Abstract:
In this study, stimulated emission (SE) and ultrafast carrier relaxation are explored in InGaN and AlGaN/GaN multiple quantum wells (MQWs). The SE threshold densities (I th) in InGaN MQWs increase with increasing QW depth. By contrast, no significant variation is observed in AlGaN/GaN MQWs with varying barrier height and growth conditions (Ga-rich or N-rich). Wavelength non-degenerate time-resolved differential transmission (TRDT) measurements reveal that increased non-radiative recombination and fast capture of carriers to the localized states below the SE energy in deeper InGaN MQWs are responsible for the increased I th. At high excitation densities SE is shown to remove carriers efficiently from the QWs with a time constant of a few picoseconds, causing carriers at higher energies to cascade down and refill these SE-emptied states. The strength and decay times of the SE feature, which are resolved from the spectrally integrated TRDT data, are seen to vary as a function of excitation energy and density. The fast, SE-accelerated decay in AlGaN MQWs occurs more than twice as fast as in InGaN MQWs for similar excitation densities. More importantly, recombination times are an order of magnitude faster in AlGaN MQWs than in InGaN MQWs.
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