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Effect of substrate temperature on structural and optoelectrical properties of silver doped zinc oxide thin films
ARTICLE IN PRESS Physica E 42 (2010) 2000–2004
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Effect of substrate temperature on structural and optoelectrical properties of silver doped zinc oxide thin films R.K. Gupta n, K. Ghosh, P.K. Kahol Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO 65897, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 7 January 2010 Received in revised form 11 February 2010 Accepted 8 March 2010 Available online 18 March 2010
Silver doped zinc oxide (ZnAgO) thin films were grown on quartz substrate using pulsed laser deposition technique. The effect of substrate temperature on structural, optical, and electrical properties was studied. The films grown at low temperatures were amorphous in nature, while films grown at high temperatures were crystalline. It is observed that the films are highly oriented along c-axis. The surface roughness of these films is observed to increase with substrate temperature; on the other hand optical transmittance is almost independent of substrate temperature. The mobility of the films increases with an increase in substrate temperature. The highest Hall mobility of 62 cm2/V s is observed for the film grown at 600 1C. These highly transparent and high mobility films could be used as bottom electrodes in optoelectronic devices. & 2010 Elsevier B.V. All rights reserved.
Keywords: Zinc oxide Pulsed laser deposition Transparent conducting oxide Mobility
1. Introduction ZnO has attracted a broad scientific interest because of its direct wide band gap (3.4 eV) [1]. ZnO is highly transparent, inexpensive, non-toxic, and chemically stable. These unique physical and chemical properties make it useful for many applications such as transparent conducting electrodes, optoelectronic devices, solar cells, thin film transistors, and gas sensors [2]. ZnO is an n-type semiconductor due to presence of interstitial zinc and oxygen deficiency. However, n-type conductivity is easily improved by substituting trivalent cations such as Al, Ga, and In on the Zn site. However, the realization of p-type conductivity (doping ZnO by either group I elements for Zn sites or group V elements for O sites) is rather difficult due to problems such as self compensating effect, deep acceptor level, and solubility of the acceptor dopants [3]. Silver is a good candidate for ZnO doping as it is a good electrical conductor with relatively lower optical absorption coefficient in the visible region [4]. It is reported that silver doping can modify the electrical and optical properties of ZnO films [5–8]. Silver doped ZnO films show some unique electrical properties such as semi-insulating, n-type and p-type behavior depending on growth conditions [5]. It is demonstrated that silver doped ZnO films could be used as transparent conducting electrodes for optoelectronic applications [9].
The effect of silver doping on electrical properties of ZnO films has been studied [10]. It is reported that crystallinity, transmittance, and electrical conductivity decrease with the amount of silver doping. A minimum resistivity of 6 10 3 Ocm was obtained for sample containing 0.7 at% of silver. Houng and Huang [11] have reported the lowest electrical resistivity of silver embedded aluminum doped ZnO films as 4.2 10 4 O cm. Khomchenko et al. [12] have fabricated silver doped ZnO films using metalorganic chemical vapour deposition method. The surface roughness of these films was 7 nm, which is higher to use as bottom transparent electrode. The effect of post-annealing on properties of RF reactive magnetron sputtering grown silver doped ZnO is also studied [13]. It is seen that as grown films are insulating in nature but after annealing in oxygen atmosphere at high temperature showed resistivity of 152 O cm, carrier concentration of 2.24 1016 cm 3, and Hall mobility of 1.83 cm2/V s. The above literature survey indicates that the electrical, structural, and optical properties of silver doped ZnO films largely depend on growth conditions as well as techniques. In this communication, the effect of substrate temperature on structural, optical, and electrical properties of silver doped ZnO films is studied in detail. These films are grown by pulsed laser deposition and characterized depending on deposition temperatures.
2. Experimental details n
Corresponding author. Tel.: + 1 417 836 6298; fax: + 1 417 836 6226. E-mail address: [email protected] (R.K. Gupta).
