Fast intrinsic emission with the participation of oxyanion and cation excitations in metal sulphates

Abstract

Several bands of fast emission (τ < 2 ns) have been observed in the spectral region of 1.6-6.0 eV in K 2 SO 4 , Rb 2 SO 4 , Cs 2 SO 4 , CaSO 4 , SrSO 4 and BaSO 4 under irradiation by single electron pulses (300 keV, 3 ns, 80 A/cm 2 ) at 80 K. In K 2 SO 4 , Cs 2 SO 4 and BaSO 4 , the photoexcitation of fast emission starts at 14-15 eV. At a crystal heating the intensity of fast emission bands decreases, while the intensity of a structureless intraband luminescence increases (especially at hν > 6 eV). On the basis of our experimental data and band structure calculations, we interpret the observed fast emissions as elecron transitions between the subbands of a split valence band. There are manifestations of the cationic states, superimposed on the states from an oxygen-sulphur medium subband, in the excitation spectra of fast emissions.1 Introduction In addition to impurity luminescence and the luminescence of free or self-trapped excitons, data on two new types of intrinsic emission in several wide-gap crystals was accumulated in the last decades. These two emissions are the so-called intraband luminescence (IBL) and crossluminescence. The IBL arises under irradiation due to the transitions of hot electrons (electron IBL) and hot holes (hole IBL) between the levels of the conduction band or the valence band of a crystal, respectively [1, 2]. The IBL is a fast, continuous and temperature-independent emission in a wide region with a structureless spectrum. The crossluminescence is connected with the radiative transitions of the holes, formed by irradiation in a cation outermost-core band, to the upper anion valence band [3,4]. This emission is also known as Auger-free or core-valence luminescence. The present study is devoted to the fast emissions that can be detected in metal sulphates in addition to IBL and crossluminescence.Several bands of fast (τ < 2 ns) emission have been observed in K 2 SO 4 under irradiation by single nanosecond electron pulses [5]. The threshold exciting photon energy of 15 eV has been detected in the excitation spectra for the most intense emission bands at 5.8 and 3.8 eV. Taking into account the band structure calculations of K 2 SO 4 [6], these fast emissions have been ascribed to the electron transitions between subbands of a splitted valence band [5]. The investigation of the nature of fast emissions and the conditions of their excitation in various metal sulphates has much potential for yielding important information about the structure of a valence band. Therefore, our efforts are aimed at the search for fast emissions and the measuring of the excitation spectra for these emissions in Me 2 SO 4 (Me: Na, K, KLi, Rb, NH 4 ) and MeSO 4 (Me: Ca, Sr, Ba) systems.

Defect creation caused by the decay of cation excitons and hot electron–hole recombination in wide-gap dielectrics

“…Contrary to the case of MgO, the energy gap between the valence subbands has been theoretically predicted [37] and experimentally detected by measuring the spectra of the hole IBL in some sulphates (e.g. in K 2 SO 4 ) [34]. The first band structure calculation did not predict the energy gap between the valence subbands of a-quartz [38].…”

“…In wide-gap metal oxides, the more intense h-IBL due to the transitions of hot holes between the levels of the v-band has also been revealed [33]. A short-wavelength edge of the h-IBL allows an estimate of the total width of the band, E v [33][34][35]. The value of E v equals 8.7, 6.6 and 6.9 eV in BeO, MgO and BaMgAl 10 O 17 , respectively.…”