Electroabsorption measurements of interfaces in a-Si:H/a-${\mathrm{SiO}}_{\mathrm{x}}$:H and a-Si:H/a-${\mathrm{SiN}}_{\mathrm{x}}$:H multilayer films

We report electroabsorption measurements on amorphous multilayer films made from alternating layers of a-Si:H and a-${\mathrm{SiO}}_{\mathrm{x}}$:H or from a-Si:H and a-${\mathrm{SiN}}_{\mathrm{x}}$:H. We observe built-in electric fields (as large as 4×${10}^5$ V/cm) associated with an asymmetric charge distribution in the layers. This asymmetry shows that the electrical properties of the interface depend strongly on the order of deposition. In the a-Si:H/a-${\mathrm{SiO}}_{\mathrm{x}}$:H system the data are well described by a conduction-band offset which is greater when silicon is deposited onto oxide than when oxide is deposited onto silicon. The difference in offsets is 170 meV when the oxide is made from a mixture of ${\mathrm{SiH}}_4$ and ${\mathrm{N}}_2$O and 220 meV when the oxide is formed entirely through the plasma oxidation of part of the underlaying a-Si:H layer. The a-Si:H/a-${\mathrm{SiN}}_{\mathrm{x}}$:H multilayers have charges concentrated at defects near the silicon-on-nitride interface. We also determine sublayer thicknesses from the refractive index. These data show that the growth rate increases dramatically when the deposition gas is changed to include ${\mathrm{N}}_2$O, due to oxidation of the a-Si:H layer. Interruption of the plasma during the gas interchange prevents the deposition of substoichiometric oxide and changes the film properties.