Origin of luminescence from porous silicon deduced by synchrotron-light-induced optical luminescence

Abstract

FOLLOWING reports of intense optical luminescence from porous silicon^1,2, the opportunity for engineering optoelectronic devices using this material^3,4 has attracted considerable attention. At present, however, the question of the origin of the luminescence has not been fully resolved⁵. The quantum-confinement model^6–8 suggests that a quantum size effect gives optical transitions, and hence luminescence, in the visible range—this idea gains support from the wavelength dependence of the luminescence on porosity. An alternative model^9,10 attributes the luminescence to siloxene-like compounds¹¹ formed on the silicon surface. A third model, which invokes hydrogenated amorphous silicon as a possible source^12,13, seems to be contradicted by X-ray absorption fine structure (XAFS) studies^14–16. Here we report optical luminescence in porous silicon and siloxene induced by soft X-rays with energies near the silicon K-edge (1,839 eV). Using the luminescence together with the total electron yield, we can obtain the XAFS spectra for the luminescent sites in both materials. Our results show that the luminescence from porous silicon does not derive from siloxene (either freshly prepared or annealed), and thus suggest that the quantum-confinement model seems to provide the only viable explanation.