s u r f a c e science
Surface Science 369 (1996) 313-320
ELSEVIER
Atomic force microscopy study of (001) SrTiO3 surfaces
B. St/iuble-POmpin, B. Ilge, V.C. Matijasevic 1 P.M.L.O. Scholte, A.J. Steinfort,
F. Tuinstra
Department of Applied Physics, Solid State Physics, TU Delft, Lorentzweg 1, 2628 CJ, Delft, The Netherlands
Received 17 March 1996; accepted for publication 27 June 1996
Abstract
By means of atomic force microscopy, we have investigated the surfaces of SrTiO3(001) substrates (used for the growth of
high-T~ superconducting films) after different heat treatments. These treatments were (i) annealing in Oa, (ii) annealing in 02, (iii)
annealing in Oa followed by deposition of SrTiO3, and (iv) annealing in UHV. Our results suggest that, independently of the type of
annealing, the surfaces of the polished SrTiO3 substrates rearrange, either by diffusion or desorption of atoms, until reaching the
energetically most favourable surface structure. For vicinal (001) substrates, this equilibrium structure corresponds to atomically
fiat terraces separated by smooth steps of one unit-cell in height, with the orientation and width of the terraces being determined by the miscut of the substrate. However, the kinetics involved in the reconstruction of the SrTiOa surface were observed
to be strongly dependent on the annealing conditions. In particular, the use of O 3 instead of 02 or UHV was found to accelerate
the reconstruction of the SrTiOa surface. Finally, this study provides an accurate characterisation of the defects present on SrTiO3
substrates.
Keywords: Atomic force microscopy; Strontium titanate; Surface defects; Surface structure, morphology, roughness, and topography;
Vicinal single crystal surfaces
1. Introduction
W i t h recent years, the u n d e r s t a n d i n g of the
m e c h a n i s m s i n v o l v e d in the n u c l e a t i o n a n d g r o w t h
of thin films of c u p r a t e s u p e r c o n d u c t o r s has p r o gressed rapidly. I n p a r t i c u l a r , it is n o w a c c e p t e d
t h a t the surface s t r u c t u r e of a s u b s t r a t e c a n
s t r o n g l y influence the m o r p h o l o g y of the subseq u e n t l y d e p o s i t e d high-T~ film. Therefore, a n accur a t e c h a r a c t e r i s a t i o n a n d u n d e r s t a n d i n g of the
surface o f s u b s t r a t e s is essential to g r o w highq u a l i t y t h i n a n d u l t r a t h i n films.
1 Present address: Conductus, 969 West Maude Avenue,
Sunnyvale, CA 94086, USA.
T h e surface of single-crystal SrTiO3 (which is
used extensively as a s u b s t r a t e for the epitaxial
g r o w t h of YBa2Cu3Or a n d r e l a t e d high-T~
s u p e r c o n d u c t o r s ) has recently been the subject of
b o t h t h e o r e t i c a l a n d e x p e r i m e n t a l studies I - 1 - 7 ] .
H o w e v e r , due to the v a r i e t y of different surface
t r e a t m e n t s a p p l i e d to the i n v e s t i g a t e d SrTiOa crystals, the p u b l i s h e d results are difficult to c o m p a r e
a n d often seem conflicting.
T h e a i m of the a t o m i c force m i c r o s c o p y ( A F M )
s t u d y p r e s e n t e d here was to investigate a n d c o m p a r e the surface m o r p h o l o g y of SrTiO3(001) after
different t r e a t m e n t s . These t r e a t m e n t s were: (i)
a n n e a l i n g in 03, (ii) a n n e a l i n g in 02, (iii) a n n e a l i n g
0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved
PI1 S0039-6028 (96) 00897-7
314
B. Stdiuble-Piimpinet al./Surface Science 369 (1996) 313-320
in 03 followed by deposition of SrTiO3, and (iv)
annealing in UHV.
operating in the tapping mode with a silicon-type
cantilever, under ambient conditions.
