Effect of thermal annealing on the optical and structural properties of γ-Al2O3 films prepared on MgO substrates by MOCVD

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Effect of thermal annealing on the optical and structural properties of γ-Al2O3 films prepared on MgO substrates by MOCVD Zhao Li, Jin Man, Xianjin Feng, Xuejian Du, Weiguang Wang, Mingxian Wang School of Physics, Shandong University, Jinan 250100, PR China Received 21 May 2015; received in revised form 16 August 2015; accepted 26 August 2015

Abstract Metalorganic chemical vapor deposition (MOCVD) of γ-Al2O3 films is performed on MgO (110) and (111) substrates by using trimethylaluminum and O2 as the precursors. The effects of post-deposition annealing on the microstructure and epitaxial relationship of the films are investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Schematic diagrams are proposed to illuminate the epitaxial relationships between the γ-Al2O3 films and MgO substrates. The γ-Al2O3 films annealed at 1000 1C exhibit the best crystalline quality, for which clear epitaxial relationships of γ-Al2O3 (110)∥MgO (110) with γ-Al2O3 [110]∥MgO [110] and γ-Al2O3 (111)∥MgO (111) with γ-Al2O3[110]∥MgO [110] have been ascertained. The average transmittance of the obtained samples in the visible range is over 85%. The optical band gaps of the γ-Al2O3 films annealed at 1000 1C on MgO (110) and (111) substrates are about 5.81 and 5.80 eV, respectively, which are a bit smaller than those of the as-deposited films. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: γ-Al2O3 films; Annealing; Microstructure; MgO substrates; Epitaxial relationship

1. Introduction Recently, aluminum oxide (Al2O3) thin films have attracted much interest due to their interesting physical and chemical properties and possible applications. Al2O3 is a well characterized compound which is thermodynamically and chemically stable and exhibits high hardness, wide band gap, high wear resistance, high transparency, low electrical conductivity and high corrosion resistance [1–5]. These properties make it useful in various fields, such as antireflective coatings for glass surfaces, [6] wear protection of metal surfaces, [7] and protective layers or hard coatings in the fields of mechanical engineering [8]. Furthermore, Al2O3 is specially applied in high power radio frequency electronic circuits, insulating layers in electronic devices, microwave devices, and refractory coatings in hostile environments with elevated temperatures [9–11].

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Corresponding author. Tel.: þ86 531 88361057. E-mail address: [email protected] (J. Ma).

Many Al2O3 polymorphs including α (rhombohedral), [12] γ (cubic), [13,14] δ (tetragonal or orthorhombic), [15,16] θ (monoclinic), [17] and κ (orthorhombic) [18] have been identified in the literatures. The α phase is the only thermodynamically stable phase. The phase formation characteristics of these polymorphs depend significantly on source materials and preparation conditions [19,20]. Al2O3 exhibits different phases with respect to temperature [21,22]. Generally, amorphous phase forms easily at low temperatures, while γ-Al2O3 phase is dominant with other metastable phases at intermediate temperatures and α-Al2O3 phase always comes into being at higher temperatures. Among the various metastable phases of Al2O3, γ-Al2O3 is of the most technological significance because it exhibits wide applications as a coating, adsorbent and soft abrasive. γ-Al2O3 is particularly important to the chemical industry, including energy generation and storage as a catalyst support material [23–25]. However, most of the prepared γ-Al2O3 films are polycrystalline, irregular in shape and contain impurities. The manufacture of single crystal γ-Al2O3 films is essential for the future development of γ-Al2O3 applications.

http://dx.doi.org/10.1016/j.ceramint.2015.08.145 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Z. Li, et al., Effect of thermal annealing on the optical and structural properties of γ-Al2O3 films prepared on MgO substrates by MOCVD, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.145

