Enhanced Wide-band Infrared Absorptivity of Black Silicon

Infrared radiative properties of materials are of fundamental significance for several applications involving thermal radiation conversion and management such as sensors, thermo-photovoltaics, thermal rectification, thermal memories, thermal logical circuits and radiative cooling[1], subsequently, attracting an increased attention in recent years [2]. In the present work, we report, on the exceptionally high absorptivity of Black Silicon (BSi) in the spectral range of thermal radiation, which can be instrumental for various thermal radiation related applications. Black Silicon (BSi) is a bottom-up nano-structured silicon surface that can be obtained by different techniques [3] including wafer-level cryogenic plasma etching [4]. It is well known since [5] that such nano-scale features lead to a significant enhancement of silicon absorptivity in the visible range [4]. Having previously shown [6][7] that we can extend such outstanding properties to the mid-infrared (MIR) by using highly doped silicon, in this work we report that the high absorptivity in fact further hinges on to the far-infrared(FIR) range. Having fabricated two wafers of BSi having n-type doping of 2x1018 cm-3 (standard doping) and 4.5x1019 cm-3 (high doping), we have found experimentally and confirmed by electromagnetic numerical simulations that for highly doped BSi, a high absorptivity is observed till 15  μm which has also been compared with similarly doped Flat Si(FSi) samples. However, beyond 15  μm, the absorptivity of highly doped BSi sample shows a growing tendency to decrease. Subsequent processing of SEM images reveals that these noteworthy radiative properties can probably be attributed to particular morphological features of heavily doped BSi at the nano-scale.

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