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Published on: July 24, 2015
n-Type behavior of graphene supported on Si/SiO(2) substrates
Hugo E Romero1, Ning Shen, Prasoon Joshi
1Department of Physics, Pennsylvania State University, University Park, Pennsylvania 16802, USA.
ACS Nano
|February 12, 2009
Summary
This study reveals that silicon dioxide (SiO2) surface states donate electrons to graphene, shifting graphene field-effect transistors (FETs) from p-type to n-type behavior after annealing. This electron donation is crucial for understanding graphene device performance.
Area of Science:
- Materials Science
- Condensed Matter Physics
- Nanotechnology
Background:
- Graphene field-effect transistors (FETs) on silicon dioxide (SiO2) substrates are crucial for electronic applications.
- Understanding the electronic properties and charge behavior in graphene FETs is essential for device optimization.
- Environmental exposure and annealing conditions significantly impact graphene's electrical characteristics.
Purpose of the Study:
- To experimentally and theoretically investigate the electronic properties of back-gated graphene FETs on Si/SiO2 substrates.
- To determine the charge neutrality point and understand the transition of graphene from p-type to n-type behavior.
- To elucidate the role of SiO2 surface states in influencing graphene's electronic properties.
Main Methods:
- Experimental characterization of graphene FETs by sweeping gate voltage to determine the charge neutrality point.
- Annealing devices at elevated temperatures (200°C) in a vacuum environment (5 x 10^-7 Torr).
- Theoretical calculations using molecular dynamics (MD) to model SiO2 structure and density functional theory (DFT) to compute electronic structure.
Main Results:
- Graphene FETs initially exhibited p-type behavior in a laboratory environment.
- Annealing for 20 hours at 200°C in vacuum induced a transition to n-type behavior.
- A final excess electron density of approximately 4 x 10^12 e/cm^2 was observed, matching theoretical predictions.
- Theoretical calculations identified significant surface states in SiO2 near the conduction band edge that donate electrons to graphene.
Conclusions:
- The SiO2 substrate possesses surface states that donate electrons to graphene, leading to n-type behavior after annealing.
- This electron donation mechanism explains the observed shift in carrier type and density in graphene FETs.
- An electrostatic model was developed to describe the gate bias dependence of carrier density in these devices.
