Far-infrared properties of helium-doped silicon

Abstract

Single crystal silicon is an important material used for semiconductor devices and also a potential material in device research. Far-infrared optical properties are studied for the helium ion implanted samples with different fluences of 5.0 × 1016, 1.0 × 1017, 2.0 × 1017, and 4.0 × 1017 cm−2. The absorption coefficient and refractive index of silicon with different helium concentrations are measured in the wave number range from 4 to 85 cm−1 and temperature range from 145 to 520 K. The results show that the absorption coefficient increases with increasing fluence of helium ions, but the refractive index decreases. The correlations between absorption coefficient and refractive index and temperature of all samples are similar. The refractive index increases with temperature in the whole measurement temperature range, but the absorption coefficient has different trends in the different wavebands. The absorption coefficient decreases with the increase of temperature in the region below 12 cm−1; however, the absorption coefficient increases when the wave number is larger than 30 cm−1. The absorption coefficient curves at different wavebands are well fitted by the Drude model. A large number of helium bubbles and defects, such as voids and dislocations have been observed in the implanted area by transmission electron microscopy (TEM), which contributes to the increase of optical absorption.