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Effect of trivalent element doping on structural and optical properties of SnO2 thin films grown by pulsed laser deposition technique
Current Applied Physics 12 (2012) S21eS24
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effect of trivalent element doping on structural and optical properties of SnO2 thin films grown by pulsed laser deposition technique Geun Woo Kim, Chang Hoon Sung, M.S. Anwar, Yong Jun Seo, Si Nae Heo, Keun Young Park, Tae Kwon Song, Bon Heun Koo* School of Nano & Advanced Materials Engineering, Changwon National University, 9 Sarim dong, Changwon 641 773, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 December 2011 Received in revised form 28 March 2012 Accepted 14 May 2012 Available online 2 June 2012
In this study, trivalent element doped SnO2 thin films were deposited on glass substrate using pulsed laser deposition method. The effect of trivalent elements (Al2O3, Bi2O3, Sb2O3 and Y2O3) doping on the structural, electrical and optical properties have been studied using X-ray diffraction (XRD), photoluminescence and resistivity measurements. XRD results indicated that all films exhibited single phase nature with (101) preferred orientation. Among all the films, the 6 wt% Sb doped film had the lowest value of the resistivity. Photoluminescence study inferred that all the films showed violeteblue emission and the intensity of this emission decrease with the increase in the doping content. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Transparent conducting oxide SnO2 Thin films X-ray diffraction Pulsed laser deposition
1. Introduction During the last several years, transparent conducting oxide (TCO) has gained much attention among the scientific community due to its excellent optical and electrical properties and chemical stability as well as their potential technological applications such as: flat-panel displays, light-emitting diodes, touch panels, defrosters, and solar cells [1e3]. However, the development of alternative TCOs has become growingly essential because of the growing concern about the storage of indium that is a nature resource. The alternatives should be composed of abundant and nontoxic material such as tin, zinc and titanium. As compared to widely used indium tin oxide (ITO), SnO2 films are chemically stabled in acidic and basic solutions, thermally stable in oxidizing environments at high temperatures and also mechanically hard. SnO2 is an n e type wide band gap semiconductor (band gap ranging from 3.6 to 4.1 eV) having rutile structure. SnO2 have been found to exhibit uncommon properties that are explored in technological applications. As being a transparent to visible wavelength of light as well as simultaneously possessing, a relatively high electron concentration and mobility makes SnO2 a key material in
* Corresponding author. Tel.: þ82 55 264 5431; fax: þ82 55 262 6486. E-mail addresses: [email protected] (G.W. Kim), bhkoo@ changwon.ac.kr (B.H. Koo). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.05.041
the field of TCOs. Several deposition techniques such as solegel paralysis method, RF sputtering method and chemical vapor deposition are used to fabricate SnO2 films [4e15]. However, interest in PLD technique paying attention on the novelty of a simple and versatile physical vapor deposition technique for ceramics, high TC superconductors, biomaterials and latter expanded to include the multi-component oxides. The films grown from the multi-component target using PLD found to reproduces the composition as that of the target. Moreover, the PLD grown films crystallize at lower substrate temperature as compared to other physical vapor deposition technique. Many works about the preparation, characteristics, electrical and optical properties of trivalent doped tin oxide thin films deposited by different techniques have been reported. However a reexamination of their properties is necessary since tin oxide thin films doped with antimony exhibit interesting electrochemical properties in different electrochemical processes, and also because their possible applications as electrodes in new generations of solar cells and other optoelectronic devices. In the present work, we have investigated the effect of trivalent ions doping on the structural, electrical and optical properties of SnO2 films fabricated using PLD technique. The XRD results infer that all the films have single phase nature with preferred orientation along (101) plane except 6 wt% Sb doped SnO2 film. From the electrical resistivity study, it is found that the resistivity of Sb doped films decreases with increase in doping and then starts increasing with further doping above 6 wt%.
