TRAPPING SITES, OPTICAL DAMAGE, AND THERMALLY STIMULATED CURRENT IN FERROELECTRIC OXIDE CRYSTALS

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TRAPPING SITES, OPTICAL DAMAGE, AND THERMALLY STIMULATED CURRENT IN

FERROELECTRIC OXIDE CRYSTALS

R. Clarke, F. Ainger, J. Burfoot

To cite this version:

R. Clarke, F. Ainger, J. Burfoot. TRAPPING SITES, OPTICAL DAMAGE, AND THERMALLY

STIMULATED CURRENT IN FERROELECTRIC OXIDE CRYSTALS. Journal de Physique Collo-

ques, 1972, 33 (C2), pp.C2-143-C2-145. �10.1051/jphyscol:1972249�. �jpa-00214985�

JOURNAL DE PHYSIQUE

Colloque C2, supplbment au no 4, Tome 33, Avril 1972, page C2-143

TRAPPING SITES, OPTICAL DAMAGE, AND THERMALLY STIMULATED CURRENT

IN FERROELECTRIC OXIDE CRYSTALS

R. CLARKE, F. W. AINGER

and J. C. BURFOOT Queen Mary College, University of London

*The Allen Clark Research Centre, Caswell, Towcester

RBsumB. - On discute le

site vide

dans les ferroklectriques

octaedres d'oxyghnes, et sa pertinence au dommage de I'index de refraction et pour des pikges des porteurs. Nous comparaissons ce dommage dans les cas de plusieurs matkriaux et en particulier pour deux niobates

seuil haut, KLN et KSN. On propose l'existence d'une correlation entre l'action des pikges et le dommage.

((

Proton filling

peut reduire la susceptibilite au dommage dans le KLN mais pas dans le KSN.

Abstract.

Empty sites

in oxide octahedral ferroelectrics are discussed, and their relevance to refractive index damageing and charge carrier trapping. Damage in several, oxide materials is compared, and two high-threshold niobates, KLN and KSN, are reported.

correlation between charge trapping and optical damage is suggested.

Proton

filling to reduce damage suscepti- bility is successful with KLN but not KSN.

Introduction.

Some octahedral ferroelectrics have useful electro-optical properties but suffer

optical damage

(refractive index changes) in a laser beam of sufficient integrated power. It has been said that optical damage is related to empty cation sites, supposedly because these provide traps for charge carriers, which are driven by internal fields due to related inhomo- geneities of the polarisation P [I], [2].

This contribution examines these suggestions and proposes that the concept of the

empty site

may have been misleading. In some of these materials we observe thermally stimulated currents but these do not correlate with the

damageable

materials (Table I).

(t

Empty sites.

The compounds of interest are composed of networks of oxygen octahedra aligned along principal symmetry axes [3].

e. g. (i) The perovskite structure ABO,, each BO, octahedron containing a transition metal ion B.

(ii) The trigonal LiNbO, structure.

(iii) The tungsten-bronze structure (Fig. 1).

The tungsten bronze structure has five distinct sites A 1, A 2, B 1, B 2, and C

the chemical formula is [(A l), (A 2), C,] [(B l), (B 2),] O,,. B sites are all occupied by metal ions, but the ten A and C sites are partially occupied depending on the compound being

Several octahedrcl oxides compared for optical damageing, empty sites, TSC, and

proton

dzjiusion.

Material Empty TSC

Proton

Laser damage CW threshold

Site Diffusion Energy cm-2 Wavelength

-

LiNbO, LiTaO,

KLN(f) no damage at

200 MJ 4 880 A

no damage at

200 MJ 4 880 A

? ?

14 J > 6 328 k

200 MJ 5 500 A

?

> 5 147 A

2 kJ > 6 328 k

Ref.

This paper

-

1101

This paper

[1 I

[21

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972249

C2-144 R. CLARKE, F. W. AINGER AND J. C . BURFOOT

FIG. 1. -The tetragonal tungsten-bronze unit cell, after Jamieson et al. [4].

discussed. The 3-sided C sites normally take small cations, e. g. Li, Be, or Mg, while the 4-sided (A 1) and 5-sided (A 2) cation sites may contain Na, K, Sr, Ba, or Li, for example. When all A sites are occupied the structure has been referred to as

filled

as distinct from

fully-filled

when all A and C sites are occupied.

A 1, A 2, or C regions not occupied have been called

<<

empty sites

~ h e s e must be- distinguished from

vacancies (i. e. imperfections in the crystal lattice).

