Microwave conductivity and conduction-electron spin-resonance linewidth of heavily doped Si: P and Si: As

Results are presented of a conduction-electron spin-resonance (CESR) study of single-crystal samples of heavily doped Si: P and Si: As. By studying the CESR line shape for samples whose thickness is comparable to the skin depth, we are able to deduce the microwave conductivity $\mathrm{Re}\sigma \left(\omega \right)$. We find in Si: P that $\mathrm{Re}\sigma \left(\omega \right)$ is less than the dc conductivity $\sigma \mathrm{}\left(0\right)\mathrm{}$ by a factor ≈ 3 at 4.2 K for samples just below the metal-insulator transition, while above the transition $\mathrm{Re}\sigma \left(\omega \right)=\sigma \mathrm{}\left(0\right)\mathrm{}$. We also compare the CESR linewidths for Si: P and Si: As and find large differences which imply, contrary to many previous studies, that Elliott-Yafet relaxation via spin-orbit coupling to the host is not involved. We propose that spin-orbit coupling to the impurities is the controlling factor.