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The effect of deposition temperature on the properties of Al-doped zinc oxide thin films
Thin Solid Films 386 Ž2001. 79᎐86
The effect of deposition temperature on the properties of Al-doped zinc oxide thin films J.F. ChangU , M.H. Hon Department of Materials Science and Engineering, National Cheng Kung Uni¨ ersity, 1 Ta Hsueh Rd., Tainan, 70101, Taiwan Received 27 January 2000; received in revised form 31 October 2000; accepted 3 December 2000
Abstract Transparent and conductive aluminum-doped zinc oxide films have been prepared by RF reactive magnetron sputtering with different substrate temperatures. The structural characteristics of the films were investigated by the X-ray diffractometry, scanning electron microscopy, atomic force microscopy and transmission electron microscopy, while the electric and optical properties of the films were studied by the Hall measurement and optical spectroscopy, respectively. It has been found that all of the films deposited were flat and smooth with a c-axis preferred orientation perpendicular to the substrate. The lowest resistivity obtained in this study was 4.16= 10y4 ⍀ cm for the films deposited at the substrate temperature of 250⬚C. By calculating the mean free path of electrons, ion impurity scattering is considered to be the dominant factor for the decrease of conductivity. Optical transmittance measurement results show a good transparency within the visible wavelength range for the films deposited at substrate temperatures above 200⬚C. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Sputtering; Optical coatings; Structural properties
1. Introduction Transparent and conductive oxide ŽTCO. films which are degenerate wide band-gap semiconductors with low specific resistance and high transparency in the visible wavelength range w1x, have found wide applications in the recent years. Al-doped ZnO ŽAZO. films have gained much attention for the TCO applications because of their comparable high optical transmittance and low electrical resistivity as compared to ITO films w2x and because of their stability under the exposure to hydrogen plasma w3x. Among the processes used to prepare ZnO:Al films, magnetron sputtering is considered to be a suitable technique due to the inherent ease with which the deposition parameters can be controlled w4x. As for sputtered films, many parameters U
Corresponding author. Tel.: q886-6-238-0208; fax: q886-6-2380208. E-mail address: [email protected] ŽJ.F. Chang..
in magnetron sputtering process including the partial pressure of oxygen, sputtering power, substrate temperature, etc. may influence the electronic and optical properties of the deposited AZO films obviously. The structural characteristics, electronic and optical properties of the AZO thin films relative to the oxygen pressure in deposition have been investigated widely w5᎐7x while the effect of substrate temperature Ts has not been studied in depth yet. Thornton w8x in the study of the sputtered metal thin films concluded that the microstructure of films would be influenced enormously by the deposition temperature. Although AZO thin films are a highly conductive oxide rather than a metal, it is believed that the deposition temperature will influence the structure and properties of the AZO films. According to this point of view, Minami et al. w9x proposed that an increase in the Hall mobility of the films improved the conductivity of the RF magnetron sputtered AZO films as the substrate temperature was increased up to 250⬚C. But both the carrier concentra-
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J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
tion and mobility were found to decrease due to a decrease in the crystallinity of the films, therefore, a further increase in substrate temperature would lead to an increase in resistivity. However, in a similar study of DC magnetron sputtered AZO films w10x, they concluded that because of the increase in mobility, the resistivity decreased as the deposition temperature was raised. Tominaga et al. w11x presumed that the decrease of the resistivity for the films deposited at a higher substrate temperature was a consequence of a deficiency of the zinc atoms promoted by zinc re-evaporated from the AZO films. From the above literature review, there remains some fundamental issues to be addressed before a complete understanding of the influence of substrate temperature on film properties is reached. In this work, the effects of substrate temperature for deposition on the structure and the electronic and optical properties of the AZO thin films will be investigated thoroughly. 2. Experiment A conventional planar RF reactive magnetron sputtering system with a 3-inch Zn-2wt.% Al target was used to prepare the films onto both Corning 1737 glass and silicon wafer substrates. The distance between the target and substrate was set at 45 mm. In order to deposit high transmittance and low resistivity films, mixed Ar q O 2 gas was introduced into the chamber and was metered by mass flow controllers for a total flow rate fixed at 15 sccm. Before deposition, the chamber was evacuated to an ultimate background pressure of 10y6 torr for 1 h and then a pre-sputtering process was employed for 10 min to clean the target surface. The experimental parameters are listed in Table 1. A conventional stylus surface roughness detector ŽAlpha-step 200. was used to measure the film thickness. The surface morphology and microstructure were investigated by scanning electron microscopy ŽFE-SEM, Philip, XL-40 FEG., atomic force microscopy ŽAFM. and transmission electron microscopy ŽTEM.. An X-ray diffractometer ŽShimadzu XD-1. with Cu᎐K ␣ radiation was used to determine crystallographic structure. For compositional analysis, X-ray photoelectron spectroscopy ŽXPS. and energy dispersive spectrometry ŽEDS. were used. The transmittance and reflectance of the films were measured by a Hitachi U-4001 specTable 1 Experimental parameters used for ZnO:Al films deposition RF power ŽW. Pressure Žtorr. Substrate temperature Ž⬚C. O2 flow fraction Ž%. Electrode distance Žmm.
