Article pubs.acs.org/cm
Enhanced Nucleation of High‑k Dielectrics on Graphene by Atomic Layer Deposition Yong Hyun Park,† Mi Hye Kim,† Soo Bin Kim,† Hae Jun Jung,† Kwanbyung Chae,† Yeong Hwan Ahn,† Ji-Yong Park,† Fabian Rotermund,*,†,‡ and Sang Woon Lee*,† †
Department of Physics and Department of Energy Systems Research, Ajou University, Suwon 16499, Korea Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea
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S Supporting Information *
ABSTRACT: Graphene has emerged as a promising 2dimensional (2D) material composed of a monolayer of carbon atoms, which is expected to be utilized for nano- and optoelectronic device applications. In order to fabricate high speed graphene transistors with low power consumption, the growth of insulating thin films with high dielectric constant (high-k) on graphene is essential. Atomic layer deposition (ALD) is one of the best deposition techniques to grow functional thin films, however, it is extremely challenging to grow high-k thin films on graphene by ALD because of the lack of surface functional groups (such as hydroxyl groups) on graphene. Here, we demonstrate that the graphene surface is fully covered by Al2O3 thin films (10−30 nm), with significantly reduced leakage current (decreased by a factor of ∼107), through simple surface treatment of the graphene in the ALD chamber prior to the deposition of the Al2O3 layer by ALD to provide surface nucleation sites on the graphene, without breaking vacuum and changing entire process temperature (100 °C). Physisorbed nuclei were created on the graphene as a form of Al2O3 with the surface treatment using trimethylaluminum (TMA) and H2O that are typical ALD precursors for Al2O3 growth. Negligible defects were generated during the graphene surface treatment, which provides promising opportunities in graphene electronics.
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INTRODUCTION Graphene has been attracting considerable attention for nanoand optoelectronic device applications due to its unique band structure, high carrier mobility, transparency, flexibility, elasticity, and mechanical strength since the first mechanical infiltration of graphene from graphite.1−7 Among various applications, graphene has been employed as a channel material for metal−oxide−semiconductor field effect transistors (MOSFETs) because of its high electron mobility (>20 000) at room temperature.8−15 Besides MOSFETs, graphene can be used in radiofrequency devices with high cutoff frequency.16 In order to fabricate high electron mobility transistors for high speed and low power consumption using graphene, top-gate transistors are preferred. Back-gate transistors have been advantageous for demonstrating new device concepts, however, the transistors have substantially high parasitic capacitances, and the integration scheme is incompatible with other electronic components.16 Thus, the development of top-gate graphene transistors is a better solution for practical applications of graphene devices. For the fabrication of top-gate graphene transistors, the growth of high-k thin films on graphene is essential as it enables high drain current and low gate leakage current. Although semiconducting graphene has been employed as a channel material for graphene transistors, metallic graphene appears to be excellent for transparent electrodes in electronic applications such as displays, solar cells, and © 2016 American Chemical Society
emerging nanoelectronic devices. In these cases, the growth of insulating thin films on the graphene is also necessary for the isolation or stacking of the electrode. Unfortunately, the growth of high-k thin films on graphene is still extremely challenging even with state-of-the-art thin film deposition techniques such as sputtering and chemical vapor deposition (CVD) that are used in the semiconductor industry because the physical and chemical properties of graphene can be degraded during the growth of high-k thin films by those processes. During the sputtering process, graphene is vulnerable to damage due to high-energy ion bombardment because the graphene is composed of only one atomic layer of carbon. Additionally, the CVD process may oxidize the graphene surface because of the high temperature (>500 °C) of the CVD process.17 In these respects, atomic layer deposition (ALD) is regarded as an optimum deposition technique for the growth of high-k dielectrics on the graphene. ALD is operated by four consecutive sequences that consist of metal precursor injection−purging−oxygen source injection−purging steps for oxide growth.18 It is known for selflimiting growth characteristics based on a chemisorption of the precursors on substrates. The unique surface reaction chemistry Received: June 19, 2016 Revised: September 26, 2016 Published: September 27, 2016 7268
DOI: 10.1021/acs.chemmater.6b02486 Chem. Mater. 2016, 28, 7268−7275
Article
Chemistry of Materials of ALD provides the growth of high quality thin films with atomic-level controllability at low temperature (20−30 nm) are usually deposited as gate dielectrics for top-gate graphene transistors, because the size of the local nuclei of high-k oxides increases with increasing thickness such that those nuclei can be connected with neighboring nuclei. However, the aforementioned approach cannot achieve high capacitance due to the thicker high-k films (>20−30 nm). For certain cases, the grown high-k oxide appears continuous because of undesired remnant organics such as poly(methyl methacrylate) (PMMA) formed in the graphene transfer process, which decreases the capacitance. Several methods were proposed to supply surface functional groups on graphene using a metal seed layer (Al, Hf, or Ti) with the evaporation process (PVD technique) followed by oxidation,29−31 or the coating of organic materials on the graphene surface for the growth of high-k thin films with ALD.20,32 Unfortunately, those methods may oxidize graphene or leave organic species on the graphene. In the meantime, a low temperature (
