- Email: [email protected]
Role of rf power on the properties of undoped SnOx films deposited by rf-PERTE at low substrate temperature
Available online at www.sciencedirect.com
Surface & Coatings Technology 202 (2008) 3893 – 3896 www.elsevier.com/locate/surfcoat
Role of rf power on the properties of undoped SnOx films deposited by rf-PERTE at low substrate temperature J. Valente a,⁎, G. Lavareda a , O. Conde b , P. Parreira a , A. Amaral c , C. Nunes de Carvalho a a
UNL-FCT, Dept. Ciência dos Materiais, Campus da Caparica, 2829-516 Caparica, Portugal b Univ. Lisboa, Fac. Ciências, Dep. Física e ICEMS, 1749-016 Lisboa, Portugal c UTL-IST, Dep. Física e ICEMS, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Received 18 October 2007; accepted in revised form 30 January 2008 Available online 8 February 2008
Abstract Transparent and conductive undoped tin oxide (SnOx) thin films were deposited at low substrate temperature (b 140 °C) by radio frequency (rf) plasma enhanced reactive thermal evaporation (rf-PERTE) of tin (Sn) in the presence of oxygen The undoped SnOx films were not submitted to any post-annealing treatments. The influence of rf power variation on the optical, electrical and structural properties of the as-grown films is presented. A variation in the films' structure was verified with the increase of rf power. Undoped SnOx films, 90 nm average thick, deposited at rf power range of 60–70 W are nanocrystalline, show a conductive behaviour, an average visible transmittance of ≥ 80% and a maximum electrical conductivity of about 34.6 (Ω cm)− 1. Films deposited at lower values of rf power (40 W) are amorphous and exhibit a semiconductive behaviour, showing an electrical conductivity of about 7.54 × 10− 1 (Ω cm)− 1. As a low substrate temperature deposition process is used, SnOx thin films can be obtained on a wide range of substrates. © 2008 Elsevier B.V. All rights reserved. Keywords: SnOx; rf-PERTE; Thin film; Transparent conductive oxide; Plasma; Low temperature
1. Introduction One of the most promising fields in materials science is related to the fundamental aspects and applications of transparent conductive thin films. Their potential applications in the industry generated a technological interest on the study of these films. Among the transparent conductors known nowadays, tin oxide is one of the most interesting due to its (1) excellent physical properties (electrical conductivity and visible transparency) and (2) several technological applications, such as gas sensors, solar cells and as electrodes for opto-electric devices. As it is known, tin oxide is an n-type semiconductor that has attracted significant attention, as thin film, because of their improved performance such as high gas sensitivity and fast oxidation/reduction response [1]. SnOx films may be deposited ⁎ Corresponding author. Tel.: +351 218419279; fax: +351 218464455/57. E-mail address: [email protected] (J. Valente). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.01.033
by a number of techniques, including spray pyrolysis [2], sputtering [3] and electron beam [4]. The high substrate temperature (or post-annealing treatments) required in these techniques to obtain SnOx with good optical, electrical and structural properties, is the main drawback. If SnOx transparent conducting thin films can be deposited at low substrate temperatures (b 140 °C), a wide range of optoelectronic SnOx based devices can be laid upon any type of substrates, from glass to flexible polymers [5]. Ultra-thin, light weight and lowpower consumption devices can be made, promising displays that can be deformable. In the present work we report on the deposition of undoped and highly transparent, conductive SnOx thin films grown at low temperature. The technique used is the rf-PERTE of tin (Sn) in the presence of oxygen [6]. A study of the influence of radio frequency power on the main properties of the films is highlighted. Optical, electrical and structural characterization of the SnOx films was carried out. Results are reported and discussed.