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Much effort has been made to obtain Al2O3 films using a variety of techniques such as r.f. magnetron sputtering, [26] plasma deposition, [27] sol–gel, [28] spray pyrolysis, [29] electron beam evaporation, [30] pulsed laser deposition [31,32] and metalorganic chemical vapor deposition (MOCVD) [33,34]. Compared with other methods, MOCVD is an attractive process which is suitable for preparation of thin films due to its advantages including easy control of deposition rate, precise control of composition and relatively large throughput up to commercial volume. Previous works show that annealing is an important method to modify the properties of films [35,36]. The structural and optical properties of the films can be altered greatly after annealing. In this paper, we report the investigations of the γ-Al2O3 films grown on MgO (110) and (111) substrates by MOCVD, focusing our attention on the effect of post-deposition annealing on the structural and optical properties of the γ-Al2O3 films. 2. Experimental details The deposition of Al2O3 films was carried out on the double-side polished MgO (110) and (111) wafers (10  10 mm2) under ultra-high vacuum using a MOCVD system with a base pressure of 2.0  10  6 Torr. Commercially available trimethylaluminum [Al(CH3)3, 6 N in purity] was used as the organometallic (OM) source. The Al precursor was transported into the reaction chamber by ultra high purity N2 (9 N), which was used as the carrier gas. The ultra high purity N2 was obtained by an AERONEX inert gas purifier (MODEL: SS2500KFI4R) made in San Diego, USA. Typical oxidizer O2 (5 N) with a flow rate of 60 sccm (sccm denotes cubic centimeter per minute at STP) was injected into the chamber using a separate delivery line. The temperature of the bubbler containing Al(CH3)3 was maintained at 16 1C with a pressure of 800 Torr. Then the molar flow rate of the Al precursor could be calculated once the flow rate of the carrier gas N2 for the OM source was fixed. During the deposition process (5 h), the flow rate of Al(CH3)3 was maintained at 3 sccm, with the substrate temperature kept at 700 1C and growth pressure at 20 Torr. The molar flow rate of the Al precursor was calculated as 2.3  10  6 mol/min. After the deposition, the samples were annealed in air at 900, 1000 and 1100 1C for an hour. The structural properties and the epitaxial relationships of the obtained films were determined by the X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) methods. The θ-2θ scans of XRD patterns were collected using a Rigaku D/Max 2200 PC diffractometer with a graphite monochrometer and Cu Kα radiation (λ¼ 0.15418 nm). The inplane Φ-scans were performed using a Philips X'Pert Pro MPD X-ray diffractometer. Cross-sectional samples were prepared by gluing two pieces together with films facing each other, followed by mechanical polishing, dimpling and ion-milling (Ar þ , 4.5 kV) at incidence angles of 10–61 to obtain a very thin area which is electron transparent. HRTEM and SAED were then performed on the cross-sectional samples using a Technai F30 transmission electron microscope operated at

Fig. 1. XRD patterns for the γ-Al2O3 films (a) before and after annealing at (b) 900, (c) 1000, and (d) 1100 1C on MgO (110) substrates.

300 kV. The optical transmittance measurements were performed using a Shimadzu TV-1900 spectrophotometer.

3. Results and discussion Fig. 1 shows the typical θ–2θ scans of XRD patterns for the Al2O3 films before and after annealing at different temperatures on MgO (110) substrates. It can be seen from Fig. 1 (a) that only a bulge located at about 2θ ¼ 30.91 is detected apart from the substrate MgO (220) peak for the as-deposited film. However, after the films being annealed at temperatures of 900, 1000 and 1100 1C, the diffraction peaks corresponding to γ-Al2O3 (220) and (440) reflections are observed in Fig. 1 (b), (c) and (d), and the locations of the γ-Al2O3 (220) and (440) peaks do not change evidently. No other diffraction peak is observed, which indicates that the annealed samples are single orientation γ-Al2O3 films with an out-of-plane relationship of γ-Al2O3 (110)∥MgO (110). The γ-Al2O3 (220) peak for the sample annealed at 1000 1C exhibits the smallest full width at half maximum (FWHM) compared with the samples annealed at 900 1C and 1100 1C, indicating the best crystalline structure. These observations imply that the annealing temperature significantly affects the structure of the films and the sample annealed at 1000 1C exhibits the best crystalline quality of γ-Al2O3. For the cubic crystal structure like γ-Al2O3, the lattice constant a and the interplanar spacing dhkl are related by a¼ dhkl(h2 þ k2 þ l2)1/2, where h, k, l are the miller indices. The lattice constant a of the γ-Al2O3 film calculated from the XRD