1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.03.010
A solid state reaction method is used to prepare silver doped zinc oxide (ZnAgO) films with 2.0 at% of silver. Required amounts
ARTICLE IN PRESS R.K. Gupta et al. / Physica E 42 (2010) 2000–2004
2001
of ZnO (Alfa Aesar, USA) and silver (Alfa Aesar, USA) were taken by molecular weight and mixed thoroughly. The well-ground mixture was heated in air at 800 1C for 10 h. The powder mixture was cold-pressed using a hydraulic press at 15 tons load, and sintered in air at 800 1C for 10 h. Thin films were grown on quartz substrate (fused quartz, Ted Pella Inc., USA) using pulsed laser deposition technique (KrF excimer laser, l ¼248 nm). The laser was operated at a pulse rate of 10 Hz with an energy of 300 mJ/pulse (spot size of 1 1 mm2). The target to substrate distance was 5.0 cm. The films were deposited at different substrate temperatures under vacuum of base pressure 1 10 6 mbar. These films were characterized with the Bruker AXS X-ray diffractometer using CuKa radiation having a wavelength of ˚ The surface morphology of the films was investigated 1.5406 A. using atomic force microscopy (Digital Instruments, Veeco-3100). The optical transmittance measurements were done using UV–vis spectrophotometer (Ocean Optics HR400). The thickness of the films was measured by height profile scanning using AFM and observed to be 56, 58, 54, 61, 63, 57, and 59 nm for films grown at 50, 100, 200, 300, 400, 500, and 600 1C, respectively [14]. The resistivity was measured using the standard four-point probe technique. The magnetic field dependence of Hall effect was measured with the field applied perpendicular to the film surface in the van der Pauw configuration [15]. For electrical measurements, contacts were prepared by depositing gold dots, followed by indium soldering. Carrier concentration and Hall mobility were calculated at room temperature using the Hall coefficient and resistivity data [16].
3. Results and discussion The effect of silver doing and substrate temperatures on XRD patterns of ZnAgO films are shown in Fig. 1. It is evident from the XRD patterns that substrate temperature affects the crystallinity of the films. Films grown at low temperatures are amorphous in nature while films grown at high temperatures are crystalline. ZnO films having different percentage of silver using magnetic sputtering is studied [11]. It is observed that these films are polycrystalline in nature with preferred orientation along c-axis at room temperature. We observed that films grown at high temperatures are highly oriented along (0 0 2) direction. The intensity of (0 0 2) peak increases with an increase in substrate temperature. This indicates that crystallinity of the deposited films improves with increasing substrate temperature. The average grain size of the film grown at different substrate temperatures was calculated using Scherrer formula [17] t¼
Cl b cos W
Fig. 1. (a) XRD patterns of ZnAgO film grown at different substrate temperature on quartz substrate (inset figure: variation of average grain particle size and d002 with substrate temperature). (b) XRD patterns of pure ZnO and ZnAgO film.
ð1Þ
where b is the full width at half maximum (FWHM) of the XRD peak, l is the X-ray wavelength, W is the diffraction angle, and C is the correction factor which is taken as unity. The estimated grain sizes were determined using the (0 0 2) diffraction peaks and are shown in Fig. 1(a). The grain size varies from 8.9 to 16.1 nm with increase in substrate temperature from 300 to 600 1C, respectively (inset of Fig. 1(a)). The effect of silver doping on XRD patterns of ZnO is shown in Fig. 1(b). We observed a shift in the (0 0 2) peak towards lower angle with silver doping, indicating expansion of c-axis lattice. The increase of lattice constant is obvious as the ionic radius of silver is larger than that of zinc. The volume expansion due to silver addition is estimated to be 1.6%. Observation of the volume expansion and absence of any impurity peaks due to silver suggest that silver atom goes to the zinc site [18]. It is also observed that the (0 0 2) peak shifts towards higher angle with increase in substrate temperature, indicating the
decrease of the c-axis lattice. The decrease in lattice parameter in c-direction with temperature and also in the interplanar spacing d as it is equal to c/2 for (0 0 2) plane in hexagonal wurtzite structure is shown in inset of Fig. 1(a) [19]. The decrease in c-axis lattice may be due to diffusion out of silver ions form zinc ion site to interstitial site at high temperature or due to stress change in the films [20]. Surface morphology of the films was studied using atomic force microscopy (AFM). Fig. 2 shows AFM images of ZnAgO films grown at different temperatures. The scan was carried out in tapping mode. The spring constant of the cantilever was 42 N/m. The cantilevered tip was oscillated close to the mechanical resonance frequency of the cantilever (typically, 200–300 kHz) with amplitudes ranging from 10 to 30 nm. It is evident from the AFM study that high substrate temperature during growth of films produces rougher films with bigger
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Fig. 2. AFM image of ZnAgO films grown at (a) 50 1C, (b) 200 1C, and (c) 600 1C.
