- Email: [email protected]
Effects of ion irradiation and annealing on optical and structural properties of CeO2 films on sapphire
ARTICLE IN PRESS
Physica B 389 (2007) 263–268 www.elsevier.com/locate/physb
Effects of ion irradiation and annealing on optical and structural properties of CeO2 films on sapphire M.Y. Chena, X.T. Zua,b,, X. Xianga, H.L. Zhangc a
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, PR China b International Center for Material Physics, Chinese Academy of Sciences, Shenyang 110015, PR China c College of Microelectronics and Solid State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, PR China Received 10 June 2006; received in revised form 22 June 2006; accepted 27 June 2006
Abstract (0 0 l) CeO2 films deposited on r-cut sapphire were irradiated by N+ ions at fluences of 1 1016, 5 1016 and 1 1017 ion/cm2 at room temperature and then annealed at 200–900 1C in O2 ambient. X-ray diffraction results showed that there was no change for the orientation and crystallinity of CeO2 film after irradiation. However, the diffraction peaks became sharper after annealing at 900 1C. Atomic force microscope measurements revealed isolated voids after ion irradiation. The optical transmittance spectra indicated a blueshift of band gap due to the valence transition of Ce3+-Ce4+ induced by thermal annealing. As-deposited films exhibited three photoluminescence bands at 300–390, 380–550, and 460–550 nm. A new emission band peaked at 670 nm appeared while the emission band at 460–550 nm disappeared after annealing. The CeO2 films show good thermal stability and irradiation resistance except some isolated voids in this work. r 2006 Elsevier B.V. All rights reserved. PACS: 74.25.Gz; 42.70a; 78.20.e Keywords: CeO2 film; XRD; AFM; Optical properties; Photoluminescence
1. Introduction Nanostructured cerium oxide with a cubic fluorite structure has been considered as one of the most important oxide materials because of its good thermal and chemical stability [1,2], special optical properties [3,4], and close lattice match with silicon [5]. Cerium oxide film has potential applications in optical, electro-optical, microelectronic, and optoelectronic devices [4]. Textured CeO2 films can be deposited using different methods, e.g., pulsed laser deposition ion beam assisted deposition, e-beam evaporation, and radio-frequency (RF) magnetron sputtering [2]. RF magnetron sputtering was cheap and simple compared with others. In addition, the thickness and uniformity of Corresponding author. Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu, 610054, PR China. Tel./fax: +86 28 83201939. E-mail address: [email protected] (X.T. Zu).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.06.162
films can be controlled properly. Up to now, many studies have concentrated on optimizing growth conditions of CeO2 films on different substrates, such as LaAlO3, r-cut Al2O3, Ag, Pd, Si, SiO2, MgO, and SrTiO3 [6–9]. There have also been some reports about the photoluminescence (PL) of CeO2 films caused by CeO/Si interface or cerium silicates for the as-deposited and annealed films on silicon substrate. [5,10,11]. The blueshift of optical band gap due to the change of cerium valence and decrease of cerium dioxide nanoparticles size have also been observed [12,13]. R-cut sapphire is a preferred substrate for the preparation of YBa2Cu3O7x (YBCO) thin films, especially for microwave applications, because of its small dielectric constant, low-loss tangent and high mechanical strength [6]. The growth condition of textured cerium dioxide has been studied as buffer layers for the good quality and hightemperature superconductor films on sapphire substrates. However, the PL properties of CeO2 film on sapphire substrate have not been investigated. Sapphire is usually
ARTICLE IN PRESS 264
M.Y. Chen et al. / Physica B 389 (2007) 263–268
used as an insulating substrate for optical and electrical instruments. Rare-earth materials have an inherent thermal stability in optical transition. Owing to this advantage, many of them have been utilized in laser and optical fiber applications. In particular, cerium atom has been utilized as a luminescence center that dominates the electroluminescence of the phosphor layer in electroluminescent displays [11]. Therefore, it is important to investigate the optical properties of CeO2 film on sapphire substrate. In this work, we report the structural and optical properties of cerium dioxide film on sapphire substrate after ion irradiation and thermal annealing. The PL of CeO2 film on sapphire substrate was studied before and after ion irradiation for the first time. This might open another application of CeO2 films on sapphire in optical-device fabrication.
3. Results and discussion
2. Experimental
D¼
CeO2 films with (0 0 1) orientation were deposited on rcut sapphire by RF magnetron sputtering at substrate temperature of 730 1C. The sputtering power was 150 W and sputtering Ar gas pressure was about 14 Pa (O2:Ar ¼ 1:2). Some as-deposited samples were annealed for 0.5 h at 200–900 1C in steps of 100 1C in flowing O2 ambient. Other as-deposited samples were irradiated with 60 keV N+ ions at the fluences of 1 1016, 5 1016 and 1 1017 ion/cm2 in a vacuum chamber at 1.8 103 Pa. Some as-irradiated samples were then annealed for 0.5 h at 200–900 1C in steps of 100 1C in flowing O2 ambient. X-ray diffraction (XRD) 2y scan of samples were obtained from a Bede D1 diffractometer operated at 40 kV and 30 mA, using Cu Ka line (1.54056 A˚) from 101 to 601 2y with steps of 0.021/s. Surface morphology measurements were performed with a SPA-300HV microscope. The optical spectra were collected using a SHIMADZU UV-2550 spectrophotometer at room temperature, with a deuterium lamp for UV and a tungsten halogen lamp for visible region. The wavelength used in the experiment ranged from 200 to 900 nm. PL measurements were carried out at room temperature using a SHIMADZU RF5301 PC spectrophotometer, with a 150 W Xe lamp as the excitation source. The ellipsometry measurements showed that the thickness of the CeO2 film is about 30 nm. TRIM 96 code simulated results indicated that the ion range is 173 nm. In this work, the XRD, optical transmittance and PL spectra showed that there is no clear change for the samples after annealing at temperatures below 900 1C. In addition, these spectra showed that the samples did not change even after irradiation up to a fluence of 1 1017 ion/cm2. Therefore, only the results for those annealed at 900 1C and irradiated at 1 1017 ion/cm2 are presented in this paper. The as-deposited samples are named as sample A. The samples annealed at 900 1C are named as sample B. The samples irradiated by N+ ions at 1 1017 ion/cm2 are named as sample C. The samples irradiated by N+ ions and annealed at 900 1C are named as sample D.
