Resumo:
The aim of this study was to investigate the influence of the etch-stop time on the structural
and optical properties of multilayer structures and their passivation to avoid aging
effects. Structural analysis by scanning electron microscopy (SEM) and the optical reflectance
data fitting procedure have shown that the inclusion of a pause during the
growth of the porous matrix promotes the formation of thicker layers (2638 nm, for 10 s
of pause), compared with the layer with low porosity without pause (2584 nm). Similar
behavior was observed for high porosity layers changing from 4500 to 4780 nm. This trend
was also observed for multilayer structures. As for porosity, an opposite behavior was observed,
decreasing in about 9.5% and 6.5% with increasing etch stop time for single layers
with low and high porosity, respectively. This phenomenon was attributed to the recovery
of hydrofluoric acid consumed during pore formation at the electrolyte-silicon interface.
To obtain 1D photonic crystals (1D-PC) of porous silicon with optical responses close
to those projected, this phenomenon must be taken into account in device development.
The fabrication of 1D-PC with a sequence starting with a layer with high porosity and
others where the first upper layer has low porosity proves that the increasing effect of
thickness and the decrease of porosity does not depend on the stacking order. However,
the thermal treatment made in air environment shows significant changes in the optical
response after the oxidation at 400°C. Since the oxidation of porous silicon depends on
the characteristics of the porous matrix, it is concluded that the etch-stop promotes the
formation of high and low porosity layers with different microstructures, so that after
thermal annealing at 1000°C in devices with the first upper layer with high porosity, the
main photonic band gap (PBG) is destroyed, i.e., the optical thickness of the high and
low porosity layers no longer obeys Bragg’s law due to the contraction-expansion effect
of the low and high porosity layers.
Three different materials were used for surface passivation: thermal silicon oxide (SiO2),
gold (Au), and titanium oxide (TiO2). Despite the passivation layer, the presence of these
elements resulted in a blue shift of the PBG. However, in the case of the deposited TiO2,
some samples showed a red shift, while in others the PBG is not changed. The red shift
was associated with the presence of xylene in the sol-gel TiO2 within the pores. Unaltered
PBG was associated with the formation of a thin pore-sealing TiO2 sol-gel layer. After
thermal treatment, the typical blue shift was observed. Despite the passivation material,
fast Fourier infrared spectroscopy (FTIR) reveals the presence of SiO2 in addition to the
Au or TiO2 phases. Energy dispersive X-ray spectroscopy analysis (EDS) showed that
Au and TiO2 were deposited in a deep concentration gradient, but with a homogeneous
distribution along the sample surface. X-ray diffraction (XRD) showed that after thermal
treatment at 450°C, the rutile and anatase phases coexist, with the latter predominating.
The stability of the optical properties after passivation was confirmed by measurements
of the samples after 20 and 36 weeks of storage. No spectral changes were observed in the
PBG position.