Gate‐Controlled Energy Barrier at a Graphene/Molecular Semiconductor Junction

Abstract

wileyonlinelibrary.comGraphene, [ 11 ] a one-atom thick zero band gap semiconductor has allowed the development of new electronic device schemes such as the graphene-barristor, [ 12 ] the graphene-vertical-fi eld-effecttransistor (VFET), [13][14][15][16] and the graphenebase hot electron transistor. [ 17 ] In organic electronics, graphene can be an ideal choice to use as the injector (or source) electrode for a VFET since its Fermi level, its corresponding work function and its available low density of states (DOS) [ 18 ] can all be easily modulated, providing a tunable energy barrier which eventually controls the device operation. In addition, the electric fi eld produced by a gate electrode will extend into the organic semiconductor, as the monolayer thickness of graphene is insuffi cient to fully screen it. [ 13,19 ] In order to increase the performance of organic thin fi lm transistors (OTFT) compared with the inorganic counterparts, a unique vertical architecture with a molecular semiconductor (C 60 ) was fi rst demonstrated using a thin and rough (with a roughness comparable to the thickness) metal electrode. [ 20 ] A clear advantage of the vertical over the lateral organic transistor geometry is that the channel length is controlled by the thickness of the organic layer and the devices can be downsized in both the lateral and the vertical directions. In the case of a perforated metallic source electrode, the electric fi eld can directly access the metal/organic interface which causes the energy level realignment (similar to the band bending in inorganic semiconductor) in the organic semiconductor. Although Ma and Yang have reported a large on/off current ratio (≈10 6 ), the high DOS and the fi xed work function of the metallic electrode (injector) limits its application to a few organic semiconductors. [ 20 ] Later on, Liu et al. introduced a carbon nanotube-based source electrode with low DOS in organic VFETs for a wide range of organic semiconductors. [ 21 ] Carbon nanotubes allow new mechanisms for transistor operation such as the tuning of the gate modulated energy barrier. Graphene, similar to carbon nanotubes in its electrical and mechanical properties, has additional advantages over them due to the large-scale availability (chemical vapor deposition (CVD)-grown graphene) of chemically inert high quality 2D surfaces. [ 14,22 ] On the other hand, fullerene (C 60 ) is one of the most widely studied molecular semiconductors [ 2,5,20,[23][24][25] and a common choice as an active media for transistors. C 60 has an energy gap ( E g = 1.7 eV), between its highest occupied molecular orbital Gate-Controlled Energy Barrier at a Graphene/Molecular Semiconductor JunctionSubir Parui , * Luca Pietrobon , David Ciudad , Saül Vélez , Xiangnan Sun , Fèlix Casanova , Pablo Stoliar , and Luis E. Hueso * The formation of an energy-barrier at a metal/molecular semiconductor junction is a universal phenomenon which limits the performance of many molecular semiconductor-based electronic devices, from fi eld-effect trans...

Vertical Organic Field‐Effect Transistors

“…[77] GE-VOFETs based on ion gel gate dielectric have also been investigated and the devices constructed with pentacene or PTCDI-C 8 as active layer exhibit high electrical performances with current density up to 100 mA cm −2 at a voltage below 1 V and a small subthreshold swing of 240 mV dec −1 . [78,79] Except for fullerene, many other organic semiconductors (n-and p-type materials), such as PCBM and P3HT, [26] PTCDI-C 13 , [34] PTCDI-C 8 , [80] PTCI-BPE, [34] pentacene, [34,80] CuPc, [34] and C 8 -BTBT, [81] have also been used in GE-VOFETs. The fullerenebased GE-VOFETs have been widely investigated in the aspects of its electrical and photoresponse properties, demonstrating improved performances with the optimization of devices structures and processing conditions.…”

“…[122] The structure of such vertical light emitting phototransistors Adv. [79] Copyright 2015, Wiley-VCH. Mater.…”

“…The fullerenebased GE-VOFETs have been widely investigated in the aspects of its electrical and photoresponse properties, demonstrating improved performances with the optimization of devices structures and processing conditions. [78,79] Except for fullerene, many other organic semiconductors (n-and p-type materials), such as PCBM and P3HT, [26] PTCDI-C 13 , [34] PTCDI-C 8 , [80] PTCI-BPE, [34] pentacene, [34,80] CuPc, [34] and C 8 -BTBT, [81] have also been used in GE-VOFETs. Benefitting from the small roughness of graphene, the thickness of active layer in devices can be downscaling to sub-100 nm and the current density of GE-VOFET is generally larger than that of CN-VOFET.…”

“…The carrier injection between graphene and organic semiconductors can be tuned by the gate thus realizing source-drain current modulating by gate voltage. [79]…”

“…Reproduced with permission. [79] Copyright 2015, Wiley-VCH. is shown in Figure 12a consisting of a vertical phototransistors and organic light emitting diode.…”