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Effects of RF power on the structural, optical and electrical properties of Al-doped zinc oxide films
Microelectronics Reliability 50 (2010) 730–733
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Effects of RF power on the structural, optical and electrical properties of Al-doped zinc oxide films Shou-Yi Kuo a,b,*, Kou-Chen Liu a, Fang-I Lai c,*, Jui-Fu Yang a, Wei-Chun Chen d, Ming-Yang Hsieh a, Hsin-I Lin a, Woei-Tyng Lin c a
Department of Electronic Engineering, Chang Gung University, Taiwan Green Technology Research Center, Chang Gung University, Taiwan Department of Electro-Optical Engineering, Yuan-Ze University, Taiwan d Instrument Technology Research Center, National Applied Research Laboratories, Taiwan b c
a r t i c l e
i n f o
Article history: Received 1 December 2009 Received in revised form 22 January 2010 Available online 28 March 2010
a b s t r a c t In this study, we discussed the effects of growth parameters on the structural and optical properties of Aldoped zinc oxide (AZO) deposited at room temperature by radio-frequency magnetron sputtering. The AZO films have been characterized in detail using X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, Hall-effect measurement system and UV–visible spectrophotometer. It was found that the morphological, structural, electrical and optical properties of AZO films are greatly dependent on sputtering power. Collision between sputter species and surface morphology play important roles in optoelectrical properties of AZO films. According to our experimental results, the AZO films can be used in versatile devices to meet various requirements. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide (ZnO), with direct wide-band-gap and wurtzite structure and strong exciton binding energy (60 meV), is one of the most important binary II–VI compounds [1–7]. It has been actively studied in various fields, with potential applications in many technological domains such as surface and bulk acoustic wave devices (SAW), optical-wave guide and acoustic-optical devices, and light-emitting diodes (LEDs) and laser diodes (LDs) [8–11]. In addition, ZnO thin films doped with Al, Ga or In have low electrical resistivity and high optical transmittance for optoelectronic and electronic devices, such as flat panel displays, solar cells and light-emitting diodes [12]. The Al-doped ZnO (AZO) transparent conducting film shows the lowest resistivity among impurity doped ZnO films [13]. Recently, ZnO has been considered as a potential substitution material for ITO and SnO2 widely used as transparent conducting materials, owing to a number of encouraging advantages namely low cost and stability under reduced hydrogen atmosphere [14,15]. A number of techniques have been employed for fabricating ZnO thin films, including chemical vapour deposition, sol–gel, spray-pyrolysis, molecular beam epitaxy, pulsed laser deposition, vacuum arc deposition, and magnetron sputtering [16– 22].
* Corresponding authors. Address: Department of Electronic Engineering, Chang Gung University, Taiwan (S.-Y. Kuo). E-mail addresses: [email protected] (S.-Y. Kuo), fi[email protected] (F.-I Lai). 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.01.042
Among these techniques, magnetron sputtering is a promising one for the deposition of transparent oxides, which permits deposition at low temperature, and gives better adhesion, larger coverage and higher film density than other methods. However, the quality of the films with regards to the crystal structure depends strongly on the sputtering conditions, such as RF power, sputtering pressure and target-to-substrate distance. The growth process during magnetron sputtering is influenced by the bombardment of the growing film with species from the sputtering target and from the plasma. In addition to the sputtered atoms with energies in the range up to 10 eV, higher energy ions from the plasma and neutral atoms reflected at the target hit the growing film. Thus, the oxide thin films deposited by sputtering are prone to have intrinsic stress. In this work, aluminum-doped zinc oxide thin films were prepared by the RF magnetron sputtering onto glass substrates at room temperature without any post-thermal treatment. The films were characterized by the X-ray diffraction (XRD) measurements, the atomic force microscopy (AFM) and the optical transmittance spectroscopy. The influence of processing parameters on the structural and optical properties of the films was investigated. 2. Experimental procedure AZO films were deposited on corning glass substrates by radiofrequency (RF) magnetron sputtering system. In the sputtering system, we use a disk of ZnO mixed with 2 wt.% Al2O3 as a target. The target-to-substrates distance was 18 cm and the diameter of the target was 3 in. A quartz crystal microbalance was employed to
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34.31
2θ (degree)
0.5226
34.29 34.28 (0002) peak position c-axis lattice constant
34.27
0.5224 0.5222
34.26
1.6 1.5
1.2
6
1.4
)
0.5228 34.30
c-axis constant (nm)
0.5230
1.3 carrier concentration (n) mobility ( ) resistivity ( )
1.2 1.1
5
1.0 0.9
-cm)
34.32
-3
(b)
1.1
4
Resistivity (10
2θ (degree)
2
Intensity (a.u.)
