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Electrical and optical properties of ITO and ITO:Zr transparent conducting films
ARTICLE IN PRESS Materials Science in Semiconductor Processing 10 (2007) 264– 269
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Electrical and optical properties of ITO and ITO:Zr transparent conducting films Bo Zhang a, Xianping Dong a,, Xiaofeng Xu b, Xinjian Wang a, Jiansheng Wu a a
Key Laboratory of Ministry of Education for High Temperature Materials and Tests, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China College of Science, Donghua University, Shanghai 200051, PR China
b
a r t i c l e i n f o
abstract
Available online 7 May 2008
ITO and ITO:Zr films were deposited on glass substrates by magnetron sputtering. Electrical and optical properties of the films at different experiment parameters such as substrate temperature, oxygen flow rate and annealing temperature were contrastively studied. The increase in substrate temperature remarkably improves the electrical and optical properties of the films. ITO:Zr films show better quality at low substrate temperature. The excessive oxygen can worsen the optical properties of the films. Better optical–electrical properties of the films can be achieved after the proper annealing treatment. Obvious Burstin–Moss effect can be revealed by transmittance spectra with different parameters, and the direct transition models show the change of optical band gap. ITO:Zr films prepared by co-sputtering show better optical–electrical properties than ITO films. & 2008 Elsevier Ltd. All rights reserved.
PACS: 07.77.K 61.16.C 79.60 Keywords: ITO Electrical properties Optical properties Optical band gap
1. Introduction Indium tin oxide (ITO) films are a highly degenerate ntype semiconductor. The semiconducting mechanism of ITO is attributed to doubly charged oxygen vacancies and substitutional Sn4+ positioned on In3+ sites. ITO films possess good electrical conductivity, high optical transparency in visible region, wide optical band gap and high IR reflectivity properties, which make them have wide applications in flat panel displays, solar cells and organic light emitting devices as the transparent electrodes [1,2]. But some critical factors, such as chemical and thermal instability at high temperature and lower surface energy, limit the wider application of ITO films, and the optical–electrical properties of ITO films should be enhanced. Methods and procedures to improve the properties of ITO films were explored, and multi-component ITO oxides
Corresponding author. Tel.:/fax: +86 21 54747471.
E-mail address: [email protected] (X. Dong). 1369-8001/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2008.03.003
with better performances were studied in order to find extensive practical application [3–6]. ITO films doped with a small amount of foreign metal not only maintain the basic properties of ITO films but also improve some special properties [7–9]. Zirconium is usually regarded as the suitable doping ion, and zirconium doping of In2O3 films deposited by atomic layer deposition has been studied by another author [10]. Therefore, the high-valence metal elements zirconium can be regarded as the donor, which replaces indium in the In2O3 matrix of ITO films and increases carrier concentration. Zr doping can lead to better crystalline structure, lower surface roughness and higher surface energy of ITO films. The better stability of zirconium oxide can improve the chemical and thermal stability of ITO films. Besides, the certain amount of Zr dopant can contribute to the activation of Sn dopant and promote Sn4+ replacement of In3+. In this paper, based on the deposition of ITO films by magnetron sputtering, Zr-doped ITO films were deposited by co-sputtering with an ITO target and a zirconium target. The influences of substrate temperature, oxygen
ARTICLE IN PRESS B. Zhang et al. / Materials Science in Semiconductor Processing 10 (2007) 264–269
flow rate and annealing temperature on electrical and optical properties of ITO and ITO:Zr films were investigated. 2. Experimental details ITO and ITO:Zr films were deposited on glass substrates by magnetron sputtering. The schematic diagram of the two-targets magnetron sputtering system used in this study is shown in Fig. 1. ITO target was hot-pressed In2O3 target containing 10 wt.% SnO2, and the diameter of all targets is 60 mm. The deposition process of ITO and ITO:Zr films was same except including zirconium target for ITO:Zr films. DC sputtering power of ITO target was 45 W, and RF sputtering power of zirconium target was 10 W. The target-to-substrate distance was about 80 mm. Typically, the base vacuum was about 1.0 104 Pa, and the pressure was stabilized at about 0.5 Pa during the deposition processing by controlling argon gas flow. Oxygen flow rate varied from 0 to 1.5 sccm by a mass flow controller. The substrates were heated from room temperature (20 1C) to 400 1C. All sputtering processes were dynamic with the substrate rotating in front of the targets. The averaged metal atomic ratios of ITO and ITO:Zr films were In:Sn ¼ 9:1 and In:Sn:Zr ¼ 9:1:0.2, respectively. After 30 min deposition, ITO and ITO:Zr films showed about the same thickness under similar deposition process, and the thickness of all the films were about 240 nm. The annealing treatment was performed using an industrial ventilated furnace in air, and the optimized annealing time was about 35 min. The film thickness was measured using an alpha-step profilometer (Dektak 6M). The sheet resistance was determined using a four-point probe system (SDY-5). The carrier concentration and mobility of the films were measured using the Hall effect in van der pau mode. Optical transmittance and reflectance spectra of ITO/glass
Fig. 1. Schematic diagram of the two-targets magnetron sputtering system.
