Silício poroso obtido por ataque químico / Porous silicon obtained by chemical etching

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

This work reports on a detailed study of porous Si layers produced by chemical etch in p+-type (001) Si wafers with solutions of HF:HNO3 at concentrations of 50:1 and 500:1 added with NaNO2 diluted at 0.1 g/L. Sets of samples with etching times from 1 to 30 min were fabricated. Porous layers etched with the 500:1 solution during 1 to 10 min were found to be much more homogeneous, and for this reason, became the main object of study. The layer thickness determined by scanning electron microscope cross-section images varied from 1 to 4 µm, and the porosity obtained by grazing incidence x-ray reflection oscillated between 35 and 55%. Atomic force microscopy analysis revealed the coexistence of at least two ranges of crystal sizes, between 35 and 150 nm and from 9 to 15 nm, distributed both in the top and in the deeper planes of the layers. This behavior is associated with a competitive and simultaneous process during de porous Si formation, also confirmed by scanning electron microscopy images. Fourier transform infrared spectra exhibited several absorption bands corresponding basically to superficial chemical bonds with hydrogen, SiHx, and oxygen, Si-O, and demonstrated the reproducibility of the chemical etching. Raman spectra were measured from 300 to 8000 cm-1 and showed a narrow peak at 520 cm-1 relative to the crystalline silicon and a large symmetrical band centered around 4500 cm-1 corresponding to the photoluminescence response. Quantitative information about the crystalline and amorphous phases of the layers was obtained from calculations using the phonon confinement model and a best fit procedure to the measured spectra. From these analyses, the correlation length of the crystallites reduced from 13 to 9 nm and the Raman line shift increased from 0.9 to 1.5 cm-1, which corresponds to residual stress values of 250 to 400 MPa, as a function of etching time. An increase in the correlation length and a decrease of the residual stress were observed after a natural oxidation process during 750 days. The amorphous phase demonstrated to be always negligible in relation to the crystalline one. The shift to higher energies and the increase in intensity of the photoluminescence band relative to the as-produced samples agreed with the decrease of the correlation length obtained from the Raman analysis. High-resolution xray diffraction spectra showed that the lattice of the crystals that compose the porous layers is distorted with a distribution of perpendicular tensile strain induced by a compressive stress in the layer plane due the presence of pores. From the fit of the more resolved spectra with a set of Voigt and Gaussian curves, three strain ranges at 4 - 9×10-4, 1,0 2.5×10-3 and 3.5 6.0×10-3 were found as a function of etching time. Stress values in the order of 102 MPa were estimated for the higher strain range, in agreement with the values obtained from Raman spectroscopy for crystallite sizes of 10 nm. Therefore, these results indicate that the strain is related to three populations of crystallite sizes that coexist in the layers. Finally, reciprocal space maps showed that all distortion in the porous Si obtained by chemical etches lies in the lattice parameter, with almost no sign of mosaicity.

ASSUNTO(S)

engenharia e tecnologia espacial espectroscopia raman infrared spectroscopy raman spectroscopy silício poroso porous silicon difração de raios x espectroscopia infravermelho characterization caracterização x ray diffaction

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