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G.W. Kim et al. / Current Applied Physics 12 (2012) S21eS24
For all trivalent ion doped SnO2 films, it is observed that 6% Sb doped SnO2 films have the lowest value of the resistivity. 2. Experimental details The polycrystalline bulk target of trivalent ion doped SnO2 was synthesized by conventional solid-state reaction technique. Pure SnO2 powder and trivalent elements (Al2O3, Sb2O, the purity was higher than 99%) were mixed completely using a ball milling for 24 h to achieve homogeneity and then pressed into a 1 inch diameter pellet at 100001 b. Finally, the pellets were sintered at 1250 C for 12 h. The KrFexcimer laser (Lambda Physik model COMPEX-201) of wavelength 248 nm and pulse duration of 20 ns was employed for the deposition on glass substrate. During the deposition, laser energy density at the target surface was kept at 1.0 J/cm2 and repetition rate at 10 Hz. The substrate temperature was maintained at 300 C and the background pressure was 60 m Torr of oxygen during the deposition. The focused laser beam was incident on the target surface at an angle of 90 . The target was rotated about 10 rpm and the substrate was mounted opposite to the target at a distance of 3.5 cm. The substrate was mounted on a heater plate using a silver paste. After deposition, the thin films were cooled slowly to room temperature, maintaining the oxygen pressure in the vacuum chamber to 60 m Torr. The structural properties of the films were analyzed by using Xray diffraction (XRD, Cu Ka1, ¼ 0.15406 nm) and the scans were performed with a 0.02 step size in the 2q range of 20e80 . The electrical properties were checked by using a four-probe sheet resistance (Mitsbishi Loresta MCP-T610 TFP) and a Hall measurement system. The PL property and optical properties of the films were studied at room temperature by using a Photoluminescence and a UVeVISeNIR spectrometer in the wavelength range of 300e800 nm.
Table 1 Comparison of ionic radius of trivalent elements with Sn4þ. Atom
Ionic radius (nm)
Sn4þ Sb3þ Bi3þ Al3þ Y3þ
0.071 0.076 0.196 0.051 0.09
Fig. 1 represents the electrical resistivity as a function of trivalent element doping in SnO2 thin films. The sheet resistance (Rs) measurements were done using a four point prove method by assuming that the thickness of the films was uniform. The resistivity of the film was calculated using the following relation: r ¼ Rs d, where d is the film thickness. It is observed that the value
of the resistivity decreases with doping up to a particular value of the doping element. Especially, when Sb2O3 doped SnO2, the resistivity obtained the lowest value .The variation in resistivity can be explained on the basis of two oxidation states of antimony (Sb3þ,Sb5þ). In this case, some of the Sn4þ ions are replaced by Sb5þ in the lattice, resulting in the generation of conduction electrons, so reducing the resistivity. Also, due to the substitution of Sn4þ by Sb3þ as their ionic radii are well matched, this substitution leads to the increase in the lattice constants due to the larger size of Sb3þ as compared with Sn4þ. Besides, ionic radius disparities exist between of trivalent elements and Sn4þ. Table 1 shows the comparison of ionic radius of trivalent elements with Sn4þ. Terrier et al. reported that at low doping level Sb exist in the Sbþ5 states, whereas, at higher doping level Sb exist in Sb3þ. Thus, the substitution of Snþ4 by Sb5þ induces a donor center very close to the conduction band of SnO2, as a result the resistivity decreases. When Sbþ3 replaces Snþ4 ions an acceptor site is created inside the band gap. If both sites are present, compensating effect may occur, as a result the resistivity would appear as devoted by the ratio between Sbþ5 and Sbþ3. On the basis of the electrical properties measurement, one can say that at the lower value of the doping, Snþ5 would predominate, as a result the resistivity decreases with increasing the level of Sb doping up to 6 wt%. With further doping of Sb over 6 wt%, Sbþ3 ions are generated and which compensate donor level which leads to increase in the resistivity value. Fig. 2 depicts the XRD pattern of undoped and trivalent element (Al2O3, Bi2O3, Sb2O3 and Y2O3) doped SnO2 thin films deposited at 300 C in 60 m Torr pressure of O2. From the XRD results, it is observed that all the films exhibit single phase polycrystalline behavior with rutile structure. None of the film shows any extra peaks corresponding to secondary phase, such as SnO, Sn2O3, Sb2O3, Al2O3 etc. The XRD results of Sb doped SnO2 films also show
Fig. 1. Electrical resistivity of trivalent elements (Al2O3, Bi2O3, Sb2O3 and Y2O3) doped SnO2 films.
