Luminescence from self-trapped excitons in BaFClBaFBr solid solutions

Journal of Electron Spectroscopy and Related Phenomena 79 (1996) 159-162

Luminescence from Self-Trapped Excitons in BaFCI-BaFBr Solid Solutions A. 0hnishi*, K. Kan'no, Y. Iwabuchi A and N. Mori A

Department of Physics, Kyoto University, Kyoto 606, Japan A Fuji Photo Film Co., Ltd., Miyanodai, Kaisei-machi, Kanagawa 258, Japan Luminescence from BaFC1, BaFBr and their mixtures BaFCh.xBrx has been investigated under VUV excitation with synchrotron radiation at temperatures 11-200 K. In BaFC1, two emission bands are found to appear at 5.36 eV and 3.37 eV, which presumably originate from serftrapped excitons (STE) situating in the on- and off-center configurations with the Clz core. In mixed crystals, two characteristic bands newly appear in the UV range (e.g., at 5.43 eV and 4.46 eV in the x =0.1 crystal), indicating preferential serf-trapping at Br- ion sites. With the increase in x, these bands continuously shift to the lower energy, and finally change into the 5.06 eV and 4.25 eV bands of BaFBr, which arise from STEs with the Br2- core situating in the on-center configuration and m, presumably, a weakly off-centered configuration, respectively.

1. INTRODUCTION

2. EXPERIMENTAL

BaFX (X=C1, Br, I) crystals are layered ionic crystals of tetragonal matlochkite (PbFC1) type [1]. In these materials, the possibility for serf-trapping of excitons in a variety of different relaxed configurations has been discussed [2-5]. As for BaFBr, we previously reported that the serf-trapped exciton (STE) of on-center type, which consists of the Brz out-of-plane Vk center (i.e., serf-trapped hole) and the bound electron, is responsible for the intrinsic luminescence band at 5.0 eV [2]. Another STE luminescence band is known to appear at 4.1 eV above 60K, with quenching of the 5.0 eV band [2,4]. We supposed that this band may come from the STE perturbed by nearby defect pairs of an impurity 02. ion and a charge-compensating (~ center [3]. However, a possibility of the off-center relaxation has been suggested by theoretical calculation [3]. In the present study, luminescence spectra from BaFCI, BaFCl:Br and BaFC1-BaFBr mixed crystals have been investigated, to see the effect of changing in the crystal environment on the m a n n e r of exciton serf-trapping.

Experiments were performed under VUV excitation using synchrotron radiation (SR) from the beam line BL-1B of UVSOR (Institute for Molecular Science, Okazaki). BaFC1, BaFBr and their mixtures were crystalhzed by a horizontal Bridgmann method from the melt containing stoichiometric amounts of BaFC1 and BaFBr. The cleaved crystals were mounted on the copper holder of a He-flow type cryostat and cooled down at l lK by thermal conduction. The SR was monochromatized with a 1 m Seya-Namioka monochromator. Luminescence was analyzed by a grating monochromator (Spex 270M) attached with a R955 photomultiplier, and corrected for the dispersion and sensitivity of the detecting system, and for the spectral distribution of the excitation hght.

"Present

3. RESULTS AND DISCUSSION Two characteristic emission bands have been generally recognized in BaFC1-BaFBr mixed crystals, under excitation with VUV light at 10.3 eV. Main results of their emis-

address: Department of Physics, Yamagata University, Yamagata 990, Japan

0368-2048/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PHS0368- 2048 (96) 02826-5

160 BaFClt.xBrx excited at 10.3eV

BaFClt.xBrx

(a) X=0 aaFCI

(a) X--0 Ba~:CI

~""

:/%, 71K

,-,,

(b) X=0.1

°

A

•

~

,o"" "*,

I021'

(d) X=0.6

O

~

';...... :

......

"

~"Jz IJ.

.

.

.

.

.

.

.

UVz

.

"

'.

.-. ............

0

O0

uJ ~

AA

~

" I ;"--'~.

"-" (el X=0.3

I

,

.----=r- :i~

"~~"'-...

102K,.,.,"

.

tO

LU (n

:

....

u~~-IZIlld)X=0.6........'~" ~ '

¢

'

.....

o,"

_ -

if) Z iii

-

100K.,

I'-

Z

_

. ~

'

"

"

!"