3. Results
2. Experimental
SrTiO3 is a widely used substrate to grow epitaxial RBa2Cu30 r ( R = Y or lanthanide) films. The
substrates investigated in our study are commerdally available (Crystal GmbH, Berlin) polished
single-crystal substrates with a nominal (001) orientation. In order to simplify the comparison, most
of the A F M results shown here (Figs. 1-5) were
obtained from the same lcm x lcm x l m m substrate which was cut into four parts. Comparable
measurements were performed on other SrTiO3
substrates and gave consistent results.
Annealing in 03 (ozone) was performed in an
U H V - M B E system adapted for reactive coevaporation, with a background pressure of 5 x 10 .6
mbar. The ozone was generated with a commercial
generator and then purified in a home-made still.
The ozone concentration was between 50 and
100%. The incidence rate of the ozone beam was
1016 molecules cm -2 S-1.
Annealing in 0 2 was performed in flowing
oxygen, in a furnace at ambient pressure.
The deposition of SrTiO3 was done by coevaporating Sr and Ti at 750°C in an ozone beam in the
reactive MBE system. By adequately timing the
deposition process, a total of 10 monolayers of
SrTiO 3 were grown on top of the previously
annealed substrates. This was confirmed by the
analysis (after film growth) of the R H E E D oscillations of the specular spot.
The miscut angles of the substrates were determined by optically aligning the sample with a laser
and then measuring the position of two different
crystal reflections using a standard four-circle
diffractometer. The direction of the surface normal
with respect to the crystal lattice was characterised
by the angles e (the vicinal angle between the
surface normal and the (001) direction) and fl (the
angle between the ( 1 0 0 ) direction and the projection of the surface normal into the (001) plane).
All images were obtained on a Nanoscope III
A F M (Digital Instruments, Santa Barbara, CA)
3.1. Annealing in 03
The first type of heat treatment used was annealing of the substrate for 15-25 min in 03 at a
temperature of 700, 750 or 900°C. The first two
temperatures are typical for the growth of cuprate
films in our reactive MBE system. Within the
investigated range, the treated SrTiO3 surface was
not found to depend significantly on the annealing
time or temperature. Typically, the substrates were
warmed to the desired annealing temperature
within 5-10 min and cooled down within approximately 30 min.
As shown in Fig. 1; the resulting surface morphology of the SrTiO3 is characterised by a series
of parallel steps indicating the existence of a slight
miscut of the substrate with respect to its nominal
(001) orientation. The height of the steps is
0.39 __0.01 nm (i.e. one lattice constant of SrTiO3)
and the terrace width is equal to 85_+ 5 nm. From
independent X-ray diffraction measurements, the
vicinal angle e of the same substrate was found to
be equal to 0.27___0.05°, which corresponds to a
terrace width of 85__15nm, in good agreement
with the above A F M results. The step ledges are
perpendicular to the projection of the surface
normal into the (001) plane. Their orientation with
respect to the crystallographic lattice is characterised by the miscut angle fi (see definition in
Section 2). As this angle fi can take on any value
(for the sample shown in Fig. 1, fi=17__5°), we
would expect that on a microscopic scale, due to
the orientation dependence of the step energy, the
step ledges should zigzag along well-defined crystallographic directions. However, within the resolution of the measurements performed here, the
observed step ledges are in general smooth, i.e.
only a limited amount of kink bunching is
observedl
In addition, the surface of SrTiO3 was characterised by different types of defects. Dislocations(see
B. Stgiuble-Pfimpinet al./Surface Science369 (1996) 313-320
(a)
3.0
i
1.5
,00
r~
r'm
O.01~
1.(iO
(b)
~o
A
o.zo
o.~0
IIN
o.~o
03,0
A'
Fig. 1. (a) AFM image of an SrTiOs(001)substrate after annealing in Oa at 750°C for 15 rain. The black arrow points to a
dislocation. The vertical profile taken along the line A-A' is
shown in (b). The profilewas averaged over a width of 200 nm.
