Recent progress and problems in high field electroluminescence of inorganic materials

摘要

High-field electroluminescence (EL) of inorganic materials are of basic and practical interest. There are two types of the high field EL; one is a thin film EL (TFEL) and the other is a powder type. Recent researches are mainly on TFEL. In this paper, recent progress on TFEL will be discussed and reviewed. Features of high field TFEL are as follows: (1) Strength of electric field is as high as 10$+6$/V/cm. This is quite different from light emitting diode (LED) of p-n junction. (2) EL phosphor materials are composed of host and luminescent centers; the host materials are wide gap semiconductors of ZnS and IIa-VIb (CaS, SrS) etc. The luminescent centers are ions which show intra- shell transitions for f-f or f-d transitions (rare-earth ions; Tb$+3$PLU$/, Tm$+3$PLU$/, Ce$+3$PLU$/ etc.) and of d-d transitions (Mn$+2$PLU$/ etc.). (3) Excitation mechanisms are not yet well understood and should be clarified, of (a) carrier generation and injection, (b) hot electron transport and distribution function, and (c) excitation mechanism of luminescent centers. Problems to be solved is color, especially blue EL materials. To solve this, a new material should be developed, and in addition, a new structure of multi- layer of quantum-well structure should be investigated.