100 3 = 10y3 Ambient ; 350 12 45
Fig. 1. XRD diffraction patterns of AZO films deposited at various substrate temperatures.
trophotometer in the wavelength range of 350᎐850 nm. The resistivity of the films was measured by a four-point probe system ŽNapson, RT-7. as well as a Hall measurement system with the van der Pauw geometry w12x. 3. Results and discussion 3.1. Structural characterization X-Ray diffraction patterns of the thin films deposited at various substrate temperatures are shown in Fig. 1. The one peak near 34.4⬚ revealed that the sputtered AZO was Ž0001. textured. The structure of sputtered ZnO thin films was generally polycrystalline with a c-axis preferred orientation due to the fact that the most densely packed Ž0001. planes in wurtzite ZnO have the lowest surface free energy w13x. From Fig. 1, the preferred orientation for the films as-deposited is increased with substrate temperature up to 250⬚C and the crystallinity of the films determined from the full width at half-maximum ŽFWHM. values shows the same trend. Usually, it is suggested that the crystallinity is enhanced as the substrate temperature is increased in deposition, however, it is not for the case of the deposited AZO films. According to the Thornton’s structure model w8x, the microstructure of the metal deposits
J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
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Fig. 2. Surface morphologies of the AZO thin films deposited at various substrate temperatures of Ža. ambient Žb. 200 Žc. 250 Žd. 300 and Že. 350⬚C.
depends mainly on pressure, power and substrate temperature. The sputtered AZO films are oxides rather than metals. While during the sputtering process the surface is exposed to the high-density plasma, therefore, the complication of kinetic mechanism in reactive sputtering system should be considered. Van de Pol et al. w14x pointed out that the calibrated Tm of ZnO should be at ; 1400 K instead of 2250 K and therefore, the Thornton’s structure model was believed to be valid for the sputtered ZnO films. Assuming the melting point of AZO is similar to that of bulk ZnO, Kim w15x attributed that the decrease of the AZO Ž0002. peak for the films deposited at high substrate temperatures to the transition region of the Thornton structure zone model where the columnar grains change their structure. In addition, a small deviation in Ž0002. peak from the regular position was also found when the substrate was unheated, indicating some residual stress inside the film may exist w16x. The surface morphologies clearly altered with the substrate temperature, as Fig. 2 shows. When the film was deposited under ambient substrate temperature, small clusters of conical grains distributed uniformly on the substrate which corresponded to the zone I area in Thornton structural zone model w8x due to limited mobility of the arrived atoms. An obvious improvement in crystallinity with faceted structures was obtained with increasing the substrate temperature as visible in Fig. 2c due to the enhanced mobility of atoms diffusing
to lower energy sites on the surface. No significant variation in morphology was obtained with further increase in substrate temperature above 250⬚C. To identify the dependence of surface structure characteristics on the substrate temperature, atomic force microscopy ŽAFM. perspective images are shown in Fig. 3. It can be easily observed that the films deposited for various substrate temperatures show a uniform grain size with a columnar structure. The surface roughness ŽRMS. increases to 12.08 nm at 250⬚C deposition temperature and then it decreases with further increase of substrate temperature. This indicates that the enhancement in adatom mobility for higher substrate temperature reduced the surface roughness. To analyze the effect of Ts on the grain morphology, TEM plan view and selective area diffraction micrographs of the films are shown in Fig. 4. The bright field image reveals the AZO films have a poly-nanocrystalline structure with a grain size of 20᎐30 nm when the substrate is not heated. By raising the substrate temperature, grain growth and an enhancement in crystallinity were observed. The diffraction patterns show the deposited AZO films exhibit a hexagonal structure, which indicates that the Al doping did not induce distinguishable effects in electron diffraction. Although Sieber w17x reported the presence of secondary spinel phases such as ZnO 2 and ZnAlO at grain boundaries by cross-sectional TEM, no phases other than ZnO were found by electron diffraction in this study.
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Fig. 3. AFM perspective morphologies and surface roughness of the AZO thin films deposited at various substrate temperatures of Ža. ambient Žb. 200 Žc. 250 Žd. 300 and Že. 350⬚C.
J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
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Table 2 Elemental composition of the ZnO:Al films deposited at various substrate temperatures analyzed by EDS Deposition temperature
Al at.%
ŽZn q O. at.%
Ar at.%
Ambient 200⬚C 250⬚C 300⬚C 350⬚C
5.11 4.51 5.55 6.26 5.00
94.27 94.40 94.59 93.08 94.09
0.62 1.08 0.86 0.66 0.91
3.2. Electrical properties
Fig. 4. TEM planar images and electronic diffraction patterns of the AZO thin films deposited at various substrate temperatures of Ža. ambient Žb. 200 and Žc. 250⬚C.
The average chemical composition and its depth uniformity in the film were determined by energy dispersive spectrometry ŽEDS. as well as by secondary ion mass spectrometry ŽSIMS. depth profile for the AZO films deposited on silicon substrate. Table 2 lists the concentration of elemental composition detected by EDS. Due to the limitations of the apparatus, the contents of Zn and O are combined together as a ŽZn q O. concentration. Szyszka w18x reported that increasing deposition temperature resulted in an increase in Al concentration, however, similar elemental composition for the various deposition conditions were obtained for our films. The compositional depth profile of Zn and O as detected by SIMS showed a uniform distribution in the AZO thin film and an excess Al concentration was found at the interface between the substrate and the film possibly due to the ease of Si᎐Al inter-diffusion by matrix effect.