particle size. This is well supported by XRD analysis, which confirms increase in average particle size with increase in growth temperature. The root mean square (rms) roughness of the films increases with temperature. The rms roughness of the films grown at 50, 200, and 600 1C is 0.52, 0.96, and 1.9 nm, respectively. The surface roughness of 7 nm is reported for silver doped films grown by metal-organic chemical vapour deposition method [13]. This indicates that our films can be used as bottom electrodes for optoelectronic applications. Fig. 3(a) shows the optical transmission spectra of ZnAgO films grown at different substrate temperatures. It is evident from the figure that these films are highly transparent in the visible region of solar spectrum, but the transmittance decreases at shorter wavelength near the ultraviolet range. The transparency of these films is more or less independent of the substrate temperature. Different observations have been reported for dependence of optical transmittance on substrate temperatures for silver doped ZnO films. Jeong et al. have reported that average optical transmittance decreases form 80% to 50% with an increase of substrate temperature from room temperature to 200 1C, whereas Sahu has observed an increase in optical transmittance with an increase in substrate temperature [21,13]. The increase of crystallinity of the films at higher temperature may be the reason for more transmittance. The effect of different metal ions doping on optical transmittance of ZnO films is investigated [22]. They observed a decrease in optical transmittance of pure ZnO from 90% to 60% by doping with Al, Ag, Sn, and Sb. Although, the optical transmittance of niobium doped ZnO fabricated by pulsed laser deposition technique shows high transmittance (485%) [23]. The average optical transmittances of our films are greater
than 85% in visible region of solar spectrum. The optical band gap (Eg) of the films were calculated using the relation ðahuÞ2 ¼ ðhuEg Þ
ð2Þ
where a is optical absorption coefficient, and hn is the photon energy. Since Eg ¼hn when (ahv)2 ¼0, an extrapolation of the linear region of (ahv)2 vs. hv plot to the photon energy axis, the energy band gap, Eg can be obtained as shown in Fig. 3(b). No relationship is observed between band gap and substrate temperature for these films. Though, increase in optical band gap of pure ZnO is evident by silver doping. Inset of Fig. 3(b) shows the band gap of ZnO films. Chen et al. [24] have reported a decrease in optical band gap of ZnO by copper doping and an increase in band gap of ZnO by silver doping. Xue et al. [4] have also observed an increase in optical band gap of ZnO by silver doping. Fig. 4 shows the variation of resistivity, carrier concentration, and Hall mobility with substrate temperature for silver doped ZnO films. The resistivity of the films decreases with an increase in substrate temperature up to 400 1C and then slightly increases with further increase in substrate temperature. The minimum resistivity of 3.0 10 3 O cm is observed for film grown at 400 1C. On the other hand, carrier concentration increases with an increase in substrate temperature up to 400 1C and then decreases with further increase in substrate temperature. The highest carrier concentration is 3.45 1019 cm 3 for film grown at 400 1C. The Hall mobility of the films increases with an increase in substrate temperature. The Hall mobility increases from 42 to 62 cm2/V s as substrate temperature increases from 50 to 600 1C. The increase in Hall mobility is believed to be due to better film
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2003
Fig. 3. Effect of substrate temperature on (a) optical transmittance and (b) optical band gap of ZnAgO films (inset figure: optical band gap of ZnO film).
