Role of Sulfur Vacancies in Luminescence of Pure CaS

Abstract

Results on EPR and luminescence of pure CaS are discussed in a new light to provide an interpretation that the same sulfur vacancies give rise to both EPR signals and fluorescence emission depending upon the relaxation times of trapped electrons. 1. I NTRODUCTON The alkaline earth sulfides, particularly CaS and SrS, have come into prominence due to the new multicolored thin film electroluminescent (EL) devices using these materials /1,2/. A long series of investigation carried out on CaS, by the present authors , had revealed some interesting aspects of sulfur vacancies. These had begun with the detection of EPR due to a photoexcited electron trapped at av~+ and of F 1 + (or v!+) needing no photoexcitation /3,4/. A trap has been revealed in thermoluminescence (TL) also, at a temperature of 226 K corresponding to an electron trap at an energy E1 of 0.32 eV below the conduction band. This TL peak also corresponds to a photo-electroluminescent peak indicating that probably electrons are more mobile than the holes which are trapped at acceptor levels in pure CaS /5/. TL glow peak analysis revealed Ca vacancies and interstitials in CaS /6/. The EPR signal has a characteristic activation energy between room temperature and about 90 K of 0.26 eV. This was later correlated with the TL trap, which was estimated to have an escape frequency S of 105 sec-1. These values of Et and S, when used to estimate lifetimes, yielded values close to the decay lifetimes of EPR state, which are very long, of the order of 10-100 secs, justifying the correlation of TL trap and EPR state. In the present paper, we shall present some results on a 490 nm fluorescence emission of the same pure CaS samples in which indications are strong that this emission also is connected with the same sulfur vacancy centres, which give rise to the EPR signals. A 580 nm fluorescence emission due to Fl+, an electron permanently trapped at a Vsl+, is also discussed. 2. EXPERIMENTAL For details of sample preparation and designation, we would refer to our earlier work /3,4/. It should suffice here to mention that pure CaS was obtained by reduction of CaSO by flowing Hz or HzS. CaS H-60, for example denotes a sample which was reduced in Hz for 60 mts. Any subsequent treatment, such as firing in Nz with 4 mole% NaCl, X-ray irradiated for 2 hrs or CaS mixed with Cao and fired, are designated as CaS:NaCl, CaS:X-ray or CaS:CaO respectively. Luminescenece emission was excited by an electron beam using a demountable system described elsewhere /7/. An EHT of 15 KV and beam current in the region of 10-lOOuA/cn? have been used. These beam currents are higher than normally used in CRT's. Nevertheless , the results deserve a close analysis, keeping these limitations in mind.