Ballistic electron emission microscopy studies of inhomogeneity in Au/CaF₂/n-Si(111) interfaces

We have performed ballistic electron emission microscopy (BEEM) measurements on the Au/CaF₂/n-Si(111) system in which calcium fluoride (CaF₂) (about 2 monolayers (ML)) was introduced at the Au/Si interface. A BEEM image clearly shows the coexistence of two types of terrace for each of which the BEEM I-V spectrum has a different shape. A typical threshold voltage of the BEEM current for one type is about 0.75 V. In contrast, the second type shows a threshold voltage of about 3.5 V, which is much higher than the value of the first type. Furthermore, the BEEM current on the second type is significantly reduced and saturates above about 6 V. We attribute the coexistence of the two types of terrace in the BEEM image to the different degrees of coverage of the CaF₂ intralayers between them. At the second type of terrace, a Au/2 ML CaF₂/1 ML CaF/Si(111) interface exists, although the first type has a Au/1 ML CaF/Si(111) interface without a CaF₂ intralayer.