Solotronics

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Solotronics, optoelectronics based on solitary dopants, is an emerging field of research and technology reaching the ultimate limit of miniaturization. It aims at exploiting quantum properties of individual ions or defects embedded in a semiconductor matrix. It has already been shown that optical control of a magnetic ion spin is feasible using the carriers confined in a quantum dot. However, a serious obstacle was the quenching of the exciton luminescence by magnetic impurities. Here we show, by photoluminescence studies on thus-far-unexplored individual CdTe dots with a single cobalt ion and CdSe dots with a single manganese ion, that even if energetically allowed, nonradiative exciton recombination through single-magnetic-ion intra-ionic transitions is negligible in such zero-dimensional structures. This opens solotronics for a wide range of as yet unconsidered systems. On the basis of results of our single-spin relaxation experiments and on the material trends, we identify optimal magnetic-ion quantum dot systems for implementation of a single-ion-based spin memory.

The term solotronics1 has been introduced to describe recent advances2,3 in fabricating and operating semiconductor optoelectronic devices based on single dopants or defects for applications in computer memories, quantum computation and on-demand photon sources. The most advanced solotronics technology has been developed for nitrogen-vacancy (N-V) defect centres in diamond, for which it has been shown that quantum states can be prepared and read out and spin can be manipulated using microwave and optical transitions4,5,6,7. Defects similar to N-V centres have also been observed in semiconductors8 such as SiC9. SiC is more compatible with present semiconductor-based technology than diamond; however, owing to the weak coupling of free carriers to defect centres, it does not allow for electrically controlled operation. More promising in this view is another solotronic system: a single magnetic ion embedded in a semiconductor quantum dot (QD)10,11,12. Here, the spin state of the single ion can be prepared and manipulated both electrically13,14 and optically12,15,16 through injection of spin-polarized carriers. The s,p-d exchange coupling between the magnetic ion and the band carrier enables an unambiguous readout of the spin projection of the ion from the energy and polarization of a photon emitted by the QD10,11. The ease of optical addressing of individual QDs enables operation on the level of single ions12,15. Multiple magnetic ions can be coupled by carriers in one QD17,18,19 or by QDs coupling through tunnelling carriers12 or photonic structures20. The use of semiconductor heterostructures opens a huge area for testing new ideas for single-ion spin operation, as it offers a band gap and strain engineering, tuning energies of optical and microwave transitions, Fermi level manipulation and integration with p-i-n structures.

A severe limitation of QDs doped with transition metal ions was attributed to the efficient recombination channel introduced by magnetic ions21,22,23,24,25,26,27,28,29 when the exciton energy is higher than the intra-ionic transition energy, which should result in quenching of exciton emission. Therefore, the only QD systems with single-magnetic ions considered so far were those where the intra-ionic transition energies exceed the exciton energy, namely Mn2+ embedded in CdTe/ZnTe and InAs/GaAs QDs10,11,12,13,14,15,16,30,31,32. On the other hand, incorporation of magnetic ions such as Cr, Fe, Co, Ni or Cu would bring physical properties like orbital momentum, reduced number of spin states, sensitivity to local strain, the Jahn-Teller effect or isotopes with zero nuclear spin, offering additional degrees of freedom for designing quantum states. Extending the studies of single magnetic ions to other QD systems such as CdSe, ZnSe, CdS, ZnS, ZnO, GaN or other wide-gap semiconductors would offer, in turn, increased photoluminescence (PL) efficiency at higher temperatures, enhancement of the exchange interaction within excitons and between excitons and ions or reduction of spin-orbit coupling and the resulting spin relaxation rates.

We report here for the first time on a single cobalt ion in a CdTe/ZnTe QD and a single manganese ion in a CdSe/ZnSe QD. The spin states of the dopant ions are mapped onto the QD optical transitions recorded in a magneto-PL measurement. We employ PL decay measurements to demonstrate that contrary to the case of systems with many magnetic ions21,22,23,24,25,26,27, the exciton emission quenching is negligible for single dopants. Through modulated, polarization-resolved PL measurements we access the single-spin relaxation and prove that all-optical control of a single magnetic moment is feasible in the systems studied. Moreover, we show that spin properties of magnetic QDs can be designed by an independent choice of the magnetic ion and the QD material. We discuss the role of the electronic configuration of d-shell, spin-orbit and hyperfine interactions and, finally, we indicate the most promising design of future QD-based solotronic systems.

The identification of the excitonic lines is confirmed by the analysis of PL spectra measured as a function of the magnetic field (Fig. 2). Zeeman shifts of bright and dark excitonic transitions for a CdSe QD with Mn2+ (Fig. 2a) can be well described (Fig. 2b) using the model proposed for a CdTe QD with Mn2+ (ref. 10). In order to account for all the observed features of the PL spectrum from a CdTe QD with a single Co2+ (Fig. 2c,d), we extend the model by introducing a strain vector that induces a zero-field splitting of the Co2+ spin states (see Methods). Figure 2e shows the scheme of excitonic optical transitions for a relatively simple case, when the strain-induced Co2+ anisotropy axis is parallel to the growth axis. In plane anisotropy of Co2+ would induce an additional excitonic mixing, analogous to the case of a neutral Mn centre (d5+h) in a InAs/GaAs QD31. Typical PL spectra of various QDs with single-magnetic ions are presented in Supplementary Note 1 and Supplementary Figs 1 and 2.

The high efficiency of radiative exciton recombination found in the present work is different with respect to the bulk DMS case, where the exciton is typically coupled to an ensemble of magnetic ions, able to absorb energy over a wide range, for example, by a collective change of spin configuration. The above results not only show that PL studies of DMS can be significantly extended by using zero-dimensional structures, but they also imply high-fidelity optical readout of a single-dopant quantum state in a QD.

The method of single-spin relaxation measurement based on initial depolarization of magnetic ions presented here has the advantage of being applicable to any QD system with magnetic ions, independently of the choice of photoexcitation energy. However, for particular systems, measurements can be improved by the identification of resonant excitation channels, which allow for efficient transfer of polarization to QDs, and consequently optical polarization of the magnetic ions. Such resonant excitation channels have been identified for (Ga,Mn)As-based quantum wells43, for CdTe QDs with single Mn ions12,15,53 and for InAs/GaAs QDs with a single Mn16. For CdSe/ZnSe QDs even quasi resonant excitation results in some degree of QD exciton polarization54. This effect and identification of resonant excitation channels for a CdSe QD with a single Mn will result in efficient optical orientation of Mn and will be an important step towards observation of coherent phenomena.

It is worth noting that strain gives the possibility of inducing a temporal evolution of spin, tuning the zero-field splitting and reducing the spin degeneracy of the ground state. Thus, it can be profitable for manipulation of ion spins. The effect of strain on Co2+ spin states is shown in Fig. 2e. In order to use strain or crystal field effects, it is desirable to control the anisotropy axis, for example, by using wurtzite structure compounds, where the c axis is expected to define the quantization axis of the zero-field spin splitting. If one wishes, in turn, to eliminate a strain-induced complexity, lattice-matched materials, such as GaAs and AlAs for growth of magnetic QDs by droplet epitaxy, are recommended.

J.K., M.P., K.G., J.-G.R., E.J. and W.P. grew and characterized samples. J.K., T.S., M.K., M.G., A.G., P.K. and W.P. performed magneto-optical experiments, data analysis and modelling. T.S., A.B. and M.G. performed single-spin relaxation measurements. T.S., J.K., J.S., M.N., A.G. and W.P. prepared the manuscript in consultation with all authors.

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