Tuning the optical and magnetic properties of II–VI quantum dots by post‐growth rapid thermal annealing

Abstract

We show that both the optical and magnetic properties of CdTe-based quantum dots (QDs) can be tuned by post-growth annealing. With increasing of annealing temperature, the photoluminescence of CdTe and CdMnTe QDs shifts towards high energies. At the same time, we observe a narrowing of the QD ensemble emission linewidth due to the improved homogeneity of the QD ensemble. We find that for annealed CdTe QDs the exciton diamagnetic shift is about two times larger that for the as-grown ones, suggesting an increase of the average dot size upon annealing. In the case of CdMnTe QDs the emission lines of individual quantum dots narrow significantly from 3 meV down to 0.25 meV showing that effect of spin fluctuations is much reduced probably due to an increase of the average dot size.1 Introduction Post-growth annealing has been widely used to change the properties of semiconductor quantum structures [1][2][3]. In particular, it has been shown that for quantum wells both the emission energy and the distribution of magnetic impurities can be modified [1,2]. The latter can lead to substantial changes of the magneto-optical properties of magnetically doped quantum well structures. In the case of Stranski-Krastanov grown III-V semiconductor quantum dots (QDs) which form on top of a twodimensional wetting layer, annealing increases the size of the QDs as well as changes their chemical composition [3]. In contrast, the epitaxial growth of II-VI QDs may occur without the formation of the wetting layer [4], so that annealing will almost certainly affect the strain distribution in and around the QDs. Additionally, since II-VI QDs are usually smaller (a few nanometers in a diameter), one could expect they will react more strongly to annealing than III-V QDs. Finally, for diluted magnetic semiconductor QDs where magnetic ions, such as Mn are incorporated into CdTe QDs [5], annealing might also tune the magnetic characteristics of such QDs.In this work we present the results of photoluminescence (PL) measured for CdMnTe and CdTe QDs subjected to rapid thermal annealing. We find that upon annealing the average size of the QDs increases and the homogeneity of the ensemble improves. In the case of CdMnTe QDs the increase of the dot size leads to a dramatic narrowing of the single dot emission linewidth (from ~3meV down to ~0.2meV). We ascribe this effect to the reduction of magnetic moment fluctuations within individual magnetic QDs.

Optically induced magnetization in diluted magnetic quantum dots

“…One can see that the polarization of EMPs increases rapidly at higher excitation energies. This observation suggests that the optically induced magnetization of the EMPs in CdMnTe depends on the QD energy [54]. It shows that the photo-induced magnetization is stronger for QDs with lower Mn concentration.…”

Exciton states and tunneling in semimagnetic asymmetric double quantum wells

“…As we reported previously, diffusion is strongly enhanced below the SiO 2 mask as compared to non-covered areas 42 and results in the increase of bandgap below the masked areas, inducing a lateral confine- ment potential up to 0.3 eV 42 depending on the sample and processing parameters. It was shown that RTA technique allows to vary the QW energy gap in a controllable way with a good optical quality 40,41,42 . QW, located between two CdMgTe barriers, incorporates only Mg atoms after RTA and is referred to as nonmagnetic well (NMW) whereas that with CdMnTe barrier contains both Mg and Mn atoms and is referred to as magnetic well (MW).…”

Rapid thermal processing of magnetic materials

“…Recently, the RTP technique has been used for tailoring the optical and magnetic properties of Mn-doped CdTe-based quantum dots (QDs) [69,70] and InAs/GaAs self-assembled QD structures (SAQDs) [71]. The preliminary studies showed that RTP led to an increase in the average dot size accompanied by narrowed emission linewidth of the individual magnetic dot.…”