Photoemission spectroscopy of heterojunctions of hydrogenated amorphous silicon with silicon oxide and nitride

The growth and electronic structure of a-Si:H/a-${\mathrm{SiN}}_{\mathrm{x}}$:H and a-Si:H/a-${\mathrm{SiO}}_{\mathrm{x}}$:H heterojunctions have been studied by photoemission spectroscopy. Si 2p core-level photoemission was used to profile the chemical composition microscopically across the interfaces. With the exception of the ${\mathrm{SiO}}_{\mathrm{x}}$-on-Si interface, which due to initial plasma oxidation is graded over ∼3 Å, the interfaces are atomically abrupt. The offset energies between the a-Si:H valence-band edge and that of a-${\mathrm{SiN}}_{\mathrm{x}}$:H and a-${\mathrm{SiO}}_{\mathrm{x}}$:H, determined by valence-band photoemission, are 1.2 and 4.0 eV, respectively. Based on the fact that the offset energy is independent of a-Si:H layer thickness down to monolayer dimensions, we concluded that hole wave functions in a-Si:H are extremely localized. From the variation of the intensity of the Si-H bonding peak, located at ∼7 eV below the Fermi level, as a function of a-Si:H overlayer thickness, we determined that there are ∼2×${10}^{15}$ ${\mathrm{cm}}^{\mathrm{-}2}$ extra H atoms incorporated at the interface region to compensate for the large lattice mismatch at the interface. The invariance of the Si $L_2$,3 absorption edge with a-Si:H overlayer thickness indicates that the range of the core-hole exciton is less than 6 Å. The downward shifts in energy of the absorption edge in ultrathin a-${\mathrm{SiN}}_{\mathrm{x}}$:H and a-${\mathrm{SiO}}_{\mathrm{x}}$:H overlayers on a-Si:H are interpreted as optical transitions in which the photoexcited electrons in the insulator overlayer tunnel into the conduction band of a-Si:H.