LiNbO3 thin films with a thickness of 200 nm were deposited onto Al2O3 substrate by RF-magnetron
sputtering technique without intentional substrate heating. The results demonstrate that post-growth
infrared pulsed light annealing of the amorphous LiNbO3 films leads to the formation of two phases,
LiNbO3 and LiNb3O8. After annealing at temperatures of 700 to 800 °C, the percentage of the nonferroelectric
phase LiNb3O8 was minimal. The surface composition of the films annealed at different
temperatures was examined by X-ray photoelectron spectroscopy. Piezoresponse force microscopy was used
to study both the vertical and the lateral polarization and to visualize the piezoelectric inactivity of
LiNb3O8 grains. A comparison of the results of PFM and XPS measurements revealed that there is a
correlation between the fraction of the piezoelectric phase and the film composition: At an annealing
temperature higher than 850 °C, the atomic ratio of lithium to niobium decreases compared to the initial
value along with a decrease of the fraction of the piezoelectric phase

Ilina, T.S.

Kiselev, D.A.

Kubasov, I.V.

Malinkovich, M.D.

Parkhomenko, Yu.N.

Savchenko, A.G.

Senatulin, B.R.

Skryleva, E.A.

Suchaneck, G.

Zhukov, R.N.

Electronic Sumy State University Institutional Repository

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 10 No 2, 02009(4pp) (2018) Том 10 № 2, 02009(4cc) (2018)   2077-6772/2018/10(2)02009(4) 02009-1  2018 Sumy State University Formation of the Microcrystalline Structure in LiNbO3 Thin Films by Pulsed Light Annealing  R.N. Zhukov1, T.S. Ilina1, E.A. Skryleva1, B.R. Senatulin1, I.V. Kubasov1, D.A. Kiselev1,*, G. Suchaneck2, M.D. Malinkovich1, Yu.N. Parkhomenko1, A.G. Savchenko1  1 National University of Science and Technology "MISiS", 4, Leninskiy Prosp., 119049 Moscow, Russian Federation 2 TU Dresden, Solid State Electronics Laboratory, 01062 Dresden, Germany   (Received 11 December 2017; revised manuscript received 28 April 2018; published online 29 April 2018)  LiNbO3 thin films with a thickness of 200 nm were deposited onto Al2O3 substrate by RF-magnetron sputtering technique without intentional substrate heating. The results demonstrate that post-growth infrared pulsed light annealing of the amorphous LiNbO3 films leads to the formation of two phases, LiNbO3 and LiNb3O8. After annealing at temperatures of 700 to 800 °C, the percentage of the non-ferroelectric phase LiNb3O8 was minimal. The surface composition of the films annealed at different temperatures was examined by X-ray photoelectron spectroscopy. Piezoresponse force microscopy was used to study both the vertical and the lateral polarization and to visualize the piezoelectric inactivity of LiNb3O8 grains. A comparison of the results of PFM and XPS measurements revealed that there is a correlation between the fraction of the piezoelectric phase and the film composition: At an annealing temperature higher than 850 °C, the atomic ratio of lithium to niobium decreases compared to the initial value along with a decrease of the fraction of the piezoelectric phase.  Keywords: Lithium niobate, Li/Nb ratio, rf-magnetron sputtering, Post-growth infrared rapid annealing, Piezoresponse force microscopy, X-ray photoelectron spectroscopy.  DOI: 10.21272/jnep.10(2).02009 PACS numbers: 77.80.Dj, 68.55. – a, 77.55.H –                                                           * dm.kiselev@gmail.com 1. INTRODUCTION  Ferroelectric materials integrated in thin film structures have become the basis for significant improvements of functional device parameters as well as for creation of a number of innovative microelectronic devices [1]. Lithium niobate (LiNbO3 or LN) is currently one of the most widely used ferroelectric materials because of its characteristic piezoelectric, electro-optical and pyroelectric properties [2]. Due to the corresponding physical effects, LiNbO3 offers a large area of applications such as waveguides, surface acoustic wave devices and nonvolatile memory elements. The combination of functional properties of bulk materials and nanoscale samples allows creating structures with specified characteristics. Thin ferroelectric layers embedded into heterostructures are of interest both to fundamental research and to electronic applications. The properties of film structures strongly depend on their fabrication, electrode materials and substrate [3, 4]. One serious problem specific to LiNbO3 is the precipitation of a second phase, lithium triniobates (LiNb3O8), in the crystal. This is especially pertinent when considering thin films where the crystal growth temperature is lower than the one of bulk crystals. Since the crystal structure of LiNb3O8 is centrosymmetric (C2v5), it possesses neither electro-optic nor ferroelectric properties. Therefore, the volume fraction of LiNb3O8 should be as small as possible and that of the stoichiometric metaniobate phase, LiNbO3, should be maximized [5]. Polarization ordering and switching dynamics are among the most important issues in the physics of ferroelectrics. They should be addressed before considering any practical applications. At present, piezoresponse force microscopy (PFM) has become a standard technique for the investigation of ferroelectric domains on the nanometer scale [6] and also for the identification of individual grains of non-ferroelectric nature in ferroelectric materials [7].  