3894
J. Valente et al. / Surface & Coatings Technology 202 (2008) 3893–3896
2. Experimental SnOx thin films were deposited by rf-PERTE of Sn in the presence of oxygen, at low substrate temperature (b 140 °C). A tungsten crucible was used for the tin evaporation. Reaction between tin and oxygen is enhanced by an oxygen plasma generated by capacitively coupled radio frequency electrode (metallic grid), placed between the resistance-heated crucible and the substrates holder (Fig. 1). The dimensions of the grid are 16 cm × 16 cm, located 10 cm from the substrates holder. That plasma promotes oxygen decomposition (dissociation) into generally more chemically active radicals that readily react (oxidation) with the evaporated tin, during its transport to the substrate [7]. The substrates used for this work were window and alkali free glasses, cleaned before loading. The base pressure in the evaporation chamber for starting the process is pin = 2.4 × 10− 3 Pa. A flux of oxygen is introduced into the chamber, controlled by a previously calibrated Matheson flowmeter (610A tube). A gate valve placed between the diffusion pump and deposition chamber, partially closed, increases the residence time of oxygen in the chamber. The evaporation of tin is performed steadily and the time of evaporation recorded to allow calculating the evaporation rate. SnOx thin films deposition parameters were the following: oxygen pressure during deposition: ≈ 6 × 10− 2 Pa; oxygen flux: ≈ 2.7 sccm; temperature of substrate: b 140 °C and deposition rate in the range: 0.1– 0.2 nm/s. Substrate temperature is measured before the rf power is turned on, with a thermocouple coupled to the substrate holder. A Dressler Ceaser 136 radio frequency power generator (13.6 MHz) was used. The rf power varied in the range 40– 70 W and is matched to the rf electrode through an automatic matching box, minimizing the reflected rf power. This variation causes a slight influence on the thickness of the SnOx films but an average thickness of 90 nm was considered. The thickness of the SnOx films was measured using a Dektak 3000 profilometer. The total transmittance, T, was measured in the range: 190–2500 nm, using a Shimadzu UV-3100 spectrophotometer, without a bare substrate across the reference beam. The aver-
Fig. 2. Variation of transmittance spectra with rf power for undoped SnOx thin films deposited by rf-PERTE on glass substrates at low substrate temperature (b140 °C).
age sheet resistance of the films was measured using a Veeco FPP-5000 four point probe. Sheet resistance values allow us to obtain the conductivity (in air) of the films deposited [8]. The conductivity as function of temperature was measured in a vacuum chamber (≈ 2 Pa), using a Keithley 617 and 228A electrometer and power supply, respectively. A Peltier junction is connected to the power supply to control the temperature in this measurement. Phase analysis and crystallinity were studied by glancing incidence X-ray diffraction (GIXRD) at 1° incidence angle to the specimen surface using a Siemens D5000 diffractometer equipped with a Cu Kα X-ray source. 3. Results and discussion The electrical characterization of undoped SnOx films deposited at rf power values in the range 40–70 W and at a substrate temperature b140 °C showed some surprising results. Films simultaneously highly transparent and conductive were obtained in the range 55–60 W. Films deposited at lower values of rf power show some slight change in visible transparency. However, in the electrical and structural properties of the films, a strong variation is observed. Our study of the undoped SnOx thin films was focused on this variation of their electrical and structural properties and a correlation between the data obtained is presented. 3.1. Optical properties of SnOx films
Fig. 1. Schematic diagram of the experimental set up for the rf-PERTE technique.
Fig. 2 shows the total transmittance spectra in the range 190– 2500 nm of undoped SnOx thin films deposited by rf-PERTE at low substrate temperature (b 140 °C) onto window glass substrates at five different rf power values. It can be seen that the films have an average visible transmittance of 80%, practically independent of the rf power. However, for films deposited at rf power of 40 W, there is a decrease in transmittance for shorter wavelengths (190–500 nm). As transparency requires oxidation of the films, we can conclude that a less efficient oxidation process of the metallic components of the film occurs at these low rf power values [9].
