Epitaxial growth of LiNbO3 on αAl2O3(0001)

Thin Solid Films 336 (1998) 163±167

Epitaxial growth of LiNbO3 on a Al2O3(0001) Franck Veignant a,*, Madeleine Gandais a, Pascal Aubert1 b, Guy Garry b a

Laboratoire de MineÂralogie-Cristallographie de Paris, UniversiteÂs Paris VI et Paris VII, CNRS UMR 7590, 4 place Jussieu, 75252 Paris Cedex 05, France b Laboratoire Central de Recherche, Thomson CSF, Domaine de Corbeville, 91404 Orsay Cedex, France

Abstract LiNbO3 ®lms have been grown on a Al2O3(0001) by pulsed laser deposition. Their structural aspects have been studied by using transmission electron microscopy. Different stages of the ®lm formation have been observed: nucleation-growth of isolated pyramids limited by the {0112} faces, coalescence of the pyramids giving rise to ¯at aggregates and formation of continuous ®lms. Mis®t dislocations relaxing the mis®t strain at the deposition temperature have been observed. However, strain states have been found. They are dependent on the ®lm morphology: inhomogeneous elastic strain ®eld in isolated pyramids, mechanical twinning and cracks in continuous ®lms. It is shown that in both cases, the strain state corresponds to the relaxation of stresses occurring at the ®lm/substrate interface during the cooling stage from deposition to room temperature, due to a large difference in thermal expansion coef®cients of LiNbO3 and a Al2O3. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Pulsed laser deposition; Transmission electron microscopy; LiNbO3; a Al2O3; Thin ®lm growth; Strains

1. Introduction Lithium niobate is a very attractive material for the realization of acoustic and optical devices because of its piezoelectricity, its high second harmonic generation and electrooptic coef®cients. The need for high-integrated devices leads to an increased interest in lithium niobate thin ®lm elaboration. Sapphire is a good candidate as a substrate for two reasons. It is favourable for the epitaxial growth of lithium niobate: both materials have close structures and crystallize in the rhombohedral system; their parameters at Ê room temperature are (in the hexagonal cell): a ˆ 5:149 A Ê Ê and c ˆ 13:862 A for LiNbO3; a ˆ 4:758 A and c ˆ 12:991 Ê for aAl2 O3 . The lattice mismatch is 8% in the (0001) A plane. Furthermore, its high acoustic wave velocity, and its low refractive indices relative to those of lithium niobate makes it advantageous for the fabrication of surface acoustic wave devices and optical waveguides, respectively. Epitaxial growth of lithium niobate on sapphire has been already successfully performed by several methods: sputtering [1], MOCVD [2], sol-gel methods [3] and pulsed laser deposition (PLD) [4]. Nevertheless, the ®lms present a number of defects and further studies are necessary to improve the quality of the devices. The present paper reports on the TEM study of LiNbO3 grown by PLD. It is

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especially centered around two aspects: the early stages of growth and the stress relaxation in this system. 2. Experimental procedure The ®lms were deposited by PLD using a 5-Hz pulsed frequency laser operating at 248 nm. The oxygen pressure in the chamber was 1.33 mbars, and the target-substrate distance of 45 mm. The substrate was a mechanically polished sapphire platelet; it was heated at 7508C during the deposition. Two types of samples have been observed: (1) thin and discontinuous ®lms obtained at low deposition times (t ˆ 20 s and t ˆ 1 min); 2) thick and continuous ®lms obtained at much longer time (t . 10 min). The latter have already been characterized by X-ray diffraction and Rutherford backscattering [5,6]. The deposition rate has been deduced to be 10 nm/min from the thickness measurement of these ®lms. Conventional transmission electron microscopy (CTEM) and high resolution transmission electron microscopy (HRTEM) were performed by using, respectively, a JEOL 2000 EX microscope and a TOPCON 002B microscope giving point to point resolution of 0.18 nm, both operating at 200 kV. Specimens parallel to the ®lm were prepared for plane view observation. They were obtained by grinding, dimpling and Ar 1 ion milling the substrate. Cross sectional specimens were also prepared. They were obtained by