where l, yB, and B are the XRD wavelength (1.54056 A˚), Bragg diffraction angle and the FLHM of the diffraction peak, respectively. The mean size of crystallites of samples B and D is 17 nm, which is larger than that of the asdeposited and as-irradiated samples (12 nm), i.e., the crystallites grown after thermal annealing. Deshpande et al. [12] have studied the relationship between the crystallite size D and the lattice parameter a of the synthesized ceria nanoparticles. They pointed out that the variation in the lattice parameter is attributed to the lattice strain induced by the introduction of Ce3+ due to the formation of oxygen vacancies. Lattice strain was observed to decrease with an increase in the particle size. Ce3+ ions concentration increased with the reduction in the particle size. Therefore, they concluded that the increase of crystallite size resulted in the decrease of Ce3+ content, lattice strain, and lattice parameter. Furthermore, CeO2 crystallizes in the fluorite structure, in which each Ce4+ cation is surrounded by eight equivalent
3.1. Structural characterization XRD spectra of CeO2 films are shown in Fig. 1, indicating the face-centered cubic (FCC) crystalline structure with (0 0 1) orientation. The spectra showed that the orientation of CeO2 films did not change after ion irradiation and thermal annealing. The full-width at halfmaximum (FWHM) of samples B and D is 0.51, narrower than that of the as-deposited and as-irradiated (samples A and C) 0.71. In addition, there is no new phase formed after ion irradiation or thermal annealing. In order to evaluate the mean grain size of CeO2 crystallites, the Scherrer formula was used [14]: 0:9 l , B cos ðyB Þ
20
Al2O3(0-224)
CeO2(002)
Al2O3(0-112)
A B C D
30
40 2/ deg
50
60
Fig. 1. XRD spectra (y–2y) of sample A (as-deposited); sample B (annealed); sample C (as-irradiated) and sample D (irradiated and annealed).
ARTICLE IN PRESS M.Y. Chen et al. / Physica B 389 (2007) 263–268
Atomic force microscopy (AFM) analysis was carried out to characterize the surface morphology of CeO2 films. The images of the as-deposited (sample A) and asirradiated (sample C) are shown in Fig. 2. The images of sample B and D are not shown because their surface morphology is similar to the images of as-deposited (sample A) and as-irradiated (sample C), respectively. In other words, there is no clear change for the morphology after annealing. Although the mean grain sizes of sample B and D are larger than those of sample A and C calculated from the spectra of XRD, their particle sizes are nearly same in AFM images. It seems that the grain sizes detected by AFM are larger than those detected by XRD. It is natural, because AFM detects only the grain on the surface. However, XRD can detect all the grains inside. However, it can be seen from Fig. 2 that the surface is smoother and some isolated voids set distributed on the film surface after ion irradiation. The voids induced by ion irradiation can still be observed even after annealing at 900 1C. Lee et al. [2] had also studied the structure, texture and surface morphology of the CeO2 film on Si (1 0 0) substrate after annealing at 800 1C for 3 h in air. They
3.3. Optical properties 3.3.1. Optical absorption Fig. 3a shows the optical transmittance spectra of the samples. The optical parameters of cerium oxide films, n, k and Eg, can be determined by spectrophotometric method 90 A B C D
80 70 Transmittance / %
3.2. Surface morphology
found that there was no change for the surface morphology of the CeO2 films but the XRD peaks of annealed films was sharper, and importantly, no significant changes of orientation. When annealing above 1100 1C, the cerium will combine with Si, and introduce new phase [11]. These phenomena revealed that CeO2 has very nice thermal stability and irradiation resistance. Therefore, when other films like superconductor, ferroelectric, and semiconductor films are deposited upon the pre-sputtered CeO2 films at high temperatures, the structure, texture and surface morphology of CeO2 films will not be damaged. In addition, CeO2 has good stability under irradiation. It might be possible to provide a structurally matched and chemically stable underlying layer for various applications.
60 50 40 30 20 10 200
300
400
(a)
500 600 Wavelength / nm
700
800
900
12 A B C D
10
( a hv)1/2/(eV)1/2
O2 anions forming the corner of a cube, with each O2– anion coordinated to four Ce4+ cations. The charge state of Ce changes from Ce4+ to Ce3+ with decreasing particle size. As a result, the coordination number of cation to anion reduces from eight (Ce4+ to O2) to seven (Ce3+ to O2), which introduces Ce3+ ions and oxygen vacancies into the crystal lattice. Ce3+ ions have a higher ionic radius (1.034 A˚) as compared with that of Ce4+ ions (0.92 A˚). Therefore, the introduction of the Ce3+ ions and the accompanying oxygen vacancies leads to a distortion of the local symmetry. This causes a change in the Ce–O bond length (lattice distortion) and the overall lattice parameter. In this work, some samples were annealed in O2 ambient. After annealing at high temperature in rich O2 ambient, the content of oxygen vacancies decreased with the charge transition of Ce3+-Ce4+. The particle size increased after annealing according to the calculated results from Scherrer formula. Therefore, it can be deduced that the lattice parameter decreased after thermal annealing.
