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The influence of deposition parameters on the optical and electrical properties of r.f.-sputter- deposited indium tin oxide films
Thin Solid Films, 138 (1986)65-70 PREPARATION AND CHARACTERIZATION
65
THE INFLUENCE OF DEPOSITION PARAMETERS ON THE O P T I C A L A N D E L E C T R I C A L P R O P E R T I E S O F R.F.-SPUTTERD E P O S I T E D I N D I U M TIN O X I D E F I L M S S. NASEEM School of Physics, Newcastle Polytechnic, Newcastle-upon-Tyne, NE1 8ST ( Gt. Brita&)
T. J. COUTTS Solar Energy Research Institute, 1617 Cole Blvd., Golden, CO 80401 (U.S.A.)
(Received February 12, 1985; accepted April 10, 1985)
The fabrication and characterization of indium tin oxide thin films prepared by r.f. sputter deposition from a target of composition 90mol.%In203-10mol.%SnO2 are described. The properties were found to depend on the rate of deposition, the post-deposition heat treatment and the composition of the sputtering gas. Using the optimum combination of these parameters, films with a resistivity of 1.36 x 10-* D cm were produced. Films with a thickness of less than 1000/~ had a visible transmittance of greater than 80% for wavelengths in the range 450-1500 nm (being greater than 90% above 550 nm).
1. INTRODUCTION Indium tin oxide (ITO) films have a band gap of approximately 3.8 eV and are therefore highly transmissive in the visible spectrum. They are als0 highly degenerate and usually have a low resistivity. These qualities have caused them to be widely used as transparent conducting elements for addressing various alphanumeric displays x. In addition, they have also been used as window layers or, more simply, as a transparent contact to various solar cells 2 4. In the latter context they often have the added benefit of acting as antireflection (AR) coatings on the solar absorber. Displays and solar cells make quite different demands on the fabrication of thin ITO films. In the former case, the requirement is simply for ITO-coated glass which will ultimately form a contact to the active element of the display (i.e. liquid crystal, electroluminescent panel etc.). In the latter case, the ITO is often a part of the active device forming the internal potential with the underlying semiconductor substrate. As such, great care must be taken to ensure that the interface between the two semiconductors is free of damage, which usually leads to inferior devices 5. Reactive ion plating 6, d.c. diode sputtering 7, r.f. sputtering 8, reactive evaporation 9, chemical vapour deposition 1°, and spray pyrolysis 11, have all been used successfully to produce transparent conducting coatings of ITO on glass. One feature which all these techniques have in common is that the glass substrate is usually heated during deposition to temperatures as high as 500 °C in order to 0040-6090/86/$3.50
© Elsevier Sequoia/Printed in The Netherlands
66
s. NASEEM, T. J. COUTTS
optimize the materials properties. For some solar cells, e.g. I T O / I n P 12, temperatures of this magnitude are known to lead to undesirable interface effects and so alternative means must be sought for the production of low sheet resistance, highly transmissive ITO films. When depositing ITO by any of the vacuum-based techniques referred to above, it is usual and desirable to use a relatively high rate of deposition. ITO conducts because of incomplete oxidation of the component metals and the formation of an oxygen vacancy defect band. Hence, a rapid rate of deposition has the beneficial effect of minimizing the quantity of background gas (in particular oxygen), from the vacuum system, incorporated in the film. For deposition onto glass, a high deposition rate has a significant economic advantage and has no technical disadvantages. Again, for I T O / I n P solar cells, a high deposition rate always leads to relatively inefficient devices5 owing to damage by any one of several possible causes 13. Thus, we have the seemingly mutually exclusive requirements of a rapid rate of deposition to minimize the sheet resistance of the solar cell window layer, and yet a low rate to avoid interface damage. In this paper we report the means by which this problem has been resolved, and also discuss the properties of ITO films as a function of their post-deposition heat treatment. 2. EXPERIMENTAL DETAILS
