Large and irregular shift of photoluminescence excitation spectra observed in photochemically etched porous silicon

Abstract

Photoluminescence excitation ͑PLE͒ spectra of porous silicon have been studied for samples subjected to postanodization photochemical etching in a HF solution. It is shown that the peak energies of the PLE spectra show a large shift of about 1 eV with increasing etching time. Furthermore, the manner in which the PLE spectra shift is irregular; they first move toward lower energies, and then back to higher energies with increasing etching time. This behavior is interpreted by assuming two independent excitation mechanisms competing with each other in luminescent Si nanocrystallites: a direct-gap absorption process similar to the ϳ4-eV absorption in bulk crystalline Si and the direct excitation of localized states in the crystallites. The latter excitation mechanism is more important in efficiently luminescent samples.

Optical Properties of Si Nanocrystals

“…First, a comparison between the porous Si``absorption edgeº and PL peak position has been widely discussed in the literature (see for instance [23,31]). An experimentally found variation of s" hw ex Y " hw det within five orders of magnitude shows that for indirect-gap nanocrystal assemblies such a correlation obviously can never be observed.…”

Activation of blue emission from oxidized porous silicon by annealing in water vapor

“…This behavior is most consistently explained on the basis of a quantum size effect. 1,5 On the other hand, the assumption of strongly localized states may be required in explaining some other important properties of this band: the broad and Gaussian line shapes of PL excitation and emission spectra, 3,6,7 a large Franck-Condon shift ͑Ͼ1 eV͒, 6,7 and the nonexponential PL decay with an emission-energy-dependent decay constant 8 ͑this may not be attributed to any structural inhomogeneity͒. 9 All of these observations can be accounted for by our donoracceptor pair recombination model 9,10 where one of the carriers is localized strongly and the other weakly in a Si crystallite.…”

Photoluminescence properties of porous silicon layers prepared by electrochemical etching in extremely dilute HF solutions

“…Since the basic structure of highly luminescent PSi is a random network of nanosize Si skeletons, quantum confinement of carriers is suggested to play a vital role in the visible PL of this material [6]. Evidence of deeply localized states has also been observed experimentally in airexposed samples [7], and the effect of double-bonded oxygen has been discussed [8]. PSi-based light emitting diodes now exhibit efficiencies higher than 1% [9].…”