Infrared absorption studies of the divacancy in silicon: New properties of the singly negative charge state

The infrared optical absorption peak at 0.34 eV in silicon, which is usually associated with the singly negative charge state of the divacancy, has been investigated in electron-irradiated samples with use of different optical excitation conditions. It is proposed that a strong Jahn-Teller distortion makes it possible to populate this charge state, when the defect initially is in the neutral charge state, either by the capture of a photoexcited free electron from the conduction band, or by the direct photoexcitation of an electron from the valence band to a defect orbital. Experimental evidence for the existence of these reactions is presented. A defect level at $E_c$-0.54 eV, frequently associated with the singly negative charge state of the divacancy, is identified as one of the levels from which these photoexcited free electrons originate. The 0.34-eV peak is attributed to an internal transition in the singly negative charge state of the divacancy center, implying the existence of a shallow defect state at approximately $E_c$-0.07 eV for this charge state. Experimental support is given for the existence of this shallow state. A tentative explanation, based on the strong Jahn-Teller distortion of the singly negative charge state, is suggested for the fact that the doubly negative charge state is not observed at temperatures below 90 K.