- Email: [email protected]
Substrate temperature effects on the electrical properties of sputtered Al doped ZnO thin films
Accepted Manuscript Substrate temperature effects on the electrical properties of sputtered Al doped ZnO thin films Deok-Kyu Kim, Hong-Bae Kim PII: DOI: Reference:
S0749-6036(15)00268-2 http://dx.doi.org/10.1016/j.spmi.2015.05.009 YSPMI 3765
To appear in:
Superlattices and Microstructures
Received Date: Accepted Date:
5 May 2015 8 May 2015
Please cite this article as: D-K. Kim, H-B. Kim, Substrate temperature effects on the electrical properties of sputtered Al doped ZnO thin films, Superlattices and Microstructures (2015), doi: http://dx.doi.org/10.1016/j.spmi. 2015.05.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Substrate temperature effects on the electrical properties of sputtered Al doped ZnO thin films Deok-Kyu Kima,* and Hong-Bae Kim b a
Advanced Development Team , Samsung Electronics Co. Ltd., Yongin 446-711, Gyeonggi, Korea
b
Department of Semiconductor Engineering, Cheongju University, Cheongju 360-746, Chungbuk, Korea
* Corresponding Author Name –Deok-Kyu Kim Telephone – 82-10-8222-7669 Fax – 82-43-229-8461 E-mail –[email protected]
Abstract Al doped ZnO (AZO) thin films were deposited on glass substrate by RF magnetron sputtering system. The dependence of structural, electrical, and optical properties on the substrate temperature variations in the range of 0 to 400 °C was investigated. The structural results reveal that the AZO films are (002) oriented and at 400 °C a considerable crystallinity enhancement of the films is observed. With increasing the substrate temperature, the resistivity is increased by decreasing of the mobility and carrier concentration. X-ray photoelectron spectroscopy (XPS) results show that the mobility and the carrier concentration are decreased by increasing the surface bonding and decreasing the Al content, respectively. In our case, the increase in substrate temperature suppressed the incorporation of Al atoms together with the decrease of oxygen vacancy. The improvement of Al doping efficiency is a very important factor to obtain better electrical properties at high substrate temperatures.
Keywords: Al doped ZnO; RF magnetron sputtering; Substrate temperature; Al content
1
1. Introduction Optoelectronics are widely applied in technological and industrial applications, such as flat panel displays [1], solar cells [2], and solid state lightings [3]. The transparent conducting oxide (TCO) materials have been extensively used in versatile fields of optoelectronics. There are specific requirements for TCO thin films depending on the type of application: high electrical conductivity and transparence. Indium tin oxide (ITO) is the most well-known TCO material but also meet some problems such as the thermal stability at high temperatures and the transparency at short wavelengths [4]. Recently, Al doped ZnO (AZO) thin film have been comprehensively investigated for replacement of ITO because of its low cost, nontoxicity, better stability in hydrogen plasma, and excellent optoelectrical properties [5]. Its easy synthetic processes are ideal for successful commercial applications. Several techniques employed to prepare AZO thin film include chemical vapor deposition [6], pulsed-laser deposition [7], sol-gel technique [8], and radio frequency (RF) sputtering [9]. Among these, RF magnetron sputtering is a powerful and flexible technique used to coat virtually any substrate in a wide range of materials [10]. The factors influencing their characteristics are power, pressure, environment, substrate position and temperature, film thickness, and post deposition treatment. In these parameters, substrate temperature usually plays a crucial role in growth of AZO films. Most of researchers have been reported that the substrate temperature improves the structural and electrical properties of AZO films [11-13]. However, studies on the degradation in electrical properties with substrate temperature through chemical bonding state in AZO thin films have not been investigated extensively. In this study, AZO thin films were deposited on glass substrate by RF magnetron sputtering under different substrate temperatures. On the basis of these results, we investigated the effects of substrate temperature on the structural, electrical, and optical properties of the films in depth. In addition, the AZO thin films were analyzed by X-ray photoelectron spectroscopy in order to reveal exactly the changes of chemical bonding state in AZO films.
2. Experimental Procedure A RF magnetron sputtering technique was used to deposit AZO thin films on glass substrates. For AZO deposition, a 99.999% purity AZO target (3inch, 98% ZnO/2% Al2O3 by wt.) was used. The glass substrate was cleaned by organic cleaning process, dried using N2 gun, and mounted in the working chamber. The system was evacuated up to ∼2 × 10−6 Torr prior to introducing argon gas at a pressure of 7 mTorr for deposition of films by RF sputtering at 25W. Presputtering is carried out during 10 min to remove oxide. The 200-nm-thick AZO thin films were deposited at three different substrate temperatures of 0, 200 and 400 °C. The thicknesses of the AZO thin films were also confirmed by a scanning electron microscopy (SEM, JSM-6400). The crystallinity of AZO films were analyzed by using an X-ray 2
diffraction (XRD, D/ MAXⅢA) measurement equipped with a Cu-source (1.5406 Å). The surface morphology and root-mean-square (rms) roughness of these films were measured at 4 × 4 um scan areas by atomic force microscopy (AFM, PUCOTECH). The optical transmission spectra were measured with a UV-visible-near-IR spectrophotometer (JASCOUV/VI S/ NI R). The resistivity could be observed by analyzing the carrier concentration and mobility of the AZO film via the Hall effect measurements using a van der Pauw geometry (ECOPI A HMS3 0 0 0). X-ray photoelectron spectroscopy (XPS, THERMOVGSCI ENTI FI CMul t i La b2 0 0 0 ) measurements were performed using a Kratos Axis Ultra DLD with a monochromatic Al Kα (1486.6 eV) source to analyze the types of chemical bonds in the AZO films.