The observed step height is equal to one unit cell of SrTiO3.
arrow in Fig. 1) were observed with a density of
6 + 1 x 107 cm -2. Such dislocations could, by inheritance, lead to the formation of spiral'shaped
islands in the subsequently deposited high-T~
superconductor film. Furthermore, line-shaped
defects running along the (100) and ( 0 1 0 )
directions of the substrate were found. The length
of these one-dimensional arrays of defects was
observed to vary between a few hundred nm and
a few #m. Typically, a height difference of one
lattice constant of SrTiO3 is observed across such
a defect. In some cases, two linear defects cross
each other, resulting in the cross-shaped defect
displayed in Fig. 2. Similar cross-shaped defects
have been reported by Sum et al. [5]. As shown
in Fig. 2, step ledges are occasionally found to
have a very wavy, almost dendritic structure.
Finally, holes with a diameter of 1 0 - 2 0 n m
315
were observed with a density of approximately
2 + 1 x 109 cm -2.
3.2. Annealing in Oe
Figs. 3 and 4 display an SrTiO3 surface after
annealing for 1 h at 750°C in 02. In order to
minimise thermal strain resulting from a possible
temperature gradient over the sample, the substrate
was warmed up to the desired temperature within
8 h and cooled down again to room temperature
within approximately 10 h.
The surface morphology shown in Fig. 3a is
characterised by irregularly shaped terraces.
Although the step ledges appear to be wavy due
to step bunching, they are locally (on a scale of
10-100 nm) parallel to crystallographic directions,
such as for example, (100), (110), ( 1 2 0 ) and
(130). The height of the steps typically varies
between 0.5, 1, 1.5, 2 and 2.5 unit cells of SrTiO3
(see, e.g., Fig. 3b), indicating the presence of
different termination layers. To exclude calibration
errors, the z-calibration of the A F M was checked
immediately before and after the recordings of the
A F M images shown in Figs. 3 and 4.
As in the case of 03 annealing, line-shaped
defects and holes were also observed after heat
treatment in 02 (see Fig. 4). Typically the holes
have the form of inverted pyramids with the sides
( 1 0 - 2 0 n m long) aligned along the (110) directions. Larger holes with the sides aligned along the
(100) directions were also observed. Comparable
square holes have been observed by Jiang et al.
[6] after annealing in hydrogen as well as by
Kawasaki et al. [3] after etching the surface of
their samples. While some of t h e holes on our
substrates were distributed randomly on the
SrTiO3 surface (approximately 2 x 10s cm-2), most
of them were found to be grouped into chains
parallel to the (100) and (010) directions (see
Fig. 4).
Finally, annealing a substrate first in O2 and
then in 03, or vice-versa, resulted in a surface
comparable to that described in Section 3.1.
3.3. Annealing in 03followed by the deposition o f
SrTi03
We attempted to examine how the SrTiO3
deposition affected the surface morphology and in
316
B. Stgiuble-Pfimpin et al./Surface Science 369 (1996) 313-320
~onN
.z,oo
0
nM
0
nM
~! .00
1.00
2.00
Fig. 2. A F M image of the same sample as displayed in Fig. 1, showing two line-shaped defects crossing each other. The linear defects
are parallel to the (100) and (010) directions.
particular whether it would improve its smoothness. Therefore, one of the treatments investigated
was annealing of the substrate in 03 as described
in Section 3.1 followed by the deposition of 10
layers of SrTiO3 in the reactive MBE system. The
resulting surface is shown in Fig. 5.