265
structure were measured with a UV–VIS–NIR scanning spectrophotometer (Lamda950). 3. Results and discussion 3.1. Electrical and optical properties with substrate temperature The crystalline and surface morphologies can directly affect the electrical and optical properties of the films. Fig. 2 shows the variations of XRD patterns of ITO and ITO:Zr films. With the increase in substrate temperature, an increase in intensity of XRD peak is observed. XRD spectra reveal that ITO:Zr films have better crystalline structure than ITO films. Zr doping changes the preferred orientation of ITO films and causes a increase in the (400) peak, meaning that compactness of the films is improved and the inter grain voids and defects are reduced [11]. AFM images of ITO and ITO:Zr films are shown in Fig. 3. All the films display a uniform grain size and void-free surface. The grain size shows an evident increasing tendency with substrate temperature. Due to Zr doping, the root mean square roughness Rrms reduces from 3.28 to 1.95 nm for ITO films deposited at 300 1C. ITO:Zr films show lower surface roughness than ITO films at the same process. Fig. 4 shows the optical spectra of ITO and ITO:Zr films including the variation of sheet resistance, and Table 1 shows carrier concentration and mobility with substrate temperature. The sheet resistance of ITO and ITO:Zr films decreases with the increase in substrate temperature and can reach about 10 O/sq. The carrier concentration and mobility, as a whole, increase with the increase in substrate temperature. The increase in carrier concentration with substrate temperature may be due to an increase in the diffusion of Sn or Zr donor atoms from interstitial locations and grain boundaries into the In location sites of the In2O3 matrix. By contrast with ITO films, ITO:Zr films show better electrical properties. The carrier concentration of ITO:Zr films is higher than that of ITO films,
Fig. 2. XRD patterns of (a) ITO and (b) ITO:Zr films with substrate temperature (oxygen flow rate at 0.3 sccm).
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Fig. 3. AFM images of (a) ITO and (b) ITO:Zr films with substrate temperature (2000 nm 2000 nm).
Fig. 4. Optical transmittance and reflection spectra and sheet resistance of (a) ITO and (b) ITO:Zr films with substrate temperature (oxygen flow rate at 0.3 sccm).
because zirconium is regarded as the other donor that replaces indium of In2O3 matrix. As a result, the electrical properties of ITO films deposited at lower substrate temperature can be remarkably improved with Zr doping by the co-sputtering technology. The optical transmittance and reflection spectra of ITO and ITO:Zr films (including glass substrate) with substrate temperature are shown in Fig. 4. The transmittance in the visual region increases with the increase in substrate temperature. The transmittance in the visual region of
ITO:Zr films is higher than that of ITO films, and a maximum transmittance in the visible region of about 89% can be achieved for ITO:Zr films (shown in Table 1). The reflection in the higher wavelength region is closely related with carrier concentration of the films [12]. Therefore, the continuous increases in reflection at the near-IR region with substrate temperature imply the sequential increases in carrier concentration. From the transmittance spectrum, we observe the fact that the near-UV absorption edge shifts toward shorter
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Table 1 Summary of the preparation conditions with optical– electrical properties and figures of merit fTC of ITO and ITO:Zr films Preparation conditions
Electrical data
Optical data
Eg (eV) Temperature ( 1C) Oxygen flow rate (sccm) Carrier concentration (1020 cm3) Mobility (cm3 s1 V1) Tmax (%) ITO (ITO:Zr) ITO (ITO:Zr) ITO (ITO:Zr) ITO (ITO:Zr) Substrate temperature 20 0.3 150 0.3 250 0.3 300 0.3 400 0.3 Oxygen flow rate 300 300 300 300 300 300
0 0.3 0.5 0.7 0.9 1.5
Annealing temperature As deposited 200 300 350 400 500
0.97 (1.75) 1.78 (2.69) 7.17 (7.84) 8.68 (8.73) 9.12 (9.23)
10.32 23.22 14.77 17.52 26.46
fTC ¼ Ta10/Rsq (103 O1)
ITO (ITO:Zr)
(16.22) (16.58) (16.71) (16.80) (27.98)
53.7 78.7 87.2 87.5 87.9
(66.1) 3.964 (4.114) 0.003 (0.15) (82.7) 4.116 (4.171) 0.75 (1.37) (87.3) 4.212 (4.231) 5.73 (6.15) (88.1) 4.239 (4.264) 8.52 (8.52) (89.5) 4.298 (4.348) 15.03 (18.14)
12.01 (14.38) 8.68 (8.73) 7.21 (7.87) 5.86 (7.82) 4.62 (5.67) 3.10 (3.61)
13.45 (12.97) 17.52 (16.80) 16.89 (16.79) 16.74 (16.76) 16.13 (15.88) 15.75 (13.54)
83.4 87.5 88.8 89.9 90.2 88.7
(73.0) 4.252 (4.281) (88.1) 4.239 (4.264) (90.4) 4.143 (4.258) (90.6) 4.133 (4.256) (91.0) 4.083 (4.187) (90.5) 4.054 (4.172)
5.51 (2.05) 8.52 (8.52) 8.09 (10.51) 7.33 (10.68) 5.84 (7.65) 3.16 (3.92)
0.97 (1.75) 5.84 (5.32) 7.15 (4.95) 5.57 (4.36) 4.74 (3.98) 2.93 (3.67)
10.32 (16.22) 16.80 (18.74) 24.28 (19.76) 30.08 (22.02) 19.11 (22.76) 17.93 (18.90)
53.7 61.4 65.9 78.8 85.4 86.9
(66.1) 3.964 (4.114) (83.9) 4.318 (4.288) (85.3) 4.345 (4.271) (88.8) 4.291 (4.217) (89.5) 4.232 (4.176) (89.9) 4.110 (4.154)
0.003 (0.15) 0.05 (3.53) 0.12 (4.24) 2.47 (5.32) 3.69 (5.34) 4.53 (5.43)
Tmax: maximum transmittance in the visible wavelength range, Eg: optical band gap.
