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Structure, interface, and luminescence of (011¯ 1) ZnO nanofilms
Thin Solid Films 519 (2010) 549–555
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
¯ Structure, interface, and luminescence of (011 1) ZnO nanofilms
Jung-Hsiung Shen a, Sung-Wei Yeh a, Hsing-Lu Huang b,⁎, Dershin Gan a a b
Department of Materials and Optoelectronic Science, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC Department of Mechanical Engineering, Chinese Military Academy, 83059 Kaohsiung, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 25 October 2009 Received in revised form 11 August 2010 Accepted 16 August 2010 Available online 21 August 2010 Keywords: Interfaces Luminescence Transmission electron microscopy (TEM) Zinc oxide
a b s t r a c t ZnO nanofilms of (011¯ 1) texture have been prepared by ion beam sputtering on the (001) surface of singlecrystal NaCl. The orientation relationship between them is determined by transmission electron microscopy. ¯ Analyses of electron diffraction patterns and interface confirm that the ZnO (011 1) plane is the interface with ¯ 1) surface shows a near-bandthe NaCl (001) surface. The photoluminescence spectrum from the ZnO (011 edge UV emission and a broad green emission. The result indicates that the inherent high surface defects of oxygen vacancies on the (011¯ 1) surface are the probable origin of the green emission. © 2010 Elsevier B.V. All rights reserved.
1. Introduction ZnO of wurtzite structure is polar along the c-axis, which leads to an undesirable red shift in the optical emission spectra [1] for the films oriented in the (0001) (c-plane) plane. Therefore, thin films oriented ¯ ¯ in the nonpolar (101 0) (m-plane) and (112 0) (a-plane) planes have been proposed as alternatives to avoid the polarity effect [2]. The surface energies of these planes have been shown to be low [3] and theoretical calculation indicated that the m-plane is the most stable ¯ one among them [4]. However, the {101 1} (p-plane) planes must also be a low energy surface as those generally form the capping surfaces between the c- and m-surfaces of a ZnO single crystal [5,6]. ZnO can be grown on various nano shapes by controlling the growing condi¯ tions [7–14]. The {101 1} surfaces have been frequently observed in nanorods, nanobelts and nanofilms [7–10]. The plane can reduce the electrostatic interaction in the formation of the deformation-free single-crystal nanohelixes [15], and is present at the intersection of ¯ the nanofingers [16]. It was also reported the surface of an (112 0)textured ZnO thin film [7] actually consists of small pyramids with ¯ four {101 1} top surfaces. ¯ So far, the reports about the ZnO (101 1) surfaces are mostly on the nanorods and nanobelts but rarely on thin films. The ZnO thin film prepared on the NaCl (001) cleavage surface at 300 °C by Henley et al. ¯ [17] is predominantly (112 0)-textured initially, but gradually over¯ taken by the (101 1)-texture as the film grows thicker. Another report ¯ of (101 1)-textured film, however, has poor quality [18].
⁎ Corresponding author. Tel.: + 886 7 746 6641; fax: + 886 7 710 4697. E-mail address: [email protected] (H.-L. Huang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.098
The energy gap of ZnO is 3.37 eV results in a near-band-edge emission of UV light of wavelength ~ 370 nm. However, a broad green emission near 520 nm has been frequently observed and the causes of it are still controversial as many experimental results are conflicting. Recent results showed that it is probably due to surface defects. Shalish et al. [19] showed that the intensity of green emission increases with decreasing diameter of ZnO rods, and in extreme case of very thin rod the UV emission can be completely quenched. Furthermore, recent results showed that capping of surfaces by SnO2 [20], surfactant [21], and dodecylamine [22], which can remove the surface defects, can reduce or quench the green emission. These also confirm that the green emission is a surface effect. Recently, measurement of cathodoluminescence (CL) from the specific crystallographic surfaces of a 5 mm ZnO single crystal [23] ¯ ¯ showed that green emissions are strong from the (1 011 ) (−p-plane) ¯ and (0001) (+c-plane) surfaces, moderate from the (101 1) (+p¯ ¯ plane), and negligible from the (0001 ) (−c-plane), and (101 0) (mplane) surfaces. In another report green emission by CL from the ¯ micrometer-sized (101 0) surface is shown to be weak, but from the ¯ micrometer-sized (101 1) surface is strong [24]. Detailed analysis shows that the green emission is due to the inherent high surface ¯ defects of oxygen vacancies on the (101 1) surface [24]. However, since the excitation volume in CL is large, the spectra may not be sufficiently representative for the surface-specific emission due to the large penetration depth. From the above reviews it is clear that the ZnO (101¯ 1) surface observed previously was very small, only nm or μm in dimension, and only in a single crystal it reached ~ 1 mm. The main purpose of this research is to use the NaCl (001) cleavage surface as substrate to ¯ prepare epitaxial ZnO film of the (101 1) surface of a much larger area. The film can then be used to evaluate the electrical, optical or other
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useful properties. The formation of the surface and the orientation relationship will be analyzed. The broad green emission of ZnO thin film, which is usually undesirable [11–14], is commonly observed and its causes are still ¯ controversial. As an application of the prepared (101 1) surface, it is used to study a possible source of the green emission, i.e., the surface defects of oxygen vacancies. Room temperature photoluminescence (PL) is used and the result analyzed. 