Self-activated Emission Research Articles

Host Sensitized White Luminescence from ZnGa2O4 : Dy3 + Phosphor

Dysprosium-doped zinc gallate phosphors with the nominal formulas were prepared by the high-temperature conventional solid-state reaction technique, the dopant concentration being varied in the range . and samples were also prepared for comparison for a dopant concentration of . Only the intrinsic self-activated emission of the host is observed in the photoluminescence emission spectra of , while both the host emission band and characteristic emission lines of are observed for and phosphors. The luminescent intensity differs in the phosphors due to the different energy-transfer rates from the respective hosts to the luminescent centers. Photoluminescent studies reveal the fact that ions replace ions in the host lattice at their octahedral sites. The source of white luminescence in the doped samples is the nonradiative resonant energy transfer via exchange interaction between the host and the activator. The CIE coordinates of the 2.5 atom % -doped sample (0.32, 0.33) matches well with achromatic white (0.33, 0.33) on the chromaticity diagram.

Copper concentration dependence of structure, morphology and optical properties of ZnS:Cu,Cl phosphor powder

ZnS:Cu,Cl phosphors were prepared by firing ZnS powder with 1 wt% NaCl and CuS nanocrystallites in the range of 40–5000 ppm at 900 °C for 2 h in an atmosphere of 3% H 2/Ar and sulfur vapor. The particle size and structure of the ZnS:Cu,Cl powders were found to depend on the Cu concentration. The powders with Cu⩾400 ppm exhibited a cubic structure and were approximately 5 μm in size as compared to the hexagonal structure of those with less Cu additions (∼2 μm). The difference, resulting from the precipitation of Cu x S in the powders as determined by X-ray photoelectron and diffuse reflectivity analyses, revealed that the powders with Cu>400 ppm showed the Cu 2p main peaks and higher absorbance in the UV–vis region due to the wide range of the bandgap energy of Cu x S (1.2–1.75 eV). Electroluminescence (EL) consisting of blue, self-activated (SA) and green emission peaking at 440, 470 and 510 nm, was observed only in the powders with Cu⩾400 ppm. Besides the activator concentration quenching effect, the variation of EL intensity with the Cu concentration was elucidated to result the from amount of Cu x S precipitates as well as charge carriers in the powders.

Deep emissions of MBE-ZnTe on tilted GaAs substrate

The photoluminescence (PL) profiles in deep region of ZnTe grown on tilted GaAs (0 0 1) substrates, 2° off toward 〈1 1 0〉, without ZnSe buffer layer have weak self-activated (SA) emissions at about 1.6–2.0 eV. PL shows the characteristic dependence on the excitation intensity and temperature. The excitation intensity dependence of the PL structure at 2.322 eV shows that the emission structure is due to the donor–acceptor pairs (DAP). The temperature dependence of the DAP emission indicates a two-step thermal quenching process. The zero-phonon line of the phonon replicas of oxygen isoelectronic traps is lowered by 5 meV from that of bulk crystal due to the difference in the thermal expansions between ZnTe and the substrate. The emissions at around 1.65 eV are the SA emissions related to Te-interstitial atoms.

Investigation of the effect of oxygen on the cathodoluminescence spectra and band gap of the ZnSxSe1-x alloy

The cathodoluminescence spectra of polycrystalline CVD ZnSxSe1−x alloy layers grown with the Zn excess are investigated. The dependence E g(x) is refined taking into account the location of exciton bands at 300 K. It is confirmed that the spectra are reconstructed on dissolving oxygen in the lattice under the effect of high pressures and temperatures of the gas-static treatment. The dependence of the self-activated emission on the alloy composition is observed for the accepted conditions of the gas-static treatment. For the compositions close to ZnSe (x 0.7), which is also most typical of the compounds themselves. These specific features are interpreted based on the calculations of the equilibrium of intrinsic point defects, which affect the location of the Fermi level under the deviation of ZnSe and ZnS from stoichiometry. The spectral regions, in which the bands of oxygen-containing SA and SAL complexes in the ZnSxSe1-x alloy can be observed, are evaluated. These regions depend on x and the oxygen concentration at the lattice sites. The behavior of exciton bands and infrared radiation emerging in the Cu (3d 9) ions is investigated after the gas-static treatment. It is shown that the variation in the ensemble of intrinsic point defects during the gas-static treatment is determined by an increase in the number of defects with a smaller volume, specifically, the SSe antisite defects. The spectra are interpreted based on the theory of nonoverlapping bands, which are induced by the isoelectronic impurity.