wavelength with substrate temperature. According to Drude’s theory, ITO film is transparent in the visible region until the fundamental absorption starts at wavelengths close to the corresponding optical band gap energy. The widening of optical band gap is due to Burstein–Moss effect [13]. Assuming that the conduction band and valence band are parabolic in nature and the Burstein–Moss shift is the predominant effect, the carrier concentration can be roughly estimated from the optical band gap shift DEBM using the following equation: g 2
¼ DEBM g
h 2
1 1 þ ð3p2 N e Þ2=3 mv mc
(1)
with the effective electron mass mc , the effective hole mass mv and carrier concentration Ne. On the basis of the above equation, the optical band gap is closely related with the carrier concentration. The near-UV absorption is closely related with the optical band gap of the films [14,15]. The direct transition model of (aE)2 versus photon energy E was established according to direct allowed transitions equation, aE ¼ A(EEg)1/2, and optical band gap energy Eg was obtained by linear extrapolation to (aE)2 ¼ 0 [16,17], where a is the absorption coefficient. The optical band gap of ITO and ITO:Zr films increases with substrate temperature (shown in Table 1), which may be due to an increase in carrier concentration with substrate temperature and as a result, the absorption edge shifts towards the near-UV range. The direct transition models show wider optical band gap of ITO:Zr films than that of ITO films,
which implies that Zr doping improves the carrier concentration of the films and widens the corresponding optical band gap. In order to evaluate the quality of the transparent conductors, the figure of merit fTC is given by fTC ¼ Ta10/Rsq, where Ta is the average transmittance in the visible range and Rsq is the sheet resistance [18,19]. Table 1 shows the figure of merit of all the films with substrate temperature. Figure of merit of ITO and ITO:Zr films increases with substrate temperature, and ITO:Zr films show higher value of figure of merit than ITO films, which reveals that ITO:Zr films have better optical–electrical properties.
3.2. Electrical and optical properties with oxygen flow rate Fig. 5 shows the optical transmittance and reflection spectra of ITO and ITO:Zr films (including glass substrate) with oxygen flow rate including the variation of sheet resistance. The sheet resistance of the films increases with the increase in oxygen flow rate. The variations of carrier concentration and mobility of the films with oxygen flow rate are shown in Table 1. The carrier concentration decreases with the increase in oxygen flow rate, which is due to filling of the oxygen vacancies and deactivating the donor. The carrier concentration of ITO:Zr films is higher than that of ITO films under the same oxygen flow rate. The transmittance in the visual region increases with the increase in oxygen flow rate, but a further increase in oxygen flow rate can result in the decrease in optical
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Fig. 5. Optical transmittance, reflection spectra and sheet resistance of (a) ITO and (b) ITO:Zr films with oxygen flow rate (substrate temperature at 300 1C).
Fig. 6. Optical transmittance, reflection spectra and sheet resistance of (a) ITO and (b) ITO:Zr films with annealing temperature.
3.3. Electrical and optical properties with annealing temperature transparency [20]. ITO:Zr films show better visual transparency than ITO films under certain oxygen flow rate, and a maximum transmittance in the visible region of about 90.6% is achieved for ITO:Zr films (shown in Table 1). The continuous decrease in carrier concentration with oxygen flow rate can be further demonstrated from the optical properties. The decrease in the reflection at the near-IR region with the oxygen flow rate is well according with the decrease in carrier concentration. The optical band gap of the films decreases with oxygen flow rate, which is due to the decrease in carrier concentration with oxygen flow rate and results in the red shift of the near-UV absorption edge. Due to higher carrier concentration in the same oxygen flow rate, the optical band gap of ITO:Zr films is wider than that of ITO films.