Fig. 5 shows the dependence of the electric properties on the substrate temperature for AZO thin films deposited on the Corning 1737 glass substrates. A minimal value for the resistivity of 4.16= 10y4 ⍀ cm was obtained at a substrate temperature 250⬚C. The conductivity is a combined contribution from both the carrier concentration and the Hall mobility of donors, but they exert an opposite influence leading to the result shown in Fig. 5. The resistivity minimum for films deposited at a substrate temperature of 250⬚C was mainly caused by a maximum in the carrier concentration. Minami et al. w19x has ascribed the decrease of resistivity with increasing temperature for substrate temperature below 250⬚C as being due to the improvement of film crystallinity, while the dramatic segregation of Al 2 O 3 at grain boundaries in the films increased the resistivity above 250⬚C w10x. Tominaga et al. w11x suggested that due to the rapid increase of zinc vapor pressure with substrate temperature, resputtering or re-evaporation of surplus zinc atoms should be considered in determining the properties of the AZO films. Thus, excess Al incorporated in the films may not only not contribute to the electronic conductivity, but conversely it may act as scattering centers which cause resistivity to increase. However, this does not seem to be a major factor for the resistivity of the films re-
Fig. 5. Resistivity Ž ., carrier concentration Ž n. and mobility Ž . for AZO thin-films as a function of substrate temperature for deposition.
J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
84
ported upon here because the EDS analysis shows a similar ratio of Zn and Al in the films. The full width at half-maximum of the X-ray spectra with respect to the variation of the substrate temperature for deposition is summarized in Fig. 6. From which the crystallinity seems to be a significant factor to the resistivity of the films w15x, though the increase in FWHM at the lowest temperature may be somewhat due to particle size induce broadening. In order to relate the fundamental electronic transport behavior to the dependence of resistivity on the substrate temperature, a scattering mechanism should be emphasized. In undoped polycrystalline ZnO the grain boundaries contain surface states due to defects or absorbed ions leading to a situation in which the electric conduction can be viewed as a function of the grain boundary barrier height w20x. For these materials, grain boundaries play an important role in determining the characteristics of carrier scattering. But for small dopant content of Al, the dopant atoms are effectively incorporated substitutionally at Zn sites in the ZnO lattice. Thus, ionized impurity scattering becomes dominant in comparison to grain boundary scattering w19,21x. The electronic mean free path Ž L. in this study was calculated using the highly degenerate electron gas model w22x as: L s Ž 3 2 .
1r3
Ž hre 2 . y1 ny2r3
Fig. 7. Binding energy of oxygen ion in the films as deposited at various substrate temperatures analyzed by XPS.
Ž1.
The mean free path of electrons demonstrates a similar dependence on the resistivity as shown in Fig. 6. This indicated that more scattering centers are formed for films deposited at higher substrate temperatures. Comparing the electron mean free path with the grain sizes of the AZO films, it is concluded that intra-grain scattering dominates the scattering, that is, ionized impurity scattering seems to play a dominant role in
Fig. 6. FWHM, mean free path Ž L. and resistivity Ž . of the AZO films as a function of substrate temperatures for deposition.
our films. In a similar result reported by Minami w19x, who stated that in the impurity-doped ZnO films with carrier concentrations of 10 20 ᎐10 21 cmy3 , the mobility is mainly dominated by the ionized impurity scattering. For the different material systems of CdIn 2 O4 and ITO thin films similar conclusions had also been drawn by Zakrzewska w23x and Tahar w24x, respectively. But since the scattering mechanism is very complex, we cannot exclude the other scattering mechanisms completely. Fig. 7 shows the binding energy of oxygen atom in the films analyzed by XPS. The peak broadening and separation in the oxygen-binding spectrum were observed for films deposited at a temperature above 250⬚C. The binding energy for oxygen in the ZnO structure is 531.3 eV, while it is 531.6 eV in the Al 2 O 3 structure w25x. It cannot be distinguished clearly between these two peaks. Nevertheless, the peaks located on the higher binding energy side are considered to be due to oxygen impurities other than lattice oxygen. Although many types of oxygen impurities, from physically absorbed to chemisorbed molecules, can be found in the literature. It is considered that the chemisorbed oxygen impurities such as O 2y, Oy and Oy 2 are responsible for the higher binding energy peak because before XPS measurements were carried out Ar ions pre-sputtering was used to clean the surface of physisorbed surface contaminants for approximately 3
J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
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The optical band-gap, Eopt , of the films can be determined from the absorption coefficient of the films using the relation for parabolic bands w28x: ␣ s ␣ d Ž h¨ y Eopt .
Fig. 8. Measured optical transmittance for the films deposited at various substrate temperatures.
1rN
Ž3.
where, h is the photon energy, N is 2 for a direct allowed transition and ␣ d is a constant. The energy gap shown in Fig. 9 was obtained by extrapolating the linear absorption edge part of the curve using Eq. Ž3.. The AZO films deposited at 250⬚C of Ts show the largest optical band-gap corresponding to the highest carrier concentration of the films. This is due to the Burstein᎐Moss effect w29,30x which described the blue-shifting of the absorption edge of a degenerate semiconductor with an increasing carrier concentration. The optical band-gap of films deposited under other conditions did not show significant shifts with similar levels of carrier concentration. 4. Conclusion
min. Thus, the decrease in conductivity at higher substrate temperatures is attributed to an increase in chemisorbed oxygen resulting in the electron traps. A similar peak-broadening phenomenon was also found in SnO 2 thin films w26x. 3.3. Optical properties The optical transmittance and reflectance of the AZO thin films were determined by a spectrophotometer within the wavelength from 350 to 850 nm. For transmission measurements the AZO films coated on the glass substrates were irradiated at a perpendicular angle of incidence with air being the reference. The reflection measurements were carried out at 45⬚ absolute reflection. Fig. 8 shows the transmittance spectrum of films deposited at various substrate temperatures on Corning 1737 glass. All of the films deposited above 200⬚C substrate temperature demonstrate transmittance of above 80% in the range of the visible spectrum, but low transmittance was found for the film deposited on an unheated substrate. This may be due to the small adatom mobility during film deposition at low substrate temperature, which leads the film morphology consisting of tapered crystallites separated by voids as a result of intergrain shadowing w8x. Consequently, a poor transmittance was obtained. The optical absorption coefficient, ␣ , of the films was calculated from the transmittance T and reflectance R with the relation w27x: 2
T s Ž 1 y R . exp Ž y␣ d . ,
Transparent and conductive ZnO:Al films were deposited successfully by RF reactive magnetron sputtering system. The effect of substrate temperature on the structure, electronic and optical properties of AZO thin films was investigated. All of the AZO thin films deposited in the experiment demonstrate a c-axis preferred orientation with the best crystallinity obtained at 250⬚C. Flat and smooth surface morphologies with a RMS roughness less than 12.08 nm were found for the as-deposited films. The lowest resistivity of 4.16= 10y4 ⍀ cm was obtained for a substrate temperature of 250⬚C due to films deposited under these conditions having the highest carrier concentration of 1.48= 10 21
Ž2.