Surface states and its influence on luminescence in ZnS nanocrystallite

In this pape,r the influence of surface effects on the self-activated (SA) luminescence in ZnS nanoparticles prepared by the wet-chemical method is presented. It is observed that the luminescence of SA decreases dramatically by rinsing with methanol. In the rinsed sample, the luminescence of SA increases more by ultraviolet (UV) light irradiation. To clarify its origin, the Raman spectra and electron paramagnetic resonance (EPR) are studied. The results demonstrate that the vibrational modes assigned to organic functional groups of –OH and –COO and –CH 3 decreases remarkably by rinsing, while the EPR signal originated from the unpaired electrons of some transition metal impurity ions including Mn 2+ increases. It is suggested that the SA centers prefer to occupy the sites near the surface and that the donor of SA emission may be partly related to the organic functional groups of –OH and –COO adsorbed on the surface. The surface-dangling bonds caused by unpaired electrons of some transition metal impurity ions play a role of surface states, leading to the quenching of the SA emissions. The organic functional groups chemically combine with these surface-dangling bonds leading to the decrease in surface states and surface nonradiative relaxation channels and to the increase in the SA emissions.

Transformation of luminescence centers in CVD ZnS films subjected to a high hydrostatic pressure

The cathodoluminescence and optical-transmission spectra of ZnS were analyzed to study the effect of a high hydrostatic gas pressure (1500 atm at 1000°C) on the equilibrium between intrinsic point defects in zinc sulfide grown by chemical vapor deposition (CVD) with an excess of zinc. The cathodoluminescence spectra were measured at 80–300 K and excitation levels of 1022 and 1026 cm−3 s−1; the optical-transmission spectra were measured at 300 K in the wavelength range 4–12 µm. It is found that exposure to a high hydrostatic gas pressure transforms the self-activated emission in the cathodoluminescence spectrum: (i) a new short-wave-length band appears at 415 nm with its intensity increasing by one to three orders of magnitude; and (ii) the long-wavelength band that peaks at 445 nm and is observed in as-grown crystals becomes quenched. Simultaneously, the cathodoluminescence band peaked at 850 nm and related to vacancies VS is no longer observed after high-pressure treatment. These effects are attributed to a partial escape of excess zinc (Zni) from crystals and additional incorporation of oxygen into lattice sites (OS). A doublet band I1, which peaked at ∼331–332 nm at 80 K and at ∼342–343 nm at 300 K and is related to excitons bound to acceptor levels of oxygen centers, was observed. This band is found to be dominant in the cathodoluminescence spectrum at an excitation level of 1026 cm−3 s−1. Traces of the ZnO phase are apparent after the high-pressure treatment in both the cathodolumi-nescence spectra (the bands at 730 and 370 nm) and the transmission spectra (narrow bands in the region of 6–7 µm).

Investigation of intrinsic defects and their distribution in CdSe/ZnSe quantum dot structures

The intrinsic defects and their distribution in CdSe/ZnSe self-assembled quantum dot heterostructures grown under variation of VI/II group beam pressure ratio are investigated by luminescent and high-resolution X-ray diffraction methods. In all samples the self-activated emission connected with donor-acceptor pairs V Zn–D is found. Analysis of excitation spectra of this band shows that vacancy related defects are mainly localised in ZnCdSe wetting layer. It is found that increase of Se beam pressure results in: (i) the increase of the number of metal vacancy related defects and their appearance on nanoisland interface; (ii) enhancement of Cd/Zn interdiffusion process; (iii) the decrease of Cd content in nanoislands and suppression of nanoisland formation. It is proposed that observed transformation of nanoisland emission band is mainly caused by enhancement of interdiffusion process.

