Porous silicon photonic crystals: surface passivation and etch-stop effect on the optical response

Abstract

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.