Photoluminescence of EuGa 2 Se 4 : Nd 3 + А rif М

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 4f5d 4f(S7/2) of Eu ions. The intracentral emission of Nd, corresponding to both transitions from F3/2 level and higher situated levels, is observed at interband excitation. The essential intensity of transitions from H11/2 and F9/2 levels is the interested peculiarity of luminescence spectra of Nd in these crystals.

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][7][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.
EuGa 2 Se 4 is the one of comparably little-studied compounds in M-Ga-S(Se) system.EuGa 2 Se 4 compound crystallizes in pseudo-orthorhombic sublattice at simultaneous existence of twinning and superstructure [9].The lattice parameters are а = 20.760Å, b=20.404Å, c = 12.200 Å [10].Eu atoms in EuGa 2 Se 4 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 EuGa 2 Se 4 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.

Experiment
EuGa 2 Se 4 compound is synthesized from binary compounds EuSe and Ga 2 Se 3 , 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 In-GaN (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 Boxcar-Integtator BC I-280.

The Results and Discussions
The PL spectra of EuGa 2 Se 4 crystals at different tem-peratures 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 Eu 2+ .The energy position of luminescence band, caused by Eu 2+ , changes in the dependence on crystal structure and percentage of Eu 2+ [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 EuGa 2 Se 4 crystal at temperature 300 K are shown on     with the half- width temperature dependence Г(Т), are obtained on the base of theoretical analysis of absorption and emission spectra in [12,13]: ( If кТ    then the expression under quadratic root in (2) can be expanded into series and limited by the first member: Then expression (2) can be rewritten in the following form: or The Equation (5) shows that half-width Г linearly depends on T .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 ЕuGa 2 Se 4 poly- crystal 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 Еu 2+ 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 Eu 2+ ions, i.e. to intracentral transitions 4f 6 5d -4f 7 ( 8 S 7/2 ) of Eu 2+ 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 4 I 9/2 -4 G 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 Nd 3+ 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 Nd 3+ , at 77 K the pumping is effective one for 4 F 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 Nd 3+ luminescence from 4 F 3/2 level on excitation wavelength.The time passes of luminescence for EuGa 2 Se 4 :Nd 3+ at 77 K are given in the Figure 6.At 77 K the excitation on 4 G 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 EuGa 2 Se 4 at small neodymium concentrations on transitions from 4 F 3/2 and 4 F 5/2 levels are equal to 50 µs and 5 µs correspondingly.
Here we need to say several words about possible excitation mechanism Nd 3+ in EuGa 2 Se 4 .In EuGa 2 Se 4 at 293 K and interband excitation Eu 2+ excites and it transfers the excitation on G-levels of neodymium.At temperature decrease Eu 2+ band narrows, the overlapping with Glevels of Nd 3+ disappears and that's why the levels causing the broad impurity emission bands which transfer the energy on 4 F 3/2 and 4 F 5/2 levels, mainly take part in energy transfer on Nd 3+ at 77 K.As a result at 77 K 4 F 3/2 level excites more effectively than G-levels as it is mentioned above.Moreover, the emission from 4 F 3/2 takes place without buildup because of the energy is transferred on it directly from impurity levels.As a whole at 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 Nd 3+ and etc.Note that luminescence intensity linearly depends on pumping power at excitation through absorption bands of Nd 3+ .
Thus, in EuGa 2 Se 4 crystals the broad band intensive luminescence is connected with intracentral transitions 4f 6 5d -4f 7 ( 8 S 7/2 ) of Eu 2+ ions.The carried investigations of triple compounds with neodymium show that nearest surrounding of Nd 3+ has the near tetragonal symmetry in investigated crystals; the luminescence of Nd 3+ effectively excites because of interband transitions; the intensive emission of Nd 3+ from levels situated higher 4 F 3/2 is observed, therefore EuGa 2 Se 4 :Nd 3+ can be used as nar- row-band luminophors in 580 -1090 nm range.

Figure 3 .
Figure 3.The temperature dependence of PL intensity of EuGa 2 Se 4 .

Figure 4 .
Figure 4.The dependence of PL band half-width of EuGa 2 Se 4 with maximum at 561 nm on temperature.

Figure 6 .
Figure 6.Time passes of PL from 4 F 3/2 level for EuGa 2 Se 4 <Nd> at 77 K; 1. hν exc > E g , λ exc = 545 nm; 2. λ exc = 598 nm.77K the efficiency of energy transfer to Nd 3+ ion by means of the interband transition goes down.The luminescence investigation results at transitions from4 F 3/2 level on 4 I 9/2 , 4 I 11/2 , 4 I 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 Figure7.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 Nd 3+ and etc.Note that luminescence intensity linearly depends on pumping power at excitation through absorption bands of Nd 3+ .Thus, in EuGa 2 Se 4 crystals the broad band intensive luminescence is connected with intracentral transitions 4f 6 5d -4f 7 ( 8 S 7/2 ) of Eu 2+ ions.The carried investigations of triple compounds with neodymium show that nearest surrounding of Nd 3+ has the near tetragonal symmetry in investigated crystals; the luminescence of Nd 3+ effectively excites because of interband transitions; the intensive emission of Nd 3+ from levels situated higher 4 F 3/2 is observed, therefore EuGa 2 Se 4 :Nd 3+ can be used as nar-