2. Experimental procedure ZnO thin films were grown by radio frequency ion beam sputtering on the NaCl (001) surface about 1 cm2 in area, which was prepared by cleaving a NaCl single crystal. The target was a 99.99% pure ZnO disc 7.5 cm in diameter. The deposition conditions were 750 V applied voltage, 15 sccm oxygen flow rate, 400 °C substrate temperature, and 20 min deposition time. The thickness of the prepared film was about 25 nm. The NaCl substrate was then dissolved in distilled water so that the thin ZnO film was left floating on the water surface. The film was caught from under with a copper grid covered with a carboncoated collodion film for transmission electron microscopy (TEM, FEI Tecnai G2 F20 at 200 kV) analysis. To determine the orientation relationship between ZnO and NaCl, the ZnO thin film was picked up before the NaCl substrate was completely dissolved. Selected area diffraction (SAD), bright field image (BFI) and dark field image (DFI) of TEM were used to analyze the microstructure. High-resolution lattice fringes images, 2-dimensional Fourier transformation, and inverse Fourier transform were used to characterize the ZnO planes and interfaces. The ZnO thin film was also analyzed by grazing-angle X-ray diffraction (XRD, Bede D1 model). Room temperature PL (Micro-PL, Jobin-Yvon T64000 with He–Cd 325 nm laser and a resolution of ~0.02 nm) was done on thin films on a soda-lime glass plate. The absorption coefficient of ZnO at 325 nm (the wave length of the source laser in PL) is ~ 2 × 105 cm−1 [25], which corresponds to a penetration depth of ~ 50 nm so that the whole film (~25 nm thick) was excited. The PL spectrum of the present ZnO thin film was compared with the ZnO nanofilm of the ¯ (112 0) surface prepared in a previous report [26]. 3. Results and discussion 3.1. TEM characterization The BFI of Fig. 1a shows that the ZnO film is continuous and that the grains are less than 20 nm in size. The SAD pattern in Fig. 1b indicates that the ZnO nanocrystals are well aligned. There are two ¯ variants of nanocrystals in the [011 1] zone, as indexed separately in Fig. 1c and d, and their composite figure is shown in Fig. 1e. The four ¯ extra {01 13} diffraction spots in the figure will be discussed later. The ¯ grazing-angle XRD pattern in Fig. 1f shows the (011 1) peak, consistent with the electron diffraction result. The TEM energy-dispersive X-ray analysis in Fig. 1g shows that no observable impurity is present in the ZnO thin film. Fig. 2 shows both the SAD patterns of ZnO, which is the same as that of Fig. 1b, and NaCl, which is in the [001] zone. The epitaxial ¯ relationship between them is [011 1]ZnO// [001]NaCl (zone axis), ¯ ¯ ¯ (2 110)ZnO//(110)NaCl and (01 12)ZnO//(1 10)NaCl (variant 1). The variant 2 has an equivalent orientation relationship. The parallel zone axes of ¯ [011 1]ZnO// [001]NaCl are consistent with the previous result [17], although neither the orientation relationship nor the interface between them was reported or analyzed in the report [17]. ¯ 3.2. Formation of the 011 1Þ surface The SAD pattern in Fig. 1b and the grazing-angle XRD pattern in Fig. 1f show that the ZnO thin film prepared on the NaCl (001) surface
¯ ¯ has the (011 1) texture. Previously only thin film of mixed (011 1) and ¯ (112 0) textures were prepared on the NaCl (001) surface at 300 °C [17]. However, the SAD pattern in Fig. 1b shows some deviations from ¯ the standard diffraction pattern in the [011 1] zone. There are four ¯ ¯ strong diffraction spots of {01 13}, which do not belong to the [011 1] zone and are not supposed to appear. In addition, the four {011¯ 2} diffraction spots are abnormally weak. Fig. 3a is a schematic figure of several related planes and directions ¯¯ in the hexagonal system. Fig. 3b is the (21 10) plane in Fig. 3a showing ¯ ¯ ¯ ¯ the [011 1] direction and the traces of the (01 11), (01 12), (01 13), and ¯ ¯¯ (0111) planes, which are perpendicular to the (2110) plane. Since a0 = 0.3250 nm and c0 = 0.5207 nm, the angles θ1 and θ2, defined in Fig. 3b, are easily calculated to be 42.7° and 31.7°, respectively. In ¯ ¯ addition, the angle ϕ between the [011 1] direction and the (011 1) ¯ plane is calculated to be 75.6°, i.e., the [011 1] zone axis is not ¯ perpendicular to the (011 1) plane. h ¯ ¯ In Fig. 3b it can be seen that 01 12Þ plane is parallel to the 011 1 zone axis, therefore it should show up as a strong diffraction spot if the electron beam comes from this direction. The (01¯ 13) spot may not show up at all as the plane makes an angle of θ1–θ2 = 11.0° from the ¯ [011 1] direction. However, contrary to that, the diffraction pattern in ¯ ¯ Fig. 1b shows that the {011 2} spots are weak but the {01 13} spots not only show up but also are strong. A reasonable explanation is that the ¯ ZnO (011 1) surface is the interface with the NaCl (001) surface. The incident electron beam, being perpendicular to the (011¯ 1) plane, ¯ actually deviates from the [011 1] zone axis by an angle Δϕ of 14.4° (=90° − 75.6°), as indicated in Fig. 3b. Therefore the angle between ¯ the (01 13) plane and the electron beam is only 3.4° (=14.4° − 11.0°), ¯ an angle small enough for the (01 13) plane to be nearly parallel to the incident electron beam and to show up as a strong diffraction spot. ¯ The (01 12) plane, however, deviates by 14.4° from the electron beam direction and its diffraction spot becomes very weak in Fig. 1b. The ¯¯ ¯ intensities of the diffraction spots of the (21 10) and (2 110) planes, which are still parallel to the electron beam, are not affected. The ¯¯ ¯ ¯ ¯ ¯ intensities of the (101 1), (1 011), (11 01) and (1 101 ) spots are not much affected and remain relatively strong in Fig. 1b. ¯ According to the above argument the four {01 13} diffraction spots in Fig. 1b are assigned to variants 1 and 2 in Fig. 1e. To further check ¯ the validity of the argument, the DFIs using the (01¯ 12)1 and (01 13)1 diffraction spots in Fig. 1b are shown in Fig. 4a and b, respectively. It is clear that some nanocrystallites contribute to both diffraction spots, which further confirms the above argument. The SEM micrograph of the bottom surface of the ZnO thin film is ¯ shown in Fig. 5. The surface is the (011 1) surface and is the interface with the NaCl (001) surface. The surface appears to be rather smooth although many small cavities ~1 μm in size are present. The cracks on the film surface formed when the film was picked up from the water surface. ¯ The result shows that the ZnO (011 1) surface, although with two variants, can then be prepared in an area as large as the NaCl (001) surface which can reach several inches in diameter. The thin film can be used to measure the electrical, optical or optoelectronic properties. It may also have potential applications as acoustic–optic [27] or piezoelectric [28] devices.