crystallinity, which increases with an increase in substrate temperature. The decrease in carrier concentration at high substrate temperature may be caused by the annealing out of point defects and interstitial impurities [25]. The annealing out of point defects and interstitial impurities result in decrease in impurity scattering and increase in the Hall mobility. Fan and Xie [26] have reported the resistivity of As-ZnO films grown at 300 1C by sputtering as 6.54 O cm. They observed As-ZnO films grown at 300 1C as n-type with mobility of 1 cm2/V s. Vaithianathan et al. [27] have reported the electrical properties of P-ZnO films grown by pulsed laser deposition. They observed carrier concentration, Hall mobility, and resistivity as about 1 1019 cm 3, 5 cm2/V s, and 1 O cm, respectively, for as grown films. We did not observe any p-type conductivity for our silver doped ZnO films grown at different substrate temperatures, but p-type conductivity is reported for silver doped ZnO films, although in very narrow temperature range [28]. The p-type conductivity is closely related with the mechanism of substitution and interstitial of silver in ZnO. Detail study on the doping amount and growth temperature are in progress, which may reveals better understanding of different behavior such as semi-insulating, n-type, and p-type properties of silver doped ZnO.
Fig. 4. Effect of substrate temperature on (a) resistivity, (b) carrier concentration, and (c) mobility of ZnAgO films (lines are for visual aid).
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4. Conclusions Highly conducting and transparent silver doped zinc oxide films were grown using pulsed laser deposition technique. The effect of substrate temperature on structural and optoelectrical properties was studied. These highly conducting, transparent, wide band gap films with very smooth surface roughness could be used for various optoelectronic applications.
Acknowledgements Authors are thankful to Mr. Rishi Patel, JVIC, Missouri State University, Missouri for recording the AFM pictures. This work is supported by National Science Foundation (award number DMR0907037). References [1] Y. Ishikawa, Y. Shimizu, T. Sasaki, N. Koshizaki, J. Colloid Interf. Sci. 300 (2006) 612. [2] H. Sheng, S. Muthukumar, N.W. Emanetoglu, Y. Lu, Appl. Phys. Lett. 80 (2003) 2132. [3] Y.W. Heo, S.J. Park, K. Ip, S.J. Pearton, D.P. Norton, Appl. Phys. Lett. 83 (2003) 1128. [4] H. Xue, X.L. Xu, Y. Chen, G.H. Zhang, S.Y. Ma, Appl. Surf. Sci. 255 (2008) 1806. [5] B.D. Ahn, H.S. Kang, J.H. Kim, G.H. Kim, H.W. Chang, S.Y. Lee, J. Appl. Phys. 100 (2006) 093701.