A Nordiko sputtering system fitted with a 10mol.~oSnO2-90mol.~oln203 target of diameter 3 in was used to prepare the ITO films for this study. The substrates used were Corning 7059 glass slides 76 mm x 26 mm x 1 mm in size which were ultrasonically cleaned by standard means before being loaded into the sputtering chamber. The target-to-substrate distance was 6 cm and the substrates were held facing downwards. The chamber was evacuated using a combination of rotary and diffusion pumps and the base pressure was held in the range (2-4) x 10 6 Torr before the sputtering gas was admitted. The sputtering gas pressure was kept at (3--5) X 10 - 3 Torr; the oxygen partial pressure, as measured with a quadrupole mass spectrometer, was (1.5-2.5)x 10-v Torr. No intentional heating or biasing of the substrates was used for these investigations. The substrate temperature was monitored initially and after the runs were completed, and the measurements indicated that the temperature never exceeded 50 °C during deposition. All the runs began with 25 min of pre-sputtering of the target at 400 W. The input power density (0.4-7 W cm-2) and the composition of the sputtering gas (argon and different mixtures of hydrogen in argon) were varied systematically. The effects of postdeposition heat treatments were also investigated. The thickness and the rate of deposition were monitored during the film growth with an Inficon thickness monitor. The thickness of each film was also measured by a Talystep and, in some cases, with the help of transmittance spectra. The transmittance was obtained using a Varian 17D spectrophotometer, while the electrical properties were obtained by Van der Pauw and four-point probe methods. The structure of the films was investigated by X-ray diffractometry.
I N F L U E N C E O F D E P O S I T I O N P A R A M E T E R S O N ITO FILMS
3.
67
RESULTS AND DISCUSSION
Films with sheet resistances of approximately 10 f~/l-/are acceptable for the display applications such as those listed earlier. Such films have been produced in this work by r.f. sputtering in an atmosphere of pure argon. These films were deposited at a rate of 4 ,~ s- 1 using a power density of approximately 7 W c m - 2 and at a pressure of 5 x 1 0 - 3 Torr. The typical visible transmittance for a film of thickness 7000 ~ was about 90~ for a wavelength of 550 nm. However, when films are deposited onto InP substrates using these conditions, a very poor quality cell results and it is necessary to use much slower rates. Because of the oxygen absorption problem described earlier, it was decided to introduce a reducing gas into the sputtering atmosphere in an attempt to lessen the problem. Films deposited very slowly (about 0.3 ~ s- 1) in argon generally had a very high sheet resistance which rendered them unacceptable for solar cell window layers because of the resulting excessive series resistance. When post-deposition annealing of films deposited very slowly was performed the sheet resistance fell, typically, from several megaohms per square to 200-300 f~/[S]. A transmittance curve of one such film (before annealing) of thickness 7000/~ is shown in Fig. 1. The transmittance was essentially unchanged upon annealing. Although sheet resistances in the above range would not contribute excessive series resistance to a solar cell, the heat treatment required would irreversibly damage the junction i z. Films deposited in various mixtures of hydrogen: argon (the compositions used being 10 voi.~o H 2 , 20 vol.~ H 2 , 30 vol.~o H 2 ) had a sheet resistance which was 100/
,
,
,
,
,
.
.
.
.
.