3. Results and Discussions The electrical properties of the AZO films were investigated by Hall measurements, which were performed under room temperature. Fig. 1 shows how the resistivity (ρ), electron concentration (n), and Hall mobility (μ) ate related with the substrate temperature for the AZO films. Hall effect measurements revealed that all the films exhibited n-type conductivity. On increasing the substrate temperature from 0 to 400 °C, the resistivity increased from 1.5 × 10-3 Ω-cm to 7.1 × 10-3 Ω-cm, while the electron concentration decreased from 4.4 × 1020 cm-3 to 1.8 × 1020 cm-3. The mobility also underwent a change from 9 cm3/Vs to 4.5 cm3/Vs and the constant. The increased resistivity at 200 and 400 °C had been results of the decrease in both electron concentration and mobility and in only electron concentration, respectively. In usual, the electron concentration and mobility with increase in substrate temperature are primarily governed by crystallinity, grain boundary and oxygen adsorption effects [14,15]. Therefore, these changes in the electron concentration and the mobility during the deposition process are discussed later in detail.
Fig. 1
The X-ray diffraction (XRD) technique was used to analyze the crystallinity of the AZO films. Fig. 2 shows the XRD results obtained from the AZO thin films for the various substrate temperatures. Regardless of substrate temperature, it was clearly observed the strong peak at 34.2 corresponding to the (002) plane in ZnO wurtzite structure and the weak peak at 62.5 attributed to the (103) plane. This implies that all the AZO thin films are polycrystalline with a hexagonal structure and have a preferred c-axis orientation perpendicular to the substrate due to the smallest surface energy. The degree of the orientation as function of substrate temperature can be obtained from the texture coefficient (TC), which is defined as [16] 3
(ℎ ) =
(ℎ ) × 100% ∑ (ℎ )
Where TC(hkl) is the texture coefficient of the (hkl) plane and I(hkl) is the measured intensity. The value of the texture coefficient indicates the maximum preferred orientation of the films along the diffraction plane, meaning that the increase in preferred orientation is associated with increase in the number of grains along that plane. As the substrate temperatures increased from 0 to 400 °C, the TC (002) of AZO film increased from 0.7 to 0.72. The increase in amount of preferred orientation associated with the increased number of grains along (002) [17]. Also, the full width a half maximum (FWHM) of (002) peak decreased with increasing the substrate temperature, this means that the crystallinity of these films enhances with substrate temperature. The improvement in higher substrate temperature is due to the increase in the mobility of the sputtered AZO atoms [18]. The average crystallite size of the AZO film is determined using the Scherrer formula [19]
D =
0.9
where D is the average crystallite size, λ is the wavelength of X-ray (1.5406 Å), θ is the Bragg’s angle and B is the FWHM of the film. The average crystallite size increased from 18.5 nm to 19.3 nm when the substrate temperature increased from 0 to 400 °C. This result agrees with the results in TC (002). From the XRD results, it can be concluded that the increased substrate temperature can improve the crystalline quality of AZO thin films. However, the resistivity of AZO thin film degrades despite of improvement of crystallinity of film, indicating that the crystalline quality does not directly affect the electrical properties in our case.
Fig. 2.
Fig. 3 presents the AFM images of a 4 × 4 μm2 scanning area as a function of the substrate temperature. It is seen that the surface morphology varies significantly with increasing substrate temperature. As the substrate temperature increased, the surface of AZO thin films exhibited rougher surfaces and the grain size became larger. The root-meansquare (rms) roughness increased from 1.89 nm to 2.67 nm with increasing the substrate temperature from 0 C to 400 °C. This result is consistent with the application requirements for LEDs for which the rms roughness must be under 3 nm. [20] In general, the increased grain size leads to decreasing the grain boundary, which hinders the movement of 4
carrier, and improving the conductivity. In our case, the contrary results appeared, so that the grain size is not critical parameter in electrical properties. Also, the rougher surface provides more opportunity for surface adsorption on the surface, resulting in degradation of electrical properties in AZO thin film. Therefore, the change of surface adsorption on the surface is observed and discussed below.
Fig. 3.