265
8 6 4 2 0 2.0
(b) Fig. 2. AFM images of (a) as-deposited (sample A); (b) as-irradiated (sample C).
2.5
3.0
3.5
4.0
4.5
hv/eV
Fig. 3. Transmission spectra (a) and the absorption coefficient as a function of photon energy (b) for samples A–D.
ARTICLE IN PRESS M.Y. Chen et al. / Physica B 389 (2007) 263–268
valence and oxygen vacancies state change, in accordance with refs [4,18]. In Fig. 1, the as-deposited sample A has nearly the same particle size as the as-irradiated one (sample C). So they have same fundamental band gap energy E g ¼ 2:53 eV. In Fig. 3b, the fundamental band gaps of samples B and D blueshift with difference of 0.38 and 0.22 eV, respectively. Sample B blueshifts more than sample D, which may be due to point defects introduced by irradiation in sample D. The point defects may create localized states within the band gap. 3.3.2. PL The PL spectra of as-deposited sample A, annealed sample B, as-irradiated sample C and irradiated and annealed sample D are presented in Fig. 4 and Fig. 5 at excitation wavelength of 235 and 306 nm, respectively. In Fig. 4, before annealing, samples A and C have three PL bands peaked at 335, 358, and 415 nm. The intensities of PL bands of as-irradiated sample (C) are weaker than those of the as-deposited one (A). After annealing, the
250
A B C D
200
Intensity/ a.u.
[15–17], which requires T(l) measurements and provides the dispersion of n(l) and k(l). The optical band gap Eg is determined by the absorption coefficients, a( ¼ 4kp/l). In this work, the transition of CeO2 is an indirect one, judging from the relationship of the absorption coefficient and band gap (not shown). So the indirect band gaps were calculated using the formula, ahn ¼ Aðhn E g Þ2 , ðahnÞ1=2 / E g , where hn is the photon energy and A a constant. From Fig. 3b, the values of Eg of samples A–D are 2.53, 2.91, 2.53, and 2.75 eV, respectively. After annealing, the optical band gap of sample B and D both showed a blueshift. Till now, there are two kinds of explanation about the reason of blueshift of the optical band gap of CeO2 films. One is the quantum-confinement effect when the particle size is down to a few nanometers [13], the other is the charge transition of Ce ions when the particle diameters are larger than a few nanometers (e.g.,X8 nm) [3,4,18]. Tsunekawa et al. [13] have observed blueshift of optical band gap with decreasing of nanoparticle size. This phenomenon has been well explained for diameters down to less than a few nanometers by the change in the electronic band structure, due to quantum-confinement effect. Furthermore, at the outermost nanocrystal surface, Ce4+ ions coexist with Ce3+ ones. When the cluster sizes decrease, the amount of Ce3+ increases. At the same time, there is an electrostatic potential effect due to a cerium valence change, which results in the blueshift of the band gap. Patsalas et al. [4,18] have also investigated the fundamental gap Eg of CeOx (Ceria nanocrystalline grains of 8–40 nm), which is due to the indirect O2p-Ce4f electronic transition along the L high-symmetry lines of the Brillouin zone. The blueshift of optical band gap is correlated with the decrease of Ce3+ content, distributing either at the CeO2 grain boundaries forming amorphous Ce2O3 or around O vacancies sites. The decrease of Ce3+ concentration will eliminate some localized states within the band gap due to the corresponding decrease of vacancies (defects) content as discussed above. These localized states have been proposed to be associated with O vacancies forming F centers [5], distributing among the gap. The strong correlation of Eg with Ce3+ explains the observed blueshift of Eg in nanocrystalline CeOx [4]. The quantum-size effect is expected to enhance the band gap of the materials due to the higher localization of energy bands with decreasing particle size (ultimately the bands are reduced to atomic states). Thus the blueshift of Eg is due to the Ce3+ content decrease, with increasing grain size, not due to the results of the quantum-size effect itself. It is well known that the fundamental band gap is mainly deduced by quantum-size effect when the nanoparticle size is between 0 and 3 nm. In our experiment, the particle size is in the range of 10–20 nm. The content of Ce3+ and oxygen vacancies decreased with growth and oxidation of the nanoparticles during annealing in rich O2 ambient. The fundamental gap Eg blueshifted, mainly induced by the
150
100
50
0 300
350
400
450 500 550 Wavelength/ nm
600
650
700
Fig. 4. PL spectra of samples A–D (EX 235 nm).
125 A B C D
100 Intensity/ a.u.
266
75
50
25
0 425
450
475
500 525 Wavelength/ nm
550
Fig. 5. PL spectra of samples A–D (EX 306 nm).