Synthesis of LiNbO3 films with an optimum composition requires control of the atomic Li/Nb ratio since lithium deficiency is characteristic for the films containing the LiNb3O8 phase. For these purpose we evaluate the Li/Nb ratio by means of a X-ray photoelectron spectroscopy (XPS) technique which was developed earlier for congruent lithium niobate (CLN) crystals and which is described in detail in a previous paper [8]. The aim of this work is to study the formation of the piezoelectric phase in LiNbO3 thin films during their infrared (IR) pulsed light annealing and to determine the temperature range in which the fraction of the active phase of LiNbO3 will be the maximum. Composition of the thin film, surface morphology and static domain structure of the LiNbO3 thin films after post-deposition IR pulsed light annealing were investigated at various temperature by PFM and XPS.  2. EXPERIMENTAL DETAILS  Al2O3 c-cut plates covered with a 150-nm-thick ITO electrode layer and a 40-nm-thick SiO2 buffer layer were used as substrates. LiNbO3 thin films were fabricated using a two-step method. At the first step, a 200-nm-thick amorphous layers was deposited using RF magnetron sputtering of a single-crystalline LiNbO3 target in Ar/O  60/40 atmosphere without intentionally heating the substrate. Secondly, the amorphous layers  R.N. ZHUKOV, T.S. ILINA, ET AL. J. NANO- ELECTRON. PHYS. 10, 02009 (2018)   02009-2 were subjected to high temperature annealing for the formation of the LiNbO3 ferroelectric structure. IR pulsed light annealing was performed at 600 °C to 900 °C for 2 min in an infrared rapid heating system ULVAC VHC-P610 with a heating rate of 5 °C/s. The final heterostructure is depicted in Fig. 1.    Fig. 1 – Schematic design of the lithium niobate thin film structure fabricated on Al2O3 substrate  The surface morphology and the static domain structure of the LiNbO3 thin films were characterized by piezoresponse force microscopy using a commercial scanning probe microscopes MFP-3D (Asylum Research, USA) with Ti/Ir coated conductive probes (Asyelec-02, Asylum Research, USA). Vertical PFM (VPFM) and lateral (LPFM) images of the samples were recorded by applying an AC voltage of VAC  3 V with a frequency of 150 kHz to the cantilever. Surface chemical composition analysis was performed by X-ray photoelectron spectroscopy. XPS measurements were performed on a VersaProbe II spectrometer (ULVAC-PHI, inc., Japan, USA) equipped with a 20 kV Ar Gas Claster Ion Source (GCIB). Monochromatic AlKα radiation (hν  1486.6 eV) operated at 50 W was used to excite photoemission. A focused X-ray beam diameter of 200 m was scanning an analysis area of 0.6  0.3 mm2. A dual beam charge compensation system was used to provide charge neutralization at the LN film surface. High resolution spectra were recorded using a pass energy of 23.5 eV at a 0.125 eV/step for C1s, Nb3d, O1s spectra and a pass energy of 29.5 eV at 0.2 eV/step for the 50 to 70 eV region (Li1s and Nb4s spectra). The binding energies (BE) were referenced by positioning the Nb 3d5/2 peaks at a binding energy of 207.1 eV. Analysis was carried out on the as-deposited surface of the samples and after Ar GCIB irradiation. The latter was performed to remove the adsorbed contaminants.  3. RESULTS  The surface morphology of the LiNbO3 films annealed at various temperatures is shown in Fig. 2. The topography images revealed an increasing with increasing annealing temperature average lateral grain size of the LiNbO3 films. There is experimental evidence that surface roughness and grain size were interrelated, i.e., the average grain size was smaller at lower roughness values. The highest grain size and roughness values were observed for LiNbO3 films annealed at 900 C. The temperature dependence of the average grain size and random mean square (Rms) roughness of the LiNbO3 thin films are shown in Fig. 3. After post-deposition annealing LiNbO3 at 900 °C, a hexagonal crystalline structure is formed, which we       Fig. 2 – Topography images of