Ballistic electron emission microscopy studies of Au/CaF ₂ /n-Si(111) interfaces

We have performed ultrahigh-vacuum (UHV) ballistic electron emission microscopy (BEEM) measurements on Au/CaF ₂ /n-Si(111) into which calcium fluoride (CaF ₂ ) (about 2 monolayers (ML)) was introduced between Au and Si. A BEEM image clearly shows that a CaF ₂ intralayer deposited at 700°C induces the coexistence of two terrace types, each with a different BEEM I-V spectrum shape. A typical threshold voltage of the BEEM current for one type is about 0.75 V. In contrast, the other type shows a threshold voltage of about 3.6 V, which is much higher than that of the first type. Furthermore, the BEEM current on the second type is significantly reduced and saturates above approximately 6 V. Ca distributions measured by Auger electron spectroscopy (AES) strongly suggest an inhomogeneous distribution of CaF ₂ coverage on 2 ML CaF ₂ (deposited at 700°C)/n-Si(111); there are two types of Si terraces, each of which has a different CaF ₂ coverage. Based on the AES measurements, we attribute the coexistence of the two terrace types in the BEEM image to the different degrees of coverage of the CaF ₂ intralayers. The second type of terrace has a Au/2 ML CaF ₂ /1 ML CaF/Si(111) heterostructure. A 2 ML CaF ₂ /1 ML CaF is an insulating intralayer which induces the threshold voltage of 3.6 V and the saturation of the BEEM current. In contrast, the first type has a Au/1 ML CaF/Si(111) heterostructure which has the threshold voltage of 0.75 V.