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OBSERVATION OF PARAMAGNETIC SILICON DANGLING ORBITALS IN LUMINESCENT POROUS SILICON

F. C. Rong,* E. H. Poindexter,* J. F. Harvey,* D. C. Morton,* R. A. Lux,* and G. J. Gerardi ** *Army Research Laboratory, Fort Monmouth New Jersey 07703 "**William Paterson College, Wayne, New Jersey 07470

ABSTRACT 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=Si 3 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.

INTRODUCTION The discovery of highly efficient photoluminescent porous silicon'. 2 has generated a great deal of interest in the scientific community, mainly because of potential realization of siliconbased visible photonic devices. The underlying physical mechanism of the photoluminescence (PL) has not yet been established. Nonetheless, one of the most viable theories 2 .3 proposes that PL is from electron-hole-pair recombination in a quantum-confined silicon crystalline nanostructure (crystallites), thereby emitting visible radiation. To further improve or maintain the efficiency of the material, competing non-radiative recombination processes should be studied. A universally accepted process is that electron-hole pairs may recombine non-radiatively at silicon dangling orbitals (often called dangling bonds) on the imperfectly terminated surface of crystalline nanostructures; thus, understanding these silicon dangling orbitals is important. In order to determine the role of dangling orbitals in non-radiative recombination processes, identification of such dangling orbitals should be firmly established. Electron paramagnetic resonance (EPR) spectroscopy is a powerful spectroscopic tool to study microscopic electronic structure of silicon dangling orbitals, such as a Pb center (.Si--Si 3, i. e., a silicon atom with dangling orbital, back-bonded to three other silicons) at the planar Si/SiO2 interface. 4 There 5have been several EPR studies on paramagnetic centers in photoluminescent porous silicon, "7 in most cases in parallel with other studies; detailed EPR analysis was not given, even though observation of dangling silicon orbitals has been generally claimed. Quite recently, a well-resolved anis