As for all SrTiO3 substrates with 03 annealing,
no step bunching was observed. Although lineshaped defects were still present, they were
observed to heal as additional material was deposited. No holes could be resolved. Furthermore, the
surface roughness was found to have increased due
to the formation of islands nucleating on the
terraces, in between two steps. These islands are
typically 0.4 nm high, which suggests that they are
SrTiO3 unit cells.
3.4. Annealing in U H V
Fig. 6 displays the surface of a SrTiO3 substrate
which was annealed for 20 min at 750°C in UHV
(background pressure of 10 -7 mbar). The warming
and cooling procedures for this sample are comparable to those used in 03 annealing described in
Section 3.1.
When imaging this sample with AFM, steps on
its surface can generally be recognised. However,
unlike the atomically fiat substrates annealed in
03, this UHV annealed substrate displays rough
step ledges and small clusters are clearly seen on
top of its terraces. This is consistent with results
by Jiang et al. 1-7] showing that higher temperatures and longer annealing times are required to
obtain an atomically fiat SrTiO3 surface by
annealing in UHV.
4. Discussion
Two possible models could explain the pronounced differences in the surface morphology of
SrTiO3 substrates after annealing at 750°C in 03
(Fig. 1) or in O2 (Fig. 3).
Model I. Each observed surface reflects a thermodynamically stable state for a given annealing
pressure and temperature. When cooling down to
room temperature, this state is frozen in.
Model H. Before annealing, the surface is disordered, probably due to the polishing of the substrate. While annealing the sample, adatoms are
desorbed and/or diffuse on the surface. Different
annealing conditions (e.g. pressure and temper-
B. Stgiuble-Pfimpin et al./Surface Science 369 (1996) 313-320
(a)
A
0
(b) ~ ] ~
0
A
0.25
0.50
0.75
o,~
0.~
1.00
m,
v.. ,',mnc.
0.~
~
A'
Fig. 3. (a) AFM image of an SrTiO3(001)substrate after annealing in 02 at 750°C for 1 h. The open arrows show step ledges
which are parallel to crystallographic directions (see text for
more details). The vertical profile taken along the (110) direction (line A-A') is shown in (b). The markers show steps with
heights of 1 and 2.5 unit cells of SrTiO3, respectively.
ature) correspond to different kinetics all leading
to the same end state, the surface structure with
lowest (surface) energy.
In order to test which of these two models can
best explain our experimental results, we checked
whether the obtained surface morphologies were
reversible under subsequent annealings. A set of
substrates was first annealed in 03 (as described
in Section 3.1) and then in 02 (as described in
Section 3.2). Another sample was first annealed in
O2 (as described in Section 3.2) and then in 03 (as
described in Section 3.1). In both cases, the surface
structure after two consecutive annealings was
always characterised by parallel steps with a step
height of one unit-cell of SrTiO3. The surface
displayed in Fig. 3, which was obtained after a
single annealing in 02 at 750°C, is therefore not a
reversible structure. As a consequence, model I
cannot explain our experimental data.
317
Model II, however, provides a consistent explanation of the results discussed so far. It should be
pointed out that the idea of a disordered SrWiO 3
surface before annealing is in agreement with the
results of Ref. [3]. Using i o n scattering spectroscopy, Kawasaki et al. [3] showed that for commercially available SrTiO3, the terminating atomic
layer was 5-25% SrO and 95-75% TiO2. Annealing
the substrates under appropriate conditions allows
the SrTiO3 surfaces to reconstruct and reach the
energetically most favourable surface structure. For
a vicinal (001) SrTiO3 substrate, this corresponds
to atomically flat terraces separated by smooth
steps of one unit-cell in height, running parallel to
each other. Our results are consistent with those
of Sum et al. [2,5], who showed that after 30 min
annealing in 0 2 at 800°C, the SrTiO3 surface was
characterised by parallel steps with rather curved
step ledges, while after 30 min at 900°C, these step
ledges were much straighter.
