Excitation of the 4f-4f emission of donor-type rare-earth centres through donor-acceptor pair states (ZnS:Tn, ZnS:Sm)

Energy transfer from donor-acceptor pair states is known to be an efficient excitation channel for 4f-4f emission, especially if the rare-earth centres themselves are the relevant donors. Information on this type of mechanism can be obtained in the case when it is possible to observe, parallel to the 4f-4f emission, luminescence arising from the radiative de-excitation of the pairs. The authors have found these two kinds of emission, under electron-beam excitation, in ZnS:Tm and ZnS:Sm crystals which, according to earlier site-selective studies performed by them, contain considerable concentrations of donor-type rare-earth centres. Characteristic differences between the two kinds of emission are found in the dependences of intensities on pumping rate; these arise from partial blocking of the energy transfer due to depopulation of the 4f ground levels. By quantitative studies of the correlation between those intensities and measurements of optical excitation spectra of 4f-4f emission the authors demonstrate that the same types of initial state are involved in the energy transfer and the pair transitions

Incorporation of rare earths into II-VI compounds during MBE growth: Optical and EXAFS studies of Sm-doped ZnTe

Sm-doped ZnTe films with thicknesses several hundred nm were grown by molecular-beam epitaxy (MBE) on GaAs substrates at temperatures between 300 and 400°C. Sm concentrations between about 1018 and about 1022 cm−3, as determined by X-ray microprobe analysis and SIMS, were incorporated by varying the temperature of the Sm source between 400 and 650°C. We report optical absorption spectra obtained from transmission measurements at photon energies below the interband edge. Two prominent absorption bands peak at about 1.45 and 2.1 eV are seen at concentrations above some 1020 cm−3. Based on a comparison with SmTe data, we assign these bands to 4f6 → 4f5 5d transitions in Sm2+ ions on sites with octahedral Te coordination. These centers are thought to be formed from zincblende interstitial sites during MBE growth. We have also performed extended X-ray absorption fine structure (EXAFS) studies at the Sm L3 absorption edge using fluorescence yield detection. The results of these studies are consistent with our incorporation model, leading to a SmTe bond length of 3.25 Å for the dominant Sm site. These Sm centers are obviously responsible for the transformation from zincblende to rocksalt structure which is seen in the RHEED patterns of such films as the total Sm concentration is increased above some 1021 cm−3

Incorporation of rare earths into II–VI compounds during molecular beam epitaxial growth: Extended x-ray absorption fine structure study of Sm-doped ZnTe

We present extended x-ray absorption fine structure measurements performed on ZnTe:Sm films, at the Sm L3 edge. The films (thickness several hundred nanometers) were grown by molecular beam epitaxy on GaAs substrates and were doped during growth, achieving Sm concentrations of the order 1021–1022 cm−3. Analysis of the experimental data by standard methods shows that the majority of the Sm atoms are located on sites with octahedral Te coordination, with a Sm–Te bond length about 3.1 Å. These sites are thought to form during growth from zinc-blende-type interstitial sites due to the high chemical affinity between the rare earth and the chalcogen atoms. This result agrees with suggestions made by us in earlier work, based on optical and structural studies. A small part of the Sm atoms (with a fraction depending on the total Sm concentration) is found to be on sites with nearest-neighbor oxygen atoms. The results indicate two subspecies of the latter type, the mean Sm–O bond lengths lying between about 2.3 and 2.5 Å. The Sm–O sites are restricted to a layer of about 50 nm thickness at the outer boundary of the doped region. This is inferred from the depth distribution of O atoms determined by elastic recoil detection analysis

Analysis of 4ff Level in Samarium-Rich MBE Grown CdSmTe Sample

Electronic states of 4f samarium ions were investigated by photoemis-sion spectroscopy in samarium-rich CdSmTe sample obtained by MBE. Thephoton energy of synchrotron radiation allowed to investigate Fano-type res-onance and antiresonance. The energy distribution curve spectra were at-tributed to the Sm 4d-4f transition. The shape of the constant initial statesspectra was compared with this one obtained for atomic samariu

The Zeeman effect on bound-exciton states of indium-related complex centres in CdTe

The PL spectrum of In-doped CdTe is known to exhibit, in many cases, a characteristic feature (the 'W line') at about 12780 cm-1 ( approximately=1.584 eV), for which there are contradictory assignments in the literature. Using bulk CdTe:In crystals we have found that this feature appears after heat treatment in a Te atmosphere, accompanied by a decrease in the D0X emission. We have measured the Zeeman effect on the W line, including its dependence on the directions of magnetic field and polarization. These data show that this line originates from the annihilation of a two-particle bound exciton localized at trigonal complexes with symmetry axes parallel to the (111) directions. By analysing level splittings and relative transition probabilities on the basis of a standard spin Hamiltonian it is found that the trigonal impurity potential is due to a 'tensional' lattice deformation and the Zeeman spectra can be described quantitatively by the parameters ge=-1.7, K=1.08, L=-0.1 and a=0. Together with the magnitude of the diamagnetic shift, these values show that in the bound excitons the electron orbitals are rather extended and the hole orbitals are rather localized. Ini-VCd, Ini-ACd, or Ini-3ACd complexes (A being small-size acceptor impurities such as Li or Na) are considered to be the most probable candidates for the centres giving rise to the W line

The configuration of Cu centers in electroluminescent SrS:Cu phosphors. An x-ray absorption fine structure and optical study

X-ray absorption fine structure and photoluminescence measurements are combined to analyze the structure of Cu centers in SrS:Cu thin-film and powder samples. We find that all the data can be understood on the basis of a model involving three kinds of Cu centers: (i) dumbbell-type Cu-Cu complexes (which are the dominant centers), (ii) substitutional centers (CuSr), and (iii) interstitial centers (Cui). The bond lengths in the dumbbell centers are determined from the x-ray absorption data. Two characteristic emission bands peaking at about 460 and 510 nm are attributed to Cui and dumbbell centers, respectively. The dependences of the emission spectrum on temperature and Cu concentration are explained by a dissociation–association reaction of the type Cu-Cu<-->CuSr + Cui, assuming center-dependent excitation efficiencies and thermal quenching rates

The configuration of Cu centers in electroluminescent SrS Cu phosphors an x ray absorption fine structure and optical study

X-ray absorption fine structure and photoluminescence measurements are combined to analyze the structure of Cu centers in SrS:Cu thin-film and powder samples. We find that all the data can be understood on the basis of a model involving three kinds of Cu centers: (i) dumbbell-type Cu-Cu complexes (which are the dominant centers), (ii) substitutional centers (CuSr), and (iii) interstitial centers (Cui). The bond lengths in the dumbbell centers are determined from the x-ray absorption data. Two characteristic emission bands peaking at about 460 and 510 nm are attributed to Cui and dumbbell centers, respectively. The dependences of the emission spectrum on temperature and Cu concentration are explained by a dissociation–association reaction of the type Cu-Cu<-->CuSr + Cui, assuming center-dependent excitation efficiencies and thermal quenching rates