LiNbO3/SiO2/ITO/Al2O3 heterostructures annealing at 650 C (a), 750 C (b), 850 C (c) and 900 C (d)  600 650 700 750 800 850 900010203040Roughness, nmAnnealing temperature, oC050100150200250300Grain size, nm  Fig. 3 – RMS roughness and average grain size for LiNbO3 thin film as a function of annealing temperature  associate with LiNb3O8 phase. Changes of the surface morphology with annealing temperature indicate that the phase conversion proceeds from the near-surface region of the film [5]. VPFM and LPFM images obtained simultaneously with the topography image are shown in Fig. 4. As known, the PFM modes can be used to characterize the polarization states in the ferroelectric materials [9]. The “bright” and “dark” regions on the VPFM images correspond to LiNbO3 grains with the out-of-plane polarization component oriented towards the lower interface and towards the surface, respectively (Fig. 4, left column). In LPFM images (Fig. 4, right column), we visualize the piezoelectric response corresponding to the polarization direction in the plane of the film, which corresponds to the piezoelectric coefficient d15. A large VFPM and LPFM signal was observed in samples LiNbO3 annealed up to 850 C. At the same time, the background contrast corresponds to “zero” piezoelectric response for both VPFM and LPFM images. It is attributed to the presence of non-ferroelectric particles, for example, the secondary phase LiNb3O8, observed previously in sol-gel derived LiNbO3 films [7].  Fig. 5 shows the piezo-histogram of a LiNbO3 film annealed at 850 C acquired from VPFM images in Fig. 4. Since piezo-histograms are statistical (b) (c) (d) (а)  FORMATION OF THE MICROCRYSTALLINE STRUCTURE… J. NANO- ELECTRON. PHYS. 10, 02009 (2018)   02009-3 distributions of the piezoelectric signal related to the domain configuration in a ferroelectric, the vertical tip displacement characterizing out-of-plane polarization indicates an effective longitudinal piezoelectric coefficient d33. The peaks observed in this distribution curve are associated with the most probable domain configuration while the peak width is a measure of a number of domain states [10].   Vertical PFM image Lateral PFM image 650 C   750 C   850 C   900 C    Fig. 4 – Vertical and lateral PFM images of the LiNbO3 thin film annealed at different temperature  The piezoresponse distribution function (Fig. 5) exhibits three peaks corresponding to certain grain contrasts. This allows determination of the percentage of the grains in which the polarization vector is oriented perpendicular to the substrate plane (“up” or “down”). As it is evidenced by the distribution, part of the grains have mainly dark contrast (region #1), which corresponds to the direction of vector Р from the substrate to the film surface. Region #2 corresponds to grains with zero piezoelectric response, i.e., they are piezoelectrically inactive. Grains where the polarization vector is directed from the free film surface to the substrate are marked as region #3 in the piezoelectric response distribution and the VPFM image. The resulting area of regions 1 and 3 is ~50 %. Therefore, during film annealing at 850 C, the z axis is oriented parallel to the normal to surface in almost 1/2 of the grains. The same procedure was done for all annealed LiNbO3 thin films. Figure 6 shows the diagram representing the ratio of piezoelectrically active grains to inactive ones (LiNbO3/LiNb3O8) calculated from VPFM histogram as a function annealing temperature.  -0,4 -0,2 0,0 0,2 0,402040608010012014031Intensity, arb. unitsVPFM signal, mV2  Fig. 5 – Distribution of the vertical piezoresponse for LiNbO3 thin film annealed at 850 C  600 650 700 750 800 850 9000255075100Grain fraction, %Annealing temperature, oCLiNbO3piezoelectrically inactive grains  Fig. 6 – Distribution of the grains fraction in LiNbO3 thin film annealed at various temperatures  The results demonstrate that the percentage of the non-ferroelectric phase LiNb3O8 is minimal after annealing at temperatures of 700 to 800 °C. Three samples of the LiNbO3 films annealed at 600, 750 and 900 °C were investigated by XPS. The peculiarity of the quantitative assessment of LiNbO3 is the very low intensity of the Li1s line and its small width which prohibits the use of survey spectra for evaluating lithium concentration using the elemental relative sensitivity factors. To assess the lithium and niobium atomic ratio we have recorded high resolution spectrum (50 to 70 eV) which contained the Li1s and Nb4s lines, measured the ratios of the integral Li1s and Nb4s intensities and calculated the atomic Li/Nb ratios by an approximate method described in detail in [8]. It is well known that the Ar-ion sputtering results in chemical damage of LiNbO3 [11]. The chemical reduction processes including the loss of oxygen and lithium and the creation of lower-valence niobium ions Nb4+ and Nb3+ are clearly visible in the Nb3d spectra as shoulder peaks at the lower binding energy side [11]. To avoid this negative phenomenon, we used an Ar gas cluster ion gun GCIB 2500Ar for surface cleaning. An acceleration voltage of 10 kV at a 2  2 mm2 raster provides complete removal of adsorbed carbon. This is indicated in the survey spectrum after 10 min Ar cluster bombardment shown in Figure 7. Such a treatment did not result in a change of the surface chemistry as seen from the absence of shoulder peaks on the Nb3d spectrum, Fig. 7 inset.  