Photostimulated luminescence in BaFBr:Eu2+ evaporated thin films

Journal of Luminescence 40&41 (1988) 819—820 North-Holland, Amsterdam

819

2~EVAPORATED THIN FILMS

PHOTOSTIMULATED LUMINESCENCE IN BaFBr:Eu

*

Hiroshi KOBAYASHI, Shosaku TANAKA, Nobuhiro IWASE

and Hideki YOSHIYAMA

Department of Electronics, Tottori University Koyama, Tottori 680, Japan

Novel BaFBr:Eu2+ phosphor films were fabricated, for the first time, by an electron beam evaporation method by using mixtures of BaF 2 and BaBr2 powder and EuBr3 (or EuF3) salt. They show efficient photostimulated luminescence (PSL). To obtain highly efficient PSL. Ts~bshould be increased above 400°C, and a mole fraction x of (BaF2)~(BaBr2)i ~ source should be chosen in a range from 0.3 to 0.7.

1. INTRODUCTION

polycrystalline (grain size

Photostimurable phosphor materials 2+ are promising materials for like an X—ray

BaFBr:Eu imaging sensor in a new system of computed

using a mixture of (BaF

1. To improve spatial resolution radiography of the X—ray imaging sensor, BaFBr:Eu2+ photo— stimurable luminescent thin films are thought to be better than conventional powder layers.

evaporation source.

However, there have been no reports on

temperature.

BaFBr:Eu2~thin films up to now.

In this

>

1 tim) thin films

with (002) orientation were successfully grown at substrate temperatures Tsub over 400°Cby 2)05(BaBr2)05 as an

In order to evaluate the PSL characteristics 2+

of the BaFBr:Eu

thin films, we measured the

photoluminescence (PL) and PSL spectra at room Figure 1(a) shows observed PL

spectra under N 2 laser

(337.1 nm) excitation.

paper, we will report on the photostimulated 2~

They have ais strong band peaking at 390 nm, which due to emission a (5d)(4f)6-(4f)7 transition

luminescence thin films, (PSL) in evaporated BaFBr:Eu

of Eu

2+.

ions.

Figure 1(b) shows observed PSL

spectra, where the films were first irradiated 2. FABRICATION METHOD

with the N 2 laser and then stimulated by the

BaFBr:Eu

2+

phosphor films were fabricated on

He—Ne laser (632.8 nm).

Increases in both the

a quartz glass substrate by an electron beam

PL and the PSL intensity could be observed by

evaporation method.

increasing Tsub (150—500°C). Probably, this

A mixture of BaF2 and

BaBr2 powder and EuBr3 (or EuF3) salt (0.1—

result indicates that the efficient charge

10 mol% as luminescent centers) was prepared and

transfer comes from an improvement in the

used as the evaporation source.

crystallinity of the films, PSL in 2+ phosphors arises because from the the charge BaFBr:Eu transfer through the conduction band between the Eu?+ luminescent centers and the F+_centers2.

Here BaF2

and x:1—xBaBr2 (x = were 0—1).mixed in several mole ratios

3. EXPERIMENTAL RESULTS AND DISCUSSION By X—ray diffraction measurements and

all of BaF

scanning electron microscope observation, we 2~ have confirmed that the BaFBr:Eu

types BaBr of crystal structures: BaF2 has CaF2—, 2 has PbC12- and BaFBr has PbFC1-type of

It is well known that crystallographically 2,

BaBr2 and BaFBr have different

5Presentaddress: Fujitsu Limited, 10—1 Morinosato—Wakamiya, Atsugi 243—01, Japan ~ 0022—2313/88/$03.50 © Elsevier Science Publishers BY. (North-Holland Physics Publishing Division)

2* evaporated thin films

H. Kobayashi et al. / BaFBr:Eu

820

I BaFBr: Eu2~(1 .OmoI°Io)film

BaFBr:Eu2 (1.0 mot°Io)film

R Tsub(0C)~k/~~~ X 1 N 2-IIn 500 (11 R.T.

~

—~

Ts

________

~ 1

_____________ _____

~C)

C

a,

300

=(“I

_________

X2.5

_____

a-

150 I I

I

I

_____________

300 150 I

2.8I

X 1.3

—

__________________________

300 400 500 Wavelength (nm)

I

I

I

X 18.8 I

300 400 500 Wavelength (nm)

FIGURE l(a) Dependence of PL spectra of the BaFBr:Eu thin films upon T during deposition.

FIGURE 1(b)

2+

Dependence of PSL spectra of the BaFBr:Eu thin films upon T b during deposition. su

sub

2+

crystal structures. fabricated 2~thin-films Weby have usingalso (BaF

BaFBr:Eu

(BaFZ)x(BaBr2)1-x 2~ x= (Eu 1

2)~(BaBr2)1~

mixtures with different mole fractions x. Figure 2 shows PL spectra of the films grown at T =500°C, measured at 20 K. In the range of sub mole fractions x of 0.4 to 0.7, the films give a

20K

—

_____

strong emission band peaking at 390 nm, which is 2+ . 2+ due to the Eu ions in the BaFBr:Eu . In

~

addition to the PL, these films also show efficient PSL. However, when the films are

~ 0.5 —

______________________

xl.2

0.55

C

a.’

xl.3

_____ ______________________

-J

with a broad emission band peaking at 600 nm, which is thought to be related to F—centers in fabricated by thethemixtures with the mole fractions of films show BaFx= 0.7 to 1, 2~centers. When x=0PL tospectra 0.3, the

O.~

xl.8

0 ______

300

2, show not to Eu films a different PL spectra, and has a

400 500 600 Wavelength (nm)

weak emission band peaking at 410 nm, which is 2+ thought to be related to Eu ions in BaBr

2. These implyIn that BaF2—BaBr2 is not results miscible. otherthe words, BaF x Br system l—x solid solutions are not formed. In conclusion, BaFBr Eu2~ films are successfully grown by the evaporation method only when the mixture of the mole fractions of x=0.4—O.7 is used. The films

FIGURE 2

2~thin films

fabricated at T PL spectra of the =500°Cby BaFBr:Eu using a mixture of (BaF ) (BaBr ) sub with x=0—l. 2 x 2 l—x

REFERENCES 1. M. Sonoda, M. Takano, J. Miyahara and H. Kato, Radiology 148 (1983) 833.

show considerably efficient PL and also PSL. This PSL can be also observed by X—ray irradiation as well as UV irradiation.

2. K. Takahashi, K. Kohda, J. Miyahara, Y. Kanemitsu, K. Amitani and S. Shionoya, J. Lumin. 31/32 (1984) 266.