A Theoretical Study On The Interfacial Electronic Structures Of Si(100)-2×1/organic Molecules

Great attention has being paid to chemical modification and functionalization of the Sisurface with organic molecule nowadays due to the multi-functions and flexiblity of “tailing”the organic molecules， the importance of Si in traditional microelectronics and thecomparatively complete theoretical system and relatively mature techniques for decades. Theinterfacial electronic properties of the Si/organic-molecules play a decisive role in thecorresponding device performances. In this thesis, density functional theory calculations wereperformed to understand the interfacial electronic structure of theSi（100）-2×1/organic-molecules, with the main results as following:The results of interfacial electronic structures for four aliphatic hydrocarbon molecules withdifferent anchoring groups （i.e. alkylene, alkynyl, alkoxy and alkylthiol） self-assemble onSi（100）-2×1through covalent bonds show that the bonding of different anchoring groups withsurface Si atom will cause different degrees of charge rearrangement, and thus different bonddipoles. What’s more, the different absorption groups will change the intrinsic properties ofthe four molecules, leading to HOMO orbit not localized on the methylene （–CH2） of alkyl,but mainly on the changed anchoring groups–alkylene （-CH-CH2）, hydroxyl （-OH） and thiol（-SH）. Anchoring groups play little influence to the VBM orbit of Si substrate. Due to theabove mentioned reasons, the calculated steps between VBM of Si substrate and HOMO ofadsorbed molecules show great differences, which is nearly2.4eV for the four studiedsystems. Different interfacial states are formed in Si（100）-2×1/organic-molecules interfacesdue to the difference of the anchoring groups. The interfacial states will provide “steps” forcharge carrier crossing the interface potential barrier and hence influence the injection andtransmission of charge carriers in the interface. At last, the calculated steps between VBM ofSi substrate and HOMO of adsorbed alkyl molecules is investigated and find the stepsbecomes narrow with the increasing of alkylene length.The work function modification，interfacial charge rearrangement and bonding dipolemoment, energy level alignment at the interface and adsorption energy are studied for systemsof4-X-phenylacetylene and4-X-styrene （X=CN or NH2） chemisoption on thehydrogen-terminated Si（100）-2×1surface. And find that the magnitude and direction of the adsorbed molecule polarities, which will cause the electrostatic potential of the self-assembledthin film convergence to two different vacuum energy levels (V_vac^leftand V_vac^right along z-axis,are mainly determined by the substituent head group. And the steps between V_vac^left and V_vac^right often dominate the change of work function of Si substrate: the cyan group will raisethe work function of Si substrate and the amino group will reduce it. Head group substitutionhas little influence on the interfacial energy level alignment. The bonding of surface Si atomswith4-X-phenylacetylene and4-X-styrene is sp2and sp3hybridization, respectively, whichwill result in some difference on the interfacial charge rearrangement and the adsorptionenergy.