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ZnO:Al ferroelectric-semiconductor heterostructure by pulsed laser deposition
Materials Letters 79 (2012) 173–176
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Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation and characterization of transparent Bi3.6Ho0.4Ti3O12/ZnO: Al ferroelectric-semiconductor heterostructure by pulsed laser deposition Sijun Luo a,⁎, Chuanbin Wang a, Song Zhang b, Rong Tu b, Qiang Shen a, Fei Chen a, Lianmeng Zhang a a b
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 13 February 2012 Accepted 5 April 2012 Available online 12 April 2012 Keywords: Ferroelectrics Semiconductors Thin films Physical vapor deposition Spectroscopy
a b s t r a c t Ho-doped Bi4Ti3O12 ferroelectric thin films with the composition of Bi3.6Ho0.4Ti3O12 (BHT) were integrated with the epitaxial (0001)-oriented Al-doped ZnO (AZO) semiconductor thin films grown on c-sapphire substrates by pulsed laser deposition using a pure phase BHT ceramic target. The structure characterizations indicated that the BHT film grown on AZO (0001) was a single phase structure of Bi-layered Aurivillius phase bismuth titanate and showed (100)-preferred orientation. The BHT/AZO heterostructure was with a smooth interface and a uniform microstructure. All samples with various thickness were transparent and the average transmittance of (500 nm)BHT/(300 nm)AZO was higher than 89% in the range of 400–800 nm. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently integrating ferroelectric oxide materials with wide band gap semiconductors has drawn worldwide attention for potential application in new devices with novel functionalities [1–8]. The multifunctional allin-one for integrated ferroelectric oxide-wide band gap semiconductor thin film heterostructures could not only achieve the miniaturization and monolithic development of electronic systems but also provide expected possibility in existing coupling effects across the interface. Exploring novel ferroelectric-wide band gap semiconductor heterostructures is important to both of the fabrication of new devices with novel functionalities and the fundamental study on application and theory. Large remanent polarization of ferroelectric thin film and excellent semiconductor characteristic of wide band gap semiconductor thin film are the essential foundation. Bi4Ti3O12 (BiT) is an Aurivillius phase Bi-layered ferroelectric oxide. Lanthanide-doped BiT thin films have attracted abundant attention in the last decade due to its fatigue-free characteristic and large remanent polarization for potential application in lead-free nonvolatile ferroelectric random access memories [9–16]. Guo et al. reported that the Ho-doped BiT thin film with the composition of Bi3.6Ho0.4Ti3O12 (BHT) showed excellent ferroelectric and fatigue performance [15]. ZnO semiconductor materials have attracted a great deal of interest owing to its characteristics of piezoelectric and wide band gap semiconductor [17]. The Al-doped ZnO (AZO)
⁎ Corresponding author. Tel.: + 86 27 87217492; fax: + 86 27 87879468. E-mail address: [email protected] (S. Luo). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.04.032
thin film, as a transparent conductive oxide film, is a good candidate electrode for semiconductor photoelectric devices like light emitting diode, solar cells and power photoelectric devices [17–20]. The integration of the BHT ferroelectric thin film with AZO transparent semiconductor thin film will open the way to the fabrication of new devices with novel functionalities, based on the thin film heterostructure composed of Bi4Ti3O12-based ferroelectric thin film and ZnO-based semiconductor thin film, useful both for application and fundamental studies. In this paper we report on a preparation of the BHT/AZO heterostructure grown on c-sapphire substrates by pulsed laser deposition (PLD), aiming at exploring a novel ferroelectric-wide band gap semiconductor thin films heterostructure for potential application in new multifunctional device. The microstructure and optical transmittance of the BHT/AZO heterostructure were investigated. 2. Experimental procedures The (0001)-oriented Al (3 wt.%)-doped ZnO (AZO) thin films were epitaxially grown on c-sapphire substrates firstly and then the Bi3.6Ho0.4Ti3O12 (BHT) films were deposited on the AZO layers. Both of the BHT and AZO films were grown by pulsed laser deposition under an oxygen partial pressure of 13 Pa using a Nd:YAG (yttrium aluminum garnet) laser beam with a wavelength of 355 nm at a repetition rate of 10 Hz. The pulsed laser energy (energy density on target) and deposition temperature for growing BHT and AZO films were 150 mJ (8.5 J/cm2) and 60 mJ (3.4 J/cm2), 700 °C and 600 °C, respectively. The thickness of BHT films and AZO films were about from 500 to 1000 nm and from 300 to 600 nm, respectively. The AZO (Al: 3 wt.%) ceramic target was
