▎ 摘　　要

High-quality thin insulating films on graphene (Gr) are essential for field-effect transistors (FETs) and other electronics applications of this material. Atomic layer deposition (ALD) is the method of choice to deposit high-kappa dielectrics with excellent thickness uniformity and conformal coverage. However, to start the growth on the sp(2) Gr surface, a chemical prefunctionalization or the physical deposition of a seed layer are required, which can effect, to some extent, the electrical properties of Gr. In this paper, we report a detailed morphological, structural, and electrical investigation of Al2O3 thin films grown by a two-steps ALD process on a large area Gr membrane residing on an Al2O3-Si substrate. This process consists of the H2O-activated deposition of a Al2O3 seed layer a few nanometers in thickness, performed in situ at 100 degrees C, followed by ALD thermal growth of Al2O3 at 250 degrees C. The optimization of the low-temperature seed layer allowed us to obtain a uniform, conformal, and pinhole-free Al2O3 film on Gr by the second ALD step. Nanoscale-resolution mapping of the current through the dielectric by conductive atomic force microscopy (CAFM) demonstrated an excellent laterally uniformity of the film. Raman spectroscopy measurements indicated that the ALD process does not introduce defects in Gr, whereas it produces a partial compensation of Gr unintentional p-type doping, as confirmed by the increase of Gr sheet resistance (from similar to 300 Omega/sq in pristine Gr to similar to 1100 Omega/sq after Al2O3 deposition). Analysis of the transfer characteristics of Gr field-effect transistors (GFETs) allowed us to evaluate the relative dielectric permittivity (E-BD = 7.45) and the breakdown electric field (EBD = 7.4 MV/cm) of the Al2O3 film as well as the transconductance and the holes field-effect mobility (similar to 1200 cm(2) V-1 s(-1)). A special focus has been given to the electrical characterization of the Al2O3-Gr interface by the analysis of high frequency capacitance-voltage measurements, which allowed us to elucidate the charge trapping and detrapping phenomena due to near-interface and interface oxide traps.