Transient absorption and luminescence of LiNbO3 ...

Integrated Ferroelectrics

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Transient absorption and luminescence of LiNbO3 and KNbO3 L. Grigorjeva , V. Pankratov , D. Millers , G. Corradi & K. Polgar To cite this article: L. Grigorjeva , V. Pankratov , D. Millers , G. Corradi & K. Polgar (2001) Transient absorption and luminescence of LiNbO3 and KNbO3 , Integrated Ferroelectrics, 35:1-4, 137-149, DOI: 10.1080/10584580108016895 To link to this article: http://dx.doi.org/10.1080/10584580108016895

Published online: 19 Aug 2006.

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I,irc,qroi?dFe’rrmeln r r n . 2001. VoI 35, pp 137-149

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Transient Absorption and Luminescence of LiNbO, and KNb03 L. GRIGORJEVA~,v. PANKRATOV”, D. MILLERS~,G. CORRADI~ and K. POLGARb “Institute of Solid State Physics, University of Latvia, 8 Kengaraga stK, LV-1063, Riga, Latvia and bInstitute of Solid State Physics and Optics, Hungarian Academy of Science, PO Box 132, H-1502 Budapest, Hungary (Received March 15, 2000; In find form Augusr 15, 2000)

The results of time-resolved optical absorption spectra in congruent and stoichiometric LiNbO, as well as KNbO, crystals are reported. The role of different polaron types in transient absorption and luminescence spectra and decay kinetics are discussed. Keywords: LiNb03; KNb03; congruent; stoichiometric; transient absorption; decay kinetics

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L. GRIGOFUEVA et al.

138/[1868]

INTRODUCTION

LiNb03 and KNb03are ferroelectric crystals which have large nonlinear optical coefficients and are the promising materids for frequency


doubliig and photorefractivedevices. The main properties of these materials are influenced by crystal structure defects (point defects, stoichiometry, impurity). On the other hand, the intrinsic electronic excitations can form trapped charge states (self-trapped or bound polarons) and these states affects the crystal properties as well. The intrinsic polarons are created from electrons and holes generated duting excitation processes and has been observed by

ESR [I] and transient optical absorption [2,3]. The transient absorption is known in many perovskite type crystals. The photo-induced absorption in KNb03 has been termed bluelight-induced infrared absorption (BLIIRA) [2]. This effect suppresses the second harmonic generation efficiency. The spectra and absorption relaxation kinetics have been

measured for different

crystals [3-51.The wide absorption band

ev) consisting of three bad resolved bands (1.0 eV, 1.6-2.3eV, 2.7-3.0

was observed. The growth of transient absorption spectra is measured in picosecond time scale IS] and the 0.62 ps time for defect formation in KNbO3 was obtained. This fact confirms that induced infrared absorption

is due to primary centers. According to theoretical study of defects in KNb03 [4]the bound hole polaron and bipolaron model was proposed

as defects due to infrared transient absorption. The recent experimental results [6] gave an evidence of a self-trapped electron on a Nb site.

TRANSIENT ABSORFTION AND LUMINESCENCE

The polaron effects were studied in detail in LI%

I s691/139

crystals by

0. F. S c h h e r et al [l]. The main polaron models are: a) 0 - center (trapped hole at regular site or at Nb vacancy neighboring). The wide optical absorption band is peaking at -2.5 eV in congruent and at -2.3


eV in stoichiometric LiNbO,; b) electrons trapped at antisite Nb;

corresponding optical absorption band at 1.6 eV; c) electron bipolarons electron pairs trapped at two Nb (one at regular site and another at antisite Nb) and the correspondingoptical band is peakingnearly 2.5 eV. The concentration of anti-site niobium is different in congruent ( L i i 4 . 9 4 ) and stoichiometric (L.dNb =l). It is reasonable to propose that

the 1.6 eV band Contribution in transient absorption spectra should be more stronger in the congruent sample. At last, it is known that in L i M F'- and F- center absorption bands are at 3.2 eV and 2.48 eV, respectively [7]. So it seems, that exist different models for centers, giving transient absorption in these materials and corresponding hypothesis are under discussion.

In this paper we present the results of time-resolved optical absorption spectra in congruent and stoichiometric L%

and in

KNb03 crystals. The common phenomena in both materials are the electron and pole polarons creation under pulse electron beam excitation. The details of this process are studied.

