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Characteristics of Al-doped ZnO thin films obtained by ultrasonic spray pyrolysis: effects of Al doping and an annealing treatment
Materials Science and Engineering B 106 (2004) 242–245
Characteristics of Al-doped ZnO thin films obtained by ultrasonic spray pyrolysis: effects of Al doping and an annealing treatment Jin-Hong Lee, Byung-Ok Park∗ Department of Inorganic Materials Engineering, Kyungpook National University, Deagu 702-701, South Korea Received 15 August 2003; accepted 9 September 2003
Abstract Transparent conducting Al-doped ZnO thin films were prepared on silica glass substrates by an ultrasonic spray pyrolysis method. The effects of Al doping and an annealing treatment on electrical and optical properties of ZnO thin films were investigated. Zinc acetate dihydrate, 2-methoxyethanol and aluminum chloride were used as a starting material, a solvent and a dopant source, respectively. The electrical conductivity of ZnO films was improved by Al doping and by annealing in a reducing atmosphere. The minimum electrical resistivity was obtained in the 3 at.% Al-doped film annealed at 500 ◦ C in nitrogen with 5% hydrogen and its value was 1.71 × 10−2 cm. The average optical transmittance of all films, regardless of a doping concentration and an annealing condition, was higher than 80% in the visible range. The optical direct band gap of films was dependent on the amount of a dopant and the annealing treatment in a reducing atmosphere. The optical direct band gap value of 3 at.% Al-doped films annealed at 500 ◦ C in nitrogen were 3.33 eV. © 2003 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Thin films; Ultrasonic spray pyrolysis methods; Electrical properties; Optical properties
1. Introduction As n-type transparent conductive thin films are wide-gap semiconductors with optical transparency in the visible range, these films have been studied in a broad range of electronic and optoelectronic devices such as gas sensors [1], transparent electrodes in display [2], heat mirrors [3] and solar cell windows [4]. Zinc oxide (ZnO) thin films have prompted a great interest as transparent conductive oxides due to their electro-optical properties, high electro-chemical stability, a large band gap, abundance in nature and absence of toxicity. Numerous ZnO thin films preparation methods have been attempted: photo-atomic layer deposition [5], rf magnetron sputtering [6], chemical vapor deposition [7], sol–gel process [8] and spray pyrolysis [9]. Among these methods, the spray pyrolysis technique has several advantages over the others such as simplicity, safety and the low cost of the apparatus and the raw materials.
∗ Corresponding author. Tel.: +82-53-950-5634; fax: +82-53-950-5645. E-mail address: [email protected] (B.-O. Park).
0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.09.040
The n-type conductivity in non-stoichiometric ZnO is well known to originate from zinc interstitials and oxygen vacancies [10]. Carriers, which act as donors, were formed by the ionization of zinc interstitials and oxygen vacancies. Between the above-mentioned two mechanisms, the formation of carriers by the ionization of zinc interstitials has been acknowledged to be predominant in intrinsic ZnO crystals. In addition, the increment in the number of carriers in ZnO crystals was achieved by thermal treatment with hydrogen [8,11] or by an appropriate doping process [12–15]. In this paper, we present the effects of Al doping and an annealing treatment in a reducing atmosphere on electrical and optical properties of ZnO thin films prepared by an ultrasonic spray pyrolysis method.
2. Experimental Transparent conducting Al-doped ZnO thin films were prepared by an ultrasonic spray pyrolysis method. An ultrasonic spray pyrolytic apparatus (vertical configuration type) was used. Zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O), methanol (CH3 OH) and aluminum chloride (AlCl3 ) were used as a starting material, a solvent and a dopant source,
J.-H. Lee, B.-O. Park / Materials Science and Engineering B 106 (2004) 242–245
243
Fig. 2. Electrical resistivity of ZnO films as a function of (a) an aluminum doping concentration and (b) a deposition and an annealing temperature. Fig. 1. XRD patterns of ZnO thin films: (a) deposited at 500 ◦ C with various aluminum doping concentrations; and (b) then annealed at 500 ◦ C with different annealing conditions.