J. Valente et al. / Surface & Coatings Technology 202 (2008) 3893–3896
3895
Table 1 Average visible transmittance (T) and electrical conductivity at 25 °C (σ), in air and vacuum, as a function of rf power for SnOx films deposited by rf-PERTE at low temperature (b140 °C) rf power (W)
T400–800 nm (%) σ25 °C (in air) (Ω cm)− 1 σ25 °C (in vacuum) (Ω cm)− 1
Fig. 3. Variation of electrical resistivity/conductivity with rf power for undoped SnOx deposited by rf-PERTE on glass, at low substrate temperature (b140 °C). Measurements were carried out in air.
3.2. Electrical properties of the SnOx films Fig. 3 shows the variation of electrical resistivity (left axis) and conductivity (right axis) as a function of rf power value for undoped SnOx films deposited at low substrate temperature b 140 °C, on window glass substrates. As can be seen, the highest electrical conductivity, σ = 34.6 (Ω cm)− 1, is obtained for undoped SnOx films deposited at rf power of 60 W. For lower or higher values of rf power, more resistive films were obtained and for films deposited at 40 W the lowest conductivity, σ = 7.5 × 10− 1 (Ω cm)− 1, is achieved. This strong variation can be attributed to an excess of oxygen vacancies in the SnOx films deposited at the lowest rf power that may result in lattice structural disorder which reduces mobility of carriers and increases light loss [10]. As electrical conduction in transparent undoped conductors is considered to be due to oxygen vacancies (non-stoichiometric films), which contribute to the generation of carriers in the film [11], it may be concluded that the ideal oxidizing conditions have been reached for the films deposited in the range 55–60 W, for the oxygen flux used. As SnOx films must fulfil both requirements i.e. high
Fig. 4. Plot of the conductivity vs. 103/T for undoped SnOx films deposited at different rf power by rf-PERTE on alkali free glass at low substrate temperature (b140 °C). Measurements were performed in vacuum.
40
50
55
60
70
70.0 7.54 × 10− 1 2.95 × 10− 4
76.0 19.5 –
76.0 26.2 –
78.0 34.6 57.0
75.0 4.90 10.3
transparency and low electrical resistivity and this condition is only attained for films deposited within this rf power range, a very careful control of this deposition parameter is required. Fig. 4 shows the variation of electrical conductivity as a function of temperature, measured in vacuum, for the undoped SnOx films deposited on alkali free glass, at 40; 60 and 70 W. It can be observed that SnOx films deposited at 60 and 70 W have an electrical conductive behaviour. Furthermore, the conductivity of the films deposited at 60 W is higher than the conductivity of SnOx films deposited at 70 W, σ(25 °C) = 57.0 and 10.3 (Ω cm)− 1, respectively. Undoped SnOx films deposited at 40 W are semiconductive, showing an electrical conductivity of σ(25 °C) = 2.95 × 10− 4 (Ω cm)− 1, about four orders of magnitude lower than the conductivity of the SnOx films grown at 60 W. The values in Table 1 confirms that the ideal nonstoichiometry is attained for undoped SnOx films deposited at 60 W, showing the highest visible transmittance associated to the highest electrical conductivity. 3.3. Structural characterization of undoped SnOx films Fig. 5 shows glancing incidence X-ray diffraction patterns of undoped SnOx films deposited on glass substrates by rf-PERTE at different values of rf power. As can be seen, the increase of the rf power from 40 to 70 W induces some crystallization of the films. SnOx films grown at 40 W are amorphous, showing the lowest electrical conductivity (Table 1). The insets on Fig. 5 clearly show the presence of broad and low intensity peaks centred at ~ 34° and ~ 52° for the 60 and 70 W samples. By
Fig. 5. Glancing incidence X-ray diffraction patterns for SnOx samples obtained by rf-PERTE at different rf power values. Labelled peaks refer to the orthorhombic phase of SnO2.