~,.....j

....

.~

:

(f) X=l BaFBr

2

3 4 5 6 PHOTON ENERGY (eV)

Fig.1. Emission spectra of BaFCll.x Brx mixed crystals under excitation at 10.3 eV. Solid curves: measured at llK. Chain curves: measured at high temperatures.

sion spectra, excitation spectra and temperature dependence of peak intensities are shown in Figs. 1-3, respectively. For comparison, we also measured the luminescence of BaFCI:Br, which is attributed to the radiative decay of localized excitons associated with Br- impurities. These results are summarized in Fig. 4. As seen in Fig. l(a), in BaFC1 (x=0), two emission bands are observed at 5.36 eV and 3.37 eV at 11 K when excited with 10.3 eV h g h t in the intrinsic absorption range. The 5.36 eV band is almost quenched at 71 K. Existence of the 3.37 eV band and its enhance-

....

. .....

:

~'i:!:!:i

ll) X=l BaFBr

I

6

7 8 9 10 PHOTON ENERGY (eV)

Fig.2. Excitation spectra for the UV1 (solid curves) and UV2 bands (dot curves) in BaFCll.xBrx mixed crystals.

m e n t upon raising temperature are in good agreement with the results reported by Radzhabov and Egranov [4], though they did not observe the 5.36 eV band. In the x=0.1 crystal, two new bands appear at 5.43 eV and 4.46 eV, in addition to a trace of the 3.37 eV band which completely disappears in the crystals of x>=0.3. They are very similar to the emission bands from relaxed excited states of localized excitons associated with Br- ions, which can be stimulated by photo-excitation into the absorption bands being located around 7.5 - 8.5 eV in the BaFCl:Br crystal (Fig. 4(a)). On the analogy of the mixed alkali halide

161

crystals, KC1-KBr [6], we suppose that the 5.51 eV band in BaFCl!Br origmates from localized excitons composed of the electron bound on the trapped hole of the (C1Br)and]or Br2- core. Thus, in the case of BaFCll-xBrx, it is suggested that major part of free holes created by photo-excitation with 10.3 eV light must be serf-trapped on the Brion sites. In fact, the maxima in the excitation spectra in the mixed crystals are located around 7.8 eV in which electron-hole pairs are selectively created at the Br- ion sites, as suggested most evidently in the Br-doped crystal. The excitation spectra, however, are much

broader without any definite structure than those for the localized exciton luminescence, as seen from comparing Fig.2(b) with Fig. 4(b). With the increase in x, the 5.43 eV band shifts continuously to the low energy side and is finally connected to the intrinsic luminescent band at 5.06 eV in BaFBr, as indicated BaFCI:Br (0.01mo1%) •

=

•

i

!

i

(a) Emission Spectra excited at 7.75eV

/" 11K

/~

,"

BaFCII.xBr x excited at 10.3eV !.. (~

I

(b) X=0.1 '~

I

'

r " . 5"43eV •.

x=,n. 3

i

,

-I

l

I

4.46eV

,

i

,

,

2

3i

UJ O Z

_

UJ Z

_

~ ~ = 7 ~

............ ,

.

.

'

"

5.19eV

.,,".... ~

5.51eV

O~

I

LL 6 O

II I

I

I __.

I

~

100

TEMPERATURE

150

200

(K)

Fig.3. Temperature dependence of peak intensities of emission bands in BaFCll.xBrx mixed crystals.

I .....

8 9 ENERGY

="

I

10 (eV) '

excited at 7.75eV

~ 5.51eV

50

4.46eV

I;o

TEMPERATURE

~ . 1 0 e V

50

I

7 PHOTON

(c) Temperature Del)endence Z UJ I-Z

o

(f) X=i BaFBr

0

6,

•

S.34eV

(d) X=0.6

5i

J I I

t = "s~*

fJ W O

4

(b) Excitation Spectra

' (K)

Fig. 4. Luminescence from localize(] excitons in BaFCI:Br (0.01tool%). (a): Emission spectra stimulated with 7.75 eV light at l l K (solid curve) and 70 K (dotted curve), (b): Excitation spectra for the 5.51 eV emission at l l K (solid curve) and the 4.46 eV emission at 70K (dotted curve), (c): Temperature dependence of emission intensities at 5.51 eV (closed circles) and 4.46 eV (open circles) stimulated with 7.75 eV light.