RF power
Mobility cm /V.sec
(a)
-3
Fig. 1a shows the XRD patterns of AZO thin films deposited by RF magnetron sputtering under various RF power condition. Magnetron sputtered AZO films usually have a hexagonal wurtzite structure with its preferred orientation along the c-axis perpendicular to the substrate surface [13,23]. By examining all the diffraction patterns of the AZO thin films obtained at various deposition parameters, the XRD spectra reveal that the ZnO films developed without the existence of secondary phases and clusters, and only the ZnO(0 0 0 2) diffraction plane is observed. No Al2O3 phase
21
3. Results and discussion
was found, which implies that Al atoms substitute Zn in the hexagonal lattice and Al ions may occupy the interstitial sites of ZnO or probably Al segregates to the non-crystalline region in grain boundaries and forms AlAO bond. According to the XRD patterns, the mean peak diffraction angle is related to the interplanar distance of atomic structure of AZO films by Bragg’s law. Fig. 1b reveals the peak position of ZnO(0 0 0 2) and derived c-axis constant as a function of RF power. As mentioned above, in fact, the highly incorporated Al3+ ions are not only in the substitutional sites but also in the interstitial sites. If Al3+ ions only substitute Zn2+ ions, the lattice parameter of AZO crystals decreases and the (0 0 0 2) peak shifts to high angle due to the smaller radius of Al3+ ions (0.53 Å) compared to Zn2+ ions (0.75 Å) [24]. Our results indicated that the aluminum-doped ZnO films suffered compressive stress with the increase of RF power. The compressive stress may be due to the interstitial Al atoms in the lattice of AZO films. In order to show significant influence of RF power on surface roughness of films, we measure AFM images (not shown here). The surface roughness of thin films was found to be 2.06 nm, 2.21 nm, 3.02 nm and 3.15 nm with RF power of 75 W, 100 W, 125 W and 150 W respectively. This indicates that the surface roughness increases with increase of RF power due to bombardment effect. Fig. 2 shows the dependence of the resistivity, mobility and carrier concentration on the RF power. As RF power was increased from 75 W to 150 W, the thickness of AZO films linearly increase from 150 nm to 300 nm. It can be reasoned that higher RF power means more argon ions in the plasma and the bombardment on the target is increased accordingly. Typically, the resistivity is decreased with increasing the film thickness. However, Fig. 2 shows a dissimilar behavior indicating other mechanisms should be taken into account. It can be seen that the resistivity of the film deposited at 75 W is 1.02 103 X cm. The resistivity is increased with the increase of RF power in the range from 75 to 125 W. Though there is no significant change in resistivity, a maximal value for the resistivity of 1.18 103 X cm is obtained at RF power of 125 W. A further increase in RF power causes resistivity to decrease slightly. The carrier concentration and Hall mobility with respect to the variation of the RF power are depicted in Fig. 2 as well. It can be seen that as RF power increases from 75 to 125 W, carrier concentrations decrease while Hall mobilities increase. The carrier concentration of 8.5 1020 cm3 and the Hall mobility of 8.2 cm2/V s are the minimum and maximum values for the film deposited at RF power of 125 W, which results in a maximum resistivity of film. Further increase of RF power, an opposite variation tendency oc-
Carrier concentration x 10 cm )
monitor film thickness during deposition. The chamber was evacuated to 3.0 107 Torr and the target–substrate distance was 18 cm. Prior to the deposition, the glass substrates were cleaned by the standard piranha process, rinsed by de-ionized water, and thoroughly dried by nitrogen gas. The working pressure of the sputter chamber was kept at 5 m Torr by an automatic pressure controller. All samples were deposited at room temperature. The flow rate of argon gas during the deposition was fixed at 15 sccm (standard cubic centimeters per minute), realized by a mass flow controller with an external control program, and the RF plasma sputtering power was varied from 75 W to 150 W. After the sputtering process, the film thickness was measured by a profilometer (Dektak3). The influence of deposition power on the structural, morphological and optical properties of these films was analyzed by XRD, X-ray photoelectron spectroscopy (XPS), AFM, Hall-effect measurement system and UV–visible spectrophotometer.
0.8 1.0
70
80
90 100 110 120 130 140 150 160
RF power (W) Fig. 1. (a) The XRD patterns and (b) variation of peak position of ZnO(0 0 0 2) and the calculated c-axis lattice constants of aluminum-doped ZnO films deposited at various RF power.
70
80
90 100 110 120 130 140 150 160
RF power (W) Fig. 2. Electrical resistivity (q), carrier concentration (n) and Hall mobility (l) of ZnO:Al films deposited on glass substrates at room temperature as a function of RF power.
