J. Benoit

IIA‐III2‐S4 ternary compounds: New host matrices for full color thin film electroluminescence displays

Strontium thiogallate (SrGa2S4) thin films have been prepared by rf reactive magnetron sputtering. This sulfur matrix permits the acceleration of charge carriers to optical energies under high electric field and provides green electroluminescence when doped with europium. The color coordinates are better than those achieved by other known green thin film electroluminescent phosphors. Strontium thiogallates as other ternary IIA‐III2‐(S,Se)4 compounds doped with Eu2+ or Ce3+ are good candidates for thin film electroluminescence phosphors of full color displays.

SrGa2S4: RE phosphors for full colour electroluminescent displays

Abstract Ternary thiogallate sulphides doped with rare-earth ions fulfill the electrical and optical requirements for thin film electroluminescent phosphors. These materials represent a breakthrough in the achievement of full colour EL displays. SrGa 2 S 4 provides a deep blue emission when doped with Ce 3+ and a saturated green with Eu 2+ . Some luminescent properties of Ce 3+ and Eu 2+ in SrGa 2 S 4 compounds are reviewed and the various techniques used to prepare thin films of this ternary compound are presented. The behaviour and performances of EL devices are discussed.

Ce-doped TiO2 insulators in thin film electroluminescent devices

Cerium-doped TiO2 thin films have been prepared by reactive RF sputtering. At low Ce concentration, X-ray diffraction indicates that the films have the anatase structure. Ce concentrations higher than 1.2 at.% result in an amorphization of the film which remains stable up to 873 K. The TiO2 electrical properties have been stabilized and improved by cerium doping, resulting in a lower conductivity (10-9 Ω-1m-1), a higher electrical breakdown strength (2 ×107 V/m), and a high value of the permittivity (45±5). The implementation of amorphous TiO2:Ce thin films as insulator layers in ZnS:Mn alternating current thin film electroluminescent devices (ACTFELD) results in a significant drop in the threshold operating voltage and a notable increase in the device brightness compared with ACTFELD containing Y2O3 or BaTa2O6 insulator layers. Rapid thermal annealing further improves the performance of the electroluminescent device.

On the excitation efficiency in ZnS:Mn thin‐film electroluminescent devices

In ac‐coupled ZnS:Mn thin‐film electroluminescent devices, the excitation efficiency ηexc depends on the insulator/ZnS interfaces and the excitation level. For optimized devices which are used in industrial production, ηexc does not depend on the transferred charge as long as the excitation level is below the saturation range. In the saturation range, ηexc decreases with increasing charge transfer. Hence the decrease of the total efficiency η at high excitation levels is partly due to the behavior of the excitation efficiency and not entirely to the decrease of the radiative efficiency ηlum. In devices with shallow insulator/ZnS interface states, however, charge injection starts at lower fields and ηexc increases strongly with the transferred charge and the applied voltage.

Applied voltage effect on the electron delocalization of excited Ce3+ in SrS:Ce electroluminescent devices

The electro-optical behavior of SrS:Ce atomic layer epitaxy thin films electroluminescent devices with Ce3+ ions in different point symmetries is analyzed. Under selective photonic excitation in the lower absorption band and for voltage below the EL threshold, it is shown that the delocalization probability of Ce3+ excited ions is much lower for Ce3+ ions in octahedral sites than in lower symmetry ones.

Excitation and Radiative Efficiencies in ZnS:Mn Thin Film Electroluminescent Devices Prepared by Reactive Radio-Frequency Magnetron Sputtering

Thin film electroluminescent (EL) devices have been prepared by reactive radio-frequency magnetron sputtering. The excitation efficiency η exc and the radiative efficiency η rad have been studied by analyzing the light response of the devices under short-pulse excitations. The influence of the preparation conditions on these partial efficiencies has been examined systematically. Partial efficiency measurements not only provide a method to characterize the quality of the active layer, but also permit us to obtain enlightening information on the electro-optical properties of the EL devices prepared by sputtering.

Influence of the electrical excitation pulse shape on the electro-optical response in SrS:Ce3+ electroluminescent devices

The electro-optical properties of SrS:Ce thin film electroluminescent devices prepared by atomic layer epitaxy have been studied by varying the shape of the electric excitation pulse. The different electric and optical responses have been explained by considering two sets of states at each interface. The shallower set of states will be populated by the electrons originating from the deeper states when the electric field is turned on. Relaxation of the carriers from the shallower states towards the deeper ones modifies the relative intensity of the different optical responses according to the shape of the excitation pulse.

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.

Excitation efficiency and field non-uniformity in ZnS-based thin-film electroluminescent devices grown by atomic layer epitaxy

Transient measurements of the excitation efficiency were performed in ZnS-based a.c. driven electroluminescent devices grown by atomic layer epitaxy. The results evidence the build-up of a bulk space-charge in the semiconductor layer from the first pulse on, even at low levels of the transferred charge (about ). At these levels, the semiconductor - insulator interfaces contribute to charge carrier injection and provide primary electrons for multiplication in the bulk. Models of space-charge generation by field emission from the bulk only are not compatible with experimental data. The multiplication rates seem rather high ; however, filamentary conduction may lead to an overestimation of .

Excitation efficiency of electrons in alternating driven ZnS: Mn electroluminescent devices

Abstract We measured directly the dependence of the manganese excitation efficiency ηexc on the excitation level and compared it to the behaviour of the intrinsic broad band ZnS emission for two different kind of devices. In well prepared, optimized devices, ηexc is constant up to the beginning of the saturation range. Within the saturation range, ηexc decreases with increasing excitation level. In specially annealed devices with shallow interface states, however, ηexc is very low at the electroluminescence threshold and increases strongly with the excitation level. In both kinds of devices, the ratio R of the broad band ZnS emission to the yellow Mn emission shows only a slight dependence on the excitation level. The results suggest that R cannot be taken as a criterion for the electron energy distribution.