Effect of interface gradient on the optical properties of multilayered porous silicon photonic structures

Abstract

The effect of gradually varying refractive index at each interface of a multilayered porous silicon photonic structure is studied experimentally and theoretically. The variation of refractive index at the interface, between any two consecutive layers, is done in small ladder-like steps (increasing/decreasing) and the resulting optical properties are compared with the standard structures with the flat interface and similar optical thickness. The proposed structures are proved to be useful in reducing the stress between the layers of high contrast, resulting in the mechanically more stable structures.

Optical characterization of one-dimensional porous silicon photonic crystals with effective refractive index gradient in depth

“…According to the results extracted by fitting the experimental spectra, the values of the n H and n L , as well as of the k H and k L are lower and they are associated with the high porosity of the H and L layers in relation to the devices PSPC1 and PSPC2. According to the results reported by many authors [28][29][30], the low extinction coefficient is mainly associated with the low interface roughness observed in highporosity layers, as well as to the absence of a porosity gradient at the H/L interface [14].…”

“…On the other hand, the experimental optical response of the 1D-PSPC reported in the literature [10][11][12][13][14][15][16] usually deviates from their simulated spectra because the anodization process generated some collateral effects such as the physical thickness shrinkage as a function of the depth of the deeper layers, an increase in porosity and interface roughness in deeper regions and corrosion of the upper layers by chemical dissolution of the silicon [12][13][14]. Because the importance of these effects, principally in lightemitting devices, such diodes, different groups have reported results about the qualitative and semiquantitative investigation of single and multilayer structures [12][13][14][15][16]. However, in most cases the experimental spectra were compared qualitatively with theoretical (simulated) spectra, losing in this way structural information 1D-PSPC.…”

Study of the omnidirectional photonic bandgap for dielectric mirrors based on porous silicon: effect of optical and physical thickness

“…To verify the mechanical stability of such structures, the surface images of the PS multilayered structure corresponding to each profile are shown as insets. The surface fractures observed on the Bragg-type structure (see inset in Figure 3c) are attributed to the high-porosity contrast between two consecutive layers [21-23]. For the sinusoidal and Gaussian refractive index profiles, the inset images (see inset in Figure 3a,b) show a flat-uncracked surface due to the gradual variation of the porosity between consecutive layers, which helps in reducing the stress and enhances the mechanical stability [21].…”

“…The surface fractures observed on the Bragg-type structure (see inset in Figure 3c) are attributed to the high-porosity contrast between two consecutive layers [21-23]. For the sinusoidal and Gaussian refractive index profiles, the inset images (see inset in Figure 3a,b) show a flat-uncracked surface due to the gradual variation of the porosity between consecutive layers, which helps in reducing the stress and enhances the mechanical stability [21]. Therefore, a significant reduction in the intensity of the reflectivity spectra observed for the Bragg-type photonic structure (Figure 3c,f), as compared to the theoretical simulations, is attributed to the cracked structure which provokes a higher dispersion of the incident light.…”

Optimization of wide-band quasi-omnidirectional 1-D photonic structures

“…The electrochemical anodization process was performed at room temperature, with an electrolyte of aqueous hydrofluoric acid (HF) (48% of wt) and ethanol (99.9% of wt) in 1:1 volumetric proportion, respectively. However, as it is not desirable to use very high porosity contrasts due to structure fragility and electrolyte diffusivity problems [36], the current densities were chosen as 35 and 305 mA∕cm 2 , with corresponding porosities of 51% and 76%, respectively. These porosities were determined through previously obtained calibration curves using a gravimetric technique [32].…”