/
80
-
70 ~ N
60
~ 5o c
~ 40 30
2O 10
O.S ' 0.6 03' O@ '. 09i ' 1.0 11i 12i ' 1.3 I./*' 1.5 Wavelength ( pm ) Fig. 1. T r a n s m i s s i o n c h a r a c t e r i s t i c s o f a 7 0 0 0 ,~ I T O film s p u t t e r d e p o s i t e d in p u r e a r g o n p l a s m a . 0
68
S. NASEEM, T. J. COUTTS
independent of the rate of deposition. Hence, for the fabrication of solar cells, an input r.f. power of 0.4 W cm -2, a total pressure of 3 x 1 0 - 3 Torr and an induced d.c. bias of 0.6 kV, corresponding to a rate of deposition of about 0.4/~ s- 1, were used. This led to cells with very low values of reverse saturation current density (2-5 pA cm 2) implying junctions suffering very little damage 14. A summary of the sheet resistance and transmittance of films deposited in the various gas compositions is shown in Table I. Clearly, the properties are optimum for the 10 vol.~o H 2 mixture although further refinement may be possible by adjusting the hydrogen content in smaller increments. It is possible tha t both sheet resistance and transmittance (reported in the table for a wavelength of 550 nm) were optimized at the same hydrogen content because of (a) a decrease in carrier mobility at concentrations greater than 10 vol.~o H2 and (b) increased light scattering or free-carrier absorption. However, these ideas have yet to be established. Films deposited at low input power in the 10vol.~oH2-90vol.~oAr plasma routinely have a resistivity of less than 2 × 10 3 fl cm (corresponding to a sheet resistance of about 300 f~/t-q for a 650/~ thick film). A typical transmittance curve is shown in Fig. 2 for a film of this thickness. Films deposited under these conditions have been shown to form excellent quality window layers in ITO/InP solar cells and have yielded total area efficiencies of greater than 16~o 14. Hence, for applications which require deposition of ITO on a sensitive surface, this technique offers great promise. ITO films deposited on borosilicate glass substrates under the above conditions have also been subjected to various heat treatments. Typically, the annealing temperature was 500 °C and an atmosphere of forming gas was used. In Fig. 3, we show the variation in sheet resistance with time. This shows that the sheet resistance decreases by a factor of more than 10 over a period of 5 min; the minimum resistivity was 1.4 x 10 -4 fl cm, corresponding to a sheet resistance of about 22 f l / O for a film 650/~ thick. The increase in the sheet resistance after 5min may again be attributable to a decrease in carrier mobility. Since the transmittance is very high, this material must be regarded as amongst the best quality ITO ever produced. Previous efforts to obtain these properties in ITO films have generally involved the use of an elevated substrate temperature during deposition, which is certainly not appropriate for the fabrication of I T O / I n P cells and may well be unsuitable for other applications using heat-sensitive substrates. In these cases the use of postdeposition heat treatment may also be unacceptable and we have shown here that this can be avoided by the use of hydrogen incorporation in the plasma, which can lead to films with a sheet resistance of the order of 200-800 f~/rl, with no deliberate substrate heating during or after deposition. In those cases where substrate damage during deposition is not problematic, high rates of deposition may be used and, in these circumstances, hydrogen incorporation makes little difference to the film properties. Presumably, the extent of oxygen incorporation is not lowered significantly by hydrogen when a high rate is used. When films deposited rapidly, with or without an H2-Ar plasma, are annealed after deposition their resistivity typically falls to somewhat less than 2 × 10 4 f~ cm. Since this is approximately equal to the values attained for unannealed films deposited slowly in the mixed sputtering gas, it can be concluded that the effects of post-deposition heat treatment
69
INFLUENCE OF DEPOSITION PARAMETERS ON ITO FILMS
TABLE I SUMMARY
OF THE FILM
PROPERTIES
AT DIFFERENT
SPUTTERING
GAS COMPOSITIONS
Gas composition
Transmittance at 550 nm (%)
Sheet resistance (~/[3)
Pure argon 10vol.%H2-90vol.~Ar 20vol.~H2-80vol.~Ar 30vol.%H2-70vol.~Ar
81.0 90.0 82.2 79.5
16.7 × l03 300.0 1.5 x l03 2.1 x 103
and deposition in a reducing atmosphere are similar. However, it was not found possible to reduce the resistivity of films deposited slowly in argon alone to less than 10- 2 D cm. Presumably, oxygen can be incorporated in the films in either strongly or weakly bound states and it appears to be possible to remove the latter by heat treatment but not the former. On the other hand, by using the mixed H2-Ar plasma, it appears to be possible to reduce the quantity of both strongly and weakly absorbed oxygen, so that post-deposition heat treatment of films prepared in such atmospheres leads to much lower resistivities. X-ray diffraction analyses of these films revealed a b.c.c, structure with preferred orientation along the (111) axis. This is consistent with the results of Fraser and Cook 7. Also, there was evidence of hexagonal In203, along with the 100
i
i
i
i
f
l
1
90
I ~50I
80
70
!001 60
i
50
rso]
i
E 40
1001 3O
20
\ ~x
5ol 10 0
I
0.6, 0 5
I
I
I
I
I
I
I
l
I
0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1,5
Wavelength
{ pm )
I
2
3
4
5
Annealing Ti,'ne (Minutes)
Fig. 2. Transmission characteristics of a 650,~ I T O film sputter deposited in a 10vol.~H2-90vol.%Ar plasma. Fig. 3. Sheet resistance of a 650 ~ I T O film as a function of the annealing time. This film was sputter deposited in a 10vol.~H 2-90vol.~Ar plasma.
