The Spectroscopy of Porous Silicon
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P. D. J. CALCOTT, K. J. NASH, L. T. CANHAM and M. J. KANE. Defence Research Agency (RSRE), St. Andrews Road, Great Malvem, Worcs. WR14 3PS, UK.
ABSTRACT The spectroscopic evidence that the main visible photoluminescence (PL) band of porous silicon (the 'S' band) originates from quantum confined crystalline silicon is presented, and arguments that claim to invalidate this evidence are analysed in detail. We find that a careful study of all the spectroscopic data provides strong support for the quantum confinement model. Additionally we consider the interesting issue of the luminescence spectrum of a single silicon quantum dot.
1. Introduction Electrochemically etched porous silicon was found in 1990 to give efficient visible photoluminescence (PL) at room temperature'. The decrease of the wavelength of this PL band (the 'S' band) compared to the bulk-Si value was attributed to an energy gap increase due to quantum confinement'. This initial proposal has since been supported by first principles calculations, by data from a range of experimental techniques, and, in particular, by spectroscopy. The spectroscopic evidence supporting the quantum confinement model is summarised in section 2. In section 3, disputes concerning the interpretation of the spectroscopic data are discussed. This discussion extends the briefer account of Ref. 2 by presenting new experimental and theoretical results. In section 4 the predicted PL spectrum of a single quantum confined crystalline silicon dot is described. 2. The Spectroscopic Evidence For Quantum Confinement In Porous Silicon Transmission electron microscopy (TEM) has demonstrated that the main constituent of luminescent porous silicon is undulating wires of crystalline silicon with mean diameter -30A (Ref. 3). First-principles electronic structure calculations showed that quantum confinement in such small crystalline silicon structures could explain the reduction in PL wavelength 4 . The high PL efficiency of the S-band is attributed primarily to the low rate of non-radiative recombination that arises from good surface passivation and from the suppression of exciton migration by localisation" 3 4, . Detailed PL spectroscopy of highly porous silicon has verified the quantum-confinement theory of the S band ". The observation of momentum-conserving phonon satellites of crystalline Si has shown that, for the S band, crystalline silicon is the luminescent material. The luminescent state, even at room temperature, is a localised exciton. The exchange splitting Ex of this exciton is manifested both in the temperature dependence of the PL lifetime, and as an energy gap in the resonantly excited PL spectrum5 '7 . To understand the PL of porous silicon, we must consider the optical transitions of crystalline Si when the law of conservation of crystal momentum in this indirect-gap 465 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
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