where T and R are the transmittance and reflectance of the films, respectively, and d is the film thickness.
Fig. 9. Optical band-gap of the films deposited at various substrate temperatures.
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J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
cmy3 . By comparing the grain size and the carrier mean free path, in-grain scattering is concluded to be the major factor in determining the scattering mechanism. All of the films deposited above 200⬚C of substrate temperature show a visible transmittance of above 80% with an optical band-gap ranging from 3.2 to 3.5 eV. Acknowledgements The authors wish to thank the National Science Council of Taiwan for partial financial support under the project NSC89-2218-E006-017. References w1x H.L. Hartnagel, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Paston Press, UK, 1995, p. 306. w2x J.-H. Lan, J. Kanicki, 1995 2nd International Workshop on Active Matrix Liquid Crystal Displays, Pennsylvania, USA, September 25᎐26, 1995, Workshop Proceeding Ž1995. 54. w3x H.C. Weller, R.H. Mauch, G.H. Bauer, Sol. Energ. Mat. Sol. C 27 Ž1992. 217. w4x L.J. Meng, M.P. dos Santos, Thin Solid Films 250 Ž1994. 26. w5x K. Ellmer, K. Diesner, R. Wendt, S. Fiechter, Solid State Phenom. 51᎐52 Ž1996. 541. w6x S. Brehme, F. Fenske, W. Fuhs, E. Nebauer, M. Poschenrieder, B. Selle, I. Sieber, Thin Solid Films 342 Ž1999. 163. w7x H. Sato, T. Minami, S. Takata, Thin Solid Films 220 Ž1992. 327. w8x J.A. Thornton, J. Vac. Sci. Technol. 11 Ž1974. 666. w9x T. Minami, H. Sato, T. Sonoda, H. Nanto, S. Takata, Thin Solid Films 171 Ž1989. 307. w10x T. Minami, H. Sato, H. Imamoto, S. Takata, Jpn. J. Appl. Phys. 31 Ž1992. L257.
w11x K. Tominaga, M. Kataoka, H. Manabe, T. Ueda, I. Mori, Thin Solid Films 290r291 Ž1996. 84. w12x L.J. van der Pauw, Philips Res. Repts. 13 Ž1958. 1. w13x Y.E. Lee, Y.J. Kim, H.J. Kim, J. Mater. Res. 13 Ž1998. 1260. w14x F.C.M. van de Pol, F.R. Blom, Th.J.A. Popma, Thin Solid Films 204 Ž1991. 349. w15x K.H. Kim, K.C. Park, D.Y. Ma, J. Appl. Phys. 81 Ž1997. 7764. w16x E.M. Bachari, G. Baud, S. Ben Amor, M. Jacquet, Thin Solid Films 348 Ž1999. 165. w17x I. Sieber, N. Wanderka, I. Urban, I. Dorfel, E. Schierhorn, F. ¨ Fenske, W. Fuhs, Thin Solid Films 330 Ž1998. 108. w18x B. Szyszka, Thin Solid Films 351 Ž1999. 163. w19x T. Minami, H. Sato, K. Ohashi, T. Tomofuji, S. Takata, J. Cryst. Growth 117 Ž1992. 370. w20x F.R. Blom, F.C.M. van de Pol, G. Bauhuis, Th.J.A. Popma, Thin Solid Films 204 Ž1991. 365. w21x S. Ghosh, A. Sarkar, S. Chaudhuri, A.K. Pal, Thin Solid Films 205 Ž1991. 64. w22x C. Kittel, Introduction to Solid State Physics, John Wiley and Sons ŽSEA., Singapore, 1986, chap. 6. w23x K. Zakrzewska, T. Pisarkiewicz, A. Czapla, Phys. Status. Solidi. A 99 Ž1987. 141. w24x R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, J. Appl. Phys. 82 Ž1997. 865. w25x K.H. Yoon, J.W. Choi, D.H. Lee, Thin Solid Films 302 Ž1997. 116. w26x H. Yan, G.H. Chen, W.K. Man, S.P. Wong, R.W.M. Kwok, Thin Solid Films 326 Ž1998. 88. w27x M. Digiulio, G. Micocci, R. Rella, P. Siciliano, A. Tepore, Thin Solid Films 148 Ž1987. 273. w28x D. Jiles, Introduction to the Electronic Properties of Materials, Chapman and Hall, 1994, chap. 9. w29x E. Burstein, Phys. Rev. 93 Ž1954. 455. w30x T.S. Moss, Proc. Phys. Soc. London Ser. B 67 Ž1954. 775.
