Evidence from spectral emissometry for conduction intraband transitions in the intrinsic regime for silicon

From emissometry measurements in lightly doped Si at elevated temperatures, we have observed an anomalous absorption band in the wavelength range of 1-5 μm. The wavelength at which the band peaks, λ≈2.3 μm, shows a negligible dependence on temperature while the peak intensity increases with temperature presumably as a result of the increasing intrinsic carrier concentration. Spitzer and Fan reported a similar absorption band in direct absorption measurements at room temperature for n-type Si with extrinsic electron concentrations of 10¹⁴ to 10¹⁹ cm^-3. No such structure was found in extrinsic p-type Si. Spitzer and Fan were unable to identify the mechanism for this anomalous absorption. In both the experiments, this absorption of free electrons is due to intraband transitions in the conduction band from the Δ₁ conduction band edge across an energy gap of E to approximately 0.5 eV to a higher lying Δ₂′ conduction band.