3896
J. Valente et al. / Surface & Coatings Technology 202 (2008) 3893–3896
comparing with the JCPDS database [12], these peaks can be assigned to the orthorhombic crystallographic structure of SnO2. As a low temperature process, the variation of rf power and the low temperature of the substrates are the only energy supply for crystallization of the films in this type of technique. Variations of the rf power values led to very different properties of the films and we verified that nanocrystallization only occurs for higher values of rf power (60–70 W), as a lack of crystallinity was obtained for SnOx films deposited at 40 W. Table 1 also shows that for the nanocrystalline SnOx films, the values of conductivity are always higher when conductivity is measured in vacuum as a function of temperature. The opposite is verified for the amorphous SnOx films. It is known that moderate heating of SnOx films in different pressure conditions (in air or in vacuum) can modify the conduction process in this material due to reversible surface chemisorption of oxygen, removing or creating charge carriers [13]. This SnOx films sensitivity to structure modifications and environmental conditions can be responsible for these variations on the conductivity of the films, which are more significant for amorphous SnOx films, suggesting a potential application of these SnOx films in the sensors field. 4. Conclusions A simple technique for preparing undoped, conductive/ semiconductive and transparent tin oxide (SnOx) thin films at low temperature (≈ 140 °C) was reported. Undoped SnOx films were deposited by rf-PERTE of tin (Sn) in the presence of oxygen. The increase of the rf power from 40 to 70 W induces some crystallization of the films with a consequent electrical behaviour variation. Undoped SnOx films grown at rf power values around 60 W are nanocrystalline, conductive, exhibiting an average total visible transmittance of 80% and a maximum electrical conductivity, σ (in air) = 34.6 (Ω cm)− 1. SnOx films deposited at lower values of rf power (40 W) are amorphous and display an evident semiconductive behaviour, with values of conductivity, σ (in air) = 7.5 × 10− 1 (Ω cm)− 1, about two orders of magnitude lower than those SnOx films deposited at 60 W. When these conductivities were measured in vacuum as a
function of temperature, different values were obtained. Higher values for the nanocrystalline SnOx films, lower for the amorphous ones. Moderate heating of SnOx in different pressure conditions can modify the conduction process in SnOx due to reversible surface chemisorption of oxygen. As a final conclusion we can say that the deposition of undoped SnOx films at low substrate temperature with reasonable optical and electrical properties was achieved. Moreover, the technique has a wide range of application since it leads to conductive or semiconductive thin films by just varying the rf power. It is a low temperature technique that allows depositing and oxidizing thin films of almost every kind of metals, on flexible substrates. Acknowledgements The authors gratefully acknowledge CENIMAT for thickness measurements facilities. This work was supported by “Fundação para a Ciência e a Tecnologia” through a Pluriannual Contract with ICEMS (IST). References [1] J.C. Jiang, K. Lian, E.I. Meletis, Thin Solid Films 411 (2002) 203. [2] E. Elangovan, K. Ramamurthi, Appl. Surf. Sci. 249 (2005) 183. [3] P. Serini, V. Briois, M.C. Horrillo, A. Traverse, L. Manes, Thin Solid Films 304 (1997) 113. [4] N.S. Choudhury, R.P. Goehner, N. Lewis, R.W. Green, Thin Solid Films 122 (1984) 231. [5] W.A. MacDonald, J. Mater. Chem. 14 (2004) 4. [6] C. Nunes de Carvalho, G. Lavareda, A. Amaral, O. Conde, A.R. Ramos, J. Non-Cryst. Solids 352 (2006) 2315. [7] T. Karasawa, Y. Miyata, Thin Solid Films 223 (1993) 135. [8] L.I. Maissel, in: L.I. Maissel, R. Glang (Eds.), Handbook of Thin Film Technology, Chapter 13, McGraw-Hill, Inc., 1970, p. 13. [9] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics Publishing, 1995, p. 137. [10] W.-K. Lee, T. Machino, T. Sugihara, Thin Solid Films 224 (1993) 105. [11] S.-K. Song, J.-S. Cho, W.-K. Choi, H.-J. Jung, D. Choi, J.-Y. Lee, H.-K. Baik, S.-K. Koh, Sens. Actuators, B, Chem. 46 (1998) 42. [12] Joint Committee on Powder Diffraction Standards, ASTM, Philadelphia, PA, 1992. [13] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics Publishing, 1995, p. 9.
