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Effect of different dopants on the properties of ZnO thin films
International Journal of Inorganic Materials 3 (2001) 1211–1213
Effect of different dopants on the properties of ZnO thin films P. Nunes a , E. Fortunato a , *, P. Vilarinho b , R. Martins a a
Department of Materials Science /CENIMAT, Faculty of Science and Technology, New University of Lisbon 2825 -114 Caparica, Portugal b Department of Ceramics and Glass Engineering /UIMC, University of Aveiro, 38 10 -193 Aveiro, Portugal
Abstract The influence of doping on the properties of zinc oxide thin films deposited by spray pyrolysis has been studied. The results show that the doping affects the properties of the films, mainly the electrical ones, function of its concentration and nature. The most significative improvement were observed for films doped with 1 at.% of indium exhibiting a resistivity of 1.9310 21 V m associated to a transmittance of 86%, characteristics required for applications on the optoelectronic devices. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Zinc oxide; Thin films; Dopant; Spray pyrolysis
1. Introduction
2. Experimental details
The development of low cost optoelectronic devices requires more and more the use of transparent conductive oxide thin films, mainly for applications such as solar cells [1], liquid crystal displays [2], heat mirrors [3], multilayers, photothermal conversion system, gas sensors [4], optical position sensors, etc. Among these materials, zinc oxide (ZnO) thin films have emerged as one of the most promising oxide materials owing to their optical and electrical properties, together with their high chemical and mechanical stability [5]. However ZnO undoped thin films are not stable due to changes in the surface conductance under oxygen chemisorption and desorption. Doping the zinc oxide can reduce this disadvantage [6]. Besides that doping leads to an increase in the conductivity of the ZnO thin films. The ZnO doping is achieved by replacing Zn 21 atoms with atoms of elements of higher valence such as In 31 , Al 31 and Ga 31 [7]. These types of oxide materials can be produced by several techniques such as sputtering [8], reactive evaporation, CVD [9] and spray pyrolysis [10]. Nevertheless, the spray pyrolysis technique is cheaper, simpler and versatile than the others and gives the possibility of obtaining films with suitable properties for optoelectronic applications.
The spray pyrolysis is a cheap and simple technique based on chemical vapour deposition process (CVD). In this technique, the precursor of the material to be deposited is in solution and sprayed into a heated substrate (T5673 K) using argon as carrier gas. The solution used was made of zinc acetate with a concentration of 0.2 M dissolved in methanol. With the aim to obtain ZnO doped, at the main solution was added InCl 3 , AlC 15 H 21 O 6 or C 15 H 21 GaO 6 with concentrations between 1 and 5 at.%, to dope with indium (In) and aluminum (Al) or gallium (Ga), respectively. A Rigaku X-ray diffraction system was used to determine the film structure. The film thickness was measured by a Sloan Dektak 3 d profilometer. For the electric characterisation was used the Van der Pauw’s technique in order to obtain the electrical resistivity ( r ), the Hall mobility ( m ) and the carrier concentration (n) of the films. A Shimadzu double-beam spectrophotometer equipped with an integrating sphere was used for optical transmittance (T ) and reflectance (R) measurements in the wavelength range of 0.4 to 0.8 mm.
3. Results
*Corresponding author. Tel.: 1351-21-294-8562; fax: 1351-21-2948558. E-mail address: [email protected] (E. Fortunato).
The X-ray diffractograms (Fig. 1a and b) shows welldefined peaks usually associated with ZnO hexagonal structure. However with the introduction of dopant we observed a change in the preferential orientation. For intrinsic films is the k101l orientation, while for the doped
1466-6049 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 01 )00129-5
1212
P. Nunes et al. / International Journal of Inorganic Materials 3 (2001) 1211 – 1213
Fig. 1. X-ray diffractograms of ZnO: (a) intrinsic and (b) doped with In, Al and Ga.
