FIGURE 2 Contact resistivity values of SrF2/Al stack associated with n-Si measured by the TLM. (A) The dark current-voltage measurement was performed on a structure consisting of a 1.5 nm SrF2 film deposited on 1-3 Ω·cm lightly doped n-type c-Si and an Al layer. . (B)The contact resistance of the structure was measured for various thicknesses of SrF2 film, and the effect of storage time in the air was also examined.
2.3. n-Si/SrF2/Al Interface.
To further investigate the cause of its high stability and thickness tolerance property. The scanning transmission electron microscopy-high-angle annular dark field (STEM-HAADF) technique was used in combination with energy-dispersive X-ray spectroscopy (EDXS) and electron-energy loss spectroscopy (EELS) to examine the interfacial structures and chemical elements at the n-Si/SrF2/Al interface. As shown in Figure 3A, the microscopy images of the stack contact interface apparently visualize a homogeneous continuous SrF2 interlayer, and n-Si is obviously recognizable. The thickness of the SrF2 film is 4 nm calculated by the resolution scale, which is in accord with the deposition. The 1 nm SiOx layer is usually produced by natural oxidation during a prolonged evacuation after the sample is transferred to the thermal evaporation chamber, which probably passivated the dangling bond of the Si surface39 The EDXS maps of the n-Si/SrF2/Al contact are presented in Figure 3b with a 20 nm resolution. The presence of the SrF2 and SiOx layers was verified by the elemental signals for Al, Sr, F, O, and Si. The top Al and bottom Si are separated by the Sr and F elements. In Figure 3C, the EELS signals of Sr and F fluctuate in the SrF2 interlayer indicating that the interlayer is a combination of SrF2 and a significant amount of Al (16 – 46%, absolute atomic percentage). This phenomenon is probably caused by the diffusion of SrF2 film to the Al layer. Al atom mixtures have the potential to improve interlayer conductivity and provide a modest thickness dependency. The fabrication of c-Si/GdF3/Al and vertical transistors using 2D materials has also shown similar behavior.40,41