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Epitaxial growth of BaTiO3 thin films at a low temperature under 300 °C with temperature-controlled BaTiO3 buffer layer
ARTICLE IN PRESS
Journal of Crystal Growth 294 (2006) 236–242 www.elsevier.com/locate/jcrysgro
Epitaxial growth of BaTiO3 thin films at a low temperature under 300 1C with temperature-controlled BaTiO3 buffer layer J. Zhu, Z. Liang, Y.R. Li, Y. Zhang, X.H. Wei School of Microelectronics and Solid-state electronics, University of electronic science and technology of China, Chengdu, 610054, PR China Received 8 December 2005; received in revised form 18 May 2006; accepted 21 May 2006 Communicated by J.B. Mullin Available online 17 August 2006
Abstract The epitaxial growth of BaTiO3 (BTO) thin films on LaAlO3 (0 0 1) substrates by laser molecular beam epitaxy was studied using realtime reflection-high-energy-electron-diffraction (RHEED). Atomic force microscopy (AFM) was employed to investigate the surface topography. When the BTO thin films were directly grown at 320 1C with a thickness of above two monolayers (ML), the RHEED patterns vanished rapidly, indicating the failure of crystallinity. However, the growth of eight ML BTO layers can be controlled by decreasing temperature from 600 1C step-by-step to 300 1C. The evolution of diffraction streaks revealed a scaling behavior with temperature and film thickness, demonstrating the evolution of the quasi-two-dimensional BTO islands growth and distribution. Furthermore, subsequently deposited 30 ML BTO thin films with a good crystallinity were obtained on the eight ML BTO buffer layers at a low temperature of 280 1C. It is found that the nucleation centers formed during decreasing temperature play a key role in determining the subsequent crystallization of BTO thin films at a lower temperature. r 2006 Elsevier B.V. All rights reserved. PACS: 79.60.Jv; 66.30.Ny; 61.14.Hg Keywords: A1. Atomic force microscopy; A1. Buffer layer; A1. Reflection high-energy electron diffraction; A3. Laser molecular beam epitaxy
1. Introduction Thin-film ferroelectric materials have been widely investigated as materials for the microelectronic devices such as dynamic random access memory, bypass capacitors, infrared detectors, and tunable-microwave devices [1]. Recently, the enhancement of ferroelectricity in BaTiO3 (BTO) thin films [2,3] leads to both of scientific and technological significance to seek an extensive understanding of the growth of these functional films. In general, the deposition temperature should be high enough to offer sufficient atomistic surface mobility, especially, to satisfy the thermodynamic and kinetic requirement for crystallization and structural stability under the optimized condition during the growth. For perovskite oxides thin films grown on LaAlO3 (LAO) substrates, the growth Corresponding author. Tel.: +86 2883202140; fax: +86 2883202569.
E-mail address: [email protected] (J. Zhu). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.05.059
temperature is always above 500 1C [4–6]. However, in modern microelectronic integration technology, the growth temperature should be low enough to prevent diffusion and chemical reaction between films and substrates occurring at high temperature, which may result in a degradation of properties of devices. Therefore, the thin films of perovskite oxides must be fabricated at low temperatures to satisfy the requirement for device applications with an atomic flat surface [7–9]. Comparing with the growth at usually accepted substrate temperatures, the atomistic properties such as hopping barriers for isolated adatoms and adatom mobility would show great differences when the growth process approaches the limit of low temperatures [10]. However, there has been little progress made to understand how to fabricate the BTO thin films under 300 1C. Reflection high-energy electron diffraction (RHEED) is one of the most useful techniques to study epitaxial growth of thin films. The real-time study of the characteristics of the diffraction patterns during the growth can provide
ARTICLE IN PRESS J. Zhu et al. / Journal of Crystal Growth 294 (2006) 236–242
useful information of the microstructure and morphology. By tracking the relative ‘‘streakiness’’ of the diffraction pattern, it is possible to qualitatively study the evolution of the atomic scale roughness [11]. In this paper, we present an in situ RHEED study of BTO thin films grown on LAO (0 0 1) substrates. The direct growth of BTO at 320 1C resulted in a failure of crystallinity which may shows a grain structure with a preferred orientation without evidence of amorphous material. However, 30 monolayer (ML) BTO crystalline thin films at a low temperature of 280 1C could be successfully fabricated on a eight ML BTO buffer layer which was firstly deposited on LAO (0 0 1) substrates by decreasing temperature from 600 1C step-bystep to 300 1C. Furthermore, it was found that the initial density of nucleation centers formed at higher temperatures plays a key role in determining the subsequent crystallization of BTO thin films at low temperatures. This method has a valuable potential to provide an approach in fabricating high quality perovskite oxide films at low temperatures. 