Investigation of Neutron-Irradiation Damage in Silicon by Transmission Electron Microscopy

Abstract

Dislocation‐free single crystals of three types of silicon have been irradiated by 6×10¹⁷ and 5×10¹⁹ nvt of fast neutrons and studied by bright‐ and dark‐field transmission electron microscopy. Mean defect image size has been found to depend on the impurity content of the silicon and annealing treatment, ranging from 22 Å in the most nearly pure silicon in the as‐irradiated state to a maximum of 40 Å in the n‐type silicon after a 30‐min anneal at 700°C. Defect density ranges from 10¹⁷ images/cm³ in the as‐irradiated state to less than 10¹⁵ after a 1000°C anneal for 30 min. The density is not noticeably affected by the type of impurity, but is approximately proportional to neutron dosage. The properties of the images formed by diffraction contrast in thin silicon foils have been studied by extending the solutions of the basic equations of the dynamical theory of diffraction contrast with anomalous absorption to defects of the small size encountered in the research. An anomalously wide black‐white image is predicted in dark field from which it is possible to determine whether a defect has interstitial or vacancy character. A ratio in the order of 2 to 1 interstitial type to vacancy type was found in the n‐type silicon; the ratio in the other two types of silicon appeared to be more nearly unity.