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Epitaxial properties of ZnO thin films on LaAlO3 substrates by pulsed laser deposition
Journal of Crystal Growth 421 (2015) 19–22
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Epitaxial properties of ZnO thin films on LaAlO3 substrates by pulsed laser deposition C.H. Jia a, S. Wang a, Y.H. Wu a, Y.H. Chen b, X.W. Sun a, W.F. Zhang a,n a b
Key Laboratory of Photovoltaic Materials of Henan Province and School of Physics & Electronics, Henan University, Kaifeng 475004, PR China Key Laboratory of Semiconductor Material Science, Institute of Semiconductors, Chinese Academy of Science, PO Box 912, Beijing, 100083, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 October 2014 Received in revised form 13 January 2015 Accepted 22 January 2015 Communicated by A. Ohtomo Available online 2 February 2015
Wurtzite ZnO thin films with different epitaxial relationships have been grown on (0 0 1)-, (0 1 1)-, and (1 1 1)LaAlO3 (LAO) single-crystal substrates by pulsed laser deposition. Nonpolar (1 1 2 0)ZnO films with two orthogonal domains were obtained on (0 0 1) LAO, in which the in-plane orientation relationship is demonstrated to be 〈0 0 0 1〉ZnO//〈1 1 0〉LAO. For ZnO on (0 1 1)- and (1 1 1)LAO substrates, a single-domain epitaxy with c axial orientation is observed, in which the in-plane relationships were 〈1 1 0 0〉ZnO//〈0 1 1〉LAO irrespective of the substrate orientations. Based on the in-plane orientation relationship, the lattice mismatch has been obtained for these three oriented ZnO/LAO heterointerfaces. & 2015 Published by Elsevier B.V.
Keywords: A1. Epitaxial growth B2. ZnO LaAlO3
1. Introduction Wurtzite ZnO semiconductor materials have attracted much attention due to its excellent optical, optoelectronic, sensing, magnetic and energy harvesting properties [1]. Recently, the coupling effects in the wurtzite-perovskite heterostructure have been intensified, since it may cause bistable ferroelectric polarization, induce new dielectric and electro-optic properties and open the way to fabricate novel devices [2–4]. Furthermore, a versatile light-switchable resistive switching memory and negative differential resistance have been observed in ZnO/Nb:SrTiO3 heterojunctions [1,5]. It is well known that the optical and electrical properties of ZnO films are sensitive to the strain induced by the lattice and thermal mismatch between the substrates and films [6]. To better understand the roles of heterointerface, it is essential to systemically study the epitaxial properties of the wurtzite– perovskite heterostructure. Generally, epitaxial ZnO films with high quality were deposited on hexagonal substrates such as GaN, AlN, ScAlMgO4, and sapphire, which have small lattice mismatch with ZnO [7–11]. LaAlO3 (LAO), one of the mostly used substrate materials for epitaxial growth of functional oxide films, has a rhombohedral structure, which can be seen as pseudocubic and perovskite structure showing different symmetries with the above-mentioned hexagonal substrates. (0 0 1) LAO single crystal has been reported for growing
n
Corresponding author. Tel.: þ 86 371 23881940; fax: þ 86 371 23880659. E-mail address: [email protected] (W.F. Zhang).
http://dx.doi.org/10.1016/j.jcrysgro.2015.01.029 0022-0248/& 2015 Published by Elsevier B.V.
nonpolar ZnO films due to small lattice mismatch, which will improve the crystalline quality by reducing the density of dislocations [12]. However, systematical research on epitaxial growth behavior of ZnO on (0 0 1)-, (0 1 1)- and (1 1 1) LaAlO3 substrates has little been reported [12–17], which could supply a useful instruction to the integration of ZnO with perovskite functional material. In this paper, we performed a comparative study on the growth behavior of ZnO films on (0 0 1)-, (0 1 1)-, and (1 1 1)LAO single crystal substrates using pulsed laser deposition, and demonstrated the epitaxial relationship by x-ray diffraction θ–2θ and Ф scanning patterns.