After the measurement of electrical property and structure property of AZO films by Hall system, XRD and AFM, XPS was applied to identify the change of chemical structure caused by substrate temperature and in order to find the elements changes in the AZO films. The O 1 s spectra showing the chemical bonding state of oxygen shows asymmetric peaks, as shown in Fig. 4. The typical O 1s peak could be consistently fitted by three nearly Gaussian. The lower binding energy (OL = 529.7 ~ 529.9 eV) and middle binding energy (OM = 531.2 ~ 531.4 eV) are associated with O2− ions in the ZnO matrix and oxygen deficiencies, respectively [21,22]. An additional high binding energy (OH = 532.5 ~ 532.7 eV) results from O–H or C–O groups near the surface of the films [23]. The increase in substrate temperature from 0 to 400 °C increased the intensity of the Zn-O bond related OL peak and decreased the intensity of the oxygenvacancy related OM peak. This indicates that the crystallinity of films is improved by increasing substrate temperatures, being agree with the XRD results. Also, the reduction of oxygen-vacancy decreases the carrier concentration and this is in accord with the electrical properties of AZO films. It was also observed that surface impurity-related O H peak increased as substrate temperature increased. This result does agree with the result of surface roughness in AZO film. The increase of the surface bonding raises the barrier potential which impedes the motion of carrier, resulting in decrease of mobility [24]. That is, the decreased mobility at 200 °C is attributed to increase of surface bonding. Therefore, the change of oxygen-vacancy and surface bonding is coincided with the behavior of carrier concentration and mobility. Generally, many researcher have been reported that the deposition of higher substrate temperature improve the crystallinity and the resistivity of film [25,26]. That is, despite of decrease of oxygen vacancy, the carrier concentration increases due to increase in Al content, indicating that the Al content is main parameter in change of the carrier concentration. However, in our case, the crystallinity of AZO film is improved but the resistivity is degraded with increasing the substrate temperature. In order to confirm the root cause of this difference, we analyzed the Al content in AZO thin film, shown in Fig. 5. As the substrate temperature increased from 0 to 400 °C, the Al content decreased from 1.7 to 0.55 at %, resulting in decrease of the carrier concentration. The reduction of Al content with the substrate temperature is attributed to increased kinetic energy. That is, when the temperature increases, the Al atoms have surplus kinetic energy and cause deeper doping, decreasing the Al content on the surface to under 1% and 5
significantly affecting the AZO surface conductivity [27,28]. Therefore, the reason why the resistivity deteriorates is because the Al content decreases and the surface bonding increase with increasing the substrate temperature. As a result, we confirmed that the increased substrate temperature resulted in improved crystalline quality but declined the Al doping efficiency. The Al content plays a major role in the conduction behavior of AZO thin films deposited at various substrate temperatures.
Fig. 4.
Fig. 5.
The wavelength dependences of the optical transmittance spectra of the AZO films subjected to the three different substrate temperatures are plotted in Fig. 6. All of the AZO films show an average transmittance of 82.3 ~ 88.8 % in the visible wavelength range from 400 to 800 nm and a sharp fundamental absorption edge in the UV region, which is suitable for display devices and solar cells as transparent electrodes [29]. It seemed that the transmittance decreased and a red shift of the absorption edge occurred with increasing the substrate temperature. The transmittance may be reduced by increasing the surface bonding, which absorbs visible photons [30]. The absorption edge is related to the optical band gap, which can be calculated from the relationship between the optical absorption coefficient (α) and the transmittance (T). The optical band gap of a direct band gap semiconductor can be determined by using Tauc model [31]:
(ℎ ) = (ℎ −
)
/
where hν is the photon energy of incident light, C is a constant and Eg is the optical band gap. The absorption coefficient (a) can be calculated from the transmittance of the films with the formula α = ln(1/T)/d, where d is the thickness of the films and T is the transmittance. Therefore, the optical band gap is obtained by extrapolating the tangential line to the photon energy axis in the plot of (αhv) as a function of hv, as shown in Fig. 7. As the substrate temperature was increased to 0, 200, 400 ◦C, the optical band gap shifted to 3.73, 3.58, and 3.428 eV, respectively. These results are consistent with those observed for the carrier concentration. It is found that there is a band gap narrowing of AZO film with increasing the substrate temperature, which is probably due to the Burstein-Moss effects [32,33]. The Fermi energy penetrates into the conduction band of the degenerate semiconductor, due to an decrease in the electron concentration from Al dopants, that leads to the energy band narrowing. 6
Fig. 6.
Fig. 7.
4. Conclusion In this study, RF-magnetron sputtering is used to fabricate AZO thin films on glass substrates under various substrate temperatures. All the thin films grew along c-axis with wurtzite structure and had a high average optical transmission (≥82%) in visible region. At higher substrate temperature, AZO thin film exhibits high resistivity connected with its low carrier concentration and mobility. We confirmed that the control of Al content in high substrate temperature is the key parameter in the fabrication of high quality AZO transparent electrodes.