Fig. 2. X-ray diffraction patterns of trivalent doped (Al2O3, Bi2O3, Sb2O3 and Y2O3) SnO2 films.
3. Results and discussion
G.W. Kim et al. / Current Applied Physics 12 (2012) S21eS24
S23
Fig. 4. The optical transmittance spectra of Sb2O3 doped SnO2 films with different film thickness.
free-exciton decay in SnO2 with an energy shift of 0.3 eV with respect to the bulk gap energy (3.6 eV) [20]. This emission is not observed in the presented work due to the limit of the PL detection range with a HeeCd 325 nm excitation source. In general, except the sharp excitonic emission, semiconductors have another extensive trapped emission, which often contains multiple luminescent centers. There are various types of surface states that give rise to different energy states inside the semiconductor band gap. Lei et al. [22] reported the green and yellow emission in Al doped SnO2 nanowires. Generally, oxygen vacancies are known to be the most common defect and act as a radiative center in the luminescence process. When Al3þ or Sb3þ ions are substituted in place of Sn4þ in the host lattice then 1 charge of the Al1Sn and Sb1Sn ion has to be compensated for somewhere in the lattice in the form of oxygen vacancy. Fig. 4 shows the transmittance spectra of Sb2O3 doped SnO2 films at 500 C and 60 m Torr of oxygen pressure with different film thickness in the visible region (300 nme800 nm). The average transmittance in the visible region was observed between 86% and 83%. Consequently, all films exhibited high transparency in the visible region. Fig. 3. Photoluminescence spectra of trivalent doped SnO2 films (a) Al2O3 doped SnO2 films (b) Sb2O3 doped SnO2 films.
the preferred orientation along (101) plane. However, Kim et al. reported that Sb doped SnO2 films have preferred orientation along (211) plane. Therefore, from the XRD study, it is observed that all the trivalent element doped SnO2 films have (101) preferred orientation. Fig. 3(a, b) shows the room temperature PL spectra of Al and Sb doped SnO2 films respectively. It can be seen from the PL spectra (see Fig. 3(a, b)), none of the films shows the near band edge emission (NBE) because of the PL detection limit. The PL of Al and Sb doped SnO2 films exhibits a strong violeteblue emission centered at 400 nm. It is observed that the intensity of violeteblue emission decreases with increase in the content of doping. In earlier reports, various groups have studied the luminescence mechanism in pure and doped SnO2, and found the complex emission band ranging from 318 to 640 nm [16e21]. S Brovelli et al. suggested that the 318 nm (3.9 eV) emission was attributed to the
Fig. 5. Variation of figure of merit for the Sb2O3 doped SnO2 films deposited with different deposition film thickness.