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sintering by hot-press method using mixed high-purity powders of ZnO (99.99%) and Al2O3 (99.99%). BHT ceramic target was prepared by using a two-step, solid state reaction combined with hot-press method. The mixed high-purity powders of Bi2O3 (99.9%), TiO2 (99.9%) and Ho2O3 (99.9%), which were weighed in stoichiometric amounts with 3 wt.% excessive Bi2O3 to compensate the loss of Bi at the high temperature, were pressed to pellet at 20 MPa and then sintered in air for the solid state reaction at 900 °C for 8 h. The resulting pellet was crushed and grinded to fine powders in the grinding bowl, and the resulting powders were sintered by hot-press at 850 °C under 30 MPa for 2 h. The microstructure of the samples was investigated using X-ray diffraction (XRD, Rigaku Ultima III) with Cu Kα radiation. Raman spectra were recorded using a Raman spectroscopy (Renishaw InVia) with an Ar + laser (wavelength 514.5 nm). The surface and cross-section morphologies of films were observed by scanning electron microscopy (SEM, Hitachi S-4800). The UV–vis transmittance spectra were performed by UV–visible-NIR spectrophotometer (Shimadzu UV-2550). All measurements were carried out at room temperature.
3. Results and discussion Fig. 1 shows XRD patterns of the BHT ceramic target and the (1000 nm)BHT/(600 nm)AZO heterostructure grown on c-sapphire substrate. The XRD patterns are indexed according to the standard
Fig. 1. X-ray diffraction patterns for (a) BHT ceramic target and (b) (1000 nm)BHT/ (600 nm)AZO heterostructure grown on c-sapphire substrate.
powder diffraction data of BiT (PDF#72-1019) [21]. From Fig. 1(a) it can been seen that the crystal structure of the BHT ceramic target consists of a single phase of Bi-layered Aurivillius phase and no impurity is detected in the sample, which suggests that the Ho 3+ ions have entered into the lattice of BiT. As shown in Fig. 1(b), besides the peak of c-sapphire substrate, only diffraction peaks corresponding to the wurtzite AZO (0002) and (0004), Aurivillius phase BHT (200)/(020), (400)/(040) and (117) reflections were detected, indicating that the bilayer oxide films are pure phase wurtzite and bismuth titanate. The sharp (0002) and (0004) reflections of AZO layer indicates that the AZO layer is well crystallized and highly (0001)-oriented. Similarly, the sharp (200) and (400) reflections of BHT film also reveals a high quality of crystalline and high (100) orientation in BHT film, which indicates that the BHT films grown on AZO (0001) buffer show (100)preferred orientation. It is acknowledged that the vector of the (major) spontaneous polarization in lanthanide-doped BiT is along its a-axis and the lanthanide-doped BiT thin films with (100) orientation show a large remanent polarization [10,14]. The (100)-preferred BHT film integrated with AZO(0001) film offers a greater potential to application in novel devices based on the functionality of ferroelectric polarization. In order to further confirm the phase structure of BHT thin film, the Raman spectrum measurement was used. The Raman spectra of the BHT ceramic target and (1000 nm)BHT/(600 nm)AZO heterostructure are illustrated in Fig. 2. Compare with Raman data of BiT [22,23], Hodoped BiT ceramic exhibits intense phonon modes at about 129, 149, 268, 325, 545, 560, 618 and 852 cm− 1 and many other weak features or overlap modes. The two peaks at 129 and 149 cm− 1 are related to the Bi atoms in the perovskite-like unit [23], and the six peaks appeared at 268, 325, 545, 560, 618 and 852 cm− 1 are caused by the internal vibrational modes of TiO6 octahedra [23,24]. The characters of position and shape of peaks for the pure phase BHT ceramic are similar to that observed in Nd- and La-doped BiT ceramics [23,24], which indicates that Ho ions has substituted Bi ions in the lattice of BiT. As shown from the phonon modes of the BHT film, there are five apparent peaks appeared at 148, 260, 535, 561 and 856 cm− 1 corresponding to the phonon modes of BHT ceramic though the peaks are remarkably broadening, weakening and overlaping compared to that of BHT ceramic. The additional phonon mode at 442 cm− 1 is induced by the increase in TiO6 octahedron tilting and structural distortion resulted from the Ho substitution in the BiT lattice, and the mode is also observed in Nd-doped BiT single crystal [24] and nanotube arrays [25]. The result suggests that the (100)-preferred BHT film grown on AZO (0001) layer is Bi-layered Aurivillius phase bismuth titanate, which is in agreement with the result from XRD analysis.