References [1] T. Minami, S. Takata, T. Kakumu, J. Vac. Sci. Technol. A 14 (1996) 1689. [2] Y. Wang, X. Zhang, L. Bai, Q. Huang, C. Wei, Y. Zhao, Appl. Phys. Lett. 100 (2012) 263508. [3] H. Y. Ryu, J. I. Shim, Opt. Express, 19 (2011) 2886. [4] C. H. Chen, W. L. Hsu, C. J. Lin, Y. H. Pai, G. R. Lin, J. Dis. Techno. 10 (2014) 814. [5] W. E. Lai, Y. H. Zhu, H. W. Zhang, Q. Y. Wen, Opt. Mater. 35 (2013) 1218. [6] W. H. Kim, W. J. Maeng, M. K. Kim, H. J. Kim, J. Electrochem. Soc. 158 (2011) D495. [7] H. Kumarakuru, D. Cherns, A. M. Collins, Ceram. Int. 40 (2014) 8389. [8] B. Efafi, M. S. Ghamsari, M. H. Majles Ara, J. Lumin. 154 (2014) 32 [9] H. W Wu, C. H. Chu, Mater. Lett. 105 (2013) 65. [10] F. H. Wang, H. P. Chang, C. C. Tseng, C. C. Huang, Surf. Coat. Technol. 205 (2011) 5269. [11] H. B. Zhou, H. Y. Zhang, M. L. Tan, Z. G. Wang, Superlatt. Microstruct. 51 (2012) 644. [12] H. W. Wu, R. Y. Yang, C. M. Hsiung, C. H. Chu, J. Mater. Sci: Mater. Electron. 24 (2013) 166. [13] Z. B. Ayadi, L. E. Mir, K. Djessas, S. Alaya, Thin Solid Films 519 (2011) 7572. [14] C.M. Mahajan, M.G. Takwale, Curr. Appl. Phys. 13 (2013) 2109. [15] A. Mosbah, M.S. Aida, J. Alloys Compd. 515 (2012) 149. 7
[16] J. Zhai, L. Zhang, X. Yao, Ceram. Int. 26 (2000) 883. [17] A. Monemdjou, Superlatt. Microstruct. 74 (2014) 19. [18] J. H. Park, Sol. Energy Mater. Sol. Cells 95 (2011) 657. [19] B. D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Reading, MA, 1978, 102. [20] B. Lu, H. Huang, X. L. Dong, X. F. Zhang, J. P. Lei, J. P. Sun, C. Dong, J. Appl. Phys. 104 (2008) 114314. [21] V. I. Nefedov, M. N. Firsov, I. S. Shaplygin, J. Electron. Spectrosc. Relat. Phenom. 26 (1982) 65. [22] J. C. Fan, J.B . Goodenough, J. Appl. Phy. 48 (1977) 3524. [23] S. major, S. Kumar, M. Bhatnagar, K. L. Chopra, Appl. Phys. Lett. 40 (1986) 394. [24] S. S. Lin, J. L. Huang. P. Sajgaik, Surf. Coat. Technol. 185 (2004) 254. [25] C. H. Ahn, S. Y. Lee, H. K. Cho, Thin Solid Films 545 (2013) 106. [26] A. Mosbah, M. S. Aida, J. Alloys Compd. 515 (2012) 149. [27] T. Y. Ma, S. C. Lee, J. Mater. Sci. 11 (2000) 305. [28] S. M. Park, T. Ikegami, K. Ebihara, P. K. Shin, Appl. Surf. Sci. 253(2006) 1522. [29] T. Prabhakar, L. Dai, L. Zhang, R. Yang, L. Li, T. Guo, Y. Yan, J. Appl. Phys. 115 (2014) 083702. [30] W, Yang, Z. Wu, Z. Liu, A. Pang, Y. L. Tu, Z. C. Feng, Thin Solid Films 519 (2010) 31. [31] J. Tauc, Mater. Res. Bull 5 (1970) 721. [32] L. Burstein, Phys. Rev. 93 (1954) 632. [33] T. S. Moss, Proc. Phys. Soc., London, Set. A 382 (1954) 775.
8
Figure Captions
Fig. 1. Changes in resistivity (ρ), electron concentration (n), and Hall mobility (μ) of AZO thin films deposited at varied substrate temperatures.
Fig. 2. XRD results for AZO thin films as a function of substrate temperature.
Fig. 3. RMS roughness for AZO thin films under various substrate temperatures. The insets are AFM images of a 4 × 4 μm2 scanning area.
Fig 4. The O1s XPS spectra of the AZO thin films with different substrate temperature. Three components from low binding energy to high binding energy, OL, OM, and OH, were used to fit the spectra.
Fig. 5. Dependence of oxygen vacancy area and surface bonding area of AZO thin films as a function of substrate temperature.
Fig. 6. Transmittance in the wavelength range of 200 – 1200 nm for AZO thin films deposited at varied substrate temperature.
Fig. 7. (αhν)2 versus optical energy gap for AZO thin films under various substrate temperature.
9
Highlights Ø
The resistivity is deteriorated with increasing the substrate temperature.
Ø
The deterioration of resistivity is not attributed to the crystallinity and the grain boundary.
Ø
The increased temperature suppressed the incorporation of Al atoms together with the decrease of oxygen vacancy.