70
s. NASEEM, T. J. COUTTS
cubic phase in the best films. This is believed to contribute to increased carrier density 15 which is consistent with experimental observations. Other oxides such as SnO, 13-SnO etc. were also present in the films and, as has previously been shown 1s, these lead to oxygen deficiency which is beneficial to the conductivity of the films. 4. CONCLUSIONS In this work we have shown the following. (i) ITO films of resistivity less than 2 x 10 -4 f2 cm can be produced by a combination of very slow sputter deposition in a reducing atmosphere and postdeposition heat treatment. This resistivity is amongst the lowest ever achieved for highly transmissive ITO. (ii) This result can also be achieved for films deposited ten times more rapidly in argon alone, provided they are also heat treated after deposition. (iii) Since very slow rates of deposition can be used, the technique is eminently suitable for deposition onto substrates sensitive to electronic surface damage caused by bombardment of ions, neutral species or electrons from the plasma. Since substrate heating is much less at these reduced rates, it should also be possible to achieve low resistivities on heat-sensitive substrates such as plastics. (iv) We hypothesize that the use of the reducing atmosphere leads to the incorporation of a smaller quantity of strongly bound (chemisorbed) oxygen while post-deposition heat treatment leads to desorption of weakly bound (physisorbed) oxygen). (v) Changes in the electrical properties on heat treatment are consistent with changes in the density and/or mobility of charge carriers. REFERENCES 1 E.W. Williams and R. Hall, in B. R. Pamplin (ed.), Luminescence and the Light Emitting Diode, International Series on Science of the Solid State, Pergamon, Oxford, 1978. 2 T.J. Coutts and N. M. Pearsall, Proc. 16th IEEE Photovoltaic Specialists" Conf., San Diego, CA, September 27-30, 1982, IEEE, New York, p. 1288. 3 S. Ashok, P.P. SharmaandS. J. Fonash, IEEETrans. ElectronDev.,27(4)(1980)725. 4 R.H. Bube, Proc. Soc. Photo-Opt. Instrum. Eng., 114 (1977) 7. 5 T.J. Coutts, N. M. Pearsall and L. Tarricone, J. Vac. Sci. Technol. B, 2 (2) (1984) 140. 6 J. Machet, J. Guile, P. Saulnier and S. Robert, Thin Solid Films, 80 (1981) 149. 7 D.B. Fraser and H. D. Cook, J. Electrochem. Soc. Solid-State Sci. Technol., (1972) 1368. 8 R. Tueta and M. Braguier, Thin Solid Films, 80 (1981) 143. 9 H.U. Habermeir, Thin Solid Films, 80(1981) 157. 10 L.A. Ryabova and Y. S. Savitskaya, Thin Solid Films, 2 (1968) 141. 11 R. Groth, Phys. Status Solidi, 14 (1966) 69. 12 T.J. Coutts, A. Schwartzlander, S. Naseem and T. Massopust, Proc. 26th Rocky Mountain Conf. on Analytical Chemistry, Denver, CO, August 1984. 13 N.M. Pearsall, T. J. Coutts, R. Hill, G. J. Russell and K. J. Lawson, Thin Solid Films, 80 (1981) 177. 14 T.J. Coutts and S. Naseem, Appl. Phys. Lett., 46 (1985) 164. 15 C. Yuanri, X. Xinghao, J. Zhaoting, P. Chuancai and X. Shuyun, Thin Solid Films, 115 (3) (1984) 195.
