ABSTRACT

The photoluminescence (PL) in temperature interval 77 - 300 K is investigated in Eu Ga₂Se₄:Nd polycrystals. It is established that broad band PL with maximum at 561nm is caused by intracentral transitions 4f⁶5d - 4f⁷(⁸S_7/2) of Eu²⁺ ions. The intracentral emission of Nd³⁺, corresponding to both transitions from ⁴F_3/2 level and higher situated levels, is observed at interband excitation. The essential intensity of transitions from ²H_11/2 and ⁴F_9/2 levels is the interested peculiarity of luminescence spectra of Nd³⁺ in these crystals.

1. Introduction

The series of MGa₂S₄(Se₄), M₂Ga₂S₄(Se₄), M₃Ga₂S₆(Se₆), M₄Ga₂S₇(Se₇), M₅Ga₂S₈(Se₈) compounds is obtained in М-Ga-S(Se) system (where М is Ba, Sr, Ca, Eu, Yb) [1- 4]. The compounds synthesized in M-Ga-S(Se) system can be combined in one group with general formula II_n-III₂-VI_m (where n = 1, 2, 3, 4, 5; m = n + 3; II are bivalent cations of Eu, Yb, Sm, Ca, Ba, Sr; III are trivalent cations of Ga, Al, In; VI are S and Se chalcogens). Many II_n-III₂-VI_m compounds [5] have large number of stoichiometric voids: the one-fourth fraction of free places unoccupied by cations in crystal lattice. The stoichiometric voids situated periodically in lattice, don’t have the defect properties.

The compounds in M-Ga-S(Se) system, M cations of which are 4f elements (lanthanides), can be the active medium of semiconductor lasers, luminescent lamps, colored display screens and other information systems [6-8]. These compounds have the forbidden band width is 4.4 eV and effectively transform the energy of electric field, X-ray and ultraviolet emissions, and also electron beams into visible light. The excitation spectrum of these compounds covers the spectral region from near ultraviolet one up to 500 nm.

EuGa₂Se₄ is the one of comparably little-studied compounds in M-Ga-S(Se) system. EuGa₂Se₄ compound crystallizes in pseudo-orthorhombic sublattice at simultaneous existence of twinning and superstructure [9]. The lattice parameters are а = 20.760 Å, b=20.404 Å, c = 12.200 Å [10]. Eu atoms in EuGa₂Se₄ structure are in partial positions 16(e), 8(a) and 8(b) in space group -Fddd having eight Se atoms as nearest neighbors. Crystal structure and some physical properties of this compound are described in [2]. The photoluminescence investigation of EuGa₂Se₄ activated by neodymium ions is of the interest for the photoluminescence mechanism revealing and the determination of trap level energy spectrum and also practical application.

2. Experiment

EuGa₂Se₄ compound is synthesized from binary compounds EuSe and Ga₂Se₃, taken in stoichiometric relations, by solid-phase reaction in graphitized ampoules evacuated up to 10⁻⁴ millimeter of mercury. The synthesis is carried out at 1300 K in one-temperature furnace during 4 hours. The annealing during 24 hours at 1000 K is carried out after the synthesis. Activation by neodymium ions is realized using neodymium fluorides doping during synthesis process. PL is investigated in temperature interval 77 - 300 K. The continuous laser diode InGaN (λ = 405 nm) is the excitation source. The registration of emission spectrum is carried out on Spectral Diffraction Luminescense device. The emission receiver is photoelectric multiplier-39A. The luminescence of the samples activated by neodymium is excited by laser on Rodamine 6G (range 550 - 620 nm). The luminescence registration is carried out by Diffraction Lattice Monochromator with Photoelectric Multiplier and BoxcarIntegtator BC I-280.

3. The Results and Discussions

The PL spectra of EuGa₂Se₄ crystals at different temperatures are shown on figure 1. The one intensive broad emission band covering the wavelength region 500 - 620 nm with maximum at 561 nm and two relatively narrow bands at 709 and 745 nm are observed in PL spectra. It is known that the broad emission band is usually observed in crystals containing Eu²⁺. The energy position of luminescence band, caused by Eu²⁺, changes in the dependence on crystal structure and percentage of Eu²⁺ [11]. The observable PL in region 700 - 900 nm (maxima at wave lengths 709 nm and 745 nm) is caused by electron-hall recombination. The excitation and emission spectra of EuGa₂Se₄ crystal at temperature 300 K are shown on figure 2. As it is seen from the figure, PL excitation spectrum is significantly broad band one and consists of overlapping bands. One of these bands at 374 nm is very clear one, but others at 404, 415, 454, 471 nm aren’t allowed bands at temperature 300 K. The dependence of PL band intensity (λ_max = 561 nm) on temperature is shown on figure 3. As it is seen from the figure the intensity in temperature region 92 - 120 K very weakly depends on temperature, in region from 120 up to 170 K slowly decreases and further temperature growth (170 - 300 K) leads to strong decrease of emission intensity, i.e. the strong temperature quenching of PL takes place. The activation energy of temperature quenching of PL is defined by high-temperature inclination (170 - 300 K). It is 0.09 eV. The temperature dependence of band half-width of PL with the maximum at 561 nm in Г(Т) and T^1/2 coordinates is shown on figure 4. It is seen that this dependence in temperature region 77 - 300 K is linear one and can be described by the use of configuration coordinate model and Boltzmann distribution. The fol-

lowing expressions, connecting Stokes shift DS, HuanRice factor s and phonon energy with the halfwidth temperature dependence Г(Т), are obtained on the base of theoretical analysis of absorption and emission spectra in [12,13]:

(1)

(2)

If then the expression under quadratic root in (2) can be expanded into series and limited by the first member:

, where (3)

Then expression (2) can be rewritten in the following form:

(4)

or

(5)

The equation (5) shows that half-width Г linearly depends on. The values of Huan-Rice factor s and Stokes shift ∆S, which are equal to 8 ± 2 and 0.33 eV correspondingly, are found by experimental results. At calculation the phonon energy in ЕuGa₂Se₄ polycrystal is considered as equal to 23 meV according to data in [14]. The red shift D is obtained using formula D = E_ex − E_em [12,13] (where E_ex is excitation state energy and E_em is emission energy of Еu²⁺ ion (4.19 eV and 2.21 eV correspondingly) D = 1.98 eV.

