Sub-Nanometer Interfacial Oxides on Highly Oriented Pyrolytic Graphite and Carbon Nanotubes Enabled by Lateral Oxide Growth
2022
Zhang, Zichen | Passlack, Matthias | Pitner, Gregory | Kuo, Cheng-Hsuan | Ueda, Scott T. | Huang, Zhifang | Kashyap, Harshil | Wang, Victor | Spiegelman, Jacob | Lam, Kai-Tak | Liang, Yu-Chia | Liew, San Lin | Hsu, Chen-Feng | Kummel, Andrew C. | Bandaru, Prabhakar
A new generation of compact and high-speed electronic devices, based on carbon, would be enabled through the development of robust gate oxides with sub-nanometer effective oxide thickness (EOT) on carbon nanotubes or graphene nanoribbons. However, to date, the lack of dangling bonds on sp² oriented graphene sheets has limited the high precursor nucleation density enabling atomic layer deposition of sub-1 nm EOT gate oxides. It is shown here that by deploying a low-temperature AlOₓ (LT AlOₓ) process, involving atomic layer deposition (ALD) of Al₂O₃ at 50 °C with a chemical vapor deposition (CVD) component, a high nucleation density layer can be formed, which templates the growth of a high-k dielectric, such as HfO₂. Atomic force microscopy (AFM) imaging shows that at 50 °C, the Al₂O₃ spontaneously forms a pinhole-free, sub-2 nm layer on graphene. Density functional theory (DFT) based simulations indicate that the spreading out of AlOₓ clusters on the carbon surface enables conformal oxide deposition. Device applications of the LT AlOₓ deposition scheme were investigated through electrical measurements on metal oxide semiconductor capacitors (MOSCAPs) with Al₂O₃/HfO₂ bilayer gate oxides using both standard Ti/Pt metal gates as well as TiN/Ti/Pd gettering gates. In this study, LT AlOₓ was used to nucleate HfO₂ and it was shown that bilayer gate oxide stacks of 2.85 and 3.15 nm were able to achieve continuous coverage on carbon nanotubes (CNTs). The robustness of the bilayer was tested through deployment in a CNT-based field-effect transistor (FET) configuration with a gate leakage of less than 10–⁸ A/μm per CNT.
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