The main objective of the RadCNT program was the characterization of fundamental mechanisms and charge transport phenomena governing the interactions between ionizing and non-ionizing radiation with carbon-based (nanotube and graphene) field-effect transistors (FETs) devices and integrated circuits (ICs). This effort was supported through the fabrication of aligned single-walled carbon nanotubes (SWCNT) FETs at the University of Southern California’s (USC) Nanotechnology Research Laboratory and through a collaboration with the Naval Research Laboratories (NRL) for radiation testing and expertise in radiation effects characterization.

(a) Schematic diagram and photograph of full wafer synthesis of aligned nanotubes on a 4-in. quartz wafer, (b) gold film deposition, (c) peeling off the gold film with nanotubes, (d) transfer of the gold film with the nanotubes onto a Si/SiO2 substrate, (e) etching away the gold film and (f) device fabrication on the transferred nanotube arrays.

The RadCNT program concentrated on understanding total ionizing dose (TID) effects on SWCNT and graphene FETs. Several TID experiments with SWCNT and graphene FETs with various gate configurations, dielectric materials and geometries were performed as part of this effort. Well-known mechanisms of radiation-induced degradation in FETs such as oxide charge buildup were confirmed in SWCNT and graphene FETs through in situ measurements following radiation exposure. The effects of ionizing radiation on charge-injection mechanisms that cause gate hysteresis in carbon-based electronics were also investigated and demonstrated experimentally for the first time in aligned SWCNT FETs.

Total ionizing dose (TID) experiments were performed in view of initial investigations in carbon-based electronics devices (both SWCNT and graphene FETs) that reported a strong dependence of the radiation response on the experimental environment. These experiments consisted of Co-60 gamma ray irradiation of aligned single-walled carbon nanotube (SWCNT) field effect transistors (FETs) fabricated with 30 nm Al2O3 gate dielectrics and individual back gate electrodes.

Results indicate net positive voltage shifts in the transfer (Id–Vgs) characteristics of FETs fabricated without a passivation layer masking the surface of the device (i.e., having the SWCNTs exposed to the testing environment). The measured shifts are attributed to contaminants that alter the condition at the surface of the device resulting in an effective p-type doping of the nanotubes. These results are consistent with previous investigations where molecular adsorption at the surface of the SWC-NTs and/or at the SiO2 surface near the CNTs results in electron traps that can modulate the nanotube carrier density when irradiated in air. On the other hand, devices fabricated with a passivation layer (i.e., 1 μm thick coating of 3612 photoresist) resulted in net negative voltage shifts characteristic of positive charge trapping in the oxide near the SWCNT/oxide interface.

The impact of ionizing radiation on charge injection mechanisms was established experimentally through in situ measurements of SWCNT FETs performed under static vacuum. Extractions of hysteresis width (h) indicated an increase of more than 20 % after 1 Mrad(SiO2) of TID exposure for worst-case conditions. This increase in h indicates a larger contribution from charge injection mechanisms as a function of ionizing radiation exposure. TCAD simulations were used to investigate the combined effect of hysteresis and radiation-induced degradation and to demonstrate its dependence on trap density, carrier lifetime and energy distribution.

The measurements of hysteresis as a function of increasing radiation exposure and gate voltage (Vg) sweep range reveal non-uniform buildup in the energy distribution of trapping centers near the SWCNT/dielectric interface. The results are consistent with extensive studies on classical bulk semiconductor/oxide interfaces (e.g., in the Si/SiO2 system).

TID experiments on graphene FETs were also performed as part of the Rad-CNT program. Graphene FETs fabricated by chemical vapor deposition (CVD) and transferred onto trimethylsiloxy (TMS)-passivated SiO2Si substrates and epitaxial graphene on 6H-SiC substrates (via Si sublimation) had similar TID responses. In both cases radiation exposure resulted in positive oxide trapped charges near the graphene/oxide interface as manifested by negative voltage shifts in the Id-Vgs characteristics, degradation of carrier mobilities and increased minimum conductivity.

A semi-empirical approach for modeling the radiation-induced degradation effective carrier mobility in graphene FETs was also developed as part of this effort. The modeling approach describes Coulomb and short-range scattering based on calculations of charge and electric field that incorporate radiation-induced oxide trapped charges. The model can correctly describe the transition of the dominant scattering mechanism as a function of effective vertical field and oxide trapped charge density. The proposed modeling approach was verified with experimental data from Co-60 irradiation of graphene FETs resulting in excellent qualitative agreement.

This work was done by Ivan Sanchez Esqueda, Cory D. Cress, Chongwu Zhou, Yuchi Che, Yue Fu, and Jonathan Ahlbin of the University of Southern California for the Defense Threat Reduction Agency. For more information, download the Technical Support Package (free white paper) below. DTRA-0011

This Brief includes a Technical Support Package (TSP).
Radiation Effects on Electronics in Aligned Carbon Nanotube Technology (RadCNT)

(reference DTRA-0011) is currently available for download from the TSP library.

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This article first appeared in the February, 2021 issue of Aerospace & Defense Technology Magazine.

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