EXPERIMENTAL The measurements have been made on the experimental equipment shown in Fig. la. The pulse electron accelerator has been used as an

L. GRIGORJEVA et al.


140/[18701

FIGURE la. The experimental set-up: 1-device for the

synchronization ;2 - Xe-flash lamp; 3 - electron beam accelerator; 4 - monochromator; 6 - photomultiplyer; 7 storage oscilloscope;

- cryogen camera; 5

FIGURE. Ib.The probing light configuration

TRANSIENT ABSORPTION AND LUMINESCENCE

[ I87 11/14 1

irradiation source. The electron pulse duration is 10 nq electron energy

- 270 keV, average density of electron beam was 10'' el cm-'. A Xe flash lamp served as a probe light source. The probe light is directed non-perpendicularly to the crystal face. Passing trough the sample the


probe light undergoes total retlection on the lateral surfirce uradiated by the electron beam and is subsequently collected on the entrance slit of the monochromator (Fig. lb). The depth of penetration of electrons in the crystal is

- 0.2 mm, therefore effects in the crystal volume are

studied.The energy of the electron beam was below 300 keV. In [8] the energy required for removing an anion from its regular lattice site is

estimated as

- 350 keV, so in our experiments the energy of electron

beam was below that the threshold energy in these materiais. Therefore as primary radiation effects only electronic excitation and the ionization of existing defects had to be taken into account. For the study of

luminescence under electron beam irradiation the Xe tlash lamp has to be switched off. The present experimental equipment allows making measurements under following conditions: i) the spectral m g e from 1

eV to 5 eV; u) the temperature range from 80 K to 400 K; iii) t i e resolution 20 ns. L%

crystals, a congruent one with a Li/Nb ratio

R4.944,a stoichiometric one with R=1.00, as well as a crystal with intermediate compoSition R= 0.995 were used in this study. The crystals have been grown by the Czochralski or the top seeded solution growth methods [9]. The stoichiometric and the intermediate crystals grown from a flux containing potassium, resulting concentration of the of order 100 ppm of the potassium concentrated m a d y in microconclutions [9].

The K N b O 3 sample is stoichiometric, undoped, was not poled and is colorless.

142/[ 1872)


RESULTS AND DISCUSSION. The irradiation of LiNb03 crystals by pulsed electron beam causes an appearance of transient absorption as well as luminescence.The transient


absorption spectra for stoichiometric LiNbO3 at room temperature are shown in Fig. 2. The transient absorption spectra are taken in the wide region - 6om 1 eV up to 4 eV and consist fiom a number of overlapping bands. The shape of the spectrum depends on crystal composition - the band peaking at 2.3-2.5 eV was better observed in congruent L N O , if compared with stoichiometric one. It is evident that different species are responsible for transient absorption observed. The decay kinetics of transient absorption (Fig. 3) is different at different photon energy. At 1.4 eV the fast initial decay of the transient absorption was followed by a

slow one. During this fast initial decay at 1.4 eV up to 65 % of transient absorption disappears. Whereas at 2.8 eV the fast initial decay of transient absorption was not observed. However, the slow decay for transient absorption at 1.4 eV and 2.8 eV is very close to each other.

This slow decay is close to the exponentialwith time constant 1.1 ps. The transient absorption spectra measured at the end of irradiation pulse shows significant optical density at 1.O eV. We can not measure the correct decay kinetics at 1.O eV because i) the decay is very fast and time resolution of equipment is not suflicient; ii) the 1.0 eV is the spectral limit of the equipment used and thus the signal measured is poor. However, fiom our experiments follows the strong fast hying transient absorption band is peaking at

- 1. 0 eV.

The transient absorption spectra memured at 93 K (Fig. 4) shows that the transient absorption band at 1.O eV is dominant, the optical


[ 1873]/143


0

0.1

-

x.&--

.Ah

0.0 1.0

I

1.5

.

1

2.0

.

1

2.5

.

1

3.0

.

l

3.5

.

0

E. eV

FIGURE 2. The transient absorption spectrum of L I W (Li/Nb=l) ~ at room temperature: 1 - under pulse; 2 - SO ns delay; 120 ns delay.

density at 1.0 eV exceeds well 1.2. Thus, the absorption centers concentration is high, roughly estimated > 10'' cme3.Since these centers are formed due to charge carriers trapping, the number of traps must be very large. It is suggested that these traps are formed by electron phonon interaction at the regular lattice sites. In this case the absorption band at 1.0 eV must be responsible for self-trapped or very heavy polarons (intrinsic polarons ). If the transient absorption observed in L i W 3

is compared with that in PbWOI [lo], it was found that in

L. GRIGORJEVA et 01.

144418741

WW04 the strong transient absorption band at

- 1.0 eV was due to a

self trapped electron (electron trapped at W in the regulat lattice site).


This band is very similar to that observed in L.iNbO3 in the present study. We suggest that the selftrapped electron polaron is responsible for a W o n of transient absorption in the region at 1.0 eV in LiNbQ3

crystals. Some details of this suggestion is discussed in a f o n d study [61.

0

loo

200

300

400

500

600

roo

wo

1. ns

3. The decay kinetics of transient absorption at room

tempemure: L

m (Li/Nb=l) at 1.4 eV (1);

eV (2);KNbCh at 1.2 eV (3).

L W 3

&i/Nb=I) at 2.8

[ 187511145


1.2

1.o

0.8

r u) C Q


U

38

0.6

0.4

o.z

t

0 . .

