Materials Sciences and Applicatio n, 2011, 2, 971-976
doi:10.4236/msa.2011.28130 Published Online August 2011 (
Copyright © 2011 SciRes. MSA
Nanostructures on Surface of SrTiO3 Single
Crystals Treated by Plasma
Nicolay Kulagin1,2*, Jablan Dojcilovic3, Ellen Hieckmann4
1Joint Venture “Firma SIFA”, Kharkov, Ukraine; 2Nikopol Institute Zaporozhye National University, Zaporozhye, Ukraine; 3 Physics
Department, University of Belgrade, Belgrade, Serbia; 4Institut für Angewandte Physik/Halbleiterphysik, Technische Universität Dres-
den, Dresden, Germany.
Email: *
Received April 6th, 2010; revised May 20th, 2010; accepted May 31st, 2011.
Modification of a surface of strontium titanate single crystals as pure as doped with Mn (Ni) or Nd (Sm) ions after
plasma treatment was studied by combination of scanning electron microscopy and atomic force microscopy techniques.
Valence shift method for characteristic X-ray lines was used for study of stoichiometry violation and oxidation state of
ions on the crystals surface after plasma treatment. One- and two-level ordered systems of unit crystallites sized of
about 10–7 - 10–10 m were discovered on samples surface after plasma treatment with energy density of about 5 - 20 (40)
J·cm–2. Oxidation state of Ti ions and stoichiometry of the surface changed essentially on background of high stability
of strontium ions valence.
Keywords: Ordered Nano-Scale Structure, Strontium Titanate, Plasma, Change In Structure And IonS Valence
1. Introduction
Strontium titanate crystals, SrTiO3, have been studied for
a long time [1-4], and interest to the crystal has recently
grown up after discovery of high-temperature supercon-
ductors and unique properties of the samples [5-7].
Selected experimental data concerning to relation of a
stoichiometric composition and properties of SrTiO3
(shortly, STO) single crystals were presented in [8-14].
Depending on the crystal composition, the cubic crystal-
line lattice parameter a (space group Pm¯3m) changes
from a = 3.9051 Å in bulk stoichiometric samples to a =
3.9102 Å in oxygen-deficient STO accompanied by a
variation of the dielectric constant
r from 360 to 200 and
less [7-14].
Variation in stoichiometric composition of the
Sr+2Ti+4O–23 crystals can results in a transition of a part of
Ti+4 ions to Ti+3 oxidation state [15-17] causing a sig-
nificant influence to dielectrical, optical and other prop-
erties of the samples. More than 20% of Ti+4 ions change
valence to Ti+3 state (electronic configuration 3d1) that
was observed in non-stoichiometric SrxTiyOz and TixOy
single crystals [17,18].
Recently, the area of investigation of bulk samples and
crystal surfaces under plasma-flow and ionizing irradia-
tion has been expanded. Experimental data presented for
materials after plasma treatment show the wide opportu
nity for modification of the surface of metals and semi-
conductors [19-21]. As a wide-band gap semiconductor,
strontium titanate could be a good object for that kind of
application, too. Preliminary results for STO surface pub-
lished in [22,23] confirm this assumption.
The paper presents analysis of the exposure of mid-
dle-energy hydrogen and helium plasma to the pure and
doped with Me and RE ions STO single crystals, we also
report on the effect of crystal growth conditions to modi-
fication of the sample surface, morphology and possible
change of the crystals properties under plasma treatment.
2. Samples and Experimental Procedures
Perfect pure (reference), standard (nominally un-doped)
and STO single crystals doped with selected iron (Me) or
rare earths (RE) ions, grown by the Verneuil technique
using different growth conditions, were chosen for the
Reference samples were grown by used of a fine and
high-purity mixture. For the samples permittivity
0 equals
(360 ± 5). Permittivity
0 for nominally un-doped samples
varied from 290 to 340. Optical absorption spectra of the
most samples consist of additional bands peaked at 430,
Nanostructures on Surface of SrTiO Single Crystals Treated by Plasma
972 3
520 and 620 nm. The refractive index changes from n =
2.4 to 2.2.
STO samples doped with Me ions (Mn and Ni) or RE
ones (Nd and Sm) were grown using technology de-
scribed in [6, 10, 16]. Magnitude of
0 for the Me doped
samples varies from 200 to 280. The studied permittivity
r for STO doped with RE ions was less and mentioned
r = ((80 – 160) ± 5). The main properties of the
examined samples are plotted in Table 1.