0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII S0 040-6090(98)012 22-X

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F. Veignant et al. / Thin Solid Films 336 (1998) 163±167

Fig. 1. Selected area diffraction pattern in [0001] zone axis of the 1-min deposited ®lm.

elaborating a sandwich of two ®lm±substrate pieces glued together with the ®lms facing each other, then cutting slices and ®nally grinding and ion milling the slices. Ion milling was performed on a GATAN PIPS operating at 5 kV. 3. Nucleation-growth of lithium niobate ®lms The epitaxial relationship has been determined for thick ®lms in previous works [5]: (0001)®lm//(0001)sub and [1120]®lm//[1120]sub. It has been con®rmed on thin ®lms by selected area diffraction pattern (Fig. 1). Two stages of the ®lm formation can be distinguished: nucleation-growth of three-dimensional islands, and island coalescence. For each stage, the determination of the 3D morphology of the ®lms has been done by performing dark ®eld images of plane view specimens far from Bragg orientation in so-called weak beam conditions (Fig. 2a,b). In these conditions, a narrow system of thickness fringes appears at the edges of the overgrowth, whose periodicity depends on the material, the diffracted beam used, and the deviation parameter to Bragg condition. Their measurement allows thus an accurate determination of the shape and the thickness of the overgrowth. The ®lm deposited at t ˆ 20 s represents the early nucleation-growth stage (Fig. 2a). It is constituted of pyramidal islands whose sides lie along k1120l. Their dimensions are well de®ned with a mean side length of 105 nm and a mean height of 40 nm. The pyramid faces have been deduced to be {0112} [7]. The ®lm deposited at 1 min corresponds to the coalescence stage (Fig. 2b). A complete change in the ®lm morphology is found. There is a trend to the vanishing of {0112} faces and the formation of the (0001) face. The

Fig. 2. Weak beam dark ®eld image in plane view using 1120 beam: (a) 20s deposited ®lm, (b) 1-min deposited ®lm.

mean height of coalesced grains has been measured to be 22 ^ 5 nm, which is signi®cantly smaller than the height of isolated islands. This result means that, at the coalescence, already deposited atoms migrate from the top towards the

Fig. 3. HRTEM image of a cross-section specimen viewed along k1120l zone axis. Mis®t dislocations are indicated by arrows.

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Ê instead of 13.86 A Ê , and indicates that temperature: 13.81 A the ®lm is in compression along the c direction [2,5], and then in extension in the (0001) plane. The hypothesis is that the LiNbO3 ®lm grows in a relaxed state, and acquires tensile stress in (0001) during the cooling stage from deposition to room temperature due to the large difference in the thermal expansion coef®cients of the ®lm and the substrate. 4.2. Thermal stress relaxation

Fig. 4. Dark-®eld image using 1120 in plane view: (a) left parts of the islands are in Bragg position, (b) right parts of the islands are in Bragg position.

island junctions, which are the most favourable sites for the reduction of surface energy. At the end of the coalescence stage, a fully continuous ®lm is obtained.

4. Stress in deposited ®lms 4.1. LiNbO3/Al2O3 interface Fig. 3 shows a HRTEM image of the interface viewed along the k1120l direction. It can be seen that the mis®t strain is relaxed at the interface by mis®t dislocations whose cores extend over two or three lattice planes. A Burger circuit has been drawn across the interface in order to measure the component bp of the Burger vector lying in the image plane and bp has been found to be 1/2k1010l. The theoretical spacing D between dislocations needed for the complete relaxation of the mis®t strain is D ˆ bp =d, where d is the mis®t parameter in the plane (0001). When D is 5.1 nm, d is then 8.8%. This value nearly corresponds to the mis®t parameter at the deposition temperature. Indeed, at Ê 7508C, lithium niobate and sapphire a parameters are 5.22 A Ê [9], respectively, which means that d ˆ 9%, [8] and 4.79 A whereas at room temperature d ˆ 8%. This result is consistent with the value of the c parameter measured by X-ray diffraction which is smaller than in the bulk crystal at room

