Oxygen Related Defect Center Red Room Temperature Photoluminescence in As Made and Oxidized Porous Silicon
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ABSTRACT Since bulk silicon does not emit light in the visible part of the spectrum, the discovery of strong visible luminescence from porous silicon has been quite surprising and has generated significant interest. This material differs from bulk silicon in that it consists of interconnected silicon nanostructures, having very large surface to volume ratios. The first emission mechanism proposed involved carrier recombination within quantum size silicon particles, but more recent work has shown that surface emission models are more likely. In this paper, an interfacial oxygen center luminescence model will be discussed, with supporting experimental data. A direct correlation between the presence of these centers and the red photoluminescence in both as-made and oxidized PoSi will also be presented. INTRODUCTION
Significant attention has recently been directed toward porous silicon due to its reported visible photoluminescence (PL) properties1 . The strong visible light emission in porous silicon (PoSi) is quite unexpected, since bulk silicon is a poor opto-electronic material, having an indirect gap where interband transitions need phonons, and resulting in radiative recombination producing light in the infrared (1.1 eV). In PoSi, Canham' attributed this red PL to quantum confinement effects in the silicon nanostructures. Several factors led to the suggested quantum confinement model, including the visible photoluminescence (PL) in the 800nm to 700nm range, PL blueshifts with increased pore widening treatments1 *2 , the broad PL emission, explained by invoking a size distribution of "undulating" wires in the porous silicon structure 1 , reports of particle sizes in the 3nm range 3'4 , and reports of two satellites of the no-phonon onset, one for absorption and one for emission in the PL in porous silicon at T=2K5 . However, all these results can also be interpreted within surface emission models, and thus cannot be viewed as unique to quantum confinement 6 . The key result necessary to the quantum confinement model linking the PL energy and the particle size does not exist, and in fact, no direct correlation has been noted experimentally 7 . Surface/interface models To address the shortcomings of the quantum confinement model, models based on surface properties and/or chemistries have been suggested. These include Hydrides/Polysilanes11, siloxene 12 , Silicon-Related Bandtail States 14 , and Oxide-Related Interfacial States 8' 9 . Although these models represent significant improvements over the quantum confinement model, those involving hydride species have serious problems when high temperature oxidation results are considered . This is because H is desorbed at these temperatures, so that the hydrogen-related species suggested as the PL source no longer exist, yet the red PL is still observed 9"10 . The most 153
Mat. Res. Soc. Symp. Proc. Vol. 405 01996 Materials Research Society
recent model suggested, which involves an oxygen-related surface localized defect similar to the non-bridging oxygen hole center (
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