On the Dependence of Band Alignment of SiO₂/Si Stack on SiO₂ Thickness: Extrinsic Or Intrinsic?

The dependence of band alignment of SiO₂/Si stack on SiO₂ thickness is restudied. The band structure of SiO₂/Si stack is investigated by time-dependent X-ray photoelectron spectroscopy (XPS) with and without electron-compensation technology. The binding energy difference ΔSi_2p between Si 2p core-levels of SiO₂ and Si, measured without electron-compensation, is found larger than that with electron-compensation, owning to the charging effect. And more severe charging effect induces larger ΔSi_2p. The ΔSi_2p measured with electron-compensation technology, however, is scarcely affected by the charging effect and thus accurate band alignment can be obtained. The band alignment of SiO₂/Si stack is found to be SiO₂ thickness dependent. And this dependence is attributed to the gap states on the SiO₂ surface and their lower charge neutrality level than the Fermi level of Si substrate, resulting in electron transfer from Si to SiO₂ and electric potential distribution across the SiO₂. As a result, the experimentally obtained dependence of ΔSi_2p on SiO₂ thickness with electron-compensation is intrinsic. The proposed explanation about the XPS results further confirms the feasibility of the gap state theory in demonstrating the band lineup of hetero-structures.