2. Experiments ZnO thin films were deposited by pulsed laser deposition (PLD), using a KrF excimer laser (COMPexPro201, Coherent) of 248 nm wavelength and 25 ns pulse duration. Laser energy and frequency were kept as 300 mJ and 3 Hz, respectively. The base vacuum was maintained at 2 10 4 Pa. The target-to-substrate distance in the deposition chamber was about 50 mm. A 2 in. ceramic ZnO target (99.99% purity) was used for ablation. The substrates used were (0 0 1)-, (0 1 1)-, and (1 1 1)- LAO single crystal wafers with sizes of 10 5 0.5 mm3. After being cleaned in an organic solution, the LAO substrates were loaded into the PLD chamber. The substrate temperature for ZnO deposition was 600 1C. The epitaxial relationships and crystal quality were determined by x-ray diffraction (XRD) in θ–2θ, Ф and ω scannings (DX-2700) with CuKα radiation.
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C.H. Jia et al. / Journal of Crystal Growth 421 (2015) 19–22
Photoluminescence (PL) spectra were performed with a Xe lamp (325 nm) and an InGaAs detector at room temperature.
3. Results and discussions Previous work has shown that nonpolar (1 1 2 0) and polar (0 0 0 1) ZnO thin films can be grown on perovskite (0 0 1)-, (0 1 1)and (1 1 1) SrTiO3 (STO) [18]. Since LAO has similar lattice structure and lattice constant to STO, the epitaxial relationships between ZnO films and LAO substrates are expected to be similar to those between ZnO and STO. For ZnO films on these three oriented LAO substrates, no foreign phases and other oriented grains were detected by XRD results shown in Fig. 1, implying that the present growth condition is perfect for studying epitaxial relationships. In order to determine the epitaxial relationship between ZnO films and LAO substrates, XRD θ–2θ and Ф scans were used to determine the out-of-plane and in-plane orientations, respectively. From the XRD θ–2θ pattern shown in Fig. 1(a), the ZnO films show pure nonpolar (1 1 2 0) orientation on (0 0 1)LAO
substrates. In single crystal (1 1 2 0)ZnO, only two crystal planes in the ZnO{1 0 1 0} family have the same angle with the surface (χ¼301), and two peaks separated by 1801 are expected in ZnO{1 0 1 0} Ф patterns, which is just the case in single domain (1 1 2 0) ZnO films on r-sapphire [19]. However, the reflections from the ZnO{1 0 1 0} family show four peaks separated by 901, as shown in Fig. 1(b), implying that two domains perpendicular to each other coexist in the film plane [20]. Furthermore, the peak positions in the Ф scans of ZnO{1 0 1 0} (2θ¼31.771, χ¼301) and LAO {1 0 1} (2θ¼33.451, χ¼451) apart from each other by 451, implying that the angle between their zone axes is 451. Thus, the crystal directions of 〈1 1 0 0〉ZnO and 〈0 1 1〉LAO are parallel to each other, as shown in Fig. 2(a and b). Based on the atomic arrangement between (1 1 2 0)ZnO and (0 0 1)LAO, the lattice mismatches can be calculated. The in-plane translational period of ZnO along pffiffiffi the c axis is cZnO ¼5.206 Å, while the one for [1 1 0]LAO direction is 2aLAO ¼ 5:363 Å, thus pffiffiffithe lattice pffiffiffi misfit is 2.9% for the projection by calculating ðððcZnO 2aLAO Þ= 2aLAOp Þ Similarly, ffiffiffi 100%Þ. pffiffiffi pffiffiffithe lattice mismatch is obtained to be 4.9% ððð 3aZnO 2aLAO Þ= 2aLAO Þ 100%Þ along the directions of 〈1 1 0 0〉ZnO. The x-ray rocking curve
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Fig. 1. X-ray θ 2θ (a, c, e), ω (inset of (a, c, e)), and Ф (b, d, f) scanning patterns of ZnO films on (0 0 1)-, (0 1 1)- and (111) LAO substrates, in which (a, b) are on (0 0 1) LAO, (c, d) are on (0 1 1) LAO, while (e, f) are on (1 1 1)LAO.
C.H. Jia et al. / Journal of Crystal Growth 421 (2015) 19–22
[010] [1100]
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Fig. 2. The atomic arrangements of ZnO films on LAO substrates, in which (a, b) are on (0 0 1)LAO with two domains, while (c, d) are on (0 1 1) and (1 1 1)LAO with a single domain. 10000 8000
Intensity (a.u.)