Ø
The control of Al content in high substrate temperature is the key parameter in the fabrication of high quality AZO.
10
S0749-6036(15)00268-2 http://dx.doi.org/10.1016/j.spmi.2015.05.009 YSPMI 3765
To appear in:
Superlattices and Microstructures
Received Date: Accepted Date:
5 May 2015 8 May 2015
Please cite this article as: D-K. Kim, H-B. Kim, Substrate temperature effects on the electrical properties of sputtered Al doped ZnO thin films, Superlattices and Microstructures (2015), doi: http://dx.doi.org/10.1016/j.spmi. 2015.05.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Substrate temperature effects on the electrical properties of sputtered Al doped ZnO thin films Deok-Kyu Kima,* and Hong-Bae Kim b a
Advanced Development Team , Samsung Electronics Co. Ltd., Yongin 446-711, Gyeonggi, Korea
b
Department of Semiconductor Engineering, Cheongju University, Cheongju 360-746, Chungbuk, Korea
* Corresponding Author Name –Deok-Kyu Kim Telephone – 82-10-8222-7669 Fax – 82-43-229-8461 E-mail –[email protected]
Abstract Al doped ZnO (AZO) thin films were deposited on glass substrate by RF magnetron sputtering system. The dependence of structural, electrical, and optical properties on the substrate temperature variations in the range of 0 to 400 °C was investigated. The structural results reveal that the AZO films are (002) oriented and at 400 °C a considerable crystallinity enhancement of the films is observed. With increasing the substrate temperature, the resistivity is increased by decreasing of the mobility and carrier concentration. X-ray photoelectron spectroscopy (XPS) results show that the mobility and the carrier concentration are decreased by increasing the surface bonding and decreasing the Al content, respectively. In our case, the increase in substrate temperature suppressed the incorporation of Al atoms together with the decrease of oxygen vacancy. The improvement of Al doping efficiency is a very important factor to obtain better electrical properties at high substrate temperatures.
Keywords: Al doped ZnO; RF magnetron sputtering; Substrate temperature; Al content
1
1. Introduction Optoelectronics are widely applied in technological and industrial applications, such as flat panel displays [1], solar cells [2], and solid state lightings [3]. The transparent conducting oxide (TCO) materials have been extensively used in versatile fields of optoelectronics. There are specific requirements for TCO thin films depending on the type of application: high electrical conductivity and transparence. Indium tin oxide (ITO) is the most well-known TCO material but also meet some problems such as the thermal stability at high temperatures and the transparency at short wavelengths [4]. Recently, Al doped ZnO (AZO) thin film have been comprehensively investigated for replacement of ITO because of its low cost, nontoxicity, better stability in hydrogen plasma, and excellent optoelectrical properties [5]. Its easy synthetic processes are ideal for successful commercial applications. Several techniques employed to prepare AZO thin film include chemical vapor deposition [6], pulsed-laser deposition [7], sol-gel technique [8], and radio frequency (RF) sputtering [9]. Among these, RF magnetron sputtering is a powerful and flexible technique used to coat virtually any substrate in a wide range of materials [10]. The factors influencing their characteristics are power, pressure, environment, substrate position and temperature, film thickness, and post deposition treatment. In these parameters, substrate temperature usually plays a crucial role in growth of AZO films. Most of researchers have been reported that the substrate temperature improves the structural and electrical properties of AZO films [11-13]. However, studies on the degradation in electrical properties with substrate temperature through chemical bonding state in AZO thin films have not been investigated extensively. In this study, AZO thin films were deposited on glass substrate by RF magnetron sputtering under different substrate temperatures. On the basis of these results, we investigated the effects of substrate temperature on the structural, electrical, and optical properties of the films in depth. In addition, the AZO thin films were analyzed by X-ray photoelectron spectroscopy in order to reveal exactly the changes of chemical bonding state in AZO films.
2. Experimental Procedure A RF magnetron sputtering technique was used to deposit AZO thin films on glass substrates. For AZO deposition, a 99.999% purity AZO target (3inch, 98% ZnO/2% Al2O3 by wt.) was used. The glass substrate was cleaned by organic cleaning process, dried using N2 gun, and mounted in the working chamber. The system was evacuated up to ∼2 × 10−6 Torr prior to introducing argon gas at a pressure of 7 mTorr for deposition of films by RF sputtering at 25W. Presputtering is carried out during 10 min to remove oxide. The 200-nm-thick AZO thin films were deposited at three different substrate temperatures of 0, 200 and 400 °C. The thicknesses of the AZO thin films were also confirmed by a scanning electron microscopy (SEM, JSM-6400). The crystallinity of AZO films were analyzed by using an X-ray 2
diffraction (XRD, D/ MAXⅢA) measurement equipped with a Cu-source (1.5406 Å). The surface morphology and root-mean-square (rms) roughness of these films were measured at 4 × 4 um scan areas by atomic force microscopy (AFM, PUCOTECH). The optical transmission spectra were measured with a UV-visible-near-IR spectrophotometer (JASCOUV/VI S/ NI R). The resistivity could be observed by analyzing the carrier concentration and mobility of the AZO film via the Hall effect measurements using a van der Pauw geometry (ECOPI A HMS3 0 0 0). X-ray photoelectron spectroscopy (XPS, THERMOVGSCI ENTI FI CMul t i La b2 0 0 0 ) measurements were performed using a Kratos Axis Ultra DLD with a monochromatic Al Kα (1486.6 eV) source to analyze the types of chemical bonds in the AZO films.
