Radiation Damage in Space The bulk properties of silicon can be dramatically affected by radiation damage. Study of the effects of radiation damage on silicon solar cells has been going on since the first satellites were put into orbit. The various radiation sources: solar wind, Van Allen belts, and cosmic rays, have been categorized in an early work published by the American Society for Testing and Materials [41]. Controlled studies using energetic electrons, ions and neutrons have provided even greater insight into the problem of radiation damage of semiconductors. The effect of energetic particles on silicon has perhaps been best characterized as the result of the study of the ion implantation of silicon. It is known that at higher energies ions undergo electronic interactions resulting in little structural damage to the silicon lattice. At lower acceleration energies or as the ion decelerates nuclear interactions occur and the displacement of atoms from their original lattice sites occurs. As the mass of the incident ion increases the damage for a given acceleration energy increases. As the energy increases the depth of damage increases. Since only a threshold energy of <50 eV is required to displace a silicon atom to a stable interstitial site accelerations of several keV to MeV will result in the displacement of thousands of lattice atoms per incoming ion. Electron, neutron and ion damage reduces electron and hole mobility and as a result electrical conductivity. Energy levels are introduced into the band gap of silicon which can lead to compensation and possibly type conversion as well as reduce minority carrier lifetime. Reductions in minority carrier lifetime and carrier mobility will both reduce solar cell efficiency. Damage is generally classified into two broad categories: light and heavy damage. Light damage is characterized by discrete, spatially isolated damage regions and heavy damage is characterized by a continuous amorphous region of damage. The significant difference between these two categories of damage lies in the ease with which this damage can be annealed out. The amorphous region produced by heavy damage will regrow via solid phase epitaxy on the underlying silicon substrate. This regrowth occurs at moderate temperatures (550-650 C) [42]. Light radiation damage is much more difficult to anneal out, and as Table II indicates temperatures as high as 750 C are required to eliminate all the deep levels associated with such damage
RkJQdWJsaXNoZXIy MTU5NjU0Mg==