Anisotropy of the Porous Silicon Photoluminescence
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Mat. Res. Soc. Symp. Proc. Vol. 452 0 1997 Materials Research Society
In the present work we show how the anisotropy in the orientation of the crystallites produces the angular dependence of the PL polarization degree in a surface plane of the PS sample. We study PL polarization of the samples made by anodization of the Si wafers of the various crystalline orientation to check whether the shape and orientation of the Si nanocrystals correlate with the crystalline axes. For (110) sample such correlation has been observed. We found that illumination during the anodization of PS changes the polarization properties of the PL. Conventional unpolarized illumination quenches the polarization memory effect in PL, while the PS prepared with polarized light-assistance shows angular dependence of the p. Finally we concern the contribution of the electronic states of the Si crystallites to the PL polarization. Under resonant excitation of (100) oriented sample p enhances and becomes dependent on the orientation of the polarization vector of the exciting light with respect to the crystalline axes. This effect cannot be explained in a frame of the electromagnetic theory [9,10] only and the correct description obviously requires the consideration of the electronic state symmetry of the Si nanocrystals. EXPERIMENTAL RESULTS AND DISCUSSION Structural investigations show that porous Si retains to some extent the initial crystalline structure of the Si wafer. The etching rate of Si is different for various crystalline directions. That causes the well-defined orientation of the pores and silicon crystallites in a porous layer correlated with the crystalline axes of the wafer [12,13]. Therefore, the crystalline orientation of the substrate determines the porous Si morphology. The PS samples are prepared from the (100), (110), and (111) Si wafers. p-type Si with a resistivity of 1-3 K1cm is used, the etching is performed in the dark. The details of the sample preparation and the optical setup can be found elsewhere [9]. The PL is excited by the linearly polarized light from the HeCd laser (442 nm). The sample is rotated to vary the polarization di-
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Figure 1. Polar diagram of PL polarization in porous Si prepared on the substrates of the various crystalline orientations. p in (100) and (111) samples is isotropic. The maximum value of p in (110) sample corresponds to the (1001 axis.
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rection of the excitation in a sample plane. The (110) surface has a low symmetry with the [100] and [110] axes in the plane. In these samples we observed the two-fold symmetry in the angular dependence of the PL polarization degree, as depicted in Figure 1. It is assumed that the direction of the maximum value of p corresponds to the preferential orientation of the elongated crystallites. This result implies that the elongated crystallites or the silicon wires in the (110) porous layer are
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