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Influence of oxygen partial pressure on the properties of undoped InOx films deposited at room temperature by rf-PERTE
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Journal of Non-Crystalline Solids 354 (2008) 1643–1647 www.elsevier.com/locate/jnoncrysol
Influence of oxygen partial pressure on the properties of undoped InOx films deposited at room temperature by rf-PERTE C. Nunes de Carvalho a
a,*
, G. Lavareda a, P. Parreira a, J. Valente a, A. Amaral b, A.M. Botelho do Rego c
Department de Cieˆncia dos Materiais, FCT-UNL, Campus da Caparica, 2829-516 Caparica, Portugal b ICEMS, IST, UTL, Av. Rovisco Pais, 1049-001 Lisboa, Portugal c CQFM, IST, UTL, Rovisco Pais, 1049-001 Lisboa, Portugal Received 11 June 2007; received in revised form 31 August 2007
Abstract Transparent and conductive/semiconductive undoped indium oxide (InOx) thin films were deposited at room temperature. The deposition technique used is the radio frequency (rf) plasma enhanced reactive thermal evaporation (rf-PERTE) of indium (In) in the presence of oxygen. The influence of oxygen partial pressure on the properties of these films is presented. The oxygen partial pressure varied between 3 · 102 and 1.3 · 101 Pa. Undoped InOx films, 100 nm thick, deposited at the oxygen partial pressure of 6 · 102 Pa show a conductive behaviour, exhibit an average visible transmittance of 81%, a band gap around 2.7 eV and an electrical conductivity of about 1100 (X cm)1. For oxygen pressures greater than 6 · 102 Pa, semiconductive films are obtained, maintaining the visible transmittance. Films deposited at lower pressures are conductive but dark. From XPS data, films deposited at an oxygen partial pressure of 6 · 102 Pa show the highest amount of oxygen in the film surface and the lowest ratio between oxygen in the oxide crystalline and amorphous phases. 2007 Elsevier B.V. All rights reserved. PACS: 73.50.-h Keywords: Conductivity; Vapor phase deposition; Optical spectroscopy; Indium tin oxide and other transparent conductors
1. Introduction Most transparent conductors consist of metal oxides that possess both good electrical conductivity and transparency in the visible and near infrared (IR) regions of the electromagnetic spectrum. The field of transparent electronics based entirely upon transparent materials has grown explosively over the last years. If transparent conductors/semiconductors thin films can be deposited at room temperature, transparent electronic devices can be laid upon any type of substrates, from glass to flexible polymers [1]. Ultra-thin, light weight and low-power consumption devices can be made, promising displays that *
Corresponding author. Tel.: +351 218419279; fax: +351 218464455/
57. E-mail address: [email protected] (C. Nunes de Carvalho). 0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.10.004
can be deformable [2]. Since the 60s, the principal commercial transparent conductive oxides currently available were films of doped SnO2, ZnO, and In2O3 but, the best performance in terms of conductivity and transmissivity associated with excellent environmental stability and good surface morphology of the ITO (In2O3 doped with Sn) films have made them the most widely used [3]. In the present work we report on the deposition of undoped and highly transparent, conductive/semiconductive InOx thin films at room temperature. The technique used is the radio frequency plasma enhanced reactive thermal evaporation technique (rf-PERTE) of indium (In) in the presence of oxygen [4]. A study of the influence of oxygen partial pressure on the main properties of the films is also presented. Among the many plasma enhanced techniques reported [5,6], our technique involves the use of a reactive thermal evaporation configuration that allows depositing materials
C. Nunes de Carvalho et al. / Journal of Non-Crystalline Solids 354 (2008) 1643–1647
in a stable process. The optical, electrical and surface chemical characterization of the InOx films was made. Results are reported and discussed. 2. Experimental InOx thin films were deposited by radio frequency (rf) plasma enhanced reactive thermal evaporation (rf-PERTE) of In, in the presence of oxygen, at room temperature. A tungsten crucible was used for the indium evaporation and the source-substrate distance is 30 cm, approximately. Oxygen (99.99%) is introduced into the deposition chamber from a steel tube through a calibrated leak valve. Reaction between indium and oxygen is enhanced by an oxygen plasma generated by a radio frequency electrode (metallic grid) placed between the resistance-heated crucible and the substrate holder, 10 cm from substrates. It is believed that plasma activates oxygen into a more reactive form that readily reacts chemically with the evaporated indium. Two types of substrates were used: alkali free glass and quartz (2.5 · 2.5 cm2), cleaned with washing agents (commercial detergent and