Fabrication of broadband anti-reflective layers by mask-free etching TiO₂ films

We present a simple way to make TiO₂ anti-reflective layers on top of silicon substrates. Surfaces of TiO₂ films have been modified by radio frequency plasma with CF₄ 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 TiO₂ layers using the finite-difference time-domain method. The anti-reflective effects are attributed to random roughness on TiO₂ 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.