Raman spectroscopy of heavily doped polycrystalline silicon thin films

Raman backscattering measurements were performed on boron and phosphorous doped polycrystalline silicon films with an average grain size varying between 150 and 2500 nm. The B- and P-doped samples exhibit free hole and electron concentrations of up to 5×10²⁰ and 2×10²¹cm^-3, respectively. The incorporation of dopants results in a shift of the Raman LO-TO line to smaller wave numbers. At B and P concentrations higher than mid 10¹⁹cm^-3 the phonon lines are asymmetric. This is discussed in terms of a resonant interaction between optical phonons and direct intraband transitions known as a Fano resonance.