Wavelength Dependent Free-Carrier Absorption Pump/Probe Spectroscopy of Silicon

Abstract

The wavelength dependence on the free carrier absorption (FCA) cross-section has been explored using pump-probe spectroscopy. Until now the determination of the cross-section over a wavelength range using a completely optical method of carrier injection has yet to be done. For the first time, measurements are done at wavelengths less than 1000 nm which contributes to the overall measurement range of 935 to 2500 nm. In most cases, the work presented here agrees with literature values that have been reported at single wavelengths for optical injection methods. However, when compared to literature that uses non-optical injection methods, such as the introduction of dopants, there is a clear discrepancy between the methods. The cross-section for non-optical methods is roughly twice that when compared to the optical method though the curvature is consistent throughout. We believe this discrepancy comes from how carriers are scattering within doped and lightly doped materials. Since the process of FCA requires a momentum-conserving scattering event to occur, the dominant scattering mechanism must influence the magnitude of the FCA cross-section in some way. FCA upon optical injection is dominated by electron-hole scattering whereas FCA upon injection by dopants is facilitated through impurity scattering. As the doping level increases, not only will there be more carriers to collide with and scatter, but the process also introduces charge donor or acceptor atoms that can act as additional scattering sites resulting in a naturally higher cross-section.