Some Perspectives on the Luminescence Mechanism via Surface-Confined States of Porous Si
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SOME PERSPECTIVES ON THE LUMINESCENCE MECHANISM VIA SURFACE-CONFINED STATES OF POROUS SI
F. KOCH, V. PETROVA-KOCH, T. MUSCHIK, A. NIKOLOV, GAVRILENKO Tech. Univ. Munich, Physik-Dept.E16, D-8046 Garching, Germany
AND V.
ABSTRACT We elaborate on the explanation of radiative emission involving surface-confined states on the nanocrystallites of porous Si. After reviewing the evidence for the existence of such states, we give a model description of the origin of visible luminescence in porous Si. The model accounts for different spectral luminescence bands with distinct energies and relaxation times. INTRODUCTION The mechanism for the photoexcited radiative emissions from porous Si is currently a hotly debated issue. Models generally are argued along three different lines schematically shown in fig.1. The line of thought, first argued by Canham [1], explains the emission in terms of radiative recombination of quantum-confined electrons and holes in columnar structures. More recently it is excitonic recombination in the undulating wire of fig.1 [2]. This concept has received supporting evidence in the work
Fig.1 The three classes of models discussed in current literature on the luminescence mechanism in porous Si (left - excitonic core state recombination in undulating cylinders; middle - trapping and emission via surface localized states; left - the luminous molecular-agent (siloxene) model). of Calcott et al. [3] where phonon-assisted radiative processes have been identified. The phonons are those necessary in crystalline Si to couple the electron valley states near the X-point, to r-point hole states. These observations are prima facie evidence for the involvement of crystalline Si in the emission process. Other authors seek to relate the emission to specific molecular agents. These luminous agents are variously argued to be polysilane [4] formed on the internal surfaces of the pores, or a molecular complex of Si, 0 and H known as siloxene (Si 6 03 H ) and its derivates [5,6]. The molecule has been resurrected from historical sources and made to provide answers for the many questions that the porous Si experiments have raised. Mat. Res. Soc. Symp. Proc. Vol. 283. ©1993 Materials Research Society
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The third category of explanation invokes surface-localized states into which the elementary excitations are first trapped prior to recombination. Proponents of this line argue that the irregularly-shaped small crystallites are not perfectly passivated. Reconstructed and disordered Si in the boundary layer provides electronic states which lower the energy and further confine the electron and hole. In analogy with the wellstudied luminescence from amorphous Si (a-Si:H) such carriers in bound states recombine radiatively. A paper by Xie et al. [7] first makes reference to luminescence via unspecified surface states. The suggestion is echoed in many other recent publications [8,9]. We have presented our model in ref. [10] and elaborate on it here, especially in view of current developments. Evidence presented to date,
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