Radiation damage in semiconductors

Radiation damage is the general alteration of the operational and detection properties of a detector, due to high doses of irradiation. In semiconductor devices, high-energy particles produce three main types of effects Lint87:

* \- Displacements. These are dislocations of atoms from their normal sites in the lattice, producing less ordered structures, with long term effects on semiconductor properties.
* \- Transient ionization. This effect produces electron-hole pairs; particle detection with semiconductors is based on this effect.
* \- Long term ionization. In insulators, the material does not return to its initial state, if the electrons and holes produced are fixed, and charged regions are induced.

Displacements determine the degradation of the bulk, and long term ionization is responsible for surface damage.

Producing displacements is a four-step process:

* \- The primary particle hits an atom in the lattice, transferring enough energy to displace it. Thus, interstitials and vacancies appear, and their pairing - the so-called Frenkel defects. In the case of high energies, nuclear reactions can occur, producing several fragments or secondary particles.
* \- The fragments of the target atom migrate through the lattice causing further displacements. The mean free path between two succesive collisions decreases towards the end of the range, so that defects produced are close enough and can interact.
* \- Thermally activated motion causes rearrangement of the lattice defects at room temperature (annealing). Part of these rearrangements are influenced by the presence of impurities in the initial material.
* \- Thermally stable defects influence the semiconductor properties, i.e. also the detector parameters.

Effects of displacements are to be seen in the increase of capture, generation and recombination rates of the non-equilibrium charge carriers. In detectors they cause changes of the internal electric field, due to the modified doping concentration, going eventually up to inverting the conduction type for very high irradiations, increase of the leakage current, changes in capacitance and resistivity, and charge collection losses.

Long term ionization effects also comprise several steps:

* \- Ionization is produced along the track of the primary ionizing particle, or sometimes in restricted regions around a nuclear reaction. Electrons and holes are created, with a certain distribution.
* \- Many of the e-h pairs produced recombine before they could move due to diffusion or the electric drift. Recombinations take place between particles produced in the same or in different events.
* \- The electrons which did not recombine in the initial phase diffuse or drift away. Some electrons end up on traps, others may escape from the insulator.
* \- The carriers trapped on levels with low ionization energies are thermally reexcited and get into the conduction or valence band; they are subject to further drift or diffusion, and leave the dielectric or are captured on deep trap levels (practically permanent).
* \- Apart from the production of trapped charge, in the energy gap new oxide-silicon interface levels are induced. These interface states are occupied by electrons or holes, depending on the position of the Fermi level at the interface.
* \- The net effect of the induced charges in the oxide is the change of the electric field in the semiconductor, in the vicinity of the interface.

All effects depend on the particle type and the incident energy.

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