Fig. 6 shows the optical transmittance and reflection spectra of ITO and ITO:Zr films (including glass substrate) with annealing temperature including the variation of sheet resistance. The sheet resistance of ITO and ITO:Zr films decreases firstly with the increase in annealing temperature and then increases at high annealing temperature. Annealing can promote crystalline growth of the films, which contributes to the activation of the dopant and reduces the number of dopant trapped at the dislocations or grain boundaries and increases carrier concentration. Carrier concentration and mobility increase as the annealing temperature increases in a certain range, which leads to the decrease in sheet resistance. Although the thermal energy at high annealing temperature can tend to promote crystallization, it can also
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contribute to some disorder influences such as thermal instability and lead to an increase in defect density. What is more, due to high-temperature annealing in air atmosphere, the excess free oxygen in air may incorporate with the films, which compensates the oxygen vacancies and decreases the carrier concentration. By contrast with ITO films, ITO:Zr films show better electrical properties at higher annealing temperature, which implies better thermal stability of ITO:Zr films. ITO:Zr films show higher sheet resistance than ITO films in the range of 200–300 1C, which results from the incorporation of excess oxygen in air due to the oxygen affinity of zirconium element, leading to a decrease in carrier concentration. The transmittance in the visual region of all the films increases with the increase in annealing temperature. The transmittance in the visual region of ITO:Zr films is higher than that of ITO films. In the near-IR range, the reflection of ITO and ITO:Zr films firstly increases and subsequently decreases with annealing temperature. In the near-UV range, the shifts of absorption edges can be observed. The variation of optical band gap arising from the shift is related to the concentration of free electrons. The optical band gap of the films increases with the increase in annealing temperature, but further increases in annealing temperature lead to a decrease in optical band gap. The direct transition models show wider optical band gap of ITO:Zr films than that of ITO films at lower and higher annealing temperatures, which also implies the variation of carrier concentration. 4. Conclusions The influences of substrate temperature, oxygen flow rate and annealing temperature on optical–electrical properties of ITO and ITO:Zr films were investigated. The transmittance in the visual region and optical band gap of the films increases with substrate temperature, and the transmittance of ITO:Zr films was higher than that of ITO films. The excessive increase in oxygen flow rate can worsen the optical transparency, and the optical band gap of the films decreases with oxygen flow rate. The proper annealing treatment in air can improve the optical–electrical properties of the films. The carrier concentration has
269
an important influence on near-IR reflection, near-UV absorption and optical band gap. Zr doping improves carrier concentration of the films and widens the corresponding optical band gap. The higher value of figure of merit of ITO:Zr films is observed, which reveals that ITO:Zr films have better optical–electrical properties than ITO films.
Acknowledgements This work has been supported by the Research Fund for Shanghai applied material (0725). The authors wish to thank Instrumental Analysis Center of SJTU for some measurements of the films. References [1] Gheidari AM, Soleimani EA, Mansorhoseini M, Mohajerzadeh S, Madani N, Kolahi WS. Mater Res Bull 2005;40:1303. [2] Yu HH, Hwang SJ, Tseng MC, Tseng CC. Opt Commun 2006;259:187. [3] Gregory OJ, You T, Crisman EE. Thin Solid Films 2005;476:344. [4] Suzuki M, Maeda Y, Muraoka M, Higuchi S, Sawada Y. Mater Sci Eng B 1998;54:43. [5] Hsu CM, Lee JW, Meen TH, Wu WT. Thin Solid Films 2005;474:19. [6] Minami T, Yamamoto T, Toda Y, Miyata T. Thin Solid Films 2000; 373:189. [7] Hsu CM, Wu WT. Appl Phys Lett 2004;85:840. [8] Ohno T, Kawahara T. Thin Solid Films 2000;373:189. [9] Nakasa A, Adachi M, Usami H, Suzuki E, Taniguchi Y. Thin Solid Films 2006;498:240. [10] Asikainen T, Ritala M, Leskela¨ M. Thin Solid Films 2003;440:152. [11] Canhola P, Martins N, Raniero L, Pereira S, Fortunato E, Ferreira I, et al. Thin Solid Films 2005;487:271. [12] Biswas PK, De A, Pramanik NC, Chakraborty PK, Ortner K, Hock V, et al. Mater Lett 2003;57:2326. [13] Hamberg I, Granqvist CG. J Appl Phys 1986;60:123. [14] Lee HC. Appl Surf Sci 2006;252:3428. [15] Manoj PK, Gopchandran KG, Koshy P, Vaidyan VK, Joseph B. Opt Mater 2006;80:615. [16] Antony A, Nisha M, Manoj R, Jayaraj MK. Appl Surf Sci 2004;225: 294. [17] Cruz LR, Legnani C, Matoso IG, Ferreira CL, Moutinho HR. Mater Res Bull 2004;39:993. [18] Nisha M, Anusha S, Antony A, Manoj R, Jayaraj MK. Appl Surf Sci 2005;252:1430. [19] Fallah HR, Ghasemi M, Hassanzadeh A, Steki H. Mater Res Bull 2007;42:487. [20] Lee HC, Park OO. Vacuum 2004;77:69.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Electrical and optical properties of ITO and ITO:Zr transparent conducting films Bo Zhang a, Xianping Dong a,, Xiaofeng Xu b, Xinjian Wang a, Jiansheng Wu a a
Key Laboratory of Ministry of Education for High Temperature Materials and Tests, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China College of Science, Donghua University, Shanghai 200051, PR China
b
a r t i c l e i n f o
abstract
Available online 7 May 2008
ITO and ITO:Zr films were deposited on glass substrates by magnetron sputtering. Electrical and optical properties of the films at different experiment parameters such as substrate temperature, oxygen flow rate and annealing temperature were contrastively studied. The increase in substrate temperature remarkably improves the electrical and optical properties of the films. ITO:Zr films show better quality at low substrate temperature. The excessive oxygen can worsen the optical properties of the films. Better optical–electrical properties of the films can be achieved after the proper annealing treatment. Obvious Burstin–Moss effect can be revealed by transmittance spectra with different parameters, and the direct transition models show the change of optical band gap. ITO:Zr films prepared by co-sputtering show better optical–electrical properties than ITO films. & 2008 Elsevier Ltd. All rights reserved.
