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Effects of bias voltage on the properties of ITO films prepared on polymer substrates
Thin Solid Films 480–481 (2005) 157 – 161 www.elsevier.com/locate/tsf
Effects of bias voltage on the properties of ITO films prepared on polymer substrates Jaehyeong Leea,*, Hakkee Junga, Donggun Limb, Keajoon Yangb, Woochang Songc, Junsin Yic a
School of Electronics and Information Engineering, Kunsan National University, San 68, Miryong-dong, Kunsan, Chonbuk 573-701, South Korea b Department of Electronic Engineering, Chungju National University, Chungju, South Korea c School of Information and Communication Engineering, Sungkyunkwan University, Suwon, South Korea Available online 30 December 2004
Abstract The ITO (indium tin oxide) thin films were deposited on acryl, glass, PET, and poly-carbonate substrates by DC reactive magnetron sputtering. The bias voltage was changed from 20 to 80 V. As the bias voltage increased, the deposition rate of ITO films decreased regardless of substrate types. The roughness of the films on PET increased with the bias voltage. The study demonstrated that the bias improved the electrical and optical properties of ITO films regardless of substrate types. The lowest electrical resistivity of 5.5104 V-cm and visible transmittance of about 80% were achieved by applying a negative bias of 60 V. D 2004 Elsevier B.V. All rights reserved. Keywords: Indium tin oxide (ITO); Bias; Magnetron sputtering; Organic substrate
1. Introduction Transparent conducting indium tin oxide (ITO) thin films on organic flexible substrates have many merits, such as light weight, small volume and can make the obtained devices be folded and easily carried. They can be used in plastic liquid crystal display devices, transparent electrostatic discharge, and electromagnetic shielding materials, flexible photovoltaic devices, unbreakable heat reflecting mirrors [1]. However, it is necessary for ITO films to be deposited at very low substrate temperature for the poor thermal endurance of polymer substrates. The low substrate temperature is not in favor of depositing good quality films. A bias applied to the substrate can attract cations in the plasma to bombard the growing film. This bombardment gives an additional energy to the molecules and clusters condensed on substrate and peel off the molecules with weak bonding from the film [2]. Danson et al. [3] believed
* Corresponding author. Tel.: +82 63 4694707; fax: +82 63 4694699. E-mail address: [email protected] (J. Lee). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.144
that this kind of bombardment was beneficial to film crystallinity and could replace the energy provided by a higher substrate temperature. In addition, it is reported that composition of an ITO film was much dependent on substrate bias [4], and proper ion impact on the deposited film caused improvement of film properties [5]. In this study, the ITO films were deposited on acrylic, glass, polycarbonate (PC), and polyethylene terephthalate (PET) at room temperature by reactive DC magnetron sputtering. The effects of bias voltage on the deposition rate, electrical, and optical properties of the films were evaluated.
2. Experimental details ITO films were deposited by conventional DC magnetron sputtering system. The sputtering conditions during deposition are shown in Table 1. The target used in this study was an In-Sn alloy (99.99% purity, 2-in in diameter) with a composition of 90:10 (wt.%, Cerac, USA). The target– substrate distance was 45 mm. The substrates used in present work were acrylic, glass, polycarbonate (PC), and
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Table 1 Sputtering condition in present work Target Substrate Target–substrate distance Base pressure Sputtering pressure Oxygen gas ratio Bias voltage Substrate temperature Thickness
In–Sn alloy (9:1) Acrylic, glass, PC, PET 45 mm 2.6103 Pa 0.67 Pa 13% 20 to 80 V 40 8C 200 nm
polyethylene terephthalate (PET). Before being introduced into the chamber, the substrate was ultrasonically cleaned in a neutral detergent solution and rinsed in deionized water at least three times to eliminate possible contamination. The vacuum chamber was evacuated by turbo-molecular pump, and the base pressure before deposition was about 2.6103 Pa. The sputtering was conducted in the mixtures of Ar–O2 atmosphere. The oxygen concentration given as the [O2]/ ([Ar]+[O2]) was 13%. Two separated mass flow controllers were used to monitor the gas flow rates of argon and oxygen. Before the sputtering deposition, presputtering was carried out in an argon atmosphere for about 10 min to remove the surface oxide layer formed during exposure to air. The gas pressure was kept at 0.67 Pa, and the sputtering power during deposition was 30 W. The substrate surface was monitored by thermocouple during sputter deposition. Although the substrates were not intentionally heated, the surface temperature reached about 40 8C. The substrate was negatively biased with DC source of 20 to 80 V. The film thickness was measured by a conventional stylus surface profiler. The film thickness is about 200 nm. Because the polymer substrate, such as PET, deforms during electron microscopic investigation, atomic force microscope (AFM) was used to investigate the microstructure of films. A compositional analysis was performed using X-ray photoemission spectroscope (XPS). The sheet resistance of the samples was measured with a four-point probe, and the resistivity of the film was calculated. Carrier concentration and Hall mobility were obtained from Hall-effect measurement by the Van der Pauw technique. A UV-visible spectrophotometer was used to record the optical transmittance of the films.