Synthesis and photoluminescent properties of ZnS nanocrystals doped with copper and halogen

A novel wet-chemical precipitation method is optimized for the synthesis of ZnS nanocrystals doped with Cu + and halogen. The nanoparticles were stabilized by capping with polyvinyl pyrrolidone (PVP). XRD studies show the phase singularity of ZnS particles having zinc-blende (cubic) structure. TEM as well as XRD line broadening indicate that the average crystallite size of undoped samples is ∼2 nm. The effects of change in stoichiometry and doping with Cu + and halogen on the photoluminescence properties of ZnS nanophosphors have been investigated. Sulfur vacancy (V S) related emission with peak maximum at 434 nm has been dominant in undoped ZnS nanoparticles. Unlike in the case of microcrystalline ZnS phosphor, incorporation of halogens in nanoparticles did not result V Zn related self-activated emission. However, emission characteristics of nanophosphors have been changed with Cu + activation due to energy transfer from vacancy centers to dopant centers. The use of halogen as co-activator helps to increase the solubility of Cu + ions in ZnS lattice and also enhances the donor–acceptor type emission efficiency. With increase in Cu + doping, Cu-Blue centers (Cu Zn −Cu i +), which were dominant at low Cu + concentrations, has been transformed into Cu-Green (Cu Zn −) centers and the later is found to be situated near the surface regions of nanoparticles. From these studies we have shown that, by controlling the defect chemistry and suitable doping, photoluminescence emission tunability over a wide wavelength range, i.e., from 434 to 514 nm, can be achieved in ZnS nanophosphors.

Silver photodoping into Al–ZnSe for application to white light emitters

Room-temperature Ag photodoping into crystalline Al-doped ZnSe grown by molecular-beam epitaxy (MBE) has been achieved. A promising approach to apply to a patterned white light emitter has also been proposed. An Al–ZnSe with Ag islands was illuminated with a HeCd laser at 300 K and annealed in the MBE chamber. Then, their photoluminescence characteristics were measured in real time. The self-activated (SA) emission, observed in as-grown Al–ZnSe, undergoes a considerable change in intensity and band energy position through Ag photodoping and subsequent annealing. The resultant emission bands centered at 2.145 eV and 2.250 eV are assigned as a donor-to-acceptor pair transition related with Ag impurities rather than SA centers. In addition, chromaticity can be controlled by an adjustment of photoirradiated Ag-doped area.

Photoluminescence Properties of Selectively Ag-Photodoped Al–ZnSe at Room Temperature

Selective Ag photodoping at room temperature (RT) has been succesfully performed on Al-doped n-type ZnSe grown by molecular beam epitaxy (MBE). Some 6-nm-thick Ag islands with a square shape (2 mm×2 mm) are deposited on the as-grown Al–ZnSe, which in turn is illuminated with a 325 nm HeCd laser at RT and the photoluminescence (PL) characteristics are measured in real time. The self-activated (SA) emission shown in Ag-undoped Al–ZnSe shifts to a shorter wavelength ΔE(I2)∼+60 meV) via a series of processes of Ag photodoping and subsequent annealing in the MBE chamber. The emission bands centered at 2.245 eV and 2.146 eV distinctly show a blue shift with the increase of the PL-excitation power, which can be assigned to a donor-to-acceptor (DA) pair transition. That is, the SA bands in Al–ZnSe appear to be converted into DA bands by Ag photodoping. The coloring is controlled to some degree by adjustment of the percentage of the Ag-doped area included in the PL-excitation spot. For example, a 40% spectrum, which corresponds to one obtained from an excitation spot consisting of an Ag doped area of ∼40% and an Ag undoped area of ∼60%, appears yellowish-white to the naked eye. We believe that it is possible to realize a patterned ZnSe-based white-light emitter with Ag islands of micrometer size using the conventional UV-lithography and lift-off processes and RT–Ag photodoping.