PACS: 07.77.K 61.16.C 79.60 Keywords: ITO Electrical properties Optical properties Optical band gap
1. Introduction Indium tin oxide (ITO) films are a highly degenerate ntype semiconductor. The semiconducting mechanism of ITO is attributed to doubly charged oxygen vacancies and substitutional Sn4+ positioned on In3+ sites. ITO films possess good electrical conductivity, high optical transparency in visible region, wide optical band gap and high IR reflectivity properties, which make them have wide applications in flat panel displays, solar cells and organic light emitting devices as the transparent electrodes [1,2]. But some critical factors, such as chemical and thermal instability at high temperature and lower surface energy, limit the wider application of ITO films, and the optical–electrical properties of ITO films should be enhanced. Methods and procedures to improve the properties of ITO films were explored, and multi-component ITO oxides
Corresponding author. Tel.:/fax: +86 21 54747471.
E-mail address: [email protected] (X. Dong). 1369-8001/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2008.03.003
with better performances were studied in order to find extensive practical application [3–6]. ITO films doped with a small amount of foreign metal not only maintain the basic properties of ITO films but also improve some special properties [7–9]. Zirconium is usually regarded as the suitable doping ion, and zirconium doping of In2O3 films deposited by atomic layer deposition has been studied by another author [10]. Therefore, the high-valence metal elements zirconium can be regarded as the donor, which replaces indium in the In2O3 matrix of ITO films and increases carrier concentration. Zr doping can lead to better crystalline structure, lower surface roughness and higher surface energy of ITO films. The better stability of zirconium oxide can improve the chemical and thermal stability of ITO films. Besides, the certain amount of Zr dopant can contribute to the activation of Sn dopant and promote Sn4+ replacement of In3+. In this paper, based on the deposition of ITO films by magnetron sputtering, Zr-doped ITO films were deposited by co-sputtering with an ITO target and a zirconium target. The influences of substrate temperature, oxygen
ARTICLE IN PRESS B. Zhang et al. / Materials Science in Semiconductor Processing 10 (2007) 264–269
flow rate and annealing temperature on electrical and optical properties of ITO and ITO:Zr films were investigated. 2. Experimental details ITO and ITO:Zr films were deposited on glass substrates by magnetron sputtering. The schematic diagram of the two-targets magnetron sputtering system used in this study is shown in Fig. 1. ITO target was hot-pressed In2O3 target containing 10 wt.% SnO2, and the diameter of all targets is 60 mm. The deposition process of ITO and ITO:Zr films was same except including zirconium target for ITO:Zr films. DC sputtering power of ITO target was 45 W, and RF sputtering power of zirconium target was 10 W. The target-to-substrate distance was about 80 mm. Typically, the base vacuum was about 1.0 104 Pa, and the pressure was stabilized at about 0.5 Pa during the deposition processing by controlling argon gas flow. Oxygen flow rate varied from 0 to 1.5 sccm by a mass flow controller. The substrates were heated from room temperature (20 1C) to 400 1C. All sputtering processes were dynamic with the substrate rotating in front of the targets. The averaged metal atomic ratios of ITO and ITO:Zr films were In:Sn ¼ 9:1 and In:Sn:Zr ¼ 9:1:0.2, respectively. After 30 min deposition, ITO and ITO:Zr films showed about the same thickness under similar deposition process, and the thickness of all the films were about 240 nm. The annealing treatment was performed using an industrial ventilated furnace in air, and the optimized annealing time was about 35 min. The film thickness was measured using an alpha-step profilometer (Dektak 6M). The sheet resistance was determined using a four-point probe system (SDY-5). The carrier concentration and mobility of the films were measured using the Hall effect in van der pau mode. Optical transmittance and reflectance spectra of ITO/glass
Fig. 1. Schematic diagram of the two-targets magnetron sputtering system.