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G.W. Kim et al. / Current Applied Physics 12 (2012) S21eS24
Fig. 5 show a plot of the figure of merit vs. deposition thickness of Sb2O3 doped SnO2 films. The figure of merit is widely used to compare the performance of various transparent conductors as first defined by Haacke [23] is, F ¼ Tr10/Rs where Tr is the optical transmittance for the range of 400e800 nm and Rs is the sheet resistance. The figure of merit increases consistently because of increase in films thickness. This can be explained as follows: usually in thin films the resistivity depends on film thickness. In bulk the resistance to the charge carrier is usually caused by photon scattering, impurity and defect scattering. In thin film, there is an additional scattering from the surface. 4. Conclusions We have successfully grown trivalent element (Al2O3, Bi2O3, Sb2O3 and Y2O3) doped SnO2 thin films using pulsed laser deposition method. The XRD results indicated that all the films have single phase nature with rutile structure and having (101) preferred orientation. The electrical resistivity has been found to decrease with doing up to a particular of doping value depending upon the doping element. PL measurements indicates that Al2O3 and Sb2O3 doped SnO2 films show violeteblue emission and the intensity of the violeteblue emission decreases with increase in the doping content. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2011-0030802). This research was supported by the MKE(The Ministry of Knowledge Economy), Korea, under the
ITRC(Information Technology Research Center) support program supervised by the NIPA(National IT Industry Promotion Agency) (NIPA-2012-H0301-12-2009). References [1] A. Goetzberger, C. Hebling, Sol. Energy Mater. Sol. Cells 62 (2000) 1. [2] P. Nelli, G. Faglia, G. Sverbeglieri, E. Cereda, G. Garbetta, A. Dieguez, A. Romano- Rodriguez, J.R. Morante, Thin Solid Films 371 (2000) 249. [3] T. Minami, S. Takata, H. Sato, H. Sonohara, J. Vac. Sci. Technol. A 13 (1995) 1095. [4] G. Frank, E. Kaur, H. Kostlin, Sol. Energy Mater. 8 (1983) 387. [5] A.C. Arias, L.S. Roman, T. Kugler, R. Toniolo, M.S. Meruvia, I.A. Hummelgen, Thin Solid Films 371 (2000) 201. [6] J. Proscia, R.G. Gordon, Thin Solid Films 214 (1992) 175. [7] T. Maruyama, K. Tabata, J. Appl. Phys. 68 (1990) 4282. [8] C. Agashe, S.S. Major, J. Mater. Sci. 31 (1996) 2965. [9] S. Shanthi, C. Subramanian, P. Ramasamy, J. Cryst. Growth 197 (1999) 858. [10] B. Thangaraju, Thin Solid Films 402 (2002) 71. [11] D. Das, R. Banerjee, Thin Solid Films 147 (1987) 321. [12] O.K. Varghese, L.K. Malhotra, J. Appl. Phys. 87 (2000) 7457. [13] Y. Onuma, Z. Wang, H. Ito, M. Nakao, K. Kamimura, Jpn. J. Appl. Phys., Part 1 37 (1998) 963. [14] B. Stjerna, E. Olsson, C.G. Granqvist, J. Appl. Phys. 76 (1994) 3797. [15] B.P. Howson, H. Barakova, A.G. Spencer, Thin Solid Films 196 (1991) 315. [16] D. Calestani, L. Lazzarini, G. Salviati, M. Zha, Cryst. Res. Technol. 40 (2005) 937. [17] J.-H. He, T.-H. Wu, C.-L. Hsin, K.-M. Li, L.-J. Chen, Y.-L. Chueh, L.-J. Chou, Z.L. Wang, Small 2 (2006) 116. [18] J.-Q. Hu, X.-L. Ma, N.-G. Shang, Z.-Y. Xie, N.-B. Wong, C.-S. Lee, S.-T. Lee, J. Phys. Chem. B 106 (2002) 3823. [19] H.-W. Kim, N.-H. Kim, J.-H. Myung, S.-H. Shim, Phys. Status Solidi A 202 (2005) 1758. [20] S. Brovelli, N. Chiodini, F. Meinardi, A. Lauria, A. Paleari, Appl. Phys. Lett. 89 (2006) 153126. [21] Wang S.-F. Guf, C.-F. Song, M.-K. Lv, Y.-X. Qi, G.-J. Zhou, D. Xu, D.-R. Yuan, Chem. Phys. Lett. 372 (2003) 451. [22] M. Lei, Q.-R. Hu, S.-L. Wang, W.-H. Tang, Mater. Lett. 64 (2010) 19. [23] G. Haacke, J. Appl. Phys. 47 (1976) 4086.
