At low temperature single CdTe quantum dot (QD) photoluminescence (PL) spectra contains four transitions resulting from the ground state, s-shell carriers recombination: the neutral exciton (X0), positively and negatively charged excitons (X+ and X-, respectively), and the biexciton (2X). With increasing temperature these PL peaks redshift, broaden, and decrease the intensity. The redshift is related to the bandgap shrinkage, while increased exciton-acoustic phonon coupling results in peaks broadening. The quenching of the PL intensity comes from thermal activation of carriers to other QDs or to excited states within the same dot.
To define the confinement conditions in the conduction and valence band separately we investigated the influence of temperature on particular charge state (X+ and X-). For CdTe QDs in ZnTe barriers, we find that the PL vanishes at about 65 K. Importantly, we observe decreasing of the normalized X+ intensity, while the X- intensity remains approximately constant. On the contrary, for CdTe QDs in ZnMgTe barriers, the PL is visible up to 115 K and the decrease of the normalized X+ intensity is substantially slower. These results point out that Mg incorporation in the barrier increases the hole confinement along the growth axis. As a consequence, the hole states are moved further apart in energy compared to those in CdTe dots in ZnTe barriers, inhibiting the thermal activation of s-shell carriers to excited states. Our result demonstrate that a proper design of the valence band structure should lead to room temperature emission from CdTe QDs.
Malgorzata M. Pilat, Lukasz Klopotowski, Piotr Wojnar, Krzysztof Fronc, Grzegorz Karczewski, Tomasz Wojtowicz, and Jacek Kossut, "Photoluminescence study of the increased hole confinement in CdTe quantum dots (Presentation Recording)," Proc. SPIE 9553, Low-Dimensional Materials and Devices, 95530J (Presented at SPIE Nanoscience + Engineering: August 12, 2015; Published: 5 October 2015); https://doi.org/10.1117/12.2188406.4519371092001.
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