Observation of Paramagnetic Silicon Dangling Orbitals in Luminescent Porous Silicon

We have detected two dominant paramagnetic centers in porous silicon by electron paramagnetic resonance (EPR). One of them is isotropic, assigned to a defect in amorphous silicon oxide in the porous silicon layer. The other is anisotropic, and is very much like a Pb center at a planar Si/SiO2 interface. This EPR center is unambiguously identified as an •Si≡Si3 moiety, a silicon with dangling orbital, back-bonded to three silicon atoms, by 29Si hyperfine structure (HFS) associated with the dangling orbital, and 29Si superHFS from three neighboring silicon atoms, as similarly observed in the usual planar surface Pb structure. The dangling orbitals are highly localized and heavily p character. The disposition of dangling orbitals is evidence that the skeletal structure of luminescent porous silicon is crystalline and has a lattice which is aligned and continuous with the wafer substrate. The possibility that these centers are the major photoluminescent killers or quenchers is not supported by our hydrogen annealing experiments.