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the mean free path length of electrons being smaller than the crystallite size. Besides, ionized impurity scattering has been considered the most determinant and appropriate mechanism in doped TCO films with carrier concentration >1020 cm3 [28]. The effect of chemisorption of oxygen on the film surface and grain boundary has to be taken into account, especially in which the films were formed at room temperature. Oxygen has been found to easily chemisorb on the film surface and grain boundaries of crystallites that capture electrons from conduction band. This allows the formation of potential barriers which would strongly scatter conduction electrons. The rougher surface of the films deposited at higher RF power resulted in higher chemisorbed oxygen atoms, and thus leading to lower carrier mobility as shown in Fig. 2. Fig. 3a and b shows the XPS analysis of Zn 2p3/2 and O 1s spectra for AZO film deposited at RF power of 150 W. Though the Al concentration is low, the Al 2p spectrum of the AZO thin film was successfully monitored. As shown in Fig. 3c, the binding energy of the AZO films is around 73.5 eV, which is lower than the binding energy (75.6 eV) of amorphous Al2O3 films. The core line of Zn 2p exhibited high symmetry and the binding energy of Zn 2p3/2 remains at 1021.8 eV, which confirms that the majority of Zn atoms remain in the same formal valence state of Zn2+ within the ZnO matrix. The component on the low binding energy side of the O 1s spectrum is attributed to O2 ions on wurtzite structure of hexagonal Zn2+ ion array, surrounded by Zn (or the substitution of Al) atoms with their full complement of nearest neighbor O2 ions. In other words, the intensity of this component is the measure of the amount of oxygen atoms in a fully oxidized stoichiometric surrounding. The high binding energy component is usually attributed to the either presence of loosely bound oxygen on the surface of the films or belongs to hydrated oxides which might incorporated from deposition chamber. It can also be associated with O2 ions in the oxygen-deficient regions within the matrix of ZnO [29]. This observation confirms the importance of bombardment effect while AZO films deposited at high RF power. The optical transmittance of the films has been measured in the wavelength range of 200–900 nm as shown in Fig. 4. The optical
curs for the carrier concentration and Hall mobility. The mobile energy of the sputtered atoms on substrate greatly depends on the RF power and it increases with the increase of RF power. Therefore, increasing RF power results in the improvement of crystallinity. This makes grains bigger in size and defects less in number, and suppressed generation of electrons by O2 vacancies and Zn interstitial atoms as well. As shown in Fig. 2, a slight increase in the resistivity of AZO films deposited from 75 W to 125 W is attributed to the improvement of crystallinity. The oxygen atoms are very sensitive to the thermalization effect, collision between moleculars, than zinc and aluminum atoms because of the smaller atomic radius and mass. At lower RF power, oxygen atoms are easily scattered out and make oxygen-deficient films. While we increase the RF power, more oxygen atoms will gain enough energy and lower the concentration of oxygen vacancies. Collisions between sputtered species might be the other cause of the decreased carrier concentration. Moreover, the resistivity of AZO films decreases slightly as we increase the RF power to 150 W. This phenomenon implies that the bombardment effects become dominant over atomic surface diffusion while the AZO film deposited at higher RF power. And the surface morphology play a key role in the electrical performance. Generally, the reciprocal mobility of the films can be expressed as
1
lH
¼
1
li
þ
1
ll
þ
1
ð1Þ
lg
where lH is Hall mobility of the films, li, ll, and lg are mobilities corresponding to ionized impurity, lattice vibration and grain boundary scattering, respectively. According to the equation of ll / 1=T, lattice vibration scattering may dominate at high temperature [25]. Since the AZO films were deposited at room temperature, it is obvious that lattice vibration scattering apparently did not play a significant role in our samples. Meanwhile, grain boundary scattering is considered as another important scattering mechanism in polycrystalline semiconductor thin film [26,27]. However, scattering by grain boundaries in our samples is neglected due to
(a)
(b)
O 1s
1015
1020
1025
1030
Binding energy (eV)
(c)
Al 2p
Relative Intensity (a.u.)
Relative Intensity (a.u.)
Relative Intensity (a.u.)
Zn 2p2/3
525
530
535
Binding energy (eV)
66 68 70 72 74 76 78 80
Binding energy (eV)
Fig. 3. XPS spectra of Zn 2p3/2 (a), O 1s (b) and (c) Al 2p binding energy of AZO thin film deposited at RF power of 150 W.
S.-Y. Kuo et al. / Microelectronics Reliability 50 (2010) 730–733
100
l2O3 ceramic targets with different RF powers. The experimental result showed that the minimum resistivity of 103 X cm and high transmittance of 87% can be obtained at low substrate temperature (room temperature). Based on the results presented in this study, the low electrical resistivity and high optical transmittance of AZO films suggested a possibility for the application in the flexible electronic devices such as transparent conducting oxide film on LEDs, solar cells, TFT-LCDs and touch panels.
bare glass substrate 75 W 100 W RF power 125 W Average transmittance 150 W (visible region 400 nm ~ 750 nm) 90
40
Transmittance (%)
Transmittance (%)
80 60
20
Acknowledgements 85
80
0 200
75
100
125
This work was supported by the National Science Council (NSC) of Taiwan under NSC-95-2112-M-182-002-MY2 and NSC-972112-M-182-004-MY3.