3.3. High-resolution lattice fringes image Fig. 6a shows that several ZnO nanoparticles are in the process of coalescence. Fig. 6b is the Fourier transformation of the square region ¯ in Fig. 6a, which shows that the particle has the zone axis of [011 1] (variant 1). The particle 1 has a small orientation difference with the central particle and their interface is marked by a dashed line in Fig. 6a ¯ and c. Across the dashed line the (1 101) fringes are bent a little and ¯ the (01 12) fringes disappear. The reconstructed image by inverse Fourier transformation performed by selecting all diffraction spots and the central spot in Fig. 6b is shown in Fig. 6c. The particle 1 can be
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Fig. 1. (a) BFI and (b) SAD pattern of the ZnO thin film. (c) and (d) are the indexes of the SAD pattern of variant 1 (closed symbols) and variant 2 (open symbols), respectively, in the ¯ ZnO [011 1] zone axis. (e) Composite figure of (c) and (d). (f) and (g) are the grazing-angle XRD and energy-dispersive X-ray spectrum of the ZnO thin film, respectively.
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Fig. 2. SAD pattern of the ZnO thin film and the NaCl substrate, which is in the [001] zone axis.
expected to reach perfect match with the central particle with a small rotation. The small particle 2 has already reached perfect match in the ¯ ¯ (1 011) and (11 01) planes. The particles 3 and 4 are still in the process ¯ of coalescence, as shown by the misaligned (11 01) planes.
¯ Fig. 4. (a) and (b) are the DFIs using the (01 12)1 and (01¯ 13)1 diffraction spots in Fig. 1b, respectively.
Fig. 7a shows two nanoparticles in contact and Fig. 7b is the Fourier transformation from the square region in Fig. 7a. The diffraction pattern indexes in Fig. 7b show that the particles belong to different variants and particle 2 is not in an exact zone. The reconstructed image in Fig. 7c shows a stepped interface between the two particles, as marked by the dashed line. Several misfit dislocations are present at the step, apparently to accommodate the misorientation between the ¯ ¯ (1 011)1 and (11 01)2 planes. This is a case of imperfect-oriented attachment of nanocrystals [29].
Fig. 3. (a) Schematic figure showing the related planes and directions of a ZnO cell. (b) ¯¯ ¯ ¯ The 21 10Þ plane in (a) showing the [011 1] direction and the traces of the (01 11), ¯ ¯ ¯ (01 12), (01 13), and (011 1) planes. The angles θ1, θ2, ϕ, and Δϕ are calculated to be 42.7°, 31.7°, 75.6°, and 14.4° (see text), respectively.
Fig. 5. SEM micrograph of the bottom surface of the ZnO thin film. The surface is the ¯ (011 1) surface, which is the interface to the NaCl (001) surface.
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Fig. 6. (a) Lattice image of nanoparticles in the process of coalescence. (b) Fourier transformation from the square region in (a) showing the diffraction pattern in the ¯ [011 1] zone axis. (c) Inverse Fourier transform from (b), in which the boundary between the central particle and particle 1 is indicated.
