Origin of Hysteresis in Carbon Nanotube Field-Effect Transistors
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ơect Transistors Origin of Hysteresis in Carbon Nanotube Field-Eơ Yael Pascal-Levi1,2, Evgeny Shifman1, Manish Pal-Chowdhury1, Itshak Kalifa1, Ida Sivan1, Tsvika Rabkin1,2, and Yuval E. Yaish1 1 Department of Electrical Engineering, Technion, Haifa, Israel, 32000 2 Russell Berrie Nanotechnology Institute, Technion, Haifa, Israel, 32000 ABSTRACT Carbon nanotube field eơect transistors (CNT FETs) have many possible applications in future nano-electronics due to their excellent electrical properties. However, one of the major challenges regarding their performance is the noticeable gate hysteresis which is often displayed in their transfer characteristics. The hysteresis phenomenon is often attributed to water-mediated charge transfer between the CNT and the dielectric layer or the CNT and the water layer itself. In this study, we implement three different experimental techniques and provide evidence that the hysteresis phenomenon of suspended CNT FETs, as well as of on-surface CNT FETs which operate at low gate voltage regimes ( Vg 3V ), is based on gate-induced, water-assisted redistribution of mobile charge on the SiO2 surface, and it is not related to charge injection from the CNT itself. Two techniques are based on the current measurements through the CNT and the third utilizes electrostatic force microscopy (EFM) setup. In addition, the applied external gate voltage affect the relaxation time of the current. This change arises from the modification of the amount of water layers which adsorb onto the dielectric surface, which caused by dielectrophoresis attraction between the water molecules and the substrate. It is found that the relaxation time, and hence the surface conductivity, are very sensitive for the first few layers, and saturates above three monolayers of water molecules. INTRODUCTION Carbon nanotube field effect transistors (CNT FETs) possess excellent electrical properties which make them promising candidate for future electronics. Their high sensitivity to various analytes have been the subject of many studies, which are mainly focused on device applications. Yet, several unresolved issues need to be clarified before further successful utilization of these devices can take place. One of these issues is the noticeable gate hysteresis that CNT FETs usually exhibit in their transfer characteristics. This phenomenon has previously been explained by several diơerent models [1-6]. Currently, the common belief is that gate hysteresis is been attributed to stationary charge traps within the dielectric, which are filled and emptied by the applied gate voltage with charge transfer to and from the CNT itself [7-11]. These findings suggest that CNTs behave as leaky FETs where charges can flow back and forth between the CNT and the surface, and that the total tube charge is changing with time. The hysteresis phenomenon was observed for suspended CNTs as well [12], and it has been shown that water molecules give a large contribution to the hysteresis. Therefore, it has been suggested that the hysteresis is due t
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