Fig. 1. Deposition rate of ITO films deposited at different bias voltages.
number of molecules peeled off from the film increase, and the deposition rate consequently decreases. Fig. 2 shows a typical AFM surface image and the roughness of ITO films deposited at various bias voltages.
3. Results and discussion Only the bias voltage of sputtering parameters was varied in order to investigate the effect of bias voltage on film properties. Films on various substrates were deposited by applying different negative bias at the O2 gas ratio of 13%. Fig. 1 illustrates the deposition rates plotted versus the bias voltage for these films with a thickness of about 200 nm. The deposition rates exhibit a decrease for the bias increasing from 20 to 80 V regardless of substrate type. As the bias voltage increases, the bombardment makes the
Fig. 2. Typical AFM image (A) and roughness (B) of ITO films, (A) AFM image of films deposited on glass at 60 V, (B) surface roughness.
J. Lee et al. / Thin Solid Films 480–481 (2005) 157–161
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Table 2 XPS compositional analysis of ITO films Bias voltage (V) 20 40 60 80
Atomic concentration X In
X Sn
XO
44.3 49.4 48.6 48.9
4.7 4.7 5.3 5.7
51.0 46.0 46.1 45.4
Since the electrical conductivity of thin ITO films may be affected by carrier scattering from a rough surface, the surface morphology of ITO films was observed. A surface morphology similar to that observed in ITO films on glass substrates was also found on PET substrate. However, asdeposited ITO films on PET were rougher in comparison with those on glass substrate. The roughness of films deposited on PET substrate increases with increasing bias voltage, whereas for glass substrate, the roughness does not change with bias voltage so much. At high bias voltage of 80 V, the deposited films are resputtered by more energetic ions, and then the film exhibits rougher surface than that for low bias voltage. Table 2 presents XPS compositional analysis of ITO films deposited at different bias voltages. A lack of oxygen is observed in all samples, and the film becomes more nonstoichiometric as the bias voltage increases from 20 V to 40 V. It suggests that the number of oxygen vacancies increases at higher bias voltage. Fig. 3 shows the electrical resistivity of the ITO films as a function of the bias voltage. The resistivity decreased rapidly with increasing the bias voltage regardless of substrate types. The reduced resistivities are attributed to the increased impurity donors, which improve the carrier density of the films. A similar result was reported by Yang et al. [6].When the bias voltage of 80 V is applied, the resistivity of ITO films increased. The minimum resistivity of the films is 5.5104 V-cm for glass substrate, which is
Fig. 3. Dependence of electrical resistivity of ITO films on bias voltage.
Fig. 4. Carrier concentration and mobility of ITO films deposited on PET substrate as a function of bias voltage.
Fig. 5. Optical transmittance spectra of ITO films deposited at different bias voltages: (A) glass substrate and (B) PET substrate.
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Fig. 6. Reflectance in the infrared region of ITO films deposited on PET substrate at different bias voltages.