Diethyldisulphide as sulphur precursor for the low temperature metalorganic vapour-phase epitaxy of ZnS: growth, morphology and cathodoluminescence results

We report on the atmospheric pressure metalorganic vapour-phase epitaxy (MOVPE) growth of ZnS using diethyldisulphide (Et 2S 2) as sulphur precursor in combination with electronic grade dimethylzinc : triethylammine (Me 2Zn : Et 3N). Et 2S 2 is used here for the first time as a substitute of diethylsulphide (Et 2S) to achieve low temperature ZnS growth in pyrolytic MOVPE. It is demonstrated that Et 2S 2 lowers the temperatures of the process by almost 150°C with respect to Et 2S, leading to growth rates in excess of 0.5 μm/h just above 400°C. Scanning electron microscopy observations and 10 K cathodoluminescence (CL) measurements performed on the as-grown samples are also reported. CL spectra show both free-to-bound and intense self-activated band emissions peaked at 3.683 and 2.58 eV, respectively. These emissions are attributed to unintentional Na and halogen (most probably Cl) doping of ZnS epilayers, likely arising from both Et 2S 2 and Me 2Zn : Et 3N precursors. Finally, monochromatic CL images of the samples demonstrate the interplay between point defects induced radiative emissions and non-radiative recombinations associated with the epilayer extended defects.

Photoluminescence properties of MgxCd1−xSe mixed single crystals

The photoluminescence spectra of MgxCd1−xSe (0.00⩽x⩽0.46) mixed single crystals grown by the chemical transport reaction technique were investigated. We observed an emission band (I2) due to excitons bound to a neutral donor, an emission band (DA) due to donor-acceptor pair recombination with LO-phonon replicas and a self-activated emission band. These emission bands were shifted to higher energy with increasing x. For the crystals with x=0.46, the donor-acceptor emission band was observed at 2.661 eV in the blue region.

Effect of chloride on the photoluminescence of ZnS:Mn thin films

ZnS:Mn thin films at various substrate temperature are grown by halide transport chemical vapor deposition. These films show blue and red photoluminescence (PL) in addition to the typical yellow-orange emission. The manganese crystal environment is characterized by electron spin resonance (ESR) spectroscopy. A computer simulation of the ESR spectra is used to quantify the number of isolated manganese and the number of clustered manganese in the crystal lattice. These data reveal that the red emission occurs in films with low manganese concentration, and, therefore, occurs from a mechanism different than those previously posed. The activation energy for Mn incorporation is measured to be Ea=137 kJ/mol. From these data, a Mn–Cl defect pair is proposed as the red emission center. Self-activated blue emission in intentionally Cl-doped ZnS films is also demonstrated. Thus both red and blue PL in ZnS thin films result from chloride impurities.

Photo and electroluminescence from terbium doped zinc sulfide films deposited by spray pyrolysis

Undoped, silver doped and silver-terbium doped zinc sulfide photoluminescent thin films have been prepared by the spray pyrolysis technique. The crystalline structure of the deposited films is hexagonal wurzite, in all cases. The films have a preferential growth with the (0 0 2) direction perpendicular to the surface of the substrate. In general, the relative chemical compositions show that the films are formed by a zinc rich ZnS material. The photoluminescent excitation measurements are in agreement with the optical transmission results indicating that the excitation mechanism is related to electron–hole pair generation across the ZnS band gap. The photoluminescent spectra show the typical bands associated with the transition from the 5D4 level to the 7F6, 7F5, 7F4 and 7F3 levels of the Tb3+ ions. The emission spectra for the undoped and silver doped films show the well known self-activated emission, centered at 460 and 490 nm, respectively. The dependence of the emission intensity on the temperature shows an energy activation of 0.1 eV for competitive non-radiative transitions. Electroluminescent devices using silver-terbium doped zinc sulfide films as active layer show emission spectra similar to those observed in photoluminescence measurements but with a reduced intensity.

Photoluminescence properties of MgxZn1−xSe single crystals

Mg x Zn 1−x Se single crystals were grown by the closed tube sublimation method. The MgxZn1−xSe single crystals crystallized into zincblende and wurtzite structures in the composition ranges of x=0.0–0.1 and x=0.2–0.6, respectively. Blue and violet emissions with LO phonon replica and self-activated emissions in the MgxZn1−xSe single crystals were observed at 10 K.