Please cite this article as: Z. Li, et al., Effect of thermal annealing on the optical and structural properties of γ-Al2O3 films prepared on MgO substrates by MOCVD, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.145

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Fig. 3. XRD off-specular Ф-scans of (a) γ-Al2O3 {400} and (b) MgO {200} planes for the Al2O3 sample on MgO (110) substrate annealed at 1000 1C at a fixed position.

Fig. 2. XRD spectra of the as-deposited and annealed Al2O3 films on MgO (111) substrates: (a) as-deposited film, (b–d) 900, 1000, 1100 1C annealed films.

data is about 8.05 Å, which is a bit bigger than that of bulk γ-Al2O3 with a value of 7.9 Å (JCPDS 10-0425). XRD spectra of the as-deposited and annealed Al2O3 films on MgO (111) substrates have been shown in Fig. 2. For the as-deposited film shown in Fig. 2(a), a weak and broad peak located at about 18.91 is observed in addition to the substrate MgO (111) and (222) peaks. For the annealed films shown in Fig. 2(b)-(d), the diffraction peaks standing for γ-Al2O3 (111) and (222) reflections are detected. Meanwhile, as the annealing temperature increases to 1000 and 1100 1C, a weak peak located at about 59.51 emerges. Since the annealing temperature is above 1000 1C which is the proper temperature for the formation of α phase Al2O3, this weak peak is corresponding to the α-Al2O3 (221) reflection. Yet the intensity of this α-Al2O3 peak is fairly weak. The γ-Al2O3 (111) peak for the sample annealed at 1000 1C exhibits the smallest FWHM compared with other samples, indicating the best crystalline structure. The sample annealed at 1000 1C exhibits the best crystalline quality, which is consistent with the results of samples on MgO (110) substrates. It was reported that the structure of γ-Al2O3 films prepared on Si (100) substrates by pulsed laser deposition (PLD) was amorphous at the low substrate temperatures (25–400 1C) and polycrystalline when the temperatures were above 500 1C [37]. Also, Cibert et al. [32] reported polycrystalline γ-Al2O3 films prepared on Si (100) and Si–SiO2 substrates at a substrate temperature of 800 1C by the PLD and plasma enhanced chemical vapor deposition (PECVD) techniques. In the present work, γ-Al2O3

films with preferred orientation were obtained after annealing not only on MgO (110) but also on MgO (111) planes. In order to examine the internal crystal structures of the 1000 1C-annealed films on MgO (110) and (111) substrates, XRD off-specular Ф-scans of the (400) reflection (ψ ¼ 451) for the γ-Al2O3 (220) film and the (200) reflection (ψ¼ 451) for the MgO (220) substrate at a fixed position are displayed in Fig. 3(a) and (b). It should be noted that the tilt angle (ψ) is the angle between the reflection plane and substrate surface plane. Curve (a) in Fig. 3 shows two diffraction peaks separated by 1801 in the whole range of  180  1801, which reveals a good in-plane alignment inside the film. From curve (b) in Fig. 3, it can be seen that the locations of MgO {200} peaks are the same as the locations of γ-Al2O3 {400} peaks, which implies the two planes have the same projection direction on the sample surface. XRD Φ-scans of the (400) reflection (ψ¼ 54.71) for the γ-Al2O3 (111) film and the (200) reflection (ψ¼ 54.71) for the MgO (111) substrate are illustrated in Fig. 4 (a) and (b), reporting three diffraction peaks separated by 1201 in the whole range of  180 to 1801. As we know, the cubic γ-Al2O3 (400) plane has two-fold symmetry along the [110] direction and three-fold symmetry along the [111] direction, which is similar to the MgO (200) plane. The measured results reveal that the films are single epitaxial films without domain structure inside the films. In order to illuminate the epitaxial relationships between the γ-Al2O3 films and MgO (110) as well as (111) substrates, schematic diagrams are proposed in Figs. 5 and 6, respectively. Diagrams (a) and (b) in Fig. 5 are the geometrical configurations of the MgO (110) and γ-Al2O3 (110) surfaces from the plan-view observation. Meanwhile, diagrams in Fig. 6 are those of the MgO (111) and γ-Al2O3 (111) surfaces. Since the radii of the Al3 þ and Mg2 þ cations are smaller than that of the O2  anion, small balls are used to represent the metallic