(XRC) analysis of (1 1 2 0) in ω scan shows that the full width at half maximum (FWHM) is about 0.971, as shown in the inset of Fig. 1(a). The FWHM is larger than that of ZnO on r-sapphire [19], which might be ascribed to the large thermal mismatch between ZnO and LAO due to the difference in thermal expansion coefficients [21]. Generally, a difference is often observed in XRC FWHM along the directions parallel and orthogonal to the in-plane c-axis of ZnO for ZnO on rsapphire [19]. However, there is no much difference in XRC FWHM at different x-ray azimuths in the present work, which is also observed by Ho etal [12]. It is supposed to be mainly due to the small lattice mismatch between ZnO and LAO along different directions. In the case of ZnO film growth on (0 1 1)-LaAlO3 substrates, ZnO films exhibit a c-axis perpendicular to the growth plane. Only six peaks are observed for ZnO{1 0 1 1} family, which has six crystal planes showing the same angle with the growth plane (χ ¼61.611), as shown in Fig. 1(c). Thus, ZnO films are single domain epitaxy on (0 1 1) LaAlO3, which is observed on (0 1 1) STO [18,22]. The peak positions in the Ф scans of ZnO{1 0 1 1} (2θ¼36.261, χ ¼61.611) and LAO {1 0 0} (2θ¼ 23.461, χ¼ 451) coincide, implying that their zone axes are parallel to each other, that is, 〈1 1 0 0〉ZnO//〈0 1 1〉LAO. The atomic arrangements are shown in Fig. 2(c). From the translational periods of ZnO and LAO along different directions, lattice pffiffiffi pthe ffiffiffi pffiffiffi mismatches are calcula ted to be 4.9% ððð 3aZnO 2aLAO Þ= 2aLAO Þ 100%Þ and 14.4% ðððaZnO aLAO Þ=aLAO Þ 100%Þ along the directions of 〈1 1 0 0〉ZnO and 〈1 1 2 0〉ZnO in the film plane, respectively. Similarly, the in-plane orientation relationships for (0 0 0 1) ZnO films on (1 1 1) LAO can also be achieved from x-ray Ф scanning. Fig. 1(d) displays 6 peaks separated by 601 for the ZnO {1 1 2 2} family, which has six crystal planes intersecting the surface at 58.031. From the relative position of ZnO{1 1 2 2} (2θ¼67.951, χ ¼58.031) and LAO{1 0 10} (2θ¼23.461, χ ¼54.741) families, the in-plane relationships is demonstrated to be 〈1 1 0 0〉ZnO//〈0 1 1〉LAO, which is observed for ZnO films on (1 1 1)STO and (1 1 1)LSAT [18,23,24]. The atomic arrangements in the heterointerface of (0 0 0 2)ZnO/(1 1 1)LAO are shown in Fig. 2(d). From the translational periods of ZnO and LAO, the lattice pffiffiffi pffiffiffi pffiffiffi mismatch is 4.9% ððð 3aZnO 2aLAO Þ= 2aLAO Þ 100%Þ along the direction of 〈1 1 0 0〉ZnO on (1 1 1)LAO. Here, it should be pointed out that the epitaxial relationships between ZnO and LAO are similar to those on STO, which is just as expected due to similar lattice structure and lattice constant between LAO and STO.
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Wavelength (nm) Fig. 3. PL spectra of ZnO films on (0 0 1)-, (0 1 1)- and (1 1 1)LAO substrates.
The quality of ZnO thin films grown on (0 1 1)-, and (1 1 1)-LAO were further examined by XRC, as shown in the inset of Fig. 1(c and e). The FWHM is 1.011 and 0.921 for ZnO films on (0 1 1)-, and (1 1 1)-LAO, respectively. The FWHM is larger than that of ZnO on r-sapphire [19], which might be ascribed to the large thermal mismatch between ZnO and LAO due to the difference in thermal expansion coefficients [21]. PL spectra were also performed to evaluate the quality of ZnO films on (0 0 1)-, (0 1 1)- and (1 1 1) LAO substrates, as shown in Fig. 3. The shape of PL spectra is almost the same for ZnO films on three oriented LAO substrates. The maximum PL intensity is observed in ZnO films on (0 0 1)LAO, which may be caused by the smallest in-plane lattice mismatch at the ZnO/(0 0 1)LAO interface and/or variation of polarization or defects [25], considering that the thicknesses of ZnO films with different orientations are almost the same.