R.N. ZHUKOV, T.S. ILINA, ET AL. J. NANO- ELECTRON. PHYS. 10, 02009 (2018)   02009-4   Fig. 7 – Survey spectrum of the LiNbO3 film annealed at 750 C after 10 min GCIB 2500Ar irradiation. Inset shows a high resolution Nb3d spectrum  The Nb3d spin-orbit doublet of BE (3d5/2) of 207.1 eV and the energy split of 2.72 eV indicate the presence of only Nb5+ ions. The binding energies Li1s, Nb4s and O1s were 55.0 ± 0.1 eV, 60.2 ± 0.1 eV and 530.2 ± 0.1 eV, respectively, the same as in the samples of CLN crystals [8]. An additional O1s peak at 532.0-532.5 eV from nonstructural oxygen with fractions of 20 to 30 % was observed on the surfaces of as-deposited samples. It disappeared after cleaning by GCIB. The Li/Nb ratios determined from the Li1s and Nb4s spectral regions, acquired after surface cleaning, were 0.85, 0.7 and 0.5 for LN films annealed at 600, 750 and 900 °C, respectively (Fig. 8). These values characterize the increase of Li deficiency with temperature. Assuming the coexistence only of two phases (LiNbO3 and LiNb3O8), we can estimate the LiNbO3 fraction from the atomic Li/Nb ratios 1.0 amounting for LiNbO3 and 0.33 for LiNb3O8, respectively. The thus obtained values were 55 % and 25 % for LN films annealed at 750 °C and 900 °C, correspondingly, in satisfactory agreement with the data in Figure 6. The maximum Li/Nb ratio was obtained for sample annealed at 600 °C, but the crystal structure was not yet formed in this sample. In general, our results show that rapid pulse light   Fig. 8 – Li1s and Nb4s spectra of the LiNbO3 films annealed at 600, 750 and 900 °C  annealing of LiNbO3 heterostructures for 1-2 minutes at temperatures of 700 °C to 800 °C leads to similar results as long-term furnace annealing for several hours reported in Ref. [5].  4. CONCLUSIONS  In summary, LiNbO3 ferroelectric films were grown onto Al2O3 substrate by RF-magnetron sputtering. A post-growth IR pulsed light annealing of the as-grown amorphous  LiNbO3 films in wide temperature range of 600-900 °C leads to the formation of two phases, LiNbO3 and LiNb3O8. From PFM and XPS data, we obtained that the percentage of the non-ferroelectric phase LiNb3O8 is minimal after annealing at temperatures of 700 to 800 °C.   ACKNOWLEDGEMENT  The studies were performed on the equipment of Center for Shared Use “Materials Science and Metallurgy” at the National University of Science and Technology “MISiS”. The research was financially supported by the Ministry of Education and Science of the Russian Federation (Federal Targeted Programme for Research and Development in Priority Areas of Development of the Russian Scientific and Technological Complex for 2014-2020, Project ID RFMEFI58716X0035). REFERENCES  1. J.F. Scott, Ferroelectric Memories (Berlin: Springer: 2000). 2. T. Volk, M. Wöhlecke, Lithium Niobate, Defects, Photorefraction and Ferroelectric Switching (Berlin: Springer: 2009). 3. D.A. Kiselev, R.N. Zhukov, A.S. BykovM.I. Voronova, K.D. Shcherbachev, M.D. Malinkovich, Yu.N. Parkhomenko, Inorg. Mater. 50 No 4, 419 (2014). 4. D.A. Kiselev, R.N. Zhukov, S.V. Ksenich I.V. Kubasov, A.A. Temirov, N.G. Timushkin, A.S. Bykov, M.D. Malinkovich, V.V. Shvartsman, D.C. Lupascu, Yu.N. Parkhomenko, J. Surf. Investig-X-Ray 10, 742 (2016). 5. H. Akazawa, M. Shimada, J. Mater. Res. 22 No 6, 1726 (2007). 6. S.V. Kalinin, A.N. Morozovska, L.Q. Chen, B.J. Rodriguez, Rep. Prog. Phys. 73 No 5, 056502 (2010). 7. F. Johann, T. Jungk, S. Lisinski, A. Hoffmann, L. Ratke, E. Soergel, Appl. Phys. Lett. 95, 202901 (2009). 8. E.A. Skryleva, I.V. Kubasov, Ph.V. Kiryukhantsev-Korneev, B.R. Senatulin, R.N. Zhukov, K.V. Zakutailov, M.D. Malinkovich, Yu.N. Parkhomenko, Appl. Surf. Sci. 389, 387 (2016). 9. A. Wu, P.M. Vilarinho, V.V. Shvartsman, G. Suchaneck, A.L. Kholkin, Nanotechnology 16, 2587 (2005). 10. M. Melo, E.B. Araújo, V.V. Shvartsman, V.Ya. Shur, A.L. Kholkin, J. Appl. Phys. 120, 054101 (2016). 11. X. Bai, Y. Shuai, Ch. Gong, Ch. Wu, W. Luo, R. Böttger, Sh. Zhou, W. Zhang, Appl. Surf. Sci. 434, 669 (2018) 

