Photoluminescence of Solid Solutions Gas 1-x sе x <er>0.1аt% (x=0.10) Irradiated with ץ-Quanta

The study investigated the photoluminescent properties of undoped and rare-earth element erbium - doped solid solutions GaS 1-x Se x <Er>0.1аt% irradiated with gamma-quanta. Erbium doping reduces the photoluminescence intensity in solid solutions. After irradiation D  = 300-1000Gy, the photoluminescence intensity increases. An increase in the photoluminescence intensity in irradiated solid solutions is explained by a decrease in the concentration of centers responsible for the fast recombination channel and associated with lattice defects. At T=77K, due to the decay of bound Frenkel pairs, Si and Vs appear in the sulfur sublattice. The Si defects are responsible for the increase in the intensity of the green luminescence band. The redistribution of photoluminescence intensity in the range of 0.520 - 0.600 µm is due to the transfer energy to rare-earth centers in activated crystals. The performed investigations allow us to conclude that doping with erbium leads the appearance of a series of emission lines in the visible region of the spectrum.


Introduction
The gallium sulfide modification is yellow crystals in the melting point of which is 950°C [1]. Gallium sulfide single crystals are stable under standard conditions and are the predominant products in most syntheses of the corresponding gallium chalcogenides. Crystal modifications of GaS has a layered hexagonal structure. The unit cell of GaS contains -S-Ga-Ga-S-fragments from two different monolayers -they are identical in the number and types of bonds and shifted relative to each other. The GaS structures consist of a sequence of pairs of Ga-S planes. The crystal structure of GaS belongs to the space group P 63/mmc with a hexagonal lattice. Its parameters are: a = 3.592 A, c = 15.495 A [2][3][4][5]. Recently, chalcogenide semiconductors activated by rare earth elements have been intensively studied. Taking this circumstance into account, this work presents the obtained experimental data, which make it possible to develop an approach to understanding the nature of luminescence in GaS1-xSex<Er>0.1аt% [6,7]. The structural and luminescence properties of chalcogenide semiconductor CaxBa1−xGa2S4 solid solutions (x=0.1-0.9) doped with 7 at% of Eu 2+ ions were studied at room temperature. The crystal structure of CaxBa1−xGa2S4 solid solutions varies with the amount of Ca 2+ and phase transition from cubic to orthorhombic takes place of x value. CaxBa1−xGa2S4:Eu 2+ solid solutions exhibit intense photoluminescence in cyan to yellow spectral region depending on x due to 5d→4f electron-dipole transitions in Eu 2+ ions [8][9]. The crystal structure of the layered semiconductor TlGaSe2 is studied using neutron diffraction at room temperature and under high pressures in to 4.6 GPa. Under ambient conditions the crystal structure of TlGaSe2 is described by monoclinic symmetry with the space group C2/c. In the pressure range P = 0.2-0.9 GPa TlGaSe2 undergoes a structural phase transition without a change in symmetry. The pressure dependences of the lattice parameters and the unit-cell volume are obtained, and the bulk moduli for both phases of TlGaSe2 are calculated [10][11]. Photoluminescence (PL) emission and excitation spectra of ZnGa2S4 are recorded. The excitation maxima for these bands are respectively to photon energies 3.3eV for ZnGa2S4. It was determined that the ZnGa2S4 compound in three narrow emission lines in the visible spectral region at 430 nm,530 nm and 675nm due in the donor acceptor recombination [12].

Experimental Procedure
The effect of -radiation with an energy of E=1.33 MeV and a dose of D = 300, 1000 Gy on the photoluminescence properties of solid solutions GaS1-xSex<Er>0.1аt% was investigated. The investigated solid solutions with a resistance of 10 9 ohm at room temperature were grown by the Bridgman method. Erbium doping was carried out during the growing process. The samples were irradiated using a Co 60 installation at room temperature. The photoluminescence spectra of the studied samples were recorded on an SDL-1 spectrometer. The spectrometer consists of a double monochromator with replaceable diffraction gratings, an illuminator with a DRSh-type lamp, a capacitor, energy receivers and an amplifier-recording device. A high-pressure mercury lamp DRSH-250-3 and DRSH-500m is used to excite luminescence. The sample is placed in a holder and illuminated by a powerful monochromatic flux, which is isolated using a light filter (λ=337.1 nm) from the spectrum of a mercury lamp. The energy receiver in the spectrometer was an FEU-39A and FEU-62 photomultiplier tubes.

Results and Discussion
The photoluminescence spectra of solid solutions GaS1-xSex undoped and doped with the rare-earth element erbium at 77K in the wavelength range from λ= 420nm to 620nm were studied (Fig. 1). At nitrogen temperature in solid solutions, the maximum of the intense band is at λ= 540nm. In addition to the main intense emission band, there is also a band with a maximum at about λ= 495 nm. The maximum observed in the luminescence spectra of solid solutions is probably due to the transition of electrons from the conduction band to acceptor levels, which are Et=0.22 and 0.43 eV above the top of the valence band. In erbium-activated solid solutions GaS1-xSex, the photoluminescence intensity decreases and narrower lines appear in the spectrum, which are related to intracenter transitions of introduced rare-earth impurities (Fig. 2.) After irradiation with -quanta with a dose of 300Gy (Fig. 3.), new high-intensity radiation peaks appear in the λ=550-560nm region in solid solutions GaS1-xSex<Er>0.1ат%.   Irradiation with a dose of D=1000 Gy (Fig. 4) leads to an increase in the line intensity in the range of λ=550-560 nm. The increase in the green luminescence intensity can be explained by the decay of bound Frenkel pairs in the sulfur sublattice, which, together with the separated pairs, are formed upon gamma irradiation of the samples. The appearance of new lines and a change in the relative line intensities known for the studied samples are interpreted as a consequence of the migration of defects arising from the displacement of sulfur atoms and their binding to rare-earth ions. At T=77K, free SI and VS arise in the sulfur sublattice due to the decay of bound Frenkel pairs. Si defects are responsible for increasing the intensity of the green luminescence band. The redistribution of the photoluminescence intensity in the range of λ= 520 -600 nm is due to the transfer of energy to the rare-earth centers in the activated crystals. The observed number of bands in the spectrum and their narrowness give reason to believe that Er +3 ions occupy mainly one position in the studied samples, forming the main erbium center. Along with the main Er +3 -center, complexes consisting of Er +3 ions, intrinsic crystal defects, or uncontrolled impurities can form in the studied samples. This is evidenced by the results of a study of photoluminescence, which allowed several erbium centers. The existence of several types of erbium centers in these crystals is associated with the substitution of impurity ions for various regular positions in the crystal lattice and various mechanisms for compensating the excess charge.

Conclusions
The performed investigations allow us to conclude that doping with erbium leads in the appearance of a series of emission lines in the visible region of the spectrum. The detected luminescence bands of solid solutions GaS1-xSex<Er>0.1аt% are the result of intra center transitions in the Er +3 ion. After irradiation with -quanta, free Si and Vs appear in the sulfur sub lattice due to the decay of bound Frenkel pairs. Si defects are responsible for an increase in the intensity of the green luminescence band. The observed number of bands in the spectrum and their narrowness give grounds to believe that Er +3 ions occupy predominantly one position in GaS1-xSex, forming an erbium center.

Conflict of Interest
No conflict of interest.