3. Results and Discussions The electrical properties of the AZO films were investigated by Hall measurements, which were performed under room temperature. Fig. 1 shows how the resistivity (ρ), electron concentration (n), and Hall mobility (μ) ate related with the substrate temperature for the AZO films. Hall effect measurements revealed that all the films exhibited n-type conductivity. On increasing the substrate temperature from 0 to 400 °C, the resistivity increased from 1.5 × 10-3 Ω-cm to 7.1 × 10-3 Ω-cm, while the electron concentration decreased from 4.4 × 1020 cm-3 to 1.8 × 1020 cm-3. The mobility also underwent a change from 9 cm3/Vs to 4.5 cm3/Vs and the constant. The increased resistivity at 200 and 400 °C had been results of the decrease in both electron concentration and mobility and in only electron concentration, respectively. In usual, the electron concentration and mobility with increase in substrate temperature are primarily governed by crystallinity, grain boundary and oxygen adsorption effects [14,15]. Therefore, these changes in the electron concentration and the mobility during the deposition process are discussed later in detail.
Fig. 1
The X-ray diffraction (XRD) technique was used to analyze the crystallinity of the AZO films. Fig. 2 shows the XRD results obtained from the AZO thin films for the various substrate temperatures. Regardless of substrate temperature, it was clearly observed the strong peak at 34.2 corresponding to the (002) plane in ZnO wurtzite structure and the weak peak at 62.5 attributed to the (103) plane. This implies that all the AZO thin films are polycrystalline with a hexagonal structure and have a preferred c-axis orientation perpendicular to the substrate due to the smallest surface energy. The degree of the orientation as function of substrate temperature can be obtained from the texture coefficient (TC), which is defined as [16] 3
(ℎ ) =
(ℎ ) × 100% ∑ (ℎ )
Where TC(hkl) is the texture coefficient of the (hkl) plane and I(hkl) is the measured intensity. The value of the texture coefficient indicates the maximum preferred orientation of the films along the diffraction plane, meaning that the increase in preferred orientation is associated with increase in the number of grains along that plane. As the substrate temperatures increased from 0 to 400 °C, the TC (002) of AZO film increased from 0.7 to 0.72. The increase in amount of preferred orientation associated with the increased number of grains along (002) [17]. Also, the full width a half maximum (FWHM) of (002) peak decreased with increasing the substrate temperature, this means that the crystallinity of these films enhances with substrate temperature. The improvement in higher substrate temperature is due to the increase in the mobility of the sputtered AZO atoms [18]. The average crystallite size of the AZO film is determined using the Scherrer formula [19]
D =
0.9
where D is the average crystallite size, λ is the wavelength of X-ray (1.5406 Å), θ is the Bragg’s angle and B is the FWHM of the film. The average crystallite size increased from 18.5 nm to 19.3 nm when the substrate temperature increased from 0 to 400 °C. This result agrees with the results in TC (002). From the XRD results, it can be concluded that the increased substrate temperature can improve the crystalline quality of AZO thin films. However, the resistivity of AZO thin film degrades despite of improvement of crystallinity of film, indicating that the crystalline quality does not directly affect the electrical properties in our case.
Fig. 2.
Fig. 3 presents the AFM images of a 4 × 4 μm2 scanning area as a function of the substrate temperature. It is seen that the surface morphology varies significantly with increasing substrate temperature. As the substrate temperature increased, the surface of AZO thin films exhibited rougher surfaces and the grain size became larger. The root-meansquare (rms) roughness increased from 1.89 nm to 2.67 nm with increasing the substrate temperature from 0 C to 400 °C. This result is consistent with the application requirements for LEDs for which the rms roughness must be under 3 nm. [20] In general, the increased grain size leads to decreasing the grain boundary, which hinders the movement of 4
carrier, and improving the conductivity. In our case, the contrary results appeared, so that the grain size is not critical parameter in electrical properties. Also, the rougher surface provides more opportunity for surface adsorption on the surface, resulting in degradation of electrical properties in AZO thin film. Therefore, the change of surface adsorption on the surface is observed and discussed below.
Fig. 3.