Atomic force microscopy (AFM-NP 206), scanning
electron microscopy (SEM-JEOL 840A, energy-dispersive
X ray line spectroscopy (EDX) techniques were used to
study the surface of the samples before and after plasma
treatment. Additionally a scanning electron microscope
with field emission gun (thermal Schottky field emitter),
type Zeiss ULTRA55 was used for the investigation of
the crystal surface after plasma treatment. All images
were obtained by an In-lense detector for secondary elec-
trons, applying an electron acceleration voltage of 15 kV.
3. Results and Discussion
3.1. Plasma Treatment
Two magneto-plasma compressors, MPC, were used for
plasma-treatment of the sample surfaces. A detailed de-
scription of experimental setups and methods can be
found in [19-21]. Briefly, the MPC is a single-stage
quasi-stationary plasma accelerator. The self-magnetic-
eld-sample shielding of the anode rods diminishes the
level of the erosion. Anode rods are connected by a car-
rier, which enables MPC flux magnitude in order of
magnitude of 1020 cm–2·s–1 and energy values of 5, 10, 15
and 20 J·cm–2 per impulse, respectively [19,20]. Time of
quasi-stable state plasma impulse varied at (5 - 50) × 10–6
s. The local temperature T on the surface of the samples
was of about 2500 K. According to data of ellipsometri-
cal measurements, thickness of the near-surface layer
involved by plasma-treatment was of about 10–6 m. and
less. SEM and AFM images of the crystal surface in ini-
tial state were similar to the pure and doped samples.
Exemplary, SEM images of the surface for pure and
selected doped STO samples (with Ni or Nd doping ions)
treated by both magneto-plasma accelerators mentioned
above plasma-flow with an energy density (dose) up 5 to
20 J·cm–2 are given in Figures 1-4. The images given in
Figures 1-2 were received after treatment at the “long
time” quasi-stationary impulse (τ ~ 150 mks). The image
shown in Figures 3-4 were observed with help of “short
time impulse” device (τ ~ 5 mks).
AFM images for pure and doped SrTiO3: Ni (Mn) and
SrTiO3: Nd samples including additional information
about the surface morphology after plasma treatment by
dose of about 10 - 20 J·cm–2 are given on Figures 5-8.
The images for other tested samples are similar.
Figure 1. SEM image of pure SrTiO3 surface after plasma
treatment by dose of about 5 J·cm2.
Figure 2. SEM image of SrTiO3: Nd surface after plasma
treatment by dose of about 10J·cm-2.
Figure 3. SEM image of SrTiO3: Nd surface after plasma
treatment by dose of about 15 J·cm2.
Systems of crystallites shown on Figures 1-8 for se-
lected samples area are very complex. For the pure sam-
ples and energy of about 10 J·cm–2, the system of unit
crystallites with size of about 10–6 m (Figure 5) on area.
with size of about several mkm were discovered. For
Copyright © 2011 SciRes. MSA
Nanostructures on Surface of SrTiO Single Crystals Treated by Plasma973
Figure 4. SEM image of SrTiO3: Nd surface after plasma
treatment by dose of about 20 J·cm2
Figure 5. AFM image of pure SiTiO3 surface after plasma
treatment of about 10 J·cm2.
Figure 6. AFM image of SiTiO3: Ni after plasma treatment
of about 15 J·cm-2.
Figure 7. AFM image of SrTiO3: Mn surface after plasma
treatment at dose 10
Figure 8. AFM image of SrTiO3: Nd surface after plasma
treatment at dose 20 J.cm2.
other samples and doze of plasma treatment we observe
quasi-ordered systems of nano-scale size crystallites
(Figures 3-4 and 6-7).The systems of crystallites with
size by about 10–9 m grown on surface of more “large”
crystallites with size by about 10–6 m (see Figure 8, too)