deionised water) before loading. The evaporation chamber is initially evacuated to a base pressure, pin = 1.5 · 103 Pa. Before the evaporation process starts, the gate valve between the diffusion pump and deposition chamber is partially closed in order to increase the residence time of oxygen into the chamber. The evaporation of indium is performed steadily and the time of evaporation recorded to calculate the evaporation rate. InOx thin films deposition parameters were the following: rf power density, P = 3.9 mW cm3 and deposition rate, r = 0.1– 0.2 nm/s. The oxygen partial pressure varied in the range 3 · 102–1.3 · 101 Pa (O2 flux variation of 1.2 to 16 sccm, respectively). This variation of deposition pressure has a slight influence on the thickness of the InOx films so, an average thickness of 100 nm was considered. The total transmittance, T, was measured in the range: 200– 1200 nm, using a Shimadzu UV-3100 spectrophotometer, without a bare substrate across the reference beam. The sheet resistance of the films was measured using a Veeco FPP-5000 four point probe. The radio frequency power generator (13.6 MHz) used was an Advanced Energy RFX-600. The conductivity versus temperature was measured using a Keithley 617 and 228 A electrometer and power supply, respectively. The XPS spectrometer used was a XSAM800 (KRATOS) operated in the fixed analyser transmission (FAT) mode, with a pass energy of 20 eV, the non-monochromatised AlKa X-radiation (hm = 1486.6 eV) and a power of 120 W (10 mA · 12 kV). Samples were kept on the sample holder by means of a metallic spring and analysed under a typical pressure in the range of 107 Pa. All sample transfers were made in air. Samples were analysed at room temperature, at take-off angle relative to the surface holder (TOA) of 0. Spectra were collected and stored in 300 channels with a step of 0.1 eV, and 60s of acquisition by sweep, using a Sun SPARC Station 4 with Vision software (Kratos). A Shirley background and
source satellites were subtracted and curve fitting for component peaks was carried out using Gaussian and Lorentzian products. No flood gun was used for neutralizing charge accumulation. As a reference for charge accumulation compensation, oxide oxygen binding energy was set to 530.0 eV [7]. For quantification purposes, sensitivity factors were 3.9 for In 3d5/2, 0.66 for O 1s and 0.25 for C 1s. 3. Results and discussion The electrical characterisation of undoped InOx films deposited at room temperature and at oxygen pressure within the range 3 · 102 to 1.3 · 101 Pa led us to the conclusion that films simultaneously transparent and conductive could only be obtained at 6 · 102 Pa. Films deposited at higher values of oxygen pressure (9 · 102 and 1.3 · 101 Pa) were much less conductive and with an evident semiconductive behaviour. InOx films deposited at 3 · 102 Pa are conductive but dark. Our study will be focused on the conductive/semiconductive InOx films and will include their optical, electrical and surface chemical analyses. A correlation between the data obtained is presented. 3.1. Visible transmittance of undoped InOx films Fig. 1 shows the variation of the total visible transmittance spectra with the oxygen partial pressure of undoped InOx thin films deposited on quartz substrates at room temperature by rf-PERTE. The transmittance spectrum of a bare substrate is presented for comparison. It can be seen that films deposited at values of oxygen pressure higher than 3 · 102 Pa have an average visible transmittance (81%) practically independent of the oxygen pressure which is confirmed by the high and almost equal values of oxygen atomic percentage present in these films (see Table 1). It can 100
80
Transmittance (%)
1644
Quartz -2 3×10 Pa -2 6×10 Pa -2 9×10 Pa -1 1.3×10 Pa
60
40
20
0 200
400
600
800
1000
1200
Wavelength (nm) Fig. 1. Variation of transmittance spectra with oxygen partial pressure of undoped InOx films deposited on quartz substrates at room temperature by rf-PERTE.
C. Nunes de Carvalho et al. / Journal of Non-Crystalline Solids 354 (2008) 1643–1647 Table 1 XPS atomic percentage (±0.7%) of Indium and Oxygen in samples prepared under different values of oxygen partial pressure
In O C O/In
3 · 102 (Pa)
6 · 102 (Pa)
9 · 102 (Pa)
1.3 · 101 (Pa)
20.2 43.5 36.3 2.15 ± 0.03
27.1 60.5 12.4 2.23 ± 0.03
28.2 59.3 12.6 2.10 ± 0.03
27.2 58.3 14.5 2.14 ± 0.03
be observed from the figure that there is a shift in the absorption edge towards the shorter wavelength as the oxygen partial pressure increases, suggesting variations in the band-gap –2.7–3.1 eV, for the lower (6 · 102 Pa) and higher (1.3 · 101 Pa) pressure values, respectively [8]. The poor transmittance (20%) of undoped InOx films deposited at 3 · 102 Pa is tentatively attributed to the large carbonaceous contamination (see Table 1) which has a relative importance, larger for InOx films deposited at the lowest values of oxygen pressure, decreasing the film transparency.
