Well-width dependence of the exciton-phonon scattering in thin InGaAs/GaAs single quantum wells

We studied the temperature dependence of the exciton dephasing time in three In0.18Ga0.82As/GaAs single quantum wells, with well thickness Lw of 1, 1.5, and 2 nm, by degenerate time-integrated four-wave-mixing (TI-FWM) using 100-fs pulses in reflection geometry. The TI-FWM correlation traces clearly show an inhomogeneous broadening in all the samples at low temperature (5 K). We also show TI-FWM traces at the heavy-hole exciton transition in the 1.5-nm wide well, for resonant excitation at different temperatures. Around 95 K, the trace shows a decay that is no longer exponential for long delay time, while an exponential trend is recovered for short positive delay at higher temperature. This indicates the transition from inhomogeneous to homogeneous broadening with increasing the temperature, according to exciton linewidth analysis. The dephasing rate is deduced consequently by dividing the FWM decay rate by a factor of 4 for T⩽80 K, and 2 for T=110 K, while the transition point is skipped. Also, we measure the density dependence of the dephasing and extrapolate the rate to zero density. The resulting 1/T2 versus temperature is shown. The acoustic and optical coefficient of the corresponding linewidth broadening have been deduced by fitting the data. We report the a (acoustic) and b (optical) phonon coefficient versus Lw, for all the investigated samples. We find an increase of a and a decrease of b with increasing Lw. This behavior is analyzed