Time-resolved photoluminescence in SrTiO3

96

Journal of Luminescence 31 & 32(1984)96-98 North-Holland, Amsterdam

TIME—RESOLVED PHOTOLUMINESCENCE IN SrTiO

3

R.

LEONELLI and -J.L.

DEpartement Canada*.

BREBNER

de physique, Université

de Montréal,

Montréal,

Québec H3C 3J7,

The visible luminescence in SrTiO3 is attributed to the recomabination of bound polarons. At times longer than 500 ~a after excitation, the emission intensity decreases following a power law, characteristic of bimolecular processes. At shorter times, the decay is exponential with a time constant less than 100 Os. Deviation from an exponential law for times less than 10 pa is attributed to defect induced disorder.

1.

INTRODUCTION SrTiO3 is an insulator (band gap energy: withemission atrong band electron— 1. At temperatures below 35 3.27 K, a cv) broad in the

phonon visible interaction is excited by photo—induced This luminescence in other

compounds

The purpose

that

~ x decay

time tails

containing

of this

the luminescence

valence

to conduction hand

transitions2.

does not appear to depend on impurity content

communication

band.

Our results

is to present

is present

time—resolved spectra

differ from those

of the emission intensity

of the exponential

and

titanate octahedra3.

part.

previously

is observed both on the

We present

a model

of

reported4 in ahort

to explain

and long

these

results.

2.

EXPERIMENTAL Nominally

pure SrTiO 3 crystals were

cut and polished into thin platelets. studied

per pulse was reduced damage.

7D20 digitisers *Work

The resistivity

at 25 Hz

(photon

energy:

to less than 1 mJ—cm2

of the two aamples

The luminescence

signal was

3.68

cv)

The energy density

to avoid saturation effects

and

captured by Tektronix 7912 AD and

and stored in an Apple 11+ microcomputer.

partially supported by the Natural

Council

Industries and were

wan above The tO~C—cm at were 300 K,illuminated giving a carrier concentration of less 8 cm3. samples by 3 os—long pulses from a

than SxlO laser operating nitrogen

crystal

obtained from ML

Sciences and Engineering

of Canada and by le Mioistére de l’Educatioo

0022—2313/84/$03OO© Llsevier Science Publishers RN. (North-Holland Physics Publishing Division)

Research

du Québec (FCAC).

R. Leommelli, i. L. Breboer / Time-resolved pltotolumioescence 6t SrTiO

97

3

3.

RESULTS Figure 1 shows luminescent energy spectra between 400 and 650 nm

for the spectral response of the grating and photomultiplier tube.

corrected The shape

of the emission band does not vary appreciably for delay times of between 5 and 500 ia following excitation.

The spectrum taken from sample a) exhibits some

structure on the low energy side.

To sample b) the emission band is broader

and the corresponding structure is smeared out.

10~

I

T

>~

4.2 }<

!::

/<~\~=a2mv 1.8

2.2

2.5

ENERGY

T

3.0

3.4

4.2 1<

________________

10_7

~5

—5

-4

-3

-2

10 10 10 10 10 10 TIME AFTER EXCITATION

(eV)

FIGDRE 1 Luminescence intensity as a function of emission energy for samples a) and b). The spectra were taken 10 ps after excitation.

=

-1

10 Cs)

FIGURE 2 Luminescence intensity as s function of time elapsed after excitation for samples a) and b). +: experimental data; full lines: theoretical fit (see section 4). The spectra were taken at an emission energy of 2.3 eV.

Figure 2 shows time—resolved spectra taken at an emission energy of 2.3 eV. One can dietinguish two different behaviours.

At times

longer than 200

after excitation, the emission decays according to a t a law where m

=

os 0.9.

shorier times, the intensity curves follow an exponential decay law in sample a).

In sample b) the abort

time decay can better be described by a modified

exponential form incorporating a power law ~ n where n

=

0.2.

At

98

4.

/

R. Leonelli, J. L. firebner

Tftne—resolved plmotolmtmomescenc-e in SrTiO

3

DISCUSSION At low temperatures, the electrons in the conduction band are rapidly

trapped to becoae polarons, possibly self—trapped on Ti—O bonds or hound to oxygen vacancies.

The broad emission band indicates strong relaxation of the

lattice around the trapping site. The ~m

behaviour of the decay curves at long times is characteristic of a

bimolecular process for which the probability of recombinatioo depends on the overlap between the electron and hole wavefunctions. 5. The exponent ma depends on the At distribution and density of the trapped species shorter times, the exponential decay can be interpreted as the recombination of either trapped polaronic excitons or of bound polarons with delocalised holes.

The ~n

pre—exponeotial factor observed in sample h) is a

feature that has also been reported in amorphous chalcogenides6.

It can be

attributed to the interaction of an electronic level with a continuum of low energy relaxation states of the lattice, with n proportionnal to the density of states7.

Those states could come from the broadening of the lattice modes due

to deformation caused by defects. The full lines in figure 2 represent a best fit of the data points with the empirical formula:

1(t)

=

T~ [t

n exp(—t/-t) +

C m 1+(t/b)

where for sample a) n

=

0.08,

sample b) n

=

55 pa, m

=

0.22,

-t

¶

97 pa. m

= =

=

0.89 and b

0.91 and b =

140 ps.

=

285 pa, and for The agreement with the

experimental data is good except in the transition region (t observed decrease of

-t

—

200 pa).

The

and broadening of the luminescence band with increasing

n is consistent with an interpretation based on disorder—induced effects.

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0. Rfroack et al., J. Phys.

2)

T. Feng, Phya. Rev. B25 (1982) 627.

C16 (1984) 883.

3)

G. Blaese and G.J. Dirkaen,

4)

L. Grabner, Phys. Rev. 177 (1969) 1315.

Chem. Phys. Lett. 62 (1972)

5)

C.J. Delbecq et al., Phys. Rev. 817 (1978) 4765.

6)

K. l4urayama and T. Ninomaya. Jap. J. of Appl. Phys.

7)

0.0. Mahan, Sol. St.

Phya. 29 (1974) 75.

19.

21 (1982) L5l2.