Please cite this article as: Z. Li, et al., Effect of thermal annealing on the optical and structural properties of γ-Al2O3 films prepared on MgO substrates by MOCVD, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.145

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Fig. 4. XRD Ф-scans of (a) γ-Al2O3 {400} and (b) MgO {200} planes for the sample on MgO (111) substrate annealed at 1000 1C.

Fig. 6. Schematic diagrams of geometrical epitaxial relationship between (a) MgO (111) substrate and (b) γ-Al2O3 film.

Fig. 5. (a) and (b) are the schematic diagrams of geometrical epitaxial relationship between MgO (110) substrate and γ-Al2O3 film.

cations. The MgO substrate exhibits the cubic structure with a lattice parameter of am ¼ 4.211 Å (JCPDS 45-0946), and the cubic structure γ-Al2O3 has a lattice parameter of aa ¼ 7.9 Å

(JCPDS 10-0425). As shown in Fig. 5, the lattice mismatch between the γ-Al2O3 [001] orientation (8.422 Å) and MgO [001] orientation (7.9 Å) is 6.2%, which is as same as that between the γ-Al2O3 [110] orientation (11.910 Å) and MgO [110] orientation (11.17 Å). The same lattice mismatch of 6.2% for the sample on MgO (111) substrate along [011] or [110] orientation is also obtained in Fig. 6. The lattice mismatch between γ-Al2O3 films and different substrates has been studied by several groups. Balakrishnan et al. [38] calculated the lattice mismatch between γ-Al2O3 and SrTiO3 substrate to be 1.2%, and that between α-Al2O3 and MgO substrate along the c-axis was 2.8%. Merckling et al. [39] found that an indirect epitaxial relationship between γ-Al2O3 and Si was set on by aligning 2 γ-Al2O3 unit cells to 3 Si unit cells, which led to an effective lattice mismatch of 2.9%. The formation of the single crystalline γ-Al2O3 structure is due to random nucleation and misfit strain evolution along the well matched crystal orientations. In our present work, the in-plane epitaxial relationships can be given as γ-Al2O3 [001]∥MgO [001] or γ-Al2O3[110]∥MgO [110] for the film on MgO (110) substrate

Please cite this article as: Z. Li, et al., Effect of thermal annealing on the optical and structural properties of γ-Al2O3 films prepared on MgO substrates by MOCVD, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.145

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Fig. 7. Cross-section (a) low magnification TEM, (b) HRTEM and (c) SAED micrographs of the interface between the γ-Al2O3 film and MgO (110) substrate.

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Fig. 8. (a) Low magnification TEM, (b) HRTEM and (c) SAED micrographs of the interface area between the γ-Al2O3 film and MgO (111) substrate.

