Study on dual-lateral-gate suspended-body single-walled carbon nanotube field-effect transistors
Highlights
► We report novel dual-gate suspended-body carbon nanotube field-effect transistors. ► Two lateral gates are self-aligned sub-100 nm away from the CNT channel. ► We analyze in detail the CNFETs in single-gate (SG) mode and dual-gate (DG) mode. ► In SG mode, we observe strong controllability and the tuning effect of dual gates. ► In DG mode, we obtain remarkably improved S, Ion and gm.
Introduction
Carbon nanotubes (CNTs) have been intensively studied for complementary metal–oxide–semiconductor (CMOS) and nano-electro-mechanical-systems (NEMS) applications due to their remarkable electrical and mechanical properties [1]. Due to the small diameter (∼1 nm), single-walled (SW) CNTs are considered as the ideal material for making one-dimensional (1-D) electronic devices.
Based on semiconducting SWCNTs, CNT field-effect transistors (CNFETs) have been realized and intensively investigated. So far, two types of CNT transistor operation have been obtained [2]. Conventional MOSFET-like CNFETs allow for carrier injection from the metal contacts into the valence/conduction band of the nanotube without a substantial Schottky barrier involved [3]. While, in Schottky barrier CNFETs, the Schottky barriers at metal contacts cannot be neglected. The gate field impacts the thickness of the Schottky barriers at the metal-CNT interfaces and enables tunneling current from metal electrodes into the CNT channel [4], [5]. Many efforts have been made to fabricate high-performance CNFETs and to understand the operating mechanism. The previously reported gate configurations of the CNFETs include: back gate (full back gate and partial back gate) [6], [7], [8], top gate (full top gate, partial top gate and multi-top gates) [9], [10], [11], dual gates (back gate as the second gate) [2] and so on. However, there are limitations of these CNFETs. In the back gated CNFETs, individual access to independent CNT devices in large scale circuits is impossible and the thick gate dielectrics limit their performance. Top gate CNFETs are not applicable for NEMS structures with suspended CNTs. The previous dual gate CNFETs have low gating efficiency since the gates are asymmetric (different length and gate dielectrics) and one gate is partially screened by the other gate.
For complex integrated circuits, individual gate access, superior gate controllability and a convenient assembly process of CNFETs are essential. CNFETs with lateral gates can offer excellent coupling between the CNT channel and the independently accessible gates. Meanwhile, dual-lateral-gate CNFETs are flexible for various CMOS and NEMS applications [12]. However, hardly any dual-lateral-gate CNFETs have been reported, probably due to the difficulties in fabricating such devices.
Two main CNT assembly methods are: in situ CNT growth and post-synthesis fabrication. In situ CNT growth onto a desired position is one of the most common assembly techniques. High temperature is needed to ensure the tube quality. However, it is not compatible with current CMOS technology [13]. Alternatively, the post-synthesis fabrication, for instance: ac-dielectrophoresis (DEP) using solution processed CNTs, has offered a promising alternative way due to its convenience, CMOS compatibility and potential for mass production [14].
Recently, we reported a precise positioning DEP method for fabricating self-aligned suspended-body CNFETs efficiently controlled by two laterally placed independent gates with sub-100 nm gate/channel distance [15].
Here, the superior electrical characteristics in single-gate (SG) mode and dual-gate (DG) mode of the suspended-body CNFETs are studied. Typical effects of the suspended-body CNFETs are investigated. The DG suspended-body CNFETs show great potential for bottom-up fabrication of CMOS and NEMS devices.
Section snippets
Device fabrication
The brief process flow of the DG CNFETs is depicted in Fig. 1 [15]. First, Ti/Pd guiding electrodes spaced 1.5 μm apart were patterned on a SiO2 (500 nm)/Si wafer (Fig. 1a). Ultra-narrow trenches (50 nm) were transferred to 100 nm LOR/50 nm PMMA layers by e-beam lithography (EBL) (Fig. 1b). The LOR thickness determines the CNT suspension height.
Twenty microlitre SWCNT solution was cast onto the substrate. Then, ac-dielectrophoresis was applied for 45 s between the guiding electrodes (Fig. 1c). SWCNTs
Results and discussion
All I–V characterizations of one of our CNFETs (gate-CNT gaps: 85 nm and 110 nm) are carried out by the PMC 150 probing system and the HP 4156C semiconductor parameter analyzer at room temperature in vacuum.
Conclusion
In conclusion, self-aligned suspended-body single-walled carbon nanotube field-effect-transistors have been demonstrated with efficient and independent electrostatic control by two laterally placed independent gates spaced less than 100 nm away from the CNT channel. We analyzed in detail the operation of the suspended body CNFETs in SG mode and DG mode. Strong interface coupling and threshold voltage variation, tuning of the second independent gate are typical effects of suspended body CNFETs in
Acknowledgements
This work was supported by the Swiss Nanotera project CABTURES and by the FP6 IST-028158 project NANORF (Hybrid Carbon Nanotube-CMOS RF Microsystems).
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