1. The Simulation of Neutron Damage within Graphene Field Effect Transistors (GFETs)
M. Edward Moore*, Edward Cazalas, Dr. Igor Jovanovic
The Pennsylvania State University
Department of Mechanical and Nuclear Engineering
University Park, PA 16802
American Nuclear Society 2014 Student Conference
Pennsylvania State University
State College, Pennsylvania, USA, April 4, 2014

2. Graphene Research Team
Edward Cazalas
G Research Assistant -PSU
Nuclear Engineering
Biddut Sarker
Post Doc. Researcher -Purdue
Physics
Michael E. Moore
U Research Assistant -PSU
Nuclear Engineering
Isaac Childres
G Research Assistant –Purdue
Physics
Chris Sopko
U Research Assistant -PSU
Nuclear Engineering
Yong P. Chen Professor –Purdue
Physics
Dr. Igor Jovanovic
Advising Professor - PSU
Nuclear Engineering

3. These attributes make GFETs attractive for various applications
What is the Motivation?
The main motivation for novel advancements in the field of radiation detection is the accurate detection of nuclear weapons materials.
Such materials are detected by achieving high sensitivity to ϒ-rays and neutrons.
Graphene Field Effect Transistors’ sensitivity to neutrons is being explored because of their potential advantages, including:
Cost effectiveness
Low-power usage
Size-scalable architecture
Light weight
Flexible
These attributes make GFETs attractive for various applications

4. GFET Operation Illustrated
As a GFET is irradiated, the radiation causes ionization within the insulating substrate.
Choice of substrate may be tailored to particular applications (i.e. gamma or neutron detection).
Ionization causes electron-hole pairs that may be drifted by the electric field toward or away from the layer of graphene. Ionization causes the substrate to increase its conductivity.
Thus, while being irradiated, the electric field distribution within the GFET changes, with the electric field near graphene increasing.

5. Multiple mechanisms exist that can induce damage to graphene.
Damage Mechanisms of Graphene
Multiple mechanisms exist that can induce damage to graphene.
The direct mechanism by which a neutron interacts directly with graphene has negligible probability.
Neutrons may also interact with the substrate, which can result in interaction products. These products have a greater probability of removing of a carbon atom or breakage of its bonds.
Removing a carbon atom from graphene only requires approximately eV.
Damage to graphene only

6. Cross Section of Natural Boron
Creation of Ions
In the BN substrate, the assumption is made that α and Li ions are produced by the capture reaction n (10B, α) 7Li
Cross Section of Natural Boron
Blue: Total
Green: Elastic scattering
Red: Absorption

7. Cross Section of Natural Silicon
Creation of Ions
In the Si substrate, the assumption is made that Si ions are produced by a collision event with a fast neutron
Cross Section of Natural Silicon
Blue: Total
Green: Elastic scattering
Red: Absorption

8. Absorption Reaction in BN
Damage by Ions
Damage will be defined as ions that leave the substrate and pass through the graphene.
Neutrons will have direct collisions with graphene, however the probability of occurrence will be approximately 10 orders of magnitude less than indirect interactions.
Because the substrates being considered are silicon and boron nitride, the ions that are going to be damaging are silicon, lithium and alphas.
Absorption Reaction in BN

9. Simulation of Ions
Experimental measurements in which GFET radiation damage is observed are predicted to require very large neutron fluence, making it impractical for direct measurements within the scope of this work. Thus, the damage is modeled by simulations:
SRIM- define the ranges of ions originating from BN
MCNP6- simulate the number of damaging ions originating from BN
simulate these ions’ energies
Geant4- illustrate Si ions’ ranges at various energies
Establishing the amount Si ions that will cause damage and their energies is currently under investigation

10. SRIM, Ranges Depth of Li Ion in BN Depth of α in BN
Since only the creation of heavy ions that possess the necessary energy to travel through their substrate and reach the graphene are of interest, the ranges of the heavy ions are determined.
(Ranges simulated with the use of SRIM-2013)

11. MCNP, Energies and Damaging Ions
The lowest recorded ion energies were 3 orders of magnitude greater than the energy needed to remove carbon.
140,000 α and 56,000 Li ions were tallied passing through the graphene, with a source of 2.0E07 neutrons.

12. 0.1MeV Neutron Interactions in 10μm of Si Substrate

13. 1MeV Neutron Interactions in 10μm of Si Substrate

14. 5MeV Neutron Interactions in 10μm of Si Substrate

15. 14.1MeV Neutron Interactions in 10μm of Si Substrate

16. From this research it can be seen that the damage of graphene is dependent upon substrate properties such as composition, density and thickness.
Further, this research also demonstrates the indirect neutron induced damage mechanisms’ creation, trajectories and energies.
Future work intends to correlate results with the effects on graphene’s electrical and mechanical properties.
Prosperities such as composition, density and thickness

17. Although the focus of this report is the damage of GFETs, the findings of this research pertain to other materials besides graphene. This research can be translated to other two dimensional materials such as molybdenum disulfide (MoS2) and tungsten(IV) disulfide (WS2). This research may also have applications with other types of field effect transistors and thin-film detectors.

18. This work is conducted in collaboration with Purdue University Department of Physics and with support of the Department of Homeland Security (DHS) and the National Science Foundation (NSF) through the Academic Research Initiative (ARI) (2009-DN-077-ARI036-02) and by the Defense Threat Reduction Agency (DTRA)

20. Cross Section of Natural Boron

21. Cross Section of Natural Carbon