2. Experiment BTO thin films were deposited by laser molecular beam epitaxy system, which is equipped with the combination of a turbo-molecular pump and a titanium-sublimation pump to achieve a base pressure of 2.0 106 Pa. A l PHYSIK KrF excimer laser (248 nm wavelength and 30 ns pulse duration) with a repetition rate of 3 Hz was used as a laser source for the ablation of the polycrystal BTO target. The laser beam was focused on the target surface with a fluence of about 3 J/cm2. The target was rotated during the ablation process to reduce possible nonuniform erosion, which has 55 mm distance away from the substrates. In order to minimize the effect caused by miscut angle (0.210.31) and surface roughness of 10 mm 10 mm LAO substrates, the combination of a chemical and thermal pretreatment was performed. Firstly, LAO substrates were etched in buffered hydrofluoric acid (BHF). The solution is the equivalent of 1:5 by volume of 49%HF:40%NH4F with pH value of about 4.7. Al-O terminated surface can be formed after BHF-treatment, which is helpful to subsequent epitaxial growth [12]. Then, the substrates were in situ annealed at 700 1C to obtain a carbon-free and high crystalline surface. At last, about 10 nm LAO buffer layers were homoepitaxially deposited on atomically flat pretreated LAO (0 0 1) substrates at 650 1C before the deposition of BTO thin films. A thermocouple bonded to the substrate accurately measured the deposition temperature with an error o75 1C [9]. Both the sample jig and the heater can be rotated in XY surface and moved in Z direction. The in situ RHEED diagnostics during the film growth was performed in anti-Bragg condition using 20 keV electron beams in [1 0 0] and [1 1 0] azimuth, respectively. The configuration of RHEED allows the beam reflection at an incidence angle of 21. The diffracted electron beams
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illuminated a phosphor screen, from which data are collected using a high-resolution digital camera. A 16 bit, 512 512 pixels charge coupled device (CCD) camera provided an adequate dynamic range. Thus, we can simultaneously observe necessary diffracted spots and to resolve spot shapes spatially. RHEED diffraction patterns and corresponding intensity oscillation curves were collected. One period of oscillation corresponds to the unit cell height of BTO. In our case, 1.5 periods can be obtained at the initial growth stage during 200 s. Thus, the deposition rate is estimated as 1.5 ML during 200 s, where 1 ML corresponds to a layer thickness of 0.403 nm, the value of the c-axis lattice constant of bulk BTO. Atomic force microscopy (AFM) (SPA-300 HV, Seiko) working in tapping mode was used to characterize the surface morphology of films at room temperature. 3. Results and discussion As we know, the quality of substrates is crucial to the epitaxial growth. Fig. 1(a) shows the RHEED pattern from pretreated LAO (0 0 1) substrate with the incident electron beam parallel to [1 0 0] direction. The diffraction pattern is dominated by three relative narrow and sharp streaks corresponding to (0 1), (0 0) and (0 1¯ ) (from left to right) Bragg reflections, which demonstrate perfectly two-dimensional surface, in agreement with the AFM observations as shown in Fig. 2. The average step height was 0.3 nm. The combination of a chemical and thermal treatment leads to perfectly crystalline of LAO substrates. In general, the flat surface would improve the quality of subsequently heteroepitaxial BTO films. BTO films were deposited on pretreated LAO substrates at a low temperature of 320 1C at 2.0 106 Pa. However, after 1 ML BTO thin film was deposited, the intensity of specular reflection became weak and other reflection streaks were not clear in comparison with that of background, as shown in Fig. 1(b). As the growth proceeded, the RHEED pattern rapidly disappeared after BTO film reached a thickness of two ML in Fig. 1(c). Thus, the observed behavior is indicative of destruction in reflection condition and the formation of amorphous layers due to the lack of enough thermal energy at a low substrate temperature. Due to the failure of the direct deposition of crystalline BTO film on LAO substrates at low temperature, a new deposition technique was performed. BTO films were firstly deposited at 600 1C, while other deposition parameters were completely as the former case. Fig. 3(a)–(d) show the evolution of RHEED patterns from 8 ML BTO buffer layers grown on LAO (0 0 1) by decreasing temperature from 600 1C to 300 1C with four steps. After each step, temperature was decreased by 100 1C, and two ML BTO layer was grown. The well resolved streak pattern could be observed at all stages during film growth, indicating a flat surface of the film. Interestingly, after the 4th two ML BTO layer was grown at 300 1C, the RHEED pattern as shown in Fig. 3(d) was still retained as compared with that
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Fig. 2. 1 mm 1 mm AFM image of the surface morphology of LAO (0 0 1) substrate. The average step height was 0.3 nm.
Fig. 1. RHEED diffraction patterns recorded with the incident electron beam parallel to [1 0 0] direction for (a) pretreated LAO (0 0 1) substrate, (b) 1 ML BTO thin film grown on LAO at 320 1C, (c) two ML BTO thin film grown on LAO at 320 1C.
