Radiation-Induced Extrinsic Photoconductivity in Li-Doped Si

A brief description is given of instrumentation and methods used in the study of radiation damage in lithium-diffused Si. Infrared photoconductivity (1 to 10 μ) and infrared spectroscopy (1 to 50 μ) were used as the probes. The defects were induced in the material by 1.5- MeV electrons with the sample temperature at 300°K during irradiation; one irradiation experiment was run at ∼110°K. No infrared-active defect bands were induced by electrons of E≲5 MeV up to radiation fluences of 3.3×1017 e/cm2. All samples studied which have lithium concentrations in the range 9×1015−2×1017 cm−3 exhibit sharp increases in resistance after heat treatment in the range 200 to 700°K. Dominant radiation-induced photoconductivity arises from levels at Ec=0.2, Ec−0.39, Ec−0.54, Ec−0.6, Ec−0.7, and Ec−0.8 eV in oxygen-rich Si and at Ec−0.2, Ec−0.54, Ec−0.8, Ec−0.92, and Ec−1.0 eV in oxygen-lean Si. The levels are found to disappear and in some cases shift with annealing in the 100–450°C temperature range. In all cases the spectrum is dominated by three or four energy levels after irradiation and after various anneals. Energy levels obtained by other workers using carrier concentration, minority carrier lifetime, and photoluminescence as the probes are compared to our measurements. Due to the complexity of the system and the present lack of adequate theory, no complete analysis of the data can be made.