A comparison between Figs. 1 and 3 and the
results of Refs. [2,5] clearly shows that the time
scale for surface reconstruction is much slower for
the annealing process in Oz than for the annealing
process in 03. In particular, the observation of
steps of half a unit-cell in height (indicative for
different termination layers) strongly suggests that
either the annealing time is too short or the
annealing temperature too low.
It is likely that the lower background pressure
used for the 03 annealing and the reactiveness of
the 03 are responsible for a higher desorption rate
and a stronger interaction with the SrTiO3 surface,
respectively. In order to investigate the importance
of the low background pressure and the role of the
ozone, a substrate was heated to 750°C in U H V
(see Section 3.4). A comparison between Fig. 1
(annealing in 03) and Fig. 6 (annealing in UHV)
clearly shows that 03 plays an important role in
accelerating the reconstruction of the SrTiO 3
surface.
Because of the reactiveness of the 03 molecules,
the existence of a stronger interaction between
SrTiO3 and 03 than between SrTiO3 and 02 is in
principle not surprising: it is well known that much
lower temperatures are required for an 03 molecule
to give up an O atom than for an 02 molecule. Furthermore, high-resolution transmission
318
B. Stgiuble-Piimpin et al./Surface Science 369 (1996) 313-320
5.0 nN
2.5
.50
nH
0,0 n M
.2S
0
0.25
0o 50
0,75
IJN
Fig. 4. A F M image of the same sample as displayed in Fig. 3, showing two chains of holes both parallel to the (100) direction. The
black arrow points to a square-shaped hole. The open arrows show step ledges which are parallel to crystallographic directions.
,0 n K
.3.1111]
.0 n M
.2.(11)
,0
nM
-1.011
o
1 .oo
z.bo
3.bo °
Fig. 5. A F M image of an SrTiO3(00l) substrate which was first annealed in 03 followed by the deposition of Ten unit-cell layers of
SrTiO3. Note that the linear defect shown in this image is healing. Furthermore, the surface roughness has increased due to the
nucleation of SrTiO3 islands.
electron microscopy results have shown that the
terminating layer of substrates having the equilibrium surface structure is TiO2 [8]. One could
therefore speculate that during the annealing,
oxygen atoms react with the uppermost SrO
atomic layer, leading to its dissolution. For com-
B. Stgiuble-Pf#npin et al./Surfaee Science 369 (1996) 313-320
319
1,0
rim
.~,0 n H
~250
0
250
1.0 nN
....~ 0
500
nl<
Fig. 6. A F M image of an SrTiO3(001 ) substrate after annealing in U H V at 750°C for 20 min. The terraces are not atomically flat.
The open arrows show step ledges which are parallel to crystallographic directions.
pleteness, it is interesting to mention that SrTiO3
substrates with a n equilibrium surface structure
were obtained by Kawasaki et al. [3] without
annealing the substrates, but by etching them with
a pH-controlled N H 4 F - H F (BHF) solution. Using
ion scattering spectroscopy, Kawasaki et al. [3]
observed that the terminating atomic layer is
TiOz with a coverage factor of 100%, and concluded that the BHF solution selectively dissolves
the SrO atomic plane.
The remaining part of this discussion will concentrate on the origin of the defects observed on
most substrates. As described in Section 3, the
most striking features are one-dimensional arrays
of defects parallel to the (100) and (010~ directions. Because such defects were also observed after
the very slow annealing in O2 (see Section 3.2 it is
unlikely that they are the result of thermal strain
induced by a temperature gradient over the
samples. The fact that these linear defects follow a
crystallographic orientation indicates that they can
be related to dislocation lines already present in
the single-crystalline SrTiO3 before it was cut and
polished. In Ref. [5], Sum et al. suggest that lineshaped defects result from the relaxation of such
dislocations during the annealing of the substrate.