[6] C.A.K. Gouvea, F. Wypych, S.G. Moraes, N. Duran, P. Peralta-Zama, Chemosphere 40 (2000) 427. [7] R.J.M. Palma, R. Gago, M. Vinnichenko, J.M.M. Durat, J. Phys. D: Appl. Phys. 37 (2004) 1554. [8] A.P. Abiyasa, S.F. Yu, S.P. Lau, W.J. Fan, S.P. Lau, Appl. Phys. Lett. 90 (2007) 231106. [9] D.R. Sahu, S.Y. Lin, J.L. Huang, Sol. Energy Mater. Sol. Cells 91 (2007) 851. [10] D.R. Sahu, Microelectron. J. 38 (2007) 1252. [11] B. Houng, C.J. Huang, Surf. Coat. Tech. 201 (2006) 3188. [12] V.S. Khomchenko, T.G. Kryshtab, A.K. Savin, L.V. Zavyalova, N.N. Roshchina, V.E. Rodionov, O.S. Lytvyn, V.I. Kushnirenko, V.B. Kachatryan, J.A.A. Adame, Superlattices Microstruct. 42 (2007) 94. [13] L. Duan, W. Gao, R. Chen, Z. Fu, Solid State Commun. 145 (2008) 479. [14] R.K. Gupta, K. Ghosh, S.R. Mishra, P.K. Kahol, Appl. Surf. Sci. 254 (2008) 4018. [15] L.J. Van der Pauw, Philips Res. Rep. 13 (1958) 1. [16] I. Yasuhiro, K. Hirokazu, Appl. Surf. Sci. 169/170 (2001) 508. [17] S.T. Tan, B.J. Chen, X.W. Sun, W.J. Fan, H.S. Kwok, X.H. Zhang, S.J. Chua, J. Appl. Phys. 98 (2005) 013505. [18] R.K. Gupta, K. Ghosh, S.R. Mishra, P.K. Kahol, Thin Solid Films 516 (2008) 3204. [19] R.K. Shukla, A. Srivastava, A. Srivastava, K.C. Dubey, J. Cryst. Growth 294 (2006) 427. [20] J. Fan, R. Freer, J. Appl. Phys. 77 (1995) 4795. [21] S.H. Jeong, B.N. Park, S.B. Lee, J.H. Boo, Surf. Coat. Technol. 193 (2005) 340. [22] A.M. Galva´n, C.T. Cruz, J. Lee, D. Bhattacharyya, J. Metson, P.J. Evans, U. Pal, J. Appl. Phys. 99 (2006) 014306. [23] J.M. Lin, Y.Z. Zhang, Z.Z. Ye, X.Q. Gu, X.H. Pan, Y.F. Yang, J.G. Lu, H.P. He, B.H. Zhao, Appl. Surf. Sci. 255 (2009) 6460. [24] Y. Chen, X.L. Xu, G.H. Zhang, H. Xue, S.Y. Ma, Physica B 404 (2009) 3645. [25] W. Wohlmuth, I. Adesida, Thin Solid Films 479 (2005) 223. [26] J.C. Fan, Z. Xie, Mater. Sci. Eng. B 150 (2008) 61. [27] V. Vaithianathan, B.T. Lee, S.S. Kim, J. Appl. Phys. 98 (2005) 043519. [28] H.S. Kang, B.D. Ahn, J.H. Kim, G.H. Kim, S.H. Lim, Appl. Phys. Lett. 88 (2006) 202108.
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Effect of substrate temperature on structural and optoelectrical properties of silver doped zinc oxide thin films R.K. Gupta n, K. Ghosh, P.K. Kahol Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO 65897, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 7 January 2010 Received in revised form 11 February 2010 Accepted 8 March 2010 Available online 18 March 2010
Silver doped zinc oxide (ZnAgO) thin films were grown on quartz substrate using pulsed laser deposition technique. The effect of substrate temperature on structural, optical, and electrical properties was studied. The films grown at low temperatures were amorphous in nature, while films grown at high temperatures were crystalline. It is observed that the films are highly oriented along c-axis. The surface roughness of these films is observed to increase with substrate temperature; on the other hand optical transmittance is almost independent of substrate temperature. The mobility of the films increases with an increase in substrate temperature. The highest Hall mobility of 62 cm2/V s is observed for the film grown at 600 1C. These highly transparent and high mobility films could be used as bottom electrodes in optoelectronic devices. & 2010 Elsevier B.V. All rights reserved.