65
THE INFLUENCE OF DEPOSITION PARAMETERS ON THE O P T I C A L A N D E L E C T R I C A L P R O P E R T I E S O F R.F.-SPUTTERD E P O S I T E D I N D I U M TIN O X I D E F I L M S S. NASEEM School of Physics, Newcastle Polytechnic, Newcastle-upon-Tyne, NE1 8ST ( Gt. Brita&)
T. J. COUTTS Solar Energy Research Institute, 1617 Cole Blvd., Golden, CO 80401 (U.S.A.)
(Received February 12, 1985; accepted April 10, 1985)
The fabrication and characterization of indium tin oxide thin films prepared by r.f. sputter deposition from a target of composition 90mol.%In203-10mol.%SnO2 are described. The properties were found to depend on the rate of deposition, the post-deposition heat treatment and the composition of the sputtering gas. Using the optimum combination of these parameters, films with a resistivity of 1.36 x 10-* D cm were produced. Films with a thickness of less than 1000/~ had a visible transmittance of greater than 80% for wavelengths in the range 450-1500 nm (being greater than 90% above 550 nm).
1. INTRODUCTION Indium tin oxide (ITO) films have a band gap of approximately 3.8 eV and are therefore highly transmissive in the visible spectrum. They are als0 highly degenerate and usually have a low resistivity. These qualities have caused them to be widely used as transparent conducting elements for addressing various alphanumeric displays x. In addition, they have also been used as window layers or, more simply, as a transparent contact to various solar cells 2 4. In the latter context they often have the added benefit of acting as antireflection (AR) coatings on the solar absorber. Displays and solar cells make quite different demands on the fabrication of thin ITO films. In the former case, the requirement is simply for ITO-coated glass which will ultimately form a contact to the active element of the display (i.e. liquid crystal, electroluminescent panel etc.). In the latter case, the ITO is often a part of the active device forming the internal potential with the underlying semiconductor substrate. As such, great care must be taken to ensure that the interface between the two semiconductors is free of damage, which usually leads to inferior devices 5. Reactive ion plating 6, d.c. diode sputtering 7, r.f. sputtering 8, reactive evaporation 9, chemical vapour deposition 1°, and spray pyrolysis 11, have all been used successfully to produce transparent conducting coatings of ITO on glass. One feature which all these techniques have in common is that the glass substrate is usually heated during deposition to temperatures as high as 500 °C in order to 0040-6090/86/$3.50
© Elsevier Sequoia/Printed in The Netherlands
66
s. NASEEM, T. J. COUTTS
optimize the materials properties. For some solar cells, e.g. I T O / I n P 12, temperatures of this magnitude are known to lead to undesirable interface effects and so alternative means must be sought for the production of low sheet resistance, highly transmissive ITO films. When depositing ITO by any of the vacuum-based techniques referred to above, it is usual and desirable to use a relatively high rate of deposition. ITO conducts because of incomplete oxidation of the component metals and the formation of an oxygen vacancy defect band. Hence, a rapid rate of deposition has the beneficial effect of minimizing the quantity of background gas (in particular oxygen), from the vacuum system, incorporated in the film. For deposition onto glass, a high deposition rate has a significant economic advantage and has no technical disadvantages. Again, for I T O / I n P solar cells, a high deposition rate always leads to relatively inefficient devices5 owing to damage by any one of several possible causes 13. Thus, we have the seemingly mutually exclusive requirements of a rapid rate of deposition to minimize the sheet resistance of the solar cell window layer, and yet a low rate to avoid interface damage. In this paper we report the means by which this problem has been resolved, and also discuss the properties of ITO films as a function of their post-deposition heat treatment. 2. EXPERIMENTAL DETAILS