Fig. 2. Variation of the electrical resistivity, carrier concentration, and Hall mobility with the dopant concentration for doped ZnO thin films.
films changes to the k002l orientation. This effect can be attributed to the role of the dopant in the nucleation process. The effect of the dopant in the nucleation process is also possible to observe in Fig. 2, where the intrinsic ZnO thin films exhibit a roughness surface which disappear in the doped films and the most significant changes were observed in the films doped with gallium. Fig. 2 shows the variation of electrical resistivity, carrier concentration and Hall mobility of doped ZnO films as a function of the dopant content. It can be observed that the resistivity decreases rapidly with the increase of dopant concentration (up to 1 at.%), being this decrease more accentuated for ZnO films doped with In. Thereafter the resistivity almost stabilises for the ZnO film doped with In, while for those doped with Al and Ga an increase in the resistivity is observed. This behaviour can be explained by the increase of defects due to the fact that some of the dopant atoms occupy interstitial sites [11], as well as by the appearance of some non-conducting Al 2 O 3 and Ga 2 O 3 oxides. Concerning the carrier concentration it is observed for all the doped films a rapid increase (up to 1 at.%), and then stabilises, for higher concentrations. Up to 1 at.% the dopant atoms provides only donors, whereas above 1 at.% the dopant atoms start to segregate at the grain boundaries. The variation of the Hall mobility is in agreement with the previous result and is mainly due to ionised impurity scattering and additional scattering due to atom segregation at the grain boundaries. Concerning the optical properties, they are also influenced by the doping, as can be seen on the variation of the absorption coefficient with the dopant concentration, presented in Fig. 5a. The data show an increase in the absorption coefficient with the dopant concentration until concentration of 1–2 at.% and them decrease slightly. This behaviour is related with the increase in the free carrier concentration (see Fig. 2). The increase on the carrier concentration is also correlated with the direct band gap (Eop ) of ZnO thin films (Fig. 3b)). This behavior is in conformity with the Burstein–
P. Nunes et al. / International Journal of Inorganic Materials 3 (2001) 1211 – 1213
1213
Fig. 3. Variation of the absorption coefficient (for l5550 nm) (a) and band gap (b) with the dopant concentration for doped ZnO thin films.
Moss shift [12], which correlates a shift to higher energies of the band gap with the increase of the carrier concentration.
with CENIMAT and by the projects TMR ERBFMRXCT970141, PRAXIS / 3 / 3. 1 / MMA / 1788 / 95 and PRAXIS / P/ CTM / 112094 / 98.
4. Conclusions
References
From the study performed it is possible to conclude that the introduction of dopant leads to significant changes in the structural properties of the ZnO thin films. However most improvements were observed in the electrical properties where the resistivity decreases two orders of magnitude with the dopant concentration until concentration of 1–2 at.%. The lowest value of resistivity was obtained for the films doped with 1 at.% of In meaning that this is the most suitable dopant for the ZnO thin film produced by spray pyrolysis. For dopant concentration higher them 2 at.% deterioration on the properties of the films were observed. The result achieved (low resistivity and absorption coefficient) gives a possibility of using these films in several optoelectronic applications. However, the properties of the ZnO thin films presented in this work, can be ‘drastically’ improved by an adequate post treatment (temperature and atmosphere).
Acknowledgements ¨ para a This work was supported by the ‘Fundac¸ao ˆ Ciencia e a Tecnologia’ through Pluriannual Contracts
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Major S, Chopra KL. Solar Energy Mater 1988;17:319. Lan J, Kanicki J. Mat Res Soc Symp 1997;424:347. Chopra KL, Major S, Panday DK. Thin Solid Films 1983;102:1. Michel HJ, Leiste H, Schierbaum KD, Halbritter J. Appl Surf Sci 1998;126:57. Hartnagel HL, Dawar AL, Jain AK, Jagadish C. Semiconducting transparent thin films, Institut of Physics Publishing, 1995. Aktaruzzaman AF, Sharma GL, Malhotra LK. Thin Solid Films 1991;198:67. Kohilci S, Nishitani M, Wada T. J Appl Phys 1994;75(4):2069. Poschenriedes M, Brehme S, Fenske F, Fuhs W, Nebauer E, Selle B, Sieber I. Tenth international conference on thin films, 1996. Dutta A, Basu S. Mater Chem Phys 1993;34:41. Abd Lefdil M, Messaoudi C, Cadene M. Thirteenth EPVSEC, 1995:2092. Nunes P, Femandes B, Fortunato E, Martins R. Thin Solid Films 1998;333:1. Sarkar A, Gosh S, Chaudhuri S, Pal AK. Thin Solid Films 1991;204:255.