575
ARTICLE IN PRESS M.Y. Chen et al. / Physica B 389 (2007) 263–268
267
Table 1 A summary of PL peaks in this work Sample A lex (nm) PL peak (nm)
235 333
Sample C
356
415
306 500
235 333
Sample B and D
356
intensities of 415 nm emission bands were weakened while the 335 and 358 nm emission bands were strengthened. In addition, a new PL band peaked at 670 nm appeared after annealing (samples B and D). The strong emission bands peaked at 500 nm of samples A and C are presented in Fig. 5 when using an excitation wavelength of 306 nm before annealing. After annealing, these PL bands disappeared. A summary of the PL bands in this work have been listed in Table 1. Previous studies of defects in CeO2 crystals [5,18] showed that the fluorite crystal structure of the material remains intact over a relatively wide range of nonstoichiometric compositions through the inclusion of oxygen vacancies. These have energy levels in the band gap lower in energy than the 4f band states, with temperature and concentration dependent occupancy. The broad PL band ranging from 350 to 650 nm of the as-grown films on silicon substrates in Ref. [5] could be the result of defects, including oxygen vacancies in the crystal with electronic energy levels below the 4f band. They also observed that the spectra of annealed samples in oxygen at 900 1C were similar to those of the as-grown samples. However, the annealed samples have lower intensities, which can be the result of a reduction in the concentration of defects related to oxygen after annealing. These defects possibly act as radiative recombination centers for electrons initially excited from the valence band to the 4f band of the oxide. This is consistent with the results in this work. As discussion above, the PL bands peaked at 415 and 500 nm may be attributed to the defects related to oxygen, including oxygen vacancies in the films in this work. After annealing at 900 1C in O2, the concentration of oxygen vacancies decreased in this work. So the PL band peaked at 415 nm decreased in Fig. 4 and the PL band peaked at 500 nm disappeared in Fig. 5. Another emission band from 300–380 nm was observed for all the samples in this work. This PL band had two peaks at 335 and 358 nm, respectively. Some reports have attributed the PL band from 320–450 nm peaked at 358 nm to the new phase of Ce2Si2O7 silicate when using silicon as the substrate of cerium oxide films [11,19]. Perhaps the emission band from 300–380 nm in this work is also due to a new phase of the compound of CeAlOx, which formed on the interface between CeO2 film and Al2O3 substrate during the RF sputtering at 730 1C. After irradiation and annealing the compound of CeAlOx always existed in the samples in this work. However, there is no new phase of CeAlOx detected except CeO2 and Al2O3 from the XRD measurements in Fig. 1. This may be because the
415
306 500
235 335
358
415
670
306 no peak
compound of CeAlOx is amorphous or its concentration is much lower than those of Al2O3 and CeO2. The origin of the weak PL band peaked at 670 nm in Fig. 4 is not clear. It may be induced by the inter-band transition of CeO2 or the transition from some localized states within the band gap to the valence band. More investigation should be carried out to ascertain the origin of this emission band. 4. Summary Cerium dioxide thin films with (0 0 1) orientation deposited on sapphire substrates were irradiated by N+ ions and then annealed at 900 1C in O2. The CeO2 films showed very nice thermal stability and irradiation resistance except some isolated voids in this work. The blueshift of band gap after annealing is mainly due to the valence transition of Ce3+-Ce4+. The O defect vacancies excited in the CeO2 film caused the emissions at 415 and 500 nm. The emission band from 300–380 nm may be due to the compound of CeAlOx. The origination of the PL band peaked at 670 nm is not clear. More investigation should be carried out to make clear the mechanism of PL of cerium oxide. Acknowledgments This study was supported financially by the NSAF Joint Foundation of China (10376006) and by Program for New Century Excellent Talents in University (NCET-04-0899) and the Ph.D. Funding Support Program of Education Ministry of China (20050614013). References [1] X.D. Wu, R.C. Dye, R.E. Muenchausen, et al., Appl. Phys. Lett. 58 (1991) 2165. [2] H.Y. Lee, S.I. Kim, Y.P. Hong, Surf. Coat. Tech. 173 (2003) 224. [3] D. Barreca, G. Bruno, A. Gasparotto, Mater. Sci. Eng. C 23 (2003) 1013. [4] P. Patsalas, S. Logothetidis, C. Metaxa, Appl. Phys. Lett. 81 (2002) 466. [5] A.H. Morshed, M.E. Moussa, S.M. Bedair, Appl. Phys. Lett. 70 (1997) 1647. [6] J. Kurian, M. Naito, Physica C 402 (2004) 31. [7] Y. Takahashi, K. Matsumoto, S. Kim, IEEE Trans. Appl. Supercond. 9 (1999) 2272. [8] Y. Yamamoto, S. Arai, T. Matsuda, et al., Jpn. J. Appl. Phys. 36 (1997) L133. [9] R. Aguir, F. Sanchez, C. Ferrater, et al., Thin Solid Films 317 (1998) 81.
ARTICLE IN PRESS 268
M.Y. Chen et al. / Physica B 389 (2007) 263–268
[10] F. Gao, G.H. Li, J.H. Zhang, et al., Chin. Phys. Lett. 18 (2001) 443. [11] W.C. Choi, H.N. Lee, E.K. Kim, Appl. Phys. Lett. 75 (1999) 2389. [12] S. Deshpande, S. Patil, S. VNT Kuchibhatla, Appl. Phys. Lett. 87 (2005) 133113. [13] S. Tsunekawa, J.T. Wang, Y. Kawazoe, J. Appl. Phys. 94 (2003) 3654. [14] A. Guinier, X-ray Diffraction, Freeman, San Francisco, CA, 1963.
[15] [16] [17] [18] [19]
D.P. Arndt, et al., Appl. Opt. 23 (1984) 3571. J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1996) 455. N. O¨zer, Sol. Energy Mater. Sol. Cells 68 (2001) 391. P. Patsalas, S. Logothetidis, Phys. Rev. B 68 (2003) 035104. L. K˛epin´ski, D. Hreniak, W. Str˛ek, J. Alloys Compounds 341 (2002) 203.