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effect of trivalent element doping on structural and optical properties of SnO2 thin films grown by pulsed laser deposition technique Geun Woo Kim, Chang Hoon Sung, M.S. Anwar, Yong Jun Seo, Si Nae Heo, Keun Young Park, Tae Kwon Song, Bon Heun Koo* School of Nano & Advanced Materials Engineering, Changwon National University, 9 Sarim dong, Changwon 641 773, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 December 2011 Received in revised form 28 March 2012 Accepted 14 May 2012 Available online 2 June 2012
In this study, trivalent element doped SnO2 thin films were deposited on glass substrate using pulsed laser deposition method. The effect of trivalent elements (Al2O3, Bi2O3, Sb2O3 and Y2O3) doping on the structural, electrical and optical properties have been studied using X-ray diffraction (XRD), photoluminescence and resistivity measurements. XRD results indicated that all films exhibited single phase nature with (101) preferred orientation. Among all the films, the 6 wt% Sb doped film had the lowest value of the resistivity. Photoluminescence study inferred that all the films showed violeteblue emission and the intensity of this emission decrease with the increase in the doping content. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Transparent conducting oxide SnO2 Thin films X-ray diffraction Pulsed laser deposition
1. Introduction During the last several years, transparent conducting oxide (TCO) has gained much attention among the scientific community due to its excellent optical and electrical properties and chemical stability as well as their potential technological applications such as: flat-panel displays, light-emitting diodes, touch panels, defrosters, and solar cells [1e3]. However, the development of alternative TCOs has become growingly essential because of the growing concern about the storage of indium that is a nature resource. The alternatives should be composed of abundant and nontoxic material such as tin, zinc and titanium. As compared to widely used indium tin oxide (ITO), SnO2 films are chemically stabled in acidic and basic solutions, thermally stable in oxidizing environments at high temperatures and also mechanically hard. SnO2 is an n e type wide band gap semiconductor (band gap ranging from 3.6 to 4.1 eV) having rutile structure. SnO2 have been found to exhibit uncommon properties that are explored in technological applications. As being a transparent to visible wavelength of light as well as simultaneously possessing, a relatively high electron concentration and mobility makes SnO2 a key material in
* Corresponding author. Tel.: þ82 55 264 5431; fax: þ82 55 262 6486. E-mail addresses: [email protected] (G.W. Kim), bhkoo@ changwon.ac.kr (B.H. Koo). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.05.041
the field of TCOs. Several deposition techniques such as solegel paralysis method, RF sputtering method and chemical vapor deposition are used to fabricate SnO2 films [4e15]. However, interest in PLD technique paying attention on the novelty of a simple and versatile physical vapor deposition technique for ceramics, high TC superconductors, biomaterials and latter expanded to include the multi-component oxides. The films grown from the multi-component target using PLD found to reproduces the composition as that of the target. Moreover, the PLD grown films crystallize at lower substrate temperature as compared to other physical vapor deposition technique. Many works about the preparation, characteristics, electrical and optical properties of trivalent doped tin oxide thin films deposited by different techniques have been reported. However a reexamination of their properties is necessary since tin oxide thin films doped with antimony exhibit interesting electrochemical properties in different electrochemical processes, and also because their possible applications as electrodes in new generations of solar cells and other optoelectronic devices. In the present work, we have investigated the effect of trivalent ions doping on the structural, electrical and optical properties of SnO2 films fabricated using PLD technique. The XRD results infer that all the films have single phase nature with preferred orientation along (101) plane except 6 wt% Sb doped SnO2 film. From the electrical resistivity study, it is found that the resistivity of Sb doped films decreases with increase in doping and then starts increasing with further doping above 6 wt%.