Such a relaxation can occur by glide of the dislocation. However, the chains of holes described in
Section 3.2 indicate that another relaxation mechanism might also play a role in the case of SrTiO3
substrates. Due to the stress field induced by
dislocations below the surface of the substrates,
favourable desorption sites for adatoms can align
along crystallographic axes. During the annealing,
material preferentially evaporates from these
desorption sites, leading to the formation of chains
of holes. As soon as enough material is evaporated,
the dislocations within the SrTiO3 could relax,
forming the one-dimensional arrays of defects
described in Section 3.1.
An attempt to smoothen the surface of the
substrates by first annealing them in 03 and then
depositing a few layers of SrTiO3 was only partially
successful. It was found that the SrTiO3 was preferentially deposited along steps, such as, for example,
those of the line-shaped defects. As a consequence,
these defects were observed to heal. However,
SrTiO3 islands were also found to nucleate on the
existing terraces, resulting in an increased roughness of the surface of the substrates.
320
B. Stiiuble-Piimpin et al./Surface Science 369 (1996) 313-320
5. Conclusions
This A F M study emphasises the importance of
an appropriate pretreatment of SrTiO3 substrates
in order to obtain smooth and well-defined surfaces. The results presented here strongly suggest
that, at high temperatures, the surface of the polished SrTiO3 substrate rearranges itself, either by
diffusion or desorption of atoms, until reaching
the energetically most favourable surface structure.
For vicinal (001) substrates, this equilibrium structure corresponds to series of smooth, one unit-cell
high steps running parallel to each other, with the
terrace width being defined by the magnitude of
the vicinal angle.
The time needed to reach this equifibrium structure was found to depend strongly on the annealing
conditions. For instance, it was observed that, for
the same temperature (750°C), annealing a substrate in 03 resulted in a smoother, better defined
surface than annealing (even for longer times) in
O2 or in UHV. In terms of rearrangement of the
SrTiO3 surface, annealing in 03 is more efficient
than annealing in Oz or in UHV. Based on our
results, it is likely that the reactiveness of 03 plays
an important role in accelerating the reconstruction of the SrTiO3 surface.
Finally, several types of defects were observed
on the substrates investigated, such as holes (diameter 10-20nm), (screw-) dislocations (density
6-t-1 x 10 7 c m - 2 ) and line-shaped defects parallel
to crystallographic orientations. The later were
found to heal after an O3 annealing followed by
the deposition of a few layers of SrTiO3. However,
after the deposition of SrTiO3, the surface roughness increased due to the nucleation of SrTiO3
islands.
Acknowledgements
We are very grateful to J. van Wingerden for
helpful and stimulating discussions leading to this
work. One of us (B.S.-P.) acknowledges financial
support from DIMES. This work was supported
by F O M and the Dutch National Research
P r o g r a m ( N O P ) for High-T~ Superconductors.
References
[ l'l N. Bickel, G. Schmidt, K. Heinz and K. Mtiller, Phys. Rev.
Lett. 62 (1989) 2009.
[2] R. Sum, R. LtRhi, H.P. Lang and H.-J. GOntherodt, Physica C 235-240 (1994) 621.
['3] M. Kawasaki, K. Takahashi, T. Maeda, R. Tsuchiya, M.
Shinohara, O. Ishiyama, T. Yonezawa, M. Yoshimoto and
H. Koinuma, Science 266 (1994) 1540.
['4] V. Ravikumar, D. Wolf and V.P. Dravid, Phys. Rev. Lett.
74 (1995) 960.
[5"] R. Sum, H.P. Lang and H.-J. Gtintherodt, Physica C 242
(1995) 174.
[6] Q.D. Jiang, D.-M. Smilgies, R. Feidenhans'l, M. Cardona
and J. Zegenhagen,Proc. EUCAS 95, Edinburgh, July 3-6,
1995, to be published.
[7] Q.D. Jiang and J. Zegenhagen, Surf. Sci., in press.
[8] J.G. Wen, C. Traeholt and H.W. Zandbergen, Physica C
205 (1993) 354.