3.4. The ZnO/NaCl interface The ZnO/NaCl interface has been shown to be (011¯ 1)ZnO/(001)NaCl ¯ ¯ and the orientation relationship is (2 110)ZnO/(110)NaCl and (01 12)ZnO/ ¯ ¯ (110)NaCl. Fig. 8a and b are the Zn layers of the ZnO (0111) plane and the NaCl (001) plane, respectively, and Fig. 8c is their composite figure overlapped according to the orientation relationship. In the figure the ¯ mismatch between the (2 110)ZnO plane (d-spacing = 0.1625 nm) and the (220)NaCl plane (d-spacing = 0.1994 nm) is −18.5%, a rather large ¯ value. However, the mismatch between every five (2 110)ZnO planes and every four (110)NaCl planes is + 1.9%. As shown in Fig. 8c the
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Fig. 7. (a) Lattice image of variant 1 and 2 crystallites in contact. (b) Fourier transformation from the square region in (a) showing the diffraction pattern. (c) Inverse Fourier transformation from (b) showing the stepped interfaces and misfit dislocations.
match of the ions in this direction is obviously not good. In the other ¯ direction, since the [01 12]ZnO direction is not perpendicular to the ¯ ¯ (01 12)ZnO plane and that the ZnO direction normal to the (01 12)ZnO ¯ plane is not specified, only the (01 12)ZnO plane is shown. In Fig. 8c, the mismatch between the fourth row of Zn2+ ions at 0.8321 nm from C and the fifth row of Cl− ions at 2 × 0.3988 nm = 0.7976 nm is a small value of 4.3%. It can be seen that the good correspondence repeats itself from the fourth to the eighth row of Zn2+ ions, and so forth. The second and third rows of Zn2+ also have good correspondence with the second and fourth rows of Na+ ions, respectively, although they have electric charges of the same sign. Theoretical calculations are necessary to understand how the various ions are relaxed on and near ¯ the interface for the formation of the (011 1)ZnO/(001)NaCl interface.
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¯ ¯¯ Fig. 9. Room temperature PL spectra of (a) the (011 1) and (b) the (21 10) ZnO nanofilms. ¯¯ The SAD pattern in the inset of (b) is that of ZnO in the (21 10) zone axis [26].
¯ Fig. 8. Schematic figures of (a) the Zn layer of the ZnO (011 1) plane, (b) the NaCl (001) plane and (c) the composite figure of (a) and (b) overlapped according to the orientation relationship.
The analysis of Fig. 8 shows that the match of ion positions are not ¯ so good at the (011 1)ZnO/(001)NaCl interface. This can explain that the interface can only be prepared within a narrow range of temperature. If the substrate temperature is lowered to 300 °C, the film turns into a ¯ ¯ mixed texture of (011 1)ZnO and (112 0)ZnO [17], which was also confirmed by us. At 450 °C, a random ring diffraction pattern of ZnO appears (result not shown), indicating that the epitaxial orientation relationship is lost. This is in contrast to other oxide such as ZrO2 [30], which can maintain the same interface with the NaCl (001) surface within a wide range of temperature.
It has been shown that green luminescence is probably a surface effect and is probably related to the ionized oxygen vacancies. Analysis show that there are more surplus charges of the oxygen on ¯ ¯ the (011 1) surface than those on the (0001 ) surface and more oxygen vacancies are to be formed on/near the surface to eliminate the surplus charge so as to form a stable surface [24]. Strong green ¯ emission shown is therefore observed on the (011 1) surface in Fig. 9a, ¯¯ as also confirmed earlier by CL [24]. However, the nonpolar (21 10) surface, which is electrically neutral and has a relatively low surface energy, should then have few defects present on the surface. As ¯ defects are known to be responsible for the green emission, the 21¯1 0Þ surface has negligible green emission in Fig. 9b. The present PL results further support that the surface defects of oxygen vacancies are probably the origin of the green emission on the (011¯ 1) surface. 4. Conclusions
3.5. Luminescence ¯ 1) (Fig. 9a) and Fig. 9 shows the PL spectra taken from the (011 (2110) (Fig. 9b) nanofilms. Both spectra exhibit a strong near-bandedge UV emission centered at 376 nm (3.30 eV) due to the free¯ exciton recombination [31]. Fig. 9a shows that the (011 1) film has a broad green emission centered at 545 nm (2.28 eV). However, the ¯¯ (21 10) film in Fig. 9b, which was prepared in a previous work [26] and confirmed by the SAD pattern in the inset, does not have the green emission at all. ¯¯
¯ ZnO nanofilm of (011 1) texture has been prepared by ion beam sputtering on the (001) surface of NaCl single crystal at 400°. Analysis ¯ of the SAD pattern shows that the interface is (011 1)ZnO/(001)NaCl. The ¯ orientation relationship between them is (2110)ZnO//(110)NaCl and ¯ ¯ (01 12)ZnO//(1 10)NaCl and two variants of ZnO are present. A stepped interface with misfit dislocations is observed between ZnO nanoparticles. ¯ The PL spectrum from the (011 1) nanofilm shows the characteristic near-band-edge UV emission and a strong and broad green emission centered around 545 nm (2.28 eV). The high surface defects
J.-H. Shen et al. / Thin Solid Films 519 (2010) 549–555 ¯ of oxygen vacancies inherent on the (011 1) surface are a probable origin of the green emission.