of Fig. 1(c). This result revealed that the surface of BTO thin film still retained crystallinity at 300 1C. Furthermore, the detailed analysis was performed to investigate the temperature-scaling behavior. The full-width at half-maximum (FWHM) of specular spot (the sharp spot at the center of the (0 0) order diffraction) and other non-specular reflectance has become broad during growth, as indicated in Fig. 3. The width of diffraction spots is inversely proportional to the effective lateral size of the ordered grains (or domains) [13]. Therefore, the increasing FWHM of streaks in Fig. 3 during decreasing temperature indicates
that the effective lateral grain size has become small. The similar results were also observed with the incident beam parallel to the [1 1 0] direction. The influence of diffuse scattering on the RHEED spots is found qualitatively as a formation of the broad halo around each spot, giving rise to two symmetric shoulders [14,15]. The RHEED streaks after two ML grown at 600 1C had a narrow FWHM and the two shoulders cannot be spatially resolved from the central elastic spike, as shown in Fig. 3(a). However, as temperature decreases, the two shoulders can be distinguished from central elastic spike by analyzing the intensity profiles taken along the AA0 line indicated in Fig. 3(b). The diffraction profile is adjusted mainly by three Lorentzians. The central one represents the specular spot, whereas the two other structures are associated with the diffuse scattering, as shown in Fig. 3(e). Considering the different strain relaxation state during growth, the evolution of momentum separation ðDqk Þ between the two shoulders obtained from Fig. 3 is illustrated in Fig. 4. Although the width of Dqk below two ML BTO layers grown at 600 1C is limited by the spatial resolution of our instrument, the increasing width of Dqk after a thickness of three ML suggests that the size of the scattering region increased during the growth at decreasing temperatures. Moreover, Dqk can be associated with a characteristic length L0 ¼ 4p=Dqk , which corresponds to the average island separation Lav [16]. It demonstrates the
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Fig. 3. Evolution of RHEED patterns from eight ML BTO buffer layers grown on LAO (0 0 1) by decreasing temperature from 600 1C to 300 1C with four steps. (a) two ML BTO thin film grown at 600 1C (b) two ML BTO thin film grown at 500 1C, (c) two ML BTO thin film grown at 400 1C and (d) two ML BTO thin film grown at 300 1C. (e) Intensity profile taken along the AA0 line in Fig. 3(b) is shown at of Fig. 3(c). Dqk denotes the separation between two shoulders.
surface island separation and distribution obey a scaling law as a function of temperature and film thickness. In other words, by decreasing temperature step-by-step, the BTO island density increases during growth, especially after a thickness of five ML as shown in Fig. 4. Our observed behavior differs from other scaling laws that suggested that the size and distribution of islands is a function of the two independent factors, namely, temperature or film thickness. The AFM image corresponding to morphology after the growth of eight ML BTO buffer layer is shown in Fig. 5. The surface shows a high density of quasi-2D islands (height from 1.2 nm to 2 nm, corresponding to 3–5 unit cell
of BTO) and a root mean square roughness o1.5 nm. Furthermore, it should be noted that the continued second layer nucleation of BTO islands seems to prefer the sites of the first layer islands. It suggests that all existing islands formed during decreasing temperature function as possible sinks for the subsequently arriving particles, and seldom particles would nucleate at the regions among islands. In other words, this behavior implies that the formation of the second layer islands could initiate on the top of the precursor islands. After eight ML crystalline BTO buffer layer are successfully deposited on LAO (1 0 0) substrate by controlling temperatures from 600 1C to 300 1C, subsequent
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Fig. 4. Momentum separation (Dqk ) as function of temperature and film thickness during growth of eight ML BTO buffer layers.
Fig. 6. RHEED patterns of (a) 10 ML, (b) 30 ML and (c) 50 ML BTO thin films grown on BTO buffer layers at a low temperature of 280 1C. Fig. 5. 1.5 mm 1.5 mm AFM image of the surface morphology of BTO buffer layers at a thickness of eight ML.
growth of thick BTO film at a lower temperature was tried. Fig. 6 shows the evolution of RHEED patterns during growth of 50 ML BTO thin film on the BTO buffer layers at 280 1C. Fig. 6(a) shows RHEED pattern of the 1st 10 ML BTO thin film. The streak shape and intensity were retained, as compared with those in Fig. 3(d). When the thickness reached 30 ML, the background intensity of the
RHEED pattern increased rapidly and the shape of all the reflectance became faint, as shown in Fig. 6(b). If the background intensity from the reflectance intensity was subtracted, the effective intensity of the reflectance would weaken. This indicates the surface roughness and in-plane disorder became strong. The RHEED patterns in Fig. 6(b) still revealed crystallinity in relative large regions of the BTO film surface. However, when we deposited BTO film with a thickness of 50 ML, the reflectance was no longer
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diffuse intensity. This is also reconfirmed by the analysis of three Lorentzians along the line AA0 at Fig. 3(d). As we know, the intensity at the center of the specular streak is mostly determined by the (2D) monolayer steps, and the intensity of the diffuse part away from the center of the specular spot should be influenced by the multi-layer steps [19]. As the temperature was decreased, multi-layer islands evidently dominated the film surface and the step density increased. Using the initial LAO surface with its bulk lattice parameter as a reference, the relative change of the streak spacing directly yields the strain state of the layer according to aLAO Dksub 1 aBTO Dkfilm Fig. 7. RHEED peak intensity of specular spot and the average intensity of background as a function of growth temperature and film thickness during the whole growth. The regions A, B, C, D, and E in Fig. 8 denote the growth temperature of 600, 500, 400, 300 and 280 1C.