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G.W. Kim et al. / Current Applied Physics 12 (2012) S21eS24
For all trivalent ion doped SnO2 films, it is observed that 6% Sb doped SnO2 films have the lowest value of the resistivity. 2. Experimental details The polycrystalline bulk target of trivalent ion doped SnO2 was synthesized by conventional solid-state reaction technique. Pure SnO2 powder and trivalent elements (Al2O3, Sb2O, the purity was higher than 99%) were mixed completely using a ball milling for 24 h to achieve homogeneity and then pressed into a 1 inch diameter pellet at 100001 b. Finally, the pellets were sintered at 1250 C for 12 h. The KrFexcimer laser (Lambda Physik model COMPEX-201) of wavelength 248 nm and pulse duration of 20 ns was employed for the deposition on glass substrate. During the deposition, laser energy density at the target surface was kept at 1.0 J/cm2 and repetition rate at 10 Hz. The substrate temperature was maintained at 300 C and the background pressure was 60 m Torr of oxygen during the deposition. The focused laser beam was incident on the target surface at an angle of 90 . The target was rotated about 10 rpm and the substrate was mounted opposite to the target at a distance of 3.5 cm. The substrate was mounted on a heater plate using a silver paste. After deposition, the thin films were cooled slowly to room temperature, maintaining the oxygen pressure in the vacuum chamber to 60 m Torr. The structural properties of the films were analyzed by using Xray diffraction (XRD, Cu Ka1, ¼ 0.15406 nm) and the scans were performed with a 0.02 step size in the 2q range of 20e80 . The electrical properties were checked by using a four-probe sheet resistance (Mitsbishi Loresta MCP-T610 TFP) and a Hall measurement system. The PL property and optical properties of the films were studied at room temperature by using a Photoluminescence and a UVeVISeNIR spectrometer in the wavelength range of 300e800 nm.
Table 1 Comparison of ionic radius of trivalent elements with Sn4þ. Atom
Ionic radius (nm)
Sn4þ Sb3þ Bi3þ Al3þ Y3þ
0.071 0.076 0.196 0.051 0.09
Fig. 1 represents the electrical resistivity as a function of trivalent element doping in SnO2 thin films. The sheet resistance (Rs) measurements were done using a four point prove method by assuming that the thickness of the films was uniform. The resistivity of the film was calculated using the following relation: r ¼ Rs d, where d is the film thickness. It is observed that the value
of the resistivity decreases with doping up to a particular value of the doping element. Especially, when Sb2O3 doped SnO2, the resistivity obtained the lowest value .The variation in resistivity can be explained on the basis of two oxidation states of antimony (Sb3þ,Sb5þ). In this case, some of the Sn4þ ions are replaced by Sb5þ in the lattice, resulting in the generation of conduction electrons, so reducing the resistivity. Also, due to the substitution of Sn4þ by Sb3þ as their ionic radii are well matched, this substitution leads to the increase in the lattice constants due to the larger size of Sb3þ as compared with Sn4þ. Besides, ionic radius disparities exist between of trivalent elements and Sn4þ. Table 1 shows the comparison of ionic radius of trivalent elements with Sn4þ. Terrier et al. reported that at low doping level Sb exist in the Sbþ5 states, whereas, at higher doping level Sb exist in Sb3þ. Thus, the substitution of Snþ4 by Sb5þ induces a donor center very close to the conduction band of SnO2, as a result the resistivity decreases. When Sbþ3 replaces Snþ4 ions an acceptor site is created inside the band gap. If both sites are present, compensating effect may occur, as a result the resistivity would appear as devoted by the ratio between Sbþ5 and Sbþ3. On the basis of the electrical properties measurement, one can say that at the lower value of the doping, Snþ5 would predominate, as a result the resistivity decreases with increasing the level of Sb doping up to 6 wt%. With further doping of Sb over 6 wt%, Sbþ3 ions are generated and which compensate donor level which leads to increase in the resistivity value. Fig. 2 depicts the XRD pattern of undoped and trivalent element (Al2O3, Bi2O3, Sb2O3 and Y2O3) doped SnO2 thin films deposited at 300 C in 60 m Torr pressure of O2. From the XRD results, it is observed that all the films exhibit single phase polycrystalline behavior with rutile structure. None of the film shows any extra peaks corresponding to secondary phase, such as SnO, Sn2O3, Sb2O3, Al2O3 etc. The XRD results of Sb doped SnO2 films also show
Fig. 1. Electrical resistivity of trivalent elements (Al2O3, Bi2O3, Sb2O3 and Y2O3) doped SnO2 films.