3.2. Conductivity of the undoped InOx films Fig. 2 shows the variation of conductivity logarithm as a function of the reciprocal of temperature during the cooling for InOx films deposited at four different oxygen pressures on alkali free glass substrates. In this conductive measurement the temperature varied between 0 C and around 70 C. As we can see the conductivity of the films deposited at 9 · 102 and 1.3 · 101 Pa decreases with the decrease in temperature, typical of a semiconductor material and the plot is linear throughout the entire temperature range, indicating that conduction is governed by a single mechanism. The low values of electrical conductivity obtained at 25 C were 2.1 · 106 and 4.3 · 108 X1 cm1 for films deposited at oxygen partial pressure of 9 · 102 4
10
3
10
2
10
Conductivity (Ω-1.cm-1)
1
10
0
10
-2
-1
3×10 Pa
-2
6×10 Pa
-3
9×10 Pa
10
-2
10
-2
10
-4
10
-1
1.3×10 Pa
-5
10
-6
10
-7
10
-8
10
1645
and 1.3 · 101 Pa, respectively. The variation in the low conductivities observed for these films may be attributed to an increase of structural disorders associated to a decrease in carriers mobility for the more resistive film [9]. Nevertheless, as we can see in Fig. 2, for transparent InOx thin films deposited at 6 · 102 Pa no dependence of electrical conductivity with temperature was observed and the electrical conductivity at 25 C is larger by more than eight orders of magnitude (1100 X1 cm1) than that of the InOx film deposited at 1.3 · 101 Pa. As electrical conduction in transparent undoped conductors is considered to be due to oxygen vacancies which contributes to the generation of carriers in the film [10] we conclude that ideal oxidizing conditions have been reached for films deposited at 6 · 102 Pa. As InOx films must fulfil both requirements (high transparency and low electrical resistivity) and this condition is only attained for films deposited at an oxygen pressure around 6 · 102 Pa, a very careful control of this deposition parameter is required. Films deposited at 3 · 102 Pa, although dark, show also a conductive behaviour with a conductivity at 25 C of 85 X1 cm1. 3.3. Surface chemical characterization of undoped InOx films The surface of samples prepared under the four different oxygen partial pressure conditions were analysed by XPS. Table 1 shows the quantitative results, expressed in atomic percentages, for indium and oxygen contents. Surprisingly, the sample prepared under 6 · 102 Pa oxygen pressure has a slightly higher amount of oxygen in the extreme surface than the other ones. Moreover, the ratio O/In shows that the atomic ratio O/In is larger than the stoichiometric ratio (1.5). This is a constant in the XPS analysis of indium oxide surfaces or indium and tin oxides [10] and can be explained by the existence of adsorbed oxygen [11], hydroxyl groups and also some oxygen bound to carbonaceous contamination. The sample which is further from the stoichiometric ratio is the one prepared under 6 · 102 Pa oxygen partial pressure. Another interesting parameter is the charge shift which, in XPS, is always positive, since photo and Auger electrons leaving the sample leave holes behind. Therefore, it increases as mobility of holes is lower, provided all the other conditions (sample position relative to the source, power, pressure in the chamber, etc.) are the same. In Table 2, the charge shift, in eV for the Four samples is shown. It is clear that the holes, left behind by the photo and Auger electrons ejected from sample, have a mobility much
-9
10
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
1000/T (K-1) Fig. 2. Variation of electrical conductivity with reciprocal of substrate temperature for undoped InOx films deposited on alkali free glass substrates at different values of oxygen partial pressure by rf-PERTE at room temperature (j non transparent films; h transparent films).
Table 2 XPS charge shifts, in eV, for the four samples
Charge shift (eV)
3 · 102 (Pa)
6 · 102 (Pa)
9 · 102 (Pa)
1.3 · 101 (Pa)
0.8
0.7
6.7
5.5
1646
C. Nunes de Carvalho et al. / Journal of Non-Crystalline Solids 354 (2008) 1643–1647
larger in the samples prepared under the lowest pressures of oxygen (3 and 6 · 102 Pa). For the other two, there is a smaller difference but, even so, we can identify the sample prepared under the intermediary flux of oxygen as the one having the poorest hole mobility. It is worth to stress here that we are not speaking of the hole mobility across all the film but just of a region around 10 nm thick. The XPS O 1s region also presents a few differences from sample to sample. Fig. 3 shows spectra for all the samples. Fig. 3 – XPS O 1s regions for the four samples: from bottom to top, increasing oxygen partial pressure. Spectra were set-off for clarity sake. Fitted curves are Lorentzian– Gaussian products with a fwhm = 1.75 ± 0.10 eV. Lorentzian percentage was set constant within a sample but different from sample to sample. The component at lower binding energies, centred at 530 eV, is assigned to the oxygen in the oxide crystalline phase (Ocr) whereas the other two peaks at 531.6 ± 0.1 and 532.9 ± 0.1 eV, are assigned to oxygen in the oxide amorphous phase (Oam) and hydroxyl groups, respectively [10]. However, these two last peaks may also contain contributions of organic oxygen and/or adsorbed molecular oxygen [7]. This last hypothesis is very remote given the ultra-high vacuum in the XPS analysis chamber. Table 3 shows the relative amount, in at.%, of each component for each sample, as well as the ratio between the components at 530 and 531.6 eV, i.e., the ratio between the crystalline and the amorphous phase.