The effect of deposition temperature on the properties of Al-doped zinc oxide thin films J.F. ChangU , M.H. Hon Department of Materials Science and Engineering, National Cheng Kung Uni¨ ersity, 1 Ta Hsueh Rd., Tainan, 70101, Taiwan Received 27 January 2000; received in revised form 31 October 2000; accepted 3 December 2000
Abstract Transparent and conductive aluminum-doped zinc oxide films have been prepared by RF reactive magnetron sputtering with different substrate temperatures. The structural characteristics of the films were investigated by the X-ray diffractometry, scanning electron microscopy, atomic force microscopy and transmission electron microscopy, while the electric and optical properties of the films were studied by the Hall measurement and optical spectroscopy, respectively. It has been found that all of the films deposited were flat and smooth with a c-axis preferred orientation perpendicular to the substrate. The lowest resistivity obtained in this study was 4.16= 10y4 ⍀ cm for the films deposited at the substrate temperature of 250⬚C. By calculating the mean free path of electrons, ion impurity scattering is considered to be the dominant factor for the decrease of conductivity. Optical transmittance measurement results show a good transparency within the visible wavelength range for the films deposited at substrate temperatures above 200⬚C. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Sputtering; Optical coatings; Structural properties
1. Introduction Transparent and conductive oxide ŽTCO. films which are degenerate wide band-gap semiconductors with low specific resistance and high transparency in the visible wavelength range w1x, have found wide applications in the recent years. Al-doped ZnO ŽAZO. films have gained much attention for the TCO applications because of their comparable high optical transmittance and low electrical resistivity as compared to ITO films w2x and because of their stability under the exposure to hydrogen plasma w3x. Among the processes used to prepare ZnO:Al films, magnetron sputtering is considered to be a suitable technique due to the inherent ease with which the deposition parameters can be controlled w4x. As for sputtered films, many parameters U
Corresponding author. Tel.: q886-6-238-0208; fax: q886-6-2380208. E-mail address: [email protected] ŽJ.F. Chang..
in magnetron sputtering process including the partial pressure of oxygen, sputtering power, substrate temperature, etc. may influence the electronic and optical properties of the deposited AZO films obviously. The structural characteristics, electronic and optical properties of the AZO thin films relative to the oxygen pressure in deposition have been investigated widely w5᎐7x while the effect of substrate temperature Ts has not been studied in depth yet. Thornton w8x in the study of the sputtered metal thin films concluded that the microstructure of films would be influenced enormously by the deposition temperature. Although AZO thin films are a highly conductive oxide rather than a metal, it is believed that the deposition temperature will influence the structure and properties of the AZO films. According to this point of view, Minami et al. w9x proposed that an increase in the Hall mobility of the films improved the conductivity of the RF magnetron sputtered AZO films as the substrate temperature was increased up to 250⬚C. But both the carrier concentra-
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 8 9 1 - 5
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J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
tion and mobility were found to decrease due to a decrease in the crystallinity of the films, therefore, a further increase in substrate temperature would lead to an increase in resistivity. However, in a similar study of DC magnetron sputtered AZO films w10x, they concluded that because of the increase in mobility, the resistivity decreased as the deposition temperature was raised. Tominaga et al. w11x presumed that the decrease of the resistivity for the films deposited at a higher substrate temperature was a consequence of a deficiency of the zinc atoms promoted by zinc re-evaporated from the AZO films. From the above literature review, there remains some fundamental issues to be addressed before a complete understanding of the influence of substrate temperature on film properties is reached. In this work, the effects of substrate temperature for deposition on the structure and the electronic and optical properties of the AZO thin films will be investigated thoroughly. 2. Experiment A conventional planar RF reactive magnetron sputtering system with a 3-inch Zn-2wt.% Al target was used to prepare the films onto both Corning 1737 glass and silicon wafer substrates. The distance between the target and substrate was set at 45 mm. In order to deposit high transmittance and low resistivity films, mixed Ar q O 2 gas was introduced into the chamber and was metered by mass flow controllers for a total flow rate fixed at 15 sccm. Before deposition, the chamber was evacuated to an ultimate background pressure of 10y6 torr for 1 h and then a pre-sputtering process was employed for 10 min to clean the target surface. The experimental parameters are listed in Table 1. A conventional stylus surface roughness detector ŽAlpha-step 200. was used to measure the film thickness. The surface morphology and microstructure were investigated by scanning electron microscopy ŽFE-SEM, Philip, XL-40 FEG., atomic force microscopy ŽAFM. and transmission electron microscopy ŽTEM.. An X-ray diffractometer ŽShimadzu XD-1. with Cu᎐K ␣ radiation was used to determine crystallographic structure. For compositional analysis, X-ray photoelectron spectroscopy ŽXPS. and energy dispersive spectrometry ŽEDS. were used. The transmittance and reflectance of the films were measured by a Hitachi U-4001 specTable 1 Experimental parameters used for ZnO:Al films deposition RF power ŽW. Pressure Žtorr. Substrate temperature Ž⬚C. O2 flow fraction Ž%. Electrode distance Žmm.