Effect of different dopants on the properties of ZnO thin films P. Nunes a , E. Fortunato a , *, P. Vilarinho b , R. Martins a a
Department of Materials Science /CENIMAT, Faculty of Science and Technology, New University of Lisbon 2825 -114 Caparica, Portugal b Department of Ceramics and Glass Engineering /UIMC, University of Aveiro, 38 10 -193 Aveiro, Portugal
Abstract The influence of doping on the properties of zinc oxide thin films deposited by spray pyrolysis has been studied. The results show that the doping affects the properties of the films, mainly the electrical ones, function of its concentration and nature. The most significative improvement were observed for films doped with 1 at.% of indium exhibiting a resistivity of 1.9310 21 V m associated to a transmittance of 86%, characteristics required for applications on the optoelectronic devices. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Zinc oxide; Thin films; Dopant; Spray pyrolysis
1. Introduction
2. Experimental details
The development of low cost optoelectronic devices requires more and more the use of transparent conductive oxide thin films, mainly for applications such as solar cells [1], liquid crystal displays [2], heat mirrors [3], multilayers, photothermal conversion system, gas sensors [4], optical position sensors, etc. Among these materials, zinc oxide (ZnO) thin films have emerged as one of the most promising oxide materials owing to their optical and electrical properties, together with their high chemical and mechanical stability [5]. However ZnO undoped thin films are not stable due to changes in the surface conductance under oxygen chemisorption and desorption. Doping the zinc oxide can reduce this disadvantage [6]. Besides that doping leads to an increase in the conductivity of the ZnO thin films. The ZnO doping is achieved by replacing Zn 21 atoms with atoms of elements of higher valence such as In 31 , Al 31 and Ga 31 [7]. These types of oxide materials can be produced by several techniques such as sputtering [8], reactive evaporation, CVD [9] and spray pyrolysis [10]. Nevertheless, the spray pyrolysis technique is cheaper, simpler and versatile than the others and gives the possibility of obtaining films with suitable properties for optoelectronic applications.
The spray pyrolysis is a cheap and simple technique based on chemical vapour deposition process (CVD). In this technique, the precursor of the material to be deposited is in solution and sprayed into a heated substrate (T5673 K) using argon as carrier gas. The solution used was made of zinc acetate with a concentration of 0.2 M dissolved in methanol. With the aim to obtain ZnO doped, at the main solution was added InCl 3 , AlC 15 H 21 O 6 or C 15 H 21 GaO 6 with concentrations between 1 and 5 at.%, to dope with indium (In) and aluminum (Al) or gallium (Ga), respectively. A Rigaku X-ray diffraction system was used to determine the film structure. The film thickness was measured by a Sloan Dektak 3 d profilometer. For the electric characterisation was used the Van der Pauw’s technique in order to obtain the electrical resistivity ( r ), the Hall mobility ( m ) and the carrier concentration (n) of the films. A Shimadzu double-beam spectrophotometer equipped with an integrating sphere was used for optical transmittance (T ) and reflectance (R) measurements in the wavelength range of 0.4 to 0.8 mm.
3. Results
*Corresponding author. Tel.: 1351-21-294-8562; fax: 1351-21-2948558. E-mail address: [email protected] (E. Fortunato).
The X-ray diffractograms (Fig. 1a and b) shows welldefined peaks usually associated with ZnO hexagonal structure. However with the introduction of dopant we observed a change in the preferential orientation. For intrinsic films is the k101l orientation, while for the doped
1466-6049 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 01 )00129-5
1212
P. Nunes et al. / International Journal of Inorganic Materials 3 (2001) 1211 – 1213
Fig. 1. X-ray diffractograms of ZnO: (a) intrinsic and (b) doped with In, Al and Ga.