Formation of the Microcrystalline Structure in LiNbO3 Thin Films by Pulsed Light Annealing

SocioEconomic Challenges Journal (Sumy State University)

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ 
Vol. 10 No 2, 02009(4pp) (2018) Том 10 № 2, 02009(4cc) (2018) 
 
 
2077-6772/2018/10(2)02009(4) 02009-1  2018 Sumy State University 
Formation of the Microcrystalline Structure in LiNbO3 Thin Films by Pulsed Light 
Annealing 
 
R.N. Zhukov1, T.S. Ilina1, E.A. Skryleva1, B.R. Senatulin1, I.V. Kubasov1, D.A. Kiselev1,*, G. Suchaneck2, 
M.D. Malinkovich1, Yu.N. Parkhomenko1, A.G. Savchenko1 
 
1 National University of Science and Technology "MISiS", 4, Leninskiy Prosp., 119049 Moscow, Russian Federation 
2 TU Dresden, Solid State Electronics Laboratory, 01062 Dresden, Germany  
 
(Received 11 December 2017; revised manuscript received 28 April 2018; published online 29 April 2018) 
 
LiNbO3 thin films with a thickness of 200 nm were deposited onto Al2O3 substrate by RF-magnetron 
sputtering technique without intentional substrate heating. The results demonstrate that post-growth 
infrared pulsed light annealing of the amorphous LiNbO3 films leads to the formation of two phases, 
LiNbO3 and LiNb3O8. After annealing at temperatures of 700 to 800 °C, the percentage of the non-
ferroelectric phase LiNb3O8 was minimal. The surface composition of the films annealed at different 
temperatures was examined by X-ray photoelectron spectroscopy. Piezoresponse force microscopy was used 
to study both the vertical and the lateral polarization and to visualize the piezoelectric inactivity of 
LiNb3O8 grains. A comparison of the results of PFM and XPS measurements revealed that there is a 
correlation between the fraction of the piezoelectric phase and the film composition: At an annealing 
temperature higher than 850 °C, the atomic ratio of lithium to niobium decreases compared to the initial 
value along with a decrease of the fraction of the piezoelectric phase. 
 
Keywords: Lithium niobate, Li/Nb ratio, rf-magnetron sputtering, Post-growth infrared rapid annealing, 
Piezoresponse force microscopy, X-ray photoelectron spectroscopy. 
 
DOI: 10.21272/jnep.10(2).02009 PACS numbers: 77.80.Dj, 68.55. – a, 77.55.H – 
 
 
                                                        
* dm.kiselev@gmail.com 
1. INTRODUCTION 
 
Ferroelectric materials integrated in thin film 
structures have become the basis for significant 
improvements of functional device parameters as well 
as for creation of a number of innovative 
microelectronic devices [1]. Lithium niobate (LiNbO3 or 
LN) is currently one of the most widely used 
ferroelectric materials because of its characteristic 
piezoelectric, electro-optical and pyroelectric properties 
[2]. Due to the corresponding physical effects, LiNbO3 
offers a large area of applications such as waveguides, 
surface acoustic wave devices and nonvolatile memory 
elements. The combination of functional properties of 
bulk materials and nanoscale samples allows creating 
structures with specified characteristics. Thin 
ferroelectric layers embedded into heterostructures are 
of interest both to fundamental research and to 
electronic applications. The properties of film 
structures strongly depend on their fabrication, 
electrode materials and substrate [3, 4]. 
One serious problem specific to LiNbO3 is the 
precipitation of a second phase, lithium triniobates 
(LiNb3O8), in the crystal. This is especially pertinent 
when considering thin films where the crystal growth 
temperature is lower than the one of bulk crystals. 
Since the crystal structure of LiNb3O8 is 
centrosymmetric (C2v5), it possesses neither electro-
optic nor ferroelectric properties. Therefore, the volume 
fraction of LiNb3O8 should be as small as possible and 
that of the stoichiometric metaniobate phase, LiNbO3, 
should be maximized [5]. 
Polarization ordering and switching dynamics are 
among the most important issues in the physics of 
ferroelectrics. They should be addressed before 
considering any practical applications. At present, 
piezoresponse force microscopy (PFM) has become a 
standard technique for the investigation of ferroelectric 
domains on the nanometer scale [6] and also for the 
identification of individual grains of non-ferroelectric 
nature in ferroelectric materials [7].  
Synthesis of LiNbO3 films with an optimum 
composition requires control of the atomic Li/Nb ratio 
since lithium deficiency is characteristic for the films 
containing the LiNb3O8 phase. For these purpose we 
evaluate the Li/Nb ratio by means of a X-ray 
photoelectron spectroscopy (XPS) technique which was 
developed earlier for congruent lithium niobate (CLN) 
crystals and which is described in detail in a previous 
paper [8]. 
The aim of this work is to study the formation of the 
piezoelectric phase in LiNbO3 thin films during their 
infrared (IR) pulsed light annealing and to determine 
the temperature range in which the fraction of the 
active phase of LiNbO3 will be the maximum. 
Composition of the thin film, surface morphology and 
static domain structure of the LiNbO3 thin films after 
post-deposition IR pulsed light annealing were 
investigated at various temperature by PFM and XPS. 
 
