Full text of DOI:10.1557/JMR.2000.0159

Copper–lithium ion exchange in LiNbO3
a)
F. Caccavale, C. Sada, and F. Segato
Instituto Nazionale Fisica della Materia, Universit a` di Padova, Dipartimento di Fisica, via Marzolo
8, 35131 Padova, Italy
L.D. Bogomolova and N.A. Krasil’nikova
Institute of Nuclear Physics, Moscow State University, 119899 Moscow, Russia
Yu.N. Korkishko, V.A. Fedorov, and T.V. Morozova
Department of Materials and Technology of Solid State Electronics, Moscow Institute of Electronic
Technology, 103498, Moscow-Zelenograd, Russia
(Received 19 October 1999; accepted 24 February 2000)
Copper-doped LiNbO waveguides were prepared by Cu–Li ion-exchange process.
3
Compositional, structural, and optical analyses were performed by secondary ion mass
spectrometry, x-ray diffraction, and m-line spectroscopy, respectively. The chemical
2+
state of Cu ions was studied by electron paramagnetic resonance, and the results
were correlated with structural modification of the LiNbO matrix. Copper
3
incorporation in the crystal took place under different regimes, and it induced a lattice
2+
rearrangement with the formation of new crystalline phases. Cu ions were
surrounded by tetragonally compressed octahedra with rhombic distortions. Cu:LiNbO3
optical waveguides were formed supporting two optical modes.
I. INTRODUCTION
In the present work, the copper ion-exchange process
is studied from the point of view of the correlation
between optical and structural properties of Cu:LiNbO3
waveguides.
Lithium niobate (LiNbO ) is a widely used material
3
in integrated optics due to its electro-optical, acousto-
optical, and nonlinear properties. Single crystals of LiNbO3
are used to realize efficient electro-optic integrated devices
for telecommunications, such as phase and intensity modu-
lators, directional couplers, and optical filters.
Thermal in-diffusion of thin metal films is the most
common technique used to realize optical waveguides in
lithium niobate. As described elsewhere (Refs. 1 and 2
and references therein), this process takes place at high
temperatures (up to 1100 °C) and requests long proc-
essing times (up to 100 h).
II. EXPERIMENTAL
Waveguiding Cu:LiNbO layers have been obtained
3
by processing X-, Y-, and Z-cut crystal plates in a melt of
dry salts with composition Na SO –CuSO (54:46 mol%).
2
4
4
Copper–lithium ion exchange has been performed at
temperatures (T) ranging from 550 to 600 °C and proc-
essing time (t) from 1 to 15 min.
Cu and Li depth profiles have been obtained by sec-
ondary ion mass spectrometry (SIMS) using an IMS-4f
CAMECA mass spectrometer. In-depth profiles have
Ion exchange is a very attractive process, as an alter-
native method to thermal diffusion, for the local doping
+
been obtained with a 14.5-keV Cs primary ion beam
of LiNbO due to the fact that it is performed at relatively
3
and by detecting negative secondary ions. A normal-
incidence electron gun has been used to compensate
charge buildup during the analysis of insulating samples.
The erosion rate has been obtained by measuring the
depth of the erosion crater at the end of each analysis
using a Tencor Alpha Step profilometer.
X-ray diffraction (XRD) analysis has been performed
in order to study the phase formation in the doped layers,
by means of a Rigaku Denki D-2 diffractometer using Cu
low temperatures (<700 °C) and short processing
3–6,8
times.
In particular, the combined proton and copper
exchange has recently attracted much attention for the
creation of a solid-state laser in LiNbO , working in the
3
7
yellow-green range. An important role in the photore-
fractive response is played by the Cu /Cu ratio, when
reverse exchange on proton and copper waveguides is
+
2+
8
performed. Thus, the possibility of the technological
control of both copper valence state concentration and
optical damage resistance provided by combined proton
and copper exchange is of great interest.
K radiation at 1.5404 Å.
