It is experimentally demonstrated in a new and direct manner that electroluminescence in thin films of ZnS: Tb3+ is excited by the direct impact of hot electrons on the Tb3+ ions. The scheme consists of monitoring as a function of applied voltage across the structure studied‐a ZnS: Tb3+, Ta2O5 sandwich‐the ratio of fluorescence intensities I(5D3)/I(5D4) originating from the 5D3 and 5D4 levels of Tb3+ at 3.25 and 2.54 eV, respectively. As the electron energy distribution is shifted to higher energies by increasing the voltage across the structure, I(5D3)/I(5D4) grows. The Tb3+ ion thus acts as a probe for the electron distribution in these films. By comparing the results obtained to simple theory, it is concluded that in considering low‐energy losses of the electrons in the active layer, inelastic scattering by low‐lying 7F levels of Tb3+ can not be the sole energy‐loss mechanism at the Tb concentration of interest. The optical phonons of ZnS must also be taken into account.

Hot‐Electron Impact Excitation of Tb3+ Luminescence in ZnS: Tb3+ Thin Films

4f–4f Luminescence of Rare‐Earth Centers in II–VI Compounds

This work explores the sensitization of luminescent lanthanide Tb3+ and Eu3+ cations by electronic structure of zinc sulfide (ZnS) semiconductor nanoparticles. Excitation spectra collected while monitoring the lanthanide emission bands reveal that the ZnS nanoparticles act as an antenna for the sensitization of Tb3+ and Eu3+. The mechanism of lanthanide ion luminescence sensitization is rationalized in terms of an energy and charge transfer between trap sites and is based on a semiempirical model, proposed by Dorenbos and co-workers (Dorenbos, P. J. Phys.: Condens Matter 2003, 15, 8417-8434; J. Lumin. 2004, 108, 301-305; J. Lumin. 2005, 111, 89-104. Dorenbos, P.; van der Kolk, E. Appl. Phys. Lett. 2006, 89, 061122-1-061122-3; Opt. Mater. 2008, 30, 1052-1057. Dorenbos, P. J. Alloys Compd. 2009, 488, 568-573; references 1-6.) to describe the energy level scheme. This model implies that the mechanisms of luminescence sensitization of Tb3+ and Eu3+ in ZnS nanoparticles are different; namely, Tb3+ acts as a hole trap, whereas Eu3+ acts as an electron trap. Further testing of this model is made by extending the studies from ZnS nanoparticles to other II-VI semiconductor materials; namely, CdSe, CdS, and ZnSe.

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Lanthanide Sensitization in II−VI Semiconductor Materials: A Case Study with Terbium(III) and Europium(III) in Zinc Sulfide Nanoparticles

In a series of experiments we have found that the field in the ZnS layer of an ac‐operated thin‐film high‐field electroluminescent device becomes clamped at values ranging from 2.1×106 to 3×106 V/cm. The device is fabricated as a sandwich with the following layers from top to bottom: thin metal; ZnS:Tb3+; Ta1.98Mo0.02O5 and Ta0.99Mo0.01. Since the electroluminescence is expected to be very strongly field dependent because the excitation proceeds via direct impact of the Tb3+ ions by hot carriers, the observed field clamping may impose a serious limitation on the efficiency that one can obtain from such structures. Evidence for field clamping is obtained by measuring as a function of peak‐to‐peak applied voltage the following: (i) Tb3+ emission intensity, (ii) the ratio of emissions originating in two different Tb3+ levels and (iii) the magnitude of a pulsed photocurrent. In addition, the clamped field may be determined quantitatively through the use of a novel technique which permits the measurements of t...

Limitation imposed by field clamping on the efficiency of high‐field ac electroluminescence in thin films

It has been shown recently that efficient electroluminescence may be obtained from thin films formed by the coevaporation of ZnS and rare‐earth fluorides. In order to assess the importance of the fluorine ions in such systems, films have been produced by coevaporating ZnS and rare‐earth metals (Tb, Sm, Nd). It is shown that large concentrations of well‐dispersed rare‐earth ions may also be obtained by this method. For ZnS: (Tb metal) the concentration which optimizes the electroluminescence power efficiency has about the same value as that for ZnS : TbF3 (2 at. % Tb), and the peak electroluminescence power efficiency is slightly lower than that obtained for ZnS : TbF3. In general, the metal‐doped systems yield electroluminescence spectra which differ from their fluoride‐doped counterparts at a similar concentration. This is taken as evidence that the fluorine ions remain in the vicinity of the rare‐earth ion. In the case of ZnS : EuF3, it is further suggested that the fluorine ions may help to control the valence of the rare‐earth ion to yield Eu3+ since independent evidence indicates that in ZnS crystals only Eu2+ is stable. Absolute photoluminescent power efficiency measurements on films of ZnS : (Tb metal) are used to show that a mechanism for electroluminescence based on electron‐hole pair formation with subsequent transfer of this energy to the Tb3+ ion may be effectively ruled out; indirect evidence for direct impact excitation of the active ion is thus obtained. For purposes of application, it appears that systems in which the rare‐earth ions are introduced by evaporation of the fluorides are superior, since the evaporation of these is simpler and the electroluminescence spectra of such devices are more reproducible.

Electroluminescence and Photoluminescence of Thin Films of ZnS Doped with Rare‐Earth Metals

Photoluminescence from evaporated films of approximately 1µ thick has been studied at room temperature as a function of the fluoride concentration. The excitation or pumping wavelength varies from 4880 to 2600Aa, while the Tb3+ emission transition at 5425Aa is monitored at the output. For pumping photon energies below the forbidden gap, the excitation spectra resemble that of the pure film or single crystal. For this case, the conversion efficiency with 4880Aa pumping in samples that contain 0.86% is close to 100% as compared to 0.8% for pure . With pumping energies above the gap, photoluminescence presumably due to energy transfer from electron‐hole pairs has also been observed; however, such a process yields conversion efficiency generally in the 10−5–10−6 range. These results support the model that electroluminescence in devices using similar films arises from direct excitation of Tb3+ levels by hot electrons as proposed by one of the authors.

D. Kahng

Papers

Photoluminescence of ZnS : TbF3 Thin Films