Keywords: Zinc oxide Pulsed laser deposition Transparent conducting oxide Mobility
1. Introduction ZnO has attracted a broad scientific interest because of its direct wide band gap (3.4 eV) [1]. ZnO is highly transparent, inexpensive, non-toxic, and chemically stable. These unique physical and chemical properties make it useful for many applications such as transparent conducting electrodes, optoelectronic devices, solar cells, thin film transistors, and gas sensors [2]. ZnO is an n-type semiconductor due to presence of interstitial zinc and oxygen deficiency. However, n-type conductivity is easily improved by substituting trivalent cations such as Al, Ga, and In on the Zn site. However, the realization of p-type conductivity (doping ZnO by either group I elements for Zn sites or group V elements for O sites) is rather difficult due to problems such as self compensating effect, deep acceptor level, and solubility of the acceptor dopants [3]. Silver is a good candidate for ZnO doping as it is a good electrical conductor with relatively lower optical absorption coefficient in the visible region [4]. It is reported that silver doping can modify the electrical and optical properties of ZnO films [5–8]. Silver doped ZnO films show some unique electrical properties such as semi-insulating, n-type and p-type behavior depending on growth conditions [5]. It is demonstrated that silver doped ZnO films could be used as transparent conducting electrodes for optoelectronic applications [9].
The effect of silver doping on electrical properties of ZnO films has been studied [10]. It is reported that crystallinity, transmittance, and electrical conductivity decrease with the amount of silver doping. A minimum resistivity of 6 10 3 Ocm was obtained for sample containing 0.7 at% of silver. Houng and Huang [11] have reported the lowest electrical resistivity of silver embedded aluminum doped ZnO films as 4.2 10 4 O cm. Khomchenko et al. [12] have fabricated silver doped ZnO films using metalorganic chemical vapour deposition method. The surface roughness of these films was 7 nm, which is higher to use as bottom transparent electrode. The effect of post-annealing on properties of RF reactive magnetron sputtering grown silver doped ZnO is also studied [13]. It is seen that as grown films are insulating in nature but after annealing in oxygen atmosphere at high temperature showed resistivity of 152 O cm, carrier concentration of 2.24 1016 cm 3, and Hall mobility of 1.83 cm2/V s. The above literature survey indicates that the electrical, structural, and optical properties of silver doped ZnO films largely depend on growth conditions as well as techniques. In this communication, the effect of substrate temperature on structural, optical, and electrical properties of silver doped ZnO films is studied in detail. These films are grown by pulsed laser deposition and characterized depending on deposition temperatures.
2. Experimental details n
Corresponding author. Tel.: + 1 417 836 6298; fax: + 1 417 836 6226. E-mail address: [email protected] (R.K. Gupta).
1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.03.010
A solid state reaction method is used to prepare silver doped zinc oxide (ZnAgO) films with 2.0 at% of silver. Required amounts
ARTICLE IN PRESS R.K. Gupta et al. / Physica E 42 (2010) 2000–2004
2001
of ZnO (Alfa Aesar, USA) and silver (Alfa Aesar, USA) were taken by molecular weight and mixed thoroughly. The well-ground mixture was heated in air at 800 1C for 10 h. The powder mixture was cold-pressed using a hydraulic press at 15 tons load, and sintered in air at 800 1C for 10 h. Thin films were grown on quartz substrate (fused quartz, Ted Pella Inc., USA) using pulsed laser deposition technique (KrF excimer laser, l ¼248 nm). The laser was operated at a pulse rate of 10 Hz with an energy of 300 mJ/pulse (spot size of 1 1 mm2). The target to substrate distance was 5.0 cm. The films were deposited at different substrate temperatures under vacuum of base pressure 1 10 6 mbar. These films were characterized with the Bruker AXS X-ray diffractometer using CuKa radiation having a wavelength of ˚ The surface morphology of the films was investigated 1.5406 A. using atomic force microscopy (Digital Instruments, Veeco-3100). The optical transmittance measurements were done using UV–vis spectrophotometer (Ocean Optics HR400). The thickness of the films was measured by height profile scanning using AFM and observed to be 56, 58, 54, 61, 63, 57, and 59 nm for films grown at 50, 100, 200, 300, 400, 500, and 600 1C, respectively [14]. The resistivity was measured using the standard four-point probe technique. The magnetic field dependence of Hall effect was measured with the field applied perpendicular to the film surface in the van der Pauw configuration [15]. For electrical measurements, contacts were prepared by depositing gold dots, followed by indium soldering. Carrier concentration and Hall mobility were calculated at room temperature using the Hall coefficient and resistivity data [16].