resolved from the background, indicative of the beginning of the formation of amorphous BTO film layer, as shown in Fig. 6(c). Thus, 30 ML BTO thin film with good crystallinity can be successfully grown on LAO (1 0 0) at a low temperature of 280 1C. The thickness of crystalline BTO film is 38 ML, namely about 15.3 nm. This thin BTO film deposited at low temperature can reduce interfacial reaction and interdiffusion in nanoscaled heterostructures such as ferroelectric superlattices. To further understand the whole growth process, the peak intensity of specular spot and the average intensity of background are plotted in Fig. 7 as a function of temperature and film thickness. The regions A, B, C, D, and E denote the behaviors at the temperature of 600, 500, 400, 300 and 280 1C, respectively. In region A, the one and a half oscillation period of specular spot intensity corresponded to the critical thickness of BTO/LAO, which is in agreement with the theoretical calculation by equilibrium theory [17]. In region B, the peak intensity of specular spot increases rapidly. Since the measurement of RHEED specular reflectance comes from the amount of reflected electrons that are in phase, the probe demonstrate the amount of surface feature of BTO film with the same height [18]. Thus, the behavior in region B could indicate that the BTO growth front has becoming smoother by mainly forming monolayer islands, which is in agreement with the relative sharp streaks in Fig. 3(b). No transmission diffraction pattern during the following growth corresponding to region C and D was shown. An intensity transfer from central Bragg peak into the diffuse scattering was not observed. However, the diffuse part of the RHEED pattern increased rapidly, indicating the presence of three-dimensional on the surface, as shown in Fig. 3(d) and (e). It can be proposed that the intensity of central specular spot should be evidently enhanced by
where Dksub and Dkfilm are the measured streak spacing for the LAO substrate and BTO thin films at a certain thickness, respectively. We found that the relaxation was deviated from the equilibrium theory in region C and D. Our observation was consistent with the reduced mobility of misfit dislocations at low temperature [20]. Moreover, the second layer nucleation in heteroepitaxy differs markedly from that in homoepitaxy. The reason is that the growth of strained 2D island should be accompanied by the relaxation of strain and the decrease of adhesion of atoms at the island perimeter [21–23]. It can lead to the increased adatom concentration on the island top. It could also lead to the enhancement of the island-on-island nucleation instead of the introduction of misfit dislocations, especially for large lattice-mismatch system (BTO/ LAO with a lattice mismatch of 5.3%). This observation is also in agreement with the scaling law shown in Fig. 4. In order to further elucidate the subsequent growth in region E, the surface of the buffer layers was treated as two distinct areas: the top of the islands and the uncovered part of the surface. At lower temperature, the arriving particles cannot obtain enough thermal energy to migrate to the right lattice sites to form crystal. It is usually believed that this occurs due to the presence of the Ehrlich–Schwoebel barrier, which hinders the ‘‘downward surface diffusion’’ [24]. However, the barrier for nucleation at the sites of already existing islands is lower than that at the sites among islands. It allows direct nucleation of the second layer or multi-layer at low temperature, which is very similar to another system [19]. At this stage, the RHEED pattern was mainly determined by the region of multi-layer islands, as shown in Fig. 6(a). Meanwhile, it should be noted that the average intensity of background also increased during growth, as shown in Fig. 7. Below a thickness of 10 ML, the background intensity would be contributed by diffuse scattering. After a thickness of 20 ML, the background intensity increased rapidly. This would be mainly contributed by plane disorder of the growth surface or directly by amorphous layers. By considering the random distribution of the arriving particles, when the particles arrived or migrated to the sites among islands due to lack of enough thermal energy,
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above two ML, the RHEED patterns vanished rapidly, indicating the failure of crystallinity. However, 30 ML BTO thin films with good crystallinity were successfully fabricated on LAO (0 0 1) substrate at a low temperature of 280 1C by inserting the temperature-controlled BTO buffer layers. Theoretical analyses were performed to investigate this low-temperature growth. It was found that the nucleation centers formed during decreasing temperature plays a key role in determining the subsequent crystallization of BTO thin films at lower temperatures.
References
Fig. 8. Available film thickness determined by disappearance of RHEED pattern vs. temperature.