Surface & Coatings Technology 202 (2008) 3893 – 3896 www.elsevier.com/locate/surfcoat
Role of rf power on the properties of undoped SnOx films deposited by rf-PERTE at low substrate temperature J. Valente a,⁎, G. Lavareda a , O. Conde b , P. Parreira a , A. Amaral c , C. Nunes de Carvalho a a
UNL-FCT, Dept. Ciência dos Materiais, Campus da Caparica, 2829-516 Caparica, Portugal b Univ. Lisboa, Fac. Ciências, Dep. Física e ICEMS, 1749-016 Lisboa, Portugal c UTL-IST, Dep. Física e ICEMS, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Received 18 October 2007; accepted in revised form 30 January 2008 Available online 8 February 2008
Abstract Transparent and conductive undoped tin oxide (SnOx) thin films were deposited at low substrate temperature (b 140 °C) by radio frequency (rf) plasma enhanced reactive thermal evaporation (rf-PERTE) of tin (Sn) in the presence of oxygen The undoped SnOx films were not submitted to any post-annealing treatments. The influence of rf power variation on the optical, electrical and structural properties of the as-grown films is presented. A variation in the films' structure was verified with the increase of rf power. Undoped SnOx films, 90 nm average thick, deposited at rf power range of 60–70 W are nanocrystalline, show a conductive behaviour, an average visible transmittance of ≥ 80% and a maximum electrical conductivity of about 34.6 (Ω cm)− 1. Films deposited at lower values of rf power (40 W) are amorphous and exhibit a semiconductive behaviour, showing an electrical conductivity of about 7.54 × 10− 1 (Ω cm)− 1. As a low substrate temperature deposition process is used, SnOx thin films can be obtained on a wide range of substrates. © 2008 Elsevier B.V. All rights reserved. Keywords: SnOx; rf-PERTE; Thin film; Transparent conductive oxide; Plasma; Low temperature
1. Introduction One of the most promising fields in materials science is related to the fundamental aspects and applications of transparent conductive thin films. Their potential applications in the industry generated a technological interest on the study of these films. Among the transparent conductors known nowadays, tin oxide is one of the most interesting due to its (1) excellent physical properties (electrical conductivity and visible transparency) and (2) several technological applications, such as gas sensors, solar cells and as electrodes for opto-electric devices. As it is known, tin oxide is an n-type semiconductor that has attracted significant attention, as thin film, because of their improved performance such as high gas sensitivity and fast oxidation/reduction response [1]. SnOx films may be deposited ⁎ Corresponding author. Tel.: +351 218419279; fax: +351 218464455/57. E-mail address: [email protected] (J. Valente). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.01.033
by a number of techniques, including spray pyrolysis [2], sputtering [3] and electron beam [4]. The high substrate temperature (or post-annealing treatments) required in these techniques to obtain SnOx with good optical, electrical and structural properties, is the main drawback. If SnOx transparent conducting thin films can be deposited at low substrate temperatures (b 140 °C), a wide range of optoelectronic SnOx based devices can be laid upon any type of substrates, from glass to flexible polymers [5]. Ultra-thin, light weight and lowpower consumption devices can be made, promising displays that can be deformable. In the present work we report on the deposition of undoped and highly transparent, conductive SnOx thin films grown at low temperature. The technique used is the rf-PERTE of tin (Sn) in the presence of oxygen [6]. A study of the influence of radio frequency power on the main properties of the films is highlighted. Optical, electrical and structural characterization of the SnOx films was carried out. Results are reported and discussed.
