Thickness-Dependent Electronic Transport in Ultrathin, Single Crystalline Silicon Nanomembranes

As distinct from bulk silicon, ultrathin silicon-on-insulator (SOI) or silicon nanomembranes (Si-NMs) offer excellent electronic and mechanical properties that are essential to the development of electronic/optoelectronic systems. Ultrathin Si-NM field effect transistors (FETs) based on p-doped SOI (100) wafers are investigated. The thickness of the Si-NMs is controllably reduced from 50 nm to 10 nm through the use of a unique etching process. Based on systematic investigation of Si-NM FETs with varying thicknesses, both insulating and metallic behaviors are observed, which can be attributed to carrier enhancement by surface-dipole doping after thickness reduction. Spectroscopy characterization and theoretical simulations reveal that this high surface-dipole density can be inverted, yielding high-density electrons regardless of the bulk p-doped nature of the material, thus significantly enhancing its conductivity. These findings offer a physical understanding of thickness dependence, which is critical to the future development of ultrathin SOI electronics, of relevance to a diverse range of semiconductor applications.