Charge fluctuations of a Cr atom probed in the optical spectra of a quantum dot

We study the emission of individual quantum dots in CdTe/ZnTe samples doped with a low concentration of Cr. In addition to dots with a photoluminescence (PL) split by the exchange interaction with a magnetic Cr atom, we observe another type of dots with a complex PL structure composed of a minimum of six lines on the exciton and biexciton and three lines on the charged excitons. In these dots, the linear polarization dependence and the magnetic field dependence of the PL behave like three similar quantum dots emitting at slightly different energies. Cross-correlation intensity measurements show that these emission lines are not independent but exchange intensities in a timescale of a few hundred nanoseconds depending on the optical excitation power. We attribute this PL structure to charge fluctuations of a Cr atom located in the vicinity the CdTe dots in the ZnTe barrier. We present a model which confirms that the presence of a single charge fluctuating between −e (${\mathrm{Cr}}^{+}$), 0 (${\mathrm{Cr}}^{2+}$), and +e (${\mathrm{Cr}}^{3+}$) and located a few nm away from the dot explains the observation of three emission energies. We finally show that the interaction between the confined carriers and the nearby fluctuating localized charge can be modified by an applied static electric field which modulates the splitting of the emission lines.

Figure 1

Example of QDs (QD1 and QD2) illustrating the different PL structure observed in the studied Cr-doped sample. (a) PL of QD1 containing a Cr atom. (b) Co and Cross circularly polarized PL of QD2 presenting a complex linear polarization structure on the exciton (biexciton) and three lines on the charged excitons. The insets show the linear polarization intensity maps of the exciton and biexciton. (c) PL excitation spectra recorded on the positively charged exciton of QD2.

Figure 2

Co- and cross-circularly polarized PL of QD3 at zero applied electric field (a) and under an applied voltage $\mathrm{V}=-2.5\mathrm{V}$ (b). The insets show the corresponding linear polarization intensity maps of the exciton and biexciton. The structure of three doublets on the neutral species is clearly revealed by the linear polarization analysis under an applied electric field.

Figure 3

(a) Linearly polarized PL spectra of QD4. The insets present the corresponding linearly polarized PL intensity map of X and ${\mathrm{X}}_2$. Six lines (three doublets) are clearly observed on the neutral species. (b) Magnetic field dependence of the PL of ${\mathrm{X}}^{+}$. (c) Magnetic field dependence of the PL of X in QD4. (d) Zoom on the anticrossings observed in the magnetic field dependence of X around ${\mathrm{B}}_z=9\mathrm{T}$. (e) Co and cross-circularly polarized PL spectra under a magnetic field ${\mathrm{B}}_z=0.5\mathrm{T}$. The large cocircularly (cross-circularly) polarized emission permits to identify the contributions of the positively (negatively) charged exciton.

Figure 4

(a) Cross-correlation of the PL intensity of the high energy (HE) and low energy (LE) lines of the positively charged exciton in QD5. The inset presents the co- and cross-circularly polarized PL of QD5. (b) Autocorrelation of the PL intensity of the high energy line of QD5 for different excitation intensities.

Figure 5

Energy of the acceptor and donor levels of Cr in ZnTe and confinement energies in the CdTe QDs. Blue arrows are possible optical transitions.

Figure 6

Calculated energy levels of the exciton as a function of the position of a charge +e or −e localized in the ZnTe barrier near the QD. The evolution of the emission energy is presented as a function of the in plane distance $\rho$ for different fixed values of $z$ ($z=0\mathrm{nm},z=1.5\mathrm{nm},z=3\mathrm{nm}$, and $z=4.5\mathrm{nm}$). The distances $\rho$ and $z$ are measured from the center of the QD. The size of the QD is ${\mathrm{l}}_z=2.5\mathrm{nm}$ and $2{\mathrm{l}}_{\rho }=10\mathrm{nm}$ (see Appendix for a detailed description of the confinement potential).

Figure 7

(a) Applied bias voltage dependence of the PL of the exciton in the reference Cr-doped QD (QD1). The inset shows the voltage dependence of the exchange induced overall splitting of the exciton. (b) Applied voltage dependence of the emission of the positively charged exciton in QD6. (c) Applied voltage dependence of the emission of X, ${\mathrm{X}}^{+}$, and ${\mathrm{X}}_2$ in QD7.

Figure 8

(a) PL of the positively charged exciton in QD5 for a resonant (res.) excitation on an excited state alone (red), a weak excitation at 532 nm (n. res.) with $P_{\text{n.res.}}=15\mu \text{W}$ (green) and a combined resonant and nonresonant excitation (blue). (b) Dependence of the intensity distribution of the positively charged exciton in QD5 for a fixed resonant excitation and variable nonresonant excitation $P_{\text{n.res.}}$. (c) Intensity distribution on the three PL lines of the positively charged exciton (HE: high energy, LE: low energy, C: central) in QD5 as a function of the resonant excitation intensity. The black line is a linear fit. (d) Intensity distribution in the PL of the charged exciton as a function of the applied bias voltage.