Fig. 2. X-ray diffraction patterns of trivalent doped (Al2O3, Bi2O3, Sb2O3 and Y2O3) SnO2 films.
3. Results and discussion
G.W. Kim et al. / Current Applied Physics 12 (2012) S21eS24
S23
Fig. 4. The optical transmittance spectra of Sb2O3 doped SnO2 films with different film thickness.
free-exciton decay in SnO2 with an energy shift of 0.3 eV with respect to the bulk gap energy (3.6 eV) [20]. This emission is not observed in the presented work due to the limit of the PL detection range with a HeeCd 325 nm excitation source. In general, except the sharp excitonic emission, semiconductors have another extensive trapped emission, which often contains multiple luminescent centers. There are various types of surface states that give rise to different energy states inside the semiconductor band gap. Lei et al. [22] reported the green and yellow emission in Al doped SnO2 nanowires. Generally, oxygen vacancies are known to be the most common defect and act as a radiative center in the luminescence process. When Al3þ or Sb3þ ions are substituted in place of Sn4þ in the host lattice then 1 charge of the Al1Sn and Sb1Sn ion has to be compensated for somewhere in the lattice in the form of oxygen vacancy. Fig. 4 shows the transmittance spectra of Sb2O3 doped SnO2 films at 500 C and 60 m Torr of oxygen pressure with different film thickness in the visible region (300 nme800 nm). The average transmittance in the visible region was observed between 86% and 83%. Consequently, all films exhibited high transparency in the visible region. Fig. 3. Photoluminescence spectra of trivalent doped SnO2 films (a) Al2O3 doped SnO2 films (b) Sb2O3 doped SnO2 films.
the preferred orientation along (101) plane. However, Kim et al. reported that Sb doped SnO2 films have preferred orientation along (211) plane. Therefore, from the XRD study, it is observed that all the trivalent element doped SnO2 films have (101) preferred orientation. Fig. 3(a, b) shows the room temperature PL spectra of Al and Sb doped SnO2 films respectively. It can be seen from the PL spectra (see Fig. 3(a, b)), none of the films shows the near band edge emission (NBE) because of the PL detection limit. The PL of Al and Sb doped SnO2 films exhibits a strong violeteblue emission centered at 400 nm. It is observed that the intensity of violeteblue emission decreases with increase in the content of doping. In earlier reports, various groups have studied the luminescence mechanism in pure and doped SnO2, and found the complex emission band ranging from 318 to 640 nm [16e21]. S Brovelli et al. suggested that the 318 nm (3.9 eV) emission was attributed to the
Fig. 5. Variation of figure of merit for the Sb2O3 doped SnO2 films deposited with different deposition film thickness.