was observed, too. Height of the unit plasma generated
peaks on surface is less than 100 nm (see Figures 5 and
7) and less than 50 nm for two-levels nano-structures
(Figures 3, 4, 6 and 8). Similar changes in STO surface
morphology were discovered for other pure and doped
STO samples (doped with Me and RE ions) after
plasma-treatment using different doses (5
20 J·cm–2).
3.2. Oxidation State of the Ions
It is well known oxidation state of ions and stoihiometry
composition are strongly relates to selected properties of
compounds. It can be assumed, that plasma treatment
changes the stoichiometric relation in samples and oxida-
tion state of ions. For verification in the oxidation state of
Ti and Sr ions and the possible changing ratio of the
atomic concentrations CSr/CTi, the method of valence
shift of X ray lines, VSXRL, described in [10,16,22] was
The dependence of the energy of the characteristic
fluorescence X ray lines, XRL, EX
+n on oxidation state of
an ion is a basis of the method. The valence shift magni-
tude, VS of XRL,
EX, is determined as the difference in
the energy of XRL for the ion Me+n and for the same ion
Men±1 with other valence:
where EX
+n (EX
+n±1) is the energy of XRL of ion in
“+n”(or “+n±1”) oxidation state, respectively [9,23]. The
accurate ab initio theoretical expressions for the energy
of TiKα1 and SrKα1 XRL for the ions in a cluster [9,18,23]
were used for detailed analysis. Me+n Me+n±1 oxidation
state transition for Me or RE ions is accompanied by
change (negative for Me+n Me+n+1 transition and posi-
tive for Me+n Me+n – 1 one) of the energy of Kα1 or Lα1
XRL of about 1eV [9-10].
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Nanostructures on Surface of SrTiO3 Single Crystals Treated by Plasma
Copyright © 2011 SciRes. MSA
XRL intensity and VS for TiK
1 and SrK
1 lines for
the original and plasma-treated samples were detected by
means of VSRL technique with help of a microanalyzer
Camebax. A simplified scheme of the setup is shown in
Figure 9. An electron gun (1) with diameter less than
10–6 m with a current of about 10–10A bombards the sam-
ple (2). As a result fluorescence X ray radiation of ions is
excited in the sample decomposed by a crystal-analyzer
(3) and registered by a counter (4). X ray intensity is de-
termined for a reference and examined samples for each
step of the crystal-analyzer [18,24].
Figure 10. Profiles of Ti K
1 XRL: perfect sample (1),
SrTiO3: Ni (Co) (2), SrTiO3: Nd (Sm) before (3) and SrTiO3:
Nd after plasma treatment (4).
As an example, the experimental profiles of TiK
XRL are given on Figure 10. Figure 10 shows XRL
profiles 1, 2, 3 and 4 corresponding to the initial perfect
SrTiO3 crystal (1), SrTiO3: Ni (2) and SrTiO3: Nd (Sm)
(3) samples and SrTiO3: Nd one after plasma treatment at
dose of about 20 J·cm–2 (4), respectively. The shift of
1 XRL for doped samples to higher energy indicates
decreasing oxidation state of separate part of Ti ions [21
-23]. The TiK
1 line profiles studied for different samples
were fitted using Lorentz functions. Calculated data
simple relation [16, 22-23]:
() /
 (2)
where Eexp —is experimental VS value and Etheor = Ex+n
Ex+n+1 is the theoretical one.
Using the Ex. (2) relative concentration of Ti3+ ions,
C(Ti+3), for the doped samples changes from 19(5) %
(SrTiO3: Ni and Co sample) to 25(5) % (SrTiO3: Nd or Sm)
assuming the ideal stoichiometric composition Sr+2Ti+4 2
of the perfect SrTiO3 crystal. For the SrTiO3: Nd sample
after plasma treatment concentration of Ti3+ ions arises to
32% as compared with the pure sample.
for VS Eexp of TiK
1 line for as-prepared doped crystals
in comparison to perfect STO are the following: 0.35 (9)
eV and 0.46 (9) eV for SrTiO3: Ni (Co) and SrTiO3: Nd
(Sm), respectively. It should be noticed that for crystals
doped Ni (Co) on the one hand and Nd (Sm) on the other
hand results were similar. We had to determine the shift
of TiK
1 line in the sample SrTiO3: Nd, after plasma
treatment with energy of about 20 J·cm–2 which is E (Ti
1) = – (0.58 0.09) eV. There is important that relative
intensity of TiK
1 line grows together with increasing VS
XRL value. Relative concentration of Ti+n ions
During the study of the tested samples, the change of
the intensity of TiK
1 and SrK
1 lines and variation of ratio
κ = ISr/ITi of the XRL intensities was observed, too. The
κ-value varies from 0.76 (0.73) to 0.95 and 1.00 for tested
samples SrTiO3: Nd, SrTiO3: Ni and perfect STO, respec-
tively (see Table 1), in close agreement with data of the
crystal density measurements [11,12].
with changed valence can be estimated with the next
It should be noted, VS for SrK
1 line for any crystal was
less than experimental error. Intensity of SrK
1 line for
sample after plasma treatment decreases in opposite to
1. Detailed investigations of TiK
1 and SrK
intensity must be a task of additional measurements.