Fig. 2. Raman spectra of BHT ceramic target and (1000 nm)BHT/(600 nm)AZO heterostructure grown on c-sapphire substrate.
S. Luo et al. / Materials Letters 79 (2012) 173–176
Fig. 3 shows the surface and cross-section morphologies of the (1000 nm)BHT/(600 nm)AZO heterostructure. As shown in Fig. 3(a), most grains are equiaxed ones in diameter of less than 100 nm, which are as the same shape as lanthanide-doped BiT grains with (100) orientation in other reports [10,14]. According to the result from XRD pattern, the other ellipse grains or catenulate grains may attribute to the grains with other orientations or the combination of neighboring (100)-oriented grains. From Fig. 3(b) the cross-section morphology shows a clear cylindrical growth pattern of BHT grains vertically grown on AZO (0001) plane. As shown in the image, the BHT/AZO heterostructure is crack-free and uniform, with a dense microstructure and has a smooth interface. The measurement of UV–vis transmittance spectrum was carried out to evaluate the application potential for BHT/AZO heterostructure in transparent devices or photoelectric devices. Fig. 4 presents the UV–vis transmittance spectra of BHT/AZO heterostructures grown on c-sapphire substrates. The dramatically dropped transmittances in the UV region are due to the fundamental absorption resulted from ZnO. The BHT/AZO heterostructures are typically transparent, and compared to (1000 nm)BHT/(600 nm)AZO, the (500 nm)BHT/AZO heterostructures show an apparently higher average transmittance. The average transmittance of (500 nm)BHT/(300 nm)AZO is higher than 89% in the range of 400–800 nm. Although the average transmittance of the (500 nm) BHT/(300 nm)AZO heterostructure is lower than the value 95% of the epitaxial (300 nm)AZO buffer (not shown), the high transmittance more than 89% supports a potential opportunity to make use of the BHT/AZO heterostructure for application in transparent or photoelectric devices.
175
Fig. 4. UV–vis transmittance spectra of BHT/AZO heterostructures grown on c-sapphire substrates.
4. Summary Bi3.6Ho0.4Ti3O12 (BHT) ferroelectric thin films were grown on epitaxial (0001)-oriented Al-doped ZnO (AZO) semiconductor thin films which buffered c-sapphire substrates by pulsed laser deposition. The BHT film grown on AZO(0001) was a single phase structure of Bi-layered Aurivillius phase bismuth titanate and showed (100)-preferred orientation. The BHT/AZO heterostructures were crack-free and uniform, with a dense microstructure and had a smooth interface. Furthermore, the BHT/AZO heterostructures were typically transparent, and the average transmittance of (500 nm)BHT/(300 nm)AZO was higher than 89% in the range of 400–800 nm. This work reveals that the transparent BHT/ AZO ferroelectric-semiconductor heterostructure would be a novel material integrated with optical, ferroelectric and semiconductor characteristics for potential application in new devices with novel functionalities. Acknowledgements This work was financially supported by the New Century Excellent Talents in University (NCET-10-0662), International Science and Technology Cooperation Project of Hubei Province (2010BFA017), International Science and Technology Cooperation Project (2009DFB50470) and International Science & Technology Cooperation Program of China (No. 2011DFA52650). References
Fig. 3. SEM (a) surface and (b) cross-section morphologies of the (1000 nm)BHT/ (600 nm)AZO heterostructure grown on c-sapphire substrate.
