Optical studies of carrier transport and fundamental absorption in 4H-SiC and Si

Abstract: The Fourier transient grating (FTG) technique and a novelspectroscopic technique, both based on free carrier absorption(FCA) probing, have been applied to study the carrierdiffusivity in 4H-SiC and the fundamental absorption edge in4H-SiC and Si, respectively.FTG is a unique technique capable of detecting diffusioncoefficient dependence over a broad injection interval rangingfrom minority carrier diffusion to the ambipolar case. In thiswork the technique is used for thin epitaxial 4H-SiC layers,increasing the time- and spatial-resolution of the experimentalsetup by factors of ~100 and ~10, respectively, in comparisonto the established Si measurements. It is found that thediffusion coefficient within the detected excitation range inn-type 4H-SiC appears to be lower than the analyticalprediction from Hall-mobility data. To explain this, it issuggested that the minority hole mobility is reduced withrespect to that of the majority one or that the hole mobilityvalue is in general lower than previously reported. Observeddifferences between the temperature dependency of the ambipolardiffusion and the Hall-prediction, on the other hand, areattributed to the unknown Hall factor for holes and theadditional carrier-carrier scattering mechanism in Hallmeasurements. Furthermore, at high excitations a substantialdecrease in the ambipolar diffusion is observed andadditionally confirmed by the holographic transient gratingtechnique. It is shown that at least half of the decrease canbe explained by incorporating into the theoretical fittingprocedure the calculated band-gap narrowing effect, taken fromthe literature. Finally, it is demonstrated that numerical datasimulation can remove miscalculations in the analytical Fourierdata analysis in the presence of Auger recombination.Measurements with variable excitation wavelength pump-probeare established in this work as a novel spectroscopic techniquefor detecting the fundamental band edge absorption in indirectband-gap semiconductors. It is shown that the techniqueprovides unique results at high carrier densities in doped orhighly excited material. In intrinsic epilayers of 4H-SiC,absorption data are obtained over a wide absorption range, atdifferent temperatures and at various polarizations withrespect to the c-axis. Experimental spectra are modeled usingthe indirect transition theory, subsequently extracting thedominat phonon energies, the approximate excitonic bindingenergy and the temperature induced band-gap narrowing (BGN)effect in the material. Measurements in highly dopedsubstrates, on the other hand, provide the first experimentalindication of the values of doping induced BGN in 4HSiC. Thefundamental absorption edge is also detected in highly dopedand excited Si at carrier concentrations exceeding theexcitonic Mott transition by several orders of magnitude. Incomparison to theoretical predictions representing the currentunderstanding of absorption behavior in dense carrier plasmas,a density dependent excess absorption is revealed at 75 K.Summarizing the mainfeatures of the subtracted absorption, itis concluded that an excitonic enhancement effect is present inSi.