Paramagnetic Resonance Absorption in Additively Colored Crystals of Alkali Halides

Abstract

A T the March, 1952, Massachusetts Institute of Technology Conference on Physical Electronics we reported on the behavior of field dependent fluorescence of zinc silicate phosphor. Fluorescence could be obtained only when the electric field had an ac component. The fluorescence observed in our samples was interpreted as being due to field emission, either cold emission or Zener tunneling at regions of high field intensity. The need for the presence of an ac component was ascribed to the existence of space charge associated with traps in the semiconducting vitreous phosphor. These assumptions also explained the anomalous behavior of the radiation as the temperature of the semiconductor was increased above 200°C.Additional experimental results have now been obtained. The samples consist of a thin strip of glass, 0.01 cm wide and about 2 cm long placed between two conducting electrodes so that the electric field is applied across the width of the strip. The glass has a low softening point, and the Zn 2 Si04:Mn phosphor is imbedded in it. The radiation is observed from a cross section in a plane perpendicular to the plane of the strip. Aluminum, silver, and graphite were used for the electrodes, the graphite giving the best performing sample as far as the efficiency of fluorescence is concerned. The vitreous mixture is a semiconductor with a resistance between 5 and 100 megohms/cm 2 at room temperature. However, the resistance is a function of the temperature and depends on the frequency of the applied potential.Observation with a low power microscope shows that the radiation is emitted from the interface between the electrode and the vitreous phosphor.The lowest potential for which radiation is observed varies between 100 v and 500 v depending on the thickness and other parameters of the sample. At a fixed potential the light output varies slightly with the frequency. For a frequency variation of the applied field from 120 cps to 16 kc/sec the light output varies by a factor less than 5. At 60 cps the light output varies roughly as the square of the potential in the range from 100 v above the low voltage cutoff to the crystal breakdown potential. The low voltage cutoff is defined arbitrarily as the potential at which radiation is first observed. Possibly a more sensitive photocell arrangement, such as a photomultiplier, would detect radiation at a potential lower than the presently observed cutoff.These observations further substantiate the hypothesis that field emission is responsible for the phenomena. The transport of charge from deep-lying energy levels across the interface barriers excites the phosphor component of the glass matrix near the potential barrier at the interface. The value of this electric field across the interface will depend, of course, on the dielectric constant of the glass matrix and on the rate of conduction of the charge through the semiconducting glass matrix to the opposite electrode.It is interesting to note that Piper and Williams 1 in their recent studies on the electroluminescence of ...

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