respectively. After zinc acetate dihydrate and aluminium chloride were dissolved in a small amount of water with methanol repectively, a starting solution was prepared by mixing these solutions. To stabilize the starting solution, a few droplets of hydrochloric acid (HCl) were added. The concentration of zinc acetate was 0.1 M and the Al/Zn ratio in the solution varied from 0 to 5 at.%. Bare glasses (Corning Inc. 1737) were used as substrates. The droplets of the solution produced by the ultrasonic generator were carried to the substrate by a mixture of nitrogen and oxygen. They were used as a carrier and a reactor gas, respectively. Their flow rates were fixed at 800 and 200 cc/min, respectively. The films were deposited in air at temperatures ranging from 350 to 500 ◦ C for 45 min. After, the films were annealed at 450 and 500 ◦ C for 30 min in nitrogen and nitrogen with 5% hydrogen. The thickness of films was measured by ␣-step and ranged from 150 to 250 nm. Structure developments and surface morphology of films were measured by an X-ray diffractometer with CuK␣ radiation and a scanning electron microscope (SEM), respectively. In addition, in order to evaluate the electrical and optical properties of films, a four-probe measurement and a UV-Vis–NIR spectroscopy were carried out, respectively.
3. Results and discussion The structural developments of ZnO thin films deposited at 500 ◦ C as a function of a doping concentration are shown in Fig. 1(a). All films were polycrystalline with a structure that belongs to the ZnO hexagonal wurtzite type [16]. It was observed that the undoped ZnO film had a preferred (0 0 2) orientation but that the intensity of the (0 0 2) peak decreased
by increasing the doping concentration. For 1 at.% doped films, (1 0 1) and (1 0 2) peaks as well as (0 0 2) and (1 0 3) peaks were observed, which indicated that the degree of the preferred (0 0 2) orientation decreased. For films doped with an amount of more than 1 at.%, the peak intensity was more reduced, so it was hard to detect (1 0 2) and (1 0 3) peaks in the film doped with 5 at.%. The crystallinity of films was aggravated by an increase in the doping concentration, which may be due to the formation of stress by the difference in ion size between zinc and aluminum (rZn2+ = 0.074 nm and rAl3+ = 0.054 nm). The electrical resistivity of ZnO films as a function of (a) a doping concentration and (b) a deposition temperature are plotted in Fig. 2. In the case of films (Fig. 2(a)) doped with various doping concentrations, a doping concentration in the film with a lower resistivity value changed with a different deposition temperature. For films deposited at 350 and 400 ◦ C, a lower resistivity value was obtained at 2 and 5 at.%, respectively. For films deposited at higher temperatures, however, lower values were obtained at 3 at.%. In the case of films (Fig. 2(b)) deposited at different temperatures, it should be noted that doped ZnO films had lower resistivity values than undoped films for all deposition temperatures. Additionally, the resistivity of films generally decreased with an increase in a deposition temperature, which was due to the improvement of the ZnO crystallinity. Films deposited at 500 ◦ C had a lower resistivity and the resistivity value of the 3 at.% doped film was 5.88 × 10−1 cm. An annealing treatment in nitrogen and nitrogen with 5% hydrogen was performed for the 3% doped films deposited at 500 ◦ C, which had the lowest resistivity value. The XRD patterns of 3 at.% doped ZnO films annealed at 500 ◦ C in reducing atmospheres are presented in Fig. 1(b). There was little difference in the XRD pattern among films annealed in nitrogen and nitrogen with 5% hydrogen and the film without an annealing as shown in Fig. 1(a). The electrical resistivity values of films annealed in reducing atmospheres were inserted in Fig. 2(b). Resistivity values of films decreased by applying an annealing treatment.
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Additionally, the higher the annealing temperature was, the lower the resistivity value was. The electrical resistivity values of films annealed at 500 ◦ C in nitrogen and nitrogen with 5% hydrogen were 1.64 × 10−1 and 1.71 × 10−2 cm, respectively. A similar behavior with the change in electrical resistivity according to Al doping concentration was observed in the Zr-doped films [17]. When a small amount of Al was introduced into the ZnO film, the Al was ionized into Al3+ and substituted for Zn2+ . Thus, one free electron was produced from one zinc atom replacement by an aluminum atom. The carrier concentration increased with the Al concentration up to 3 at.%. At more than 3 at.%, however, the carrier concentration decreased because the increasing Al atoms formed some kind of neutral defects and these neutralized Al atoms did not contribute free electrons. The amount of electrically active Al in the film was reduced when the Al doping is high. The diminution in the electrical resistivity of films was also observed by the annealing treatment. This is attributed to an increase in the number of carriers by the ionization of oxygen vacancies followed by oxygen annihilation from the ZnO crystals and by desorption of oxygen in the grain boundaries which act as traps for the carriers [18]. The optical transmittance spectra of ZnO films as a function of (a) a doping concentration and (b) an annealing condition in the wavelength range (300–800 nm) are plotted in Fig. 3. It was shown that all films were highly transparent over the visible region. For films (Fig. 3(a)) doped with a various concentration, the average optical transmittance values of all films were higher than 80% in the visible range. For films (Fig. 3(b)) annealed in reducing atmospheres, the average transmittance values were somewhat higher than those of films untreated an annealing. There was little difference in the average optical transmittance values between the films annealed in a different atmosphere. The undoped ZnO thin film had a sharp absorption onset at about 380 nm.
