Selectively grown vertical silicon nanowire p-n ⁺ photodiodes via aqueous electroless etching

A facile method to selectively grow vertically-aligned silicon nanowires (SiNWs) which can inherit the doping concentration from its mother wafer, with controllable length, is demonstrated using the combination of photolithography and aqueous electroless etching. The use of SU-8-2002, a chemically and mechanically robust photoresist (PR) material, provided a high selectivity for the etching reaction on the exposed surface of 1-μm-thick n ⁺ doped p-type (1 0 0) Si substrate, resulting in the fabrication of ∼30-μm-long vertically-aligned SiNW photodiode arrays on the desired locations, while the areas covered with SU-8-2002 remained unreacted. Optical and field emission scanning electron microscope analyses confirmed that SiNWs were selectively grown while retaining the shape of the PR patterns. The electrical and optical measurements of the fabricated p-n ⁺ junction SiNW photodiodes were compared to those of reference planar p-n ⁺ junction Si photodiodes: the current density of the p-n ⁺ junction SiNW photodiodes was approximately 3 times greater than that of the planar counterpart at the forward bias of 5 V, which can be attributed to the high density of defect states on the rough surfaces of the synthesized SiNWs, leading to the increased recombination efficiencies for the injected carriers. In addition, the photoresponse of the p-n ⁺ SiNW photodiode arrays was 3.4 times higher than that of the planar device at -3.5 V due to the increase in the light scattering.