[1] Ahn CH, Rabe KM, Triscone JM. Ferroelectricity at the nanoscale: local polarization in oxide thin films and heterostructures. Science 2004;303:488–91. [2] Posadas A, Yau JB, Ahn CH, Han J, Gariglio S, Johnston K, et al. Epitaxial growth of multiferroic YMnO3 on GaN. Appl Phys Lett 2005;87:171915. [3] Chye Y, Liu T, Li D, Lee K, Lederman D, Myers TH. Molecular beam epitaxy of YMnO3 on c-plane GaN. Appl Phys Lett 2006;88:132903. [4] Hao L, Zhu J, Liu Y, Wang S, Zeng H, Liao X, et al. Integration and electrical properties of epitaxial LiNbO3 ferroelectric film on n-type GaN semiconductor. Thin Solid Films 2012;520:3035–8. [5] Bellingeri E, Marre D, Pallecchi I, Pellegrino L, Siri AS. High mobility in ZnO thin films deposited on perovskite substrates with a low temperature nucleation layer. Appl Phys Lett 2005;86:012109. [6] Siddiqui J, Cagin E, Chen D, Phillips JD. ZnO thin-film transistors with polycrystalline (Ba, Sr)TiO3 gate insulators. Appl Phys Lett 2006;88:212903. [7] Wei XH, Li YR, Zhu J, Huang W, Zhang Y, Luo WB, et al. Epitaxial properties of ZnO thin films on SrTiO3 substrates grown by laser molecular beam epitaxy. Appl Phys Lett 2007;90:151918. [8] Wu J, Wang J. Diodelike and resistive hysteresis behavior of heterolayered BiFeO3/ZnO ferroelectric thin films. J Appl Phys 2010;108:094107. [9] Park BH, Kang BS, Bu SD, Noh TW, Lee J, Jo W. Lanthanum-substituted bismuth titanate for use in non-volatile memories. Nature 1999;401:682–4. [10] Lee HN, Hesse D, Zakharov N, Gösele U. Ferroelectric Bi3.25La0.75Ti3O12 films of uniform a-axis orientation on silicon substrates. Science 2002;296:2006–9.
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[11] Garg A, Barber ZH, Dawber M, Scott JF, Snedden A, Lightfoot P. Orientation dependence of ferroelectric properties of pulsed-laser-ablated Bi4 − xNdxTi3O12 films. Appl Phys Lett 2003;83:2414–6. [12] Watanabe T, Funakubo H. Controlled crystal growth of layered-perovskite thin films as an approach to study their basic properties. J Appl Phys 2006;100: 051602. [13] Watanabe T, Funakubo H, Osada M, Uchida H, Okada I, Rodriguez BJ, et al. Probing intrinsic polarization properties in bismuth-layered ferroelectric films. Appl Phys Lett 2007;90:112914. [14] Hu GD, Fan SH, Cheng X. Anisotropy of ferroelectric and piezoelectric properties of Bi3.15Pr0.85Ti3O12 thin films on Pt(100)/Ti/SiO2/Si substrates. J Appl Phys 2007;101:054111. [15] Guo D, Li M, Wang J, Liu J, Yu B, Yang B. Ferroelectric properties of Bi3.6Ho0.4Ti3O12 thin films prepared by sol–gel method. Appl Phys Lett 2007;91:232905. [16] Xue KH, Araujo CAP, Celinska J. A comparative study on Bi4Ti3O12 and Bi3.25La0.75Ti3O12 ferroelectric thin films derived by metal organic decomposition. J Appl Phys 2010;107: 104123. [17] Özgür Ü, Alivov YI, Liu C, Teke A, Reshchikov MA, Doğan S, et al. A comprehensive review of ZnO materials and devices. J Appl Phys 2005;98:041301.
