A systematic study is presented of the effects of silicon dopant type, resistivity, current density, and hydrofluoric acid concentration on the formation and properties of porous silicon. Cross-section transmission electron microscopy revealed the presence of two distinct microstructures. The structure formed is determined by the doping level with the transition occurring near degeneracy. A model of the anodisation process is presented which is based on the semiconducting properties of the material and which explains the formation of the two different types of porous structure observed.
The detailed structure of porous Si (PS) layers formed in p-type wafers with resistivities 0.01-25 Omega cm has been investigated using reflectance, transmission, ellipsometry and photoluminescence techniques. Marked differences were observed in the optical properties of PS formed in degenerate or non-degenerate Si and these results are correlated with the results of other techniques. The optical techniques together with effective medium modelling have been shown to be useful non-destructive methods for either assessment of PS density or detection of unsuspected phases. The degenerate PS layers consistently showed good retention of the single-crystal characteristics of the starting wafer, only c-Si and voids being detected. For these samples, good agreement was obtained between optical and gravimetric densities. However, the non-degenerate PS had much greater variability, with greater loss of crystallinity and significant incorporation of oxygen, due to partial oxidation having occurred on or immediately after anodisation. Oxide fractions have been determined both optically and gravimetrically, with up to 50% oxide being detected in some samples. Non-degenerate PS samples with high oxygen concentrations appeared to be in the form of a chemical mixture, SiOx, from interpretation of the optical constants. Photoluminescence measurements together with the other techniques indicated a complex mixture of phases in the latter samples-voids, alpha -Si:O (and/or alpha -Si:H), an unknown amorphous phase and silicon oxide. This complex structure probably contributes to the observed instability of thick non-degenerate PS layers when heated in UHV as part of the cleaning procedure prior to epitaxial growth, all degenerate samples being able to withstand heat treatment.
Visible electroluminescence (EL) has been obtained from porous silicon cathodically biased in an aqueous electrolyte containing either the persulphate or the peroxide ion. EL efficiencies of up to 0.1% have been obtained from porous silicon formed on both n-type and p-type substrates for the application of only a few volts bias. In subdued lighting, the EL is easily visible to the naked eye at excitation densities of 0.1 W cm−2. EL is obtained only from porous silicon capable of giving photoluminescence (PL); the EL and PL spectra are broadly similar in width and peak wavelength. The EL spectra are reversibly shifted to shorter wavelengths as the magnitude of the bias is increased. In contrast with the previously reported EL under anodic conditions, this cathodic EL process does not irreversibly oxidize the porous silicon skeleton.
Depending on the dopant concentration, two distinct types of porous silicon can be formed during the anodization of silicon in hydrofluoric acid. A range of samples of both types of porous silicon has been investigated using x-ray double crystal diffraction techniques. The crystal lattice of porous silicon is found to be tetragonally distorted. In the plane of the substrate, the interplanar spacing of the porous film is identical to that of the substrate but is increased in the direction normal to it. The increase is typically 700 ppm in the type of film formed on heavily doped silicon and 6000 ppm in that on lightly doped silicon. We propose that stresses, generated by the growth of a native oxide on the surface of the pores, are responsible for the observed increase in lattice parameter. The different interplanar spacings of the two types of film are related to the observed differences in their oxygen contents which are a consequence of their different surface area to volume ratios.
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