Z-cut (C-plane), X-cut (M-plane), Y-cut (A-plane) LiNbO3 and LiTaO3 substrates were annealed at high temperature over 1000 °C in air. On all annealed substrates, atomic scale step structures with uniform height were observed, which demonstrates that the surfaces of the substrates become atomically smooth. The step height on Z-cut substrates was 0.25 ± 0.02 nm, which was well accordance with the distance between oxygen layers along the c-axis of the hexagonal unit cell of LiNbO3 and LiTaO3 crystals. The step heights on X-cut and on Y-cut substrates were 0.45 ± 0.04 nm and 0.50 ± 0.05 nm, respectively, which corresponded well with the distances between A-planes and M-planes in the unit cell.
©2002 Optical Society of America
LiNbO3 and LiTaO3 has been widely used as the most important crystals for acoustic and optical applications due to their large electro-, acousto-, and nonlinear optical coefficients. Whatever the applications discussed above are in bulk form or thin film form, optical waveguides have been fabricated for use in integrated devices. In the waveguides, optical loss due to surface scattering resulted from rough surface are of major concern . Thus, for the waveguide with low optical loss, atomically smooth surface on the crystals is most desired.
LiNbO3 and LiTaO3 have also been gaining increasing attention as important and versatile substrates for the realization of integrated optoelectronics devices. On LiNbO3 substrates, various potential applications such as the epitaxial growth of ZnO for light source in conjunction with high speed optical modulators , the epitaxial growth of diamond-like carbon films for high SAW velocity , and the fabrication of multi-layered thin film optical waveguides with a step index profile  have been widely investigated. LiTaO3 substrate has been utilized for the epitaxial growth of ZnO for high efficiency SAW device owing to high electromechanical coupling factor and the epitaxial growth of high quality LiNbO3 film  due to its lattice constant (0.3% in a-axis) and the thermal expansion coefficient very close to that of LiNbO3.
It is generally well known that atomic scale flatness of the substrate surface leads to enhance the epitaxial growth of film, resulting in sharp interface, atomically smooth film surface and good device quality. Until now, for several ceramic substrates including SrTiO3  and a–Al2O3 , atomically smooth surface has been obtained by annealing or chemical etching. But, LiNbO3 and LiTaO3 substrates are used as it is mechanically mirror polished. A mechanically polished surface has a lot of protrusions and depressions formed as a result of polishing procedure. In order to prepare the low optical loss waveguide and high quality epitaxial films, it is very important to realize the LiNbO3 and LiTaO3 substrates with atomically smooth surface.
In addition, the realization of atomically flat surface enables to predict the topmost layer through the interpretation between the height of atomic step formed on the surface and the crystal structure. This surface characterization must improve not only the control and the understanding of the epitaxial growth on the surface, but also the understanding of the operation of acoustic and electro-optic devices.
In this report, we demonstrate the preparation of atomically smooth surface on LiNbO3 and LiTaO3 substrates and predict the topmost layer on the surfaces.
As-supplied (mirror polished) commercial Z-, X-, Y-cut LiNbO3 and LiTaO3 substrates were annealed at the temperature over 1000 °C in air. It is well known that in the temperature range between 550 °C and 900 °C lithium triniobate (LiNb3O8) normally precipitates on the substrate of LiNbO3 with congruent composition irrespective of the oxygen, N2, or Ar atmosphere. The LiNb3O8 transforms reversibly to LiNbO3 at the temperature above 900 °C, for which LiNb3O8 phase vanishes and the surface becomes a single phase LiNbO3 . Therefore, the annealing of substrates was performed at the temperature over 1000 °C in air to prevent the precipitation of LiNb3O8 on the surface of LiNbO3 substrate. The annealing conditions of LiTaO3 substrates were also the same as those of LiNbO3 substrate. The surface morphology of the annealed substrates was examined by atomic force microscopy (AFM) using a SPI-3700 system of Seiko Corp. The AFM observation was performed at room temperature in air using a microfabricated Si3N4 cantilever in a contact mode.
3. Results and discussion
Figure 1 shows the AFM image and the cross sectional profile of the surface of as-supplied Z-cut LiNbO3 substrate. The as-supplied commercial substrates are typically prepared by mechanochemical polishing using an alkaline solution containing colloidal silica particles. This polishing procedure leaves damages such as scratches and corrugations on atomic scale on the topmost surface of substrate. As shown in Fig. 1, the surface of the as-supplied substrate is rugged and many irregular small corrugations ranging from 0.3 nm to 1.4 nm are observed. The existence of such small corrugations implies that a lot of atomic planes different from the original plane exist on the surface. Accordingly it is very difficult to analyze the atomic species and their alignments on the surface of the substrate. The peak to valley roughness on the surface is estimated to be 1.4 nm over a 2 μm2 area, which is similar to that observed on X- and Y-cut substrates.
Figure 2 represents the surface images and the cross sectional profiles on (a) Z-cut, (b) X-cut and (c) Y-cut LiNbO3 substrate annealed at 1000 °C for 5 hrs in air. The roughness of the surface drastically improved to atomically flat type. Irregular small corrugations disappeared, while atomic steps and atomically flat terraces appeared. Atomic step structure is unavoidable because the geometrical misorientation less than 0.5 ° on the present as-polished substrate is inevitable due to cutting machinery precision. The annealed substrates were as ultrasmooth as the atomic steps could be observed. The atomic step height on annealed Z-cut substrate was as low as 0.25 ± 0.02 nm, which was uniform on whole substrate. For an accurate estimation of step height, the heights of 30 steps were measured and averaged. The average height was 0.25 nm and the standard deviation was 0.02 nm. This means that the surface roughness on the annealed substrate was improved five times compared with that on the as-supplied substrate. Also, no precipitates were observed on the annealed substrate, which implies that no LiNb3O8 were precipitated on the substrate during the annealing procedure. Also on annealed X- and Y-cut substrates atomic step structure and atomically flat terraces were obtained as shown in Fig. 2(b) and (c). The atomic step heights were 0.45 ± 0.04 nm and 0.50 ± 0.05 nm, respectively, which indicates that the surface became atomically smoother by three times than that on the as-supplied substrate. No precipitates could be also observed on annealed X- and Y-cut substrate.