␣
Optical characterization has been performed by means
of modal (m-line) spectroscopy technique with an au-
tomatized prism-coupling method. Angles at which the
guided modes are selectively excited by a He–Ne polar-
ized laser beam (␭ ס 633 nm) were measured in order to
a)
Address all correspondence to this author.
e-mail: sada@padova.infm.it
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F. Caccavale et al.: Copper–lithium ion exchange in LiNbO3
calculate the corresponding effective indeces. The refrac- in the transition region moves toward the substrate for
tive index profile of the waveguides was reconstructed by increasing processing time. Moreover, the peak is con-
an inverse Wentzel, Kramers, and Brillouin (WKB) pro- fined in the first micron of the doped layer, suggesting
9
cedure. Optical absorption spectroscopy has been per- that some structural rearrangement takes place.
formed in the 250–2000-nm range with a Cary Varian
double spectrophotometer.
On the other hand, XRD measurements performed on
Cu:LiNbO samples indicate the formation of crystalline
3
2
+
Cu incorporation has been studied by electron phases different from the LiNbO one. The x-ray rocking
3
paramagnetic resonance (EPR) performed on Brucker curves of X-, Y-, and Z-cut Cu:LiNbO samples are re-
3
ESP-300 and RE-1306 spectrometers, working in the x- ported in Fig. 3. The observed large mismatch (about
band frequency (∼ 9.5 GHz). The measurements have been 5%) with respect to the pure LiNbO cell indicates the
3
performed at room temperature and at 77 K. Spectra were generation of new compounds, such as CuNb O , and
2
6
recorded as a function of angle for X and Z cuts. The CuNbO , or a solid solution, such as Cu Li1−kxNbO3.
3
kx
2+
absolute number of Cu ions has been determined with The intensity of the XRD peaks grows with the increase
respect to Cu spectrum in a powder CuCl и 2H O ref- of exchange time, whereas their positions remain un-
2+
2
2
erence sample.
changed.
2+
EPR analysis of Cu ions has been performed in order
to clarify how the copper incorporation in LiNbO took
3
2+
place. The EPR spectra of Cu ions in ion-exchanged
X-cut and Z-cut substrates have been recorded for various
processing parameters. Results obtained on five exam-
III. RESULTS
Copper and lithium depth profiles of a Cu:LiNbO Z-cut
3
sample are reported in Fig. 1 (T ס 570 °C, t ס 2 min).
Three regions in the Cu depth profiles, corresponding
to different regimes in which copper incorporation
takes place, could be identified. In the upper layer (about
400-nm thick), the profile shows a steplike form and a
complete depletion of lithium occurs. In the second tran-
sition region, a peak occurs at 500 nm. In the third region
copper–lithium ion exchange takes place. No substantial
differences could be observed between X- and Z-cut
samples.
Copper SIMS depth profiles for different processing
times are reported in Fig. 2. It can be observed that the
depth of the exchanged layers is of order of a few microns,
and it increases (about 10–25%) for increasing processing
time. It is also worth noting that the Cu peak
FIG. 2. Copper SIMS depth profiles for Cu:LiNbO3 Z-cut samples
(ion-exchange parameters: T ס 570 °C, t ס 1, 2, and 3 min).
FIG. 1. Copper and lithium SIMS depth profiles for a Cu:LiNbO Z-cut
FIG. 3. Diffractograms of copper ion-exchanged structures formed on
3
sample (ion-exchange parameters: T ס 570 °C, t ס 2 min).
X-, Y-, and Z-cut LiNbO (LN).
3
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F. Caccavale et al.: Copper–lithium ion exchange in LiNbO3
ined samples are reported in Table I. Figure 4 shows the
visible at room temperature. Those spectra were recorded
at 77 K, where the linewidth is reduced and the intensity
is enhanced.
EPR line for the magnetic field, H, parallel to the c axis
of the crystal lattice. Whatever the ion-exchange condi-
tions, the intense structureless line is always present in
the spectra recorded at room temperature for crystals
doped with copper.
Figures 5 and 6 show the EPR spectra of the Cu1
sample recorded at 77 K for H parallel to two different
directions in the Z plane. As it can be seen, the positions
of HFS lines depend on the direction. When the magnetic
field rotates in the Z plane, the g value which corresponds
to the center of HFS quartet ranges from 2.31 to 2.35 and
the separation between HFS components varies from 60
to 72 G. The maximum g value corresponds to the maxi-
mum separation. For H parallel to the c axis the g and A
values are g ס 2.05 and A ס 173 G. When the mag-
Both the peak-to-peak width (⌬H) and g-value of this
line do not depend on the orientation of crystallographic
axes in the magnetic field and are equal to 220 ± 25 G
and 2.15 ± 0.1, respectively. No variation of ⌬H has been
observed at 77 K. The extra line which is seen in Fig. 4
at H ס 3600 G is due to one of the several transitions
10
related to the presence of iron impurity in LiNbO3.
