Electrochemically controlled fabrication of lightly doped porous Si nanowire arrays with excellent antireflective and self-cleaning properties

The doping level and morphology of porous Si nanowire (SiNW) arrays are critical for their electrical, optical and surface properties, but preparation of lightly doped porous SiNW arrays with uniform length still remains a challenge. By integrating electroless chemical etching with a proposed electrochemical route, lightly doped single-crystalline porous SiNW arrays with uniform length were prepared from Si wafers with resistivity of 1-770 Ω cm for the first time. Scanning electron microscopy and transmission electron microscopy images show that the size of pores in the NWs is enlarged by increasing the duration of electrochemical process. Based on current-voltage measurements, thermionic emission is proposed to be responsible for the pore formation mechanism. The photoluminescence spectrum of the porous SiNWs shows an obvious peak centered at 680 nm, which is attributed to the quantum confinement effect due to porous structures, evidenced by the shift of Raman peak from 520.7 to 519.7 cm−1. Reflectivity spectra show the average reflectivity of SiNW arrays after electrochemical treatments was further decreased to less than 1.8% in the 350-600 nm wavelength range. Contact angle measurements show that the porous SiNW arrays possess superhydrophobic characteristics with a contact angle of up to 164°. Compared with previous studies, the proposed integrated route can not only prepare uniform, lightly doped, and porous SiNW arrays, but also provide an efficient way to independently control the lengths of NWs and sizes of nanopores in them. The SiNW arrays can be employed as an excellent antireflective and self-cleaning substrate for high efficiency opto-electronic devices.