3894
J. Valente et al. / Surface & Coatings Technology 202 (2008) 3893–3896
2. Experimental SnOx thin films were deposited by rf-PERTE of Sn in the presence of oxygen, at low substrate temperature (b 140 °C). A tungsten crucible was used for the tin evaporation. Reaction between tin and oxygen is enhanced by an oxygen plasma generated by capacitively coupled radio frequency electrode (metallic grid), placed between the resistance-heated crucible and the substrates holder (Fig. 1). The dimensions of the grid are 16 cm × 16 cm, located 10 cm from the substrates holder. That plasma promotes oxygen decomposition (dissociation) into generally more chemically active radicals that readily react (oxidation) with the evaporated tin, during its transport to the substrate [7]. The substrates used for this work were window and alkali free glasses, cleaned before loading. The base pressure in the evaporation chamber for starting the process is pin = 2.4 × 10− 3 Pa. A flux of oxygen is introduced into the chamber, controlled by a previously calibrated Matheson flowmeter (610A tube). A gate valve placed between the diffusion pump and deposition chamber, partially closed, increases the residence time of oxygen in the chamber. The evaporation of tin is performed steadily and the time of evaporation recorded to allow calculating the evaporation rate. SnOx thin films deposition parameters were the following: oxygen pressure during deposition: ≈ 6 × 10− 2 Pa; oxygen flux: ≈ 2.7 sccm; temperature of substrate: b 140 °C and deposition rate in the range: 0.1– 0.2 nm/s. Substrate temperature is measured before the rf power is turned on, with a thermocouple coupled to the substrate holder. A Dressler Ceaser 136 radio frequency power generator (13.6 MHz) was used. The rf power varied in the range 40– 70 W and is matched to the rf electrode through an automatic matching box, minimizing the reflected rf power. This variation causes a slight influence on the thickness of the SnOx films but an average thickness of 90 nm was considered. The thickness of the SnOx films was measured using a Dektak 3000 profilometer. The total transmittance, T, was measured in the range: 190–2500 nm, using a Shimadzu UV-3100 spectrophotometer, without a bare substrate across the reference beam. The aver-
Fig. 2. Variation of transmittance spectra with rf power for undoped SnOx thin films deposited by rf-PERTE on glass substrates at low substrate temperature (b140 °C).
age sheet resistance of the films was measured using a Veeco FPP-5000 four point probe. Sheet resistance values allow us to obtain the conductivity (in air) of the films deposited [8]. The conductivity as function of temperature was measured in a vacuum chamber (≈ 2 Pa), using a Keithley 617 and 228A electrometer and power supply, respectively. A Peltier junction is connected to the power supply to control the temperature in this measurement. Phase analysis and crystallinity were studied by glancing incidence X-ray diffraction (GIXRD) at 1° incidence angle to the specimen surface using a Siemens D5000 diffractometer equipped with a Cu Kα X-ray source. 3. Results and discussion The electrical characterization of undoped SnOx films deposited at rf power values in the range 40–70 W and at a substrate temperature b140 °C showed some surprising results. Films simultaneously highly transparent and conductive were obtained in the range 55–60 W. Films deposited at lower values of rf power show some slight change in visible transparency. However, in the electrical and structural properties of the films, a strong variation is observed. Our study of the undoped SnOx thin films was focused on this variation of their electrical and structural properties and a correlation between the data obtained is presented. 3.1. Optical properties of SnOx films
Fig. 1. Schematic diagram of the experimental set up for the rf-PERTE technique.
Fig. 2 shows the total transmittance spectra in the range 190– 2500 nm of undoped SnOx thin films deposited by rf-PERTE at low substrate temperature (b 140 °C) onto window glass substrates at five different rf power values. It can be seen that the films have an average visible transmittance of 80%, practically independent of the rf power. However, for films deposited at rf power of 40 W, there is a decrease in transmittance for shorter wavelengths (190–500 nm). As transparency requires oxidation of the films, we can conclude that a less efficient oxidation process of the metallic components of the film occurs at these low rf power values [9].
J. Valente et al. / Surface & Coatings Technology 202 (2008) 3893–3896
3895
Table 1 Average visible transmittance (T) and electrical conductivity at 25 °C (σ), in air and vacuum, as a function of rf power for SnOx films deposited by rf-PERTE at low temperature (b140 °C) rf power (W)
T400–800 nm (%) σ25 °C (in air) (Ω cm)− 1 σ25 °C (in vacuum) (Ω cm)− 1
Fig. 3. Variation of electrical resistivity/conductivity with rf power for undoped SnOx deposited by rf-PERTE on glass, at low substrate temperature (b140 °C). Measurements were carried out in air.