O 1s
Table 3 Characterization, in atomic percentage (±1%), of the oxygen components at binding energies of 530, 531.6 and 532.9 eV for the four samples
530.0 (eV) 531.6 (eV) 532.9 (eV) 530/531.6
3 · 102 (Pa)
6 · 102 (Pa)
9 · 102 (Pa)
1.3 · 101 (Pa)
48.8 45.3 5.9 1.1
45.5 45.4 9.2 1.0
52.2 37.5 10.3 1.4
52.2 44.8 3.0 1.2
This table suggests that, in the surface region, the lower the crystalline phase, the higher the conductivity of the InOx films, suggesting that the decrease of carriers mobility associated with the increase of disorder in the amorphous phase (semiconductive films) is overcompensated by the increase in carrier concentration (conductive films) consequence of the presence of an ideal number of oxygen vacancies. The appearance of this amorphous phase is typical characteristic of thin films deposited at room temperature [12]. Optimal condition for obtaining undoped transparent conductive InOx films is at an oxygen partial pressure of 6 · 102 Pa. For lower values of this pressure the relative importance of contaminants is larger and decreases film transparency. 3.4. Table re´sume´ Table 4 summarizes the results of visible transmissivity, conductivity and XPS data for undoped InOx films produced by rf-PERTE at room temperatures at different values of oxygen partial pressure.
Intensity, a.u.
4. Summary
535
530
525
Binding Energy, eV Fig. 3. XPS O 1s regions for the four samples: from bottom to top, increasing oxygen partial pressure. Spectra were set-off for clarity sake. Fitted curves are Lorentzian–Gaussian products with a fwhm = 1.75 ± 0.10 eV. Lorentzian percentage was set constant within a sample but different from sample to sample.
A simple technique for preparing undoped, conductive/ semiconductive and transparent thin films of indium oxide (InOx) at room temperature was reported. Undoped InOx films were deposited by rf-plasma enhanced reactive thermal evaporation (rf-PERTE) of indium (In), in the presence of oxygen. At an oxygen pressure of 6 · 102 Pa, undoped InOx films are conductive and exhibit the following characteristics: an average total visible transmittance of 80% and an electrical conductivity of 1100 (X cm)1 for films of about 100 nm thick. InOx films deposited at 3 · 102 Pa are also conductive (85 X1 cm1) but dark. InOx films deposited at higher oxygen pressure (9 · 102 and 1.3 · 101 Pa) show an evident semiconductive behaviour, with values of conductivity lower by more than eight orders of magnitude than that of the InOx film deposited at 6 · 102 Pa. From XPS data it has been shown that undoped InOx films deposited at the 6 · 102 Pa oxygen partial pressure have the highest amount of oxygen in the surface with the highest fraction assigned to oxygen in the oxide amorphous phase. The poor transmittance (20%) of undoped InOx films deposited at 3 · 102 Pa is tentatively attributed to the large carbonaceous contamina-
C. Nunes de Carvalho et al. / Journal of Non-Crystalline Solids 354 (2008) 1643–1647
1647
Table 4 Average total visible transmittance, hTi (400–800 nm), electrical conductivity, r, XPS charge shift and Ocr/Oam atomic ratio as a function of oxygen partial pressure for undoped InOx films deposited by rf-PERTE at room temperature Oxygen p.p. (Pa) 2
3 · 10 6 · 102 9 · 102 1.3 · 101
hTi 400–800 nm (%)
r (25 C) (X cm)1
Average Thickness (nm)
XPS charge shift (eV)
Ocr/Oam (atomic ratio)
20 81 82 81
85 1100 2.1 · 106 4.3 · 108
100 100 100 100
0.8 0.7 6.7 5.5
1.1 1.0 1.4 1.2
tion which has a relative importance, larger when the oxygen pressure is lower. As a final conclusion we can say that undoped, conductive/semiconductive transparent InOx films can be deposited at room temperature by rf-PERTE, varying the oxygen partial pressure during the deposition process, leading to the formation of InOx films with great potential for device applications. Acknowledgements The authors gratefully acknowledge CENIMAT for thickness measurements facilities. This work was supported by ‘Fundac¸a˜o para a Cieˆncia e a Tecnologia’ through a Pluriannual Contract with CFM (IST) and by a POCTI project, SICAL ref. CTM/41317/01. References [1] C. Liu, T. Matsutani, N. Yamamoto, M. Kiuchi, Europhys. Lett. 59 (4) (2002) 606.
[2] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature 432 (2004) 488. [3] B.G. Lewis, D.C. Paine, MRS Bull. 25 (8) (2000) 22. [4] C. Nunes de Carvalho, G. Lavareda, E. Fortunato, A. Amaral, Thin Solid Films 427 (2003) 215. [5] S.K. Ghandhi, R. Siviy, J.M. Borrego, Appl. Phys. Lett. 34 (12) (1979) 833. [6] D. Lee, S.-J. Lee, K. Song, H. Baik, J. Vac. Sci. Technol. A21 (4) (2003) 1069. [7] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Rumble, NIST X-ray Photoelectron Spectroscopy Database, NIST Standard Reference Database 20, Version 3.3 (Web Version), http://srdata.nist.gov/xps/ [2003]. [8] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics Publishing, 1995, 288. [9] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics Publishing, 1995, 178. [10] G. Hass, M.H. Francombe, R.W. Hoffman, Physics of Thin Films, vol. 9, Academic Press, London, 1977, 9. [11] C. Nunes de Carvalho, A.M. Botelho do Rego, A. Amaral, P. Brogueira, G. Lavareda, Surf. Coatings Technol. 124 (2000) 70. [12] W.-K. Lee, T. Machino, T. Sugihara, Thin Solid Films 224 (1993) 105.