265
structure were measured with a UV–VIS–NIR scanning spectrophotometer (Lamda950). 3. Results and discussion 3.1. Electrical and optical properties with substrate temperature The crystalline and surface morphologies can directly affect the electrical and optical properties of the films. Fig. 2 shows the variations of XRD patterns of ITO and ITO:Zr films. With the increase in substrate temperature, an increase in intensity of XRD peak is observed. XRD spectra reveal that ITO:Zr films have better crystalline structure than ITO films. Zr doping changes the preferred orientation of ITO films and causes a increase in the (400) peak, meaning that compactness of the films is improved and the inter grain voids and defects are reduced [11]. AFM images of ITO and ITO:Zr films are shown in Fig. 3. All the films display a uniform grain size and void-free surface. The grain size shows an evident increasing tendency with substrate temperature. Due to Zr doping, the root mean square roughness Rrms reduces from 3.28 to 1.95 nm for ITO films deposited at 300 1C. ITO:Zr films show lower surface roughness than ITO films at the same process. Fig. 4 shows the optical spectra of ITO and ITO:Zr films including the variation of sheet resistance, and Table 1 shows carrier concentration and mobility with substrate temperature. The sheet resistance of ITO and ITO:Zr films decreases with the increase in substrate temperature and can reach about 10 O/sq. The carrier concentration and mobility, as a whole, increase with the increase in substrate temperature. The increase in carrier concentration with substrate temperature may be due to an increase in the diffusion of Sn or Zr donor atoms from interstitial locations and grain boundaries into the In location sites of the In2O3 matrix. By contrast with ITO films, ITO:Zr films show better electrical properties. The carrier concentration of ITO:Zr films is higher than that of ITO films,
Fig. 2. XRD patterns of (a) ITO and (b) ITO:Zr films with substrate temperature (oxygen flow rate at 0.3 sccm).
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Fig. 3. AFM images of (a) ITO and (b) ITO:Zr films with substrate temperature (2000 nm 2000 nm).
Fig. 4. Optical transmittance and reflection spectra and sheet resistance of (a) ITO and (b) ITO:Zr films with substrate temperature (oxygen flow rate at 0.3 sccm).
because zirconium is regarded as the other donor that replaces indium of In2O3 matrix. As a result, the electrical properties of ITO films deposited at lower substrate temperature can be remarkably improved with Zr doping by the co-sputtering technology. The optical transmittance and reflection spectra of ITO and ITO:Zr films (including glass substrate) with substrate temperature are shown in Fig. 4. The transmittance in the visual region increases with the increase in substrate temperature. The transmittance in the visual region of
ITO:Zr films is higher than that of ITO films, and a maximum transmittance in the visible region of about 89% can be achieved for ITO:Zr films (shown in Table 1). The reflection in the higher wavelength region is closely related with carrier concentration of the films [12]. Therefore, the continuous increases in reflection at the near-IR region with substrate temperature imply the sequential increases in carrier concentration. From the transmittance spectrum, we observe the fact that the near-UV absorption edge shifts toward shorter
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267
Table 1 Summary of the preparation conditions with optical– electrical properties and figures of merit fTC of ITO and ITO:Zr films Preparation conditions
Electrical data
Optical data
Eg (eV) Temperature ( 1C) Oxygen flow rate (sccm) Carrier concentration (1020 cm3) Mobility (cm3 s1 V1) Tmax (%) ITO (ITO:Zr) ITO (ITO:Zr) ITO (ITO:Zr) ITO (ITO:Zr) Substrate temperature 20 0.3 150 0.3 250 0.3 300 0.3 400 0.3 Oxygen flow rate 300 300 300 300 300 300
0 0.3 0.5 0.7 0.9 1.5
Annealing temperature As deposited 200 300 350 400 500
0.97 (1.75) 1.78 (2.69) 7.17 (7.84) 8.68 (8.73) 9.12 (9.23)
10.32 23.22 14.77 17.52 26.46
fTC ¼ Ta10/Rsq (103 O1)
ITO (ITO:Zr)
(16.22) (16.58) (16.71) (16.80) (27.98)
53.7 78.7 87.2 87.5 87.9
(66.1) 3.964 (4.114) 0.003 (0.15) (82.7) 4.116 (4.171) 0.75 (1.37) (87.3) 4.212 (4.231) 5.73 (6.15) (88.1) 4.239 (4.264) 8.52 (8.52) (89.5) 4.298 (4.348) 15.03 (18.14)
12.01 (14.38) 8.68 (8.73) 7.21 (7.87) 5.86 (7.82) 4.62 (5.67) 3.10 (3.61)
13.45 (12.97) 17.52 (16.80) 16.89 (16.79) 16.74 (16.76) 16.13 (15.88) 15.75 (13.54)
83.4 87.5 88.8 89.9 90.2 88.7
(73.0) 4.252 (4.281) (88.1) 4.239 (4.264) (90.4) 4.143 (4.258) (90.6) 4.133 (4.256) (91.0) 4.083 (4.187) (90.5) 4.054 (4.172)
5.51 (2.05) 8.52 (8.52) 8.09 (10.51) 7.33 (10.68) 5.84 (7.65) 3.16 (3.92)
0.97 (1.75) 5.84 (5.32) 7.15 (4.95) 5.57 (4.36) 4.74 (3.98) 2.93 (3.67)
10.32 (16.22) 16.80 (18.74) 24.28 (19.76) 30.08 (22.02) 19.11 (22.76) 17.93 (18.90)
53.7 61.4 65.9 78.8 85.4 86.9
(66.1) 3.964 (4.114) (83.9) 4.318 (4.288) (85.3) 4.345 (4.271) (88.8) 4.291 (4.217) (89.5) 4.232 (4.176) (89.9) 4.110 (4.154)
0.003 (0.15) 0.05 (3.53) 0.12 (4.24) 2.47 (5.32) 3.69 (5.34) 4.53 (5.43)
Tmax: maximum transmittance in the visible wavelength range, Eg: optical band gap.
