Abstract
Achieving a broadband antireflection property from material surfaces is one of the highest priorities for those who want to improve the efficiency of solar cells or the sensitivity of photo-detectors. To lower the reflectance of a surface, we are concerned with the study of the optical response of flat-top and patterned-topped cone shaped silicon gratings, based on previous work exploring pyramid gratings. Through rigorous numerical methods such as Finite Different Time Domain, we first designed several flat-top structures that theoretically demonstrate an antireflective character within the middle infrared region. From the opto-geometrical parameters such as period, depth and shape of the pattern determined by numerical analysis, these structures have been fabricated using controlled slope plasma etching processes. In order to extend the antireflective properties up to the visible wavelengths, patterned-topped cones have been fabricated as well. Afterwards, optical characterizations of several samples were carried out. Thus, the performances of the flat-top and patterned-topped cones have been compared in the visible and mid infrared range.
Highlights
Over the last few years, antireflective gratings have been widely investigated with the emergence of photovoltaics and photosensing
While the first level is efficient for the 3-5 μm wavelength range, by scaling down the geometric parameters of the second level of periodicity following a ratio of M/2P, it is possible to decrease the reflection in the visible region [Fig. 9]
The main objective was to demonstrate an antireflective coating with less than 2% reflectance for the 0.45-0.75 μm and 3-5 μm wavelength bands
Summary
Over the last few years, antireflective gratings have been widely investigated with the emergence of photovoltaics and photosensing. Periodic structures allow a more tunable antireflective band and their optical properties are easy to compute through electromagnetic analysis They are slightly less effective in terms of antireflective efficiency than the former ones [15]. It has been demonstrated as well that adding a second periodicity level by patterning the top flat pyramid allows theoretically to extend the antireflective band down to the 0.7-2.5 μm region [17]. This idea came from the fact that those structures have a constant behavior when a scale factor (multiplying all the geometrical dimensions with the same value) is applied. Performances of the patterned-topped cone gratings have been compared with the flat-topped ones
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