Femtosecond Extreme Ultraviolet Photoemission Spectroscopy: Observation of Ultrafast Charge Transfer at the n-TiO2/p-Si(100) Interface with Controlled TiO2 Oxygen Vacancies

Abstract

Wide band gap heterostructures can be accurately fabricated at room temperature to exhibit remarkable electrical properties that facilitate the transfer of electrons across the heterojunction while blocking the transfer of holes. The present investigation focuses on engineering the electronic structure of a TiO2 overlayer on a p-type doped Si(100) substrate by controlling the concentration of TiO2 oxygen vacancies. TiO2 films are deposited on p-Si(100) in an ultrahigh vacuum setup by evaporation of Ti atoms at a constant rate in a variable O2 atmosphere. The concentration of oxygen vacancies and consequently the degree of n-type doping of TiO2 is tuned by controlling the oxygen background pressure during the TiO2 formation. To investigate the electronic structure and the concentration of defects in the TiO2 layer as well as to characterize the TiO2/p-Si(100) band alignment we used photoelectron spectroscopy employing femtosecond extreme ultraviolet laser pulses produced via high harmonic generation. Furthermore, using a pump-probe technique in conjunction with photoemission spectroscopy, an ultrafast electron injection from the p-Si(100) substrate into the defect-rich TiO2 layer is observed with a time constant of 450 fs, as well as the subsequent charge carrier recombination. The latter is revealed to be affected by the oxygen defects when investigated with femtosecond resolution. No charge transfer is observed when defect-poor TiO2 films are prepared on the p-Si(100) substrate. This might be attributed to a change in the energy band alignment at the TiO2/Si(100) interface that reduces the built-in potential across the heterojunction and consequently reduces the driving force responsible for the injection of electrons into the TiO2 layer.