100 3 = 10y3 Ambient ; 350 12 45
Fig. 1. XRD diffraction patterns of AZO films deposited at various substrate temperatures.
trophotometer in the wavelength range of 350᎐850 nm. The resistivity of the films was measured by a four-point probe system ŽNapson, RT-7. as well as a Hall measurement system with the van der Pauw geometry w12x. 3. Results and discussion 3.1. Structural characterization X-Ray diffraction patterns of the thin films deposited at various substrate temperatures are shown in Fig. 1. The one peak near 34.4⬚ revealed that the sputtered AZO was Ž0001. textured. The structure of sputtered ZnO thin films was generally polycrystalline with a c-axis preferred orientation due to the fact that the most densely packed Ž0001. planes in wurtzite ZnO have the lowest surface free energy w13x. From Fig. 1, the preferred orientation for the films as-deposited is increased with substrate temperature up to 250⬚C and the crystallinity of the films determined from the full width at half-maximum ŽFWHM. values shows the same trend. Usually, it is suggested that the crystallinity is enhanced as the substrate temperature is increased in deposition, however, it is not for the case of the deposited AZO films. According to the Thornton’s structure model w8x, the microstructure of the metal deposits
J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
81
Fig. 2. Surface morphologies of the AZO thin films deposited at various substrate temperatures of Ža. ambient Žb. 200 Žc. 250 Žd. 300 and Že. 350⬚C.
depends mainly on pressure, power and substrate temperature. The sputtered AZO films are oxides rather than metals. While during the sputtering process the surface is exposed to the high-density plasma, therefore, the complication of kinetic mechanism in reactive sputtering system should be considered. Van de Pol et al. w14x pointed out that the calibrated Tm of ZnO should be at ; 1400 K instead of 2250 K and therefore, the Thornton’s structure model was believed to be valid for the sputtered ZnO films. Assuming the melting point of AZO is similar to that of bulk ZnO, Kim w15x attributed that the decrease of the AZO Ž0002. peak for the films deposited at high substrate temperatures to the transition region of the Thornton structure zone model where the columnar grains change their structure. In addition, a small deviation in Ž0002. peak from the regular position was also found when the substrate was unheated, indicating some residual stress inside the film may exist w16x. The surface morphologies clearly altered with the substrate temperature, as Fig. 2 shows. When the film was deposited under ambient substrate temperature, small clusters of conical grains distributed uniformly on the substrate which corresponded to the zone I area in Thornton structural zone model w8x due to limited mobility of the arrived atoms. An obvious improvement in crystallinity with faceted structures was obtained with increasing the substrate temperature as visible in Fig. 2c due to the enhanced mobility of atoms diffusing
to lower energy sites on the surface. No significant variation in morphology was obtained with further increase in substrate temperature above 250⬚C. To identify the dependence of surface structure characteristics on the substrate temperature, atomic force microscopy ŽAFM. perspective images are shown in Fig. 3. It can be easily observed that the films deposited for various substrate temperatures show a uniform grain size with a columnar structure. The surface roughness ŽRMS. increases to 12.08 nm at 250⬚C deposition temperature and then it decreases with further increase of substrate temperature. This indicates that the enhancement in adatom mobility for higher substrate temperature reduced the surface roughness. To analyze the effect of Ts on the grain morphology, TEM plan view and selective area diffraction micrographs of the films are shown in Fig. 4. The bright field image reveals the AZO films have a poly-nanocrystalline structure with a grain size of 20᎐30 nm when the substrate is not heated. By raising the substrate temperature, grain growth and an enhancement in crystallinity were observed. The diffraction patterns show the deposited AZO films exhibit a hexagonal structure, which indicates that the Al doping did not induce distinguishable effects in electron diffraction. Although Sieber w17x reported the presence of secondary spinel phases such as ZnO 2 and ZnAlO at grain boundaries by cross-sectional TEM, no phases other than ZnO were found by electron diffraction in this study.
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J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
Fig. 3. AFM perspective morphologies and surface roughness of the AZO thin films deposited at various substrate temperatures of Ža. ambient Žb. 200 Žc. 250 Žd. 300 and Že. 350⬚C.
J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
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Table 2 Elemental composition of the ZnO:Al films deposited at various substrate temperatures analyzed by EDS Deposition temperature
Al at.%
ŽZn q O. at.%
Ar at.%
Ambient 200⬚C 250⬚C 300⬚C 350⬚C
5.11 4.51 5.55 6.26 5.00
94.27 94.40 94.59 93.08 94.09
0.62 1.08 0.86 0.66 0.91
3.2. Electrical properties
Fig. 4. TEM planar images and electronic diffraction patterns of the AZO thin films deposited at various substrate temperatures of Ža. ambient Žb. 200 and Žc. 250⬚C.
The average chemical composition and its depth uniformity in the film were determined by energy dispersive spectrometry ŽEDS. as well as by secondary ion mass spectrometry ŽSIMS. depth profile for the AZO films deposited on silicon substrate. Table 2 lists the concentration of elemental composition detected by EDS. Due to the limitations of the apparatus, the contents of Zn and O are combined together as a ŽZn q O. concentration. Szyszka w18x reported that increasing deposition temperature resulted in an increase in Al concentration, however, similar elemental composition for the various deposition conditions were obtained for our films. The compositional depth profile of Zn and O as detected by SIMS showed a uniform distribution in the AZO thin film and an excess Al concentration was found at the interface between the substrate and the film possibly due to the ease of Si᎐Al inter-diffusion by matrix effect.