3.2. Electrical properties of the SnOx films Fig. 3 shows the variation of electrical resistivity (left axis) and conductivity (right axis) as a function of rf power value for undoped SnOx films deposited at low substrate temperature b 140 °C, on window glass substrates. As can be seen, the highest electrical conductivity, σ = 34.6 (Ω cm)− 1, is obtained for undoped SnOx films deposited at rf power of 60 W. For lower or higher values of rf power, more resistive films were obtained and for films deposited at 40 W the lowest conductivity, σ = 7.5 × 10− 1 (Ω cm)− 1, is achieved. This strong variation can be attributed to an excess of oxygen vacancies in the SnOx films deposited at the lowest rf power that may result in lattice structural disorder which reduces mobility of carriers and increases light loss [10]. As electrical conduction in transparent undoped conductors is considered to be due to oxygen vacancies (non-stoichiometric films), which contribute to the generation of carriers in the film [11], it may be concluded that the ideal oxidizing conditions have been reached for the films deposited in the range 55–60 W, for the oxygen flux used. As SnOx films must fulfil both requirements i.e. high
Fig. 4. Plot of the conductivity vs. 103/T for undoped SnOx films deposited at different rf power by rf-PERTE on alkali free glass at low substrate temperature (b140 °C). Measurements were performed in vacuum.
40
50
55
60
70
70.0 7.54 × 10− 1 2.95 × 10− 4
76.0 19.5 –
76.0 26.2 –
78.0 34.6 57.0
75.0 4.90 10.3
transparency and low electrical resistivity and this condition is only attained for films deposited within this rf power range, a very careful control of this deposition parameter is required. Fig. 4 shows the variation of electrical conductivity as a function of temperature, measured in vacuum, for the undoped SnOx films deposited on alkali free glass, at 40; 60 and 70 W. It can be observed that SnOx films deposited at 60 and 70 W have an electrical conductive behaviour. Furthermore, the conductivity of the films deposited at 60 W is higher than the conductivity of SnOx films deposited at 70 W, σ(25 °C) = 57.0 and 10.3 (Ω cm)− 1, respectively. Undoped SnOx films deposited at 40 W are semiconductive, showing an electrical conductivity of σ(25 °C) = 2.95 × 10− 4 (Ω cm)− 1, about four orders of magnitude lower than the conductivity of the SnOx films grown at 60 W. The values in Table 1 confirms that the ideal nonstoichiometry is attained for undoped SnOx films deposited at 60 W, showing the highest visible transmittance associated to the highest electrical conductivity. 3.3. Structural characterization of undoped SnOx films Fig. 5 shows glancing incidence X-ray diffraction patterns of undoped SnOx films deposited on glass substrates by rf-PERTE at different values of rf power. As can be seen, the increase of the rf power from 40 to 70 W induces some crystallization of the films. SnOx films grown at 40 W are amorphous, showing the lowest electrical conductivity (Table 1). The insets on Fig. 5 clearly show the presence of broad and low intensity peaks centred at ~ 34° and ~ 52° for the 60 and 70 W samples. By
Fig. 5. Glancing incidence X-ray diffraction patterns for SnOx samples obtained by rf-PERTE at different rf power values. Labelled peaks refer to the orthorhombic phase of SnO2.
