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Media type:
E-Article
Title:
(Invited) Enabling High Quality Dielectric Passivation on Monolayer WS2 Using a Sacrificial Graphene Oxide Template
Contributor:
Wyndaele, Pieter-Jan;
de Marneffe, Jean-Francois;
Sergeant, Stefanie;
de la Rosa, Cesar Javier;
Brems, Steven;
Caro, Arantxa Maestre;
De Gendt, Stefan
Description:
<jats:p> Two-dimensional transition metal dichalcogenides (2D TMDC’s) hold a wide variety of applications, among which microelectronic devices. 2D TMDC’s are promising alternatives for today’s silicon-based technology but suffer from various integration challenges. Among others, direct dielectric growth on the 2D surface occurs poorly due to their self-passivated surfaces resulting in island-like growth initiated at defect sites. This work focuses on enabling uniform high-k dielectric nucleation and growth on 2D TMDC’s via a sacrificial, graphene oxide-based buffer layer. Essentially, the role of the sacrificial graphene buffer layer is twofold: 1) serve as a passivation layer, protecting the underlying 2D TMDC (i.e., WS<jats:sub>2</jats:sub>) during processing and 2) act as a nucleation layer, enabling uniform atomic layer deposition (ALD) high-k dielectric (i.e,. HfO<jats:sub>2</jats:sub>) growth. A graphene layer is transferred on monolayer WS<jats:sub>2</jats:sub>, after which polymeric transfer residues are cleaned via a combination of wet treatments and a dry hydrogen downstream plasma exposure. The cleaned graphene is functionalized via a dry UV/O<jats:sub>3</jats:sub> oxidative treatment, which enriches its basal plane with carbon-oxygen single bond functionalities by sacrificing its π-network. This way, the graphene carbon lattice itself remains intact and retains its passivation properties. Moreover, a substrate dependency is observed where graphene transferred on WS<jats:sub>2</jats:sub> requires longer UV/O<jats:sub>3</jats:sub> exposure to ensure functionalization compared to a SiO<jats:sub>2</jats:sub> substrate and can be explained by UV-light induced, ultrafast charge transfer between the graphene and WS<jats:sub>2</jats:sub> monolayer. The carbon-oxygen groups formed on graphene’s basal plane act as nucleation sites during a subsequent HfO<jats:sub>2</jats:sub> ALD process. A constant roughness is noted for the HfO<jats:sub>2</jats:sub> film grown on the graphene oxide capped WS<jats:sub>2</jats:sub>, indicating uniform material deposition even in the early stages of the ALD process. This is in sharp contrast when HfO<jats:sub>2</jats:sub> is directly grown on the WS2 monolayer, whose roughness stabilizes only after many ALD cycles (up to 256), demonstrating the poor, island-like growth mechanism. In addition, a similar hafnium areal density is measured for the graphene oxide capped WS<jats:sub>2</jats:sub> compared bare SiO<jats:sub>2</jats:sub>, which was selected to represent a well-functionalized surface and thus confirming the efficiency of the graphene oxide-based nucleation layer. For devices incorporating the graphene oxide buffer layer concept, a crucial question is to what extent the graphene oxide layer gives rise to leakage currents by providing a conductive path between source and drain. Electrical evaluation of a GFET by means of I-V measurements reveal a significant shift in Dirac point towards positive gate voltage for transferred graphene, which is typically associated with p-type doping originating from the polymeric transfer residues still present on the surface. The Dirac point shifts towards 0-point voltage after final hydrogen downstream plasma cleaning, confirming the effectiveness of the established cleaning protocol. Moreover, Graphene’s conductivity is suppressed after UV/O<jats:sub>3</jats:sub> to the nA range, as caused by the formation of carbon-oxygen functionalities and associated sacrifice of its π-network. The residual current may be further suppressed by extending the UV/O<jats:sub>3</jats:sub> treatment, but nevertheless demonstrates that the graphene oxide buffer layer will not give rise to significant leakage currents. </jats:p>