The Visible and the Infrared Luminescence Bands as a Tool for Characterization of the Porous Silicon Bandstructure
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The visible and the infrared photoluminescence bands in porous Si have been studied at low temperature for two series of samples: one in which the size of the crystallites has been varied and another in which the degree of surface degradation has been changed. It is shown that the relation of the two bands can be explored for characterization of the porous Si bandstructure. The size- and the surface dependence of the valence band and of the conduction band related states is discussed. A model is proposed for explanation.
The optical properties of porous Si, with size of the the crystallites < 100 A and an internal surface area > 100 m2/cm 3 , change drastically in comparison to those of bulk Si. The absorption edge shifts to the blue on a scale of electronvolts [1]. Most striking is the observation of six to seven orders of magnitude enhanced room temperature photoluminescence, tunable in the visible spectral region [2]. On the way to understand the properties of this novel material there are several complications: - The shape of the crystallites is not defined and their size is dispersed. Therefore the determination of an absolute crystallite size is a difficult task. -The surface of the crystallites is not ideal. Its perturbations are manifested in the inhomogeneous broadening of the IR vibrational modes [3] and in the existence of paramagnetic surface defects [4]. The appearance of shallow and deep surface states affects both the absorbtion edge and the PL [5]. -In contrast to the absorption edge of semiconductor nanocrystals with direct gap like CdSe and
CdS, the absorption edge of the porous Si is featureless [5]. The most likely reason for this is the indirect gap of the small Si crystallite, as predicted by recent theory [6]. However, the optical properties of porous Si are reproducible and their changes systematical. Moreover, a puzzling similarity exists between the PL of porous Si and colloidal Si nanoparticles
[7,8].
To extract information about the physics of the porous Si while bypassing some of the difficulties listed above, we explore the relation between the energy position of the visible and the infrared band to the conduction band- AEC and the valence band shift AEV in respect to the Si bulk values, as it was discussed in [10, 5,11 and 12]. As first reported by Pickering at al [9] and known from many later publications [2, 13-21] the two emission bands are easily observable at low temperature. Very important is the fact that they have the same excitation spectra [15] showing that they originate from the same ensemble of crystallites. Here we try to separate the effects of the crystallite size from that of the surface state density on both the energy position and the intensity of the two bands. The following samples were prepared for that purpose. To vary the size a series of samples was made by electrochemical etching of p- (100) Si-substrates at - lmnAcm (initial crystallite size - 100A [26]) and poststripping the skeleton down to the smallest possible size for which the skeleton still forms
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