Please cite this article as: Z. Li, et al., Effect of thermal annealing on the optical and structural properties of γ-Al2O3 films prepared on MgO substrates by MOCVD, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.145

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Fig. 9. Optical transmittance spectra of the as-deposited γ-Al2O3 samples and those annealed at 1000 1C on MgO (a) (110) and (b) (111), as well as those of their respective MgO substrates, with the plots of (αhν)2 vs. hν in the inset.

and γ-Al2O3[011]∥MgO [011] or γ-Al2O3[110]∥MgO [110] for the film on MgO (111) substrate. Fig. 7(a) and (b) show the low magnification cross-sectional transmission electron microscopy (XTEM) and HRTEM images of the interface area between the γ-Al2O3 film and MgO (110) substrate, respectively. The incident electron beam is parallel to the [001] direction of the MgO substrate. The thickness of the film is measured to be about 206 nm from the low magnification XTEM image, and the deposition rate is about 0.68 nm/min. The substrate/film interface in atomic scale is indicated by the black dash line in the HRTEM image. Since the crystalline γ-Al2O3 film is obtained from amorphous structure after annealing, a transitional region with a thickness of several nanometers can be seen across the interface. The transitional region acts as a buffer layer, which has an effect on the strain relief and reduces the formation of misfit dislocations. The presence of the transitional region improves the film quality, which may be analogous to the effect of the buffer layer in the heteroepitaxial growth of GaN material. For the substrate area shown in Fig. 7(b), the lattice fringes with an angular separation of 451 are labeled, which are corresponding to MgO (200) and (220) planes. For the film area, the HRTEM

image exhibits the γ-Al2O3 (220) and ( 220) lattice fringes parallel and perpendicular respectively to the interface. It is shown that the film is structurally uniform single crystalline of cubic γ-Al2O3 with a growth direction of [110] perpendicular to the (220) plane of MgO substrate. The corresponding SAED pattern of the same sample annealed at 1000 1C is displayed in Fig. 7(c), from which the distance between the diffraction spot and the central point is inversely proportional to the interplanar spacing dhkl. The diffraction spots of γ-Al2O3 (400), (800), (220), (440) and (840) as well as MgO (200), (220) and (420) planes can be seen clearly. The calculated lattice constants of γ-Al2O3 and MgO are aa’ ¼ 8.09 Å and am’¼ 4.30 Å, respectively. The lattice mismatch is about 5.9%, which is close to the theoretical value of 6.2%. The results of HRTEM micrograph and SAED pattern confirm the epitaxial relationship of γ-Al2O3 (110)∥MgO (110) with γ-Al2O3 [110]∥MgO [110], which are consistent with the XRD analyses. The low magnification XTEM image of the γ-Al2O3 film and MgO (111) substrate interface region is illustrated in Fig. 8 (a), from which the thickness of the film is measured to be 211 nm and the deposition rate is about 0.70 nm/min. Fig. 8 (b) shows the HRTEM image of the interface area taken from [ 112] axis of the MgO substrate, indicating that the γ-Al2O3 sample has a well defined interface, which is marked by the white dash line. For the γ-Al2O3 film portion, the lattice fringes corresponding to γ-Al2O3 (111) and ( 311) with an angular separation of 58.61 are marked, which is in good accordance with the standard calculated value of 58.51. It can be seen that the γ-Al2O3 film annealed at 1000 1C is an epitaxial film and the growth direction is along γ-Al2O3 [111]. The growth of γ-Al2O3 (111) epitaxial films on different substrates by molecular beam epitaxy (MBE) has also been reported by other researchers. Okada et al. [40] reported epitaxial growth of γ-Al2O3 (111) films on Si (111) substrates using the MBE method with Al-N2O mixed source. Merckling et al. [39] successfully grew epitaxial γ-Al2O3 layers on both Si (111) and Si (100) substrates by MBE and gave an in-plane epitaxial relationship of γ-Al2O3 [110]∥Si [110]. The SAED pattern of this sample is illustrated in Fig. 8(c), showing the diffraction spots of γ-Al2O3 (111), ( 220) and ( 311) planes clearly, which are corresponding to the planes observed in the HRTEM image. The HRTEM and SAED analyses of the interface for this sample show a cube-on-cube epitaxial relationship of γ-Al2O3 (111)∥MgO (111) with γ-Al2O3 [110]∥MgO [110]. Fig. 9 illustrates the optical transmittance spectra of the asdeposited and 1000 1C annealed γ-Al2O3 samples on MgO (a) (110) and (b) (111) substrates as a function of wavelength ranging from 200 to 800 nm, as well as those of the MgO substrates. According to the curves, the average transmittance for each sample in the visible wavelength range (400–800 nm) is over 85%. The spectra show a sharp fall in transmittance at the wavelength corresponding to the optical band gap of the material. Here, the “optical band gap” is defined as the energy corresponding to the onset of significant optical absorption [41]. The optical band gap is related to the absorption coefficient (α) and photon energy (hν) and can be calculated using the following relation [42]:

