Demonstration of photoluminescence in nonanodized silicon (original) (raw)

20. The Role of Oxygen in the Photoluminescence of Porous Silicon: Some Recent Observations

Physica Status Solidi A, 2000

The role of oxygen in the luminescence from porous silicon is examined using a synchrotron spec-troscopic technique often known as XEOL (X-ray Excited Optical Luminescence) and associated optical XAFS (X-ray Absorption Fine Structures using photoluminescence yield). The tunability of synchrotron light permits the probing of the inner valence and the core levels of silicon and oxygen in porous silicon. These processes are associated with site specificity and probing depth variations depending on the absorption coefficient. Luminescence excited with photons in the inner valence region (15±40 eV) and at the oxygen K-edge are reported. From these results together with previous and new Si K-edge and L 3,2-edge studies, a picture of the role of oxygen emerges. Introduction Although there has been considerable work on the role of oxygen in the luminescence from porous silicon [PS], the debate continues [1±9]. It is now generally accepted that oxygen does play some role: chemical capping of the nanocrystallite (stabilizing nanocrystallites and introducing interfacial states tailing into the band gap) [1, 2] and forming SiO 2 encapsulating layers in which defects produce luminescence [6±9]. However, the details of these roles are still not fully understood. In this paper, we present new results from a synchrotron radiation (SR) induced photoluminescence study. The tunability of the SR light permits the study of site-specific photolumines-cence under favorable experimental conditions as well as variation in probing depth. This technique is often referred to as XEOL (X-ray excited optical luminescence) [10]. Using the high energy resolution of absorption spectroscopy and the significantly different spectral behavior between silicon and silicon oxide (large chemical shift), one can preferentially excite atomic Si and O sites, and one can further select chemically different sites: Si atoms bonded to oxygen and those that do not. It is also important to note that in the photon energy region of interest to porous silicon, the soft X-ray is totally absorbed by the PS specimen. This property has some ramifications. Thus it is useful to examine how the incident photon is attenuated in porous silicon in terms of the one-absorption length (1/e transmission of incident photon, 63% absorption) of Si and SiO 2 , components of porous silicon. Table 1 lists the attenuation length of the inci