Strong Geometrical Dependence of the Absorption of Light in Arrays of Semiconductor Nanowires

We demonstrate experimentally that arrays of base-tapered InP nanowires on top of an InP substrate form a broad band and omnidirectional, absorbing medium. These characteristics are due to the specific geometry of the nanowires. Almost perfect absorption of light (higher than 97%) occurs In the system. We describe the strong optical absorption by finite difference time-domain simulations and present the first study of the influence of the geometry of the nanowires on the enhancement of the optical absorption by arrays. Cylindrical nanowires present the highest absorption normalized to the volume fraction of the semiconductor. The absolute absorption in layers of conical nanowires,is' higher than that in cylindrical nanowires but requires a larger volume fraction of semiconducting material. Base-tapered nanowires, with a cylindrical top and a conical base, represent an intermediate geometry. These results set the basis for an optimized optical design of nanowire solar cells.