they would lose the possibility to form independent nucleus. As growth proceeded, the RHEED diffraction was strongly influenced by the amorphous layers, as shown in Fig. 6(c). Undoubtedly, the substrate temperature is a determined factor of crystal growth for a given system. If the temperature is further decreased, the additional energy provided by already existing nucleation sites cannot compensate for the barriers, the island-on-island growth mode was no longer maintained. The relationship between temperature and film thickness determined by disappearance of RHEED patterns (using the same eight ML BTO buffer layers) is displayed in Fig. 8. As shown in Fig. 8, below 280 1C, the available film thickness was decreased rapidly. When temperature is approaching 250 1C, the diffraction condition failed below a thickness of two ML. In this case, we infer the amorphous layers formed at the whole surface. However, by using the temperature-controlled BTO buffer layers, the much thicker BTO thin films can be indeed grown at lower temperatures from 300 1C to 280 1C, as compared with the direct growth at 320 1C. 4. Conclusions In conclusion, the epitaxial growth of BaTiO3 (BTO) thin films on LaAlO3 (0 0 1) substrates by laser molecular beam epitaxy was studied using (RHEED). When the BTO thin films were directly grown at 320 1C with a thickness of
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Journal of Crystal Growth 294 (2006) 236–242 www.elsevier.com/locate/jcrysgro
Epitaxial growth of BaTiO3 thin films at a low temperature under 300 1C with temperature-controlled BaTiO3 buffer layer J. Zhu, Z. Liang, Y.R. Li, Y. Zhang, X.H. Wei School of Microelectronics and Solid-state electronics, University of electronic science and technology of China, Chengdu, 610054, PR China Received 8 December 2005; received in revised form 18 May 2006; accepted 21 May 2006 Communicated by J.B. Mullin Available online 17 August 2006
Abstract The epitaxial growth of BaTiO3 (BTO) thin films on LaAlO3 (0 0 1) substrates by laser molecular beam epitaxy was studied using realtime reflection-high-energy-electron-diffraction (RHEED). Atomic force microscopy (AFM) was employed to investigate the surface topography. When the BTO thin films were directly grown at 320 1C with a thickness of above two monolayers (ML), the RHEED patterns vanished rapidly, indicating the failure of crystallinity. However, the growth of eight ML BTO layers can be controlled by decreasing temperature from 600 1C step-by-step to 300 1C. The evolution of diffraction streaks revealed a scaling behavior with temperature and film thickness, demonstrating the evolution of the quasi-two-dimensional BTO islands growth and distribution. Furthermore, subsequently deposited 30 ML BTO thin films with a good crystallinity were obtained on the eight ML BTO buffer layers at a low temperature of 280 1C. It is found that the nucleation centers formed during decreasing temperature play a key role in determining the subsequent crystallization of BTO thin films at a lower temperature. r 2006 Elsevier B.V. All rights reserved. PACS: 79.60.Jv; 66.30.Ny; 61.14.Hg Keywords: A1. Atomic force microscopy; A1. Buffer layer; A1. Reflection high-energy electron diffraction; A3. Laser molecular beam epitaxy
1. Introduction Thin-film ferroelectric materials have been widely investigated as materials for the microelectronic devices such as dynamic random access memory, bypass capacitors, infrared detectors, and tunable-microwave devices [1]. Recently, the enhancement of ferroelectricity in BaTiO3 (BTO) thin films [2,3] leads to both of scientific and technological significance to seek an extensive understanding of the growth of these functional films. In general, the deposition temperature should be high enough to offer sufficient atomistic surface mobility, especially, to satisfy the thermodynamic and kinetic requirement for crystallization and structural stability under the optimized condition during the growth. For perovskite oxides thin films grown on LaAlO3 (LAO) substrates, the growth Corresponding author. Tel.: +86 2883202140; fax: +86 2883202569.
E-mail address: [email protected] (J. Zhu). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.05.059
temperature is always above 500 1C [4–6]. However, in modern microelectronic integration technology, the growth temperature should be low enough to prevent diffusion and chemical reaction between films and substrates occurring at high temperature, which may result in a degradation of properties of devices. Therefore, the thin films of perovskite oxides must be fabricated at low temperatures to satisfy the requirement for device applications with an atomic flat surface [7–9]. Comparing with the growth at usually accepted substrate temperatures, the atomistic properties such as hopping barriers for isolated adatoms and adatom mobility would show great differences when the growth process approaches the limit of low temperatures [10]. However, there has been little progress made to understand how to fabricate the BTO thin films under 300 1C. Reflection high-energy electron diffraction (RHEED) is one of the most useful techniques to study epitaxial growth of thin films. The real-time study of the characteristics of the diffraction patterns during the growth can provide
ARTICLE IN PRESS J. Zhu et al. / Journal of Crystal Growth 294 (2006) 236–242