162

by a line. Hereafter, we call the series of these bands as "UV1 band". Contrary to the UV1 band, the 4.46 eV band is weakened to disappear for x>0.3, as far as temperature is low enough. This band, however, seems to be connected to the 4.25 eV band of BaFBr (dotted curve in (f)) which is prominent at high temperatures, as indicated by another line in the figure. Hereafter we call these bands as "UV2 band". Anti-correlation in the temperature dependence of emission intensities between the UV1 and UV2 bands is remarkable, as clearly seen in Fig. 3. In addition, a weak emission band, probably due to 0 z- impurities, is sometimes observed around 2-3 eV. The emission bands mentioned above are stimulated well in the VUV range above 7 eV (Fig.2). In BaFC1 and BaFBr, two dips are clearly observed in the excitation spectra, as indicated by arrows in the figure. Their positions (8.6 eV and 8.8 eV in the chloride, 7.6 eV and 8.1 eV in the bromide) correspond to the absorption band peaks due to the n=l exciton; i.e., so-called halogen doublet. Hence, a threshold of the excitation spectrum for the intrinsic luminescence could be expected around 8.6 eV in BaFC1 and 7.6 eV in BaFBr. Appreciable humps, however, were observed in the region of the low energy tail of the fundamental absorption, as depicted by shadows. The reason is not clear at present, though they are supposedly related to stimulating some kind of localized exciton. As for the mixed crystals, excitation spectra for the UVI bands (solid curves) and those for the UV~. bands (dotted curves) are almost similar to each other, except for the low energy part around 8 eV in the x=0.1 and x=0.3 crystals. It is suggested that localized excitons will change into band excitons around x=0.1-0.3. For x=0.6, new dips appear at 7.75 eV and 8.24 eV, being connected to the halogen doublet of BaFBr with increasing x. The present results suggest that the UV1 and UV2 bands should be attributed to two different STEs associated with the Brz cores. It should be noted that the spectral changes of emission and excitation in BaFCh.xBrx are somewhat similar to those in KCh-xBrx where

three different types (I, II and III) of STE luminescence appear against x [6,7]. Such variations in the spectra may reflect the change in the manner of exciton self-tapping. A configuration of on-center type, like the type I STE in alkali halides, is reasonably imagmed to be responsible for the UVI bands of the mixtures, the 5.05 eV band of BaFBr, and also for the 5.36 eV band of BaFC1. On the other hand, a configuration situating in a manner similar to that of the type II STE, i.e., a weakly off-centered configuration, will be responsible for the UV~ band and for the 4.25 eVbandofBaFBr. As for the 3.37 e V b a n d o f BaFCI, a strongly off-centered configuration, like the type III STE in alkali halides, should be assumed to explain the origin of its remarkably large Stokes-shift, though there remains some ambiguity for its intrinsic nature. Anyway, ODMR study will be necessary for determinmg structural models for the luminescent states. ACKNOWLEDGMENT

This work was supported by the Joint Studies Program of the Institute for Molecular Science, UVSOR Facility. REFERENCES

1. B. W. Liebich and D. Nicollm ; Acta. Cryst. B33 (1977), 2790. 2. A. Ohnishi, K. Kan'no, Y. Iwabuchi and N. Mori ; Nucl. Instr. and Meth. in Phys. Res. B91 (1994), 210. 3. R. C. Baetzold ; Phys. Rev. B40 (1989), 3246. 4. E. A. Radzhabov and A. V. Eranov ; J. Phys : Condens. Matter 6 (1994), 5639. 5. H. H. Ruter, H. v. Seggern. R. Reininger and V. Saile; Phys. Rev. Lett. 65 (1990), 2438. 6. K. Tanaka, K. Kan'no and Y. Nakai; J. Phys. Soc. Jpn. 59 (1990), 1474. 7. K. Kan'no, K. Tanaka and T. Hayashi; Rev. Solid State Science 4 (1990), 383.