After the measurement of electrical property and structure property of AZO films by Hall system, XRD and AFM, XPS was applied to identify the change of chemical structure caused by substrate temperature and in order to find the elements changes in the AZO films. The O 1 s spectra showing the chemical bonding state of oxygen shows asymmetric peaks, as shown in Fig. 4. The typical O 1s peak could be consistently fitted by three nearly Gaussian. The lower binding energy (OL = 529.7 ~ 529.9 eV) and middle binding energy (OM = 531.2 ~ 531.4 eV) are associated with O2− ions in the ZnO matrix and oxygen deficiencies, respectively [21,22]. An additional high binding energy (OH = 532.5 ~ 532.7 eV) results from O–H or C–O groups near the surface of the films [23]. The increase in substrate temperature from 0 to 400 °C increased the intensity of the Zn-O bond related OL peak and decreased the intensity of the oxygenvacancy related OM peak. This indicates that the crystallinity of films is improved by increasing substrate temperatures, being agree with the XRD results. Also, the reduction of oxygen-vacancy decreases the carrier concentration and this is in accord with the electrical properties of AZO films. It was also observed that surface impurity-related O H peak increased as substrate temperature increased. This result does agree with the result of surface roughness in AZO film. The increase of the surface bonding raises the barrier potential which impedes the motion of carrier, resulting in decrease of mobility [24]. That is, the decreased mobility at 200 °C is attributed to increase of surface bonding. Therefore, the change of oxygen-vacancy and surface bonding is coincided with the behavior of carrier concentration and mobility. Generally, many researcher have been reported that the deposition of higher substrate temperature improve the crystallinity and the resistivity of film [25,26]. That is, despite of decrease of oxygen vacancy, the carrier concentration increases due to increase in Al content, indicating that the Al content is main parameter in change of the carrier concentration. However, in our case, the crystallinity of AZO film is improved but the resistivity is degraded with increasing the substrate temperature. In order to confirm the root cause of this difference, we analyzed the Al content in AZO thin film, shown in Fig. 5. As the substrate temperature increased from 0 to 400 °C, the Al content decreased from 1.7 to 0.55 at %, resulting in decrease of the carrier concentration. The reduction of Al content with the substrate temperature is attributed to increased kinetic energy. That is, when the temperature increases, the Al atoms have surplus kinetic energy and cause deeper doping, decreasing the Al content on the surface to under 1% and 5
significantly affecting the AZO surface conductivity [27,28]. Therefore, the reason why the resistivity deteriorates is because the Al content decreases and the surface bonding increase with increasing the substrate temperature. As a result, we confirmed that the increased substrate temperature resulted in improved crystalline quality but declined the Al doping efficiency. The Al content plays a major role in the conduction behavior of AZO thin films deposited at various substrate temperatures.
Fig. 4.
Fig. 5.
The wavelength dependences of the optical transmittance spectra of the AZO films subjected to the three different substrate temperatures are plotted in Fig. 6. All of the AZO films show an average transmittance of 82.3 ~ 88.8 % in the visible wavelength range from 400 to 800 nm and a sharp fundamental absorption edge in the UV region, which is suitable for display devices and solar cells as transparent electrodes [29]. It seemed that the transmittance decreased and a red shift of the absorption edge occurred with increasing the substrate temperature. The transmittance may be reduced by increasing the surface bonding, which absorbs visible photons [30]. The absorption edge is related to the optical band gap, which can be calculated from the relationship between the optical absorption coefficient (α) and the transmittance (T). The optical band gap of a direct band gap semiconductor can be determined by using Tauc model [31]:
(ℎ ) = (ℎ −
)
/
where hν is the photon energy of incident light, C is a constant and Eg is the optical band gap. The absorption coefficient (a) can be calculated from the transmittance of the films with the formula α = ln(1/T)/d, where d is the thickness of the films and T is the transmittance. Therefore, the optical band gap is obtained by extrapolating the tangential line to the photon energy axis in the plot of (αhv) as a function of hv, as shown in Fig. 7. As the substrate temperature was increased to 0, 200, 400 ◦C, the optical band gap shifted to 3.73, 3.58, and 3.428 eV, respectively. These results are consistent with those observed for the carrier concentration. It is found that there is a band gap narrowing of AZO film with increasing the substrate temperature, which is probably due to the Burstein-Moss effects [32,33]. The Fermi energy penetrates into the conduction band of the degenerate semiconductor, due to an decrease in the electron concentration from Al dopants, that leads to the energy band narrowing. 6
Fig. 6.
Fig. 7.
4. Conclusion In this study, RF-magnetron sputtering is used to fabricate AZO thin films on glass substrates under various substrate temperatures. All the thin films grew along c-axis with wurtzite structure and had a high average optical transmission (≥82%) in visible region. At higher substrate temperature, AZO thin film exhibits high resistivity connected with its low carrier concentration and mobility. We confirmed that the control of Al content in high substrate temperature is the key parameter in the fabrication of high quality AZO transparent electrodes.
