Photoexcitation of Si-Si Radiative Surface States in Nanocrystallites
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M.H. NAYFEH, N. RIGAKIS, Z. YAMANI Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL, USA ABSTRACT Recently, intrinsic localized excitonic radiative surface states belonging to interacting dangling bonds of expanded, unstrained Si-Si dimers on the surface of nanocrystallites have been predicted. Those are connected, via a potential barrier, to the nonradiative delocalized excitonic states on the unexpanded, strained dimer. We present a theoretical analysis of the photoexcitation pathways involved in populating the radiative states. We include both direct above barrier, and indirect excitation from the photoexcited delocalized excitonic states via quantum tunneling and thermal activation. Our calculation gives an enhancement in the efficiency for sizes below a critical size of 1.4 nm, the size for which the outer trapped states become stable against tunneling and thermal activation. It is as if the material were changed from an indirect gap to a somewhat direct gap material. The results show that the photoluminescence exhibits a large Stokes shift resulting from the expansion of the radiative dimers. Moreover, the emission bandwidth is found to be quite wide especially for the ultra small crystallites (0.8 nm) where it encompasses nearly all of the visible spectrum, and the near infrared. Spectra simulated over size distributions are presented. INTRODUCTION Recently, the existence of intrinsic localized surface states in silicon nanocrystallites [1,2] which might behave as luminescent systems has been presented. They were proposed as a contributor to the optical effect of porous silicon [3-9] especially for very small nanocrystallites where quantum confinement becomes more pronounced. In the model, the optical activity is produced by interacting dangling bonds in the form of expanded, unstrained Si - Si dimers bonds. The structure, absorption, emission, and time behavior of those depend on the confinement energy (on size) [1,2]. Radiative surface-related states have been suggested in the past [3], but there has been no information about their nature or origin. The new studies [lI]demonstrated, using empirical tight binding and first principle local density calculations, that such states indeed exist under the form of "self-trapped excitons", namely Si-Si dimers on the surface of nanocrystallites. Those are stabilized because of the widening of the gap induced by the quantum confinement. The study of the mechanism for accessing and populating these states is important for comparison with experiment[0-1 1]. We discuss inthis article the various pathways for excitation of and emission from these states. We include both direct above barrier excitation, and indirect excitation from the photoexcited delocalized excitonic states via quantum tunneling and thermal activation. Our calculation give a dramatic enhancement in the efficiency for sizes below a critical size of -1.4 rim, the size for which the outer states become stable against tunneling. It is as if the material were
changed from an in
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