GaAs surface modifications under light irradiation in vacuum

Electrical Contacts to Organic Molecular Films by Metal Evaporation: Effect of Contacting Details

“…In contrast to indirect evaporation, the high leakage currents and inferior rectifying behavior observed in the junctions prepared by direct evaporation (Figure ) indicate that the metal atoms and radiation emitted from the crucible damage the GaAs surface. This radiation, which includes X-rays (as a result of electron−metal interaction), stray electrons from the electron-beam source, and electrons backscattered from the molten metal target, can be intense enough to lead to compositional changes of the GaAs surface . Therefore, it is clear that molecules, adsorbed on that surface, will be damaged as well.…”

“…While contamination is almost excluded if evaporation is performed in a vacuum better than 10 -6 mbar, this necessitates placing the samples at unrealistically large distances from the crucible. In addition, one needs to consider that radiation emitted from the metal source can arrive at the sample surface and affect the substrate and/or the molecules adsorbed on it. , …”

“…This radiation, which includes X-rays (as a result of electron-metal interaction), stray electrons from the electronbeam source, and electrons backscattered from the molten metal target, can be intense enough to lead to compositional changes of the GaAs surface. 60 Therefore, it is clear that molecules, adsorbed on that surface, will be damaged as well.…”

“…We ascribe the differences between BH values of bare junctions contacted by the same directly or indirectly deposited metal to the effects of radiation emitted from the metal source that arrives at the samples' surface during direct evaporation, 60 but not during indirect evaporation. Such radiation, which includes X-rays (as a result of electron-metal interaction) can be intense enough to lead to compositional changes of the GaAs surface 60 and can facilitate surface oxidation. Such oxidation decreases the negative charge on the GaAs surface and, thus, the band bending and Schottky barrier, compared to what is the case for junctions prepared with indirectly evaporated metal.…”

“…In addition, one needs to consider that radiation emitted from the metal source can arrive at the sample surface and affect the substrate and/or the molecules adsorbed on it. 21,60 Introducing an inert gas into the chamber after achieving a base vacuum pressure <10 -6 mbar, will decrease the total vacuum in the chamber while keeping the concentration level of the contaminants similar to that in the system with the high base vacuum. Introducing Ar as the inert gas, to reduce the vacuum by bringing the pressure in the chamber to ∼10 -3 mbar, as done here, decreases the mean free path of the evaporated atoms/clusters to ca.…”