Results of lattice refinement presented in [21] confirm
the preservation of the cubic symmetry of the SrTiO3: Nd
lattice after 10, 20 and 40 J·cm–2 plasma-flow treatment
with decreasing unit cell parameter to a = 3.8922(6) Å.
3.3. Discussion
Based on the first experimentally obtained data a conclu-
sion can be drawn that the surface of STO pure as well as
doped with Me and RE changed significantly by plasma
irradiation. Complex systems of crystallites with typical
size of about 10–6 - 10–9 m appear on the surface of each
tested sample. However, as can be seen by comparison of
SEM and AFM pictures for selected samples (Figures
1-8), the geometry and size of crystallites depend on the
Figure 9. Scheme of X ray microanalyzer: electron gun (1),
sample (2), crystal-analyzer (3), detector (4).
Nanostructures on Surface of SrTiO Single Crystals Treated by Plasma 975
Table 1. Selected properties of the examined SrTiO3 single crystals.
Crystal\Properties Cimp, [wt%] 0 CSr/CTi, [a.u.] C(Ti3+), [%]
SrTiO3 pure 10-4 360 5 1.0 -
SrTiO3: Mn (3 0.1)·10-2 220 5 0.86 0.03 15 3
SrTiO3: Ni (4 0.1)·10-2 230 5 0.85 0.03 12 3
SrTiO3: Sm (6 0.2)·10-2 200 5 0.82 0.03 20 3
SrTiO3: Nd (6 0.2)·10-2 190 5 0.80 0.03 21 3
kind of the sample, growth technology, concentration of
impurities and the magnitude of permittivity of the sam-
ples initial state (see Table 1). Transition of certain part
of Ti4+ ions to Ti3+ oxidation state for the crystals doped
with Me or RE ions is accompanied by the change in
optical and crystallographic parameters. Appearance of
Ti3+ ions with 3d1 configuration leads to appearance of a
3d electronic level in forbidden energy zone of the crys-
tal. As a result, conductivity of the sample increases in
opposite to decreasing refractive index. We can propose
that geometry and size of crystallites formed at the crys-
tal-plate surface under plasma-flow could be managed,
probably, by the changing conductivity (Ti+3 ions con-
Plasma treatment results in change of the surface
stoichiometric composition. Additionally, as was shown
in [22] the lattice parameter of the polycrystalline mate-
rial decreases with growing plasma energy density (and
increasing Ti+3 ions concentration). And at the first ap-
proximation, the plasma treatment may be considered as
re-crystallization procedure of the surface. A study of the
properties of the unit crystallites, system of pyramid-like
crystallites and two-levels nano-structures on the surface
of single-crystalline strontium titanate after plasma-
treatment there is the task for a future investigation, too.
4. Conclusions
Following conclusions can be drawn analyzing AFM,
SEM, and X ray valence shift data for SrTiO3 single
crystal surface after plasma effects. Plasma-flow treat-
ment with energy density up 5 to 20 (40) J·cm–2, creates
structures of nano-scale size crystallites on the surface of
the STO crystals as pure as doped with Me or RE ions.
Appearance of polycrystalline layer indicates that tem-
perature over plasma is more than point of crystalliza-
Plasma treatment effects in appearance of systems of
unit crystallites with size in order up 10–6 to 10–9 m de-
pending on crystal conductivity, time and energy of
plasma impulse. For certain conditions the area of cre-
ated crystallites may be called as “quasi-ordered system”.
For selected conditions we discover appearance of
two-level systems of crystallites when unit one with size
of about 10–10 m grown on ordered structures with size of
about 10–6 m. Density of crystallites on ordered areas is
about of 1016 - 1018 m–2. Change in stoichiometry of the
STO surface and well-known data of properties variation
for the bulk crystals opens the wide area for design of
directly changed properties of STO samples.
5. Acknowlegements
Authors thank to Dr. M. Mitrović from Faculty of Phys-
ics, University of Belgrade, Dr. M. Mitrić from Vinča
Institute in Belgrade and Dr. I. Garkusha from IPP NTU
KhFTI NAS Ukraine for help with plasma-irradiation
and study of the samples.
This work is supported by Ukrainian-Germany joint
project, Ukr 07/006 - M/83-2008. The Ukrainian co-auth-
or are indebted for support of the Ministry of Education
and Science of Ukraine, Project 231-1.
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