Rectifying behavior of silicon–phthalocyanine junctions investigated with scanning tunneling microscopy/spectroscopy

Abstract

We present ultrahigh vacuum (UHV) scanning tunneling microscopy/spectroscopy (STM/STS) and ultraviolet photoemission spectroscopy (UPS) measurements on ultrathin films of nickel–phthalocyanine (Ni–PC) deposited onto clean Si(111)7×7 substrates. STS spectra refer to a sample with 5 Å of Ni–PC nominal thickness. The current-voltage (I-V) measurements were taken at constant tip-sample separation showing high reproducibility at different nanometer size topographical features of the sample. In particular, three typical spectra have been repeatedly observed: (i) I-V curves with a slow increase of the current up to +1.9 eV (I < 5 nA) where the current steeply increases (I>50 nA), (ii) highly noisy I-V curves symmetric with respect to the applied bias and, (iii) curves which show a marked rectifying behavior with negligible current (I<10 pA) at negative (sample to tip) voltages and steeply increasing current by applying a positive bias (I>50 nA for V>0.5 V). We explain the different electronic behaviors in terms of the different morphological features observed by means of simultaneous STM imaging (different Ni–PC crystallites height, and different Ni–PC molecular plane orientation with respect to the substrate) and starting from the density of states below the Fermi energy of the Ni–PC/Si(111)7×7 interface as measured with UPS. The diodelike behavior observed is explained in terms of resonant tunneling of electrons from the tip through the PC layer to the silicon substrate, and also in terms of the intrinsic asymmetry of the tip/vacuum gap/Ni–PC/Si(111)7×7 junction.