References [1] T. Minami, S. Takata, T. Kakumu, J. Vac. Sci. Technol. A 14 (1996) 1689. [2] Y. Wang, X. Zhang, L. Bai, Q. Huang, C. Wei, Y. Zhao, Appl. Phys. Lett. 100 (2012) 263508. [3] H. Y. Ryu, J. I. Shim, Opt. Express, 19 (2011) 2886. [4] C. H. Chen, W. L. Hsu, C. J. Lin, Y. H. Pai, G. R. Lin, J. Dis. Techno. 10 (2014) 814. [5] W. E. Lai, Y. H. Zhu, H. W. Zhang, Q. Y. Wen, Opt. Mater. 35 (2013) 1218. [6] W. H. Kim, W. J. Maeng, M. K. Kim, H. J. Kim, J. Electrochem. Soc. 158 (2011) D495. [7] H. Kumarakuru, D. Cherns, A. M. Collins, Ceram. Int. 40 (2014) 8389. [8] B. Efafi, M. S. Ghamsari, M. H. Majles Ara, J. Lumin. 154 (2014) 32 [9] H. W Wu, C. H. Chu, Mater. Lett. 105 (2013) 65. [10] F. H. Wang, H. P. Chang, C. C. Tseng, C. C. Huang, Surf. Coat. Technol. 205 (2011) 5269. [11] H. B. Zhou, H. Y. Zhang, M. L. Tan, Z. G. Wang, Superlatt. Microstruct. 51 (2012) 644. [12] H. W. Wu, R. Y. Yang, C. M. Hsiung, C. H. Chu, J. Mater. Sci: Mater. Electron. 24 (2013) 166. [13] Z. B. Ayadi, L. E. Mir, K. Djessas, S. Alaya, Thin Solid Films 519 (2011) 7572. [14] C.M. Mahajan, M.G. Takwale, Curr. Appl. Phys. 13 (2013) 2109. [15] A. Mosbah, M.S. Aida, J. Alloys Compd. 515 (2012) 149. 7
[16] J. Zhai, L. Zhang, X. Yao, Ceram. Int. 26 (2000) 883. [17] A. Monemdjou, Superlatt. Microstruct. 74 (2014) 19. [18] J. H. Park, Sol. Energy Mater. Sol. Cells 95 (2011) 657. [19] B. D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Reading, MA, 1978, 102. [20] B. Lu, H. Huang, X. L. Dong, X. F. Zhang, J. P. Lei, J. P. Sun, C. Dong, J. Appl. Phys. 104 (2008) 114314. [21] V. I. Nefedov, M. N. Firsov, I. S. Shaplygin, J. Electron. Spectrosc. Relat. Phenom. 26 (1982) 65. [22] J. C. Fan, J.B . Goodenough, J. Appl. Phy. 48 (1977) 3524. [23] S. major, S. Kumar, M. Bhatnagar, K. L. Chopra, Appl. Phys. Lett. 40 (1986) 394. [24] S. S. Lin, J. L. Huang. P. Sajgaik, Surf. Coat. Technol. 185 (2004) 254. [25] C. H. Ahn, S. Y. Lee, H. K. Cho, Thin Solid Films 545 (2013) 106. [26] A. Mosbah, M. S. Aida, J. Alloys Compd. 515 (2012) 149. [27] T. Y. Ma, S. C. Lee, J. Mater. Sci. 11 (2000) 305. [28] S. M. Park, T. Ikegami, K. Ebihara, P. K. Shin, Appl. Surf. Sci. 253(2006) 1522. [29] T. Prabhakar, L. Dai, L. Zhang, R. Yang, L. Li, T. Guo, Y. Yan, J. Appl. Phys. 115 (2014) 083702. [30] W, Yang, Z. Wu, Z. Liu, A. Pang, Y. L. Tu, Z. C. Feng, Thin Solid Films 519 (2010) 31. [31] J. Tauc, Mater. Res. Bull 5 (1970) 721. [32] L. Burstein, Phys. Rev. 93 (1954) 632. [33] T. S. Moss, Proc. Phys. Soc., London, Set. A 382 (1954) 775.
8
Figure Captions
Fig. 1. Changes in resistivity (ρ), electron concentration (n), and Hall mobility (μ) of AZO thin films deposited at varied substrate temperatures.
Fig. 2. XRD results for AZO thin films as a function of substrate temperature.
Fig. 3. RMS roughness for AZO thin films under various substrate temperatures. The insets are AFM images of a 4 × 4 μm2 scanning area.
Fig 4. The O1s XPS spectra of the AZO thin films with different substrate temperature. Three components from low binding energy to high binding energy, OL, OM, and OH, were used to fit the spectra.
Fig. 5. Dependence of oxygen vacancy area and surface bonding area of AZO thin films as a function of substrate temperature.
Fig. 6. Transmittance in the wavelength range of 200 – 1200 nm for AZO thin films deposited at varied substrate temperature.
Fig. 7. (αhν)2 versus optical energy gap for AZO thin films under various substrate temperature.
9
Highlights Ø
The resistivity is deteriorated with increasing the substrate temperature.
Ø
The deterioration of resistivity is not attributed to the crystallinity and the grain boundary.
Ø
The increased temperature suppressed the incorporation of Al atoms together with the decrease of oxygen vacancy.
Ø
The control of Al content in high substrate temperature is the key parameter in the fabrication of high quality AZO.
10