Fig. 5 shows the dependence of the electric properties on the substrate temperature for AZO thin films deposited on the Corning 1737 glass substrates. A minimal value for the resistivity of 4.16= 10y4 ⍀ cm was obtained at a substrate temperature 250⬚C. The conductivity is a combined contribution from both the carrier concentration and the Hall mobility of donors, but they exert an opposite influence leading to the result shown in Fig. 5. The resistivity minimum for films deposited at a substrate temperature of 250⬚C was mainly caused by a maximum in the carrier concentration. Minami et al. w19x has ascribed the decrease of resistivity with increasing temperature for substrate temperature below 250⬚C as being due to the improvement of film crystallinity, while the dramatic segregation of Al 2 O 3 at grain boundaries in the films increased the resistivity above 250⬚C w10x. Tominaga et al. w11x suggested that due to the rapid increase of zinc vapor pressure with substrate temperature, resputtering or re-evaporation of surplus zinc atoms should be considered in determining the properties of the AZO films. Thus, excess Al incorporated in the films may not only not contribute to the electronic conductivity, but conversely it may act as scattering centers which cause resistivity to increase. However, this does not seem to be a major factor for the resistivity of the films re-
Fig. 5. Resistivity Ž ., carrier concentration Ž n. and mobility Ž . for AZO thin-films as a function of substrate temperature for deposition.
J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
84
ported upon here because the EDS analysis shows a similar ratio of Zn and Al in the films. The full width at half-maximum of the X-ray spectra with respect to the variation of the substrate temperature for deposition is summarized in Fig. 6. From which the crystallinity seems to be a significant factor to the resistivity of the films w15x, though the increase in FWHM at the lowest temperature may be somewhat due to particle size induce broadening. In order to relate the fundamental electronic transport behavior to the dependence of resistivity on the substrate temperature, a scattering mechanism should be emphasized. In undoped polycrystalline ZnO the grain boundaries contain surface states due to defects or absorbed ions leading to a situation in which the electric conduction can be viewed as a function of the grain boundary barrier height w20x. For these materials, grain boundaries play an important role in determining the characteristics of carrier scattering. But for small dopant content of Al, the dopant atoms are effectively incorporated substitutionally at Zn sites in the ZnO lattice. Thus, ionized impurity scattering becomes dominant in comparison to grain boundary scattering w19,21x. The electronic mean free path Ž L. in this study was calculated using the highly degenerate electron gas model w22x as: L s Ž 3 2 .
1r3
Ž hre 2 . y1 ny2r3
Fig. 7. Binding energy of oxygen ion in the films as deposited at various substrate temperatures analyzed by XPS.
Ž1.
The mean free path of electrons demonstrates a similar dependence on the resistivity as shown in Fig. 6. This indicated that more scattering centers are formed for films deposited at higher substrate temperatures. Comparing the electron mean free path with the grain sizes of the AZO films, it is concluded that intra-grain scattering dominates the scattering, that is, ionized impurity scattering seems to play a dominant role in
Fig. 6. FWHM, mean free path Ž L. and resistivity Ž . of the AZO films as a function of substrate temperatures for deposition.
our films. In a similar result reported by Minami w19x, who stated that in the impurity-doped ZnO films with carrier concentrations of 10 20 ᎐10 21 cmy3 , the mobility is mainly dominated by the ionized impurity scattering. For the different material systems of CdIn 2 O4 and ITO thin films similar conclusions had also been drawn by Zakrzewska w23x and Tahar w24x, respectively. But since the scattering mechanism is very complex, we cannot exclude the other scattering mechanisms completely. Fig. 7 shows the binding energy of oxygen atom in the films analyzed by XPS. The peak broadening and separation in the oxygen-binding spectrum were observed for films deposited at a temperature above 250⬚C. The binding energy for oxygen in the ZnO structure is 531.3 eV, while it is 531.6 eV in the Al 2 O 3 structure w25x. It cannot be distinguished clearly between these two peaks. Nevertheless, the peaks located on the higher binding energy side are considered to be due to oxygen impurities other than lattice oxygen. Although many types of oxygen impurities, from physically absorbed to chemisorbed molecules, can be found in the literature. It is considered that the chemisorbed oxygen impurities such as O 2y, Oy and Oy 2 are responsible for the higher binding energy peak because before XPS measurements were carried out Ar ions pre-sputtering was used to clean the surface of physisorbed surface contaminants for approximately 3
J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
85
The optical band-gap, Eopt , of the films can be determined from the absorption coefficient of the films using the relation for parabolic bands w28x: ␣ s ␣ d Ž h¨ y Eopt .
Fig. 8. Measured optical transmittance for the films deposited at various substrate temperatures.
1rN
Ž3.
where, h is the photon energy, N is 2 for a direct allowed transition and ␣ d is a constant. The energy gap shown in Fig. 9 was obtained by extrapolating the linear absorption edge part of the curve using Eq. Ž3.. The AZO films deposited at 250⬚C of Ts show the largest optical band-gap corresponding to the highest carrier concentration of the films. This is due to the Burstein᎐Moss effect w29,30x which described the blue-shifting of the absorption edge of a degenerate semiconductor with an increasing carrier concentration. The optical band-gap of films deposited under other conditions did not show significant shifts with similar levels of carrier concentration. 4. Conclusion
min. Thus, the decrease in conductivity at higher substrate temperatures is attributed to an increase in chemisorbed oxygen resulting in the electron traps. A similar peak-broadening phenomenon was also found in SnO 2 thin films w26x. 3.3. Optical properties The optical transmittance and reflectance of the AZO thin films were determined by a spectrophotometer within the wavelength from 350 to 850 nm. For transmission measurements the AZO films coated on the glass substrates were irradiated at a perpendicular angle of incidence with air being the reference. The reflection measurements were carried out at 45⬚ absolute reflection. Fig. 8 shows the transmittance spectrum of films deposited at various substrate temperatures on Corning 1737 glass. All of the films deposited above 200⬚C substrate temperature demonstrate transmittance of above 80% in the range of the visible spectrum, but low transmittance was found for the film deposited on an unheated substrate. This may be due to the small adatom mobility during film deposition at low substrate temperature, which leads the film morphology consisting of tapered crystallites separated by voids as a result of intergrain shadowing w8x. Consequently, a poor transmittance was obtained. The optical absorption coefficient, ␣ , of the films was calculated from the transmittance T and reflectance R with the relation w27x: 2
T s Ž 1 y R . exp Ž y␣ d . ,
Transparent and conductive ZnO:Al films were deposited successfully by RF reactive magnetron sputtering system. The effect of substrate temperature on the structure, electronic and optical properties of AZO thin films was investigated. All of the AZO thin films deposited in the experiment demonstrate a c-axis preferred orientation with the best crystallinity obtained at 250⬚C. Flat and smooth surface morphologies with a RMS roughness less than 12.08 nm were found for the as-deposited films. The lowest resistivity of 4.16= 10y4 ⍀ cm was obtained for a substrate temperature of 250⬚C due to films deposited under these conditions having the highest carrier concentration of 1.48= 10 21
Ž2.