wavelength with substrate temperature. According to Drude’s theory, ITO film is transparent in the visible region until the fundamental absorption starts at wavelengths close to the corresponding optical band gap energy. The widening of optical band gap is due to Burstein–Moss effect [13]. Assuming that the conduction band and valence band are parabolic in nature and the Burstein–Moss shift is the predominant effect, the carrier concentration can be roughly estimated from the optical band gap shift DEBM using the following equation: g 2
¼ DEBM g
h 2
1 1 þ ð3p2 N e Þ2=3 mv mc
(1)
with the effective electron mass mc , the effective hole mass mv and carrier concentration Ne. On the basis of the above equation, the optical band gap is closely related with the carrier concentration. The near-UV absorption is closely related with the optical band gap of the films [14,15]. The direct transition model of (aE)2 versus photon energy E was established according to direct allowed transitions equation, aE ¼ A(EEg)1/2, and optical band gap energy Eg was obtained by linear extrapolation to (aE)2 ¼ 0 [16,17], where a is the absorption coefficient. The optical band gap of ITO and ITO:Zr films increases with substrate temperature (shown in Table 1), which may be due to an increase in carrier concentration with substrate temperature and as a result, the absorption edge shifts towards the near-UV range. The direct transition models show wider optical band gap of ITO:Zr films than that of ITO films,
which implies that Zr doping improves the carrier concentration of the films and widens the corresponding optical band gap. In order to evaluate the quality of the transparent conductors, the figure of merit fTC is given by fTC ¼ Ta10/Rsq, where Ta is the average transmittance in the visible range and Rsq is the sheet resistance [18,19]. Table 1 shows the figure of merit of all the films with substrate temperature. Figure of merit of ITO and ITO:Zr films increases with substrate temperature, and ITO:Zr films show higher value of figure of merit than ITO films, which reveals that ITO:Zr films have better optical–electrical properties.
3.2. Electrical and optical properties with oxygen flow rate Fig. 5 shows the optical transmittance and reflection spectra of ITO and ITO:Zr films (including glass substrate) with oxygen flow rate including the variation of sheet resistance. The sheet resistance of the films increases with the increase in oxygen flow rate. The variations of carrier concentration and mobility of the films with oxygen flow rate are shown in Table 1. The carrier concentration decreases with the increase in oxygen flow rate, which is due to filling of the oxygen vacancies and deactivating the donor. The carrier concentration of ITO:Zr films is higher than that of ITO films under the same oxygen flow rate. The transmittance in the visual region increases with the increase in oxygen flow rate, but a further increase in oxygen flow rate can result in the decrease in optical
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Fig. 5. Optical transmittance, reflection spectra and sheet resistance of (a) ITO and (b) ITO:Zr films with oxygen flow rate (substrate temperature at 300 1C).
Fig. 6. Optical transmittance, reflection spectra and sheet resistance of (a) ITO and (b) ITO:Zr films with annealing temperature.
3.3. Electrical and optical properties with annealing temperature transparency [20]. ITO:Zr films show better visual transparency than ITO films under certain oxygen flow rate, and a maximum transmittance in the visible region of about 90.6% is achieved for ITO:Zr films (shown in Table 1). The continuous decrease in carrier concentration with oxygen flow rate can be further demonstrated from the optical properties. The decrease in the reflection at the near-IR region with the oxygen flow rate is well according with the decrease in carrier concentration. The optical band gap of the films decreases with oxygen flow rate, which is due to the decrease in carrier concentration with oxygen flow rate and results in the red shift of the near-UV absorption edge. Due to higher carrier concentration in the same oxygen flow rate, the optical band gap of ITO:Zr films is wider than that of ITO films.