lower than the resistivity of 1103 V-cm for films deposited on water-cooled mylar by conventional magnetron sputtering, as reported by Mansingh and Vasant Kumar [7]. There is still considerable discrepancy on the role of the substrate on film properties. Mukherjee [8] deposited ITO on acrylic and polycarbonate substrates and reported that the substrate type had little or no influence on film properties. In contrast, Kulkarni et at. [9] studied ITO films growth on glass, polycarbonate, and PET substrate and reported clear differences in film conductivity dependent upon substrate type. Additional insight into the effects of surface roughness on film quality should be gained from future experiments using an AFM. Fig. 4 shows the carrier concentration and the Hall mobility of the ITO films deposited on PET substrate as a function of the bias voltage. The dependence of carrier concentration on bias voltage was contributed by oxygen content in the film. In the case of 40 V, the carrier concentration increased because of the increase of oxygen deficiency, as seen in Table 2. On the other hand, the mobility of the films did not change so much. Therefore, in this case, the resistivity mainly depends on the carrier concentration, and the bias voltage of 40 V did not change the mobility. For the bias voltage increasing from 40 to 60 V, the carrier concentration shows to be saturated, and the mobility is enhanced, hence, the resistivity decreases. The lower mobility at the bias voltage of 80 V correlates to a larger surface roughness, as seen in Fig. 2(b). Honda et al. [10] proposed a model of deposition process with bias voltage. For low bias voltage (V biasb20 V), oxygen atoms, which might be damaged largely due to light mass in the deposited film, are resputtered by energetic Ar ions. Then the film with much oxygen deficiency was formed. However, for high bias voltage (V biasN40 V), oxygen atoms and also indium atoms are resputtered by more energetic ions. Then the film had a composition relatively
close to stoichiometry. In addition, they reported that the substrate negative voltage is available to control the oxygen composition, and the proper bias voltage which is 20 V in this study effectively produced the oxygen deficiency. Fig. 5 shows the optical transmittance of ITO films deposited on glass and PET substrate as a function of bias voltages. In the visible region, the transmittance improves with increasing the bias voltages regardless of substrate type. Possibly, with increasing the negative bias, the density of the films increased due to the enhancement of the ion bombardment, which makes the transmittance increase. Fig. 6 shows the IR reflectance of ITO films deposited at different bias voltages. The IR reflectance of the ITO films increases significantly with increasing bias voltage, reached a maximum at 40 V, and then reduced with further increase of the bias voltage. Frank et al. [11] showed that the IR reflectance R can be expressed by R¼1
4e0 c0 1 e dN l
ð1Þ
where c 0 is the velocity of light, e 0 is the permittivity of free space, e is the electronic charge, N is the carrier concentration, d is the film thickness, and l is the mobility of the free carriers. According to Eq. (1), the IR reflectance increases with the product of the carrier concentration N and carrier mobility l. As discussed previously, the ITO films deposited at the bias voltage of 40 V have smaller resistivities and higher values of the product Nl than those deposited at 20 V. However, the product of Nl, after reaching a maximum, starts decreasing again with further bias voltage.
4. Conclusions ITO films have been deposited by DC reactive magnetron sputtering by using metal In–Sn alloy target in an Ar–O2 gas mixture. The influences of substrate bias voltage on electrical and optical properties of the films were studied. Highly transparent ITO films with a visible transmittance of 80% and a low resistivity of 5.5104 V-cm have been obtained by applying a negative bias of 60 V to the substrate. In the wavelength range of 300–900 nm, the transmittance was improved with increasing a bias voltage.
Acknowledgement This work was supported by Korea Industrial Technology Foundation.
References [1] J.M. Margolis, Conductive Polymers and Plastics, Chapman & Hall, London, 1989.
J. Lee et al. / Thin Solid Films 480–481 (2005) 157–161 [2] Z.W. Yang, S.H. Han, T.L. Yang, Lina Ye, D.H. Zhang, H.L. Ma, C.F. Cheng, Thin Solid Films 366 (2000) 4. [3] N. Danson, I. Safi, G.W. Hall, R.P. Howson, Surf. Coat. Technol. 99 (1998) 147. [4] R.D. Bland, G.J. Kominiak, D.M. Mattox, J. Vac. Sci. Technol. 11 (1974) 671. [5] E. Kubota, Y. Shigesato, M. Igarashi, K. Suzuki, Jpn. J. Appl. Phys. 33 (1994) 4997. [6] Z. Yang, Shenghao Han, Tianlin Yang, Lina Ye, Honglei Ma, Chuanfu Cheng, Appl. Surf. Sci. 161 (2000) 279.
161
[7] A. Mansingh, C.V.R. Vasant Kumar, Thin Solid Films 167 (1988) L11. [8] A. Mukherjee, Vacuum 39 (1989) 537. [9] A.K. Kulkarni, K.H. Schulz, T.S. Lim, M. Khan, Thin Solid Films 308 (1997) 1. [10] S. Honda, K. Chihara, M. Watamori, K. Oura, Appl. Surf. Sci. 113/114 (1997) 408. [11] G. Frank, E. Kauer, H. Kfstlin, Thin Solid Films 77 (1981) 107.