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G.W. Kim et al. / Current Applied Physics 12 (2012) S21eS24
Fig. 5 show a plot of the figure of merit vs. deposition thickness of Sb2O3 doped SnO2 films. The figure of merit is widely used to compare the performance of various transparent conductors as first defined by Haacke [23] is, F ¼ Tr10/Rs where Tr is the optical transmittance for the range of 400e800 nm and Rs is the sheet resistance. The figure of merit increases consistently because of increase in films thickness. This can be explained as follows: usually in thin films the resistivity depends on film thickness. In bulk the resistance to the charge carrier is usually caused by photon scattering, impurity and defect scattering. In thin film, there is an additional scattering from the surface. 4. Conclusions We have successfully grown trivalent element (Al2O3, Bi2O3, Sb2O3 and Y2O3) doped SnO2 thin films using pulsed laser deposition method. The XRD results indicated that all the films have single phase nature with rutile structure and having (101) preferred orientation. The electrical resistivity has been found to decrease with doing up to a particular of doping value depending upon the doping element. PL measurements indicates that Al2O3 and Sb2O3 doped SnO2 films show violeteblue emission and the intensity of the violeteblue emission decreases with increase in the doping content. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2011-0030802). This research was supported by the MKE(The Ministry of Knowledge Economy), Korea, under the
ITRC(Information Technology Research Center) support program supervised by the NIPA(National IT Industry Promotion Agency) (NIPA-2012-H0301-12-2009). References [1] A. Goetzberger, C. Hebling, Sol. Energy Mater. Sol. Cells 62 (2000) 1. [2] P. Nelli, G. Faglia, G. Sverbeglieri, E. Cereda, G. Garbetta, A. Dieguez, A. Romano- Rodriguez, J.R. Morante, Thin Solid Films 371 (2000) 249. [3] T. Minami, S. Takata, H. Sato, H. Sonohara, J. Vac. Sci. Technol. A 13 (1995) 1095. [4] G. Frank, E. Kaur, H. Kostlin, Sol. Energy Mater. 8 (1983) 387. [5] A.C. Arias, L.S. Roman, T. Kugler, R. Toniolo, M.S. Meruvia, I.A. Hummelgen, Thin Solid Films 371 (2000) 201. [6] J. Proscia, R.G. Gordon, Thin Solid Films 214 (1992) 175. [7] T. Maruyama, K. Tabata, J. Appl. Phys. 68 (1990) 4282. [8] C. Agashe, S.S. Major, J. Mater. Sci. 31 (1996) 2965. [9] S. Shanthi, C. Subramanian, P. Ramasamy, J. Cryst. Growth 197 (1999) 858. [10] B. Thangaraju, Thin Solid Films 402 (2002) 71. [11] D. Das, R. Banerjee, Thin Solid Films 147 (1987) 321. [12] O.K. Varghese, L.K. Malhotra, J. Appl. Phys. 87 (2000) 7457. [13] Y. Onuma, Z. Wang, H. Ito, M. Nakao, K. Kamimura, Jpn. J. Appl. Phys., Part 1 37 (1998) 963. [14] B. Stjerna, E. Olsson, C.G. Granqvist, J. Appl. Phys. 76 (1994) 3797. [15] B.P. Howson, H. Barakova, A.G. Spencer, Thin Solid Films 196 (1991) 315. [16] D. Calestani, L. Lazzarini, G. Salviati, M. Zha, Cryst. Res. Technol. 40 (2005) 937. [17] J.-H. He, T.-H. Wu, C.-L. Hsin, K.-M. Li, L.-J. Chen, Y.-L. Chueh, L.-J. Chou, Z.L. Wang, Small 2 (2006) 116. [18] J.-Q. Hu, X.-L. Ma, N.-G. Shang, Z.-Y. Xie, N.-B. Wong, C.-S. Lee, S.-T. Lee, J. Phys. Chem. B 106 (2002) 3823. [19] H.-W. Kim, N.-H. Kim, J.-H. Myung, S.-H. Shim, Phys. Status Solidi A 202 (2005) 1758. [20] S. Brovelli, N. Chiodini, F. Meinardi, A. Lauria, A. Paleari, Appl. Phys. Lett. 89 (2006) 153126. [21] Wang S.-F. Guf, C.-F. Song, M.-K. Lv, Y.-X. Qi, G.-J. Zhou, D. Xu, D.-R. Yuan, Chem. Phys. Lett. 372 (2003) 451. [22] M. Lei, Q.-R. Hu, S.-L. Wang, W.-H. Tang, Mater. Lett. 64 (2010) 19. [23] G. Haacke, J. Appl. Phys. 47 (1976) 4086.