Drastically increased absorption in vertical semiconductor nanowire arrays: A non-absorbing dielectric shell makes the difference

Geometrical optics, electrostatics, and nanophotonic resonances in absorbing nanowire arrays

“…Vertical arrays [1,2] of III-V semiconductor nanowires (NWs) have recently gained considerable interest as a platform for next-generation photovoltaics [3][4][5][6][7][8][9][10][11][12] and photodetection [13]. Strain relaxation in the radial direction [14] in the NW geometry allows for greater freedom in choosing lattice mismatched materials for heterostructures [15] than in thin-film devices.…”

“…Indeed, an NW array can absorb more efficiently than a corresponding bulk thinfilm [7][8][9], but screening of the incident light from the interior of the NWs can also occur, leading instead to a weak absorption [11,12]. Furthermore, strongly absorbing optical resonances can sometimes show up in the NWs [12,16,17]. Thus, there is a need to further clarify the interaction of light with NW arrays.…”

“…When we shift to the electrostatic limit where D ≪ λ, the incident light is strongly screened from the interior of the NWs, due to the large refractive index contrast between the NWs and the air exterior [12]. To continue the analytical derivation, we assume a sparse NW array with f ≪ 1.…”

Reflection measurements to reveal the absorption in nanowire arrays

“…However, the NWs are often fabricated on a native substrate that is thick and efficient at absorbing light, and therefore opaque. This allows for reflection measurements only [5,11,12]. At first glance, such measurements seem to only be able to reveal how much of the incident light is absorbed in the whole sample [5], without distinguishing between absorption in the NWs and in the substrate.…”

“…The NWs can in principle be embedded in a polymer and peeled off the substrate [12-14] to allow for an assessment of the absorption in the NWs through a combination of reflection and transmission measurements. However, the change in the dielectric surrounding of the NWs due to the polymer can alter the absorption properties of the NWs [11].Here, we study theoretically and experimentally the reflection of light from an InP NW array. We analyze the light scattering in the NW layer based on a multiplescattering description, which allows for identification of the varying reflection contributions with respect to the total reflectance.…”