150
RF power (W)
300
400
500
733
600
700
800
900
Wavelength (nm) Fig. 4. The optical transmittance of AZO thin films as a function of RF power in a wide spectral range from 200 nm to 900 nm. Shown in the inset is the average transmittance in the visible region.
transmittance of the films grown at different RF power all exhibit an average transmittance in the visible range over 82% (inset of Fig. 4) and a sharp fundamental absorption edge. Fig. 4 indicates that the transmittance of AZO films on glass gradually decreases with an increasing RF power. This characteristic may be caused by thicker thickness, increased scattering, reflection and optical absorption of the films, owing to larger surface roughness and amorphous contents of the films deposited at higher RF powers. The average optical transmittance of the AZO films with the lowest resistivity of 1.02 103 X cm was 86.7%. Accordingly, the increase in surface roughness decreases the effective thickness of conducting path in film and then increases the electrical resistivity. In addition, oxygen adsorbed on the surface of crystallites traps electrons, decreases carrier concentration and also decreases the Hall mobility by increasing potential height at the surface of crystallites [30]. With increasing RF power, the deteriorated surface may result in the increase of effective surface area of films, and the number of absorption sites for oxygen causing worse resistivity. The decrease of optical transmittance of AZO film with increasing RF power also can be ascribed to the enhancement of scattering and absorption of light caused by the decrease of grain size and the increase of surface roughness. 4. Conclusion In summary, transparent and conducting ZnO films doped with Al have been deposited on corning glass by RF-sputtering ZnO:A-
References [1] Thomas DG. J Phys Chem Solids 1960;15:86. [2] Ekem N, Korkmaz S, Pat S, Balbag MZ, Cetin EN, Ozmumcu M. Int J Hydrogen Energy 2009;34:5218–22. [3] Reynolds DC, Look DC, Jogai B, Litton CW, Cantwell G, Harsch WC. Phys Rev B 1999;60:2340. [4] Lim JH, Kang CK, Kim KK, Park IK, Hwang DK, Park SJ. Adv Mater 2006;18:2720. [5] Srikant V, Clarke DR. J Appl Phys 1998;83:5447. [6] Kim DC, Kong BH, Cho HK. J Mater Sci Mater Electron 2008;19:760. [7] Xu QA, Zhang JW, Ju KR, Yang XD, Hou XJ. J Cryst Growth 2006;289:44. [8] Wang WS, Wu TT, Chou TH, Chen YY. Nanotechnology 2009;20:135503. [9] Lee JY, Lee JH, Kim HS, Lee CH, Ahn HS, Cho HK, et al. Thin Solid Films 2009;517:5157–60. [10] Fan G, Ning J, Yang J. Optics Laser Technol 2007;39:421–3. [11] Lee YH, Song MH, Ju BK, Shin DK, Oh MH. J Vac Sci Technol 1997;B15:512–5. [12] Tseng JY, Chen YT, Yang MY, Wang CY, Li PC, et al. Thin solid films 2009;517:6310–4. [13] Yang W, Liu Z, Peng DL, Zhang F, Huang H, Xie Y, et al. Appl Surf Sci 2009;255:5669–73. [14] Kuo SY, Chen WC, Lai FI, Cheng CP, Kuo HC, Wang SC, et al. J Cryst Growth 2006;287:78. [15] Lee KE, Wang MS, Kim EJ, Hahn SH. Curr Appl Phys 2009;9:683–7. [16] Li BS, Liu YC, Chu ZS, Shen DZ, Lu YM, Zhang JY, et al. J Appl Phys 2002;91:501–5. [17] Alam MJ, Cameron DC. J Vac Sci Technol 2001;A19:1642–6. [18] Fiddes AJC, Durose K, Brinkman AW, Woods J, Coates PD, Banister AJ. J Cryst Growth 1996;159:210–3. [19] Ohgaki T, Ohashi N, Kakemoto H, Wada S, Adachi Y, Haneda H, et al. J Appl Phys 2003;93:1961–5. [20] Ryu YR, Zhu S, Budai JD, Chandrasekhar HR, Miceli PF, White HW. J Appl Phys 2000;88:201–4. [21] Minami T, Ida S, Miyata T. Thin Solid Films 2002;416:92–6. [22] Kim KH, Park KC, Ma DY. J Appl Phys 1997;81:7764–72. [23] Chang JF, Wang L, Hon MH. J Cryst Growth 2000;211:93. [24] Park KC, Ma DY, Kim KH. Thin Solid Films 1997;305:201. [25] Kireev PS. Semiconductor Physics. Moscow: Mir; 1978. [26] Morris JE, Ridge MI, Bishop CA, Howson RP. J Appl Phys 1980;51:1847. [27] Chaudhuri S, Bhattacharyya J, Pal AK. Thin Solid Films 1987;148:279. [28] Ellmer K. J Phys D Appl Phys 2001;34:3097. [29] Chen M, Pei ZL, Wang X, Sun C, Wen LS. J Vac Sci Technol 2001;19:3. [30] Fu EG, Zhuang DM, Zhang G, Ming Z, Yang WF, Liu JJ. Microelectron J 2004;35:383–7.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Effects of RF power on the structural, optical and electrical properties of Al-doped zinc oxide films Shou-Yi Kuo a,b,*, Kou-Chen Liu a, Fang-I Lai c,*, Jui-Fu Yang a, Wei-Chun Chen d, Ming-Yang Hsieh a, Hsin-I Lin a, Woei-Tyng Lin c a
Department of Electronic Engineering, Chang Gung University, Taiwan Green Technology Research Center, Chang Gung University, Taiwan Department of Electro-Optical Engineering, Yuan-Ze University, Taiwan d Instrument Technology Research Center, National Applied Research Laboratories, Taiwan b c