The drop-off of the values of shorter wavelengths was related to the fundamental absorption by band-to-band transition. Meanwhile, the wavelength of the absorption edge decreased with an increase in doping concentration. The optical transmittance of films is known to be dependent upon the surface morphology. The SEM photographs of 3 at.% doped ZnO films are displayed in Fig. 4. The films deposited at 500 ◦ C had round shaped grains of about 70 nm. For the film annealed in nitrogen, the shape of grains was
Fig. 3. Optical transmittance spectra of ZnO films as a function of (a) an aluminum doping concentration and (b) an annealing condition.
Fig. 4. SEM images of the 3 at.% Al-doped ZnO thin films deposited for 500 ◦ C and then annealed in nitrogen and nitrogen with 5% hydrogen.
J.-H. Lee, B.-O. Park / Materials Science and Engineering B 106 (2004) 242–245
Fig. 5. Optical direct band gaps of ZnO films deposited at 500 ◦ C and then annealed at 500 ◦ C in different annealing conditions.
245
fluences of Al doping and an annealing treatment on electrical and optical properties of ZnO thin films were studied. Al-doped films had a lower resistivity than undoped films. For doped ZnO films, the 3 at.% doped film deposited at 500 ◦ C had a lower resistivity than any other film and its value was 5.8 × 10−1 cm. The optical transmittance of all films was higher than 80%. The optical direct band gap of films increased from 3.28 to 3.30 eV by doping more than 3 at.%. Additionally, the annealing treatment in reducing atmospheres led to an increase in the electrical conductivity and the optical direct band gap but had little effect on the optical transmittance. The resistivity value of the 3 at.% doped film annealed at 500 ◦ C in nitrogen with 5% hydrogen became 1.71 × 10−2 cm and the direct band gap of the film in nitrogen was approximately 3.33 eV.
References oval and the size became somewhat small. On the other hand, the film annealed in nitrogen with 5% hydrogen had warm-like grains. By applying an annealing treatment, the change in the shape and the size of grains may be due to the removal of oxygen from zinc oxide. Optical direct band gaps of ZnO films as a function of a doping concentration and an annealing condition are shown in Fig. 5. In the case of films without the annealing treatment, the direct band gap value of the undoped ZnO film was approximately 3.28 eV. The value of films was scarcely affected by the doping concentration up to 2 at.% but increased with doping over 3%. The value of films doped with more than 3% was about 3.30 eV. The 4 at.% doped film, which was 3.31 eV, had a higher value than other films. Additionally, the value was also dependent on the annealing treatment. The value of films increased by the annealing treatment in nitrogen and nitrogen with 5% hydrogen and their values were 3.33 and 3.32 eV, respectively. The increment in the direct band gap value of ZnO films by doping of aluminum may be attributed to the Burstein–Moss effect [19] that the optical direct band gap increased followed by heavy doping of dopants. Moreover, the annealing treatment makes the number of carriers increase. This, like the Burstein–Moss effect, affects the direct band gap. 4. Conclusions Highly conducting and transparent Al-doped ZnO thin films were deposited by ultrasonic spray pyrolysis. The in-
