Sep 28, 2012
Lift off for nanotube transistors
Single-walled carbon nanotube transistors containing a silicon oxynitride gate dielectric layer might be ideal for use in the harsh, ionizing environment of space. So say researchers at the US Naval Research Laboratory who have performed experiments proving that the devices are resistant to doses of gamma radiation as high as 2 Mrad.
Ordinary single-walled nanotube thin-film transistors are susceptible to ionizing radiation, in the same way that silicon-based field-effect transistors are. The root cause of the effect is hole trapping in the silicon oxide gate dielectric layer. To overcome the problem, Cory Cress and colleagues have developed so-called radiation-hardened SWCNT-TFTs comprising a silicon oxynitride (SiON) gate dielectric layer instead, which is less susceptible to radiation-induced changes.
“The SiON layer appears to be less sensitive to radiation thanks to fewer trapped charges in the material and because it has a tendency to trap both electron and holes. This yields a net neutral radiation-induced charge build-up that has little effect on the transport properties of the SWCNTs.”
The researchers are the first to show that such SWCNT-based transistors are resistant to Co-60 gamma radiation to a total dose of 2 Mrad
The Earth’s magnetic field traps energetic charged particles in two toroidal-shaped radiation belts, known as the Van Allen belts. As spacecraft orbit our planet, they repeatedly pass through these belts, exposing them to high doses of ionizing radiation from energetic electrons and protons. The energy spectrum and incident angles of this radiation yield a fairly uniform dose, explains Cress, which researchers can simulate using Co-60 gamma rays in the laboratory.
Hardened gate dielectric layer
As semiconductors, SWCNTs are not permanently affected by gamma radiation because the gamma rays interact with electrons in the material, which leads to a rapid charge excitation that quickly relaxes, leaving the SWCNT unharmed. “However, gamma rays can indirectly disrupt the crystalline lattice of these nanotubes,” said Cress, “by first exciting an electron to an energy in excess of the displacement threshold (in the 90 -120 keV range for SWCNTs). This subsequently displaces a carbon atom from the lattice.”
The probability of these events happening in our materials for a 2 Mrad dose appears to be very small, he adds.
For most majority-carrier electronic devices, the gate dielectric and isolation layers trap charges (positive in the case of SiO2) when exposed to radiation, something that ultimately leads to poor device performance. “Our devices are not affected thanks to the hardened gate dielectric layer we have developed,” Cress told nanotechweb.org.
The future: ballistic devices
The team investigated SWCNT-TFTs that operate in the “diffusive transport” regime, in which charge carriers are multiply scattered by nearby defects that include SWCNT lattice defects, SWCNT-SWCNT boundaries and phonons. These defects thus control charge transport and how the nanotube devices respond to radiation. “Future SWCNT field effect transistors will possess short channels where transport is ballistic throughout the devices,” added Cress. “As a result, the properties of the SWCNT-metal contacts will play a more dominant role in device performance and will need to be much better understood. We are therefore focusing our future research efforts on transistors that operate in the ballistic transport regime.”
The researchers report their current work in IEEE Trans. Nucl. Sci., MRS Commun. and Electronics.
About the author
Belle Dumé is contributing editor at nanotechweb.org.