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Elektrolumineszenz von II–VI-Verbindungen

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Festkörperprobleme V

Part of the book series: Advances in Solid State Physics ((ASSP,volume 5))

Abstract

The direct conversion of electrical energy into light energy when applying electric fields on II–VI-compounds has been studied on powders, thin films, and single crystals. As to the mechanism of II–VI-luminescence no final theory has yet been developed which can explain all experimental facts. This review tries to give an outline of the current trends of both experimental work and theoretical explanation.

To investigate the electroluminescence on powders excited by ac-fields (Destriau-effect) two ways are used: 1. the light of a great number of grains with numerous light spots is enregistered (macroscopic measurements), 2. the emission of single light spots is observed in a microscope (microscopic measurements).

One of the most important microscopic observation is that the light spots generally emit in one half period of the field only, whereas other spots emit whenever the field has the opposite direction. A close relation between the direction of emitting lines and the crystal structure, and of irregularities of the crystal lattice has been found. In ZnS powders the best electroluminescence is obtained when both cubic and hexagonal phases alternate in the lattice and when copper precipitates at the disturbed lattice sites. The intensity of the time integrated light emission I of a single spot depending on the applied voltage U follows approximately the relation I∼ exp (−B/U), whereas the intensity I of many dissimilar lines observed simultaneously obeys a law I∼ exp (−B/U 1/2) over 12 orders of magnitude.

The time integrated brightness of single spots increases sublinearily with the frequency of the applied field and saturates at higher voltages. The brightness of a number of dissimilar spots, however, increases first linearily with increasing frequency and saturates then, provided there are not too much different competint recombination centers. If there are more than one type of recombination centers with different capture cross section of free charges and different energetic distances from the bands, as Cu, Mn, Co, Fe, Ni in ZnS have, the phenomenon of delayed recombination influences the spectral distribution and the frequency dependence: electrons and holes, which are liberated by the electric field, are separated within a half period of the ac-field. The holes are quickly captured by acceptors with high capture cross section. After reversing the electric field the returning electrons will recombine either within the centers with high capture cross section for defectelectrons, when the frequency is high and the lattice temperature low, or they will recombine according to a fermi-distribution of holes, if the frequency is low and the temperature sufficiently high. This phenomenon gives the possibility of studying the hole migration in luminescence and to determine capture cross sections and energetic distances of levels from their bands by statistical and kinetical methods. So far the recombination properties of the Destriau-effect are quite well understood.

However, as far as the excitation mechanism of the Destriau-effect is concerned, there are several competing models and assumptions: alternating injection of holes and/or electrons from copper precipitations or Cu2S in ZnS, creation of free electrons by direct action of the electric field (tunnel effect), and acceleration and excitation of luminescence centers by electron-impurity-collision (hot electron model). No direct proof of one or the other excitation mechanism of the Destriau-effect in powders has yet been delivered. Better understanding of the excitation mechanism has been achieved with thin films and single crystals. Theoretical considerations show that pn-homojunctions in binary II–VI-crystals can be obtained, when the ratio of the radii of cation and anion is close to unity. If this is not the case, the selfcompensation by lattice defects prevents the creation of efficient n- or p-conduction. Efficient pn-injection luminescence has indeed been obtained in CdTe since it had been discovered that phosphorus provides very shallow acceptor centers. External quantum efficiencies of CdTe as high as 12% at 77°K have been measured at 77°K within a spectral range of 8300 to 8800 Å, but not lasering action has yet been observed. In order to shift the emission into the visible region of the spectrum II–VI-crystals with more than two components have been prepared. In Cd x Zn1−x Te pn-homojunctions have been obtained by diffusing simultaneously Zn and P into n-type CdTe: (Al)-crystals. The highest quantum efficiency of this kind of diodes ever measured was 4%, a band gap as high as 2.1 eV has been reached. One of the advantages of these crystals is the possibility of applying good ohmic contacts, which can stand high currents.

Similar diodes have been made from ZnSxTe1−x crystals, which also have been found having amphoteric conductivity, as predicted by theoretical considerations. Electroluminescence of heterojunctions has been observed for instance in ZnSe−Cu2Se and ZnS−Cu2S. In this case the luminescent semiconductors are coated by chemically different semiconducting materials of opposite conductivity and different bandgap, which does not form a complete series of solid solutions with the luminescent semiconductors. Thin insulating films between the p-type and the n-type semiconductor enhance the luminescence considerably, thus the working mechanism is assumed to be a p-tunneling, process. The quantum efficiency of heterojunctions is much lower than the efficiency of pn-homojunctions.

Thin films of wide bandgap material like ZnSe and ZnS show electroluminescence as well as photo-electroluminescence. As in single crystals a thin semi-isolating sheet at the electrodes is decisive for the working of electroluminescence in thin films. In photo-electroluminescence of thin films the electroluminescence emission is triggered by incident radiation and can reach higher intensities than those of indicent radiation. The activator Manganese plays an important, but not yet, clarified role.

Apart from the injection luminescence in pn-homojunctions and pn-heterojunctions, impact ionisation electroluminescence, thus luminescence caused by “hot” electrons has been identified in ZnSe. In the same material tunnel injection through thin insulating films, injection electroluminescence in inversion layers and tunnel injection electroluminescence at the tips of conducting spikes occur.

Therefore, there is certainly not only one single excitation mechanism of electroluminescence in II–VI-compounds, but at least five which can clearly be distinguished by qualitative criteria, as far as single crystals are concerned. This gives the answer to the question whether electrons and holes in electroluminescence of II–VI-compounds are always in thermal equilibrium with the lattice or not.

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  1. Institut für Elektronenmikroskopie am Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin-Dahlem

    H.. -E. Gumlich

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Fritz Sauter

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© 1966 Friedr. Vieweg & Sohn Braunschweig

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Gumlich, H.E. (1966). Elektrolumineszenz von II–VI-Verbindungen. In: Sauter, F. (eds) Festkörperprobleme V. Advances in Solid State Physics, vol 5. Springer, Berlin, Heidelberg. https://doi.org/10.1007/BFb0119278

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  • DOI: https://doi.org/10.1007/BFb0119278

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