Appl Phys B (2009) 95: 125–130
DOI 10.1007/s00340-009-3442-y
Raman spectroscopy study of compositional inhomogeneity
in lithium tantalate crystals
S.M. Kostritskii · M. Aillerie · P. Bourson · D. Kip
Received: 30 September 2008 / Revised version: 18 November 2008 / Published online: 3 March 2009
© Springer-Verlag 2009
Abstract Raman spectra of LiTaO
3
single crystals with
various stoichiometries were measured to investigate the
compositional uniformity of these crystals. Raman spectra
mapping demonstrates a spatial variation of the widths of
the phonon bands for stoichiometric, congruent, and quasi-
congruent samples. A significant radial compositional in-
homogeneity is found to be a common feature of commer-
cially available wafers having a near-congruent crystal com-
position (i.e., x
c
= {[Li
2
O]/([Li
2
O]+[Ta
2
O
5
])}× 100% =
47.85–48.50%) grown by the single-crucible Czochralski
method. A maximum value of the composition gradient
∇ x
c
for a radial inhomogeneity of 0.163 and 0.036%/cm
is measured for thin wafers diced from so-called congru-
ent (vendors’ value of x
c
= 48.50%) and quasi-congruent
(x
c
= 47.88%) crystals, respectively. In crystals grown from
highly Li-rich melts (starting composition 54.5 mol% Li
2
O),
a drastic spatial dependence of Raman bandwidths, indi-
cating a significant gradual compositional inhomogeneity
throughout the crystal, is found, which is due to a change
of the melt composition during crystal growth. In contrast,
the Raman bandwidths of near-stoichiometric crystals fabri-
S.M. Kostritskii
MPTE Department, Moscow Institute of Electronic Technology,
Moscow, 124498 Zelenograd, Russia
M. Aillerie ( ) · P. Bourson
Laboratoire Matériaux Optiques, Photonique et Systèmes, UMR
CNRS 7132, University of Metz and Supelec, 2 rue E. Belin,
57070 Metz, France
e-mail: aillerie@metz.supelec.fr
D. Kip
Institute of Physics and Physical Technologies, Clausthal
University of Technology, Leibnizstr. 4,
38678 Clausthal-Zellerfeld, Germany
cated by a vapor transport equilibrium (VTE) technique are
found to be constant, i.e. these crystals are practically com-
positionally uniform. This conclusion has been confirmed
by mapping the photoluminescence intensity, evidencing
∇ x
c
≤ 0.006%/cm in near-stoichiometric VTE-treated crys-
tals.
PACS 61.50.Nw · 77.84.Dy · 78.30.-j · 78.55.Hx
1 Introduction
Lithium tantalate (LiTaO
3
, LT) is an advanced ferroelec-
tric material for, e.g., surface acoustic wave (SAW), electro-
optic (EO) and nonlinear optical devices. On the other hand,
waveguide technology in LT crystals has attracted consid-
erable interest for new applications such as integrated opti-
cal devices [1]. Because the refractive index of LT crystals
was found to depend strongly on crystal composition [2, 3],
growth of LT crystals with compositional uniformity is im-
portant, as even a relatively small crystal compositional in-
homogeneity has dramatic influences on the performance of
various integrated optical devices [4]. However, the differ-
ence in compositional uniformity of crystals fabricated by
different methods, which are in common use today, has not
been clarified yet.
Since the specific Raman bandwidths in a LT crystal
generally depend on the Li deficiency (i.e., on stoichiom-
etry) [5, 6], it is possible to determine the composition by
accurate measurement of the full width at half maximum
(FWHM) of certain phonon bands. In this work, mapping of
Raman spectra is used to investigate the compositional uni-
formity of LT crystals having stoichiometric, congruent, and