4. Conclusions Nonpolar and polar ZnO thin films have been grown on (0 0 1)-, (0 1 1)-, and (1 1 1)LAO substrates by pulsed laser deposition. ZnO films exhibit nonpolar (1 1 2 0)-orientation with two domains and in-plane orientation relationship of 〈0 0 0 1〉ZnO//〈1 1 0〉LAO on (0 0 1) LAO. For (0 1 1)- and (1 1 1)LAO substrates, single-domain epitaxy with c axial orientation and the in-plane relationship of 〈1 1 0 0〉ZnO//〈0 1 1〉LAO was observed. From the in-plane orientation
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C.H. Jia et al. / Journal of Crystal Growth 421 (2015) 19–22
relationship, the lattice mismatches are obtained for these three oriented ZnO/LAO heterointerfaces.
Acknowledgments This work is supported by the National Natural Science Foundation of China (nos. 51202057, 61350012), Foundation of He’nan Educational Committee (12A480001), the Program for Innovative Research Team in Science and Technology in University of Henan Province (IRTSTHN) (Grant no. 2012 IRTSTHN004). References [1] A. Bera, H. Peng, J. Lourembam, Y. Shen, X.W. Sun, T. Wu, Adv. Funct. Mater. 23 (2013) 4977. [2] K.C. Sekhar, J.P.B. Silva, K. Kamakshi, M. Pereira, M.J.M. Gomes, Appl. Phys. Lett. 102 (2013) 212903. [3] Z.W. Li, M.X. Zhou, W.F. Ding, H. Zhou, B. Chen, J.G. Wan, J.M. Liu, G.H. Wang, Appl. Phys. Lett. 100 (2012) 262903. [4] V.M. Voora, T. Hofmann, M. Brandt, M. Lorentz, M. Grundmann, N. Ashkenov, H. Schmidt, N. Ianno, M. Schubert, Phys. Rev. B 81 (2010) 195307. [5] C.H. Jia, X.W. Sun, G.Q. Li, Y.H. Chen, W.F. Zhang, Appl. Phys. Lett. 104 (2014) 043501. [6] X.H. Wei, Y.R. Li, J. Zhu, W. Huang, Y. Zhang, W.B. Luo, H. Ji, Appl. Phys. Lett. 90 (2007) 151918. [7] K.H. Lee, P.C. Chang, T.P. Chen, S.P. Chang, H. Wu. Shiu, L.Y. Chang, C.H. Chen, S.J. Chang, Appl. Phys. Lett. 102 (2013) 072104.
[8] Z.F. Shi, Y.T. Zhang, X.J. Cui, B. Wu, S.W. Zhuang, F. Yang, X.T. Yang, B.L. Zhang, G.T. Zhang, Appl. Phys. Lett. 104 (2014) 131109. [9] S.G. Zhang, X.W. Zhang, Z.G. Yin, J.X. Wang, F.T. Si, H.L. Gao, J.J. Dong, X. Liu, J. Appl. Phys. 112 (2012) 013112. [10] A. Tsukazaki, A. Ohtomo, M. Kawasaki, J. Phys. D: Appl. Phys. 47 (2014) 034003. [11] S. Brochen, M. Lafossas, I.C. Robin, P. Ferret, F. Gemain, J. Pernot, G. Feuillet, J. Appl. Phys. 115 (2014) 113508. [12] Y.T. Ho, W.L. Wang, C.Y. Peng, M.H. Liang, J.S. Tian, C.W. Lin, L. Chang, Appl. Phys. Lett. 93 (2008) 121911. [13] T.C. Yen, W.L. Wang, C.Y. Peng, J.S. Tian, Y.T. Ho, L. Chang, J. Vac. Sci & Tech A 32 (2014) 02B103. [14] J.S. Tian, Y.H. Wu, W.L. Wang, T.C. Yen, Y.T. Ho, L. Chang, Mater. Lett. 109 (2013) 237. [15] Y.T. Ho, W.L. Wang, C.Y. Peng, J.S. Tian, Y. Sen Shih, T.C. Yen, L. Chang, Phys. Status Solidi: Rapid Res. Lett. 6 (2012) 114. [16] W.L. Wang, C.Y. Peng, Y.T. Ho, L. Chang, Thin Solid Films 518 (2010) 2967. [17] Y.T. Ho, W.L. Wang, C.Y. Peng, W.C. Chen, M.H. Liang, J.S. Tian, L. Chang, Phys. Status Solidi: Rapid Res. Lett. 3 (2009) 109. [18] C.H. Jia, Y.H. Chen, X.L. Liu, S.Y. Yang, W.F. Zhang, Z.G. Wang, Nano Res. Lett. 8 (2013) 23. [19] S.K. Han, S.K. Hong, J.W. Lee, J.Y. Lee, J.H. Song, Y.S. Nam, S.K. Chang, T. Minegishi, T. Yao, J. Crystal Growth 309 (2007) 121. [20] Y.L. Wu, L.W. Zhang, G.L. Xie, J.L. Zhu, Y.H. Chen, Appl. Phys. Lett. 92 (2008) 012115. [21] J.S. Tian, M.H. Liang, Y.T. Ho, Y.A. Liu, L. Chang, J. Crystal Growth 310 (2008) 777. [22] E. Bellingeri, D. Marre, I. Pallecchi, L. Pellegrino, A.S. Siri, Appl. Phys. Lett. 86 (2005) 012109. [23] T. Nakamura, H. Minoura, H. Muto, Thin Solid Films 405 (2002) 109. [24] M.J. Ying, X.L. Du, Y.Z. Liu, Z.T. Zhou, Z.Q. Zeng, Z.X. Mei, J.F. Jia, H. Chen, Q.K. Xue, Z. Zhang, Appl. Phys. Lett. 87 (2005) 202107. [25] F. Fabbri, M. Villani, A. Catellani, A. Calzolari, G. Cicero, D. Calestani, G. Calestani, A. Zappettini, B. Dierre, T. Sekiguchi, G. Salviati, Sci. Rep. 4 (2014) 5158.