Characterization of silicon doped with sodium upon high-voltage implantation

The depth distribution profiles of sodium atoms in silicon upon high-voltage implantation (ion energy, 300 keV; implantation dose, 5 x 10(14) and 3 x 10(15) cm (-2)) are investigated before and after annealing at temperatures in the range T (ann) = 300-900A degrees C (t (ann) = 30 min). Ion implantation is performed with the use of a high-resistivity p-Si (rho= 3-5 k Omega cm) grown by floating-zone melting. After implantation, the depth distribution profiles are characterized by an intense tail attributed to the incorporation of sodium atoms into channels upon their scattering from displaced silicon atoms. At an implantation dose of 3 x 10(15) ions/cm(2), which is higher than the amorphization threshold of silicon, a segregation peak is observed on the left slope of the diffusion profile in the vicinity of the maximum after annealing at a temperature T (ann) = 600A degrees C. At an implantation dose of 5 x 10(14) ions/cm(2), which is insufficient for silicon amorphization, no similar peak is observed. Annealing at a temperature T (ann) = 700A degrees C leads to a shift of the profile toward the surface of the sample. Annealing performed at temperatures T (ann) a parts per thousand yen 800A degrees C results in a considerable loss of sodium atoms due to their diffusion toward the surface of the sample and subsequent evaporation. After annealing, only a small number of implanted atoms that are located far from the region of the most severe damages remain electrically active. It is demonstrated that, owing to the larger distance between the diffusion source and the surface of the sample, the superficial density of electrically active atoms in the diffusion layer upon high-voltage implantation of sodium ions is almost one order of magnitude higher than the corresponding density observed upon low-voltage implantation (50-70 keV). In this case, the volume concentration of donors near the surface of the sample increases by a factor of 5-10. The measured values of the effective diffusion parameters of sodium at annealing temperatures in the range T (ann) = 525-900A degrees C are as follows: D (0) = 0.018 cm(2)/s and E (a) = 1.29 eV/kT. These parameters are almost identical to those previously obtained in the case of low-voltage implantation.