Study On The Structure And Electronic States At The Interface Between Organic And Inorganic Semiconductors

Si and SiO₂ are advanced materials with many existing or potential advanced technological applications in microelectronics and mechanical structures, and they have been investigated for many decades. The performance of silicon-based devices has been steadily improved by scaling down device dimensions according to Moore's Law, and a nearly exponential growth of microelectronics capabilities has been achieved.However, retaining this top-down miniaturization trend is becoming exceedingly hard due to fundamental physical and technological limitations. By contrast, the use of organic molecules as a building block for nanoscale devices has attracted much attention because the precisely controlled nanostructures may be formed cheaply by utilizing self-assembly of molecules. Therefore, combining the organic molecules with the conventional silicon substrate enables us to explore miniaturization devices. It is likely that the performance of such devices is strongly influenced by the organic compound, the structure and electronic structures of the substrate surface and interface between organic and inorganic. Only a few reports have focused on the electronic states of molecule-silicon interfaces. Thus, the direct measurement of the molecule-silicon interfacial electronic states is required.Small aromatic organic semiconductor molecules, such as tetracene(C₁₈H₁₂),perylene(C20H12) and pentacene(C22Hi4), are attracting increasing attention due to their high purity and well-ordered crystalline structures by sublimation techniques. In the present study, tetracene and perylene are adsorbed on the clean Si(111)-(7×7) and SiO₂/Si surface slowly in the ultra-high vacuum system, and investigated by ultraviolet photoelectron spectroscopy (UPS).The UPS measurement of tetracene on Si(111)-(7×7) substrate shows that seven emission features from the organic material are located at 1.82,3.36,4.60, 5.77,6.78,9.02 and 11.0eV, respectively, below the Fermi level. The UPS measurement of tetracene on SiO₂/Si(111) substrate shows that seven emission features from the organic material are located at 1.73,3.14,4.45,5.60,6.64,7.88 and 8.92eV, respectively, below the Fermi level. The UPS measurement of perylene on Si(111)-(7×7) substrate shows that six emission features from the organic material are located at 0.7,2.3,4.0,6.0,7.2 and 9.0eV, respectively, below the Fermi level. The UPS measurement of perylene on SiO2/Si(111) substrate shows that five emission features from the organic material are located at 3.0,4.5,6.1,7.5 and 9.3eV, respectively, below the Fermi level. With increasing the coverage, emission features from the organic material shift in binding energy indicating the interaction between the organic molecule and the substrate near the interface. More shift on Si(111)-(7×7) substrate shows that stronger interaction between the organic molecule and the Si(111)-(7×7) substrate. From the UPS measurements, the work function of the sample surface was found to decrease with increasing molecular coverage in the sub-monolayer range. With further increasing the coverage, there is a small increase in work function. The minimum in work function is observed at one monolayer. The change in work function suggests the formation of interface dipole.