Physica B 389 (2007) 263–268 www.elsevier.com/locate/physb
Effects of ion irradiation and annealing on optical and structural properties of CeO2 films on sapphire M.Y. Chena, X.T. Zua,b,, X. Xianga, H.L. Zhangc a
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, PR China b International Center for Material Physics, Chinese Academy of Sciences, Shenyang 110015, PR China c College of Microelectronics and Solid State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, PR China Received 10 June 2006; received in revised form 22 June 2006; accepted 27 June 2006
Abstract (0 0 l) CeO2 films deposited on r-cut sapphire were irradiated by N+ ions at fluences of 1 1016, 5 1016 and 1 1017 ion/cm2 at room temperature and then annealed at 200–900 1C in O2 ambient. X-ray diffraction results showed that there was no change for the orientation and crystallinity of CeO2 film after irradiation. However, the diffraction peaks became sharper after annealing at 900 1C. Atomic force microscope measurements revealed isolated voids after ion irradiation. The optical transmittance spectra indicated a blueshift of band gap due to the valence transition of Ce3+-Ce4+ induced by thermal annealing. As-deposited films exhibited three photoluminescence bands at 300–390, 380–550, and 460–550 nm. A new emission band peaked at 670 nm appeared while the emission band at 460–550 nm disappeared after annealing. The CeO2 films show good thermal stability and irradiation resistance except some isolated voids in this work. r 2006 Elsevier B.V. All rights reserved. PACS: 74.25.Gz; 42.70a; 78.20.e Keywords: CeO2 film; XRD; AFM; Optical properties; Photoluminescence
1. Introduction Nanostructured cerium oxide with a cubic fluorite structure has been considered as one of the most important oxide materials because of its good thermal and chemical stability [1,2], special optical properties [3,4], and close lattice match with silicon [5]. Cerium oxide film has potential applications in optical, electro-optical, microelectronic, and optoelectronic devices [4]. Textured CeO2 films can be deposited using different methods, e.g., pulsed laser deposition ion beam assisted deposition, e-beam evaporation, and radio-frequency (RF) magnetron sputtering [2]. RF magnetron sputtering was cheap and simple compared with others. In addition, the thickness and uniformity of Corresponding author. Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu, 610054, PR China. Tel./fax: +86 28 83201939. E-mail address: [email protected] (X.T. Zu).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.06.162
films can be controlled properly. Up to now, many studies have concentrated on optimizing growth conditions of CeO2 films on different substrates, such as LaAlO3, r-cut Al2O3, Ag, Pd, Si, SiO2, MgO, and SrTiO3 [6–9]. There have also been some reports about the photoluminescence (PL) of CeO2 films caused by CeO/Si interface or cerium silicates for the as-deposited and annealed films on silicon substrate. [5,10,11]. The blueshift of optical band gap due to the change of cerium valence and decrease of cerium dioxide nanoparticles size have also been observed [12,13]. R-cut sapphire is a preferred substrate for the preparation of YBa2Cu3O7x (YBCO) thin films, especially for microwave applications, because of its small dielectric constant, low-loss tangent and high mechanical strength [6]. The growth condition of textured cerium dioxide has been studied as buffer layers for the good quality and hightemperature superconductor films on sapphire substrates. However, the PL properties of CeO2 film on sapphire substrate have not been investigated. Sapphire is usually
ARTICLE IN PRESS 264
M.Y. Chen et al. / Physica B 389 (2007) 263–268
used as an insulating substrate for optical and electrical instruments. Rare-earth materials have an inherent thermal stability in optical transition. Owing to this advantage, many of them have been utilized in laser and optical fiber applications. In particular, cerium atom has been utilized as a luminescence center that dominates the electroluminescence of the phosphor layer in electroluminescent displays [11]. Therefore, it is important to investigate the optical properties of CeO2 film on sapphire substrate. In this work, we report the structural and optical properties of cerium dioxide film on sapphire substrate after ion irradiation and thermal annealing. The PL of CeO2 film on sapphire substrate was studied before and after ion irradiation for the first time. This might open another application of CeO2 films on sapphire in optical-device fabrication.
3. Results and discussion
2. Experimental
D¼
CeO2 films with (0 0 1) orientation were deposited on rcut sapphire by RF magnetron sputtering at substrate temperature of 730 1C. The sputtering power was 150 W and sputtering Ar gas pressure was about 14 Pa (O2:Ar ¼ 1:2). Some as-deposited samples were annealed for 0.5 h at 200–900 1C in steps of 100 1C in flowing O2 ambient. Other as-deposited samples were irradiated with 60 keV N+ ions at the fluences of 1 1016, 5 1016 and 1 1017 ion/cm2 in a vacuum chamber at 1.8 103 Pa. Some as-irradiated samples were then annealed for 0.5 h at 200–900 1C in steps of 100 1C in flowing O2 ambient. X-ray diffraction (XRD) 2y scan of samples were obtained from a Bede D1 diffractometer operated at 40 kV and 30 mA, using Cu Ka line (1.54056 A˚) from 101 to 601 2y with steps of 0.021/s. Surface morphology measurements were performed with a SPA-300HV microscope. The optical spectra were collected using a SHIMADZU UV-2550 spectrophotometer at room temperature, with a deuterium lamp for UV and a tungsten halogen lamp for visible region. The wavelength used in the experiment ranged from 200 to 900 nm. PL measurements were carried out at room temperature using a SHIMADZU RF5301 PC spectrophotometer, with a 150 W Xe lamp as the excitation source. The ellipsometry measurements showed that the thickness of the CeO2 film is about 30 nm. TRIM 96 code simulated results indicated that the ion range is 173 nm. In this work, the XRD, optical transmittance and PL spectra showed that there is no clear change for the samples after annealing at temperatures below 900 1C. In addition, these spectra showed that the samples did not change even after irradiation up to a fluence of 1 1017 ion/cm2. Therefore, only the results for those annealed at 900 1C and irradiated at 1 1017 ion/cm2 are presented in this paper. The as-deposited samples are named as sample A. The samples annealed at 900 1C are named as sample B. The samples irradiated by N+ ions at 1 1017 ion/cm2 are named as sample C. The samples irradiated by N+ ions and annealed at 900 1C are named as sample D.