a r t i c l e
i n f o
Article history: Received 1 December 2009 Received in revised form 22 January 2010 Available online 28 March 2010
a b s t r a c t In this study, we discussed the effects of growth parameters on the structural and optical properties of Aldoped zinc oxide (AZO) deposited at room temperature by radio-frequency magnetron sputtering. The AZO films have been characterized in detail using X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, Hall-effect measurement system and UV–visible spectrophotometer. It was found that the morphological, structural, electrical and optical properties of AZO films are greatly dependent on sputtering power. Collision between sputter species and surface morphology play important roles in optoelectrical properties of AZO films. According to our experimental results, the AZO films can be used in versatile devices to meet various requirements. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide (ZnO), with direct wide-band-gap and wurtzite structure and strong exciton binding energy (60 meV), is one of the most important binary II–VI compounds [1–7]. It has been actively studied in various fields, with potential applications in many technological domains such as surface and bulk acoustic wave devices (SAW), optical-wave guide and acoustic-optical devices, and light-emitting diodes (LEDs) and laser diodes (LDs) [8–11]. In addition, ZnO thin films doped with Al, Ga or In have low electrical resistivity and high optical transmittance for optoelectronic and electronic devices, such as flat panel displays, solar cells and light-emitting diodes [12]. The Al-doped ZnO (AZO) transparent conducting film shows the lowest resistivity among impurity doped ZnO films [13]. Recently, ZnO has been considered as a potential substitution material for ITO and SnO2 widely used as transparent conducting materials, owing to a number of encouraging advantages namely low cost and stability under reduced hydrogen atmosphere [14,15]. A number of techniques have been employed for fabricating ZnO thin films, including chemical vapour deposition, sol–gel, spray-pyrolysis, molecular beam epitaxy, pulsed laser deposition, vacuum arc deposition, and magnetron sputtering [16– 22].
* Corresponding authors. Address: Department of Electronic Engineering, Chang Gung University, Taiwan (S.-Y. Kuo). E-mail addresses: [email protected] (S.-Y. Kuo), fi[email protected] (F.-I Lai). 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.01.042
Among these techniques, magnetron sputtering is a promising one for the deposition of transparent oxides, which permits deposition at low temperature, and gives better adhesion, larger coverage and higher film density than other methods. However, the quality of the films with regards to the crystal structure depends strongly on the sputtering conditions, such as RF power, sputtering pressure and target-to-substrate distance. The growth process during magnetron sputtering is influenced by the bombardment of the growing film with species from the sputtering target and from the plasma. In addition to the sputtered atoms with energies in the range up to 10 eV, higher energy ions from the plasma and neutral atoms reflected at the target hit the growing film. Thus, the oxide thin films deposited by sputtering are prone to have intrinsic stress. In this work, aluminum-doped zinc oxide thin films were prepared by the RF magnetron sputtering onto glass substrates at room temperature without any post-thermal treatment. The films were characterized by the X-ray diffraction (XRD) measurements, the atomic force microscopy (AFM) and the optical transmittance spectroscopy. The influence of processing parameters on the structural and optical properties of the films was investigated. 2. Experimental procedure AZO films were deposited on corning glass substrates by radiofrequency (RF) magnetron sputtering system. In the sputtering system, we use a disk of ZnO mixed with 2 wt.% Al2O3 as a target. The target-to-substrates distance was 18 cm and the diameter of the target was 3 in. A quartz crystal microbalance was employed to
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S.-Y. Kuo et al. / Microelectronics Reliability 50 (2010) 730–733
34.31
2θ (degree)
0.5226
34.29 34.28 (0002) peak position c-axis lattice constant
34.27
0.5224 0.5222
34.26
1.6 1.5
1.2
6
1.4
)
0.5228 34.30
c-axis constant (nm)
0.5230
1.3 carrier concentration (n) mobility ( ) resistivity ( )
1.2 1.1
5
1.0 0.9
-cm)
34.32
-3
(b)
1.1
4
Resistivity (10
2θ (degree)
2
Intensity (a.u.)
