Exciton dynamics in GaAs/(Al,Ga)As core-shell nanowires with shell quantum dots

Abstract

We study the dynamics of excitons in GaAs/(Al,Ga)As core-shell nanowires by continuous-wave and timeresolved photoluminescence and photoluminescence excitation spectroscopy. Strong Al segregation in the shell of the nanowires leads to the formation of Ga-rich inclusions acting as quantum dots. At 10 K, intense light emission associated with these shell quantum dots is observed. The average radiative lifetime of excitons confined in the shell quantum dots is 1.7 ns. We show that excitons may tunnel toward adjacent shell quantum dots and nonradiative point defects. We investigate the changes in the dynamics of charge carriers in the shell with increasing temperature, with particular emphasis on the transfer of carriers from the shell to the core of the nanowires. We finally discuss the implications of carrier localization in the (Al,Ga)As shell for fundamental studies and optoelectronic applications based on core-shell III-As nanowires. arXiv:1603.01111v3 [cond-mat.mes-hall]

Impact of Outer Shell Structure and Localization Effects on Charge Carrier Dynamics in GaAs/(In,Ga)As Nanowire Core–Shell Quantum Wells

“…In contrast, for the samples with an AlAs outer shell, the quenching is much less pronounced for temperatures above 80 K. Even at room temperature, the intensity still amounts to about 1% of the low‐temperature intensity. The value is comparable to those typically observed for planar (In,Ga)As/(Al,Ga)As(001) QWs and GaAs/(Al,Ga)As core–shell NWs . These results show that the additional AlAs barrier in the outer shell structure markedly improves the luminous efficiency at room temperature.…”

“…The second channel with meV is more difficult to interpret. The value of the activation energy is about twice as large as those reported for closely lattice‐matched GaAs/(Al,Ga)As and (In,Ga)As/(In,Al)As core–shell NWs, which was speculated to be related to a specific recombination center at the {110} interfaces . In the present case, it seems unlikely that this nonradiative channel is related to an interfacial defect, since we obtain exactly the same value of E 2 for samples with an interface to an outer GaAs or AlAs shell.…”

“…The resulting potential fluctuations can explain differences in emission energy of several 10 meV. For (Al,Ga)As shells, pronounced exciton localization was observed as well, both by other groups and us, and found to result from Al segregation effects beyond random alloy fluctuations . Even for the growth of planar (In,Ga)As QWs by MBE, it was shown that roughness and composition inhomogeneities are stronger for growth on the (110) plane compared to the more common (001) plane .…”

“…This result indicates an increased broadening of excitons in k ‐space possibly due to a high carrier temperature that was observed previously for NWs . The radiative lifetime of the localized exciton is 3.5 ns, longer than lifetimes reported for GaAs/(Al,Ga)As shell quantum dots ranging from 450 ps to 1.7 ns . Exciton localization in the In 0.15 Ga 0.85 As shell is thus remarkably strong, suggesting a comparatively strong compositional disorder.…”

“…Figure a presents PL spectra taken on the same sample as in Figure at temperatures between 10 and 185 K with an excitation power of 700 µW. For this power, no PL signal could be detected at temperatures above 185 K. For all temperatures up to 185 K, a single peak dominates the PL spectrum, which is centered at 1.375 eV at 10 K. An additional shoulder at slightly higher energies corresponding to the band observed in Figure is detected up to 100 K. Figure b,c show PL spectra taken on samples with outer shell structures containing AlAs for temperatures between 10 and 300 K. The binary compound AlAs was chosen to avoid additional localization effects in (Al,Ga)As shells . For the sample with a GaAs spacer (Figure b), the spectra show a dominant peak centered at 1.39 eV, corresponding to emission from the In 0.15 Ga 0.85 As shell QW.…”