Hot-electron transport through Au/CaF₂/Si(111) structure studied by ballistic electron emission spectroscopy

The influence of calcium fluoride (CaF₂) [about two monolayers (ML)] in Au/CaF₂/Si(111) heterostructure on hot-electron transport across the CaF₂ intralayers has been studied using ballistic electron emission microscopy (BEEM) and Fourier transform infrared spectroscopy (FTIR). The BEEM current-voltage spectra show that the electron transport property is strongly affected by the CaF₂ intralayers. The threshold voltage V₀ for the onset of the BEEM current for an insulating CaF ₂ intralayer, which is about 3.6 V, is obtained only for the sample in which CaF₂ was deposited at 700°C. In contrast, the threshold voltage of the sample in which CaF₂ was deposited at 550°C .is determined to be about 0.74 V. The FTIR spectra of these CaF₂ layers show that Ca-Si-F bonds exist in the latter, but not in the former. The existence of Ca-Si-F bonds implies that the CaF₂ heteroepitaxial growth at 550°C is unsuccessful in obtaining a high-quality CaF₂ layer and will induce many defects in the CaF₂ layer and/or at the interface. The defect-induced states in the CaF₂ intralayers allow hot electrons to travel through the intralayers even below 3.6 eV and lie in a position of the threshold voltage for the onset of the BEEM current to be about 0.74 V.