Mechanisms formation of electron-hole trap centers in LiKSO 4 crystall

The mechanisms of creation of electron-hole trapping centers in LiKSO 4 have been investigated by the methods of vacuum and thermal activation spectroscopy. It is shown that electron-hole trapping centers are formed during the trapping of electrons by anionic complexes and localization of a hole in the lattice in the form of the radical SO − 4 . The appearance of phosphorescence at 3.0-3.1 eV, 2.6-2.7 eV and 2.3-2.4 eV conﬁrms the creation of electron-hole trap centers.


Introduction
The LiKSO 4 crystall and other lithium containing mixed sulfates of alkali metals activated by various ions are practically used as phosphors, dosimeters and various sensors. To increase the sensitivity of such active crystals, it is necessary to study the recombination processes under excitation process. The regularities of the relaxation processes depend on the energy-band structure of the crystals. Quantum-chemical calculations of the authors of [1][2][3] showed that the band gap of the crystals LiKSO 4 , LiNaSO 4 , and LiRbSO 4 is 5.2 ÷ 5.8 eV.
The results obtained are in satisfactory agreement with the experimental data for these crystals [4][5][6][7]. Calculations have shown that the upper part of the valence band consists of 3 subbands formed from the 2p state of oxygen of the anionic complex SO − 4 . The conduction band consists of the s state of cations, the 3p state of sulfur, and the 2p state of oxygen, which are not filled with electrons. During the relaxation of an excited LiKSO 4 crystal, holes formed in three subbands of the valence band rise above the top of valence band and are localized in the form of local states. Depending on the crystallographic direction of the S-O bond, nonequivalently located holes SO − 4 are formed. When electrons recombine from the conduction band with nonequivalently localized SO − 4 holes, intrinsic emissions arise. By measuring the creation of intrinsic emission, it is possible to determine at what excitation energies effectively created different intrinsic emissions.
The main intrinsic emission at 3.7-3.8 eV appears upon excitation by lowenergy photons with energies 5.1-6.2 eV and high-energy photons with energies of 6.9-10.5 eV. It was assumed that the appearance of emission 3.7-3.8 eV is associated with the recombination of electrons in the s state of the conduction band with localized holes relaxed from the first, second and third subbands in the 2p state of oxygen above the top of the valence band [8].
Along with the intrinsic emission in the irradiated crystals, there appear long-wavelength emissions associated with the creation of electron-hole trapping centers. It was assumed that long-wavelength recombination emissions at 3.0-3.1 eV, 2.6-2.7 eV and 2.3-2.4 eV is associated with the creation of electron-hole trapping centers.
The main task of this work is to study the nature of long-wavelength emission in the LiKSO 4 crystal.

Objects and methods of research
Crystals LiKSO 4 are grown at a temperature of 50°C from a saturated aqueous solution by the method of slow evaporation. Plates 3-5 mm thick were cut from the crystal. We have investigated samples of crystals and powders with a chemical purity of 99.99% (Sigma Aldrich) LiKSO 4 . Crystals LiKSO 4 and powders were investigated by the methods of photoluminescence, X-ray luminescence, vacuumultraviolet and thermal activation spectroscopy. A deuterium lamp D200VUV (Heraeus Noblelight, Germany) with a photon energy of 6.2 ÷ 11.5 eV and an Xenon lamp XBO 150 W (OSRAM, Germany) with a photon energy of 1.5 ÷ 6.2 eV were used for excitation in the ultraviolet region. To measure the emission and excitation spectra in the spectral region of 1.5 ÷ 6.2 eV, we used the Solar CM 2203 spectrofluorometer. The excitation and emission spectra in the 4 ÷ 11.5 eV spectrum were measured on a vacuum monochromator assembled according to the Seya-Namioka scheme in a wide range temperatures of 15 ÷ 400 K. Registration of emission was carried out through an MDR-41 monochromator using a photomultiplier (Photomultiplier tube) 1P28 (Hamamatsu, Japan). The excitation spectrum is corrected for the spectral intensity distribution of the exciting radiation.      Thus, we have found for the first time that electron-hole trapping centers are created in irradiated crystals in parallel with the short-wavelength emission bands that arise during the recombination of electrons with localized holes, which appear in the form of long-wavelength tunneling recombination emission of 3.0-3.1 eV, 2.7-2.6 eV and 2.4-2.3 eV. Figure 4 shows the spectrum of generation of recombination emission 3.0-3.1 eV. Figure 4 shows that recombination emission at 3.0-3.1 eV is generated in the spectral region at 5.9-6.0 eV and above 9-12 eV. Emission of 3.0-3.1 eV is created in the fundamental region of the LiKSO 4 crystal, where free electron-hole pairs are created.

The results of the experiment and their discussion
Thus, all three bands of recombination emission are excited in the same spectral regions. In our previous publications [9], we assumed that in the photon energy range at 6.0-6.2 eV and 9-12 eV, long-wavelength recombination emission at 3.0-3.1 eV is generated rather than excited. Based on our previous results [9], it can be assumed that the bands of recombination emission in LiKSO 4 at 3.0-3.1 eV, 2.7 eV and 2.4 eV upon excitation by photons with energies of 5.5-12, 0 eV are created (Figure 4). And the excitation spectrum at 3.25 eV, 3.9-4.0 eV and 4.4-4.5 eV in LiKSO 4 for the recombination emission band at 3.0-3.1 eV, 2.7 eV and 2.4 eV corresponds to an intracenter electronic transition at the trapping centers. As suggested in previous works [9], trapping centers are created when electrons are localized on anionic complexes according to the reaction: Holes created in subzones are autolocalized as SO   Figure 5 shows the phosphorescence spectrum of the LiKSO 4 crystal measured after the termination of X-ray radiation for 20 min. at 80 K. It can be seen from Figure 5 that phosphorescence is detected in those spectral regions where we detected recombination emissions at 3.0-3.1 eV, 2.7 eV and 2.4 eV. The appearance of phosphorescence in these long-wavelength ranges proves the existence of electron-hole trapping centers.

Conclusion
In irradiated LiKSO 4 crystals, the appearance of long-wavelength recombination emission bands at 3.0-3.1 eV, 2.6-2.7 eV, and 2.3-2.4 eV is associated with the formation of electron-hole trapping centers. The electron-hole trapping centers are formed during the trapping of electrons by the anionic complexes SO 2− 4 and self-trapping of the hole in the form of the radical SO − 4 . The appearance of phosphorescence after the termination of X-ray irradiation proves the existence of electron-hole trapping centers in these spectral regions.