useful information of the microstructure and morphology. By tracking the relative ‘‘streakiness’’ of the diffraction pattern, it is possible to qualitatively study the evolution of the atomic scale roughness [11]. In this paper, we present an in situ RHEED study of BTO thin films grown on LAO (0 0 1) substrates. The direct growth of BTO at 320 1C resulted in a failure of crystallinity which may shows a grain structure with a preferred orientation without evidence of amorphous material. However, 30 monolayer (ML) BTO crystalline thin films at a low temperature of 280 1C could be successfully fabricated on a eight ML BTO buffer layer which was firstly deposited on LAO (0 0 1) substrates by decreasing temperature from 600 1C step-bystep to 300 1C. Furthermore, it was found that the initial density of nucleation centers formed at higher temperatures plays a key role in determining the subsequent crystallization of BTO thin films at low temperatures. This method has a valuable potential to provide an approach in fabricating high quality perovskite oxide films at low temperatures. 2. Experiment BTO thin films were deposited by laser molecular beam epitaxy system, which is equipped with the combination of a turbo-molecular pump and a titanium-sublimation pump to achieve a base pressure of 2.0 106 Pa. A l PHYSIK KrF excimer laser (248 nm wavelength and 30 ns pulse duration) with a repetition rate of 3 Hz was used as a laser source for the ablation of the polycrystal BTO target. The laser beam was focused on the target surface with a fluence of about 3 J/cm2. The target was rotated during the ablation process to reduce possible nonuniform erosion, which has 55 mm distance away from the substrates. In order to minimize the effect caused by miscut angle (0.210.31) and surface roughness of 10 mm 10 mm LAO substrates, the combination of a chemical and thermal pretreatment was performed. Firstly, LAO substrates were etched in buffered hydrofluoric acid (BHF). The solution is the equivalent of 1:5 by volume of 49%HF:40%NH4F with pH value of about 4.7. Al-O terminated surface can be formed after BHF-treatment, which is helpful to subsequent epitaxial growth [12]. Then, the substrates were in situ annealed at 700 1C to obtain a carbon-free and high crystalline surface. At last, about 10 nm LAO buffer layers were homoepitaxially deposited on atomically flat pretreated LAO (0 0 1) substrates at 650 1C before the deposition of BTO thin films. A thermocouple bonded to the substrate accurately measured the deposition temperature with an error o75 1C [9]. Both the sample jig and the heater can be rotated in XY surface and moved in Z direction. The in situ RHEED diagnostics during the film growth was performed in anti-Bragg condition using 20 keV electron beams in [1 0 0] and [1 1 0] azimuth, respectively. The configuration of RHEED allows the beam reflection at an incidence angle of 21. The diffracted electron beams
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illuminated a phosphor screen, from which data are collected using a high-resolution digital camera. A 16 bit, 512 512 pixels charge coupled device (CCD) camera provided an adequate dynamic range. Thus, we can simultaneously observe necessary diffracted spots and to resolve spot shapes spatially. RHEED diffraction patterns and corresponding intensity oscillation curves were collected. One period of oscillation corresponds to the unit cell height of BTO. In our case, 1.5 periods can be obtained at the initial growth stage during 200 s. Thus, the deposition rate is estimated as 1.5 ML during 200 s, where 1 ML corresponds to a layer thickness of 0.403 nm, the value of the c-axis lattice constant of bulk BTO. Atomic force microscopy (AFM) (SPA-300 HV, Seiko) working in tapping mode was used to characterize the surface morphology of films at room temperature. 3. Results and discussion As we know, the quality of substrates is crucial to the epitaxial growth. Fig. 1(a) shows the RHEED pattern from pretreated LAO (0 0 1) substrate with the incident electron beam parallel to [1 0 0] direction. The diffraction pattern is dominated by three relative narrow and sharp streaks corresponding to (0 1), (0 0) and (0 1¯ ) (from left to right) Bragg reflections, which demonstrate perfectly two-dimensional surface, in agreement with the AFM observations as shown in Fig. 2. The average step height was 0.3 nm. The combination of a chemical and thermal treatment leads to perfectly crystalline of LAO substrates. In general, the flat surface would improve the quality of subsequently heteroepitaxial BTO films. BTO films were deposited on pretreated LAO substrates at a low temperature of 320 1C at 2.0 106 Pa. However, after 1 ML BTO thin film was deposited, the intensity of specular reflection became weak and other reflection streaks were not clear in comparison with that of background, as shown in Fig. 1(b). As the growth proceeded, the RHEED pattern rapidly disappeared after BTO film reached a thickness of two ML in Fig. 1(c). Thus, the observed behavior is indicative of destruction in reflection condition and the formation of amorphous layers due to the lack of enough thermal energy at a low substrate temperature. Due to the failure of the direct deposition of crystalline BTO film on LAO substrates at low temperature, a new deposition technique was performed. BTO films were firstly deposited at 600 1C, while other deposition parameters were completely as the former case. Fig. 3(a)–(d) show the evolution of RHEED patterns from 8 ML BTO buffer layers grown on LAO (0 0 1) by decreasing temperature from 600 1C to 300 1C with four steps. After each step, temperature was decreased by 100 1C, and two ML BTO layer was grown. The well resolved streak pattern could be observed at all stages during film growth, indicating a flat surface of the film. Interestingly, after the 4th two ML BTO layer was grown at 300 1C, the RHEED pattern as shown in Fig. 3(d) was still retained as compared with that
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Fig. 2. 1 mm 1 mm AFM image of the surface morphology of LAO (0 0 1) substrate. The average step height was 0.3 nm.
Fig. 1. RHEED diffraction patterns recorded with the incident electron beam parallel to [1 0 0] direction for (a) pretreated LAO (0 0 1) substrate, (b) 1 ML BTO thin film grown on LAO at 320 1C, (c) two ML BTO thin film grown on LAO at 320 1C.
