Thickness-dependent change in the valence band offset of the SiO₂/Si interface studied using synchrotron-radiation photoemission spectroscopy

We have studied the thickness-dependent change in the valence band offset (VBO) of the SiO₂/Si(001) interface using synchrotron-radiation photoemission spectroscopy with soft and hard X-rays. The SiO₂-film thickness (T_ox) and X-ray irradiation time (t_irrad) were systematically parameterized to distinguish between the "intrinsic" T_ox effects in the VBOs and the "extrinsic" differential charging phenomena in SiO₂ films on Si substrates. The results revealed that at a spontaneous time (t_irrad ≈ 5 s) that suppresses the differential charging phenomena as much as possible, the experimental VBO abruptly increases as a function of T_ox and gradually saturates to the traditional VBO value range determined by the internal photoemission and photoconduction measurements. This effect is not attributed to the differential charging phenomena, but rather it is attributed to the "intrinsic" T_ox-dependent change in the VBO. The two possible physical behaviors include electronic polarization and image charge. We have derived the electronic polarization contribution from experimental data by carefully describing the effects of the long-range image charges based on the classical dielectric-screening model.