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Epitaxial properties of ZnO thin films on LaAlO3 substrates by pulsed laser deposition C.H. Jia a, S. Wang a, Y.H. Wu a, Y.H. Chen b, X.W. Sun a, W.F. Zhang a,n a b
Key Laboratory of Photovoltaic Materials of Henan Province and School of Physics & Electronics, Henan University, Kaifeng 475004, PR China Key Laboratory of Semiconductor Material Science, Institute of Semiconductors, Chinese Academy of Science, PO Box 912, Beijing, 100083, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 October 2014 Received in revised form 13 January 2015 Accepted 22 January 2015 Communicated by A. Ohtomo Available online 2 February 2015
Wurtzite ZnO thin films with different epitaxial relationships have been grown on (0 0 1)-, (0 1 1)-, and (1 1 1)LaAlO3 (LAO) single-crystal substrates by pulsed laser deposition. Nonpolar (1 1 2 0)ZnO films with two orthogonal domains were obtained on (0 0 1) LAO, in which the in-plane orientation relationship is demonstrated to be 〈0 0 0 1〉ZnO//〈1 1 0〉LAO. For ZnO on (0 1 1)- and (1 1 1)LAO substrates, a single-domain epitaxy with c axial orientation is observed, in which the in-plane relationships were 〈1 1 0 0〉ZnO//〈0 1 1〉LAO irrespective of the substrate orientations. Based on the in-plane orientation relationship, the lattice mismatch has been obtained for these three oriented ZnO/LAO heterointerfaces. & 2015 Published by Elsevier B.V.
Keywords: A1. Epitaxial growth B2. ZnO LaAlO3
1. Introduction Wurtzite ZnO semiconductor materials have attracted much attention due to its excellent optical, optoelectronic, sensing, magnetic and energy harvesting properties [1]. Recently, the coupling effects in the wurtzite-perovskite heterostructure have been intensified, since it may cause bistable ferroelectric polarization, induce new dielectric and electro-optic properties and open the way to fabricate novel devices [2–4]. Furthermore, a versatile light-switchable resistive switching memory and negative differential resistance have been observed in ZnO/Nb:SrTiO3 heterojunctions [1,5]. It is well known that the optical and electrical properties of ZnO films are sensitive to the strain induced by the lattice and thermal mismatch between the substrates and films [6]. To better understand the roles of heterointerface, it is essential to systemically study the epitaxial properties of the wurtzite– perovskite heterostructure. Generally, epitaxial ZnO films with high quality were deposited on hexagonal substrates such as GaN, AlN, ScAlMgO4, and sapphire, which have small lattice mismatch with ZnO [7–11]. LaAlO3 (LAO), one of the mostly used substrate materials for epitaxial growth of functional oxide films, has a rhombohedral structure, which can be seen as pseudocubic and perovskite structure showing different symmetries with the above-mentioned hexagonal substrates. (0 0 1) LAO single crystal has been reported for growing
n
Corresponding author. Tel.: þ 86 371 23881940; fax: þ 86 371 23880659. E-mail address: [email protected] (W.F. Zhang).