Effects of bias voltage on the properties of ITO films prepared on polymer substrates Jaehyeong Leea,*, Hakkee Junga, Donggun Limb, Keajoon Yangb, Woochang Songc, Junsin Yic a
School of Electronics and Information Engineering, Kunsan National University, San 68, Miryong-dong, Kunsan, Chonbuk 573-701, South Korea b Department of Electronic Engineering, Chungju National University, Chungju, South Korea c School of Information and Communication Engineering, Sungkyunkwan University, Suwon, South Korea Available online 30 December 2004
Abstract The ITO (indium tin oxide) thin films were deposited on acryl, glass, PET, and poly-carbonate substrates by DC reactive magnetron sputtering. The bias voltage was changed from 20 to 80 V. As the bias voltage increased, the deposition rate of ITO films decreased regardless of substrate types. The roughness of the films on PET increased with the bias voltage. The study demonstrated that the bias improved the electrical and optical properties of ITO films regardless of substrate types. The lowest electrical resistivity of 5.5104 V-cm and visible transmittance of about 80% were achieved by applying a negative bias of 60 V. D 2004 Elsevier B.V. All rights reserved. Keywords: Indium tin oxide (ITO); Bias; Magnetron sputtering; Organic substrate
1. Introduction Transparent conducting indium tin oxide (ITO) thin films on organic flexible substrates have many merits, such as light weight, small volume and can make the obtained devices be folded and easily carried. They can be used in plastic liquid crystal display devices, transparent electrostatic discharge, and electromagnetic shielding materials, flexible photovoltaic devices, unbreakable heat reflecting mirrors [1]. However, it is necessary for ITO films to be deposited at very low substrate temperature for the poor thermal endurance of polymer substrates. The low substrate temperature is not in favor of depositing good quality films. A bias applied to the substrate can attract cations in the plasma to bombard the growing film. This bombardment gives an additional energy to the molecules and clusters condensed on substrate and peel off the molecules with weak bonding from the film [2]. Danson et al. [3] believed
* Corresponding author. Tel.: +82 63 4694707; fax: +82 63 4694699. E-mail address: [email protected] (J. Lee). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.144
that this kind of bombardment was beneficial to film crystallinity and could replace the energy provided by a higher substrate temperature. In addition, it is reported that composition of an ITO film was much dependent on substrate bias [4], and proper ion impact on the deposited film caused improvement of film properties [5]. In this study, the ITO films were deposited on acrylic, glass, polycarbonate (PC), and polyethylene terephthalate (PET) at room temperature by reactive DC magnetron sputtering. The effects of bias voltage on the deposition rate, electrical, and optical properties of the films were evaluated.
2. Experimental details ITO films were deposited by conventional DC magnetron sputtering system. The sputtering conditions during deposition are shown in Table 1. The target used in this study was an In-Sn alloy (99.99% purity, 2-in in diameter) with a composition of 90:10 (wt.%, Cerac, USA). The target– substrate distance was 45 mm. The substrates used in present work were acrylic, glass, polycarbonate (PC), and
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Table 1 Sputtering condition in present work Target Substrate Target–substrate distance Base pressure Sputtering pressure Oxygen gas ratio Bias voltage Substrate temperature Thickness
In–Sn alloy (9:1) Acrylic, glass, PC, PET 45 mm 2.6103 Pa 0.67 Pa 13% 20 to 80 V 40 8C 200 nm
polyethylene terephthalate (PET). Before being introduced into the chamber, the substrate was ultrasonically cleaned in a neutral detergent solution and rinsed in deionized water at least three times to eliminate possible contamination. The vacuum chamber was evacuated by turbo-molecular pump, and the base pressure before deposition was about 2.6103 Pa. The sputtering was conducted in the mixtures of Ar–O2 atmosphere. The oxygen concentration given as the [O2]/ ([Ar]+[O2]) was 13%. Two separated mass flow controllers were used to monitor the gas flow rates of argon and oxygen. Before the sputtering deposition, presputtering was carried out in an argon atmosphere for about 10 min to remove the surface oxide layer formed during exposure to air. The gas pressure was kept at 0.67 Pa, and the sputtering power during deposition was 30 W. The substrate surface was monitored by thermocouple during sputter deposition. Although the substrates were not intentionally heated, the surface temperature reached about 40 8C. The substrate was negatively biased with DC source of 20 to 80 V. The film thickness was measured by a conventional stylus surface profiler. The film thickness is about 200 nm. Because the polymer substrate, such as PET, deforms during electron microscopic investigation, atomic force microscope (AFM) was used to investigate the microstructure of films. A compositional analysis was performed using X-ray photoemission spectroscope (XPS). The sheet resistance of the samples was measured with a four-point probe, and the resistivity of the film was calculated. Carrier concentration and Hall mobility were obtained from Hall-effect measurement by the Van der Pauw technique. A UV-visible spectrophotometer was used to record the optical transmittance of the films.