[1] I. Stambolova, K. Konstantinov, S. Vassilev, P. Peshev, Ts. Tsacheva, Mater. Chem. Phys. 63 (2000) 104. [2] M. Chen, Z.L. Pei, C. Sun, J. Gong, R.F. Huang, L.S. Wen, Mater. Sci. Eng. B 85 (2001) 212. [3] K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1. [4] Z.-C. Jin, I. Hamberg, C.G. Granqvist, B.E. Sernelius, K.-F. Berggren, Thin Solid Films 164 (1988) 381. [5] Y. Yamamoto, K. Saito, K. Takahashi, M. Konagai, Sol. Energ. Mater. Sol. Cells 65 (2001) 125. [6] D. Song, P. Widenborg, W. Chin, A.G. Aberle, Sol. Energ. Mater Sol. Cells 73 (2002) 1. [7] B.M. Ataev, A.M. Bagamadova, V.V. Mamedov, A.K. Omaev, M.R. Rabadanov, J. Cryst. Growth 198–199 (1999) 1222. [8] J.H. Lee, K.H. Ko, B.O. Park, J. Cryst. Growth 247 (2003) 119. [9] T.Y. Ma, S.C. Lee, J. Mater. Sci.: Mater. Electron. 11 (2000) 305. [10] M.H. Sukkar, H.L. Tuller, Adv. Ceram. 7 (1984) 71. [11] P. Nunes, E. Fortunato, R. Martins, Inter. J. Inorg. Mater. 3 (2001) 1125. [12] J.H. Lee, B.O. Park, Thin Solid Films 426 (2003) 94. [13] A. Sanchez-Juarez, A. Tiburcio-Silver, A. Ortiz, Sol. Energ. Mater. Sol. Cells 52 (1998) 301. [14] A. Tiburcio-Silver, A. Sanchez-Juarez, A. Avila-Garcia, Sol. Energ. Mater. Sol. Cells 55 (1998) 3. [15] S.A. Nasser, H.H. Afify, S.A. El-Hakim, M.K. Zayed, Thin Solid Films 315 (1998) 327. [16] Joint Committee on Powder Diffraction Standards, Powder Diffraction File, International Center for Diffraction Data, Swarthmore, PA, card 36–1451, 1988. [17] G.K. Paul, S. Bandyopadhyay, S.K. Sen, S. Sen, Mater. Chem. Phys. 79 (2003) 71. [18] S. Major, A. Banerjee, K.L. Chopra, Thin Solid Films 122 (1984) 31. [19] B.E. Sernelius, K.-F. Berggren, Z.-C. Jin, I. Hamberg, C.G. Granqvist, Phys. Rev. B 37 (17) (1988) 10244.
Characteristics of Al-doped ZnO thin films obtained by ultrasonic spray pyrolysis: effects of Al doping and an annealing treatment Jin-Hong Lee, Byung-Ok Park∗ Department of Inorganic Materials Engineering, Kyungpook National University, Deagu 702-701, South Korea Received 15 August 2003; accepted 9 September 2003
Abstract Transparent conducting Al-doped ZnO thin films were prepared on silica glass substrates by an ultrasonic spray pyrolysis method. The effects of Al doping and an annealing treatment on electrical and optical properties of ZnO thin films were investigated. Zinc acetate dihydrate, 2-methoxyethanol and aluminum chloride were used as a starting material, a solvent and a dopant source, respectively. The electrical conductivity of ZnO films was improved by Al doping and by annealing in a reducing atmosphere. The minimum electrical resistivity was obtained in the 3 at.% Al-doped film annealed at 500 ◦ C in nitrogen with 5% hydrogen and its value was 1.71 × 10−2 cm. The average optical transmittance of all films, regardless of a doping concentration and an annealing condition, was higher than 80% in the visible range. The optical direct band gap of films was dependent on the amount of a dopant and the annealing treatment in a reducing atmosphere. The optical direct band gap value of 3 at.% Al-doped films annealed at 500 ◦ C in nitrogen were 3.33 eV. © 2003 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Thin films; Ultrasonic spray pyrolysis methods; Electrical properties; Optical properties
1. Introduction As n-type transparent conductive thin films are wide-gap semiconductors with optical transparency in the visible range, these films have been studied in a broad range of electronic and optoelectronic devices such as gas sensors [1], transparent electrodes in display [2], heat mirrors [3] and solar cell windows [4]. Zinc oxide (ZnO) thin films have prompted a great interest as transparent conductive oxides due to their electro-optical properties, high electro-chemical stability, a large band gap, abundance in nature and absence of toxicity. Numerous ZnO thin films preparation methods have been attempted: photo-atomic layer deposition [5], rf magnetron sputtering [6], chemical vapor deposition [7], sol–gel process [8] and spray pyrolysis [9]. Among these methods, the spray pyrolysis technique has several advantages over the others such as simplicity, safety and the low cost of the apparatus and the raw materials.