http://dx.doi.org/10.1016/j.jcrysgro.2015.01.029 0022-0248/& 2015 Published by Elsevier B.V.
nonpolar ZnO films due to small lattice mismatch, which will improve the crystalline quality by reducing the density of dislocations [12]. However, systematical research on epitaxial growth behavior of ZnO on (0 0 1)-, (0 1 1)- and (1 1 1) LaAlO3 substrates has little been reported [12–17], which could supply a useful instruction to the integration of ZnO with perovskite functional material. In this paper, we performed a comparative study on the growth behavior of ZnO films on (0 0 1)-, (0 1 1)-, and (1 1 1)LAO single crystal substrates using pulsed laser deposition, and demonstrated the epitaxial relationship by x-ray diffraction θ–2θ and Ф scanning patterns.
2. Experiments ZnO thin films were deposited by pulsed laser deposition (PLD), using a KrF excimer laser (COMPexPro201, Coherent) of 248 nm wavelength and 25 ns pulse duration. Laser energy and frequency were kept as 300 mJ and 3 Hz, respectively. The base vacuum was maintained at 2 10 4 Pa. The target-to-substrate distance in the deposition chamber was about 50 mm. A 2 in. ceramic ZnO target (99.99% purity) was used for ablation. The substrates used were (0 0 1)-, (0 1 1)-, and (1 1 1)- LAO single crystal wafers with sizes of 10 5 0.5 mm3. After being cleaned in an organic solution, the LAO substrates were loaded into the PLD chamber. The substrate temperature for ZnO deposition was 600 1C. The epitaxial relationships and crystal quality were determined by x-ray diffraction (XRD) in θ–2θ, Ф and ω scannings (DX-2700) with CuKα radiation.
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C.H. Jia et al. / Journal of Crystal Growth 421 (2015) 19–22
Photoluminescence (PL) spectra were performed with a Xe lamp (325 nm) and an InGaAs detector at room temperature.
3. Results and discussions Previous work has shown that nonpolar (1 1 2 0) and polar (0 0 0 1) ZnO thin films can be grown on perovskite (0 0 1)-, (0 1 1)and (1 1 1) SrTiO3 (STO) [18]. Since LAO has similar lattice structure and lattice constant to STO, the epitaxial relationships between ZnO films and LAO substrates are expected to be similar to those between ZnO and STO. For ZnO films on these three oriented LAO substrates, no foreign phases and other oriented grains were detected by XRD results shown in Fig. 1, implying that the present growth condition is perfect for studying epitaxial relationships. In order to determine the epitaxial relationship between ZnO films and LAO substrates, XRD θ–2θ and Ф scans were used to determine the out-of-plane and in-plane orientations, respectively. From the XRD θ–2θ pattern shown in Fig. 1(a), the ZnO films show pure nonpolar (1 1 2 0) orientation on (0 0 1)LAO
substrates. In single crystal (1 1 2 0)ZnO, only two crystal planes in the ZnO{1 0 1 0} family have the same angle with the surface (χ¼301), and two peaks separated by 1801 are expected in ZnO{1 0 1 0} Ф patterns, which is just the case in single domain (1 1 2 0) ZnO films on r-sapphire [19]. However, the reflections from the ZnO{1 0 1 0} family show four peaks separated by 901, as shown in Fig. 1(b), implying that two domains perpendicular to each other coexist in the film plane [20]. Furthermore, the peak positions in the Ф scans of ZnO{1 0 1 0} (2θ¼31.771, χ¼301) and LAO {1 0 1} (2θ¼33.451, χ¼451) apart from each other by 451, implying that the angle between their zone axes is 451. Thus, the crystal directions of 〈1 1 0 0〉ZnO and 〈0 1 1〉LAO are parallel to each other, as shown in Fig. 2(a and b). Based on the atomic arrangement between (1 1 2 0)ZnO and (0 0 1)LAO, the lattice mismatches can be calculated. The in-plane translational period of ZnO along pffiffiffi the c axis is cZnO ¼5.206 Å, while the one for [1 1 0]LAO direction is 2aLAO ¼ 5:363 Å, thus pffiffiffithe lattice pffiffiffi misfit is 2.9% for the projection by calculating ðððcZnO 2aLAO Þ= 2aLAOp Þ Similarly, ffiffiffi 100%Þ. pffiffiffi pffiffiffithe lattice mismatch is obtained to be 4.9% ððð 3aZnO 2aLAO Þ= 2aLAO Þ 100%Þ along the directions of 〈1 1 0 0〉ZnO. The x-ray rocking curve
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Fig. 1. X-ray θ 2θ (a, c, e), ω (inset of (a, c, e)), and Ф (b, d, f) scanning patterns of ZnO films on (0 0 1)-, (0 1 1)- and (111) LAO substrates, in which (a, b) are on (0 0 1) LAO, (c, d) are on (0 1 1) LAO, while (e, f) are on (1 1 1)LAO.
