Direct observation of electron emission site on boron-doped polycrystalline diamond thin films using an ultra-high-vacuum scanning tunneling microscope

Surface morphology characteristics and electron emission properties of boron-doped polycrystalline diamond thin films grown by microwave plasma-assisted chemical-vapor deposition were investigated using an ultra-high-vacuum scanning tunneling microscope. Small secondary grains with average size less than 10 nm were observed to be on top of the isolated crystal surfaces. The current imaging tunneling spectroscopy (CITS) study revealed that high electron emission occurred at some crystalline facets while others showed relatively no measurable electron emission. The CITS study at high magnification also indicated that the electron emission was initiated at the grain boundaries rather than at the top of the grains. This observation suggests that the electrons transport through the grain-boundary conductive channels and preferentially emit at the low electron affinity facets.