to proton irradiation in the cases of undoped and n-type a-Si:Hs, and after that both decrease with increasing the irradiation fluence. However, Si ion irradiation does not induce the anomalous enhancement and induce a monotonic decrease in DC and PC in the low fluence regime. It is shown from the Seebeck effect analysis that the anomalous enhancement is caused by generation of donor-centers which have metastable nature at room temperature. The decrease in DC and PC is ascribed to the carrier removal effect and the carrier lifetime decrease accompanied by accumulation of dangling bonds, respectively. However, further irradiation causes the loss of photoconduction and the drastic increase in DC. This indicates that the dominant conduction mechanism changes from the band transport to the hopping transport due to excessive accumulation of dangling bonds. The change in the dominant conduction mechanism occurs at above about 10^-4 dpa (displacement per atom) and is independent of the majority carrier concentration before irradiation. It is concluded that the conductivity variations caused by self-ion irradiation can be systematically categorized according to the ratio of the nuclear energy deposition to the electronic energy deposition of incident ions.