The constancy of energy position of broad band emission maximum at 561 nm (figure 1) with temperature variation and temperature dependence of band half-width evidence about belonging of this emission band to Eu²⁺ ions, i.e. to intracentral transitions 4f⁶5d - 4f⁷(⁸S_7/2) of Eu²⁺ ions.

The excitation spectra registered in the region of matrix self-absorption edge (figure 5) evidence about fact, that the excitation on transition band-band isn’t less effective, than one on intracentral transition ⁴I_9/2 - ⁴G_5/2. Note that the beginning of monotonously growing excitation spectrum part lies below than the region of matrix self-absorption edge, i.e. the transfer of excitation energy from small donor levels to Nd³⁺ ion ones is observed. The curve 2 is obtained under the conditions when the luminescence intensity linearly depends on pumping intensity; the curve 3 corresponds to nonlinear dependence of luminescence on pumping intensity (more detail about nonlinear dependence of luminescence intensity on pumping one can see below).

It is interesting to note that the pumping at room temperature into matrix absorption band is more effective one for excitation of high-situated levels of Nd³⁺, at 77 K the pumping is effective one for ⁴F_3/2 level. As a whole, the efficiency for excitation by means of matrix at 77 K is less than at room temperature.

Note that dependence of buildup effect of Nd³⁺ luminescence from ⁴F_3/2 level on excitation wavelength. The time passes of luminescence for EuGa₂Se₄:Nd³⁺ at 77 K are given in the figure 6. At 77 K the excitation on ⁴G_5/2/λ_exc = 598 nm level (curve 2) gives the clearly expressed buildup effect during ~15 µs. The transition to interband excitation, increase of pumping frequency hν_exc > E_g, λ_exc = 545 nm (curve 1) leads to disappearance of this effect. At room temperature the buildup isn’t observed in both cases. The luminescence decay times for both cases of excitation are similar ones (~50 µs). At 77 K the luminescence decay times for EuGa₂Se₄ at small neodymium concentrations on transitions from ⁴F_3/2 and ⁴F_5/2 levels are equal to 50 µs and 5 µs correspondingly. Here we need to say several words about possible excitation mechanism Nd³⁺ in EuGa₂Se₄. In EuGa₂Se₄ at 293 K and interband excitation Eu²⁺ excites and it transfers the excitation on G-levels of neodymium. At temperature decrease Eu²⁺ band narrows, the overlapping with Glevels of Nd³⁺ disappears and that’s why the levels causing the broad impurity emission bands which transfer the energy on ⁴F_3/2 and ⁴F_5/2 levels, mainly take part in energy transfer on Nd³⁺ at 77 K. As a result at 77 K ⁴F_3/2 level excites more effectively than G-levels as it is mentioned above. Moreover, the emission from ⁴F_3/2 takes place without buildup because of the energy is transferred on it directly from impurity levels. As a whole at

77 K the efficiency of energy transfer to Nd³⁺ ion by means of the interband transition goes down. The luminescence investigation results at transitions from ⁴F_3/2 level on ⁴I_9/2, ⁴I_11/2, ⁴I_13/2 evidence about the fact that in considered crystals not one type of luminescent centers exists as Stark component number exceeds the maximum possible one. This is also confirmed by the dependence of emission spectra on excitation wave length, on delay value of registration time moment and on sample history.

The effect of intensity quick saturation with pumping growth is the interest peculiarity of neodymium luminescence at interband excitation in the investigated crystals [15]. The dependence of PL intensity on pumping power is given on figure 7. In this case the excitation wave length is 552 nm and emission registration wave length is λ_reg = 780 nm. It is seen that emission intensity nonlinearly depends on pumping power. The saturation is observed at room temperature and at liquid nitrogen temperature. This effect can be connected with pumping energy absorption by free carriers, growth of pumping reflection at increase of free carrier number, saturation of intermediate agents by means of witch the excitation is transferred from matrix to Nd³⁺ and etc. Note that luminescence intensity linearly depends on pumping power at excitation through absorption bands of Nd³⁺.

Thus, in EuGa₂Se₄ crystals the broad band intensive luminescence is connected with intracentral transitions 4f⁶5d - 4f⁷(⁸S_7/2) of Eu²⁺ ions. The carried investigations of triple compounds with neodymium show that nearest surrounding of Nd³⁺ has the near tetragonal symmetry in investigated crystals; the luminescence of Nd³⁺ effectively excites because of interband transitions; the intensive emission of Nd³⁺ from levels situated higher ⁴F_3/2 is observed, therefore EuGa₂Se₄:Nd³⁺ can be used as nar-

row-band luminophors in 580 - 1090 nm range.

4. Acknowledgements

This work was supported by the Science Development Foundation under the President of the Republic of Azerbaijan, Grant № EİF-2011-1(3)-82/01/1.

REFERENCES