3. Results and discussion The effect of silver doing and substrate temperatures on XRD patterns of ZnAgO films are shown in Fig. 1. It is evident from the XRD patterns that substrate temperature affects the crystallinity of the films. Films grown at low temperatures are amorphous in nature while films grown at high temperatures are crystalline. ZnO films having different percentage of silver using magnetic sputtering is studied [11]. It is observed that these films are polycrystalline in nature with preferred orientation along c-axis at room temperature. We observed that films grown at high temperatures are highly oriented along (0 0 2) direction. The intensity of (0 0 2) peak increases with an increase in substrate temperature. This indicates that crystallinity of the deposited films improves with increasing substrate temperature. The average grain size of the film grown at different substrate temperatures was calculated using Scherrer formula [17] t¼
Cl b cos W
Fig. 1. (a) XRD patterns of ZnAgO film grown at different substrate temperature on quartz substrate (inset figure: variation of average grain particle size and d002 with substrate temperature). (b) XRD patterns of pure ZnO and ZnAgO film.
ð1Þ
where b is the full width at half maximum (FWHM) of the XRD peak, l is the X-ray wavelength, W is the diffraction angle, and C is the correction factor which is taken as unity. The estimated grain sizes were determined using the (0 0 2) diffraction peaks and are shown in Fig. 1(a). The grain size varies from 8.9 to 16.1 nm with increase in substrate temperature from 300 to 600 1C, respectively (inset of Fig. 1(a)). The effect of silver doping on XRD patterns of ZnO is shown in Fig. 1(b). We observed a shift in the (0 0 2) peak towards lower angle with silver doping, indicating expansion of c-axis lattice. The increase of lattice constant is obvious as the ionic radius of silver is larger than that of zinc. The volume expansion due to silver addition is estimated to be 1.6%. Observation of the volume expansion and absence of any impurity peaks due to silver suggest that silver atom goes to the zinc site [18]. It is also observed that the (0 0 2) peak shifts towards higher angle with increase in substrate temperature, indicating the
decrease of the c-axis lattice. The decrease in lattice parameter in c-direction with temperature and also in the interplanar spacing d as it is equal to c/2 for (0 0 2) plane in hexagonal wurtzite structure is shown in inset of Fig. 1(a) [19]. The decrease in c-axis lattice may be due to diffusion out of silver ions form zinc ion site to interstitial site at high temperature or due to stress change in the films [20]. Surface morphology of the films was studied using atomic force microscopy (AFM). Fig. 2 shows AFM images of ZnAgO films grown at different temperatures. The scan was carried out in tapping mode. The spring constant of the cantilever was 42 N/m. The cantilevered tip was oscillated close to the mechanical resonance frequency of the cantilever (typically, 200–300 kHz) with amplitudes ranging from 10 to 30 nm. It is evident from the AFM study that high substrate temperature during growth of films produces rougher films with bigger
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Fig. 2. AFM image of ZnAgO films grown at (a) 50 1C, (b) 200 1C, and (c) 600 1C.