∗ Corresponding author. Tel.: +82-53-950-5634; fax: +82-53-950-5645. E-mail address: [email protected] (B.-O. Park).
0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.09.040
The n-type conductivity in non-stoichiometric ZnO is well known to originate from zinc interstitials and oxygen vacancies [10]. Carriers, which act as donors, were formed by the ionization of zinc interstitials and oxygen vacancies. Between the above-mentioned two mechanisms, the formation of carriers by the ionization of zinc interstitials has been acknowledged to be predominant in intrinsic ZnO crystals. In addition, the increment in the number of carriers in ZnO crystals was achieved by thermal treatment with hydrogen [8,11] or by an appropriate doping process [12–15]. In this paper, we present the effects of Al doping and an annealing treatment in a reducing atmosphere on electrical and optical properties of ZnO thin films prepared by an ultrasonic spray pyrolysis method.
2. Experimental Transparent conducting Al-doped ZnO thin films were prepared by an ultrasonic spray pyrolysis method. An ultrasonic spray pyrolytic apparatus (vertical configuration type) was used. Zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O), methanol (CH3 OH) and aluminum chloride (AlCl3 ) were used as a starting material, a solvent and a dopant source,
J.-H. Lee, B.-O. Park / Materials Science and Engineering B 106 (2004) 242–245
243
Fig. 2. Electrical resistivity of ZnO films as a function of (a) an aluminum doping concentration and (b) a deposition and an annealing temperature. Fig. 1. XRD patterns of ZnO thin films: (a) deposited at 500 ◦ C with various aluminum doping concentrations; and (b) then annealed at 500 ◦ C with different annealing conditions.
respectively. After zinc acetate dihydrate and aluminium chloride were dissolved in a small amount of water with methanol repectively, a starting solution was prepared by mixing these solutions. To stabilize the starting solution, a few droplets of hydrochloric acid (HCl) were added. The concentration of zinc acetate was 0.1 M and the Al/Zn ratio in the solution varied from 0 to 5 at.%. Bare glasses (Corning Inc. 1737) were used as substrates. The droplets of the solution produced by the ultrasonic generator were carried to the substrate by a mixture of nitrogen and oxygen. They were used as a carrier and a reactor gas, respectively. Their flow rates were fixed at 800 and 200 cc/min, respectively. The films were deposited in air at temperatures ranging from 350 to 500 ◦ C for 45 min. After, the films were annealed at 450 and 500 ◦ C for 30 min in nitrogen and nitrogen with 5% hydrogen. The thickness of films was measured by ␣-step and ranged from 150 to 250 nm. Structure developments and surface morphology of films were measured by an X-ray diffractometer with CuK␣ radiation and a scanning electron microscope (SEM), respectively. In addition, in order to evaluate the electrical and optical properties of films, a four-probe measurement and a UV-Vis–NIR spectroscopy were carried out, respectively.
3. Results and discussion The structural developments of ZnO thin films deposited at 500 ◦ C as a function of a doping concentration are shown in Fig. 1(a). All films were polycrystalline with a structure that belongs to the ZnO hexagonal wurtzite type [16]. It was observed that the undoped ZnO film had a preferred (0 0 2) orientation but that the intensity of the (0 0 2) peak decreased
by increasing the doping concentration. For 1 at.% doped films, (1 0 1) and (1 0 2) peaks as well as (0 0 2) and (1 0 3) peaks were observed, which indicated that the degree of the preferred (0 0 2) orientation decreased. For films doped with an amount of more than 1 at.%, the peak intensity was more reduced, so it was hard to detect (1 0 2) and (1 0 3) peaks in the film doped with 5 at.%. The crystallinity of films was aggravated by an increase in the doping concentration, which may be due to the formation of stress by the difference in ion size between zinc and aluminum (rZn2+ = 0.074 nm and rAl3+ = 0.054 nm). The electrical resistivity of ZnO films as a function of (a) a doping concentration and (b) a deposition temperature are plotted in Fig. 2. In the case of films (Fig. 2(a)) doped with various doping concentrations, a doping concentration in the film with a lower resistivity value changed with a different deposition temperature. For films deposited at 350 and 400 ◦ C, a lower resistivity value was obtained at 2 and 5 at.%, respectively. For films deposited at higher temperatures, however, lower values were obtained at 3 at.%. In the case of films (Fig. 2(b)) deposited at different temperatures, it should be noted that doped ZnO films had lower resistivity values than undoped films for all deposition temperatures. Additionally, the resistivity of films generally decreased with an increase in a deposition temperature, which was due to the improvement of the ZnO crystallinity. Films deposited at 500 ◦ C had a lower resistivity and the resistivity value of the 3 at.% doped film was 5.88 × 10−1 cm. An annealing treatment in nitrogen and nitrogen with 5% hydrogen was performed for the 3% doped films deposited at 500 ◦ C, which had the lowest resistivity value. The XRD patterns of 3 at.% doped ZnO films annealed at 500 ◦ C in reducing atmospheres are presented in Fig. 1(b). There was little difference in the XRD pattern among films annealed in nitrogen and nitrogen with 5% hydrogen and the film without an annealing as shown in Fig. 1(a). The electrical resistivity values of films annealed in reducing atmospheres were inserted in Fig. 2(b). Resistivity values of films decreased by applying an annealing treatment.
