For a flat surface having an AR overlayer, using Fresnel’s reflection formula, we measured the

reflectance at different wavelengths. It is observed that with varying film thickness, the position of the reflection minima shifts, while a change in the refractive index modifies buy Torin 1 the amount of surface reflectance [25]. Although similar trends are quite evident, the experimentally observed average surface reflectance turns out to be much lower over the spectral range under consideration. In order to explain these results, let us first try to understand the role of the Si template which is practically an ensemble of ion beam-fabricated self-organized conical nanofacets at the top of the Si substrate. It is known that grating on any surface can be used to achieve arbitrary refractive index if the geometry of the grating structures can be tuned. For instance, if we consider a binary grating, its effective refractive index, n eff, can be expressed as n eff = (n 1 - 1)DC + 1, where n 1 is the refractive index of the grating and DC is the duty cycle and is defined as the ratio of the grating

line width to the grating period [26]. If the surrounding medium is taken as air and the grating is of the same material as the substrate, the optimized duty cycle (to meet the AR criterion) can be expressed as where n 2 is the refractive index GDC-0199 cell line of the substrate [26]. Such binary gratings are expected to exhibit the AR property over a very narrow spectral range. This range can be broadened by continuous tuning of the refractive index (n eff) between the two surrounding media. This would essentially mean

a continuous change in DC along the depth (from the apex towards the base of the facets) of the grating lines, which is possible to be achieved by having tapered/conical gratings. When the grating and the substrate materials are the same, the matching of refractive index at the substrate interfaces can exhibit highly Celecoxib improved AR property [27]. This explains the enhanced AR performance observed here for the faceted Si surface formed on the Si substrate. Following the same argument, further improved AR performance is expected due to the conformal growth of an AZO overlayer on nanofaceted Si template. Indeed, the experimental findings confirm the same where increasing AZO thickness leads to a systematic red shift in the reflection minima. However, such small variations in the thickness may not be sufficient to cause any significant difference in depth-dependent change of the effective refractive index for the AZO-coated faceted Si template which corroborates well with the experimentally measured reflectance minima values.