Photoconductivity Studies of Defects inp-Type Silicon: Boron Interstitial and Aluminum Interstitial Defects

Two defects introduced in $p$-type silicon by 1.5-MeV electron irradiation are studied by means of infrared photoconductivity, including the measurement of the stress-induced dichroism. They are identified as being dopant atoms in interstitial positions produced by the silicon interstitial-impurity atom replacement mechanism proposed by Watkins. They are introduced by room-temperature irradiations as well as by irradiations performed at 77, 20.4, and 4.2°K. They disappear during annealing at temperatures ∼250-300°C. The symmetry of these defects $C_{3v}$ is deduced from the low-temperature stress-induced dichroism of the photoconductivity which is associated with electronic reorientation among different configurations. This $C_{3v}$ symmetry can be explained by distortion of a possible Jahn-Teller type of a configuration in which the dopant atom was originally in a tetrahedral position. The defect response to the stress is determined by the value of the term in the piezospectroscopic defect tensor which characterizes the relative change in defect energy per unit strain. This value is ≃ -12 eV/(unit strain). Numerical values of the dichroic ratios show that the photoconductivity transition which is observed corresponds to a distribution of dipole moments which is an ellipsoid of rotation about the trigonal axis of the defect. They also allow the determination of this distribution.