Origin of porous silicon photoluminescence: Evidence for a surface bound oxyhydride-like emitter

Abstract

Time-dependent excitation spectroscopy coupled with quantum chemical calculations is used to demonstrate that the photoluminescence (PL) resulting from the ultraviolet optical pumping of an etched porous silicon (PS) surface results from a silicon oxyhydride-like fluorophor bound to the PS surface. The time-dependent PL, in both aqueous (${\mathrm{H}\mathrm{F}/\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{H}\mathrm{F}/\mathrm{C}\mathrm{H}}_{3}{\mathrm{O}\mathrm{H}/\mathrm{H}}_{2}\mathrm{O}$) and nonaqueous [MeCN/HF (anhydrous)] etching media, has been monitored both in situ, during the etching cycle and before the PS sample is removed from the etching solution, and ex situ, after removal of the PS sample from the etching solution. The early appearance in time of the PS luminescence is consistent with the formation of a surface bound emitter created on a time scale $(l~10\mathrm{s})$ much shorter than that needed for pore formation. Laser excitation spectra (PLE) over the wavelength range extending from 193 to 400 nm produce an almost identical time-dependent PL emission feature between 550 and 700 nm. Influenced strongly by the chemical composition of the etch solution, an intermediate ``green'' emitter can be excited with select laser pumping wavelengths and observed to transform to the final ``orange-red'' luminescent product. In conjunction with experiments whose focus has been to compare the time-dependent PL after ArF (193 nm) and ${\mathrm{N}}_{2}$ (337 nm) laser excitation (PLE), the data suggest the pumping of an excited-state manifold for a molecule-like species followed by rapid relaxation via nonradiative transitions down the manifold and the subsequent emission of radiation at much longer wavelength. Detailed quantum chemical modeling supports this interpretation and suggests a correlation to changes in the bonding associated with electronic transitions that involve silanone-like ground electronic singlet states and their low-lying triplet excitons. Especially important are those changes involving SiO related bonds. A substantial shift in the excited-state manifold, relative to the ground state, correlates with the character of the observed PL spectra as the excitation to a manifold of states greatly shifted from the ground electronic state produces a considerable redshift of the PL spectrum $(\ensuremath{\sim}600--800\mathrm{nm})$ compared to the known peak wavelength of the PLE (excitation) spectrum at 350 nm. The combination of quantum chemical modeling and time-dependent spectroscopic studies also suggests that the multiexponential PL decay commonly observed as a function of increasing wavelength (550--750 nm) after excitation at 355 nm results primarily from nonradiative cascade. The optical detection of magnetic resonance (ODMR) spectrum obtained for PS and associated with a triplet exciton is assigned to an oxyhydride-like emitter possessing silicon-oxygen and silicon-hydroxide fluorophors similar to the much more complex annealed siloxene. Calculated infrared spectra are correlated with experimentally observed features and are consistent with a surface-based oxyhydride-like emitting fluorophor. A recent analysis that associates the linewidth of the triplet ODMR spectrum with an inhomogeneous distribution of quantum confined crystallites is shown to be in error. We demonstrate that the correct extension of the arguments used in this analysis provides clear evidence for the existence of a common radiative center associated with a molecule-like species bound to the surface of the PS framework. The results obtained in this study are thus not consistent with quantum confinement and suggest a surface bound emitter as the source of the PS photoluminescence.