Ballistic-electron-emission microscopy at epitaxial metal/semiconductor interfaces

The invention of ballistic-electron-emission microscopy (BEEM) has made it possible to study hot electron transport across interfaces with a spatial resolution unparalleled before. In order to exploit the limits of the method, we have applied BEEM experiments carried out in UHV and at 77 K to epitaxial CoSi2 films on silicon. CoSi2/Si may be considered as a model system for the metal/semiconductor interface, because its atomic structure can be rather well controlled experimentally and has been well characterized by transmission electron microscopy. This overview contains a discussion of the various processes leading to contrast in BEEM images for CoSi2/Si interfaces. The BEEM current may be affected by (a) the atomic surface structure or surface defects, both of which can change the tunneling distribution, (b) inelastic and elastic scattering processes within the metal films and (c) interface scattering or variations of the Schottky barrier height, resulting from interfacial defects. Scattering processes will be shown to be dominant in the case of CoSi2/Si(111) interfaces, since the Schottky barrier height is not measurably affected by interfacial dislocations and other defects. Here, the ultimate resolution limits of the BEEM technique have been reached, in the sense that individual point defects can be resolved. The CoSi2/Si(100) interface represents a more complicated case, where extended defects lead to significant barrier lowering, whereas interface scattering is obscured by the strong modification of the tunneling distribution by surface reconstructions.