Paper LB-WeA3
Functionalizing Graphene for ALD using a Simple Wet Chemical Treatment

Wednesday, October 20, 2010, 2:40 pm, Room Cimmaron

Realizing graphene-based technologies requires both large-area graphene production and integration with scalable high-k gate dielectrics. Thermal atomic layer deposition (ALD) provides a means to achieve high-quality ultrathin dielectric films below 300 °C, but the hydrophobic and chemically inert nature of the graphene surface inhibits direct application of ALD oxides. Several methods have been utilized to functionalize the surface of graphene for ALD including: deposition of a metal seed layer^[1], an ozone pretreatment^[2], and using a low-κ polymer seed layer^[3]. Although some success has been achieved, these techniques are complex and often result in degradation of the graphene mobility. In this work, we present a simple wet chemical surface treatment which functionalizes the surface of graphene and results in conformal, uniform Al₂O₃ films.

Epitaxial graphene was synthesized on semi-insulating, on-axis 4H- and 6H-SiC substrates (16x16mm²) using an Axitron/Epigress VP508 SiC growth reactor at temperatures from 1600 to 1650 °C for a duration of 60 to 180 min. Samples were grown in an Ar ambient at a constant pressure of 100 mbar. All samples underwent an optimized wet chemical surface treatment before oxide deposition to render the graphene susceptible to ALD process. This ex-situ treatment consisted of an HF dip (25 wt. %) for 2 min. at room temperature followed by soaking in SC1 (1 H₂O₂:1 NH₄OH:5 H₂O) at 80 °C for 10 min. Samples were then rinsed in DI and blown dry in N₂. ALD of Al₂O₃ was carried out in a Cambridge NanoTech 200 at 225 °C with trimethylaluminum and deionized (DI) water precursors, under conditions which yielded a growth rate of ~1.1 Å/pulse cycle. Initial pulse sequences and final oxide thickness (10-30 nm) were varied to determine the effect on oxide coverage. Al₂O₃ coverage and uniformity was characterized with atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy, while mobility changes were ascertained with van der Pauw Hall measurements.

Results show that a 30 nm conformal, uniform Al₂O₃ film can be obtained using a simple wet chemical surface treatment prior to ALD. Mobility was maintained indicating little impact of the pretreatment or deposited oxide on the underlying graphene properties. While the initial pulse sequence had minimal effect on the coverage and uniformity of thick films, it is a significant factor in obtaining conformal thin films (< 30 nm). Better coverage with thinner films was achieved using 10 initial water pulses. Key to the success of this approach is the ability to nucleate on both terraces and step edges. Variations in coverage as a function of oxide thickness and deposition temperature will also be discussed.

Since graphene growth on Si-face SiC substrates results in a surface morphology with step heights on the order of 5-10 nm, only the 30 nm oxide was completely uniform. We hypothesize that the steps inhibit deposition of ultrathin conformal films (≤ 10 nm) needed for scaling of devices. Therefore, we have deposited thin Al₂O₃ films on step-free graphene mesas^[4] to elucidate the affect of steps on oxide uniformity and obtain the ultimate limit of this wet chemical approach.

[1] J.A. Robinson, et.al. ACSNano 4(5) 2667-2672 (2010).

[2] B.Lee, et.al. ECS Transactions 19(5) 225-230 (2009).

[3] D.B. Farmer, et. al. Nano Letters 9(12) 4474-4478 (2009).

[4] L.O.Nyakiti, et.al. presented at STEG conference, Amelia Island, FL 2010.