A Nordiko sputtering system fitted with a 10mol.~oSnO2-90mol.~oln203 target of diameter 3 in was used to prepare the ITO films for this study. The substrates used were Corning 7059 glass slides 76 mm x 26 mm x 1 mm in size which were ultrasonically cleaned by standard means before being loaded into the sputtering chamber. The target-to-substrate distance was 6 cm and the substrates were held facing downwards. The chamber was evacuated using a combination of rotary and diffusion pumps and the base pressure was held in the range (2-4) x 10 6 Torr before the sputtering gas was admitted. The sputtering gas pressure was kept at (3--5) X 10 - 3 Torr; the oxygen partial pressure, as measured with a quadrupole mass spectrometer, was (1.5-2.5)x 10-v Torr. No intentional heating or biasing of the substrates was used for these investigations. The substrate temperature was monitored initially and after the runs were completed, and the measurements indicated that the temperature never exceeded 50 °C during deposition. All the runs began with 25 min of pre-sputtering of the target at 400 W. The input power density (0.4-7 W cm-2) and the composition of the sputtering gas (argon and different mixtures of hydrogen in argon) were varied systematically. The effects of postdeposition heat treatments were also investigated. The thickness and the rate of deposition were monitored during the film growth with an Inficon thickness monitor. The thickness of each film was also measured by a Talystep and, in some cases, with the help of transmittance spectra. The transmittance was obtained using a Varian 17D spectrophotometer, while the electrical properties were obtained by Van der Pauw and four-point probe methods. The structure of the films was investigated by X-ray diffractometry.
I N F L U E N C E O F D E P O S I T I O N P A R A M E T E R S O N ITO FILMS
3.
67
RESULTS AND DISCUSSION
Films with sheet resistances of approximately 10 f~/l-/are acceptable for the display applications such as those listed earlier. Such films have been produced in this work by r.f. sputtering in an atmosphere of pure argon. These films were deposited at a rate of 4 ,~ s- 1 using a power density of approximately 7 W c m - 2 and at a pressure of 5 x 1 0 - 3 Torr. The typical visible transmittance for a film of thickness 7000 ~ was about 90~ for a wavelength of 550 nm. However, when films are deposited onto InP substrates using these conditions, a very poor quality cell results and it is necessary to use much slower rates. Because of the oxygen absorption problem described earlier, it was decided to introduce a reducing gas into the sputtering atmosphere in an attempt to lessen the problem. Films deposited very slowly (about 0.3 ~ s- 1) in argon generally had a very high sheet resistance which rendered them unacceptable for solar cell window layers because of the resulting excessive series resistance. When post-deposition annealing of films deposited very slowly was performed the sheet resistance fell, typically, from several megaohms per square to 200-300 f~/[S]. A transmittance curve of one such film (before annealing) of thickness 7000/~ is shown in Fig. 1. The transmittance was essentially unchanged upon annealing. Although sheet resistances in the above range would not contribute excessive series resistance to a solar cell, the heat treatment required would irreversibly damage the junction i z. Films deposited in various mixtures of hydrogen: argon (the compositions used being 10 voi.~o H 2 , 20 vol.~ H 2 , 30 vol.~o H 2 ) had a sheet resistance which was 100/
,
,
,
,
,
.
.
.
.
.
