Model for persistent photoconductivity in doping-modulated amorphous silicon superlattices

Doping superlattices prepared by depositing successively 105-Å-thick layers of n-type, intrinsic, and p-type amorphous hydrogenated silicon (n-i-p-i structures), exhibit persistent photoconductivity at room temperature. Persistent photoconductivity (PPC) is the excess dark conductivity that remains after the sample has been exposed to white, heat-filtered light and that decays on a time-scale of hours or days. We prove that PPC is a bulk effect and that the amount of light necessary to induce a certain level of PPC decreases exponentially with temperature. This temperature dependence corresponds to an activation energy of ∼0.4 eV for the creation of ‘‘state PPC’’; a slightly higher activation energy (∼0.7 eV) for the quenching of PPC is deduced from the annealing behavior. Above around 400 K the sample can be transformed into a second metastable state C, which does not exhibit PPC. The fact that the generation of PPC is thermally activated is incompatible with an explanation of this effect simply in terms of metastable carriers which are spatially separated in the space-charge fields of the n-i-p-i structure. We propose instead that holes are trapped in acceptorlike centers, AX, in the p regions of the sample, whereas the balancing concentration of electrons, confined to the n layers by the n-i-p-i fields and thus prevented from recombining with the holes, are responsible for PPC. The AX centers possess, in analogy to the DX centers in ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Al}}_{\mathrm{x}}$As, a strong electron-lattice coupling which accounts for the thermal activation energies of hole capture and ionization.