particle size. This is well supported by XRD analysis, which confirms increase in average particle size with increase in growth temperature. The root mean square (rms) roughness of the films increases with temperature. The rms roughness of the films grown at 50, 200, and 600 1C is 0.52, 0.96, and 1.9 nm, respectively. The surface roughness of 7 nm is reported for silver doped films grown by metal-organic chemical vapour deposition method [13]. This indicates that our films can be used as bottom electrodes for optoelectronic applications. Fig. 3(a) shows the optical transmission spectra of ZnAgO films grown at different substrate temperatures. It is evident from the figure that these films are highly transparent in the visible region of solar spectrum, but the transmittance decreases at shorter wavelength near the ultraviolet range. The transparency of these films is more or less independent of the substrate temperature. Different observations have been reported for dependence of optical transmittance on substrate temperatures for silver doped ZnO films. Jeong et al. have reported that average optical transmittance decreases form 80% to 50% with an increase of substrate temperature from room temperature to 200 1C, whereas Sahu has observed an increase in optical transmittance with an increase in substrate temperature [21,13]. The increase of crystallinity of the films at higher temperature may be the reason for more transmittance. The effect of different metal ions doping on optical transmittance of ZnO films is investigated [22]. They observed a decrease in optical transmittance of pure ZnO from 90% to 60% by doping with Al, Ag, Sn, and Sb. Although, the optical transmittance of niobium doped ZnO fabricated by pulsed laser deposition technique shows high transmittance (485%) [23]. The average optical transmittances of our films are greater
than 85% in visible region of solar spectrum. The optical band gap (Eg) of the films were calculated using the relation ðahuÞ2 ¼ ðhuEg Þ
ð2Þ
where a is optical absorption coefficient, and hn is the photon energy. Since Eg ¼hn when (ahv)2 ¼0, an extrapolation of the linear region of (ahv)2 vs. hv plot to the photon energy axis, the energy band gap, Eg can be obtained as shown in Fig. 3(b). No relationship is observed between band gap and substrate temperature for these films. Though, increase in optical band gap of pure ZnO is evident by silver doping. Inset of Fig. 3(b) shows the band gap of ZnO films. Chen et al. [24] have reported a decrease in optical band gap of ZnO by copper doping and an increase in band gap of ZnO by silver doping. Xue et al. [4] have also observed an increase in optical band gap of ZnO by silver doping. Fig. 4 shows the variation of resistivity, carrier concentration, and Hall mobility with substrate temperature for silver doped ZnO films. The resistivity of the films decreases with an increase in substrate temperature up to 400 1C and then slightly increases with further increase in substrate temperature. The minimum resistivity of 3.0 10 3 O cm is observed for film grown at 400 1C. On the other hand, carrier concentration increases with an increase in substrate temperature up to 400 1C and then decreases with further increase in substrate temperature. The highest carrier concentration is 3.45 1019 cm 3 for film grown at 400 1C. The Hall mobility of the films increases with an increase in substrate temperature. The Hall mobility increases from 42 to 62 cm2/V s as substrate temperature increases from 50 to 600 1C. The increase in Hall mobility is believed to be due to better film
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2003
Fig. 3. Effect of substrate temperature on (a) optical transmittance and (b) optical band gap of ZnAgO films (inset figure: optical band gap of ZnO film).
crystallinity, which increases with an increase in substrate temperature. The decrease in carrier concentration at high substrate temperature may be caused by the annealing out of point defects and interstitial impurities [25]. The annealing out of point defects and interstitial impurities result in decrease in impurity scattering and increase in the Hall mobility. Fan and Xie [26] have reported the resistivity of As-ZnO films grown at 300 1C by sputtering as 6.54 O cm. They observed As-ZnO films grown at 300 1C as n-type with mobility of 1 cm2/V s. Vaithianathan et al. [27] have reported the electrical properties of P-ZnO films grown by pulsed laser deposition. They observed carrier concentration, Hall mobility, and resistivity as about 1 1019 cm 3, 5 cm2/V s, and 1 O cm, respectively, for as grown films. We did not observe any p-type conductivity for our silver doped ZnO films grown at different substrate temperatures, but p-type conductivity is reported for silver doped ZnO films, although in very narrow temperature range [28]. The p-type conductivity is closely related with the mechanism of substitution and interstitial of silver in ZnO. Detail study on the doping amount and growth temperature are in progress, which may reveals better understanding of different behavior such as semi-insulating, n-type, and p-type properties of silver doped ZnO.
Fig. 4. Effect of substrate temperature on (a) resistivity, (b) carrier concentration, and (c) mobility of ZnAgO films (lines are for visual aid).
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4. Conclusions Highly conducting and transparent silver doped zinc oxide films were grown using pulsed laser deposition technique. The effect of substrate temperature on structural and optoelectrical properties was studied. These highly conducting, transparent, wide band gap films with very smooth surface roughness could be used for various optoelectronic applications.
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