Then LiTaO3 substrates which are as important as LiNbO3 in optoelectronics were annealed at high temperature over 1000 °C in air. After annealing, Z-, X-, Y-cut substrates became atomically flat easily revealing the atomic step structure as shown in Fig. 3. The step heights were 0.25 ± 0.02 nm, 0.45 ± 0.04 nm, and 0.50 ± 0.05 nm on Z-, X- and Y-cut LiTaO3 substrates, respectively.
Explanations for how the surfaces are smoothed with forming a terrace-and-step structure are based on the faceting process [9,10]. Faceting is a process by which an unstable surface decomposes into two or more planar surfaces found on the equilibrium crystal shape in order to reduce the overall surface free energy. The driving force for faceting is the reduction in surface free energy caused by the formation of the equilibrium crystal surface with lower surface energy. Faceting is introduced by annealing or chemical etching, which is dependent on the material and the surface orientation. In case of thermal faceting induced by annealing, a thermodynamically unstable surface is converted to the equilibrium crystal surfaces by rearrangement of surface atoms. Also, either hill-and-valley or terrace-and-step surface structure is developed from faceting, depending on the orientation of the original surface. For a vicinal surface, on which the surface normal deviates slightly from that of a surface observed in the equilibrium state, a terrace-and-step structure is produced predominantly. As the angle between the original surface normal and that found on the equilibrium form increases, faceting results in a hill-and-valley morphology.
Commercial substrates are typically prepared by cutting and polishing a particular surface orientation. As shown in Fig. 1, the surface of the commercial LiNbO3 and LiTaO3 substrates has irregularly a lot of protrusions and depressions with the height in the range of 0.3~1.4nm, which indicates the existence of the unstable surface atomic planes with the orientations different from the particular surface orientation. When the substrate is annealed at high temperature, a lot of the unstable surface atomic planes are decomposed into the stable surfaces with the particular orientation via the faceting process in order to diminish the overall surface free energy, which gives rise to the improvement of surface smoothness. On the other hand, the surface on commercial substrate has the inevitable misorientation angles smaller than 0.5° inclined from a particular surface orientation due to a cutting machinery precision, which implies that the surface will be a vicinal surface. Accordingly, faceting resulted in terrace-and-step structures on the surfaces of LiNbO3 and LiTaO3 substrates.
From the experimental results, it is found that the atomically rough surfaces of commercial LiNbO3 and LiTaO3 substrates can be smoothed on atomic scale via the thermal faceting arose from a simple annealing in air and a terrace-and-step morphology is produced on the surfaces regardless of the surface orientation of X-, Y-, Z-cut.
The topmost layer on the annealed substrates can be also elucidated via the relationship between characteristic height of atomic step on the ultrasmooth terrace and crystallographic orientation on the substrate, which is of great importance in the viewpoint of not only the control and the understanding of the epitaxial growth on the surface, but also the understanding of the operation of acoustic and electro-optic devices. LiNbO3 and LiTaO3 have the same crystal structure and their lattice parameters are (in the hexagonal cell): a = 0.5149 nm, c = 1.3862 nm for LiNbO3; a = 0.5154 nm, c = 1.3783 nm for LiTaO3. The structure consists of planar sheet of oxygen atoms, the spacing of which is 0.231 nm along c-axis, in hexagonal unit cell configuration.
Figure 4(a) represents the packed sequence of atoms along c-axis in LiNbO3 and LiTaO3 crystals. The step height observed on the annealed Z-cut LiNbO3 and LiTaO3 substrates was 0.25 ± 0.02 nm. This height corresponds with the distance between oxygen triple layers, which suggests that the topmost layer of the annealed Z-cut substrates constituted of oxygen layers. The topmost layers of annealed X- and Y-cut substrates is studied using the schematic diagram of the ideal arrangement of the atoms on the (0001) basal plane in unit cell displayed in Fig. 4(b). The step heights observed on annealed X- and Y-cut substrates were 0.45 ± 0.04 nm and 0.50 ± 0.05 nm, which are in good agreement with the distance between A-planes in X-cut substrate and M-planes in Y-cut substrate, respectively. This means that the topmost layer on these substrates is composed of Li, Nb(or Ta), and oxygen.
Atomically smooth surfaces with atomic step structure were obtained on LiNbO3 and LiTaO3 substrates by high temperature annealing over 1000 °C in air. From the step heights it could be supposed the atoms constituting the topmost layer on the surfaces. Using the substrates with this ultrasmooth surface, it is expected the development of high quality optoelectronic devices based on LiNbO3 and LiTaO3 substrates. Furthermore, the substrates with straight step structure make it possible the formation of nanowire along the step edge and the position control of nanodot at the step edge.
This work was financially supported by Research Center for Electronic Ceramics (RCEC) of Dong-Eui University funded by Korea Science and Engineering Foundation (KOSEF), Ministry of Science and Technology (MOST) and the Busan Metropolitan City Government.
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