Journal of Non-Crystalline Solids 354 (2008) 1643–1647 www.elsevier.com/locate/jnoncrysol
Influence of oxygen partial pressure on the properties of undoped InOx films deposited at room temperature by rf-PERTE C. Nunes de Carvalho a
a,*
, G. Lavareda a, P. Parreira a, J. Valente a, A. Amaral b, A.M. Botelho do Rego c
Department de Cieˆncia dos Materiais, FCT-UNL, Campus da Caparica, 2829-516 Caparica, Portugal b ICEMS, IST, UTL, Av. Rovisco Pais, 1049-001 Lisboa, Portugal c CQFM, IST, UTL, Rovisco Pais, 1049-001 Lisboa, Portugal Received 11 June 2007; received in revised form 31 August 2007
Abstract Transparent and conductive/semiconductive undoped indium oxide (InOx) thin films were deposited at room temperature. The deposition technique used is the radio frequency (rf) plasma enhanced reactive thermal evaporation (rf-PERTE) of indium (In) in the presence of oxygen. The influence of oxygen partial pressure on the properties of these films is presented. The oxygen partial pressure varied between 3 · 102 and 1.3 · 101 Pa. Undoped InOx films, 100 nm thick, deposited at the oxygen partial pressure of 6 · 102 Pa show a conductive behaviour, exhibit an average visible transmittance of 81%, a band gap around 2.7 eV and an electrical conductivity of about 1100 (X cm)1. For oxygen pressures greater than 6 · 102 Pa, semiconductive films are obtained, maintaining the visible transmittance. Films deposited at lower pressures are conductive but dark. From XPS data, films deposited at an oxygen partial pressure of 6 · 102 Pa show the highest amount of oxygen in the film surface and the lowest ratio between oxygen in the oxide crystalline and amorphous phases. 2007 Elsevier B.V. All rights reserved. PACS: 73.50.-h Keywords: Conductivity; Vapor phase deposition; Optical spectroscopy; Indium tin oxide and other transparent conductors
1. Introduction Most transparent conductors consist of metal oxides that possess both good electrical conductivity and transparency in the visible and near infrared (IR) regions of the electromagnetic spectrum. The field of transparent electronics based entirely upon transparent materials has grown explosively over the last years. If transparent conductors/semiconductors thin films can be deposited at room temperature, transparent electronic devices can be laid upon any type of substrates, from glass to flexible polymers [1]. Ultra-thin, light weight and low-power consumption devices can be made, promising displays that *
Corresponding author. Tel.: +351 218419279; fax: +351 218464455/
57. E-mail address: [email protected] (C. Nunes de Carvalho). 0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.10.004
can be deformable [2]. Since the 60s, the principal commercial transparent conductive oxides currently available were films of doped SnO2, ZnO, and In2O3 but, the best performance in terms of conductivity and transmissivity associated with excellent environmental stability and good surface morphology of the ITO (In2O3 doped with Sn) films have made them the most widely used [3]. In the present work we report on the deposition of undoped and highly transparent, conductive/semiconductive InOx thin films at room temperature. The technique used is the radio frequency plasma enhanced reactive thermal evaporation technique (rf-PERTE) of indium (In) in the presence of oxygen [4]. A study of the influence of oxygen partial pressure on the main properties of the films is also presented. Among the many plasma enhanced techniques reported [5,6], our technique involves the use of a reactive thermal evaporation configuration that allows depositing materials
C. Nunes de Carvalho et al. / Journal of Non-Crystalline Solids 354 (2008) 1643–1647
in a stable process. The optical, electrical and surface chemical characterization of the InOx films was made. Results are reported and discussed. 2. Experimental InOx thin films were deposited by radio frequency (rf) plasma enhanced reactive thermal evaporation (rf-PERTE) of In, in the presence of oxygen, at room temperature. A tungsten crucible was used for the indium evaporation and the source-substrate distance is 30 cm, approximately. Oxygen (99.99%) is introduced into the deposition chamber from a steel tube through a calibrated leak valve. Reaction between indium and oxygen is enhanced by an oxygen plasma generated by a radio frequency electrode (metallic grid) placed between the resistance-heated crucible and the substrate holder, 10 cm from substrates. It is believed that plasma activates oxygen into a more reactive form that readily reacts chemically with the evaporated indium. Two types of substrates were used: alkali free glass and quartz (2.5 · 2.5 cm2), cleaned with washing agents (commercial detergent and deionised water) before loading. The evaporation chamber is initially evacuated to a base pressure, pin = 1.5 · 