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Additionally, the higher the annealing temperature was, the lower the resistivity value was. The electrical resistivity values of films annealed at 500 ◦ C in nitrogen and nitrogen with 5% hydrogen were 1.64 × 10−1 and 1.71 × 10−2 cm, respectively. A similar behavior with the change in electrical resistivity according to Al doping concentration was observed in the Zr-doped films [17]. When a small amount of Al was introduced into the ZnO film, the Al was ionized into Al3+ and substituted for Zn2+ . Thus, one free electron was produced from one zinc atom replacement by an aluminum atom. The carrier concentration increased with the Al concentration up to 3 at.%. At more than 3 at.%, however, the carrier concentration decreased because the increasing Al atoms formed some kind of neutral defects and these neutralized Al atoms did not contribute free electrons. The amount of electrically active Al in the film was reduced when the Al doping is high. The diminution in the electrical resistivity of films was also observed by the annealing treatment. This is attributed to an increase in the number of carriers by the ionization of oxygen vacancies followed by oxygen annihilation from the ZnO crystals and by desorption of oxygen in the grain boundaries which act as traps for the carriers [18]. The optical transmittance spectra of ZnO films as a function of (a) a doping concentration and (b) an annealing condition in the wavelength range (300–800 nm) are plotted in Fig. 3. It was shown that all films were highly transparent over the visible region. For films (Fig. 3(a)) doped with a various concentration, the average optical transmittance values of all films were higher than 80% in the visible range. For films (Fig. 3(b)) annealed in reducing atmospheres, the average transmittance values were somewhat higher than those of films untreated an annealing. There was little difference in the average optical transmittance values between the films annealed in a different atmosphere. The undoped ZnO thin film had a sharp absorption onset at about 380 nm.
The drop-off of the values of shorter wavelengths was related to the fundamental absorption by band-to-band transition. Meanwhile, the wavelength of the absorption edge decreased with an increase in doping concentration. The optical transmittance of films is known to be dependent upon the surface morphology. The SEM photographs of 3 at.% doped ZnO films are displayed in Fig. 4. The films deposited at 500 ◦ C had round shaped grains of about 70 nm. For the film annealed in nitrogen, the shape of grains was
Fig. 3. Optical transmittance spectra of ZnO films as a function of (a) an aluminum doping concentration and (b) an annealing condition.
Fig. 4. SEM images of the 3 at.% Al-doped ZnO thin films deposited for 500 ◦ C and then annealed in nitrogen and nitrogen with 5% hydrogen.
J.-H. Lee, B.-O. Park / Materials Science and Engineering B 106 (2004) 242–245
Fig. 5. Optical direct band gaps of ZnO films deposited at 500 ◦ C and then annealed at 500 ◦ C in different annealing conditions.
245
fluences of Al doping and an annealing treatment on electrical and optical properties of ZnO thin films were studied. Al-doped films had a lower resistivity than undoped films. For doped ZnO films, the 3 at.% doped film deposited at 500 ◦ C had a lower resistivity than any other film and its value was 5.8 × 10−1 cm. The optical transmittance of all films was higher than 80%. The optical direct band gap of films increased from 3.28 to 3.30 eV by doping more than 3 at.%. Additionally, the annealing treatment in reducing atmospheres led to an increase in the electrical conductivity and the optical direct band gap but had little effect on the optical transmittance. The resistivity value of the 3 at.% doped film annealed at 500 ◦ C in nitrogen with 5% hydrogen became 1.71 × 10−2 cm and the direct band gap of the film in nitrogen was approximately 3.33 eV.
