W. Busse

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.

The luminescence of the semimagnetic semiconductor Zn1-xMnxSe

Zn1-xMnxSe (0.016<or=x<or=0.445) shows luminescence emission not only in the yellow spectral range (4T1 to 6A1) but also in the near infrared. The excitation spectra of the yellow and of the infrared band are compared at different concentrations x and different temperatures T. Both emission bands are connected by energy transfer. While the yellow emission is well described by a transition within the 3d shell of tetrahedrally coordinated Mn2+ the nature of the infrared emission is not yet clear.

Thin film electroluminescence of Zn1−xMnxS1−yTey

Zn1−xMnxS1−yTey thin films show, besides the yellow emission due to the internal transition 4T1(G)→6A1(S) of tetrahedrally coordinated Mn2+ by 4 S, an additional luminescence emission peaking at 635 nm which is attributed to the presence of Te. This red emission band is most pronounced both in electroluminescence and in photoluminescence at the concentrations 0.01<x<0.03 and 0.03<y<0.06. The red emission is observed only in the presence of both Mn and Te.

II-VI COMPOUNDS CODOPED WITH TRANSITION METALS AND RARE EARTHS : ZNS : MN2+, SM3+ BULK CRYSTALS SITE SELECTION AND ENERGY TRANSFER

Abstract Single crystals of ZnS codoped with both Mn2+ and Sm3+ have been studied using time resolved spectroscopy, site selective and decay time measurements. The results obtained on differently prepared samples are compared with the findings observed on ZnS bulk crystals doped either with manganese or samarium. The dominating sites and energy transfer processes are discussed. EPR measurements of the Mn2+ sites lead to the conclusion that crystals codoped with Li+ are less strained compared to those doped with Sm3+ and Mn2+ only. The stability of the luminescent centers has been verified in a host lattice damaged by Ar-implantation. A new energy transfer channel has been found in the codoped samples giving rise to a very slow Sm3+ decay component of 5 ms. Mn diffusion into a ZnS:Sm,Li crystal provides to the formation of Sm-Mn complexes characterized by shorter Sm3+ decay times.

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.

Dynamical Jahn-Teller effect on the triplet states 4T2 of Mn++ in polymorphic ZnS

Abstract The zero phonon lines and phonon-assisted lines appearing in the band centered at 23200 cm -1 of Mn ++ in cubic ZnS with stacking faults and in ZnS with wurzite structure have been analyzed by using dye laser site selection spectroscopy. A model is proposed which correctly describes the observed fine structure pattern as well as the relative dipole strengths.

Photoluminescence dynamics of Co-doped Zn1-xCdxSe and ZnSxSe1-xcrystals

Abstract Intra-shell d-d relaxation processes of Co 2+ centres are investigated by means of time-integrated and time-resolved photoluminescence spectroscopy. The composition dependence of the luminescence and the decay of the Co L-line in ZnCdSe and ZnSSe alloys is presented for the first time. Additionally, new Co-related infrared luminescence lines were observed near 1.6 eV. Taking into account the energy position of the L-line and the relaxation dynamics of the green and the infrared luminescence, the L-line is explained as an internal doublet-quartet d-d transition of the Co-centres.

The influence of II–VI lattice damage on rare earth sites as determined by site selective spectroscopy and cathodoluminescence: argon implanted ZnS:Sm3+

Abstract Using site selective and time resolved spectroscopy more than 10 different Sm sites have been discriminated. Beside the direct 4f-intra-shell excitation of the rare earth (RE) impurities a second indirect excitation channel via the host lattice is active with increasing laser power due to two-photon processes. The RE sites differ mainly in their ability to gain energy from this second channel. This knowledge is used to study the influence of host lattice damage — caused by Ar-implantation — on the Sm3+ luminescence. Using cathodoluminescence (CL) and photoluminescence (PL) it is shown that the RE emission is less affected by the damage than the near-band-edge (NBE), self-activated (SA) and donor-acceptor (DA) emission. No new Sm sites have been created by the implantation and the direct 4f-4f excitation channel is still efficient, whereas the energy transfer (ET) via the ZnS lattice is disturbed leading to an intensity decrease. These results are taken as an evidence for the complex nature and the relative stability of RE centers in a II–VI lattice.

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.

The influence of Cd and Zn on the luminescence of the semimagnetic semiconductor CdZnMnTe

The quaternary semimagnetic semiconductors CdxZnyMnzTe show red and infrared luminescence emission bands which are assigned to the internal transition 4Te1(G) to 6A1(S) within the 3d shell of Mn2+ in different crystalline environments. The intensities and the peak energies of the emission and excitation bands depend upon both the temperature and the ratio x:y. When the Cd ions are replaced by Zn a shift of 30 meV is observed which is small compared with the shift due to replacing the Te anions by S or Se.