C.H. Jia et al. / Journal of Crystal Growth 421 (2015) 19–22
[010] [1100]
[0001]
[010] [0001]
cZnO
[1100] [011]
3 aZnO
(011)LAO (0001)ZnO
[1100] aLAO
(001)LAO (1120)ZnO
3 aZnO
aLAO
[1100] [011]
aZnO [1120] [100]
cZnO [100]
[100] (001)LAO (1120)ZnO
21
2 aLAO
aLAO
(111)LAO (0001)ZnO
2 aLAO
[1120] [211]
aZnO
Fig. 2. The atomic arrangements of ZnO films on LAO substrates, in which (a, b) are on (0 0 1)LAO with two domains, while (c, d) are on (0 1 1) and (1 1 1)LAO with a single domain. 10000 8000
Intensity (a.u.)
(XRC) analysis of (1 1 2 0) in ω scan shows that the full width at half maximum (FWHM) is about 0.971, as shown in the inset of Fig. 1(a). The FWHM is larger than that of ZnO on r-sapphire [19], which might be ascribed to the large thermal mismatch between ZnO and LAO due to the difference in thermal expansion coefficients [21]. Generally, a difference is often observed in XRC FWHM along the directions parallel and orthogonal to the in-plane c-axis of ZnO for ZnO on rsapphire [19]. However, there is no much difference in XRC FWHM at different x-ray azimuths in the present work, which is also observed by Ho etal [12]. It is supposed to be mainly due to the small lattice mismatch between ZnO and LAO along different directions. In the case of ZnO film growth on (0 1 1)-LaAlO3 substrates, ZnO films exhibit a c-axis perpendicular to the growth plane. Only six peaks are observed for ZnO{1 0 1 1} family, which has six crystal planes showing the same angle with the growth plane (χ ¼61.611), as shown in Fig. 1(c). Thus, ZnO films are single domain epitaxy on (0 1 1) LaAlO3, which is observed on (0 1 1) STO [18,22]. The peak positions in the Ф scans of ZnO{1 0 1 1} (2θ¼36.261, χ ¼61.611) and LAO {1 0 0} (2θ¼ 23.461, χ¼ 451) coincide, implying that their zone axes are parallel to each other, that is, 〈1 1 0 0〉ZnO//〈0 1 1〉LAO. The atomic arrangements are shown in Fig. 2(c). From the translational periods of ZnO and LAO along different directions, lattice pffiffiffi pthe ffiffiffi pffiffiffi mismatches are calcula ted to be 4.9% ððð 3aZnO 2aLAO Þ= 2aLAO Þ 100%Þ and 14.4% ðððaZnO aLAO Þ=aLAO Þ 100%Þ along the directions of 〈1 1 0 0〉ZnO and 〈1 1 2 0〉ZnO in the film plane, respectively. Similarly, the in-plane orientation relationships for (0 0 0 1) ZnO films on (1 1 1) LAO can also be achieved from x-ray Ф scanning. Fig. 1(d) displays 6 peaks separated by 601 for the ZnO {1 1 2 2} family, which has six crystal planes intersecting the surface at 58.031. From the relative position of ZnO{1 1 2 2} (2θ¼67.951, χ ¼58.031) and LAO{1 0 10} (2θ¼23.461, χ ¼54.741) families, the in-plane relationships is demonstrated to be 〈1 1 0 0〉ZnO//〈0 1 1〉LAO, which is observed for ZnO films on (1 1 1)STO and (1 1 1)LSAT [18,23,24]. The atomic arrangements in the heterointerface of (0 0 0 2)ZnO/(1 1 1)LAO are shown in Fig. 2(d). From the translational periods of ZnO and LAO, the lattice pffiffiffi pffiffiffi pffiffiffi mismatch is 4.9% ððð 3aZnO 2aLAO Þ= 2aLAO Þ 100%Þ along the direction of 〈1 1 0 0〉ZnO on (1 1 1)LAO. Here, it should be pointed out that the epitaxial relationships between ZnO and LAO are similar to those on STO, which is just as expected due to similar lattice structure and lattice constant between LAO and STO.