where l, yB, and B are the XRD wavelength (1.54056 A˚), Bragg diffraction angle and the FLHM of the diffraction peak, respectively. The mean size of crystallites of samples B and D is 17 nm, which is larger than that of the asdeposited and as-irradiated samples (12 nm), i.e., the crystallites grown after thermal annealing. Deshpande et al. [12] have studied the relationship between the crystallite size D and the lattice parameter a of the synthesized ceria nanoparticles. They pointed out that the variation in the lattice parameter is attributed to the lattice strain induced by the introduction of Ce3+ due to the formation of oxygen vacancies. Lattice strain was observed to decrease with an increase in the particle size. Ce3+ ions concentration increased with the reduction in the particle size. Therefore, they concluded that the increase of crystallite size resulted in the decrease of Ce3+ content, lattice strain, and lattice parameter. Furthermore, CeO2 crystallizes in the fluorite structure, in which each Ce4+ cation is surrounded by eight equivalent
3.1. Structural characterization XRD spectra of CeO2 films are shown in Fig. 1, indicating the face-centered cubic (FCC) crystalline structure with (0 0 1) orientation. The spectra showed that the orientation of CeO2 films did not change after ion irradiation and thermal annealing. The full-width at halfmaximum (FWHM) of samples B and D is 0.51, narrower than that of the as-deposited and as-irradiated (samples A and C) 0.71. In addition, there is no new phase formed after ion irradiation or thermal annealing. In order to evaluate the mean grain size of CeO2 crystallites, the Scherrer formula was used [14]: 0:9 l , B cos ðyB Þ
20
Al2O3(0-224)
CeO2(002)
Al2O3(0-112)
A B C D
30
40 2/ deg
50
60
Fig. 1. XRD spectra (y–2y) of sample A (as-deposited); sample B (annealed); sample C (as-irradiated) and sample D (irradiated and annealed).
ARTICLE IN PRESS M.Y. Chen et al. / Physica B 389 (2007) 263–268
Atomic force microscopy (AFM) analysis was carried out to characterize the surface morphology of CeO2 films. The images of the as-deposited (sample A) and asirradiated (sample C) are shown in Fig. 2. The images of sample B and D are not shown because their surface morphology is similar to the images of as-deposited (sample A) and as-irradiated (sample C), respectively. In other words, there is no clear change for the morphology after annealing. Although the mean grain sizes of sample B and D are larger than those of sample A and C calculated from the spectra of XRD, their particle sizes are nearly same in AFM images. It seems that the grain sizes detected by AFM are larger than those detected by XRD. It is natural, because AFM detects only the grain on the surface. However, XRD can detect all the grains inside. However, it can be seen from Fig. 2 that the surface is smoother and some isolated voids set distributed on the film surface after ion irradiation. The voids induced by ion irradiation can still be observed even after annealing at 900 1C. Lee et al. [2] had also studied the structure, texture and surface morphology of the CeO2 film on Si (1 0 0) substrate after annealing at 800 1C for 3 h in air. They
3.3. Optical properties 3.3.1. Optical absorption Fig. 3a shows the optical transmittance spectra of the samples. The optical parameters of cerium oxide films, n, k and Eg, can be determined by spectrophotometric method 90 A B C D
80 70 Transmittance / %
3.2. Surface morphology
found that there was no change for the surface morphology of the CeO2 films but the XRD peaks of annealed films was sharper, and importantly, no significant changes of orientation. When annealing above 1100 1C, the cerium will combine with Si, and introduce new phase [11]. These phenomena revealed that CeO2 has very nice thermal stability and irradiation resistance. Therefore, when other films like superconductor, ferroelectric, and semiconductor films are deposited upon the pre-sputtered CeO2 films at high temperatures, the structure, texture and surface morphology of CeO2 films will not be damaged. In addition, CeO2 has good stability under irradiation. It might be possible to provide a structurally matched and chemically stable underlying layer for various applications.
60 50 40 30 20 10 200
300
400
(a)
500 600 Wavelength / nm
700
800
900
12 A B C D
10
( a hv)1/2/(eV)1/2
O2 anions forming the corner of a cube, with each O2– anion coordinated to four Ce4+ cations. The charge state of Ce changes from Ce4+ to Ce3+ with decreasing particle size. As a result, the coordination number of cation to anion reduces from eight (Ce4+ to O2) to seven (Ce3+ to O2), which introduces Ce3+ ions and oxygen vacancies into the crystal lattice. Ce3+ ions have a higher ionic radius (1.034 A˚) as compared with that of Ce4+ ions (0.92 A˚). Therefore, the introduction of the Ce3+ ions and the accompanying oxygen vacancies leads to a distortion of the local symmetry. This causes a change in the Ce–O bond length (lattice distortion) and the overall lattice parameter. In this work, some samples were annealed in O2 ambient. After annealing at high temperature in rich O2 ambient, the content of oxygen vacancies decreased with the charge transition of Ce3+-Ce4+. The particle size increased after annealing according to the calculated results from Scherrer formula. Therefore, it can be deduced that the lattice parameter decreased after thermal annealing.
