In spite of major efforts for more than 50 years, the understanding of radiation damage in silicon by hadrons remains a big challenge. Reasons are the large number of radiation-induced states in the silicon band gap, the formation of defect clusters and the difficulties of their simulation. It is well known that Si recoils from the interaction of neutrons and energetic electrons and hadrons produce locally dense cascades of Si vacancies, V, and interstitials, I, with densities as high as cm−3 [1]. Inside these clusters the mobile vacancies can agglomerate to multi-vacancy states, in particular to the divacancy, V , which is immobile at room temperature. In [2] it has been estimated that there are about 12 V states per cluster, which have typical dimensions of (5 nm) .
The V defect is known to come in four charge states: . The neutral V state, which has an energy eV, can trap a hole or an electron resulting in states with energies eV or eV, respectively, where denotes the energy of the valence and of the conduction band. The negative V state can trap electrons, and the resulting state has the energy eV [3]. As discussed in [4], [5], [6], the V state gives rise to absorption bands at wavelengths of m and m, and the V state at m. From the absence of photocurrent for illumination with m light [4], it is concluded that the photons induce transitions from the V ground state to an excited state with energy . A qualitative term scheme, deduced from the results of EPR experiments is shown in [5]. The NIR absorption data of [4] (Fig. 2) also show that the overall intensity of the m band is approximately independent of temperature. However, the mean photon energy, , decreases from about 730 meV ( m) at 100 K to 690 meV ( m) at 300 K, and the full-width-half-maximum, , increases from 90 meV to 110 meV.
The annealing of the V in silicon with different doping and impurities has been studied by several groups. An example are the results from [5] for 20 min isochronal annealing and temperatures up to 330 C: Significant annealing starts around 150 C, the 50% point is around 240 C, and complete annealing is reached at 280 C. From the publications it is however not clear if the overall intensity or the maximum of the m band is shown, nor if or change during annealing.
In this work the light transmission for photon energies, , between 0.62 and 1.3 eV has been measured for phosphorous-doped high-ohmic Si irradiated by reactor neutrons to 1 MeV neutron equivalent fluences of (1, 5, 10) cm−2 and 15 min isochronal annealing in C steps up to C. From the transmission the absorbance has been obtained. For the radiation-induced absorbance, , the absorption data for non-irradiated Si from [7] were subtracted. The spectra for 0.62 eV eV were fitted by a Breit–Wigner function for the V excitation and a phenomenological background parametrization to determine the values of the V intensity, , and of the Breit–Wigner parameters and , as a function of and .
The next section gives an overview over the Si samples and the measurement techniques. This is followed by a discussion of the analysis method. Finally the results are presented and their relevance discussed.