Four weak lines of the hyperfine structure (HFS), at-
c
c
netic field rotates in the X plane, the maximum A value
(192 G) is observed at the minimum g value (2.03) for
some direction. For a rotation of magnetic field away
from this direction by ∼ 90° in the X plane g and A values
are equal 2.35 and 70 G, respectively. It is impossible to
follow the angular dependence of line positions because
of very low intensity of the HFS spectrum for both
2+
tributed to the interaction between the Cu unpaired
63.65
3
electron and
Cu nuclei (I ס ⁄
2
), are superimposed on
a single g ס 2.15 signal when H is parallel to the c axis
−5
of the crystal. As far as their intensities are less than 10
of the total intensity of the spectrum, the HFS lines are
planes. The spectra of powder LiNbO samples consist of
2
+
3
TABLE I. Concentration of Cu ions obtained by EPR in Cu:LiNbO3
samples.
the g ס 2.15 line on which the HFS spectrum is super-
imposed. The width of the g ס 2.15 line is 220 ± 25 G.
The HFS spectrum consists of 12 hyperfine lines forming
three groups of four components.
Exchange
temp. (°C)
Processing
time (min)
Concentration of
Cu ions (10 cm )
2
+
18
−2
Sample
Cu1
Cu2
Cu3
Cu4
Cu5
580
600
585
560
560
5
2
2
2
15
7
The optical characterization of the sample showed that
the doped layers are waveguides at 633 nm, supporting
two optical modes. The reconstruction of the refractive
index profile shows that the processing temperature does
not play an appreciable role in the determination of the
5.2
4.6
3
6.8
FIG. 4. EPR line for magnetic field H parallel to the c axis of the
lattice.
FIG. 5. EPR spectra for a Cu:LiNbO3 sample (T ס 580 °C, t ס
5 min) recorded at 77 K. H is parallel to a direction in the Z plane.
1
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F. Caccavale et al.: Copper–lithium ion exchange in LiNbO3
The rearrangement of a lithium niobate cell, invoked
both to preserve local charge compensation and to ac-
commodate copper ions, leads to the formation of new
phases, as confirmed by XRD measurements.
From EPR analysis it results that both the g ס
2
+
2
.15 line and HFS spectra are attributed to Cu ions.
The single g ס 2.15 line arises from interacting
2+
Cu ions whereas the HFS spectrum is due to isolated
2+
Cu ions. The contribution of the HFS specturm to the
2+
total concentration of Cu ions is negligible. The doses
2+
ni (i ס 1, . . . , 5) of Cu ions given in Table I were
obtained by n ס N /(2S ), where S is the area of the
i
i
i
i
sample before crushing and N represents the number of
i
2+
Cu ions calculated by comparing a standard sample to
the Cu:LiNbO EPR spectra. The factor 2 is because the
3
ion exchange has been performed on both sides of the
samples. The accuracy of measurements of relative con-
centrations is about ±10%. The lack of an hyperfine
structure for the g ס 2.15 line proves that spin–spin
interactions predominate over hyperfine interactions. The
degree to which the EPR spectrum is affected by dipole–
dipole and exchange interactions depends on the distance
between the ions and their dislocation in the crystal lat-
FIG. 6. EPR spectra for a Cu:LiNbO3 sample (T ס 580 °C, t ס
min) recorded at 77 K. H is parallel to a direction in the Z plane
different from that shown in Fig. 5.
5
2+
tice. If Cu ions are uniformly distributed through the
crystal, dipolar interactions cause a broadening of
the resonance line and a dependence of its width
from the orientation of the crystal in magnetic field.