103 Pa. Before the evaporation process starts, the gate valve between the diffusion pump and deposition chamber is partially closed in order to increase the residence time of oxygen into the chamber. The evaporation of indium is performed steadily and the time of evaporation recorded to calculate the evaporation rate. InOx thin films deposition parameters were the following: rf power density, P = 3.9 mW cm3 and deposition rate, r = 0.1– 0.2 nm/s. The oxygen partial pressure varied in the range 3 · 102–1.3 · 101 Pa (O2 flux variation of 1.2 to 16 sccm, respectively). This variation of deposition pressure has a slight influence on the thickness of the InOx films so, an average thickness of 100 nm was considered. The total transmittance, T, was measured in the range: 200– 1200 nm, using a Shimadzu UV-3100 spectrophotometer, without a bare substrate across the reference beam. The sheet resistance of the films was measured using a Veeco FPP-5000 four point probe. The radio frequency power generator (13.6 MHz) used was an Advanced Energy RFX-600. The conductivity versus temperature was measured using a Keithley 617 and 228 A electrometer and power supply, respectively. The XPS spectrometer used was a XSAM800 (KRATOS) operated in the fixed analyser transmission (FAT) mode, with a pass energy of 20 eV, the non-monochromatised AlKa X-radiation (hm = 1486.6 eV) and a power of 120 W (10 mA · 12 kV). Samples were kept on the sample holder by means of a metallic spring and analysed under a typical pressure in the range of 107 Pa. All sample transfers were made in air. Samples were analysed at room temperature, at take-off angle relative to the surface holder (TOA) of 0. Spectra were collected and stored in 300 channels with a step of 0.1 eV, and 60s of acquisition by sweep, using a Sun SPARC Station 4 with Vision software (Kratos). A Shirley background and
source satellites were subtracted and curve fitting for component peaks was carried out using Gaussian and Lorentzian products. No flood gun was used for neutralizing charge accumulation. As a reference for charge accumulation compensation, oxide oxygen binding energy was set to 530.0 eV [7]. For quantification purposes, sensitivity factors were 3.9 for In 3d5/2, 0.66 for O 1s and 0.25 for C 1s. 3. Results and discussion The electrical characterisation of undoped InOx films deposited at room temperature and at oxygen pressure within the range 3 · 102 to 1.3 · 101 Pa led us to the conclusion that films simultaneously transparent and conductive could only be obtained at 6 · 102 Pa. Films deposited at higher values of oxygen pressure (9 · 102 and 1.3 · 101 Pa) were much less conductive and with an evident semiconductive behaviour. InOx films deposited at 3 · 102 Pa are conductive but dark. Our study will be focused on the conductive/semiconductive InOx films and will include their optical, electrical and surface chemical analyses. A correlation between the data obtained is presented. 3.1. Visible transmittance of undoped InOx films Fig. 1 shows the variation of the total visible transmittance spectra with the oxygen partial pressure of undoped InOx thin films deposited on quartz substrates at room temperature by rf-PERTE. The transmittance spectrum of a bare substrate is presented for comparison. It can be seen that films deposited at values of oxygen pressure higher than 3 · 102 Pa have an average visible transmittance (81%) practically independent of the oxygen pressure which is confirmed by the high and almost equal values of oxygen atomic percentage present in these films (see Table 1). It can 100
80
Transmittance (%)
1644
Quartz -2 3×10 Pa -2 6×10 Pa -2 9×10 Pa -1 1.3×10 Pa
60
40
20
0 200
400
600
800
1000
1200
Wavelength (nm) Fig. 1. Variation of transmittance spectra with oxygen partial pressure of undoped InOx films deposited on quartz substrates at room temperature by rf-PERTE.
C. Nunes de Carvalho et al. / Journal of Non-Crystalline Solids 354 (2008) 1643–1647 Table 1 XPS atomic percentage (±0.7%) of Indium and Oxygen in samples prepared under different values of oxygen partial pressure
In O C O/In
3 · 102 (Pa)
6 · 102 (Pa)
9 · 102 (Pa)
1.3 · 101 (Pa)
20.2 43.5 36.3 2.15 ± 0.03
27.1 60.5 12.4 2.23 ± 0.03
28.2 59.3 12.6 2.10 ± 0.03
27.2 58.3 14.5 2.14 ± 0.03
be observed from the figure that there is a shift in the absorption edge towards the shorter wavelength as the oxygen partial pressure increases, suggesting variations in the band-gap –2.7–3.1 eV, for the lower (6 · 102 Pa) and higher (1.3 · 101 Pa) pressure values, respectively [8]. The poor transmittance (20%) of undoped InOx films deposited at 3 · 102 Pa is tentatively attributed to the large carbonaceous contamination (see Table 1) which has a relative importance, larger for InOx films deposited at the lowest values of oxygen pressure, decreasing the film transparency.