References oval and the size became somewhat small. On the other hand, the film annealed in nitrogen with 5% hydrogen had warm-like grains. By applying an annealing treatment, the change in the shape and the size of grains may be due to the removal of oxygen from zinc oxide. Optical direct band gaps of ZnO films as a function of a doping concentration and an annealing condition are shown in Fig. 5. In the case of films without the annealing treatment, the direct band gap value of the undoped ZnO film was approximately 3.28 eV. The value of films was scarcely affected by the doping concentration up to 2 at.% but increased with doping over 3%. The value of films doped with more than 3% was about 3.30 eV. The 4 at.% doped film, which was 3.31 eV, had a higher value than other films. Additionally, the value was also dependent on the annealing treatment. The value of films increased by the annealing treatment in nitrogen and nitrogen with 5% hydrogen and their values were 3.33 and 3.32 eV, respectively. The increment in the direct band gap value of ZnO films by doping of aluminum may be attributed to the Burstein–Moss effect [19] that the optical direct band gap increased followed by heavy doping of dopants. Moreover, the annealing treatment makes the number of carriers increase. This, like the Burstein–Moss effect, affects the direct band gap. 4. Conclusions Highly conducting and transparent Al-doped ZnO thin films were deposited by ultrasonic spray pyrolysis. The in-
[1] I. Stambolova, K. Konstantinov, S. Vassilev, P. Peshev, Ts. Tsacheva, Mater. Chem. Phys. 63 (2000) 104. [2] M. Chen, Z.L. Pei, C. Sun, J. Gong, R.F. Huang, L.S. Wen, Mater. Sci. Eng. B 85 (2001) 212. [3] K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1. [4] Z.-C. Jin, I. Hamberg, C.G. Granqvist, B.E. Sernelius, K.-F. Berggren, Thin Solid Films 164 (1988) 381. [5] Y. Yamamoto, K. Saito, K. Takahashi, M. Konagai, Sol. Energ. Mater. Sol. Cells 65 (2001) 125. [6] D. Song, P. Widenborg, W. Chin, A.G. Aberle, Sol. Energ. Mater Sol. Cells 73 (2002) 1. [7] B.M. Ataev, A.M. Bagamadova, V.V. Mamedov, A.K. Omaev, M.R. Rabadanov, J. Cryst. Growth 198–199 (1999) 1222. [8] J.H. Lee, K.H. Ko, B.O. Park, J. Cryst. Growth 247 (2003) 119. [9] T.Y. Ma, S.C. Lee, J. Mater. Sci.: Mater. Electron. 11 (2000) 305. [10] M.H. Sukkar, H.L. Tuller, Adv. Ceram. 7 (1984) 71. [11] P. Nunes, E. Fortunato, R. Martins, Inter. J. Inorg. Mater. 3 (2001) 1125. [12] J.H. Lee, B.O. Park, Thin Solid Films 426 (2003) 94. [13] A. Sanchez-Juarez, A. Tiburcio-Silver, A. Ortiz, Sol. Energ. Mater. Sol. Cells 52 (1998) 301. [14] A. Tiburcio-Silver, A. Sanchez-Juarez, A. Avila-Garcia, Sol. Energ. Mater. Sol. Cells 55 (1998) 3. [15] S.A. Nasser, H.H. Afify, S.A. El-Hakim, M.K. Zayed, Thin Solid Films 315 (1998) 327. [16] Joint Committee on Powder Diffraction Standards, Powder Diffraction File, International Center for Diffraction Data, Swarthmore, PA, card 36–1451, 1988. [17] G.K. Paul, S. Bandyopadhyay, S.K. Sen, S. Sen, Mater. Chem. Phys. 79 (2003) 71. [18] S. Major, A. Banerjee, K.L. Chopra, Thin Solid Films 122 (1984) 31. [19] B.E. Sernelius, K.-F. Berggren, Z.-C. Jin, I. Hamberg, C.G. Granqvist, Phys. Rev. B 37 (17) (1988) 10244.