Please cite this article as: Z. Li, et al., Effect of thermal annealing on the optical and structural properties of γ-Al2O3 films prepared on MgO substrates by MOCVD, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.145

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αhν = A ( hν − Eg )

where A is a constant and Eg is the optical band gap of the material. Therefore the values of Eg can be determined by plotting (αhν)2 vs. hν and then extrapolating the linear regions of the plots to the energy axis. The plots of (αhν)2 vs. hν for the as-deposited and annealed films are shown in the insets of Fig. 9(a) and (b). The optical band gaps of the γ-Al2O3 films annealed at 1000 1C on MgO (110) and (111) substrates are 5.81 and 5.80 eV, respectively, which are a bit smaller than the values of the respective as-deposited films (5.89 and 5.83 eV), implying improvements of film crystalline quality after annealing at 1000 1C. Some first-principal calculations were performed on the electronic and structural properties of different phase alumina [43,44]. The calculated band gap of γ-Al2O3 is about 2.2–2.7 eV smaller than that of α-Al2O3, which is in accordance with our result. Amirsalari et al. [45] studied the effects of calcination temperature on the band gap of γ-Al2O3 and obtained values of 5.18–5.36 eV. Dhonge et al. [46] reported a band gap of 5.63 eV for aluminum oxide films deposited by spray pyrolysis. Our results show that the obtained films have excellent optical transparency in the visible wavelength region. High optical transmittance within visible spectral region is a key property of transparent oxide materials with a main application in optoelectronic devices. 4. Conclusion Al2O3 films have been successfully prepared on MgO (110) and (111) substrates at 700 1C by the MOCVD method. γ-Al2O3 epitaxial films were obtained after thermal annealing. The annealing temperature extraordinarily affected the structure of the γ-Al2O3 films and the samples annealed at 1000 1C exhibited the best crystallization. Schematic diagrams were proposed to illuminate the epitaxial relationships between the γ-Al2O3 films and MgO substrates. HRTEM and SAED measurements confirmed the epitaxial relationships to be γ-Al2O3 (110)∥MgO (110) with γ-Al2O3[110]∥MgO [110] and γ-Al2O3 (111)∥MgO (111) with γ-Al2O3 [110]∥MgO [110]. The average transmittance of the as-deposited and 1000 1C annealed samples in visible range was over 85%. The optical band gaps of the γ-Al2O3 films annealed at 1000 1C were 5.81 and 5.80 eV, which were a bit smaller than those of the asdeposited films, implying better crystalline qualities. γ-Al2O3 films with low defects and high transparency have broad application prospects in film field effect transistors, high frequency devices, optoelectronic devices, etc. Acknowledgment This work is supported by the National Natural Science Foundation of China (Grant no. 51272138). References [1] R.F. Bunshah, Handbook of hard coatings, Noyes Publications, New Jersey, 2001.

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Please cite this article as: Z. Li, et al., Effect of thermal annealing on the optical and structural properties of γ-Al2O3 films prepared on MgO substrates by MOCVD, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.145