Atomic layer deposition of high-k dielectrics for graphene-based high frequency and flexible electronics

Gabriele FISICHELLA, IMM-CNR

The integration of dielectric materials with Graphene (Gr) is essential for several device schemes, such as top- or back-gated Gr Field Effect Transistors (GFETs) as well as for new concept Gr vertical devices, such as the Gr Base Hot Electron Transistors (GBHET). Thickness uniformity, conformal coverage and high structural/electrical quality of the insulator/Gr interface are key requirements for optimal device performances. They can strongly depend on the deposition technique, the substrate quality, and specific processing limitations. In this sense, Atomic Layer Deposition (ALD) is the ideal technique to achieve such kind of control on dielectric fabrication. In addition, ALD is intrinsically a non-degradative method for Gr and various other soft materials such as plastic substrates, an important advantage, particularly if compared with other deposition techniques.

First, we will discuss the main physical and chemical issues related to ALD of high-k dielectrics on Gr. Due to the surface inertness of Gr, chemical pre‑functionalization or ex‑situ physical deposition of a seed layer, are commonly employed, which can in turn degrade Gr or result in an insufficient film quality. Here we investigated a fully-in-situ activated ALD process for Al2O3 on Gr, starting with a seed layer deposition at 100°C with trimethylaluminum (TMA) and H2O precursors, followed by Al2O3 thermal growth at 250°C. The structural and electrical quality of Al2O3 as well as the effects of the dielectric deposition on the Gr properties were extensively analyzed by atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, local electrical measurements by conductive atomic force microscopy (CAFM) and on properly designed test devices. Finally, top gated Al2O3/Gr GFETs with different geometries have been fabricated and employed to investigate the impact of the dielectric integration on the Gr mobility.

Plasma-assisted ALD of Al2O3 at low temperatures (100°C) on plastics substrates (PEN) was also developed for the fabrication of locally back-gated flexible GFETs with large area Gr channel, to be employed for environmental sensing applications. The key aspects of this process will be discussed and a study of the morphological, structural and electrical properties of the dielectric layer will be presented. The electrical performances (transconductance, mobility) of the large area back-gated Gr transistors will be finally presented.

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