Acknowledgements This work was financially supported by the National Science Council of Taiwan and by the Center for Nanoscience and Nanotechnology of National Sun Yet-sen University. References [1] G. Vaschenko, D. Patel, C.S. Menoni, N.F. Gardner, J. Sun, W. Götz, C.N. Tomé, B. Clausen, Phys. Rev. B 64 (2001) 241308. [2] H. Matsui, H. Tabata, J. Appl. Phys. 99 (2006) 124307. [3] N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, J. Cryst. Growth 130 (1993) 269. [4] B. Meyer, D. Marx, Phys. Rev. B 67 (2003) 35403. [5] G. Dhanaraj, M. Dudley, D. Bliss, M. Callahan, M. Harris, J. Cryst. Growth 297 (2006) 74. [6] R.A. Laudise, A.A. Ballman, J. Phys. Chem. 64 (1960) 688. [7] J. Zúñiga-Pérez, V. Muñoz-Sanjosé, E. Palacios-Lidón, J. Colchero, Phys. Rev. Lett. 95 (2005) 226105. [8] G.D. Yuan, W.J. Zhang, J.S. Jle, X. Fan, J.A. Zaplen, Y.H. Leung, L.B. Luo, P.F. Wang, C.S. Lee, S.T. Lee, Nano Lett. 8 (2008) 2591. [9] G. Zhang, A. Nakamura, T. Aoki, J. Temmyo, Phys. Status Solidi C 3 (2006) 722. [10] Z.L. Wang, X.Y. Kong, Y. Ding, P. Gao, W.L. Hughes, R. Yang, Y. Zhang, Adv. Funct. Mater. 14 (2004) 943.
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Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
¯ Structure, interface, and luminescence of (011 1) ZnO nanofilms
Jung-Hsiung Shen a, Sung-Wei Yeh a, Hsing-Lu Huang b,⁎, Dershin Gan a a b
Department of Materials and Optoelectronic Science, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC Department of Mechanical Engineering, Chinese Military Academy, 83059 Kaohsiung, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 25 October 2009 Received in revised form 11 August 2010 Accepted 16 August 2010 Available online 21 August 2010 Keywords: Interfaces Luminescence Transmission electron microscopy (TEM) Zinc oxide
a b s t r a c t ZnO nanofilms of (011¯ 1) texture have been prepared by ion beam sputtering on the (001) surface of singlecrystal NaCl. The orientation relationship between them is determined by transmission electron microscopy. ¯ Analyses of electron diffraction patterns and interface confirm that the ZnO (011 1) plane is the interface with ¯ 1) surface shows a near-bandthe NaCl (001) surface. The photoluminescence spectrum from the ZnO (011 edge UV emission and a broad green emission. The result indicates that the inherent high surface defects of oxygen vacancies on the (011¯ 1) surface are the probable origin of the green emission. © 2010 Elsevier B.V. All rights reserved.
1. Introduction ZnO of wurtzite structure is polar along the c-axis, which leads to an undesirable red shift in the optical emission spectra [1] for the films oriented in the (0001) (c-plane) plane. Therefore, thin films oriented ¯ ¯ in the nonpolar (101 0) (m-plane) and (112 0) (a-plane) planes have been proposed as alternatives to avoid the polarity effect [2]. The surface energies of these planes have been shown to be low [3] and theoretical calculation indicated that the m-plane is the most stable ¯ one among them [4]. However, the {101 1} (p-plane) planes must also be a low energy surface as those generally form the capping surfaces between the c- and m-surfaces of a ZnO single crystal [5,6]. ZnO can be grown on various nano shapes by controlling the growing condi¯ tions [7–14]. The {101 1} surfaces have been frequently observed in nanorods, nanobelts and nanofilms [7–10]. The plane can reduce the electrostatic interaction in the formation of the deformation-free single-crystal nanohelixes [15], and is present at the intersection of ¯ the nanofingers [16]. It was also reported the surface of an (112 0)textured ZnO thin film [7] actually consists of small pyramids with ¯ four {101 1} top surfaces. ¯ So far, the reports about the ZnO (101 1) surfaces are mostly on the nanorods and nanobelts but rarely on thin films. The ZnO thin film prepared on the NaCl (001) cleavage surface at 300 °C by Henley et al. ¯ [17] is predominantly (112 0)-textured initially, but gradually over¯ taken by the (101 1)-texture as the film grows thicker. Another report ¯ of (101 1)-textured film, however, has poor quality [18].
⁎ Corresponding author. Tel.: + 886 7 746 6641; fax: + 886 7 710 4697. E-mail address: [email protected] (H.-L. Huang). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.098
The energy gap of ZnO is 3.37 eV results in a near-band-edge emission of UV light of wavelength ~ 370 nm. However, a broad green emission near 520 nm has been frequently observed and the causes of it are still controversial as many experimental results are conflicting. Recent results showed that it is probably due to surface defects. Shalish et al. [19] showed that the intensity of green emission increases with decreasing diameter of ZnO rods, and in extreme case of very thin rod the UV emission can be completely quenched. Furthermore, recent results showed that capping of surfaces by SnO2 [20], surfactant [21], and dodecylamine [22], which can remove the surface defects, can reduce or quench the green emission. These also confirm that the green emission is a surface effect. Recently, measurement of cathodoluminescence (CL) from the specific crystallographic surfaces of a 5 mm ZnO single crystal [23] ¯ ¯ showed that green emissions are strong from the (1 011 ) (−p-plane) ¯ and (0001) (+c-plane) surfaces, moderate from the (101 1) (+p¯ ¯ plane), and negligible from the (0001 ) (−c-plane), and (101 0) (mplane) surfaces. In another report green emission by CL from the ¯ micrometer-sized (101 0) surface is shown to be weak, but from the ¯ micrometer-sized (101 1) surface is strong [24]. Detailed analysis shows that the green emission is due to the inherent high surface ¯ defects of oxygen vacancies on the (101 1) surface [24]. However, since the excitation volume in CL is large, the spectra may not be sufficiently representative for the surface-specific emission due to the large penetration depth. From the above reviews it is clear that the ZnO (101¯ 1) surface observed previously was very small, only nm or μm in dimension, and only in a single crystal it reached ~ 1 mm. The main purpose of this research is to use the NaCl (001) cleavage surface as substrate to ¯ prepare epitaxial ZnO film of the (101 1) surface of a much larger area. The film can then be used to evaluate the electrical, optical or other
