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

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Abstract
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 CaF2 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.

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