On the Nature of Fluorescent Centers and Traps in Zinc Sulfide

Abstract

The impurity levels in the energy diagram of a zinc sulfide phosphor are considered to be localized S2− levels lifted above the filled S2− band due to the presence of monovalent positive or trivalent negative activator ions in the lattice. Electron traps are formed similarly by the substitution of S ions by monovalent negative ions or of Zn2+ ions by trivalent positive ions. The energy produced when electrons recombine with trapped holes or when holes recombine with trapped electrons is either emitted directly as light or is first transferred to impurity ions. The elements of the iron group give rise to electron traps. The killing action of these elements is explained by assuming that the energy liberated by recombination between holes and electrons in these traps is transferred to the killer ions. The excited ions return to the ground state radiationless because of the presence of many electronic levels between the excited and the ground state. The effect of heat and infrared radiation on the luminescence is discussed. It is shown that, in a phosphor, energy may be transferred by electrons through the conduction band or by holes through the occupied S2− band.

Some Optical Properties of Powder and Crystal Halophosphate Phosphors

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Developments in Luminescence and Display Materials Over the Last 100 Years as Reflected in Electrochemical Society Publications

“…Froelich 29 reported yellow and red emissions from ZnS:Cu phosphors. Klasens 30 discussed the nature of fluorescent centers and traps in ZnS with S 2Ϫ impurity. Hoogenstraaten and Klasens 31 presented the luminescence of ZnS:Cu,Co phosphors and described the role of ''killer'' centers, namely, additional impurities responsible for the drop of the fluorescence efficiency.…”

X-ray studies and luminescence of ZnS-phosphor doped with silver and cobalt

“…One band was found to be activator dependent and independent of coactivators. This band is attributed to the transition from the conduction band to the activator level which is just situated above the valance band (classical RiehlSchon-Klasen model [1][2][3]. On the other hand, the second band depends on both activator and coactivator.…”

“…The first band is produced by the recombination of an excited electron either from the conduction band or some shallow traps just below the bottom of the conduction band with a positive hole captured at the acceptor level of silver. The Riehl-Schon-Klasen model [1][2][3] can be applied to this band in the light of the information given about the width of the forbidden gap (3.7 eV for wiirtzite ZnS form) and the position of the acceptor level introduced by silver in the energy gap (0.85 eV above the valence band). The second band of longer wavelength is produced by a radiative transition of an excited electron captured at a donor level associated with an impurity coactivator (probably bromine, iodine, oxygen or lattice defects situated at 0.28eV below the lower edge of the conduction band) to recombine with a positive hole captured at the acceptor level of silver (i.e.…”

“…In this case, it is possible to correlate the average spacing between activator impurity atoms with the spacing between certain hostcrystal atoms, because the number of suitable interstitial sites in hexagonal ZnS is roughly equal to the number of zinc atoms. If one assumes, as a first approximation, a uniform distribution of the zinc host-crystal atoms in ZnS, the average centre to centre spacings between zinc atoms is given by [22] Xzn-zn ~ (oNvNA/Mw)-~/3cm (3) where o is the density of the host crystal (gcm 3), Nv is the number of zinc atoms per simple molecules, NA is Avogadro's number and Mw is the gramme molecular weight per simple molecule. The average distance between activator atoms, assuming a uniform distribution, is…”