The empty site is held to occur in principle in a perfect lattice at absolute zero. The concept of an empty site may be defined intuitively by figure 1. The structures of several tungsten-bronzes have been formally inves- tigated by Jamieson [4] but we are not aware that any more precise definition of an empty site has been given.

The question arises, whether the empty site of the idealised type described above could produce effects such as the TSC. We have made observations on potassium lithium niobate (KLN), both ferroelectric (f) and non-ferroelectric (nf) [5], and our measurement of conductivity in KLN (nf),

3.97 A, has shown a deep donor level 2.07 eV below the conduction band.

This is confirmed by our TSC measurement 151. We find that the band gap corresponding to the absorption edge is 3.2 eV.

Johnston [6] associates optical damage in LiNbO, with moderately deep electron traps

traps arise from Nb-excess stacking faults, Nb being substitutionally incorporated in a Li site. It is thus possible that in oxide ferroelectrics, empty sites do not, per se, contribute to optical damage, but depending on the cations available, may make the structure type prone to stoichiometry variation and subsequent charge trapping.

The band theory of deep levels is not well developed because the effective-mass method is applicable only to shallow levels. Di Domenico and Wemple [3] have given a first order calculation of the band structure applicable to oxygen octahedra ferroelectrics. All are principally determined by the octahedra which are taken to control the lower conduction band, via the d-orbitals of Nb5+, and the upper valence bands, via the 2 p orbitals of 0-. It is clear that the treatment in terms of the octahedra takes into account the structure as it exists, including empty sites, and gives a valence and conduction band because it is designed to do so.

There is no question of localised microscopic behaviour of charge carriers being represented by them especially in view of the density of empty sites which, for the stoichiometric case, should be about ~ m - ~ . Typical trap densities may be 1014-17 ~ m - ~ .

Thermally stimulated current.

Expressions have been given [7], [8] describing the conductivity peak when a material containing traps is heated at a constant rate, both for fast and slow retrapping. In KLN (nf), we observe a TSC peak at about 460 OC indicating a trap depth of 2.07 eV. In potassium strontium niobate (KSN) a peak at 475 OC coincides with an abrupt expan- sion in the c-dimension, which is not repeated once emptied. Conductivity measurements indicate a trap depth of 0.4 eV associated with this. Possibly the changes in potential when the traps are emptied affect the thermal vibrations thus altering the thermal expansion curve at 475 OC.

<<

Proton

filling.

Levinstein et. al.

have devised a procedure for reducing the degree of optical damage in some materials by the diffusion of hydrogen under electric fields at high temperatures, (cr field- annealing

The entry of hydrogen is demonstrated by the appearance of infrared OH absorption lines near 3 495 cm-I and a reduction in d. c. conductivity.

It seems that the process fills in such a way as to remove carrier sources. We attempted to field-anneal KLN (nf) and KSN and found that hydrogen infusion occurred only in the former

no OH absorption could be detected in KSN before or after the annealing process. We have found that KSN and KLN both require high threshold energies for optical damage (see Table I).

Summary.

Briefly, we have examined whether

empty sites can be closely related to TSC or to optical

damage, particularly in KLN and KSN. We find evi-

dence of electron levels, which, in the case of KLN (nf)

are deep but it is not clear that they can be associated

with specific empty sites. It seems that empty sites do

not correlate directly with optical damage although

it is probable that charge carrier trapping and TSC do,

in view of the fact that damage in KSN can be erased

with white light. We suggest that the type of structure,

and its particular form of variation from stoichiometry,

is more important in determining the susceptibility to

TRAPPING SITES, AND THERMALLY STIMULATED CURRENT C2-145

optical damage than

empty sites

which can exist (Chemistry Dept., University College, London) for in the stoichiometric material. his help in laser damage experiments and R. Clarke acknowledges the support of the Science Research

We thank P. Mitchell Council.

References

[l]

ASHKIN (A.), BOYD

JOHNSTON (W. D.),

1966, 9, 72. [7]

NICHOLAS

H.) and WOODS (J.),

CHEN (F.

[3]

DI DOMENICO,

and WEMPLE

COWELL (T. A.

and WOODS (T.),

Phys., 1969, 40, 720. 1967, 18, 1045.

[4]

JAMIESON

ABRAHAMS

and BERNSTEIN

LEVINSTEIN

J.), BALLMAN (A. A.)

(J.

L.),

[5]

AINGER (F.

BESWICK (J. A.)

VAN UITERT (L.

LEVINSTEIN (J. H.)

1971, 2, 183. Res. Bull., 1968, 3 , 47.