2. EXPERIMENTAL DETAILS 
 
Al2O3 c-cut plates covered with a 150-nm-thick ITO 
electrode layer and a 40-nm-thick SiO2 buffer layer were 
used as substrates. LiNbO3 thin films were fabricated 
using a two-step method. At the first step, a 200-nm-
thick amorphous layers was deposited using RF 
magnetron sputtering of a single-crystalline LiNbO3 
target in Ar/O  60/40 atmosphere without intentionally 
heating the substrate. Secondly, the amorphous layers 
 R.N. ZHUKOV, T.S. ILINA, ET AL. J. NANO- ELECTRON. PHYS. 10, 02009 (2018) 
 
 
02009-2 
were subjected to high temperature annealing for the 
formation of the LiNbO3 ferroelectric structure. IR 
pulsed light annealing was performed at 600 °C to 
900 °C for 2 min in an infrared rapid heating system 
ULVAC VHC-P610 with a heating rate of 5 °C/s. The 
final heterostructure is depicted in Fig. 1. 
 
 
 
Fig. 1 – Schematic design of the lithium niobate thin 
film structure fabricated on Al2O3 substrate 
 
The surface morphology and the static domain 
structure of the LiNbO3 thin films were characterized 
by piezoresponse force microscopy using a commercial 
scanning probe microscopes MFP-3D (Asylum Research, 
USA) with Ti/Ir coated conductive probes (Asyelec-02, 
Asylum Research, USA). Vertical PFM (VPFM) and 
lateral (LPFM) images of the samples were recorded by 
applying an AC voltage of VAC  3 V with a frequency of 
150 kHz to the cantilever. 
Surface chemical composition analysis was 
performed by X-ray photoelectron spectroscopy. XPS 
measurements were performed on a VersaProbe II 
spectrometer (ULVAC-PHI, inc., Japan, USA) equipped 
with a 20 kV Ar Gas Claster Ion Source (GCIB). 
Monochromatic AlKα radiation (hν  1486.6 eV) 
operated at 50 W was used to excite photoemission. A 
focused X-ray beam diameter of 200 m was scanning 
an analysis area of 0.6  0.3 mm2. A dual beam charge 
compensation system was used to provide charge 
neutralization at the LN film surface. High resolution 
spectra were recorded using a pass energy of 23.5 eV at 
a 0.125 eV/step for C1s, Nb3d, O1s spectra and a pass 
energy of 29.5 eV at 0.2 eV/step for the 50 to 70 eV 
region (Li1s and Nb4s spectra). The binding energies 
(BE) were referenced by positioning the Nb 3d5/2 peaks 
at a binding energy of 207.1 eV. Analysis was carried 
out on the as-deposited surface of the samples and after 
Ar GCIB irradiation. The latter was performed to 
remove the adsorbed contaminants. 
 
3. RESULTS 
 
The surface morphology of the LiNbO3 films annealed at 
various temperatures is shown in Fig. 2. The 
topography images revealed an increasing with 
increasing annealing temperature average lateral grain 
size of the LiNbO3 films. There is experimental evidence 
that surface roughness and grain size were interrelated, 
i.e., the average grain size was smaller at lower 
roughness values. 
The highest grain size and roughness values were 
observed for LiNbO3 films annealed at 900 C. The 
temperature dependence of the average grain size and 
random mean square (Rms) roughness of the LiNbO3 
thin films are shown in Fig. 3. 
After post-deposition annealing LiNbO3 at 900 °C, a 
hexagonal crystalline structure is formed, which we  
  
  
 
Fig. 2 – Topography images of LiNbO3/SiO2/ITO/Al2O3 
heterostructures annealing at 650 C (a), 750 C (b), 850 C 
(c) and 900 C (d) 
 