Fig. 6 shows the optical transmittance and reflection spectra of ITO and ITO:Zr films (including glass substrate) with annealing temperature including the variation of sheet resistance. The sheet resistance of ITO and ITO:Zr films decreases firstly with the increase in annealing temperature and then increases at high annealing temperature. Annealing can promote crystalline growth of the films, which contributes to the activation of the dopant and reduces the number of dopant trapped at the dislocations or grain boundaries and increases carrier concentration. Carrier concentration and mobility increase as the annealing temperature increases in a certain range, which leads to the decrease in sheet resistance. Although the thermal energy at high annealing temperature can tend to promote crystallization, it can also
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contribute to some disorder influences such as thermal instability and lead to an increase in defect density. What is more, due to high-temperature annealing in air atmosphere, the excess free oxygen in air may incorporate with the films, which compensates the oxygen vacancies and decreases the carrier concentration. By contrast with ITO films, ITO:Zr films show better electrical properties at higher annealing temperature, which implies better thermal stability of ITO:Zr films. ITO:Zr films show higher sheet resistance than ITO films in the range of 200–300 1C, which results from the incorporation of excess oxygen in air due to the oxygen affinity of zirconium element, leading to a decrease in carrier concentration. The transmittance in the visual region of all the films increases with the increase in annealing temperature. The transmittance in the visual region of ITO:Zr films is higher than that of ITO films. In the near-IR range, the reflection of ITO and ITO:Zr films firstly increases and subsequently decreases with annealing temperature. In the near-UV range, the shifts of absorption edges can be observed. The variation of optical band gap arising from the shift is related to the concentration of free electrons. The optical band gap of the films increases with the increase in annealing temperature, but further increases in annealing temperature lead to a decrease in optical band gap. The direct transition models show wider optical band gap of ITO:Zr films than that of ITO films at lower and higher annealing temperatures, which also implies the variation of carrier concentration. 4. Conclusions The influences of substrate temperature, oxygen flow rate and annealing temperature on optical–electrical properties of ITO and ITO:Zr films were investigated. The transmittance in the visual region and optical band gap of the films increases with substrate temperature, and the transmittance of ITO:Zr films was higher than that of ITO films. The excessive increase in oxygen flow rate can worsen the optical transparency, and the optical band gap of the films decreases with oxygen flow rate. The proper annealing treatment in air can improve the optical–electrical properties of the films. The carrier concentration has
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an important influence on near-IR reflection, near-UV absorption and optical band gap. Zr doping improves carrier concentration of the films and widens the corresponding optical band gap. The higher value of figure of merit of ITO:Zr films is observed, which reveals that ITO:Zr films have better optical–electrical properties than ITO films.
Acknowledgements This work has been supported by the Research Fund for Shanghai applied material (0725). The authors wish to thank Instrumental Analysis Center of SJTU for some measurements of the films. References [1] Gheidari AM, Soleimani EA, Mansorhoseini M, Mohajerzadeh S, Madani N, Kolahi WS. Mater Res Bull 2005;40:1303. [2] Yu HH, Hwang SJ, Tseng MC, Tseng CC. Opt Commun 2006;259:187. [3] Gregory OJ, You T, Crisman EE. Thin Solid Films 2005;476:344. [4] Suzuki M, Maeda Y, Muraoka M, Higuchi S, Sawada Y. Mater Sci Eng B 1998;54:43. [5] Hsu CM, Lee JW, Meen TH, Wu WT. Thin Solid Films 2005;474:19. [6] Minami T, Yamamoto T, Toda Y, Miyata T. Thin Solid Films 2000; 373:189. [7] Hsu CM, Wu WT. Appl Phys Lett 2004;85:840. [8] Ohno T, Kawahara T. Thin Solid Films 2000;373:189. [9] Nakasa A, Adachi M, Usami H, Suzuki E, Taniguchi Y. Thin Solid Films 2006;498:240. [10] Asikainen T, Ritala M, Leskela¨ M. Thin Solid Films 2003;440:152. [11] Canhola P, Martins N, Raniero L, Pereira S, Fortunato E, Ferreira I, et al. Thin Solid Films 2005;487:271. [12] Biswas PK, De A, Pramanik NC, Chakraborty PK, Ortner K, Hock V, et al. Mater Lett 2003;57:2326. [13] Hamberg I, Granqvist CG. J Appl Phys 1986;60:123. [14] Lee HC. Appl Surf Sci 2006;252:3428. [15] Manoj PK, Gopchandran KG, Koshy P, Vaidyan VK, Joseph B. Opt Mater 2006;80:615. [16] Antony A, Nisha M, Manoj R, Jayaraj MK. Appl Surf Sci 2004;225: 294. [17] Cruz LR, Legnani C, Matoso IG, Ferreira CL, Moutinho HR. Mater Res Bull 2004;39:993. [18] Nisha M, Anusha S, Antony A, Manoj R, Jayaraj MK. Appl Surf Sci 2005;252:1430. [19] Fallah HR, Ghasemi M, Hassanzadeh A, Steki H. Mater Res Bull 2007;42:487. [20] Lee HC, Park OO. Vacuum 2004;77:69.