/
80
-
70 ~ N
60
~ 5o c
~ 40 30
2O 10
O.S ' 0.6 03' O@ '. 09i ' 1.0 11i 12i ' 1.3 I./*' 1.5 Wavelength ( pm ) Fig. 1. T r a n s m i s s i o n c h a r a c t e r i s t i c s o f a 7 0 0 0 ,~ I T O film s p u t t e r d e p o s i t e d in p u r e a r g o n p l a s m a . 0
68
S. NASEEM, T. J. COUTTS
independent of the rate of deposition. Hence, for the fabrication of solar cells, an input r.f. power of 0.4 W cm -2, a total pressure of 3 x 1 0 - 3 Torr and an induced d.c. bias of 0.6 kV, corresponding to a rate of deposition of about 0.4/~ s- 1, were used. This led to cells with very low values of reverse saturation current density (2-5 pA cm 2) implying junctions suffering very little damage 14. A summary of the sheet resistance and transmittance of films deposited in the various gas compositions is shown in Table I. Clearly, the properties are optimum for the 10 vol.~o H 2 mixture although further refinement may be possible by adjusting the hydrogen content in smaller increments. It is possible tha t both sheet resistance and transmittance (reported in the table for a wavelength of 550 nm) were optimized at the same hydrogen content because of (a) a decrease in carrier mobility at concentrations greater than 10 vol.~o H2 and (b) increased light scattering or free-carrier absorption. However, these ideas have yet to be established. Films deposited at low input power in the 10vol.~oH2-90vol.~oAr plasma routinely have a resistivity of less than 2 × 10 3 fl cm (corresponding to a sheet resistance of about 300 f~/t-q for a 650/~ thick film). A typical transmittance curve is shown in Fig. 2 for a film of this thickness. Films deposited under these conditions have been shown to form excellent quality window layers in ITO/InP solar cells and have yielded total area efficiencies of greater than 16~o 14. Hence, for applications which require deposition of ITO on a sensitive surface, this technique offers great promise. ITO films deposited on borosilicate glass substrates under the above conditions have also been subjected to various heat treatments. Typically, the annealing temperature was 500 °C and an atmosphere of forming gas was used. In Fig. 3, we show the variation in sheet resistance with time. This shows that the sheet resistance decreases by a factor of more than 10 over a period of 5 min; the minimum resistivity was 1.4 x 10 -4 fl cm, corresponding to a sheet resistance of about 22 f l / O for a film 650/~ thick. The increase in the sheet resistance after 5min may again be attributable to a decrease in carrier mobility. Since the transmittance is very high, this material must be regarded as amongst the best quality ITO ever produced. Previous efforts to obtain these properties in ITO films have generally involved the use of an elevated substrate temperature during deposition, which is certainly not appropriate for the fabrication of I T O / I n P cells and may well be unsuitable for other applications using heat-sensitive substrates. In these cases the use of postdeposition heat treatment may also be unacceptable and we have shown here that this can be avoided by the use of hydrogen incorporation in the plasma, which can lead to films with a sheet resistance of the order of 200-800 f~/rl, with no deliberate substrate heating during or after deposition. In those cases where substrate damage during deposition is not problematic, high rates of deposition may be used and, in these circumstances, hydrogen incorporation makes little difference to the film properties. Presumably, the extent of oxygen incorporation is not lowered significantly by hydrogen when a high rate is used. When films deposited rapidly, with or without an H2-Ar plasma, are annealed after deposition their resistivity typically falls to somewhat less than 2 × 10 4 f~ cm. Since this is approximately equal to the values attained for unannealed films deposited slowly in the mixed sputtering gas, it can be concluded that the effects of post-deposition heat treatment
69
INFLUENCE OF DEPOSITION PARAMETERS ON ITO FILMS
TABLE I SUMMARY
OF THE FILM
PROPERTIES
AT DIFFERENT
SPUTTERING
GAS COMPOSITIONS
Gas composition
Transmittance at 550 nm (%)
Sheet resistance (~/[3)
Pure argon 10vol.%H2-90vol.~Ar 20vol.~H2-80vol.~Ar 30vol.%H2-70vol.~Ar
81.0 90.0 82.2 79.5
16.7 × l03 300.0 1.5 x l03 2.1 x 103
and deposition in a reducing atmosphere are similar. However, it was not found possible to reduce the resistivity of films deposited slowly in argon alone to less than 10- 2 D cm. Presumably, oxygen can be incorporated in the films in either strongly or weakly bound states and it appears to be possible to remove the latter by heat treatment but not the former. On the other hand, by using the mixed H2-Ar plasma, it appears to be possible to reduce the quantity of both strongly and weakly absorbed oxygen, so that post-deposition heat treatment of films prepared in such atmospheres leads to much lower resistivities. X-ray diffraction analyses of these films revealed a b.c.c, structure with preferred orientation along the (111) axis. This is consistent with the results of Fraser and Cook 7. Also, there was evidence of hexagonal In203, along with the 100
i
i
i
i
f
l
1
90
I ~50I
80
70
!001 60
i
50
rso]
i
E 40
1001 3O
20
\ ~x
5ol 10 0
I
0.6, 0 5
I
I
I
I
I
I
I
l
I
0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1,5
Wavelength
{ pm )
I
2
3
4
5
Annealing Ti,'ne (Minutes)
Fig. 2. Transmission characteristics of a 650,~ I T O film sputter deposited in a 10vol.~H2-90vol.%Ar plasma. Fig. 3. Sheet resistance of a 650 ~ I T O film as a function of the annealing time. This film was sputter deposited in a 10vol.~H 2-90vol.~Ar plasma.