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

E, r V

FIGURE 4. The transient absorption spectrum of L 93 K: 30 ns delay (1) and 120 ns delay (2).

W & i l ) at

It can be noted that in the congruent LiNbO3 crystal we observe only a trace of transient absorption at 1.0 eV, however a strong

absorption within the region 1.6 eV - 3.0eV. Thus, we conclude the fast retrapping of polarons occurs in the crystal containing imperfections.

This retrapping of primary intrinsic polarons is responsible for the very f&st decay of transient absorption at 1.0 eV and due to low time

resolution in our experiments - only the weak transient absorption at I .O eV was observed.Trapping of intrinsic polarons at lattice imperfections

leads to the formation of secondary defects responsible for the transient


146418761

absorption above -1.5 eV. On the other hand the contribution of hole polarons in the transient absorption is anticipated. However, fiom the present study we can not recognize location of this absorption band. The transient absorption ofKNI>o3 was measured only at RT.


The spectra matches well with those discussed in a former study [3-41. The strong absorption band at -1.0 eV was observed for all KNbO3 crystals studied. This band is very similar to that observed in LiNb03 and seems to be due to the same kinds of polarons. The decay kinetics of

transient absorption in -03

are close to those in Lslbo, (see Fig. 2).

However, the hction of fast decaying absorption at 1.4 eV for KNb03 is larger (Fig. 2) and possibly slightly slower than for L a o 3 . The slow decay stage for both materials is close to the exponential, with nearly the same time constant. Thus the relaxation processes in both materials are very close. We suggest, that main processes of polarons self-trapping, retrapping and recombination occur within niobium-oxygen complexes and are only slightly affected fiom alkali ions. The l u m i n m c e in LiNb03 was studied under electron pulse excitation. The luminescence spectrum covers the wide spectral region fiom 1.5 eV up to 4.0 eV and contains overhpping bands, possibly the same described in [ 1 I]. Decay of luminescence is so fast, that we were not able to meawe the kinetics. The wide spectrum gives an evidence, that for lumin-ce

one well defined excited state is not responsible. It

seemsthat some distribution of excited states on energy take place. The intensity of luminescence depends on temperature and below -250 K the intensity decreased. The decrease of luminescence intensity

at low temperatures indicates that excited states are created via

TRANSIENT ABSORVTION AND LUMINESCENCE

[I 877]/147

recombination and the recombination competes with electron and/or hole capturing on traps. Since the transient absorption complete decay is in microseconds and luminescence decay does not exceed 20 ns, it is concluded that significant number of generated electrons and holes forms absorption


centers and do not participate in luminescence. The only exception may

be the absorption at 1.O eV. It seems that a fraction of this band can be responsible for luminescence centers creation. On the other hand in the luminescence centers creation must participate electronsand holes. Thus, the absorption band due to hole polarons have a fast decay component. Since the fast decay is observed only within the region at 1.0 eV the absorption bands due to electron and hole polarons either overlaps or we do not recognize the hole polaron absorption. The

luminmce decay shows that the creation of

luminescence centers is fast as well. This fast creation eliminates the long distance migration of relaxed or partially-relaxed electron and hole polarons before luminescence excited state formation. It was concluded, that under electron beam irradiation a fraction of electrons and holes are generated at shon distances, possibly they are geminate pairs (electron and hole generated in a single ionization went).

These pairs recombine

immediately and are responsible for the luminescence, whereas well separated electrons and holes undergoe a complicated relaxation via polaron states and formation of centers responsible for transient absorption observed.


148/[18781

CONCLUSIONS

The &ion

of transient absorption at 1.0 eV in

Lib03

and

KNb@ is due to electron polarons. The part of transient absorption

above 1.5 eV is due to secondary defects. These defects are formed fiom Downloaded by [Oulu University Library] at 06:51 15 December 2015

primary intrinsic polarons trapped at lattice imperfections. The main

relaxation processes in LiNbO3 and KNbO3 occur within niobiumoxygen complexes and are only slightly affected by the alkali ion.

The excited states responsible for luminescence are created via recombination. In the creation of these excited states participate pore spaced electrons and holes, possibly geminate pairs.

Acknowledgments

This work has been supported by Latvian Council for Science (grant 96.0662)and Hungarian Science and Research Foundation (OTKA pant T24092). The authors would like to thank R.T.Williams for stimulating discussions.

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[9] K. Polgar, A. Peter, L. Covacs, G. Corradi and Zs. Szaller. J. Cryst. Growth, 177, 21 1 ( 1997). [lo] L. Grigorjeva, D. Millers, S. Chernov, M. Nikl, Y.Usuki, V. Pankratov. Nucl Instr. and Methods in Physics Research B, (2000) in press. [ I l l P. H. Bunton, E. D. Thoma, Y. C. Zhang, R. F. Haglung, and R. T. Williams. Materials Science Forums, 239-241,333-336 (1997).