where T and R are the transmittance and reflectance of the films, respectively, and d is the film thickness.
Fig. 9. Optical band-gap of the films deposited at various substrate temperatures.
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J.F. Chang, M.H. Hon r Thin Solid Films 386 (2001) 79᎐86
cmy3 . By comparing the grain size and the carrier mean free path, in-grain scattering is concluded to be the major factor in determining the scattering mechanism. All of the films deposited above 200⬚C of substrate temperature show a visible transmittance of above 80% with an optical band-gap ranging from 3.2 to 3.5 eV. Acknowledgements The authors wish to thank the National Science Council of Taiwan for partial financial support under the project NSC89-2218-E006-017. References w1x H.L. Hartnagel, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Paston Press, UK, 1995, p. 306. w2x J.-H. Lan, J. Kanicki, 1995 2nd International Workshop on Active Matrix Liquid Crystal Displays, Pennsylvania, USA, September 25᎐26, 1995, Workshop Proceeding Ž1995. 54. w3x H.C. Weller, R.H. Mauch, G.H. Bauer, Sol. Energ. Mat. Sol. C 27 Ž1992. 217. w4x L.J. Meng, M.P. dos Santos, Thin Solid Films 250 Ž1994. 26. w5x K. Ellmer, K. Diesner, R. Wendt, S. Fiechter, Solid State Phenom. 51᎐52 Ž1996. 541. w6x S. Brehme, F. Fenske, W. Fuhs, E. Nebauer, M. Poschenrieder, B. Selle, I. Sieber, Thin Solid Films 342 Ž1999. 163. w7x H. Sato, T. Minami, S. Takata, Thin Solid Films 220 Ž1992. 327. w8x J.A. Thornton, J. Vac. Sci. Technol. 11 Ž1974. 666. w9x T. Minami, H. Sato, T. Sonoda, H. Nanto, S. Takata, Thin Solid Films 171 Ž1989. 307. w10x T. Minami, H. Sato, H. Imamoto, S. Takata, Jpn. J. Appl. Phys. 31 Ž1992. L257.
w11x K. Tominaga, M. Kataoka, H. Manabe, T. Ueda, I. Mori, Thin Solid Films 290r291 Ž1996. 84. w12x L.J. van der Pauw, Philips Res. Repts. 13 Ž1958. 1. w13x Y.E. Lee, Y.J. Kim, H.J. Kim, J. Mater. Res. 13 Ž1998. 1260. w14x F.C.M. van de Pol, F.R. Blom, Th.J.A. Popma, Thin Solid Films 204 Ž1991. 349. w15x K.H. Kim, K.C. Park, D.Y. Ma, J. Appl. Phys. 81 Ž1997. 7764. w16x E.M. Bachari, G. Baud, S. Ben Amor, M. Jacquet, Thin Solid Films 348 Ž1999. 165. w17x I. Sieber, N. Wanderka, I. Urban, I. Dorfel, E. Schierhorn, F. ¨ Fenske, W. Fuhs, Thin Solid Films 330 Ž1998. 108. w18x B. Szyszka, Thin Solid Films 351 Ž1999. 163. w19x T. Minami, H. Sato, K. Ohashi, T. Tomofuji, S. Takata, J. Cryst. Growth 117 Ž1992. 370. w20x F.R. Blom, F.C.M. van de Pol, G. Bauhuis, Th.J.A. Popma, Thin Solid Films 204 Ž1991. 365. w21x S. Ghosh, A. Sarkar, S. Chaudhuri, A.K. Pal, Thin Solid Films 205 Ž1991. 64. w22x C. Kittel, Introduction to Solid State Physics, John Wiley and Sons ŽSEA., Singapore, 1986, chap. 6. w23x K. Zakrzewska, T. Pisarkiewicz, A. Czapla, Phys. Status. Solidi. A 99 Ž1987. 141. w24x R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, J. Appl. Phys. 82 Ž1997. 865. w25x K.H. Yoon, J.W. Choi, D.H. Lee, Thin Solid Films 302 Ž1997. 116. w26x H. Yan, G.H. Chen, W.K. Man, S.P. Wong, R.W.M. Kwok, Thin Solid Films 326 Ž1998. 88. w27x M. Digiulio, G. Micocci, R. Rella, P. Siciliano, A. Tepore, Thin Solid Films 148 Ž1987. 273. w28x D. Jiles, Introduction to the Electronic Properties of Materials, Chapman and Hall, 1994, chap. 9. w29x E. Burstein, Phys. Rev. 93 Ž1954. 455. w30x T.S. Moss, Proc. Phys. Soc. London Ser. B 67 Ž1954. 775.