Transport of positrons in the electrically biased metal-oxide-silicon system

This paper describes a study of the effect of an external electric field on the behavior of positrons in metal-oxide-silicon (MOS) systems. Doppler broadening measurements of the annihilation radiation were performed on capacitors with identical thermally grown SiO${}_{\mathrm{2}}$ layers and with Al, W and Au layers as a gate. The data were analyzed by the combined use of the shape- and wing-parameters of the photo peak. The observed effects of the electric field are due to the field-driven transport of positrons through the SiO${}_{\mathrm{2}},$ silicon and the interfaces. By applying a field of the order of 1 MV/cm the positrons can be efficiently transported through the approximately 100 nm thick SiO${}_{\mathrm{2}}$ layer. From the transport behavior of the positrons it is concluded that the positron affinity is higher for SiO${}_{\mathrm{2}}$ than for silicon and for the gate metal. By properly choosing the direction of the field, the positrons implanted into the SiO${}_{\mathrm{2}}$ layer are collected either at the Si/SiO${}_{\mathrm{2}}$ interface or at the SiO${}_{\mathrm{2}}$ /gate interface. For negative gate bias the positrons implanted into the substrate, that diffuse back to the SiO${}_{\mathrm{2}},$ are transported through the oxide layer and injected into the gate metal. This is the first time that field-assisted transport of positrons across an insulating layer has been demonstrated.