3896
J. Valente et al. / Surface & Coatings Technology 202 (2008) 3893–3896
comparing with the JCPDS database [12], these peaks can be assigned to the orthorhombic crystallographic structure of SnO2. As a low temperature process, the variation of rf power and the low temperature of the substrates are the only energy supply for crystallization of the films in this type of technique. Variations of the rf power values led to very different properties of the films and we verified that nanocrystallization only occurs for higher values of rf power (60–70 W), as a lack of crystallinity was obtained for SnOx films deposited at 40 W. Table 1 also shows that for the nanocrystalline SnOx films, the values of conductivity are always higher when conductivity is measured in vacuum as a function of temperature. The opposite is verified for the amorphous SnOx films. It is known that moderate heating of SnOx films in different pressure conditions (in air or in vacuum) can modify the conduction process in this material due to reversible surface chemisorption of oxygen, removing or creating charge carriers [13]. This SnOx films sensitivity to structure modifications and environmental conditions can be responsible for these variations on the conductivity of the films, which are more significant for amorphous SnOx films, suggesting a potential application of these SnOx films in the sensors field. 4. Conclusions A simple technique for preparing undoped, conductive/ semiconductive and transparent tin oxide (SnOx) thin films at low temperature (≈ 140 °C) was reported. Undoped SnOx films were deposited by rf-PERTE of tin (Sn) in the presence of oxygen. The increase of the rf power from 40 to 70 W induces some crystallization of the films with a consequent electrical behaviour variation. Undoped SnOx films grown at rf power values around 60 W are nanocrystalline, conductive, exhibiting an average total visible transmittance of 80% and a maximum electrical conductivity, σ (in air) = 34.6 (Ω cm)− 1. SnOx films deposited at lower values of rf power (40 W) are amorphous and display an evident semiconductive behaviour, with values of conductivity, σ (in air) = 7.5 × 10− 1 (Ω cm)− 1, about two orders of magnitude lower than those SnOx films deposited at 60 W. When these conductivities were measured in vacuum as a
function of temperature, different values were obtained. Higher values for the nanocrystalline SnOx films, lower for the amorphous ones. Moderate heating of SnOx in different pressure conditions can modify the conduction process in SnOx due to reversible surface chemisorption of oxygen. As a final conclusion we can say that the deposition of undoped SnOx films at low substrate temperature with reasonable optical and electrical properties was achieved. Moreover, the technique has a wide range of application since it leads to conductive or semiconductive thin films by just varying the rf power. It is a low temperature technique that allows depositing and oxidizing thin films of almost every kind of metals, on flexible substrates. Acknowledgements The authors gratefully acknowledge CENIMAT for thickness measurements facilities. This work was supported by “Fundação para a Ciência e a Tecnologia” through a Pluriannual Contract with ICEMS (IST). References [1] J.C. Jiang, K. Lian, E.I. Meletis, Thin Solid Films 411 (2002) 203. [2] E. Elangovan, K. Ramamurthi, Appl. Surf. Sci. 249 (2005) 183. [3] P. Serini, V. Briois, M.C. Horrillo, A. Traverse, L. Manes, Thin Solid Films 304 (1997) 113. [4] N.S. Choudhury, R.P. Goehner, N. Lewis, R.W. Green, Thin Solid Films 122 (1984) 231. [5] W.A. MacDonald, J. Mater. Chem. 14 (2004) 4. [6] C. Nunes de Carvalho, G. Lavareda, A. Amaral, O. Conde, A.R. Ramos, J. Non-Cryst. Solids 352 (2006) 2315. [7] T. Karasawa, Y. Miyata, Thin Solid Films 223 (1993) 135. [8] L.I. Maissel, in: L.I. Maissel, R. Glang (Eds.), Handbook of Thin Film Technology, Chapter 13, McGraw-Hill, Inc., 1970, p. 13. [9] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics Publishing, 1995, p. 137. [10] W.-K. Lee, T. Machino, T. Sugihara, Thin Solid Films 224 (1993) 105. [11] S.-K. Song, J.-S. Cho, W.-K. Choi, H.-J. Jung, D. Choi, J.-Y. Lee, H.-K. Baik, S.-K. Koh, Sens. Actuators, B, Chem. 46 (1998) 42. [12] Joint Committee on Powder Diffraction Standards, ASTM, Philadelphia, PA, 1992. [13] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics Publishing, 1995, p. 9.