Optical properties of porous silicon surface
Introduction
Porous dielectrics or semiconductors were never considered as good optical materials, and for obvious reasons: their inhomogeneity creates the light scattering and therefore the dissipation of optical energy. In particular, the traditional antireflecting coatings for solar cells, as any other antireflecting layers, are optically homogeneous and made of materials with the refractive index lower than that of a cell semiconductor (see, for example, [1], [2], [3]). However, the development of science and technology of the nanostructured materials (i.e. materials with the structural details smaller than the light wavelength and thus characterized by the averaged optical constants), the new approach to the mentioned problem appeared [4], [5] based on the use of nanostructured materials with the components having the different refractive indexes, which gives a possibility to tune an average index to the necessary value. This paper presents investigation of one of the types of nanostructural antireflecting layers—namely, the nanoporous one. The results obtained show that these layers, although exhibit some light scattering, could essentially reduce the reflection and therefore are promising to be used in solar cell engineering.
Section snippets
Samples and methods
The nanoporous (P-Si) layers on Si crystalline samples were prepared by electrochemical etching of (100) oriented p-type silicon wafers. The HF–ethanol solution was used (HF:H2O:C2H5OH=1:1:2), anodization current density between 5 and 75 mA/cm2 and the fixed etching time 10 min. An example of the surface structure of a sample thus obtained is shown in Fig. 1 (AFM image showing the details of quasi-pyramidal shape with the size of 20–60 nm). Depending on the etching current, the details of the
Results and discussion
The data of reflection measurements from the P-Si layer on c-Si surface made at normal light incidence are presented in Fig. 3. The upper spectrum (squares) refers to the specular reflection; the specular reflection for other angles of incidence is not much different, having the same order of magnitude (between 5 and 8%). It is necessary to remind that reflection of the visible light from pure c-Si surface is of order of 35% due to large refractive index of the material (varies from 5.5 at 400
Conclusions
Simple method of estimation of the optical parameters of porous semiconductor layer on the balk material of the same kind is given. The results of calculation and measurements show that the classical approach to optical materials as highly homogeneous ones might be questioned on the basis of the achievements and ideas of the nanotechnology, which give opportunities to tailor the material up to the specific needs and applications.
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On sabbatical leave from CINVESTAV-IPN.