265
8 6 4 2 0 2.0
(b) Fig. 2. AFM images of (a) as-deposited (sample A); (b) as-irradiated (sample C).
2.5
3.0
3.5
4.0
4.5
hv/eV
Fig. 3. Transmission spectra (a) and the absorption coefficient as a function of photon energy (b) for samples A–D.
ARTICLE IN PRESS M.Y. Chen et al. / Physica B 389 (2007) 263–268
valence and oxygen vacancies state change, in accordance with refs [4,18]. In Fig. 1, the as-deposited sample A has nearly the same particle size as the as-irradiated one (sample C). So they have same fundamental band gap energy E g ¼ 2:53 eV. In Fig. 3b, the fundamental band gaps of samples B and D blueshift with difference of 0.38 and 0.22 eV, respectively. Sample B blueshifts more than sample D, which may be due to point defects introduced by irradiation in sample D. The point defects may create localized states within the band gap. 3.3.2. PL The PL spectra of as-deposited sample A, annealed sample B, as-irradiated sample C and irradiated and annealed sample D are presented in Fig. 4 and Fig. 5 at excitation wavelength of 235 and 306 nm, respectively. In Fig. 4, before annealing, samples A and C have three PL bands peaked at 335, 358, and 415 nm. The intensities of PL bands of as-irradiated sample (C) are weaker than those of the as-deposited one (A). After annealing, the
250
A B C D
200
Intensity/ a.u.
[15–17], which requires T(l) measurements and provides the dispersion of n(l) and k(l). The optical band gap Eg is determined by the absorption coefficients, a( ¼ 4kp/l). In this work, the transition of CeO2 is an indirect one, judging from the relationship of the absorption coefficient and band gap (not shown). So the indirect band gaps were calculated using the formula, ahn ¼ Aðhn E g Þ2 , ðahnÞ1=2 / E g , where hn is the photon energy and A a constant. From Fig. 3b, the values of Eg of samples A–D are 2.53, 2.91, 2.53, and 2.75 eV, respectively. After annealing, the optical band gap of sample B and D both showed a blueshift. Till now, there are two kinds of explanation about the reason of blueshift of the optical band gap of CeO2 films. One is the quantum-confinement effect when the particle size is down to a few nanometers [13], the other is the charge transition of Ce ions when the particle diameters are larger than a few nanometers (e.g.,X8 nm) [3,4,18]. Tsunekawa et al. [13] have observed blueshift of optical band gap with decreasing of nanoparticle size. This phenomenon has been well explained for diameters down to less than a few nanometers by the change in the electronic band structure, due to quantum-confinement effect. Furthermore, at the outermost nanocrystal surface, Ce4+ ions coexist with Ce3+ ones. When the cluster sizes decrease, the amount of Ce3+ increases. At the same time, there is an electrostatic potential effect due to a cerium valence change, which results in the blueshift of the band gap. Patsalas et al. [4,18] have also investigated the fundamental gap Eg of CeOx (Ceria nanocrystalline grains of 8–40 nm), which is due to the indirect O2p-Ce4f electronic transition along the L high-symmetry lines of the Brillouin zone. The blueshift of optical band gap is correlated with the decrease of Ce3+ content, distributing either at the CeO2 grain boundaries forming amorphous Ce2O3 or around O vacancies sites. The decrease of Ce3+ concentration will eliminate some localized states within the band gap due to the corresponding decrease of vacancies (defects) content as discussed above. These localized states have been proposed to be associated with O vacancies forming F centers [5], distributing among the gap. The strong correlation of Eg with Ce3+ explains the observed blueshift of Eg in nanocrystalline CeOx [4]. The quantum-size effect is expected to enhance the band gap of the materials due to the higher localization of energy bands with decreasing particle size (ultimately the bands are reduced to atomic states). Thus the blueshift of Eg is due to the Ce3+ content decrease, with increasing grain size, not due to the results of the quantum-size effect itself. It is well known that the fundamental band gap is mainly deduced by quantum-size effect when the nanoparticle size is between 0 and 3 nm. In our experiment, the particle size is in the range of 10–20 nm. The content of Ce3+ and oxygen vacancies decreased with growth and oxidation of the nanoparticles during annealing in rich O2 ambient. The fundamental gap Eg blueshifted, mainly induced by the
150
100
50
0 300
350
400
450 500 550 Wavelength/ nm
600
650
700
Fig. 4. PL spectra of samples A–D (EX 235 nm).
125 A B C D
100 Intensity/ a.u.
266
75
50
25
0 425
450
475
500 525 Wavelength/ nm
550
Fig. 5. PL spectra of samples A–D (EX 306 nm).