Fig. 2. Variation of the electrical resistivity, carrier concentration, and Hall mobility with the dopant concentration for doped ZnO thin films.
films changes to the k002l orientation. This effect can be attributed to the role of the dopant in the nucleation process. The effect of the dopant in the nucleation process is also possible to observe in Fig. 2, where the intrinsic ZnO thin films exhibit a roughness surface which disappear in the doped films and the most significant changes were observed in the films doped with gallium. Fig. 2 shows the variation of electrical resistivity, carrier concentration and Hall mobility of doped ZnO films as a function of the dopant content. It can be observed that the resistivity decreases rapidly with the increase of dopant concentration (up to 1 at.%), being this decrease more accentuated for ZnO films doped with In. Thereafter the resistivity almost stabilises for the ZnO film doped with In, while for those doped with Al and Ga an increase in the resistivity is observed. This behaviour can be explained by the increase of defects due to the fact that some of the dopant atoms occupy interstitial sites [11], as well as by the appearance of some non-conducting Al 2 O 3 and Ga 2 O 3 oxides. Concerning the carrier concentration it is observed for all the doped films a rapid increase (up to 1 at.%), and then stabilises, for higher concentrations. Up to 1 at.% the dopant atoms provides only donors, whereas above 1 at.% the dopant atoms start to segregate at the grain boundaries. The variation of the Hall mobility is in agreement with the previous result and is mainly due to ionised impurity scattering and additional scattering due to atom segregation at the grain boundaries. Concerning the optical properties, they are also influenced by the doping, as can be seen on the variation of the absorption coefficient with the dopant concentration, presented in Fig. 5a. The data show an increase in the absorption coefficient with the dopant concentration until concentration of 1–2 at.% and them decrease slightly. This behaviour is related with the increase in the free carrier concentration (see Fig. 2). The increase on the carrier concentration is also correlated with the direct band gap (Eop ) of ZnO thin films (Fig. 3b)). This behavior is in conformity with the Burstein–
P. Nunes et al. / International Journal of Inorganic Materials 3 (2001) 1211 – 1213
1213
Fig. 3. Variation of the absorption coefficient (for l5550 nm) (a) and band gap (b) with the dopant concentration for doped ZnO thin films.
Moss shift [12], which correlates a shift to higher energies of the band gap with the increase of the carrier concentration.
with CENIMAT and by the projects TMR ERBFMRXCT970141, PRAXIS / 3 / 3. 1 / MMA / 1788 / 95 and PRAXIS / P/ CTM / 112094 / 98.
4. Conclusions
References
From the study performed it is possible to conclude that the introduction of dopant leads to significant changes in the structural properties of the ZnO thin films. However most improvements were observed in the electrical properties where the resistivity decreases two orders of magnitude with the dopant concentration until concentration of 1–2 at.%. The lowest value of resistivity was obtained for the films doped with 1 at.% of In meaning that this is the most suitable dopant for the ZnO thin film produced by spray pyrolysis. For dopant concentration higher them 2 at.% deterioration on the properties of the films were observed. The result achieved (low resistivity and absorption coefficient) gives a possibility of using these films in several optoelectronic applications. However, the properties of the ZnO thin films presented in this work, can be ‘drastically’ improved by an adequate post treatment (temperature and atmosphere).
Acknowledgements ¨ para a This work was supported by the ‘Fundac¸ao ˆ Ciencia e a Tecnologia’ through Pluriannual Contracts
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Major S, Chopra KL. Solar Energy Mater 1988;17:319. Lan J, Kanicki J. Mat Res Soc Symp 1997;424:347. Chopra KL, Major S, Panday DK. Thin Solid Films 1983;102:1. Michel HJ, Leiste H, Schierbaum KD, Halbritter J. Appl Surf Sci 1998;126:57. Hartnagel HL, Dawar AL, Jain AK, Jagadish C. Semiconducting transparent thin films, Institut of Physics Publishing, 1995. Aktaruzzaman AF, Sharma GL, Malhotra LK. Thin Solid Films 1991;198:67. Kohilci S, Nishitani M, Wada T. J Appl Phys 1994;75(4):2069. Poschenriedes M, Brehme S, Fenske F, Fuhs W, Nebauer E, Selle B, Sieber I. Tenth international conference on thin films, 1996. Dutta A, Basu S. Mater Chem Phys 1993;34:41. Abd Lefdil M, Messaoudi C, Cadene M. Thirteenth EPVSEC, 1995:2092. Nunes P, Femandes B, Fortunato E, Martins R. Thin Solid Films 1998;333:1. Sarkar A, Gosh S, Chaudhuri S, Pal AK. Thin Solid Films 1991;204:255.