of Fig. 1(c). This result revealed that the surface of BTO thin film still retained crystallinity at 300 1C. Furthermore, the detailed analysis was performed to investigate the temperature-scaling behavior. The full-width at half-maximum (FWHM) of specular spot (the sharp spot at the center of the (0 0) order diffraction) and other non-specular reflectance has become broad during growth, as indicated in Fig. 3. The width of diffraction spots is inversely proportional to the effective lateral size of the ordered grains (or domains) [13]. Therefore, the increasing FWHM of streaks in Fig. 3 during decreasing temperature indicates
that the effective lateral grain size has become small. The similar results were also observed with the incident beam parallel to the [1 1 0] direction. The influence of diffuse scattering on the RHEED spots is found qualitatively as a formation of the broad halo around each spot, giving rise to two symmetric shoulders [14,15]. The RHEED streaks after two ML grown at 600 1C had a narrow FWHM and the two shoulders cannot be spatially resolved from the central elastic spike, as shown in Fig. 3(a). However, as temperature decreases, the two shoulders can be distinguished from central elastic spike by analyzing the intensity profiles taken along the AA0 line indicated in Fig. 3(b). The diffraction profile is adjusted mainly by three Lorentzians. The central one represents the specular spot, whereas the two other structures are associated with the diffuse scattering, as shown in Fig. 3(e). Considering the different strain relaxation state during growth, the evolution of momentum separation ðDqk Þ between the two shoulders obtained from Fig. 3 is illustrated in Fig. 4. Although the width of Dqk below two ML BTO layers grown at 600 1C is limited by the spatial resolution of our instrument, the increasing width of Dqk after a thickness of three ML suggests that the size of the scattering region increased during the growth at decreasing temperatures. Moreover, Dqk can be associated with a characteristic length L0 ¼ 4p=Dqk , which corresponds to the average island separation Lav [16]. It demonstrates the
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Fig. 3. Evolution of RHEED patterns from eight ML BTO buffer layers grown on LAO (0 0 1) by decreasing temperature from 600 1C to 300 1C with four steps. (a) two ML BTO thin film grown at 600 1C (b) two ML BTO thin film grown at 500 1C, (c) two ML BTO thin film grown at 400 1C and (d) two ML BTO thin film grown at 300 1C. (e) Intensity profile taken along the AA0 line in Fig. 3(b) is shown at of Fig. 3(c). Dqk denotes the separation between two shoulders.
surface island separation and distribution obey a scaling law as a function of temperature and film thickness. In other words, by decreasing temperature step-by-step, the BTO island density increases during growth, especially after a thickness of five ML as shown in Fig. 4. Our observed behavior differs from other scaling laws that suggested that the size and distribution of islands is a function of the two independent factors, namely, temperature or film thickness. The AFM image corresponding to morphology after the growth of eight ML BTO buffer layer is shown in Fig. 5. The surface shows a high density of quasi-2D islands (height from 1.2 nm to 2 nm, corresponding to 3–5 unit cell
of BTO) and a root mean square roughness o1.5 nm. Furthermore, it should be noted that the continued second layer nucleation of BTO islands seems to prefer the sites of the first layer islands. It suggests that all existing islands formed during decreasing temperature function as possible sinks for the subsequently arriving particles, and seldom particles would nucleate at the regions among islands. In other words, this behavior implies that the formation of the second layer islands could initiate on the top of the precursor islands. After eight ML crystalline BTO buffer layer are successfully deposited on LAO (1 0 0) substrate by controlling temperatures from 600 1C to 300 1C, subsequent
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Fig. 4. Momentum separation (Dqk ) as function of temperature and film thickness during growth of eight ML BTO buffer layers.
Fig. 6. RHEED patterns of (a) 10 ML, (b) 30 ML and (c) 50 ML BTO thin films grown on BTO buffer layers at a low temperature of 280 1C. Fig. 5. 1.5 mm 1.5 mm AFM image of the surface morphology of BTO buffer layers at a thickness of eight ML.
growth of thick BTO film at a lower temperature was tried. Fig. 6 shows the evolution of RHEED patterns during growth of 50 ML BTO thin film on the BTO buffer layers at 280 1C. Fig. 6(a) shows RHEED pattern of the 1st 10 ML BTO thin film. The streak shape and intensity were retained, as compared with those in Fig. 3(d). When the thickness reached 30 ML, the background intensity of the
RHEED pattern increased rapidly and the shape of all the reflectance became faint, as shown in Fig. 6(b). If the background intensity from the reflectance intensity was subtracted, the effective intensity of the reflectance would weaken. This indicates the surface roughness and in-plane disorder became strong. The RHEED patterns in Fig. 6(b) still revealed crystallinity in relative large regions of the BTO film surface. However, when we deposited BTO film with a thickness of 50 ML, the reflectance was no longer
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diffuse intensity. This is also reconfirmed by the analysis of three Lorentzians along the line AA0 at Fig. 3(d). As we know, the intensity at the center of the specular streak is mostly determined by the (2D) monolayer steps, and the intensity of the diffuse part away from the center of the specular spot should be influenced by the multi-layer steps [19]. As the temperature was decreased, multi-layer islands evidently dominated the film surface and the step density increased. Using the initial LAO surface with its bulk lattice parameter as a reference, the relative change of the streak spacing directly yields the strain state of the layer according to aLAO Dksub 1 aBTO Dkfilm Fig. 7. RHEED peak intensity of specular spot and the average intensity of background as a function of growth temperature and film thickness during the whole growth. The regions A, B, C, D, and E in Fig. 8 denote the growth temperature of 600, 500, 400, 300 and 280 1C.