600 650 700 750 800 850 900
0
10
20
30
40
R
o
u
g
h
n
es
s,
 n
m
Annealing temperature, 
o
C
0
50
100
150
200
250
300
G
ra
in
 s
iz
e,
 n
m
 
 
Fig. 3 – RMS roughness and average grain size for 
LiNbO3 thin film as a function of annealing temperature 
 
associate with LiNb3O8 phase. Changes of the surface 
morphology with annealing temperature indicate that 
the phase conversion proceeds from the near-surface 
region of the film [5]. 
VPFM and LPFM images obtained simultaneously 
with the topography image are shown in Fig. 4. As 
known, the PFM modes can be used to characterize the 
polarization states in the ferroelectric materials [9]. The 
“bright” and “dark” regions on the VPFM images 
correspond to LiNbO3 grains with the out-of-plane 
polarization component oriented towards the lower 
interface and towards the surface, respectively (Fig. 4, 
left column). In LPFM images (Fig. 4, right column), we 
visualize the piezoelectric response corresponding to the 
polarization direction in the plane of the film, which 
corresponds to the piezoelectric coefficient d15. A large 
VFPM and LPFM signal was observed in samples 
LiNbO3 annealed up to 850 C. At the same time, the 
background contrast corresponds to “zero” piezoelectric 
response for both VPFM and LPFM images. It is 
attributed to the presence of non-ferroelectric particles, 
for example, the secondary phase LiNb3O8, observed 
previously in sol-gel derived LiNbO3 films [7].  
Fig. 5 shows the piezo-histogram of a LiNbO3 film 
annealed at 850 C acquired from VPFM images in 
Fig. 4. Since piezo-histograms are statistical 
(b) 
(c) (d) 
(а) 
 FORMATION OF THE MICROCRYSTALLINE STRUCTURE… J. NANO- ELECTRON. PHYS. 10, 02009 (2018) 
 
 
02009-3 
distributions of the piezoelectric signal related to the 
domain configuration in a ferroelectric, the vertical tip 
displacement characterizing out-of-plane polarization 
indicates an effective longitudinal piezoelectric 
coefficient d33. The peaks observed in this distribution 
curve are associated with the most probable domain 
configuration while the peak width is a measure of a 
number of domain states [10]. 
 
 Vertical PFM image Lateral PFM image 
6
5
0
 
C
 
  
7
5
0
 
C
 
  
8
5
0
 
C
 
  
9
0
0
 
C
 
  
 
Fig. 4 – Vertical and lateral PFM images of the LiNbO3 thin 
film annealed at different temperature 
 
The piezoresponse distribution function (Fig. 5) 
exhibits three peaks corresponding to certain grain 
contrasts. This allows determination of the percentage 
of the grains in which the polarization vector is oriented 
perpendicular to the substrate plane (“up” or “down”). 
As it is evidenced by the distribution, part of the grains 
have mainly dark contrast (region #1), which 
corresponds to the direction of vector Р from the 
substrate to the film surface. Region #2 corresponds to 
grains with zero piezoelectric response, i.e., they are 
piezoelectrically inactive. Grains where the polarization 
vector is directed from the free film surface to the 
substrate are marked as region #3 in the piezoelectric 
response distribution and the VPFM image. 
The resulting area of regions 1 and 3 is ~50 %. 
Therefore, during film annealing at 850 C, the z axis is 
oriented parallel to the normal to surface in almost 1/2 
of the grains. The same procedure was done for all 
annealed LiNbO3 thin films. Figure 6 shows the 
diagram representing the ratio of piezoelectrically active 
grains to inactive ones (LiNbO3/LiNb3O8) calculated 
from VPFM histogram as a function annealing 
temperature. 
 
-0,4 -0,2 0,0 0,2 0,4
0
20
40
60
80
100
120
140
31I
n
te
n
si
ty
, 
ar
b
. 
u
n
it
s
VPFM signal, mV
2
 
 
Fig. 5 – Distribution of the vertical piezoresponse for 
LiNbO3 thin film annealed at 850 C 
 
600 650 700 750 800 850 900
0
25
50
75
100
G
ra
in
 f
ra
ct
io
n
, 
%
Annealing temperature, 
o
C
LiNbO
3
piezoelectrically inactive grains
 
 
Fig. 6 – Distribution of the grains fraction in LiNbO3 thin 
film annealed at various temperatures 
 