RF power
Mobility cm /V.sec
(a)
-3
Fig. 1a shows the XRD patterns of AZO thin films deposited by RF magnetron sputtering under various RF power condition. Magnetron sputtered AZO films usually have a hexagonal wurtzite structure with its preferred orientation along the c-axis perpendicular to the substrate surface [13,23]. By examining all the diffraction patterns of the AZO thin films obtained at various deposition parameters, the XRD spectra reveal that the ZnO films developed without the existence of secondary phases and clusters, and only the ZnO(0 0 0 2) diffraction plane is observed. No Al2O3 phase
21
3. Results and discussion
was found, which implies that Al atoms substitute Zn in the hexagonal lattice and Al ions may occupy the interstitial sites of ZnO or probably Al segregates to the non-crystalline region in grain boundaries and forms AlAO bond. According to the XRD patterns, the mean peak diffraction angle is related to the interplanar distance of atomic structure of AZO films by Bragg’s law. Fig. 1b reveals the peak position of ZnO(0 0 0 2) and derived c-axis constant as a function of RF power. As mentioned above, in fact, the highly incorporated Al3+ ions are not only in the substitutional sites but also in the interstitial sites. If Al3+ ions only substitute Zn2+ ions, the lattice parameter of AZO crystals decreases and the (0 0 0 2) peak shifts to high angle due to the smaller radius of Al3+ ions (0.53 Å) compared to Zn2+ ions (0.75 Å) [24]. Our results indicated that the aluminum-doped ZnO films suffered compressive stress with the increase of RF power. The compressive stress may be due to the interstitial Al atoms in the lattice of AZO films. In order to show significant influence of RF power on surface roughness of films, we measure AFM images (not shown here). The surface roughness of thin films was found to be 2.06 nm, 2.21 nm, 3.02 nm and 3.15 nm with RF power of 75 W, 100 W, 125 W and 150 W respectively. This indicates that the surface roughness increases with increase of RF power due to bombardment effect. Fig. 2 shows the dependence of the resistivity, mobility and carrier concentration on the RF power. As RF power was increased from 75 W to 150 W, the thickness of AZO films linearly increase from 150 nm to 300 nm. It can be reasoned that higher RF power means more argon ions in the plasma and the bombardment on the target is increased accordingly. Typically, the resistivity is decreased with increasing the film thickness. However, Fig. 2 shows a dissimilar behavior indicating other mechanisms should be taken into account. It can be seen that the resistivity of the film deposited at 75 W is 1.02 103 X cm. The resistivity is increased with the increase of RF power in the range from 75 to 125 W. Though there is no significant change in resistivity, a maximal value for the resistivity of 1.18 103 X cm is obtained at RF power of 125 W. A further increase in RF power causes resistivity to decrease slightly. The carrier concentration and Hall mobility with respect to the variation of the RF power are depicted in Fig. 2 as well. It can be seen that as RF power increases from 75 to 125 W, carrier concentrations decrease while Hall mobilities increase. The carrier concentration of 8.5 1020 cm3 and the Hall mobility of 8.2 cm2/V s are the minimum and maximum values for the film deposited at RF power of 125 W, which results in a maximum resistivity of film. Further increase of RF power, an opposite variation tendency oc-
Carrier concentration x 10 cm )
monitor film thickness during deposition. The chamber was evacuated to 3.0 107 Torr and the target–substrate distance was 18 cm. Prior to the deposition, the glass substrates were cleaned by the standard piranha process, rinsed by de-ionized water, and thoroughly dried by nitrogen gas. The working pressure of the sputter chamber was kept at 5 m Torr by an automatic pressure controller. All samples were deposited at room temperature. The flow rate of argon gas during the deposition was fixed at 15 sccm (standard cubic centimeters per minute), realized by a mass flow controller with an external control program, and the RF plasma sputtering power was varied from 75 W to 150 W. After the sputtering process, the film thickness was measured by a profilometer (Dektak3). The influence of deposition power on the structural, morphological and optical properties of these films was analyzed by XRD, X-ray photoelectron spectroscopy (XPS), AFM, Hall-effect measurement system and UV–visible spectrophotometer.
0.8 1.0
70
80
90 100 110 120 130 140 150 160
RF power (W) Fig. 1. (a) The XRD patterns and (b) variation of peak position of ZnO(0 0 0 2) and the calculated c-axis lattice constants of aluminum-doped ZnO films deposited at various RF power.
70
80
90 100 110 120 130 140 150 160
RF power (W) Fig. 2. Electrical resistivity (q), carrier concentration (n) and Hall mobility (l) of ZnO:Al films deposited on glass substrates at room temperature as a function of RF power.
