Abstract

The development of nanocircuits for space applications is an emerging technology area for NASA. We will preset our recent research results on the performance of both nanomaterials and actively running nanocircuits during heavy ion irradiation. The radiation experiments were performed at the National Superconducting Cyclotron Laboratory at Michigan State University whose available primary beams and beam energies well match the energy spectra of abundant charged particles in galactic cosmic rays. Primary beams of Krypton having mass numbers of 78 and 86 respectively were used in two sets of experiments, representative of the high-Z heavy ions encountered in a space radiation environment. Introduction Energetic heavy ions having many-tens to hundreds of MeV per nucleon are a serious source of non-recoverable electronic upsets for satellites. They are highly penetrating and capable of generating penetrating secondary ions in all but the most massive radiation shielding. As the increased use of miniaturized equipment corresponds to a decreased availability to carry any shielding, radiation damage by heavy ions will become an increasing challenge. The development of nanocircuits for space applications is an emerging technology area for NASA. We will present our recent research results on the performance of both nanomaterials and actively running nanocircuits during heavy ion irradiation. The radiation experiments were performed at the National Superconducting Cyclotron Laboratory at Michigan State University. The National Superconducting Cyclotron Laboratory is particularly suited for such studies because its available beam energies well match the energy spectra of abundant charged particles in galactic cosmic rays. Primary beams of Krypton having mass numbers of 78 and 86 respectively were used in two sets of experiments, representative of the high-Z heavy ions encountered in a space radiation environment. 12th NASA Symposium on VLSI Design, Coeur d’Alene, Idaho, USA, Oct. 4-5, 2005

Introduction

Energetic heavy ions having many-tens to hundreds of MeV per nucleon are a serious source of non-recoverable electronic upsets for satellites. They are highly penetrating and capable of generating penetrating secondary ions in all but the most massive radiation shielding. As the increased use of miniaturized equipment corresponds to a decreased availability to carry any shielding, radiation damage by heavy ions will become an increasing challenge.

The development of nanocircuits for space applications is an emerging technology area for NASA. We will present our recent research results on the performance of both nanomaterials and actively running nanocircuits during heavy ion irradiation. The radiation experiments were performed at the National Superconducting Cyclotron Laboratory at Michigan State University. The National Superconducting Cyclotron Laboratory is particularly suited for such studies because its available beam energies well match the energy spectra of abundant charged particles in galactic cosmic rays. Primary beams of Krypton having mass numbers of 78 and 86 respectively were used in two sets of experiments, representative of the high-Z heavy ions encountered in a space radiation environment.

In one set of experiments gallium nitride (GaN) nanowires and nanocircuits were irradiated using a primary beam of Krypton having mass number 78 ( 78 Kr) at 140.32 MeV per nucleon (MeV/u), delivered by the coupled cyclotron facility at the National Superconducting Cyclotron Laboratory (NSCL) The experiments were performed in the NSCL S1 Vault Single Event Excitation Test Facility (SEETF) Vault end station, which is specially equipped with electronic and beam operations connections to a remote control room 1 . This enabled the first real-time characterization of the electronic performance of a GaN-based field effect transistor (FET) during 78 Kr irradiation. Irradiations of GaN nanowires were also carried out separately under the same beam conditions.

In another set of experiments, carbon nanomaterials including single and multi wall carbon nanotubes and electrospun carbon nanofibers, with well-graphitized vapor grown carbon fibers as controls, were irradiated using a primary beam of Krypton having mass number 86 ( 86 Kr) at 142A MeV. All of these nanomaterials are hollow care graphitic structures with varying wall structures.

(1) Summary Of Real -Time Electronic Performance Of Irradiated Gan Nanocircuit

We will summarize real-time GaN nanocircuit performance during irradiation previously reported in References (Error! Bookmark not defined.,2) to show the potential that nanowire based devices may have in radiation environments. A GaN nanowire FET design using an n-type semiconducting channel 3 was used in the radiation experiments ( Figure 1 ). Nanotube/nanowire FET designs achieve transistor functionality via unconventional Schottky barrier modulation at the contacts rather than by standard channel modulation 4, 5 . The nanowire was connected to contacts pads using electron beam lithography and wire-bonded to a dual-in-line package.. The I-V characteristics of the FET were taken before, during and after irradiation with different gate-source voltages (V GS ) and swept source-drain voltages (V SD ), using a Keithley 487 Picoammeter/Voltage Source and a Hewlett Packard 6633A System DC Power Supply. Measured pre radiation I-V characteristics at 0, 3, 6, and 9 Volts V GS demonstrated that the nanowire was n-type and its conduction could be altered by varying V GS .

Real time electronic measurements during 78 Kr heavy ion irradiation were implemented in LabVIEW 6 using a remote-connection computer during the experiments. All measurements were made in air at room temperature. Irradiation was initiated at a low level intensity of 10 2 particles per second (pps), and subsequently increased to 10 6 pps with intermediate intensities of 10 4 pps and 10 5 pps. The gate voltage was also increased through 0, 3, 6, and 9 Volts. Each combination of conditions was maintained for 600 seconds, therefore the beam fluence per run ranged from 3 x 10 4 to 3 x 10 7 particles/cm 2 .

The selected radiation levels corresponded to ones that have caused electronic upsets in conventional circuits during 78 Kr irradiation 7 and were the highest available at the SEETF facility.

The real time I-V characteristics of the active GaN nanowire FET during 78 Kr irradiation are shown in Figure 2 particles/ cm 2 , the post-radiation electronic performance of the GaN FET was again measured, first at 9 Volts V GS followed by 12 Volts V GS . Normal electronic function was observed under these post-irradiation high bias conditions.

(2) Pre-And Post Irradiation Characterization Of Gan Nanowires

The stable performance of the GaN FET nanocircuit is determined by both the nanowire properties and the nanocircuit architecture. The GaN nanowires were grown in a direct reaction of metal gallium vapor with flowing ammonia at 850-900°C without a catalyst, resulting in diameters of ~50-100 nm 8, 9 . They displayed a new two-phase coaxial crystalline homostructure with an inner wurtzite phase crystal structure and a coaxial outer zinc-blende phase crystal structure, reported in Reference (10) ..The inner wurtzite phase diameter was typically 30 to 40 nm and the outer zincblende layer was 20 to 60 nm, with an overall nanowire diameter of 50 to 100 nm. High resolution transmission electron microscopy (JEOL FS2200) with nanodiffraction, selected area electron diffraction, energy dispersive spectroscopy, electron energy loss spectroscopy, and fast Fourier transforms, of over thirty GaN nanowires grown under the above conditions was used to identify the two crystalline phases.

HRTEM images of the GaN nanowires are shown in Figure 3 (a-b). Most GaN nanowires displayed the smooth and highly crystalline inner core and outer layer, with a sharp ~1-3 atomic layer interface, shown in Figure 3 (a). However GaN nanowires with irregular outer layers were sometimes observed, as shown in Figure 3 (b).