[18] Agura H, Suzuki A, Matsushita T, Aoki T, Okuda M. Low resistivity transparent conducting Al-doped ZnO films prepared by pulsed laser deposition. Thin Solid Films 2003;445:263–7. [19] Tanaka H, Ihara K, Miyata T, Sato H, Minami T. Low resistivity polycrystalline ZnO:Al thin films prepared by pulsed laser deposition. J Vac Sci Technol A 2004;22:1757–62. [20] Zhu H, Hupkes J, Bunte E, Gerber A, Huang SM. Influence of working pressure on ZnO:Al films from tube targets for silicon thin film solar cells. Thin Solid Films 2010;518:4997–5002. [21] Dorrian JF, Newnham RE, Smith DK, Kay MI. Crystal structure of Bi4Ti3O12. Ferroelectric 1971;3:17–27. [22] Graves PR, Hua G, Myhra S, Thompson JG. The Raman modes of the Aurivillius phases: temperature and polarization dependence. J Solid State Chem 1995;114:112–22. [23] Kim YI, Nahm SH, Yoon DJ, Gregory DH. Site preference of La in Bi3.75La0.25Ti3O12 using neutron powder diffraction and Raman scattering. J Electroceram 2005;14:265–71. [24] Liang K, Qi Y, Lu C. Temperature-dependent Raman scattering in ferroelectric Bi4 − xNdxTi3O12 (x= 0, 0.5, 0.85) single crystals. J Raman Spectrosc 2009;40:2088–91. [25] Zhou D, Gu H, Hu Y, Qian Z, Hu Z, Yang K, et al. Raman scattering, electronic, and ferroelectric properties of Nd modified Bi4Ti3O12 nanotube arrays. J Appl Phys 2010;107:094105.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation and characterization of transparent Bi3.6Ho0.4Ti3O12/ZnO: Al ferroelectric-semiconductor heterostructure by pulsed laser deposition Sijun Luo a,⁎, Chuanbin Wang a, Song Zhang b, Rong Tu b, Qiang Shen a, Fei Chen a, Lianmeng Zhang a a b
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 13 February 2012 Accepted 5 April 2012 Available online 12 April 2012 Keywords: Ferroelectrics Semiconductors Thin films Physical vapor deposition Spectroscopy
a b s t r a c t Ho-doped Bi4Ti3O12 ferroelectric thin films with the composition of Bi3.6Ho0.4Ti3O12 (BHT) were integrated with the epitaxial (0001)-oriented Al-doped ZnO (AZO) semiconductor thin films grown on c-sapphire substrates by pulsed laser deposition using a pure phase BHT ceramic target. The structure characterizations indicated that the BHT film grown on AZO (0001) was a single phase structure of Bi-layered Aurivillius phase bismuth titanate and showed (100)-preferred orientation. The BHT/AZO heterostructure was with a smooth interface and a uniform microstructure. All samples with various thickness were transparent and the average transmittance of (500 nm)BHT/(300 nm)AZO was higher than 89% in the range of 400–800 nm. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently integrating ferroelectric oxide materials with wide band gap semiconductors has drawn worldwide attention for potential application in new devices with novel functionalities [1–8]. The multifunctional allin-one for integrated ferroelectric oxide-wide band gap semiconductor thin film heterostructures could not only achieve the miniaturization and monolithic development of electronic systems but also provide expected possibility in existing coupling effects across the interface. Exploring novel ferroelectric-wide band gap semiconductor heterostructures is important to both of the fabrication of new devices with novel functionalities and the fundamental study on application and theory. Large remanent polarization of ferroelectric thin film and excellent semiconductor characteristic of wide band gap semiconductor thin film are the essential foundation. Bi4Ti3O12 (BiT) is an Aurivillius phase Bi-layered ferroelectric oxide. Lanthanide-doped BiT thin films have attracted abundant attention in the last decade due to its fatigue-free characteristic and large remanent polarization for potential application in lead-free nonvolatile ferroelectric random access memories [9–16]. Guo et al. reported that the Ho-doped BiT thin film with the composition of Bi3.6Ho0.4Ti3O12 (BHT) showed excellent ferroelectric and fatigue performance [15]. ZnO semiconductor materials have attracted a great deal of interest owing to its characteristics of piezoelectric and wide band gap semiconductor [17]. The Al-doped ZnO (AZO)
⁎ Corresponding author. Tel.: + 86 27 87217492; fax: + 86 27 87879468. E-mail address: [email protected] (S. Luo). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.04.032
thin film, as a transparent conductive oxide film, is a good candidate electrode for semiconductor photoelectric devices like light emitting diode, solar cells and power photoelectric devices [17–20]. The integration of the BHT ferroelectric thin film with AZO transparent semiconductor thin film will open the way to the fabrication of new devices with novel functionalities, based on the thin film heterostructure composed of Bi4Ti3O12-based ferroelectric thin film and ZnO-based semiconductor thin film, useful both for application and fundamental studies. In this paper we report on a preparation of the BHT/AZO heterostructure grown on c-sapphire substrates by pulsed laser deposition (PLD), aiming at exploring a novel ferroelectric-wide band gap semiconductor thin films heterostructure for potential application in new multifunctional device. The microstructure and optical transmittance of the BHT/AZO heterostructure were investigated. 2. Experimental procedures The (0001)-oriented Al (3 wt.%)-doped ZnO (AZO) thin films were epitaxially grown on c-sapphire substrates firstly and then the Bi3.6Ho0.4Ti3O12 (BHT) films were deposited on the AZO layers. Both of the BHT and AZO films were grown by pulsed laser deposition under an oxygen partial pressure of 13 Pa using a Nd:YAG (yttrium aluminum garnet) laser beam with a wavelength of 355 nm at a repetition rate of 10 Hz. The pulsed laser energy (energy density on target) and deposition temperature for growing BHT and AZO films were 150 mJ (8.5 J/cm2) and 60 mJ (3.4 J/cm2), 700 °C and 600 °C, respectively. The thickness of BHT films and AZO films were about from 500 to 1000 nm and from 300 to 600 nm, respectively. The AZO (Al: 3 wt.%) ceramic target was
