Fig. 1: Properties and features of the studied ZnO/Si and TiO₂/Si heterojunctions.

a Schematic band diagram of a type II MOx/Si heterojunction. Boron-doped, p-type Si is used as the substrate for ALD deposition of metal oxides, ZnO and TiO₂, which are natively n-type doped. The alignment of the Fermi levels creates the band banding in the MOx. The heterojunction band alignment is staggered (type II). b ALD allows for the deposition of precise MOx layer thicknesses, here illustrated by the colors of 100 mm Si wafers with different MOx thicknesses \(d\) as indicated. c Demonstration of the hydrophilic effect for MOx induced by UV light at 365, 300, and 275 nm wavelengths. The image shown is for water droplets before (left), and after (right) UV illumination of a TiO₂ surface with 365 nm light. d Restoration after UV exposure (365 nm @3.2(4) J cm⁻²) can be done by annealing for one hour at ∼100 °C. The advancing contact angle is shown here for a 70 nm thick TiO₂ layer on a Si substrate. Error bars represent standard deviations (STD) with a minimum of three sample measurements from the same wafer. e Illumination of the MOx/Si heterojunctions by UV light through a mask (left) allows for selective wetting of complex geometries (right) upon dipping in water.