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useful properties. The formation of the surface and the orientation relationship will be analyzed. The broad green emission of ZnO thin film, which is usually undesirable [11–14], is commonly observed and its causes are still ¯ controversial. As an application of the prepared (101 1) surface, it is used to study a possible source of the green emission, i.e., the surface defects of oxygen vacancies. Room temperature photoluminescence (PL) is used and the result analyzed. 2. Experimental procedure ZnO thin films were grown by radio frequency ion beam sputtering on the NaCl (001) surface about 1 cm2 in area, which was prepared by cleaving a NaCl single crystal. The target was a 99.99% pure ZnO disc 7.5 cm in diameter. The deposition conditions were 750 V applied voltage, 15 sccm oxygen flow rate, 400 °C substrate temperature, and 20 min deposition time. The thickness of the prepared film was about 25 nm. The NaCl substrate was then dissolved in distilled water so that the thin ZnO film was left floating on the water surface. The film was caught from under with a copper grid covered with a carboncoated collodion film for transmission electron microscopy (TEM, FEI Tecnai G2 F20 at 200 kV) analysis. To determine the orientation relationship between ZnO and NaCl, the ZnO thin film was picked up before the NaCl substrate was completely dissolved. Selected area diffraction (SAD), bright field image (BFI) and dark field image (DFI) of TEM were used to analyze the microstructure. High-resolution lattice fringes images, 2-dimensional Fourier transformation, and inverse Fourier transform were used to characterize the ZnO planes and interfaces. The ZnO thin film was also analyzed by grazing-angle X-ray diffraction (XRD, Bede D1 model). Room temperature PL (Micro-PL, Jobin-Yvon T64000 with He–Cd 325 nm laser and a resolution of ~0.02 nm) was done on thin films on a soda-lime glass plate. The absorption coefficient of ZnO at 325 nm (the wave length of the source laser in PL) is ~ 2 × 105 cm−1 [25], which corresponds to a penetration depth of ~ 50 nm so that the whole film (~25 nm thick) was excited. The PL spectrum of the present ZnO thin film was compared with the ZnO nanofilm of the ¯ (112 0) surface prepared in a previous report [26]. 3. Results and discussion 3.1. TEM characterization The BFI of Fig. 1a shows that the ZnO film is continuous and that the grains are less than 20 nm in size. The SAD pattern in Fig. 1b indicates that the ZnO nanocrystals are well aligned. There are two ¯ variants of nanocrystals in the [011 1] zone, as indexed separately in Fig. 1c and d, and their composite figure is shown in Fig. 1e. The four ¯ extra {01 13} diffraction spots in the figure will be discussed later. The ¯ grazing-angle XRD pattern in Fig. 1f shows the (011 1) peak, consistent with the electron diffraction result. The TEM energy-dispersive X-ray analysis in Fig. 1g shows that no observable impurity is present in the ZnO thin film. Fig. 2 shows both the SAD patterns of ZnO, which is the same as that of Fig. 1b, and NaCl, which is in the [001] zone. The epitaxial ¯ relationship between them is [011 1]ZnO// [001]NaCl (zone axis), ¯ ¯ ¯ (2 110)ZnO//(110)NaCl and (01 12)ZnO//(1 10)NaCl (variant 1). The variant 2 has an equivalent orientation relationship. The parallel zone axes of ¯ [011 1]ZnO// [001]NaCl are consistent with the previous result [17], although neither the orientation relationship nor the interface between them was reported or analyzed in the report [17]. ¯ 3.2. Formation of the 011 1Þ surface The SAD pattern in Fig. 1b and the grazing-angle XRD pattern in Fig. 1f show that the ZnO thin film prepared on the NaCl (001) surface
¯ ¯ has the (011 1) texture. Previously only thin film of mixed (011 1) and ¯ (112 0) textures were prepared on the NaCl (001) surface at 300 °C [17]. However, the SAD pattern in Fig. 1b shows some deviations from ¯ the standard diffraction pattern in the [011 1] zone. There are four ¯ ¯ strong diffraction spots of {01 13}, which do not belong to the [011 1] zone and are not supposed to appear. In addition, the four {011¯ 2} diffraction spots are abnormally weak. Fig. 3a is a schematic figure of several related planes and directions ¯¯ in the hexagonal system. Fig. 3b is the (21 10) plane in Fig. 3a showing ¯ ¯ ¯ ¯ the [011 1] direction and the traces of the (01 11), (01 12), (01 13), and ¯ ¯¯ (0111) planes, which are perpendicular to the (2110) plane. Since a0 = 0.3250 nm and c0 = 0.5207 nm, the angles θ1 and θ2, defined in Fig. 3b, are easily calculated to be 42.7° and 31.7°, respectively. In ¯ ¯ addition, the angle ϕ between the [011 1] direction and the (011 1) ¯ plane is calculated to be 75.6°, i.e., the [011 1] zone axis is not ¯ perpendicular to the (011 1) plane. h ¯ ¯ In Fig. 3b it can be seen that 01 12Þ plane is parallel to the 011 1 zone axis, therefore it should show up as a strong diffraction spot if the electron beam comes from this direction. The (01¯ 13) spot may not show up at all as the plane makes an angle of θ1–θ2 = 11.0° from the ¯ [011 1] direction. However, contrary to that, the diffraction pattern in ¯ ¯ Fig. 1b shows that the {011 2} spots are weak but the {01 13} spots not only show up but also are strong. A reasonable explanation is that the ¯ ZnO (011 1) surface is the interface with the NaCl (001) surface. The incident electron beam, being perpendicular to the (011¯ 1) plane, ¯ actually deviates from the [011 1] zone axis by an angle Δϕ of 14.4° (=90° − 75.6°), as indicated in Fig. 3b. Therefore the angle between ¯ the (01 13) plane and the electron beam is only 3.4° (=14.4° − 11.0°), ¯ an angle small enough for the (01 13) plane to be nearly parallel to the incident electron beam and to show up as a strong diffraction spot. ¯ The (01 12) plane, however, deviates by 14.4° from the electron beam direction and its diffraction spot becomes very weak in Fig. 1b. The ¯¯ ¯ intensities of the diffraction spots of the (21 10) and (2 110) planes, which are still parallel to the electron beam, are not affected. The ¯¯ ¯ ¯ ¯ ¯ intensities of the (101 1), (1 011), (11 01) and (1 101 ) spots are not much affected and remain relatively strong in Fig. 1b. ¯ According to the above argument the four {01 13} diffraction spots in Fig. 1b are assigned to variants 1 and 2 in Fig. 1e. To further check ¯ the validity of the argument, the DFIs using the (01¯ 12)1 and (01 13)1 diffraction spots in Fig. 1b are shown in Fig. 4a and b, respectively. It is clear that some nanocrystallites contribute to both diffraction spots, which further confirms the above argument. The SEM micrograph of the bottom surface of the ZnO thin film is ¯ shown in Fig. 5. The surface is the (011 1) surface and is the interface with the NaCl (001) surface. The surface appears to be rather smooth although many small cavities ~1 μm in size are present. The cracks on the film surface formed when the film was picked up from the water surface. ¯ The result shows that the ZnO (011 1) surface, although with two variants, can then be prepared in an area as large as the NaCl (001) surface which can reach several inches in diameter. The thin film can be used to measure the electrical, optical or optoelectronic properties. It may also have potential applications as acoustic–optic [27] or piezoelectric [28] devices.
3.3. High-resolution lattice fringes image Fig. 6a shows that several ZnO nanoparticles are in the process of coalescence. Fig. 6b is the Fourier transformation of the square region ¯ in Fig. 6a, which shows that the particle has the zone axis of [011 1] (variant 1). The particle 1 has a small orientation difference with the central particle and their interface is marked by a dashed line in Fig. 6a ¯ and c. Across the dashed line the (1 101) fringes are bent a little and ¯ the (01 12) fringes disappear. The reconstructed image by inverse Fourier transformation performed by selecting all diffraction spots and the central spot in Fig. 6b is shown in Fig. 6c. The particle 1 can be
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Fig. 1. (a) BFI and (b) SAD pattern of the ZnO thin film. (c) and (d) are the indexes of the SAD pattern of variant 1 (closed symbols) and variant 2 (open symbols), respectively, in the ¯ ZnO [011 1] zone axis. (e) Composite figure of (c) and (d). (f) and (g) are the grazing-angle XRD and energy-dispersive X-ray spectrum of the ZnO thin film, respectively.
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Fig. 2. SAD pattern of the ZnO thin film and the NaCl substrate, which is in the [001] zone axis.
expected to reach perfect match with the central particle with a small rotation. The small particle 2 has already reached perfect match in the ¯ ¯ (1 011) and (11 01) planes. The particles 3 and 4 are still in the process ¯ of coalescence, as shown by the misaligned (11 01) planes.
¯ Fig. 4. (a) and (b) are the DFIs using the (01 12)1 and (01¯ 13)1 diffraction spots in Fig. 1b, respectively.
Fig. 7a shows two nanoparticles in contact and Fig. 7b is the Fourier transformation from the square region in Fig. 7a. The diffraction pattern indexes in Fig. 7b show that the particles belong to different variants and particle 2 is not in an exact zone. The reconstructed image in Fig. 7c shows a stepped interface between the two particles, as marked by the dashed line. Several misfit dislocations are present at the step, apparently to accommodate the misorientation between the ¯ ¯ (1 011)1 and (11 01)2 planes. This is a case of imperfect-oriented attachment of nanocrystals [29].
Fig. 3. (a) Schematic figure showing the related planes and directions of a ZnO cell. (b) ¯¯ ¯ ¯ The 21 10Þ plane in (a) showing the [011 1] direction and the traces of the (01 11), ¯ ¯ ¯ (01 12), (01 13), and (011 1) planes. The angles θ1, θ2, ϕ, and Δϕ are calculated to be 42.7°, 31.7°, 75.6°, and 14.4° (see text), respectively.
Fig. 5. SEM micrograph of the bottom surface of the ZnO thin film. The surface is the ¯ (011 1) surface, which is the interface to the NaCl (001) surface.
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Fig. 6. (a) Lattice image of nanoparticles in the process of coalescence. (b) Fourier transformation from the square region in (a) showing the diffraction pattern in the ¯ [011 1] zone axis. (c) Inverse Fourier transform from (b), in which the boundary between the central particle and particle 1 is indicated.
