Displacement Damage

Overview

Incident energetic particles on a solid experience ionizing and non-ionizing energy loss (NIEL). For charged particles the ionizing energy loss predominates and results in the production of electron-hole pairs. As second effect massive particles transfer momentum on the atoms in the solid, thereby displacing the atoms from their positions in the crystal lattice. The resulting unoccupied lattice site is called vacancy. The displaced atom eventually settles in a non-lattice position, a so called interstitial. Often the energy of the incoming particle is high enough to displace several atoms and the displaced atoms themselves can displace other atoms on their way through the crystal creating a cluster of defects.
Particles that give rise to displacement damage are mainly protons, neutrons, electrons, and heavy ions. The major particle of concern for displacement damage in the natural space environment is the proton.
Defects in the periodicity of the lattice give rise to new energy levels within the bandgap of a semiconductor resulting in a change of the optical and electrical properties of the material. Those levels lead to (i) generation of electron-hole pairs which increases the leakage current, (ii) act as recombination centers of electron-hole pairs which causes gain degradation in bipolar transistors, (iii) temporarily trap electrons which reduces the charge-transfer efficiency in CCDs, (iv) compensate donors or acceptors which alternates device properties that depend on the carrier concentration, (v) let electrons tunnel though barriers by means of defect levels which causes a reverse current in pn junctions, and (vi) act as scattering centers for charge carriers which decreases their mobility1.

 

1 J.R. Sour et al, IEEE Transactions on Nuclear Science, Vol. 50, No. 3, 653 (2003)

Figure 1: D-711 neutron generators

Figure 2: Typical test board for irradiation

Figure 3: JULIC cyclotron at Forschungszentrum Jülich

Test capabilities at Fraunhofer INT

The testing of displacement damage is usually done with either protons or neutrons.
Fraunhofer INT offers two neutron generators (Fig. 1) which are suitable for investigating displacement-damage effects in electronic or optical components. The energies of the fast neutrons are 2.5 MeV or 14.1 MeV, respectively. The generators can be regarded as a point source and produce up to 3·1010 neutrons/s in 4π. The fluence is monitored online during the irradiation with a calibrated uranium fission chamber.
Additionally, Fraunhofer INT has access to a dedicated irradiation area at the cyclotron JULIC at Forschungszentrum Jülich FZJ (Fig. 3). The protons have an energy of 39 MeV at the irradiation area. Irradiations are done in air. Fraunhofer INT has access to the cyclotron about every two months.