Fig. 1. Deposition rate of ITO films deposited at different bias voltages.
number of molecules peeled off from the film increase, and the deposition rate consequently decreases. Fig. 2 shows a typical AFM surface image and the roughness of ITO films deposited at various bias voltages.
3. Results and discussion Only the bias voltage of sputtering parameters was varied in order to investigate the effect of bias voltage on film properties. Films on various substrates were deposited by applying different negative bias at the O2 gas ratio of 13%. Fig. 1 illustrates the deposition rates plotted versus the bias voltage for these films with a thickness of about 200 nm. The deposition rates exhibit a decrease for the bias increasing from 20 to 80 V regardless of substrate type. As the bias voltage increases, the bombardment makes the
Fig. 2. Typical AFM image (A) and roughness (B) of ITO films, (A) AFM image of films deposited on glass at 60 V, (B) surface roughness.
J. Lee et al. / Thin Solid Films 480–481 (2005) 157–161
159
Table 2 XPS compositional analysis of ITO films Bias voltage (V) 20 40 60 80
Atomic concentration X In
X Sn
XO
44.3 49.4 48.6 48.9
4.7 4.7 5.3 5.7
51.0 46.0 46.1 45.4
Since the electrical conductivity of thin ITO films may be affected by carrier scattering from a rough surface, the surface morphology of ITO films was observed. A surface morphology similar to that observed in ITO films on glass substrates was also found on PET substrate. However, asdeposited ITO films on PET were rougher in comparison with those on glass substrate. The roughness of films deposited on PET substrate increases with increasing bias voltage, whereas for glass substrate, the roughness does not change with bias voltage so much. At high bias voltage of 80 V, the deposited films are resputtered by more energetic ions, and then the film exhibits rougher surface than that for low bias voltage. Table 2 presents XPS compositional analysis of ITO films deposited at different bias voltages. A lack of oxygen is observed in all samples, and the film becomes more nonstoichiometric as the bias voltage increases from 20 V to 40 V. It suggests that the number of oxygen vacancies increases at higher bias voltage. Fig. 3 shows the electrical resistivity of the ITO films as a function of the bias voltage. The resistivity decreased rapidly with increasing the bias voltage regardless of substrate types. The reduced resistivities are attributed to the increased impurity donors, which improve the carrier density of the films. A similar result was reported by Yang et al. [6].When the bias voltage of 80 V is applied, the resistivity of ITO films increased. The minimum resistivity of the films is 5.5104 V-cm for glass substrate, which is
Fig. 3. Dependence of electrical resistivity of ITO films on bias voltage.
Fig. 4. Carrier concentration and mobility of ITO films deposited on PET substrate as a function of bias voltage.
Fig. 5. Optical transmittance spectra of ITO films deposited at different bias voltages: (A) glass substrate and (B) PET substrate.
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Fig. 6. Reflectance in the infrared region of ITO films deposited on PET substrate at different bias voltages.