Atomic Layer Epitaxy of ZnS by Low-Pressure Horizontal Metalorganic Chemical Vapor Deposition

The self-limiting growth in atomic layer epitaxy (ALE) of ZnS on (100)-GaAs substrate was investigated using a horizontal low-pressure metalorganic chemical vapor deposition (MOCVD) system. The growth rate per cycle was kept constant at one monolayer, independent of the substrate temperature which was in the range of 125° C to 200° C. Under optimal growth conditions, ZnS layers grown in the ALE mode showed good surface morphology. Photoluminescence (PL) showed three peaks; a near-band-edge emission at 349 nm, a self-activated emission at 434 nm and a broad one at around 500 and 600 nm, which is considered to be due to relaxation through dislocation formation.

Photo- and cathodoluminescence of ZnSSe quantum well heterostructures grown by MOVPE

Lateral- and depth-resolved cathodoluminescence as well as photoluminescence of MOVPE-grown single- and multi quantum well ZnSSe-based heterostructures in the temperature range of 14–300 K were investigated. The intensity ratios between different emission peaks and their dependencies on the electron penetration depth together with the temperature dependence of photoluminescence spectra permit concluding that the transport of non-equilibrium carriers or excitons in the structure is hindered by potential fluctuations created by microvariations of the sulphur content. The deep centres are localized mostly in the near-substrate region which is indicated by an increase of the self-activated emission at 2.2 eV with increasing electron beam penetration depth. Further improvement of layer homogeneity as well as a sufficient distance between the quantum well and the substrate together with minimization of the interlayer strain is necessary to improve the radiative properties of ZnSe-based heterostructures.

Crystal Growth of ZnSe in Closed Tube by Chemical Transport with Iodine

The crystal growth of ZnSe was performed by chemical transport using iodine as the transport agent. The growth parameters were : source zone temperature ; 1098-1123 K, temperature difference between source and growth zones ; 25 - 50 K, and amount of iodine (M) ; 0.10∼30 kg/m 3 . The transport rate was measured experimentally and calculated by using thermodynamical data and gas-phase transport equation. With an increase of M, the observed transport rate was divided into three regions ; a gradual increase at M 5.0 kg/m 3 where fine granular, block-form and needle-like crystals resulted, respectively. The observed transport rate was almost along the calculated one which reproduced the steep increase in the transport rate at M> 5.0 kg/m 3 . The results indicate that the transport was limited by the diffusion and laminar flow in the gas phase at M≤5.0 kg/m 3 and then modified by the thermal convection at M>5.0 kg/m 3 . All the grown crystals only showed the self-activated emission peak in photoluminescence spectra indicating the presence of non-radiative recombination centers. Those centers were attributed to the defects caused from the Se 2 overpressure and the iodine incorporation in the crystals.

Characterization of Al-doped n-type ZnSe prepared under a zinc-rich growth condition by low-pressure organometallic chemical vapor deposition

High conductivity n-type ZnSe with ϱ = 0.01 ωcm and n = 2.4 × 10 18 cm −3 is obtained on (100) GaAs substrates by low pressure organometallic chemical vapor deposition. The 14 meV full width at half maximum of the 77 K photoluminescence near-band-edge emission shows a high quality of as-grown Al-doped ZnSe epilayers. With a suitable Al doping level, a strong photoluminescence intensity of near-band-edge emission is obtained. The behavior of near-band-edge emission and of self-activated emission related to the incorporation of aluminum are discussed in this paper.

Dependence of electrical and optical properties of iodine-doped cubic ZnCdS films on solid composition

Iodine-doped cubic ZnCdS films have been grown on GaAs by low-pressure metalorganic vapor phase epitaxy using ethyl iodide as a donor dopant source. And the electrical and optical properties have been investigated. It has been found that the maximum electron density of the doped-ZnCdS films is about 10 19 cm -3. In the photoluminescence properties, near-band-edge emissions and self-activated emissions relating to the iodine donors are observed. Furthermore, the electrical properties greatly depend on the solid composition. It has been found that the electron density changes abruptly at the solid composition of about 0.5. From the photoluminescence measurements and the secondary ion mass spectroscopy analysis, this has been attributed to the increase in the concentration of donor-compensation centers there.