Intense photocurrent from Mo-doped TiO2 film with depletion layer array.

A novel bilayer structure of TiO2 film was found capable of yielding fairly strong photocurrent under visible light. The base layer was lightly doped with Mo and then etched by reactive ion beam, and was finally covered by an undoped TiO2 surface layer. Because of Fermi level drop at the interface of the trenches, such a deposition-etching-redeposition process implanted an array of depletion layer into TiO2 film successfully. Microstructures, crystallite parameters, and the absorption property were investigated with scanning electron microscope, atomic force microscopy, X-ray diffraction, and ultraviolet-visible spectroscopy in order. Photocurrent density was collected on an electrochemical workstation under visible light. The results indicate that carrier collection probability near depletion layer was enhanced significantly owing to high parallel diffusivity. Under visible light, current density demonstrates a marked increase as etching depth grows. At an etching depth around 660 nm, photocurrent density achieved is 56 times larger than TiO2 film. Depletion layer at vertical trench edges may have a much bigger universal value than anticipated for various doping cases of wide-bandgap films.