Ultrafast spectroscopy of group III-nitride semiconductor heterostructures

This thesis reports a comprehensive study of ultrafast dynamics in heterostructures based on group III-nitrides, a new and important class of semiconductor materials. Various relaxation mechanisms in GaN, InGaN and AlGaN epilayer and multiple quantum well (MQW) structures were measured using wavelength degenerate and non-degenerate time-resolved differential transmission (TRDT) and reflection (TRDR) techniques.; The degenerate TRDT technique was used to monitor the capture of electrons from the three-dimensional barriers to the two-dimensional quantum wells for the first time in a nitride-based MQW structure. The sub-picosecond capture times in InGaN and AlGaN MQWs obtained from non-degenerate TRDT increased slightly with QW depth. Non-degenerate TRDT measurements for AlGaN/GaN MQW structures revealed significantly faster carrier cooling and recombination compared to the InGaN MQWs, partly due to the increased non-radiative decay. Optical Stark effect, two-photon absorption, and bandgap renormalization in different layers of the MQW samples were also resolved during non-degenerate measurements.; At high excitation densities, stimulated emission (SE) in InGaN and AIGaN/GaN MQWs is shown to remove carriers efficiently from the QWs with a time constant of a few picoseconds. The SE threshold densities (I_th) in InGaN/In_xGa_{1− x}N MQWs increased with increasing QW depth (x). By contrast, no significant variation was observed in Al_yGa _1−yN/GaN MQWs with varying barrier height (y). Non-degenerate TRDT measurements revealed that increased non-radiative recombination and fast capture of carriers to the localized states below the SE energy in deeper InGaN MQWs were responsible for the increased I_th.; For the first time, zone-folded longitudinal acoustic phonons (ZFLAPs) were generated and controlled in InGaN MQWs. Terahertz coherent acoustic phonons were generated by the abrupt screening of the strain-induced piezoelectric (PZE) field by the laser-induced carriers. TRDT measurements revealed increasing transmission modulation due to ZFLAPs with increasing QW depth. The modulation was at least two orders of magnitude larger than in the GaAs/AlAs superlattices at comparable excitation densities. Lack of higher order zone-folded modes resulted in the first ever complete cancellation and amplification of coherent acoustic phonons in any semiconductor structure using a two-pump, degenerate TRDT technique. Bulk-mode coherent acoustic phonons were also generated, and their transport through a GaN layer and reflection from the GaN/air interface were observed using TRDR.; Additionally, a prism coupling technique was used to measure ordinary and extraordinary refractive indices of Al_xGa _1−xN thin films in the whole Al compositional range, with accuracies superior to the other limited number measurements in the literature. The large variations in the reported AlGaN refractive indices are shown to originate from varying bowing parameters, predominantly due to differing growth conditions. A Sellmeier dispersion formula, parameterized by the bandgap, is used to reconcile these discrepancies.; A comprehensive picture of ultrafast dynamics in group III-nitride semiconductor heterostructures is created by measuring many important relaxation processes in the 100 fs-1 ns range simultaneously. Many fundamental parameters are also measured, and all these results provide necessary information for understanding basic fundamental physics and designing novel optoelectronic devices based on nitrides.