3.4. The ZnO/NaCl interface The ZnO/NaCl interface has been shown to be (011¯ 1)ZnO/(001)NaCl ¯ ¯ and the orientation relationship is (2 110)ZnO/(110)NaCl and (01 12)ZnO/ ¯ ¯ (110)NaCl. Fig. 8a and b are the Zn layers of the ZnO (0111) plane and the NaCl (001) plane, respectively, and Fig. 8c is their composite figure overlapped according to the orientation relationship. In the figure the ¯ mismatch between the (2 110)ZnO plane (d-spacing = 0.1625 nm) and the (220)NaCl plane (d-spacing = 0.1994 nm) is −18.5%, a rather large ¯ value. However, the mismatch between every five (2 110)ZnO planes and every four (110)NaCl planes is + 1.9%. As shown in Fig. 8c the
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Fig. 7. (a) Lattice image of variant 1 and 2 crystallites in contact. (b) Fourier transformation from the square region in (a) showing the diffraction pattern. (c) Inverse Fourier transformation from (b) showing the stepped interfaces and misfit dislocations.
match of the ions in this direction is obviously not good. In the other ¯ direction, since the [01 12]ZnO direction is not perpendicular to the ¯ ¯ (01 12)ZnO plane and that the ZnO direction normal to the (01 12)ZnO ¯ plane is not specified, only the (01 12)ZnO plane is shown. In Fig. 8c, the mismatch between the fourth row of Zn2+ ions at 0.8321 nm from C and the fifth row of Cl− ions at 2 × 0.3988 nm = 0.7976 nm is a small value of 4.3%. It can be seen that the good correspondence repeats itself from the fourth to the eighth row of Zn2+ ions, and so forth. The second and third rows of Zn2+ also have good correspondence with the second and fourth rows of Na+ ions, respectively, although they have electric charges of the same sign. Theoretical calculations are necessary to understand how the various ions are relaxed on and near ¯ the interface for the formation of the (011 1)ZnO/(001)NaCl interface.
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¯ ¯¯ Fig. 9. Room temperature PL spectra of (a) the (011 1) and (b) the (21 10) ZnO nanofilms. ¯¯ The SAD pattern in the inset of (b) is that of ZnO in the (21 10) zone axis [26].
¯ Fig. 8. Schematic figures of (a) the Zn layer of the ZnO (011 1) plane, (b) the NaCl (001) plane and (c) the composite figure of (a) and (b) overlapped according to the orientation relationship.
The analysis of Fig. 8 shows that the match of ion positions are not ¯ so good at the (011 1)ZnO/(001)NaCl interface. This can explain that the interface can only be prepared within a narrow range of temperature. If the substrate temperature is lowered to 300 °C, the film turns into a ¯ ¯ mixed texture of (011 1)ZnO and (112 0)ZnO [17], which was also confirmed by us. At 450 °C, a random ring diffraction pattern of ZnO appears (result not shown), indicating that the epitaxial orientation relationship is lost. This is in contrast to other oxide such as ZrO2 [30], which can maintain the same interface with the NaCl (001) surface within a wide range of temperature.
It has been shown that green luminescence is probably a surface effect and is probably related to the ionized oxygen vacancies. Analysis show that there are more surplus charges of the oxygen on ¯ ¯ the (011 1) surface than those on the (0001 ) surface and more oxygen vacancies are to be formed on/near the surface to eliminate the surplus charge so as to form a stable surface [24]. Strong green ¯ emission shown is therefore observed on the (011 1) surface in Fig. 9a, ¯¯ as also confirmed earlier by CL [24]. However, the nonpolar (21 10) surface, which is electrically neutral and has a relatively low surface energy, should then have few defects present on the surface. As ¯ defects are known to be responsible for the green emission, the 21¯1 0Þ surface has negligible green emission in Fig. 9b. The present PL results further support that the surface defects of oxygen vacancies are probably the origin of the green emission on the (011¯ 1) surface. 4. Conclusions
3.5. Luminescence ¯ 1) (Fig. 9a) and Fig. 9 shows the PL spectra taken from the (011 (2110) (Fig. 9b) nanofilms. Both spectra exhibit a strong near-bandedge UV emission centered at 376 nm (3.30 eV) due to the free¯ exciton recombination [31]. Fig. 9a shows that the (011 1) film has a broad green emission centered at 545 nm (2.28 eV). However, the ¯¯ (21 10) film in Fig. 9b, which was prepared in a previous work [26] and confirmed by the SAD pattern in the inset, does not have the green emission at all. ¯¯
¯ ZnO nanofilm of (011 1) texture has been prepared by ion beam sputtering on the (001) surface of NaCl single crystal at 400°. Analysis ¯ of the SAD pattern shows that the interface is (011 1)ZnO/(001)NaCl. The ¯ orientation relationship between them is (2110)ZnO//(110)NaCl and ¯ ¯ (01 12)ZnO//(1 10)NaCl and two variants of ZnO are present. A stepped interface with misfit dislocations is observed between ZnO nanoparticles. ¯ The PL spectrum from the (011 1) nanofilm shows the characteristic near-band-edge UV emission and a strong and broad green emission centered around 545 nm (2.28 eV). The high surface defects
J.-H. Shen et al. / Thin Solid Films 519 (2010) 549–555 ¯ of oxygen vacancies inherent on the (011 1) surface are a probable origin of the green emission.
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