3.2. Conductivity of the undoped InOx films Fig. 2 shows the variation of conductivity logarithm as a function of the reciprocal of temperature during the cooling for InOx films deposited at four different oxygen pressures on alkali free glass substrates. In this conductive measurement the temperature varied between 0 C and around 70 C. As we can see the conductivity of the films deposited at 9 · 102 and 1.3 · 101 Pa decreases with the decrease in temperature, typical of a semiconductor material and the plot is linear throughout the entire temperature range, indicating that conduction is governed by a single mechanism. The low values of electrical conductivity obtained at 25 C were 2.1 · 106 and 4.3 · 108 X1 cm1 for films deposited at oxygen partial pressure of 9 · 102 4
10
3
10
2
10
Conductivity (Ω-1.cm-1)
1
10
0
10
-2
-1
3×10 Pa
-2
6×10 Pa
-3
9×10 Pa
10
-2
10
-2
10
-4
10
-1
1.3×10 Pa
-5
10
-6
10
-7
10
-8
10
1645
and 1.3 · 101 Pa, respectively. The variation in the low conductivities observed for these films may be attributed to an increase of structural disorders associated to a decrease in carriers mobility for the more resistive film [9]. Nevertheless, as we can see in Fig. 2, for transparent InOx thin films deposited at 6 · 102 Pa no dependence of electrical conductivity with temperature was observed and the electrical conductivity at 25 C is larger by more than eight orders of magnitude (1100 X1 cm1) than that of the InOx film deposited at 1.3 · 101 Pa. As electrical conduction in transparent undoped conductors is considered to be due to oxygen vacancies which contributes to the generation of carriers in the film [10] we conclude that ideal oxidizing conditions have been reached for films deposited at 6 · 102 Pa. As InOx films must fulfil both requirements (high transparency and low electrical resistivity) and this condition is only attained for films deposited at an oxygen pressure around 6 · 102 Pa, a very careful control of this deposition parameter is required. Films deposited at 3 · 102 Pa, although dark, show also a conductive behaviour with a conductivity at 25 C of 85 X1 cm1. 3.3. Surface chemical characterization of undoped InOx films The surface of samples prepared under the four different oxygen partial pressure conditions were analysed by XPS. Table 1 shows the quantitative results, expressed in atomic percentages, for indium and oxygen contents. Surprisingly, the sample prepared under 6 · 102 Pa oxygen pressure has a slightly higher amount of oxygen in the extreme surface than the other ones. Moreover, the ratio O/In shows that the atomic ratio O/In is larger than the stoichiometric ratio (1.5). This is a constant in the XPS analysis of indium oxide surfaces or indium and tin oxides [10] and can be explained by the existence of adsorbed oxygen [11], hydroxyl groups and also some oxygen bound to carbonaceous contamination. The sample which is further from the stoichiometric ratio is the one prepared under 6 · 102 Pa oxygen partial pressure. Another interesting parameter is the charge shift which, in XPS, is always positive, since photo and Auger electrons leaving the sample leave holes behind. Therefore, it increases as mobility of holes is lower, provided all the other conditions (sample position relative to the source, power, pressure in the chamber, etc.) are the same. In Table 2, the charge shift, in eV for the Four samples is shown. It is clear that the holes, left behind by the photo and Auger electrons ejected from sample, have a mobility much
-9
10
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
1000/T (K-1) Fig. 2. Variation of electrical conductivity with reciprocal of substrate temperature for undoped InOx films deposited on alkali free glass substrates at different values of oxygen partial pressure by rf-PERTE at room temperature (j non transparent films; h transparent films).
Table 2 XPS charge shifts, in eV, for the four samples
Charge shift (eV)
3 · 102 (Pa)
6 · 102 (Pa)
9 · 102 (Pa)
1.3 · 101 (Pa)
0.8
0.7
6.7
5.5
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larger in the samples prepared under the lowest pressures of oxygen (3 and 6 · 102 Pa). For the other two, there is a smaller difference but, even so, we can identify the sample prepared under the intermediary flux of oxygen as the one having the poorest hole mobility. It is worth to stress here that we are not speaking of the hole mobility across all the film but just of a region around 10 nm thick. The XPS O 1s region also presents a few differences from sample to sample. Fig. 3 shows spectra for all the samples. Fig. 3 – XPS O 1s regions for the four samples: from bottom to top, increasing oxygen partial pressure. Spectra were set-off for clarity sake. Fitted curves are Lorentzian– Gaussian products with a fwhm = 1.75 ± 0.10 eV. Lorentzian percentage was set constant within a sample but different from sample to sample. The component at lower binding energies, centred at 530 eV, is assigned to the oxygen in the oxide crystalline phase (Ocr) whereas the other two peaks at 531.6 ± 0.1 and 532.9 ± 0.1 eV, are assigned to oxygen in the oxide amorphous phase (Oam) and hydroxyl groups, respectively [10]. However, these two last peaks may also contain contributions of organic oxygen and/or adsorbed molecular oxygen [7]. This last hypothesis is very remote given the ultra-high vacuum in the XPS analysis chamber. Table 3 shows the relative amount, in at.%, of each component for each sample, as well as the ratio between the components at 530 and 531.6 eV, i.e., the ratio between the crystalline and the amorphous phase.