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sintering by hot-press method using mixed high-purity powders of ZnO (99.99%) and Al2O3 (99.99%). BHT ceramic target was prepared by using a two-step, solid state reaction combined with hot-press method. The mixed high-purity powders of Bi2O3 (99.9%), TiO2 (99.9%) and Ho2O3 (99.9%), which were weighed in stoichiometric amounts with 3 wt.% excessive Bi2O3 to compensate the loss of Bi at the high temperature, were pressed to pellet at 20 MPa and then sintered in air for the solid state reaction at 900 °C for 8 h. The resulting pellet was crushed and grinded to fine powders in the grinding bowl, and the resulting powders were sintered by hot-press at 850 °C under 30 MPa for 2 h. The microstructure of the samples was investigated using X-ray diffraction (XRD, Rigaku Ultima III) with Cu Kα radiation. Raman spectra were recorded using a Raman spectroscopy (Renishaw InVia) with an Ar + laser (wavelength 514.5 nm). The surface and cross-section morphologies of films were observed by scanning electron microscopy (SEM, Hitachi S-4800). The UV–vis transmittance spectra were performed by UV–visible-NIR spectrophotometer (Shimadzu UV-2550). All measurements were carried out at room temperature.
3. Results and discussion Fig. 1 shows XRD patterns of the BHT ceramic target and the (1000 nm)BHT/(600 nm)AZO heterostructure grown on c-sapphire substrate. The XRD patterns are indexed according to the standard
Fig. 1. X-ray diffraction patterns for (a) BHT ceramic target and (b) (1000 nm)BHT/ (600 nm)AZO heterostructure grown on c-sapphire substrate.
powder diffraction data of BiT (PDF#72-1019) [21]. From Fig. 1(a) it can been seen that the crystal structure of the BHT ceramic target consists of a single phase of Bi-layered Aurivillius phase and no impurity is detected in the sample, which suggests that the Ho 3+ ions have entered into the lattice of BiT. As shown in Fig. 1(b), besides the peak of c-sapphire substrate, only diffraction peaks corresponding to the wurtzite AZO (0002) and (0004), Aurivillius phase BHT (200)/(020), (400)/(040) and (117) reflections were detected, indicating that the bilayer oxide films are pure phase wurtzite and bismuth titanate. The sharp (0002) and (0004) reflections of AZO layer indicates that the AZO layer is well crystallized and highly (0001)-oriented. Similarly, the sharp (200) and (400) reflections of BHT film also reveals a high quality of crystalline and high (100) orientation in BHT film, which indicates that the BHT films grown on AZO (0001) buffer show (100)preferred orientation. It is acknowledged that the vector of the (major) spontaneous polarization in lanthanide-doped BiT is along its a-axis and the lanthanide-doped BiT thin films with (100) orientation show a large remanent polarization [10,14]. The (100)-preferred BHT film integrated with AZO(0001) film offers a greater potential to application in novel devices based on the functionality of ferroelectric polarization. In order to further confirm the phase structure of BHT thin film, the Raman spectrum measurement was used. The Raman spectra of the BHT ceramic target and (1000 nm)BHT/(600 nm)AZO heterostructure are illustrated in Fig. 2. Compare with Raman data of BiT [22,23], Hodoped BiT ceramic exhibits intense phonon modes at about 129, 149, 268, 325, 545, 560, 618 and 852 cm− 1 and many other weak features or overlap modes. The two peaks at 129 and 149 cm− 1 are related to the Bi atoms in the perovskite-like unit [23], and the six peaks appeared at 268, 325, 545, 560, 618 and 852 cm− 1 are caused by the internal vibrational modes of TiO6 octahedra [23,24]. The characters of position and shape of peaks for the pure phase BHT ceramic are similar to that observed in Nd- and La-doped BiT ceramics [23,24], which indicates that Ho ions has substituted Bi ions in the lattice of BiT. As shown from the phonon modes of the BHT film, there are five apparent peaks appeared at 148, 260, 535, 561 and 856 cm− 1 corresponding to the phonon modes of BHT ceramic though the peaks are remarkably broadening, weakening and overlaping compared to that of BHT ceramic. The additional phonon mode at 442 cm− 1 is induced by the increase in TiO6 octahedron tilting and structural distortion resulted from the Ho substitution in the BiT lattice, and the mode is also observed in Nd-doped BiT single crystal [24] and nanotube arrays [25]. The result suggests that the (100)-preferred BHT film grown on AZO (0001) layer is Bi-layered Aurivillius phase bismuth titanate, which is in agreement with the result from XRD analysis.
