| In recent years, organic semiconductor thin films (OSTFs) have attracted considerableattention. Comparing with traditional inorganic semiconductor thin films, OSTFs possess theadvantages of low-cost deposition, flexibility, and large area processing. As active layers,OSTFs have potential applications in electronic devices, such as organic field-effecttransistors (OFETs), organic photovoltaic cells, and organic light-emitting diodes. AlthoughOFETs are not expected to compete with silicon metal-oxide-semiconductor field-effecttransistors (MOSFETs) in the production of high-end products, they can be used in lost-costflexible memory cards, smart price tags, pixel drivers for displays, and so on. Organic vapordeposition (OVD) is an effective way to fabricate active layers for OFETs. The performanceof OFETs depends strongly on the morphology and molecular orientation of the first fewmolecular layers near gate dielectrics, which are in turn governed by the initial growth stageof OSTFs. Until now, many efforts have concentrated on growth mechanism, morphology,and growth dynamics of OSTFs. However, the studies on the initial growth stage havelagged far behind due to the limitations of spatial and temporal scales in direct experimentalobservation. Therefore, the investigations on the initial growth stage of OSTFs during OVDprocess become apparently urgent.On the other hand, in high-end electronic circuits, modern digital logic is based onsilicon complementary metal-oxide-semiconductor (CMOS) technology, where the logicgates consist of silicon metal-oxide-semiconductor field effect transistors (MOSFETs). Fordecades, making MOSFETs smaller has been the key to improve the digital logic. However,MOSFETs scaling is approaching the limits due to the physical limitation of silicon as wellas the short-channel effects. To continue the development in this field, a suitable substitute ofsilicon is necessary as the channel material of field effect transistors (FETs). Graphene is atwo-dimensional sheet of carbon atoms tightly packed into a honeycomb lattice. If a FET hasa thin barrier and a thin gate-controlled region, it will be robust against short-channel effectsdown to very short gate lengths. Therefore, the possibility of making channels at atomiclayer thick is considered as the most attractive feature of graphene for use in FETs. Moreover,graphene has an extremely high carrier mobility (μ). These impressive properties make graphene a very promising channel material for high-speed FETs. However, due to theabsence of a bandgap, pristine graphene-based FETs have a very low on/off current ratio(Ion/Ioff), leading to high static power dissipation. Therefore, opening a sizable bandgap ofgraphene without degrading the μ value is the prerequisite for use in FETs.Based on above considerations, by using thermodynamic modeling and moleculardynamics simulations, we investigate the initial growth stage of OSTFs during OVD process.Furthermore, by using first-principles density functional theory (DFT) calculations, wesystematically study the effect of dual-doping from substrates and organic molecules on thebandgap opening of bilayer graphene (BLG), and develop a dual-doped BLG that satisfiesthe technical requirements. The main results are divided into three parts as following:Firstly, a unified thermodynamic model is established to characterize the initial growthstage of disk-like and rod-like organic molecules during OVD process. Under high substratetemperature and low deposition rate, the initially grown parallel cluster can transfer to thenormal one at a critical molecular number Nc, which is determined by the competitionbetween surface energy of molecule γf(surface energy of the molecular surface normal toπ-π interaction direction for disk-like molecule or parallel to the molecular axis for rod-likemolecule) and that of substrate γsub. By thermodynamic analyses and molecular dynamicssimulations for the OVD process of phthalocyanine, we further confirm this parallel-normaltransformation mechanism. When N <Nc, a parallel cluster is grown by self-assembly of thelying down molecules driven by γf. At N≥Nctogether with γf>(5/7)γsub, the grown parallelcluster tilts down to form a normal one. The results show that the orientation transformationcan be controlled by surface energy to get a stable normal orientation for use in OFETs.Secondly, our first-principles DFT calculations have quantitatively identified that BLGis n-doped in N,N-dimethyl paraphenylenediamine/BLG (DMPD/BLG) system while it isp-doped in tetracyanoethylene/BLG (TCNE/BLG) system. The opposite p-doping fromamorphous SiO2substrate with O2–on its surface increases the bandgap of DMPD/BLGsystem from106to253meV. Similarly, the bandgap of TCNE/BLG system is enhancedfrom98to211meV by the opposite n-doping from4H-SiC(0001) with a C buffer layer. Theincreased bandgap can improve Ion/Ioffvalue of FETs. Meanwhile, velue of BLG is largelymaintained due to the weak interaction at the interface. Thus, this work provides thescientific basis for further development in BLG-based FETs.Thirdly, through n-doping from decamethylcobaltocene (DMC) and p-doping fromfunctionalized amorphous SiO2gate dielectric, the bandgap opening in BLG is predicted by first-principles DFT calculations. With DMC monolayer on BLG and maximal O2on amorphous SiO2, the dual-doped BLG presents a bandgap of390394meV and a Dirac levelshift of5952meV. The former is close to the technical requirement of400meV, whilethe latter lies in the gate voltage accessible range of300meV. The high value largelyremains with Ion/Ioffsatisfying the technical requirement of104107. No external electricfield is employed in this method, avoiding a complex fabrication step for preparing adual-gate structure and a substantial reduction in and Ion/Ioffvalues induced by adding anextra gate. These impressive properties promise this dual-doped BLG as the channel of asingle-gate FET used in modern digital logic. |
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