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.