O 1s
Table 3 Characterization, in atomic percentage (±1%), of the oxygen components at binding energies of 530, 531.6 and 532.9 eV for the four samples
530.0 (eV) 531.6 (eV) 532.9 (eV) 530/531.6
3 · 102 (Pa)
6 · 102 (Pa)
9 · 102 (Pa)
1.3 · 101 (Pa)
48.8 45.3 5.9 1.1
45.5 45.4 9.2 1.0
52.2 37.5 10.3 1.4
52.2 44.8 3.0 1.2
This table suggests that, in the surface region, the lower the crystalline phase, the higher the conductivity of the InOx films, suggesting that the decrease of carriers mobility associated with the increase of disorder in the amorphous phase (semiconductive films) is overcompensated by the increase in carrier concentration (conductive films) consequence of the presence of an ideal number of oxygen vacancies. The appearance of this amorphous phase is typical characteristic of thin films deposited at room temperature [12]. Optimal condition for obtaining undoped transparent conductive InOx films is at an oxygen partial pressure of 6 · 102 Pa. For lower values of this pressure the relative importance of contaminants is larger and decreases film transparency. 3.4. Table re´sume´ Table 4 summarizes the results of visible transmissivity, conductivity and XPS data for undoped InOx films produced by rf-PERTE at room temperatures at different values of oxygen partial pressure.
Intensity, a.u.
4. Summary
535
530
525
Binding Energy, eV Fig. 3. XPS O 1s regions for the four samples: from bottom to top, increasing oxygen partial pressure. Spectra were set-off for clarity sake. Fitted curves are Lorentzian–Gaussian products with a fwhm = 1.75 ± 0.10 eV. Lorentzian percentage was set constant within a sample but different from sample to sample.
A simple technique for preparing undoped, conductive/ semiconductive and transparent thin films of indium oxide (InOx) at room temperature was reported. Undoped InOx films were deposited by rf-plasma enhanced reactive thermal evaporation (rf-PERTE) of indium (In), in the presence of oxygen. At an oxygen pressure of 6 · 102 Pa, undoped InOx films are conductive and exhibit the following characteristics: an average total visible transmittance of 80% and an electrical conductivity of 1100 (X cm)1 for films of about 100 nm thick. InOx films deposited at 3 · 102 Pa are also conductive (85 X1 cm1) but dark. InOx films deposited at higher oxygen pressure (9 · 102 and 1.3 · 101 Pa) show an evident semiconductive behaviour, with values of conductivity lower by more than eight orders of magnitude than that of the InOx film deposited at 6 · 102 Pa. From XPS data it has been shown that undoped InOx films deposited at the 6 · 102 Pa oxygen partial pressure have the highest amount of oxygen in the surface with the highest fraction assigned to oxygen in the oxide amorphous phase. The poor transmittance (20%) of undoped InOx films deposited at 3 · 102 Pa is tentatively attributed to the large carbonaceous contamina-
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Table 4 Average total visible transmittance, hTi (400–800 nm), electrical conductivity, r, XPS charge shift and Ocr/Oam atomic ratio as a function of oxygen partial pressure for undoped InOx films deposited by rf-PERTE at room temperature Oxygen p.p. (Pa) 2
3 · 10 6 · 102 9 · 102 1.3 · 101
hTi 400–800 nm (%)
r (25 C) (X cm)1
Average Thickness (nm)
XPS charge shift (eV)
Ocr/Oam (atomic ratio)
20 81 82 81
85 1100 2.1 · 106 4.3 · 108
100 100 100 100
0.8 0.7 6.7 5.5
1.1 1.0 1.4 1.2
tion which has a relative importance, larger when the oxygen pressure is lower. As a final conclusion we can say that undoped, conductive/semiconductive transparent InOx films can be deposited at room temperature by rf-PERTE, varying the oxygen partial pressure during the deposition process, leading to the formation of InOx films with great potential for device applications. Acknowledgements The authors gratefully acknowledge CENIMAT for thickness measurements facilities. This work was supported by ‘Fundac¸a˜o para a Cieˆncia e a Tecnologia’ through a Pluriannual Contract with CFM (IST) and by a POCTI project, SICAL ref. CTM/41317/01. References [1] C. Liu, T. Matsutani, N. Yamamoto, M. Kiuchi, Europhys. Lett. 59 (4) (2002) 606.
[2] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature 432 (2004) 488. [3] B.G. Lewis, D.C. Paine, MRS Bull. 25 (8) (2000) 22. [4] C. Nunes de Carvalho, G. Lavareda, E. Fortunato, A. Amaral, Thin Solid Films 427 (2003) 215. [5] S.K. Ghandhi, R. Siviy, J.M. Borrego, Appl. Phys. Lett. 34 (12) (1979) 833. [6] D. Lee, S.-J. Lee, K. Song, H. Baik, J. Vac. Sci. Technol. A21 (4) (2003) 1069. [7] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Rumble, NIST X-ray Photoelectron Spectroscopy Database, NIST Standard Reference Database 20, Version 3.3 (Web Version), http://srdata.nist.gov/xps/ [2003]. [8] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics Publishing, 1995, 288. [9] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadish, Semiconducting Transparent Thin Films, Institute of Physics Publishing, 1995, 178. [10] G. Hass, M.H. Francombe, R.W. Hoffman, Physics of Thin Films, vol. 9, Academic Press, London, 1977, 9. [11] C. Nunes de Carvalho, A.M. Botelho do Rego, A. Amaral, P. Brogueira, G. Lavareda, Surf. Coatings Technol. 124 (2000) 70. [12] W.-K. Lee, T. Machino, T. Sugihara, Thin Solid Films 224 (1993) 105.