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Register a new userThe effect of the dielectric end groups on the positive bias stress stability of N2200 organic field effect transistors
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Bias stress degradation in conjugated polymer field-effect transistors is a fundamental problem in these disordered materials and can be traced back to interactions of the material with environmental species,1,2,3 as well as fabrication-induced defects.4,5 However, the effect of the end groups of the polymer gate dielectric and the associated dipole-induced disorder on bias stress stability has not been studied so far in high-performing n-type materials, such as N2200.6,7 In this work, the performance metrics of N2200 transistors are examined with respect to dielectrics with different end groups (Cytop-M and Cytop-S8). We hypothesize that the polar end groups would lead to increased dipole-induced disorder, and worse performance.1,9,10 The long-time annealing scheme at lower temperatures used in the paper is assumed to lead to better crystallization by allowing the crystalline domains to reorganize in the presence of the solvent.11 It is hypothesized that the higher crystallinity could narrow down the range at which energy carriers are induced and thus decrease the gate dependence of the mobility. The results show that the dielectric end groups do not influence the bias stress stability of N2200 transistors. However, long annealing times result in a dramatic improvement in bias stress stability, with the most stable devices having a mobility that is only weakly dependent on or independent of gate voltage.
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The environmental stability of the layered semiconductor black phosphorus (bP) remains a challenge. Passivation of the bP surface with phosphorus oxide, POx, grown by a reactive ion etch with oxygen plasma is known to improve photoluminescence efficiency of exfoliated bP flakes. We apply phosphorus oxide passivation in the fabrication of bP field effect transistors using a gate stack consisting of a POx layer grown by reactive ion etching followed by atomic layer deposition of Al2O3. We observe room temperature top-gate mobilities of 115 cm2/Vs in ambient conditions, which we attribute to the low defect density of the bP/POx interface.
This paper has been withdrawn by the author.
Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are good candidates for high-performance flexible electronics. However, most demonstrations of such flexible field-effect transistors (FETs) to date have been on the micron scale, not benefitting from the short-channel advantages of 2D-TMDs. Here, we demonstrate flexible monolayer MoS2 FETs with the shortest channels reported to date (down to 50 nm) and remarkably high on-current (up to 470 uA/um at 1 V drain-to-source voltage) which is comparable to flexible graphene or crystalline silicon FETs. This is achieved using a new transfer method wherein contacts are initially patterned on the rigid TMD growth substrate with nanoscale lithography, then coated with a polyimide (PI) film which becomes the flexible substrate after release, with the contacts and TMD. We also apply this transfer process to other TMDs, reporting the first flexible FETs with MoSe2 and record on-current for flexible WSe2 FETs. These achievements push 2D semiconductors closer to a technology for low-power and high-performance flexible electronics.
We demonstrate controllable shift of the threshold voltage and the turn-on voltage in pentacene thin film transistors and rubrene single crystal field effect transistors (FET) by the use of nine organosilanes with different functional groups. Prior to depositing the organic semiconductors, the organosilanes were applied to the SiO2 gate insulator from solution and form a self assembled monolayer (SAM). The observed shift of the transfer characteristics range from -2 to 50 V and can be related to the surface potential of the layer next to the transistor channel. Concomitantly the mobile charge carrier concentration at zero gate bias reaches up to 4*10^12/cm^2. In the single crystal FETs the measured transfer characteristics are also shifted, while essentially maintaining the high quality of the subthreshold swing. The shift of the transfer characteristics is governed by the built-in electric field of the SAM and can be explained using a simple energy level diagram. In the thin film devices, the subthreshold region is broadened, indicating that the SAM creates additional trap states, whose density is estimated to be of order 1*10^12/cm^2.
The surface conductivity is measured by a four-probe technique for pentacene and rubrene single-crystals laminated on polarized and nearly unpolarized molecular monolayers with application of perpendicular electric fields. The polarization of the self-assembled monolayers (SAMs) shifts the threshold gate voltage, while maintaining a very low subthreshold swing of the single-crystal devices (0.11 V/decade). The results, excluding influences of parasitic contacts and grain boundaries, demonstrate SAM-induced nanoscale charge injection up to ~10^12 cm^-2 at the surface of the organic single crystals.
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