575
ARTICLE IN PRESS M.Y. Chen et al. / Physica B 389 (2007) 263–268
267
Table 1 A summary of PL peaks in this work Sample A lex (nm) PL peak (nm)
235 333
Sample C
356
415
306 500
235 333
Sample B and D
356
intensities of 415 nm emission bands were weakened while the 335 and 358 nm emission bands were strengthened. In addition, a new PL band peaked at 670 nm appeared after annealing (samples B and D). The strong emission bands peaked at 500 nm of samples A and C are presented in Fig. 5 when using an excitation wavelength of 306 nm before annealing. After annealing, these PL bands disappeared. A summary of the PL bands in this work have been listed in Table 1. Previous studies of defects in CeO2 crystals [5,18] showed that the fluorite crystal structure of the material remains intact over a relatively wide range of nonstoichiometric compositions through the inclusion of oxygen vacancies. These have energy levels in the band gap lower in energy than the 4f band states, with temperature and concentration dependent occupancy. The broad PL band ranging from 350 to 650 nm of the as-grown films on silicon substrates in Ref. [5] could be the result of defects, including oxygen vacancies in the crystal with electronic energy levels below the 4f band. They also observed that the spectra of annealed samples in oxygen at 900 1C were similar to those of the as-grown samples. However, the annealed samples have lower intensities, which can be the result of a reduction in the concentration of defects related to oxygen after annealing. These defects possibly act as radiative recombination centers for electrons initially excited from the valence band to the 4f band of the oxide. This is consistent with the results in this work. As discussion above, the PL bands peaked at 415 and 500 nm may be attributed to the defects related to oxygen, including oxygen vacancies in the films in this work. After annealing at 900 1C in O2, the concentration of oxygen vacancies decreased in this work. So the PL band peaked at 415 nm decreased in Fig. 4 and the PL band peaked at 500 nm disappeared in Fig. 5. Another emission band from 300–380 nm was observed for all the samples in this work. This PL band had two peaks at 335 and 358 nm, respectively. Some reports have attributed the PL band from 320–450 nm peaked at 358 nm to the new phase of Ce2Si2O7 silicate when using silicon as the substrate of cerium oxide films [11,19]. Perhaps the emission band from 300–380 nm in this work is also due to a new phase of the compound of CeAlOx, which formed on the interface between CeO2 film and Al2O3 substrate during the RF sputtering at 730 1C. After irradiation and annealing the compound of CeAlOx always existed in the samples in this work. However, there is no new phase of CeAlOx detected except CeO2 and Al2O3 from the XRD measurements in Fig. 1. This may be because the
415
306 500
235 335
358
415
670
306 no peak
compound of CeAlOx is amorphous or its concentration is much lower than those of Al2O3 and CeO2. The origin of the weak PL band peaked at 670 nm in Fig. 4 is not clear. It may be induced by the inter-band transition of CeO2 or the transition from some localized states within the band gap to the valence band. More investigation should be carried out to ascertain the origin of this emission band. 4. Summary Cerium dioxide thin films with (0 0 1) orientation deposited on sapphire substrates were irradiated by N+ ions and then annealed at 900 1C in O2. The CeO2 films showed very nice thermal stability and irradiation resistance except some isolated voids in this work. The blueshift of band gap after annealing is mainly due to the valence transition of Ce3+-Ce4+. The O defect vacancies excited in the CeO2 film caused the emissions at 415 and 500 nm. The emission band from 300–380 nm may be due to the compound of CeAlOx. The origination of the PL band peaked at 670 nm is not clear. More investigation should be carried out to make clear the mechanism of PL of cerium oxide. Acknowledgments This study was supported financially by the NSAF Joint Foundation of China (10376006) and by Program for New Century Excellent Talents in University (NCET-04-0899) and the Ph.D. Funding Support Program of Education Ministry of China (20050614013). References [1] X.D. Wu, R.C. Dye, R.E. Muenchausen, et al., Appl. Phys. Lett. 58 (1991) 2165. [2] H.Y. Lee, S.I. Kim, Y.P. Hong, Surf. Coat. Tech. 173 (2003) 224. [3] D. Barreca, G. Bruno, A. Gasparotto, Mater. Sci. Eng. C 23 (2003) 1013. [4] P. Patsalas, S. Logothetidis, C. Metaxa, Appl. Phys. Lett. 81 (2002) 466. [5] A.H. Morshed, M.E. Moussa, S.M. Bedair, Appl. Phys. Lett. 70 (1997) 1647. [6] J. Kurian, M. Naito, Physica C 402 (2004) 31. [7] Y. Takahashi, K. Matsumoto, S. Kim, IEEE Trans. Appl. Supercond. 9 (1999) 2272. [8] Y. Yamamoto, S. Arai, T. Matsuda, et al., Jpn. J. Appl. Phys. 36 (1997) L133. [9] R. Aguir, F. Sanchez, C. Ferrater, et al., Thin Solid Films 317 (1998) 81.
ARTICLE IN PRESS 268
M.Y. Chen et al. / Physica B 389 (2007) 263–268
[10] F. Gao, G.H. Li, J.H. Zhang, et al., Chin. Phys. Lett. 18 (2001) 443. [11] W.C. Choi, H.N. Lee, E.K. Kim, Appl. Phys. Lett. 75 (1999) 2389. [12] S. Deshpande, S. Patil, S. VNT Kuchibhatla, Appl. Phys. Lett. 87 (2005) 133113. [13] S. Tsunekawa, J.T. Wang, Y. Kawazoe, J. Appl. Phys. 94 (2003) 3654. [14] A. Guinier, X-ray Diffraction, Freeman, San Francisco, CA, 1963.
[15] [16] [17] [18] [19]
D.P. Arndt, et al., Appl. Opt. 23 (1984) 3571. J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1996) 455. N. O¨zer, Sol. Energy Mater. Sol. Cells 68 (2001) 391. P. Patsalas, S. Logothetidis, Phys. Rev. B 68 (2003) 035104. L. K˛epin´ski, D. Hreniak, W. Str˛ek, J. Alloys Compounds 341 (2002) 203.