The results demonstrate that the percentage of the 
non-ferroelectric phase LiNb3O8 is minimal after 
annealing at temperatures of 700 to 800 °C. 
Three samples of the LiNbO3 films annealed at 600, 
750 and 900 °C were investigated by XPS. The 
peculiarity of the quantitative assessment of LiNbO3 is 
the very low intensity of the Li1s line and its small 
width which prohibits the use of survey spectra for 
evaluating lithium concentration using the elemental 
relative sensitivity factors. To assess the lithium and 
niobium atomic ratio we have recorded high resolution 
spectrum (50 to 70 eV) which contained the Li1s and 
Nb4s lines, measured the ratios of the integral Li1s and 
Nb4s intensities and calculated the atomic Li/Nb ratios 
by an approximate method described in detail in [8]. 
It is well known that the Ar-ion sputtering results in 
chemical damage of LiNbO3 [11]. The chemical 
reduction processes including the loss of oxygen and 
lithium and the creation of lower-valence niobium ions 
Nb4+ and Nb3+ are clearly visible in the Nb3d spectra as 
shoulder peaks at the lower binding energy side [11]. To 
avoid this negative phenomenon, we used an Ar gas 
cluster ion gun GCIB 2500Ar for surface cleaning. An 
acceleration voltage of 10 kV at a 2  2 mm2 raster 
provides complete removal of adsorbed carbon. This is 
indicated in the survey spectrum after 10 min Ar cluster 
bombardment shown in Figure 7. Such a treatment did 
not result in a change of the surface chemistry as seen 
from the absence of shoulder peaks on the Nb3d 
spectrum, Fig. 7 inset. 
 R.N. ZHUKOV, T.S. ILINA, ET AL. J. NANO- ELECTRON. PHYS. 10, 02009 (2018) 
 
 
02009-4 
 
 
Fig. 7 – Survey spectrum of the LiNbO3 film annealed 
at 750 C after 10 min GCIB 2500Ar irradiation. Inset 
shows a high resolution Nb3d spectrum 
 
The Nb3d spin-orbit doublet of BE (3d5/2) of 207.1 
eV and the energy split of 2.72 eV indicate the presence 
of only Nb5+ ions. The binding energies Li1s, Nb4s and 
O1s were 55.0 ± 0.1 eV, 60.2 ± 0.1 eV and 530.2 ± 0.1 
eV, respectively, the same as in the samples of CLN 
crystals [8]. 
An additional O1s peak at 532.0-532.5 eV from 
nonstructural oxygen with fractions of 20 to 30 % was 
observed on the surfaces of as-deposited samples. It 
disappeared after cleaning by GCIB. 
The Li/Nb ratios determined from the Li1s and Nb4s 
spectral regions, acquired after surface cleaning, were 
0.85, 0.7 and 0.5 for LN films annealed at 600, 750 and 
900 °C, respectively (Fig. 8). 
These values characterize the increase of Li deficiency 
with temperature. Assuming the coexistence only of 
two phases (LiNbO3 and LiNb3O8), we can estimate the 
LiNbO3 fraction from the atomic Li/Nb ratios 1.0 
amounting for LiNbO3 and 0.33 for LiNb3O8, 
respectively. The thus obtained values were 55 % and 
25 % for LN films annealed at 750 °C and 900 °C, 
correspondingly, in satisfactory agreement with the 
data in Figure 6. The maximum Li/Nb ratio was 
obtained for sample annealed at 600 °C, but the crystal 
structure was not yet formed in this sample. 
In general, our results show that rapid pulse light 
 
 
Fig. 8 – Li1s and Nb4s spectra of the LiNbO3 films 
annealed at 600, 750 and 900 °C 
 
annealing of LiNbO3 heterostructures for 1-2 minutes 
at temperatures of 700 °C to 800 °C leads to similar 
results as long-term furnace annealing for several hours 
reported in Ref. [5]. 
 
4. CONCLUSIONS 
 
In summary, LiNbO3 ferroelectric films were grown 
onto Al2O3 substrate by RF-magnetron sputtering. A post-
growth IR pulsed light annealing of the as-grown 
amorphous  LiNbO3 films in wide temperature range of 
600-900 °C leads to the formation of two phases, LiNbO3 
and LiNb3O8. From PFM and XPS data, we obtained that 
the percentage of the non-ferroelectric phase LiNb3O8 is 
minimal after annealing at temperatures of 700 to 800 °C.  
 
ACKNOWLEDGEMENT 
 
The studies were performed on the equipment of 
Center for Shared Use “Materials Science and 
Metallurgy” at the National University of Science and 
Technology “MISiS”. The research was financially 
supported by the Ministry of Education and Science of 
the Russian Federation (Federal Targeted Programme 
for Research and Development in Priority Areas of 
Development of the Russian Scientific and 
Technological Complex for 2014-2020, Project ID 
RFMEFI58716X0035).
 
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Formation of the Microcrystalline Structure in LiNbO3 Thin Films by Pulsed Light Annealing

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