Modification of metal / semiconductor junctions with molecular monolayers: structural analysis and electrical characterization

Metal/semiconductor (MS) junctions are fundamental in classical microelectronic devices. With device fabrication size approaching atomic scales, electronic performance will become unpredictable as a result of quantum effects becoming relevant in charge transport. The objective of this thesis is to investigate the potential application of thin organic films to modify MS junctions and to modulate their electrical properties. Knowing how the electronic parameters in a device change over time is necessary for commercial viability. I therefore studied the long-term structural and electronic stability of metal-monolayer-silicon junctions, with respect to silicon oxide formation (monitored by x-ray photoelectron spectroscopy (XPS)). Simple straight-chain n-alkyl (CH3-(CH2)11-Si≡) and phenyl-terminated (C6H5-(CH2)3-Si≡) monolayers were compared. Both samples had a significant change in surface, optical and electronic properties upon oxide formation. Although phenyl-terminated samples oxidized quicker than n-alkyl ones, their electrical properties were more similar to its original measurement before oxidation. There is a wide variety of deposition techniques available for placing metal contacts onto organically modified semiconductors, which are complex and costly. The investigation on monolayers that could withstand simple and inexpensive physical vapour deposition provides an alternative, molecular approach to overcome this technical challenge I discovered that phenyl-terminated monolayers have a significantly greater density than n-alkyl monolayers, based on XPS. This correlated with reducing metal penetration into the monolayer and improvements in electronic properties preservation of the molecular junctions, as observed with ballistic electron emission spectroscopy. In fact, molecular dipole moment (perpendicular vector to the surface) can also alter the charge transport in an MS junction. I have prepared a diverse series of monolayers on silicon (n-alkyl, thioether, phenyl and ether) and discovered a linear relationship between dipole moment, and both the barrier height and ideality factor. The calculated dipole moment has been normalized to account for the monolayer density on silicon which greatly improved the aforementioned linear correlation. A simple mathematical model to predict experimental current versus voltage behavior was then proposed. It was determined that relatively negative dipole moment (parallel to the direction of R-S≡) affect the charge transport pathways to a greater extent than neutral ones.