D. Suisky

Rare earths in II–VI compounds: Non-linear optical excitation processes at low and high doping levels

Abstract Based on a careful separation of the observed samarium sites in ZnS : Sm 3+ bulk crystals, the optical properties of the dominant rare earth (RE) centers and their relative stability with respect to changes in the host lattice are reported. Based on non-linear optical processes at high excitation densities, the direct 4f-4f excitation channel is compared with energy transfer via the crystal. Developing a theoretical model for the non-exponential decay behaviour, connected with the indirect excitation channel, conclusions will be drawn on the complex nature of the RE sites involving other defects and impurities. Also in crystals with very low RE concentrations (10 17 cm -3 ) these complex centers are observed and can be described by the same model.

Magneto-optical study of ZnSe/(Zn,Mn)Se and ZnSe/(Zn,Cd,Mn)Se quantum well structures and superlattices

Abstract Quantum well structures with semimagnetic barriers, ZnSe /( Zn , Mn ) Se , or semimagnetic wells, ( Zn , Cd , Mn ) Se / ZnSe , are studied by luminescence and reflection spectroscopy in an external magnetic field up to 7.5 T.A. unique offset dependence on the manganese concentration and a type conversion of the ZnSe /( Zn , Mn ) Se heterostructures due to the giant Zeeman splitting of the barriers are found. In the ( Zn , Cd , Mn ) Se / ZnSe structures, a strong reduced energy transfer from the exciton states to the Mn internal transition was found, which is further suppressed by an increasing magnetic field. Therefore (Zn,Cd,Mn)Se quantum wells can be proposed to be suitable and effective for tuneable blue-green laser devices in an external magnetic field. The field dependent energy splitting both of well and barrier excitons are significantly anisotropic with respect to the orientation of the magnetic field B parallel or perpendicular to the growth axis. This magneto-optical anisotropy can be explained by the mixing of the heavy and light hole states in the framework of a multi-band envelope-function approximation.

Discrimination of Sm centers in ZnS by site selective laser spectroscopy

Abstract Evidence of seven Sm centers in a ZnS:Sm, Li crystal has been found. The relative emission intensities due to different centers depend on exciting laser power.

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.

Electric Field Effect on Multiphonon Transitions at Deep Centres

It has been demonstrated both experimentally and theoretically that multiphonon transitions from deep levels into band states, under certain conditions, are extremely sensitive to an applied electric field. The effect has been clearly seen in the case of the EL2 centre in GaAs /1/ and of the Au-related neutral acceptor in Si /2,3/. In the case of GaAs a good level of theoretical understanding has already been reached by the work of Makram-Ebeid et. al. /1/ although there remain unresolved problems also in this case. In comparison to GaAs the situation for Si is more complex due to the indirect gap, the multi-valley structure and effective mass anisotropy of the conduction band. It is the purpose of this paper to develop a theory of the electric field effect on multiphonon emission which applies also to the more complex situation of Si. Further we will present experimental results on field dependent emission rates from the neutral Au-acceptor and the A-centre in Si and give an adequate theoretical interpretation for them. The authors of /1/ constructed their theory of multiphonon transitions in an electric field by extending the theory of tunneling between gap and band states to the case where multiphonon transitionscan take place in addition, i.e. where tunneling processes become phonon-assisted. In this paper we start from the standard theory of multiphonon transitions and extend it to the case where an electric field is present. In this way the various field effects considered so far, like phonon assisted tunneling and Poole-Frenkel effect, are automatically taken into account, but additional effects are also regarded like tunneling enhanced multiphonon transitions. Stark effect and field induced line shape broadening of the deep level. In part 2 of the paper we present the theory. In part 3 we describe the experimental procedure. A comparison between theory and experiment is made in part 4. Some unresolved problems in the case of the EL2 centre in GaAs are also adressed in this part.

Magneto-optical anisotropy of semimagnetic ADQW structures

CdTe and ZnSe based semimagnetic asymmetric double quantum wells (ADQWs) have been fabricated. Pronounced magneto-optical anisotropies were found in either case on varying the angle between the sample normal and the direction of an applied magnetic field. The energies and wavefunctions of the excitons are calculated and are compared to the experimental results from ADQWs with strongly and weakly coupled wells.

Energy transfer processes in rare earth doped ZnS under non-linear excitation

ZnS: Sm3+ single crystals are studied by site selective, time resolved and decay time measurements. It will be shown that beside the direct 4f-4f excitation a slow energy transfer component occurs that can be attributed to the blue self-activated emission in ZnS. Analysing the decay behaviour of various Sm3+ sites in detail a fast transfer component could be selected which is active for only some of the Sm3+ sites. The nature of the energy transfer processes will be discussed basing on decay fits and conclusions will be drawn on the differences between Sm3+ centers, which provides to a better understanding of the luminescence data.

Non-radiative transition rates: Accepting and promoting phonon modes in an n-mode model

Abstract The emission rate from deep levels due to multiphonon processes is calculated in the presence of an electric field adopting an N-mode model for the lattice vibrations. A closed analytical formula is obtained for any number of promoting and accepting modes. Experimental data on Si: Au are interpreted by means of this formula.

Determination of deep centre parameters from electroabsorption spectra

Abstract A theory is presented for the electric field induced changes of optical absorption spectra due to multiphonon assisted transitions from deep level into band states. The electroabsorption line shape is determined by an interplay between the final state Franz-Keldysh oscillations and the multiphonon oscillations.It is demonstrated that deep centre parameters can be derived with high accuracy from experimental electroabsorption spectra.