ZnO/(001)LAO 6000 4000
ZnO/(011)LAO
2000
ZnO/(111)LAO 0 350
400
450
500
550
600
Wavelength (nm) Fig. 3. PL spectra of ZnO films on (0 0 1)-, (0 1 1)- and (1 1 1)LAO substrates.
The quality of ZnO thin films grown on (0 1 1)-, and (1 1 1)-LAO were further examined by XRC, as shown in the inset of Fig. 1(c and e). The FWHM is 1.011 and 0.921 for ZnO films on (0 1 1)-, and (1 1 1)-LAO, respectively. The FWHM is larger than that of ZnO on r-sapphire [19], which might be ascribed to the large thermal mismatch between ZnO and LAO due to the difference in thermal expansion coefficients [21]. PL spectra were also performed to evaluate the quality of ZnO films on (0 0 1)-, (0 1 1)- and (1 1 1) LAO substrates, as shown in Fig. 3. The shape of PL spectra is almost the same for ZnO films on three oriented LAO substrates. The maximum PL intensity is observed in ZnO films on (0 0 1)LAO, which may be caused by the smallest in-plane lattice mismatch at the ZnO/(0 0 1)LAO interface and/or variation of polarization or defects [25], considering that the thicknesses of ZnO films with different orientations are almost the same.
4. Conclusions Nonpolar and polar ZnO thin films have been grown on (0 0 1)-, (0 1 1)-, and (1 1 1)LAO substrates by pulsed laser deposition. ZnO films exhibit nonpolar (1 1 2 0)-orientation with two domains and in-plane orientation relationship of 〈0 0 0 1〉ZnO//〈1 1 0〉LAO on (0 0 1) LAO. For (0 1 1)- and (1 1 1)LAO substrates, single-domain epitaxy with c axial orientation and the in-plane relationship of 〈1 1 0 0〉ZnO//〈0 1 1〉LAO was observed. From the in-plane orientation
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relationship, the lattice mismatches are obtained for these three oriented ZnO/LAO heterointerfaces.
Acknowledgments This work is supported by the National Natural Science Foundation of China (nos. 51202057, 61350012), Foundation of He’nan Educational Committee (12A480001), the Program for Innovative Research Team in Science and Technology in University of Henan Province (IRTSTHN) (Grant no. 2012 IRTSTHN004). References [1] A. Bera, H. Peng, J. Lourembam, Y. Shen, X.W. Sun, T. Wu, Adv. Funct. Mater. 23 (2013) 4977. [2] K.C. Sekhar, J.P.B. Silva, K. Kamakshi, M. Pereira, M.J.M. Gomes, Appl. Phys. Lett. 102 (2013) 212903. [3] Z.W. Li, M.X. Zhou, W.F. Ding, H. Zhou, B. Chen, J.G. Wan, J.M. Liu, G.H. Wang, Appl. Phys. Lett. 100 (2012) 262903. [4] V.M. Voora, T. Hofmann, M. Brandt, M. Lorentz, M. Grundmann, N. Ashkenov, H. Schmidt, N. Ianno, M. Schubert, Phys. Rev. B 81 (2010) 195307. [5] C.H. Jia, X.W. Sun, G.Q. Li, Y.H. Chen, W.F. Zhang, Appl. Phys. Lett. 104 (2014) 043501. [6] X.H. Wei, Y.R. Li, J. Zhu, W. Huang, Y. Zhang, W.B. Luo, H. Ji, Appl. Phys. Lett. 90 (2007) 151918. [7] K.H. Lee, P.C. Chang, T.P. Chen, S.P. Chang, H. Wu. Shiu, L.Y. Chang, C.H. Chen, S.J. Chang, Appl. Phys. Lett. 102 (2013) 072104.
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