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S.-Y. Kuo et al. / Microelectronics Reliability 50 (2010) 730–733
the mean free path length of electrons being smaller than the crystallite size. Besides, ionized impurity scattering has been considered the most determinant and appropriate mechanism in doped TCO films with carrier concentration >1020 cm3 [28]. The effect of chemisorption of oxygen on the film surface and grain boundary has to be taken into account, especially in which the films were formed at room temperature. Oxygen has been found to easily chemisorb on the film surface and grain boundaries of crystallites that capture electrons from conduction band. This allows the formation of potential barriers which would strongly scatter conduction electrons. The rougher surface of the films deposited at higher RF power resulted in higher chemisorbed oxygen atoms, and thus leading to lower carrier mobility as shown in Fig. 2. Fig. 3a and b shows the XPS analysis of Zn 2p3/2 and O 1s spectra for AZO film deposited at RF power of 150 W. Though the Al concentration is low, the Al 2p spectrum of the AZO thin film was successfully monitored. As shown in Fig. 3c, the binding energy of the AZO films is around 73.5 eV, which is lower than the binding energy (75.6 eV) of amorphous Al2O3 films. The core line of Zn 2p exhibited high symmetry and the binding energy of Zn 2p3/2 remains at 1021.8 eV, which confirms that the majority of Zn atoms remain in the same formal valence state of Zn2+ within the ZnO matrix. The component on the low binding energy side of the O 1s spectrum is attributed to O2 ions on wurtzite structure of hexagonal Zn2+ ion array, surrounded by Zn (or the substitution of Al) atoms with their full complement of nearest neighbor O2 ions. In other words, the intensity of this component is the measure of the amount of oxygen atoms in a fully oxidized stoichiometric surrounding. The high binding energy component is usually attributed to the either presence of loosely bound oxygen on the surface of the films or belongs to hydrated oxides which might incorporated from deposition chamber. It can also be associated with O2 ions in the oxygen-deficient regions within the matrix of ZnO [29]. This observation confirms the importance of bombardment effect while AZO films deposited at high RF power. The optical transmittance of the films has been measured in the wavelength range of 200–900 nm as shown in Fig. 4. The optical
curs for the carrier concentration and Hall mobility. The mobile energy of the sputtered atoms on substrate greatly depends on the RF power and it increases with the increase of RF power. Therefore, increasing RF power results in the improvement of crystallinity. This makes grains bigger in size and defects less in number, and suppressed generation of electrons by O2 vacancies and Zn interstitial atoms as well. As shown in Fig. 2, a slight increase in the resistivity of AZO films deposited from 75 W to 125 W is attributed to the improvement of crystallinity. The oxygen atoms are very sensitive to the thermalization effect, collision between moleculars, than zinc and aluminum atoms because of the smaller atomic radius and mass. At lower RF power, oxygen atoms are easily scattered out and make oxygen-deficient films. While we increase the RF power, more oxygen atoms will gain enough energy and lower the concentration of oxygen vacancies. Collisions between sputtered species might be the other cause of the decreased carrier concentration. Moreover, the resistivity of AZO films decreases slightly as we increase the RF power to 150 W. This phenomenon implies that the bombardment effects become dominant over atomic surface diffusion while the AZO film deposited at higher RF power. And the surface morphology play a key role in the electrical performance. Generally, the reciprocal mobility of the films can be expressed as
1
lH
¼
1
li
þ
1
ll
þ
1
ð1Þ
lg
where lH is Hall mobility of the films, li, ll, and lg are mobilities corresponding to ionized impurity, lattice vibration and grain boundary scattering, respectively. According to the equation of ll / 1=T, lattice vibration scattering may dominate at high temperature [25]. Since the AZO films were deposited at room temperature, it is obvious that lattice vibration scattering apparently did not play a significant role in our samples. Meanwhile, grain boundary scattering is considered as another important scattering mechanism in polycrystalline semiconductor thin film [26,27]. However, scattering by grain boundaries in our samples is neglected due to
(a)
(b)
O 1s
1015
1020
1025
1030
Binding energy (eV)
(c)
Al 2p
Relative Intensity (a.u.)
Relative Intensity (a.u.)
Relative Intensity (a.u.)
Zn 2p2/3
525
530
535
Binding energy (eV)
66 68 70 72 74 76 78 80
Binding energy (eV)
Fig. 3. XPS spectra of Zn 2p3/2 (a), O 1s (b) and (c) Al 2p binding energy of AZO thin film deposited at RF power of 150 W.
S.-Y. Kuo et al. / Microelectronics Reliability 50 (2010) 730–733
100
l2O3 ceramic targets with different RF powers. The experimental result showed that the minimum resistivity of 103 X cm and high transmittance of 87% can be obtained at low substrate temperature (room temperature). Based on the results presented in this study, the low electrical resistivity and high optical transmittance of AZO films suggested a possibility for the application in the flexible electronic devices such as transparent conducting oxide film on LEDs, solar cells, TFT-LCDs and touch panels.
bare glass substrate 75 W 100 W RF power 125 W Average transmittance 150 W (visible region 400 nm ~ 750 nm) 90
40
Transmittance (%)
Transmittance (%)
80 60
20
Acknowledgements 85
80
0 200
75
100
125
This work was supported by the National Science Council (NSC) of Taiwan under NSC-95-2112-M-182-002-MY2 and NSC-972112-M-182-004-MY3.
