Cobalt introduced into wide gap semiconductor is known to quench its excitonic photoluminescence (PL). Therefore magneto-photoluminescence study of cobalt based dilute magnetic semiconductors is an experimental challenge. Using (Zn,Co)Te and (Cd,Co)Te layers grown for this work it was, however, possible to study the giant Zeeman effect both in reflectivity and photoluminescence spectroscopy.

About 1 μm thick layers were grown using molecular beam epitaxy (MBE) on GaAs substrates, at conditions optimized for growth of nonmagnetic ZnTe and CdTe. Using reflection high-energy electron diffraction (RHEED) we observe, that cobalt only weakly affects growth of (Zn,Co)Te layers with x_Co = 0.56%, but the same cobalt flux leads to degradation of (Cd,Co)Te. As revealed by RHEED, good quality (Cd,Co)Te layer was obtained for lower concentration of x_Co = 0.2%. Concentration of cobalt was determined from analysis of the giant Zeeman effect observed using magneto-reflectivity of excitonic transitions (Fig.1) [1-4].

Figure 1. Exciton energy splitting vs magnetic field for: (a) – (Zn,Co)Te, (b) – (Cd,Co)Te. The solid line is a fit of a modified Brillouin function from Ref. [2]

Low temperature (1.5-10 K) PL was excited using 405 nm laser. Free exciton photoluminescence was observed at high excitation power only for (Zn,Co)Te (x_Co = 0.56%). Bound exciton transitions were observed for both (Zn,Co)Te (x_Co = 0.56%) and (Cd,Co)Te (x_Co = 0.2%). PL of (Cd,Co)Te was significantly weaker so that for higher Co concentration in (Cd,Co)Te (bad RHEED quality) the near band gap PL was not observed at all.

Under magnetic field in Faraday configuration PL lines are split and circularly polarized due to the giant Zeeman effect. Additionally, total PL intensity strongly increases with magnetic field, which shows that recombination channel through magnetic ions is less effective at high magnetic fields. For (Zn,Co)Te (x_Co = 0.56%) we observe twofold increase, whereas for (Cd,Co)Te (x_Co = 0.2%) integrated PL intensity increases by a factor of 30 (at B = 5 T compared to B = 0 T, shown in Fig.2).

Figure 2. Photoluminescence spectra in σ+ polarization for: (a) – (Zn,Co)Te, (b) – (Cd,Co)Te. Increase of PL intensity with magnetic field is particularly visible for (Cd,Co)Te.

Such difference observed for this two samples has to be a consequence of more efficient PL quenching in (Cd,Co)Te as compared to (Zn,Co)Te. The intraionic Co²⁺ transitions ⁴A₂ - ⁴T₁ (⁴P) are in resonance [5] with energy gap of CdTe, so energy transfer from excitons to cobalt ions is significantly faster for (Cd,Co)Te than for (Zn,Co)Te.

[1] M. Papaj, J. Kobak, J.-G. Rousset, E. Janik, A. Golnik, P. Kossacki and W. Pacuski,
Acta Phys. Pol. A122, 1010 (2012).

[2] M. Zielinski, C. Rigaux, A. Lemaitre, A. Mycielski, and J. Deportes, Phys. Rev. B 53, 674 (1996);

[3] M. Zielinski, C. Rigaux, A. Mycielski, and M. Menant, Phys. Rev. B 63, 035202 (2000);

[4] H. Alawadhi, I. Miotkowski, V. Souw, M. McElfresh, A. K. Ramdas, S. Miotkowska, Phys. Rev. B 63, 155201 (2001)

[5] J. M. Baranowski, J. W. Allen, and G. L. Pearson, Phys. Rev. 160, 627 (1967).

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