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phys. stat. sol. (c) 4, No. 6 (2007) 1967
2 Materials and methods
A highly doped p + -silicon, oriented, 0.01 Ω cm resistivity, 400 µm thick was used as substrate to
realize the Thue-Morse structures. Samples were fabricated in dark light at room temperature using a
solution of 30% volumetric fraction of aqueous HF (50% wt) and 70% of Ethanol. Before anodization,
we have removed the thin film of native oxide from the silicon wafer by rapid rinsing it in a diluted HF
solution. High porosity layers, with an average refractive index nH ≅ 1.3 and a thickness d ≅ 135 nm,
were obtained applying a current density of 150 mA/cm 2 for 0.88 s. Low porosity layers, with an effective
refractive index nL ≅ 1.96 and a thickness of d ≅ 90 nm, were obtained with a current density of 5
mA/cm 2 for 0.53 s. The thickness d of each layer was designed to satisfy the Bragg condition dn= λB/4
where n is the average refractive index and λB=700 nm. Thicknesses and porosities have been estimated
by variable angle spectroscopic ellipsometry measurements on the single PSi layers.
An Y optical reflection probe (Avantes), connected to a white light source and to an optical spectrum
analyzer (Ando, AQ6315A), has been used for the reflectivity measurements at normal incidence. The
reflectivity spectra were measured between 600 and 1600 nm with a resolution of 0.2 nm.
Reflectivity measurements on exposure to volatile substances have been performed in a steel test chamber
equipped with an optical access through a quartz window and in/out channels for gas feeding.
3 Results and discussion
A scheme of the porous silicon Thue-Morse sequences realized is reported in Fig. 1: the number of the
layers increases, as 2 n where n is the Thue-Morse order, while the thickness of the devices is dSn = 2dSn-1
for n > 1. The realized samples S0-S7 have thicknesses spanning the range between 0.135 µm and 14.4
µm.
Fig. 1 Scheme of the porous silicon Thue-Morse sequences realized and characterized in the present work.
In Figs. 2 and 3 the experimental (solid line) and calculated (dash line) normal incidence reflectivity
spectra are reported for S3 (2-a), S4 (2-b), S5 (2-c), S6 (3-d), and S7 (3-e) Thue-Morse structures which
are constituted by 8, 16, 32, 64 and 128 layers, respectively. The reflectivity spectra have been reproduced
by a transfer matrix method [9] including the wavelength dispersion of silicon. At the increasing
of the layers number we can observe a worsening of the agreement between the experimental and calcu-
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