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The Formation of H2* by Electron-Irradiation of Hydrogenated Si

To clarify the formation mechanism of H2* in Si, we studied the dependence of the concentration of H2* on the electron-irradiation dose in hydrogenated Si crystals. Specimens were prepared from high-purity, B-doped and C-doped Si crystals. They were doped with H by annealing in H2 gas at 1300°C followed by quenching. They were irradiated by 3 MV electrons at room temperature (RT). We measured their optical absorption spectra at 7 K by an fourier-transform infrared absorption spectroscopy (FT-IR) spectrometer. The intensity of the 1838 cm-1 peak which is due to H2* was found to be the weakest in C-doped Si, medium in B-doped Si and the strongest in high-purity Si. This tendency correlates with that of the intensity of the 2122 cm-1 peak which is due to a complex of a vacancy and two H atoms. The intensity of the 1838 cm-1 peak linearly depends on the electron dose at low doses. These results suggest two mechanisms for the formation of H2* from H2, namely, a combination of a vacancy and an H2 or a combination of a Frenkel pair and an H2.