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s. NASEEM, T. J. COUTTS
cubic phase in the best films. This is believed to contribute to increased carrier density 15 which is consistent with experimental observations. Other oxides such as SnO, 13-SnO etc. were also present in the films and, as has previously been shown 1s, these lead to oxygen deficiency which is beneficial to the conductivity of the films. 4. CONCLUSIONS In this work we have shown the following. (i) ITO films of resistivity less than 2 x 10 -4 f2 cm can be produced by a combination of very slow sputter deposition in a reducing atmosphere and postdeposition heat treatment. This resistivity is amongst the lowest ever achieved for highly transmissive ITO. (ii) This result can also be achieved for films deposited ten times more rapidly in argon alone, provided they are also heat treated after deposition. (iii) Since very slow rates of deposition can be used, the technique is eminently suitable for deposition onto substrates sensitive to electronic surface damage caused by bombardment of ions, neutral species or electrons from the plasma. Since substrate heating is much less at these reduced rates, it should also be possible to achieve low resistivities on heat-sensitive substrates such as plastics. (iv) We hypothesize that the use of the reducing atmosphere leads to the incorporation of a smaller quantity of strongly bound (chemisorbed) oxygen while post-deposition heat treatment leads to desorption of weakly bound (physisorbed) oxygen). (v) Changes in the electrical properties on heat treatment are consistent with changes in the density and/or mobility of charge carriers. REFERENCES 1 E.W. Williams and R. Hall, in B. R. Pamplin (ed.), Luminescence and the Light Emitting Diode, International Series on Science of the Solid State, Pergamon, Oxford, 1978. 2 T.J. Coutts and N. M. Pearsall, Proc. 16th IEEE Photovoltaic Specialists" Conf., San Diego, CA, September 27-30, 1982, IEEE, New York, p. 1288. 3 S. Ashok, P.P. SharmaandS. J. Fonash, IEEETrans. ElectronDev.,27(4)(1980)725. 4 R.H. Bube, Proc. Soc. Photo-Opt. Instrum. Eng., 114 (1977) 7. 5 T.J. Coutts, N. M. Pearsall and L. Tarricone, J. Vac. Sci. Technol. B, 2 (2) (1984) 140. 6 J. Machet, J. Guile, P. Saulnier and S. Robert, Thin Solid Films, 80 (1981) 149. 7 D.B. Fraser and H. D. Cook, J. Electrochem. Soc. Solid-State Sci. Technol., (1972) 1368. 8 R. Tueta and M. Braguier, Thin Solid Films, 80 (1981) 143. 9 H.U. Habermeir, Thin Solid Films, 80(1981) 157. 10 L.A. Ryabova and Y. S. Savitskaya, Thin Solid Films, 2 (1968) 141. 11 R. Groth, Phys. Status Solidi, 14 (1966) 69. 12 T.J. Coutts, A. Schwartzlander, S. Naseem and T. Massopust, Proc. 26th Rocky Mountain Conf. on Analytical Chemistry, Denver, CO, August 1984. 13 N.M. Pearsall, T. J. Coutts, R. Hill, G. J. Russell and K. J. Lawson, Thin Solid Films, 80 (1981) 177. 14 T.J. Coutts and S. Naseem, Appl. Phys. Lett., 46 (1985) 164. 15 C. Yuanri, X. Xinghao, J. Zhaoting, P. Chuancai and X. Shuyun, Thin Solid Films, 115 (3) (1984) 195.