The thermal stress energy stored in the ®lm being thickness dependent, stress effects are expected to be different in thin ®lms than in continuous thick ®lms. In the early islands, thermal stresses lead to an inhomogeneous elastic strain ®eld which is shown in dark ®eld images by the asymmetry of the thickness fringes to the top of the island. The effect is evidenced at best close to Bragg orientation. Fig. 4a,b shows the images performed in these conditions using a 1120 beam. In Fig. 4a, the left parts of the islands are in bright contrast whereas the right parts are at extinction. This feature means that (1120) planes are in Bragg orientation in the left parts and not in the right ones. To bring the right parts into Bragg orientation, it is necessary to tilt the sample by a small angle (Fig. 4b). The set of images described above reveals that the (1120) diffracting planes are not normal to (0001) as they should be in either a bulk crystal or in a ®lm uniformly extended. They are inclined with different inclination in the left and the right part of the pyramids. Knowing the sense of the tilt angle, it is possible to deduce the sense of inclination of the planes. It is found that they are inclined towards the center, at the top of the pyramids. Such a strain ®eld involves a decrease of the a parameter from the ®lm/substrate interface, where a is larger than in the bulk, to the top of the pyramid. The relaxation of the interface strain is thus allowed by the presence of the pyramids free surfaces. In continuous thicker ®lms, stress relaxation occurs by twinning and cracking. Cracking occurs when the ®lms reach a thickness of approximately 100 nm. At this thickness, the width of the cracks is smaller than 10 nm, and the ®lms remain strained (Fig. 5a). At the opposite, they have a width of several tens of nanometers in ®lms thicker than approximately 180 nm (Fig. 5b). In this case cracking is ef®cient enough to fully relax the lithium niobate ®lm, whose parameters reach their bulk values. Mechanical twins characterization has been made by examining both cross section and plane view specimens. HRTEM image in cross section clearly shows the twin/ matrix system and allows the determination of the twin law (Fig. 6): the twin lattice is deduced from the matrix lattice by mirror symmetry relative to the (0112) plane. In a plane view specimen, twins are evidenced in dark ®eld images performed with the 0114 beam belonging to the twin lattice exclusively (Fig. 7): twins appear as lamellae having a mean width of 60 nm, and a length ranging between 100 and 400 nm. It is important to note that twins are always

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Fig. 7. Dark ®eld image using 0114 twin beam showing twin lamellae in bright contrast.

mechanical twinning [10]; (2) twins play a role in crack formation [10,11]. 5. Conclusion

Fig. 5. Cracks: (a) bright ®eld image of a 110-nm thick ®lm in plan view, (b) optical microscopy image of a ®lm thicker than 180 nm.

bordered by cracks, which suggests that they are favourable sites for crack initiation. Our observations are consistent with the results of deformation experiments performed on bulk lithium niobate. These works show that (1) in the range of temperature below 7508C, dislocation motion cannot be activated, and the main process of stress relaxation is the

The observation of the early stages of growth indicates that the ®lm formation results from nucleation-growth of 3D islands and then coalescence of these islands. Atomic surface migration during the coalescence stage and its importance for the modi®cation of the ®lm morphology have been evidenced. Concerning the LiNbO3 interface, it has been shown that a network of mis®t dislocations has been formed during the growth stage at 7508C and was ef®cient enough to relax the mis®t strain at this stage. Nevertheless, interface induced stresses have been produced during the cooling stage from deposition to room temperature. It has been shown that they are relaxed by inhomogeneous elastic strain in discontinuous thin ®lms, and by mechanical twinning and cracking in continuous thick ®lms. References

Fig. 6. HRTEM image of a cross-section specimen in k1120l zone axis showing the twin/matrix interface.

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