Graphene on a hydrophobic substrate: Doping reduction and hysteresis suppression under ambient conditions (0910.2596v1)

Abstract: The intrinsic doping level of graphene prepared by mechanical exfoliation and standard lithography procedures on thermally oxidized silicon varies significantly and seems to depend strongly on processing details and the substrate morphology. Moreover, transport properties of such graphene devices suffer from hysteretic behavior under ambient conditions. The hysteresis presumably originates from dipolar adsorbates on the substrate or graphene surface. Here, we demonstrate that it is possible to reliably obtain low intrinsic doping levels and to strongly suppress hysteretic behavior even in ambient air by depositing graphene on top of a thin, hydrophobic self assembled layer of hexamethyldisilazane (HMDS). The HMDS serves as a reproducible template that prevents the adsorption of dipolar substances. It may also screen the influence of substrate deficiencies.

• The paper shows that HMDS-treated substrates significantly reduce intrinsic doping and nearly eliminate hysteresis in graphene under ambient conditions.
• The research employs an extended HMDS treatment on SiO₂ to form a self-assembled hydrophobic barrier that limits water-induced p-doping.
• The improved charge carrier mobility and lower back-gate voltage requirements pave the way for more reliable graphene FETs and scalable device fabrication.

Doping Reduction and Hysteresis Suppression in Graphene Utilizing Hydrophobic Substrates

The paper "Graphene on a hydrophobic substrate: Doping reduction and hysteresis suppression under ambient conditions" explores a significant challenge in the field of graphene-based electronics: the variation in intrinsic doping levels and the presence of hysteresis in graphene devices. These issues are primarily influenced by substrate morphology and environmental conditions. The authors address this by introducing a novel substrate preparation methodology incorporating hexamethyldisilazane (HMDS) to mitigate these effects.

Graphene, known for its exceptional electronic properties, is often prepared by the mechanical exfoliation of graphite onto silicon dioxide (SiO$_2$) substrates. Despite its potential in electronics, significant challenges exist in controlling doping levels and minimizing hysteresis, which affect device performance and reproducibility. These properties are sensitive to substrate conditions, processing methods, and environmental contaminants, notably water-induced p-doping, which highlights the need for advancements in substrate preparation.

The authors propose the use of a thin, hydrophobic HMDS layer as a substrate modification method to improve the transport characteristics of graphene. The HMDS layer serves as a barrier, mitigating the adsorption of dipolar substances, such as water, which are implicated in doping and hysteresis. By forming a self-assembled layer on SiO$_2$, HMDS provides a consistent, hydrophobic surface that reduces these undesirable effects.

The experimental methodology involved cleaning the SiO$_2$ substrates followed by hydrophobizing them with a long HMDS treatment. Graphene was then transferred and characterized using standard electronic measurements. A comparison was made between graphene samples on HMDS-modified substrates and those on untreated SiO$_2$.

Key findings indicate that graphene on HMDS-modified substrates exhibits significantly reduced intrinsic doping levels and nearly eliminated hysteresis, even under ambient conditions. Furthermore, the charge carrier mobility observed in these samples was notably higher compared to those on bare SiO$_2$. The charge neutrality point was attained at low back-gate voltages, reinforcing the efficacy of HMDS in stabilizing the electronic properties of graphene.

The paper's findings have crucial implications in the development of reliable graphene-based field-effect transistors (FETs) and other electronic applications. The use of HMDS-treated substrates presents a reproducible method to enhance graphene's electronic properties without necessitating vacuum conditions or extensive post-processing treatments. This advancement paves the way for more consistent graphene device fabrication, with potential applications in a range of electronic and optoelectronic systems.

Further exploration is needed to examine the long-term stability and scalability of these methods in industrial applications. The paper also encourages future investigations into alternative hydrophobic treatments or combinations of surface modifications that can further enhance graphene's properties. Moreover, understanding and controlling the interactions at the interface level will remain a focal area to refine approaches for optimizing graphene's performance in ambient conditions.