W. Heimbrodt

Magneto-optical study of asymmetric CdTe/(Cd,Mn)Te double-quantum-well nanostructures

Abstract A strained CdTe/(Cd,Mn)Te double-quantum-well nanostructure, grown by MBE on InSb substrate, is studied by CW and ps luminescence, reflection, and excitation spectroscopy in magnetic fields up to 5 T. The two CdTe quantum-wells of the sample have different well widths and are separated by a thin (Cd,Mn)Te barrier. A fast exciton transfer is observed from the higher-energy lh and hh states of the narrow well into the hh state of the broad well. The exciton energies in the wells and barriers and their magnetic-field induced splitting can consistently be explained by model calculations, taking into account the strain known from X-ray diffraction measurements. The decay times of the various luminescence bands are determined.

Optical study of the antiferromagnetic phase transition in (Cd,Mn)S at highest Mn concentration

Abstract Luminescence emission and excitation measurements are used to study the antiferromagnetic phase transition in (Cd,Mn)S single crystals and thin films in the case of highest Mn concentrations 0.8 ≤ xMn ≤ 1. The spin-freezing temperature TN(xMn) is determined from the temperature dependence of the Mn2+ EPR linewidth. The samples having rocksalt or zincblende/wurtzite structure show luminescence band near 1.5 and 1.8 eV, respectively. Below TN(xMn) a significant shift of the corresponding Mn2+ d-d excitation bands to higher energies is found, which is ascribed to the different spin-ordering induced energy relaxation in ground and excited states of the Mn2+ ions. The superexchange interaction parameters for the various excited Mn2+ states are obtained in a mean-field approximation. The measured luminescence decay parameters indicate the energy transfer between the Mn2+ ions to be also influenced by the antiferromagnetic spin ordering.

Exciton transfer in CdTe(Cd,Mn)Te-double quantum well structures

Abstract A series of asymmetric CdTe ( Cd,Mn ) Te double quantum well structures, grown by MBE on InSb substrates, are studied by luminescence, excitation, and reflection spectroscopy in magnetic fields up to 9T. The well widths are 4 and 8 nm; respectively, the width of the inner barrier varies in the range 2.5 to 15 nm. The exciton transfer from the narrow into the wide well is found to be strongly dependent on the width of the inner barrier. The decay of the various luminescence bands after 5 ps-pulse excitation at various excitation energies and the dependence of the exciton transfer on a magnetic field are investigated. The results can be explained on the basis of an effective-lifetime model assuming an independent tunneling of electrons and holes.

Optical properties of mercurycadmium telluride based superlattices

Abstract Optical spectra of the dielectric function as well as absorption and reflection spectra are calculated for HgTe/CdTe superlattices over a wide range of photon energies including E 0 , E 1 and E 2 derived transitions. Electronic structure data are obtained by means of tight binding theory. Theoretical results are compared with experimental data measured on a 70 A-70 A HgTe/CdTe superlattice.

Luminescence study of CdTe/(Cd,Mn)Te-MQW structures in magnetic fields

Abstract Strained CdTe/(Cd,Mn)Te-MQW structures, grown by MBE on InSb substrates, are investigated by luminescence and reflection spectroscopy. Free and donor-bound (D 0 X) exciton and donor-acceptor-pair (DAP) transitions are studied in dependence on magnetic field strength and orientation. The D 0 X binding energy is found to decrease with increasing magnetic field. magnetic polaron effects are observed both for D 0 X and DAP transitions. In the case of D 0 X spin-pinning with respect to the normal axis of the quantum well is seen. A discussion of the results on the basis of a Kronig-Penney model calculation taking into account the strain effects yields a large valence band offset, and a valence/conduction band offset ratio of 1.6 to 1.8 is determined.

MBE growth and characterization of (Zn, Mn)Se/ZnSe and MnSe/ZnSe structures on (001) GaAs

Abstract The MBE growth of ZnSe, (Zn,Mn)Se, MnSe, and related quantum well structures has been studied. Results are presented concerning the effect of growth interruptions and optical properties of SQW and MQW structures.

Exciton tunnelling and resonances in semimagnetic double-quantum-well structures

Abstract We present a study of tunnelling processes between CdTe CdMnTe quantum wells by means of steady-state and time-resolved spectroscopy. The energy difference of the involved states and the barrier height can be controlled by an external magnetic field and resonances of the heavy-hole and exciton levels can be achieved. The heavy-hole (hh) resonance is essential for the type and direction of the transfer process. An efficient transfer of excitons is possible even without emission of LO phonons.

Magneto-optical study of Mn containing broad gap II–VI quantum well structures and superlattices

Abstract Semimagnetic ZnSe/(Zn,Mn)Se single and multiple quantum well structures are studied by luminescence and reflection spectroscopy in a magnetic field up to 7.5 T. Using the strain-induced heavy and light hole splitting of the barrier and well excitons the valence band offset is determined in the framework of a Kronig-Penney model calculation. A unique offset dependence on the manganese concentration and a type conversion due to the giant Zeeman splitting are found.

Spin-flip Raman spectroscopy of nitrogen acceptors in ZnSe layers with different biaxial strains

Abstract Resonant spin-flip Raman scattering (SFR) was used to investigate systematically the effects of strain on the J = 3 2 spin states of nitrogen acceptors in p-type ZnSe grown on GaAs. Specimens from several MBE and MOVPE sources were studied and, by choosing specimens with different thicknesses of ZnSe, the in-plane biaxial strain could be chosen to be tensile or compressive, according to the extent of relaxation. For large tensile or large compressive strain, the acceptor states that lead to scattering are, respectively, the light- and heavy-hole states, which can be distinguished by the very different anisotropy of the corresponding SFR signals as the direction of the magnetic field is altered. The investigation also included specimens with a range of intermediate layer thicknesses for which the strain splitting of the hole states is comparable with the Zeeman splitting. In this case the field-induced lh/hh mixing is significant and the spectra are very sensitive to the exact value of the strain. The spin-Hamiltonian parameters κ ( = − 0.50 ± 0.05) and q ( = − 0.05 ± 0.01) were determined accurately by using specimens with small strains.

Dimensional dependence of magneto-optical properties of ZnMnSe MBE layers

A set of samples have been grown consisting of ZnMnSe MBE layers with x Mn = 0.12 and various layer thicknesses between 500 nm and 3 monolayers. Luminescence, excitation and reflection spectroscopy were performed in a magnetic field up to 7.5 T. Whereas for thick ZnMnSe layers of 500 and 50 nm direct excitons are to be seen, in the case of thin layers d < 10 nm only spatially indirect excitons are observable in luminescence with holes localized mainly in the ZnMnSe layers and the electrons localized in the adjacent ZnSe layers. These interface localized spatially indirect excitons exhibit a reduced Zeeman splitting compared to the direct excitons observable in the thick layers, due to the reduced number of Mn ions inside of the exciton volume. The corresponding ZnMnSe gap energy shift shows, however, a strong increase with decreasing layer thickness. The enhancement of paramagnetic response is only partly due to the interface. We suggest a strong increase of the exchange integrals for the dimensional crossover.