resolved from the background, indicative of the beginning of the formation of amorphous BTO film layer, as shown in Fig. 6(c). Thus, 30 ML BTO thin film with good crystallinity can be successfully grown on LAO (1 0 0) at a low temperature of 280 1C. The thickness of crystalline BTO film is 38 ML, namely about 15.3 nm. This thin BTO film deposited at low temperature can reduce interfacial reaction and interdiffusion in nanoscaled heterostructures such as ferroelectric superlattices. To further understand the whole growth process, the peak intensity of specular spot and the average intensity of background are plotted in Fig. 7 as a function of temperature and film thickness. The regions A, B, C, D, and E denote the behaviors at the temperature of 600, 500, 400, 300 and 280 1C, respectively. In region A, the one and a half oscillation period of specular spot intensity corresponded to the critical thickness of BTO/LAO, which is in agreement with the theoretical calculation by equilibrium theory [17]. In region B, the peak intensity of specular spot increases rapidly. Since the measurement of RHEED specular reflectance comes from the amount of reflected electrons that are in phase, the probe demonstrate the amount of surface feature of BTO film with the same height [18]. Thus, the behavior in region B could indicate that the BTO growth front has becoming smoother by mainly forming monolayer islands, which is in agreement with the relative sharp streaks in Fig. 3(b). No transmission diffraction pattern during the following growth corresponding to region C and D was shown. An intensity transfer from central Bragg peak into the diffuse scattering was not observed. However, the diffuse part of the RHEED pattern increased rapidly, indicating the presence of three-dimensional on the surface, as shown in Fig. 3(d) and (e). It can be proposed that the intensity of central specular spot should be evidently enhanced by
where Dksub and Dkfilm are the measured streak spacing for the LAO substrate and BTO thin films at a certain thickness, respectively. We found that the relaxation was deviated from the equilibrium theory in region C and D. Our observation was consistent with the reduced mobility of misfit dislocations at low temperature [20]. Moreover, the second layer nucleation in heteroepitaxy differs markedly from that in homoepitaxy. The reason is that the growth of strained 2D island should be accompanied by the relaxation of strain and the decrease of adhesion of atoms at the island perimeter [21–23]. It can lead to the increased adatom concentration on the island top. It could also lead to the enhancement of the island-on-island nucleation instead of the introduction of misfit dislocations, especially for large lattice-mismatch system (BTO/ LAO with a lattice mismatch of 5.3%). This observation is also in agreement with the scaling law shown in Fig. 4. In order to further elucidate the subsequent growth in region E, the surface of the buffer layers was treated as two distinct areas: the top of the islands and the uncovered part of the surface. At lower temperature, the arriving particles cannot obtain enough thermal energy to migrate to the right lattice sites to form crystal. It is usually believed that this occurs due to the presence of the Ehrlich–Schwoebel barrier, which hinders the ‘‘downward surface diffusion’’ [24]. However, the barrier for nucleation at the sites of already existing islands is lower than that at the sites among islands. It allows direct nucleation of the second layer or multi-layer at low temperature, which is very similar to another system [19]. At this stage, the RHEED pattern was mainly determined by the region of multi-layer islands, as shown in Fig. 6(a). Meanwhile, it should be noted that the average intensity of background also increased during growth, as shown in Fig. 7. Below a thickness of 10 ML, the background intensity would be contributed by diffuse scattering. After a thickness of 20 ML, the background intensity increased rapidly. This would be mainly contributed by plane disorder of the growth surface or directly by amorphous layers. By considering the random distribution of the arriving particles, when the particles arrived or migrated to the sites among islands due to lack of enough thermal energy,
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above two ML, the RHEED patterns vanished rapidly, indicating the failure of crystallinity. However, 30 ML BTO thin films with good crystallinity were successfully fabricated on LAO (0 0 1) substrate at a low temperature of 280 1C by inserting the temperature-controlled BTO buffer layers. Theoretical analyses were performed to investigate this low-temperature growth. It was found that the nucleation centers formed during decreasing temperature plays a key role in determining the subsequent crystallization of BTO thin films at lower temperatures.
References
Fig. 8. Available film thickness determined by disappearance of RHEED pattern vs. temperature.
they would lose the possibility to form independent nucleus. As growth proceeded, the RHEED diffraction was strongly influenced by the amorphous layers, as shown in Fig. 6(c). Undoubtedly, the substrate temperature is a determined factor of crystal growth for a given system. If the temperature is further decreased, the additional energy provided by already existing nucleation sites cannot compensate for the barriers, the island-on-island growth mode was no longer maintained. The relationship between temperature and film thickness determined by disappearance of RHEED patterns (using the same eight ML BTO buffer layers) is displayed in Fig. 8. As shown in Fig. 8, below 280 1C, the available film thickness was decreased rapidly. When temperature is approaching 250 1C, the diffraction condition failed below a thickness of two ML. In this case, we infer the amorphous layers formed at the whole surface. However, by using the temperature-controlled BTO buffer layers, the much thicker BTO thin films can be indeed grown at lower temperatures from 300 1C to 280 1C, as compared with the direct growth at 320 1C. 4. Conclusions In conclusion, the epitaxial growth of BaTiO3 (BTO) thin films on LaAlO3 (0 0 1) substrates by laser molecular beam epitaxy was studied using (RHEED). When the BTO thin films were directly grown at 320 1C with a thickness of
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