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 *mem6040@psu.edu American Nuclear Society 2014 Student Conference Pennsylvania State University State College, Pennsylvania, USA, April 4, 2014

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

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

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.

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 18 - 20 eV. Damage to graphene only

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

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

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

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

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)

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.

0.1MeV Neutron Interactions in 10μm of Si Substrate

1MeV Neutron Interactions in 10μm of Si Substrate

5MeV Neutron Interactions in 10μm of Si Substrate

14.1MeV Neutron Interactions in 10μm of Si Substrate

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

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.

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)

Cross Section of Natural Boron

Cross Section of Natural Carbon