Infrared Spectroscopy of Divacancy-Associated Radiation-Induced Absorption Bands in Silicon

Abstract

Defect infrared-absorption bands induced by fast neutrons in $n$-type, $p$-type, and intrinsic silicon were investigated by the piezospectroscopic method. The divacancy-associated bands at 1.8 and 3.3 μ together with two sharp subbands at 3.45 and 3.61 μ were observed in all types of Si when the measurements were made below 150°K. When measurements were made at 300°K the 1.8-μ band was observed in all samples while the 3.3-μ band was observed only in $n$-type Si. The 3.9-μ band which has been identified to be associated with the + 1 charge state of the divacancy defect was not observed. Detailed temperature-dependence studies of the fine structures of the 3.3-μ band give no evidence of phonon structure. The fine structures of the 3.3-μ band are not associated with a single -2 charge state of the divacancy as previously reported but rather are due to three different optical transitions. We conclude that the charge state of the divacancy defect responsible for the observation of these bands is not the same everywhere in the sample. Detailed low-temperature-stress studies of the absorption bands utilizing polarized light lead to large dichroism and small splitting for the 3.4- and 3.61-μ sharp bands, with smaller dichroism and large splitting for the 1.8-μ band. Stress-induced splitting of the bands at 78°K leads to a confirmation of the model of the divacancy defect deduced from electron-paramagnetic-resonance (EPR) studies by Watkins and Corbett. In terms of a one-electron molecular-orbital model of energy levels the bands at 1.8, 3.45, and 3.61 μ are identified as associated with the $X-Y$ dipole-moment transitions. The 3.3-μ band arises from an optical transition due to ionizations of electrons from the weak-orbital-bonding state to the nearest conduction-band edge.