150
RF power (W)
300
400
500
733
600
700
800
900
Wavelength (nm) Fig. 4. The optical transmittance of AZO thin films as a function of RF power in a wide spectral range from 200 nm to 900 nm. Shown in the inset is the average transmittance in the visible region.
transmittance of the films grown at different RF power all exhibit an average transmittance in the visible range over 82% (inset of Fig. 4) and a sharp fundamental absorption edge. Fig. 4 indicates that the transmittance of AZO films on glass gradually decreases with an increasing RF power. This characteristic may be caused by thicker thickness, increased scattering, reflection and optical absorption of the films, owing to larger surface roughness and amorphous contents of the films deposited at higher RF powers. The average optical transmittance of the AZO films with the lowest resistivity of 1.02 103 X cm was 86.7%. Accordingly, the increase in surface roughness decreases the effective thickness of conducting path in film and then increases the electrical resistivity. In addition, oxygen adsorbed on the surface of crystallites traps electrons, decreases carrier concentration and also decreases the Hall mobility by increasing potential height at the surface of crystallites [30]. With increasing RF power, the deteriorated surface may result in the increase of effective surface area of films, and the number of absorption sites for oxygen causing worse resistivity. The decrease of optical transmittance of AZO film with increasing RF power also can be ascribed to the enhancement of scattering and absorption of light caused by the decrease of grain size and the increase of surface roughness. 4. Conclusion In summary, transparent and conducting ZnO films doped with Al have been deposited on corning glass by RF-sputtering ZnO:A-
References [1] Thomas DG. J Phys Chem Solids 1960;15:86. [2] Ekem N, Korkmaz S, Pat S, Balbag MZ, Cetin EN, Ozmumcu M. Int J Hydrogen Energy 2009;34:5218–22. [3] Reynolds DC, Look DC, Jogai B, Litton CW, Cantwell G, Harsch WC. Phys Rev B 1999;60:2340. [4] Lim JH, Kang CK, Kim KK, Park IK, Hwang DK, Park SJ. Adv Mater 2006;18:2720. [5] Srikant V, Clarke DR. J Appl Phys 1998;83:5447. [6] Kim DC, Kong BH, Cho HK. J Mater Sci Mater Electron 2008;19:760. [7] Xu QA, Zhang JW, Ju KR, Yang XD, Hou XJ. J Cryst Growth 2006;289:44. [8] Wang WS, Wu TT, Chou TH, Chen YY. Nanotechnology 2009;20:135503. [9] Lee JY, Lee JH, Kim HS, Lee CH, Ahn HS, Cho HK, et al. Thin Solid Films 2009;517:5157–60. [10] Fan G, Ning J, Yang J. Optics Laser Technol 2007;39:421–3. [11] Lee YH, Song MH, Ju BK, Shin DK, Oh MH. J Vac Sci Technol 1997;B15:512–5. [12] Tseng JY, Chen YT, Yang MY, Wang CY, Li PC, et al. Thin solid films 2009;517:6310–4. [13] Yang W, Liu Z, Peng DL, Zhang F, Huang H, Xie Y, et al. Appl Surf Sci 2009;255:5669–73. [14] Kuo SY, Chen WC, Lai FI, Cheng CP, Kuo HC, Wang SC, et al. J Cryst Growth 2006;287:78. [15] Lee KE, Wang MS, Kim EJ, Hahn SH. Curr Appl Phys 2009;9:683–7. [16] Li BS, Liu YC, Chu ZS, Shen DZ, Lu YM, Zhang JY, et al. J Appl Phys 2002;91:501–5. [17] Alam MJ, Cameron DC. J Vac Sci Technol 2001;A19:1642–6. [18] Fiddes AJC, Durose K, Brinkman AW, Woods J, Coates PD, Banister AJ. J Cryst Growth 1996;159:210–3. [19] Ohgaki T, Ohashi N, Kakemoto H, Wada S, Adachi Y, Haneda H, et al. J Appl Phys 2003;93:1961–5. [20] Ryu YR, Zhu S, Budai JD, Chandrasekhar HR, Miceli PF, White HW. J Appl Phys 2000;88:201–4. [21] Minami T, Ida S, Miyata T. Thin Solid Films 2002;416:92–6. [22] Kim KH, Park KC, Ma DY. J Appl Phys 1997;81:7764–72. [23] Chang JF, Wang L, Hon MH. J Cryst Growth 2000;211:93. [24] Park KC, Ma DY, Kim KH. Thin Solid Films 1997;305:201. [25] Kireev PS. Semiconductor Physics. Moscow: Mir; 1978. [26] Morris JE, Ridge MI, Bishop CA, Howson RP. J Appl Phys 1980;51:1847. [27] Chaudhuri S, Bhattacharyya J, Pal AK. Thin Solid Films 1987;148:279. [28] Ellmer K. J Phys D Appl Phys 2001;34:3097. [29] Chen M, Pei ZL, Wang X, Sun C, Wen LS. J Vac Sci Technol 2001;19:3. [30] Fu EG, Zhuang DM, Zhang G, Ming Z, Yang WF, Liu JJ. Microelectron J 2004;35:383–7.