Fig. 2. Raman spectra of BHT ceramic target and (1000 nm)BHT/(600 nm)AZO heterostructure grown on c-sapphire substrate.
S. Luo et al. / Materials Letters 79 (2012) 173–176
Fig. 3 shows the surface and cross-section morphologies of the (1000 nm)BHT/(600 nm)AZO heterostructure. As shown in Fig. 3(a), most grains are equiaxed ones in diameter of less than 100 nm, which are as the same shape as lanthanide-doped BiT grains with (100) orientation in other reports [10,14]. According to the result from XRD pattern, the other ellipse grains or catenulate grains may attribute to the grains with other orientations or the combination of neighboring (100)-oriented grains. From Fig. 3(b) the cross-section morphology shows a clear cylindrical growth pattern of BHT grains vertically grown on AZO (0001) plane. As shown in the image, the BHT/AZO heterostructure is crack-free and uniform, with a dense microstructure and has a smooth interface. The measurement of UV–vis transmittance spectrum was carried out to evaluate the application potential for BHT/AZO heterostructure in transparent devices or photoelectric devices. Fig. 4 presents the UV–vis transmittance spectra of BHT/AZO heterostructures grown on c-sapphire substrates. The dramatically dropped transmittances in the UV region are due to the fundamental absorption resulted from ZnO. The BHT/AZO heterostructures are typically transparent, and compared to (1000 nm)BHT/(600 nm)AZO, the (500 nm)BHT/AZO heterostructures show an apparently higher average transmittance. The average transmittance of (500 nm)BHT/(300 nm)AZO is higher than 89% in the range of 400–800 nm. Although the average transmittance of the (500 nm) BHT/(300 nm)AZO heterostructure is lower than the value 95% of the epitaxial (300 nm)AZO buffer (not shown), the high transmittance more than 89% supports a potential opportunity to make use of the BHT/AZO heterostructure for application in transparent or photoelectric devices.
175
Fig. 4. UV–vis transmittance spectra of BHT/AZO heterostructures grown on c-sapphire substrates.
4. Summary Bi3.6Ho0.4Ti3O12 (BHT) ferroelectric thin films were grown on epitaxial (0001)-oriented Al-doped ZnO (AZO) semiconductor thin films which buffered c-sapphire substrates by pulsed laser deposition. The BHT film grown on AZO(0001) was a single phase structure of Bi-layered Aurivillius phase bismuth titanate and showed (100)-preferred orientation. The BHT/AZO heterostructures were crack-free and uniform, with a dense microstructure and had a smooth interface. Furthermore, the BHT/AZO heterostructures were typically transparent, and the average transmittance of (500 nm)BHT/(300 nm)AZO was higher than 89% in the range of 400–800 nm. This work reveals that the transparent BHT/ AZO ferroelectric-semiconductor heterostructure would be a novel material integrated with optical, ferroelectric and semiconductor characteristics for potential application in new devices with novel functionalities. Acknowledgements This work was financially supported by the New Century Excellent Talents in University (NCET-10-0662), International Science and Technology Cooperation Project of Hubei Province (2010BFA017), International Science and Technology Cooperation Project (2009DFB50470) and International Science & Technology Cooperation Program of China (No. 2011DFA52650). References
Fig. 3. SEM (a) surface and (b) cross-section morphologies of the (1000 nm)BHT/ (600 nm)AZO heterostructure grown on c-sapphire substrate.
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