Fabrication of broadband anti-reflective layers by mask-free etching TiO films.

We present a simple way to make TiO anti-reflective layers on top of silicon substrates. Surfaces of TiO films have been modified by radio frequency plasma with CF as an etchant. Mask-free etching process on the polycrystalline films leads to the formation of random sub-wavelength textures. The reflection of the etched samples are significantly suppressed in the wavelength range of 400~800 nm (2.9~4.6%, 3% compared with 34% on bare silicon at the wavelength of 600 nm). We have numerically simulated the optical properties of TiO layers using the finite-difference time-domain method. The anti-reflective effects are attributed to random roughness on TiO surfaces. The etching porcess increases the surface roughness, therefore, the gradient of refractive index between air and silicon substrate is reduced. As a result, the Fresnel reflection is supressed. Our results demonstrate an efficient way of anti-reflective coating for solar cells.