Moreover, the exchange interaction narrows the line. As
mentioned above the g ס 2.15 line appears isotropic
with a constant g value when magnetic field rotates in
effective refractive indices. From optical absorption
measurements it was possible to identify a small broad-
2
2
ened band around 800 nm, ascribed to the E → T
g
g
2+
transition of Cu ion octahedrally coordinated with oxy-
gen ions, and a strong optical absorption at 400 nm. The
both Z and X planes of the ion-exchange LiNbO . This
3
+
last band suggests the presence of Cu in small concen-
permits one to assume that the symmetry axes of com-
0
3,11,12
2+
tration. No evdence of Cu has been found.
The
plexes of Cu ions formed in the diffusion layer are
+
2+
presence of Cu ions can be a consequence of Cu re-
duction or can be due to the presence of an electron
associated with a Cu center or trapped on it. Work is
in progress to clarify whether the waveguiding properties
oriented randomly with respect to the axes of the crystal
2+
lattice and with each other. We assume that Cu ions,
which are responsible for the g ס 2.15 line, are located
near the surface of the crystal where the total copper
concentration is high. The thickness of the diffusion layer
in which the majority of copper is accumulated is of
2+
7
2+
+
are due to Cu or Cu ions.
2+
order of 2 ␮m. Assuming that the Cu ions are uni-
IV. DISCUSSION
formly distributed through a such layer, the surface con-
2
+
22
It results from the previously described analysis that
copper takes part in ion exchange mainly in the 2+ oxi-
centration of Cu ions results to vary from 1.5 × 10 to
22
−3
3.5 × 10 cm . It follows from Table I that the con-
+
2+
dation state. However, the possibility that Cu ions are
centration of Cu ions in the exchanged layer is very
also involved in the process cannot be excluded. In the
first case, copper incorporation induces a re-ordering of
high and increases with processing time at constant tem-
perature and with exchange temperature at constant proc-
essing time.
2+
+
the matrix due to the nonisovalent Cu –2Li exchange.
2+
2+
Moreover, compositional analyses suggest that Cu in-
diffusion results are partially limited by the presence of
blocking forces, the origin of which could be ascribed to
a high concentration gradient of the species involved in
the process. As a consequence, the copper accumulation
The HFS spectrum belongs to Cu ions, which are
located in the tail of the depth profile of copper distri-
2+
bution where the concentration of Cu ions is low. In
contrast to the single g ס 2.15 line, the HFS spectrum
depends on the orientation of the crystal axes in the mag-
netic field. It has been shown above that the minimum
(
corresponding to the concentration peak shown in
2+
Fig. 2) plays the role of a reservoir for the further Cu
in-diffusion toward the substrate.
g value (2.03) obtained for Cu ions in LiNbO is close
3
to the g value of the free electron (g ). In addition, the
e
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F. Caccavale et al.: Copper–lithium ion exchange in LiNbO3
2
+
maximum hyperfine separation (192 G) occurs along the
orientation dependent, and Cu ions are surrounded by
tetragonally compressed octahedron with rhombic distor-
minimum g-value direction. Such a situation is observed
2
+
2
2
13
for Cu1 ions with ground-state |3r − z >. It is
tions. Cu:LiNbO optical waveguides are formed sup-
porting two optical modes. Work is in progress to clarify
whether waveguides properties are due to Cu incorpo-
ration or they are the consequence of the reduction of
Cu to Cu , as observed in glass matrices.
3
4
known that 3d ions occupy the Li and Nb sites in
2+
LiNbO . It can be expected that during the Li–Cu ion-
3
2+
+
exchange process Cu ions occupy Li sites. When the
Cu ion enters substitutionally into the Li site, an
octahedral coordination of the Cu ion is expected, since
2
+
+
2+
+
16
2
+
+
Li ions occupy the octahedral sites in LiNbO crystal
3
1
5
ACKNOWLEDGMENT
lattice. Starting from an octahedral coordination of
2+
2
2
Cu ion, a ground-state |3r − z > will result from a
The authors would like to acknowledge NATO Grant
HTECH.LG No. 974730 and INTAS Project 97-009.
13
tetragonal contraction of an octahedron along one axis,
which is one of the principal axes of the g and A tensors
z axis). Since this axis lies in the X plane and g < g with
(
z
c
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different from the real angles between H and the crys-
tal axes.
1
2
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3
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18
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7
2+
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1
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+
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1124
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