lower than the resistivity of 1103 V-cm for films deposited on water-cooled mylar by conventional magnetron sputtering, as reported by Mansingh and Vasant Kumar [7]. There is still considerable discrepancy on the role of the substrate on film properties. Mukherjee [8] deposited ITO on acrylic and polycarbonate substrates and reported that the substrate type had little or no influence on film properties. In contrast, Kulkarni et at. [9] studied ITO films growth on glass, polycarbonate, and PET substrate and reported clear differences in film conductivity dependent upon substrate type. Additional insight into the effects of surface roughness on film quality should be gained from future experiments using an AFM. Fig. 4 shows the carrier concentration and the Hall mobility of the ITO films deposited on PET substrate as a function of the bias voltage. The dependence of carrier concentration on bias voltage was contributed by oxygen content in the film. In the case of 40 V, the carrier concentration increased because of the increase of oxygen deficiency, as seen in Table 2. On the other hand, the mobility of the films did not change so much. Therefore, in this case, the resistivity mainly depends on the carrier concentration, and the bias voltage of 40 V did not change the mobility. For the bias voltage increasing from 40 to 60 V, the carrier concentration shows to be saturated, and the mobility is enhanced, hence, the resistivity decreases. The lower mobility at the bias voltage of 80 V correlates to a larger surface roughness, as seen in Fig. 2(b). Honda et al. [10] proposed a model of deposition process with bias voltage. For low bias voltage (V biasb20 V), oxygen atoms, which might be damaged largely due to light mass in the deposited film, are resputtered by energetic Ar ions. Then the film with much oxygen deficiency was formed. However, for high bias voltage (V biasN40 V), oxygen atoms and also indium atoms are resputtered by more energetic ions. Then the film had a composition relatively
close to stoichiometry. In addition, they reported that the substrate negative voltage is available to control the oxygen composition, and the proper bias voltage which is 20 V in this study effectively produced the oxygen deficiency. Fig. 5 shows the optical transmittance of ITO films deposited on glass and PET substrate as a function of bias voltages. In the visible region, the transmittance improves with increasing the bias voltages regardless of substrate type. Possibly, with increasing the negative bias, the density of the films increased due to the enhancement of the ion bombardment, which makes the transmittance increase. Fig. 6 shows the IR reflectance of ITO films deposited at different bias voltages. The IR reflectance of the ITO films increases significantly with increasing bias voltage, reached a maximum at 40 V, and then reduced with further increase of the bias voltage. Frank et al. [11] showed that the IR reflectance R can be expressed by R¼1
4e0 c0 1 e dN l
ð1Þ
where c 0 is the velocity of light, e 0 is the permittivity of free space, e is the electronic charge, N is the carrier concentration, d is the film thickness, and l is the mobility of the free carriers. According to Eq. (1), the IR reflectance increases with the product of the carrier concentration N and carrier mobility l. As discussed previously, the ITO films deposited at the bias voltage of 40 V have smaller resistivities and higher values of the product Nl than those deposited at 20 V. However, the product of Nl, after reaching a maximum, starts decreasing again with further bias voltage.
4. Conclusions ITO films have been deposited by DC reactive magnetron sputtering by using metal In–Sn alloy target in an Ar–O2 gas mixture. The influences of substrate bias voltage on electrical and optical properties of the films were studied. Highly transparent ITO films with a visible transmittance of 80% and a low resistivity of 5.5104 V-cm have been obtained by applying a negative bias of 60 V to the substrate. In the wavelength range of 300–900 nm, the transmittance was improved with increasing a bias voltage.
Acknowledgement This work was supported by Korea Industrial Technology Foundation.
References [1] J.M. Margolis, Conductive Polymers and Plastics, Chapman & Hall, London, 1989.
J. Lee et al. / Thin Solid Films 480–481 (2005) 157–161 [2] Z.W. Yang, S.H. Han, T.L. Yang, Lina Ye, D.H. Zhang, H.L. Ma, C.F. Cheng, Thin Solid Films 366 (2000) 4. [3] N. Danson, I. Safi, G.W. Hall, R.P. Howson, Surf. Coat. Technol. 99 (1998) 147. [4] R.D. Bland, G.J. Kominiak, D.M. Mattox, J. Vac. Sci. Technol. 11 (1974) 671. [5] E. Kubota, Y. Shigesato, M. Igarashi, K. Suzuki, Jpn. J. Appl. Phys. 33 (1994) 4997. [6] Z. Yang, Shenghao Han, Tianlin Yang, Lina Ye, Honglei Ma, Chuanfu Cheng, Appl. Surf. Sci. 161 (2000) 279.
161
[7] A. Mansingh, C.V.R. Vasant Kumar, Thin Solid Films 167 (1988) L11. [8] A. Mukherjee, Vacuum 39 (1989) 537. [9] A.K. Kulkarni, K.H. Schulz, T.S. Lim, M. Khan, Thin Solid Films 308 (1997) 1. [10] S. Honda, K. Chihara, M. Watamori, K. Oura, Appl. Surf. Sci. 113/114 (1997) 408. [11] G. Frank, E. Kauer, H. Kfstlin, Thin Solid Films 77 (1981) 107.