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 SiOxlayers 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