- Email: [email protected]
Low resistivity and high mobility tin-doped indium oxide films
MaterialsLetters North-Holland
18 (1993) 123-127
Low resistivity and high mobility tin-doped indium oxide films I.A. Rauf l MP Group, CavendishLaboratory,MadingleyRoad, Cambridge, UK Received 24 April 1993; in tinal form 19 September t993; accepted 22 September 1993
Tin-doped indium oxide Sims have been prepared by e-beam reactive evaporation using a done-con~ning process. The lowest resistivity of 4.4x IO-’ Q m is lower by a factor of about 4 than previously reported values. Despite such a low resistivity the optical transparency is h&b (above 70% for film-substrate composite) and the mobility is 0.0103 m* V-’ s-’ which is comparable to the mobility observed for flux-grown single crystals of indium oxide.
1. Introduction
There is high interest in the optical and electrical properties of undoped and tin-doped indium oxide films because they are wide band gap (3.5-4 eV) materials exhibiting high visible transmission, fairly good electrical conductivity and high infrared reflectivity. However, transparency in the visible region is strongly affected by the electrical properties of the films. Good electrical properties can be achieved but often at the expense of transmission. The required quality of the films increases with increasing sophistication of application. Achievement of the lowest possible resistivity is of practical significance in that it provides some freedom in selecting the film thickness to achieve high optical transmission while still maintaining iow sheet resistance. Much effort has been expended to obtain low resistivities. The lowest resistivity reported ( 1] in the literature is of the order of 1.6~ lop6 R m. This Letter reports the preparation of polycrystalline tindoped indium oxide films with very low resistivity and high mobility.
r Present address: Department of Materials and MetaUtugicai Engineering, Nicol Hall, Queen’s University, Kingston, Ontario, K7N 3N6, Canada. 0167-577x/93/$ 06.00 0 1993 Elsevier Science Publishers B.V.
2, Experimental Thin films of tin-doped indium oxide were prepared by reactive e-beam evaporation as follows: tin oxide and indium oxide powders were mixed using a mortar and pestle in different proportions (ranging from 4.9 to 6.8 at% Sn). These mixtures were pressed to form pellets, which were sintered in air at 675 *C for 24 h and the sintered pellets were then loaded into a crucible for subsequent evaporation. Fused quartz slides, cleaned with acetone in an ultrasonic bath for I.5 min, were placed at a distance of 30 cm above the crucible and then the chamber was evacuated to its base pressure of less than 5 x 10m4 Pa, The slides were then heated to various deposition temperatures by means of a substrate heater (described in detail in ref. [ 21). Oxygen was then admitted to the chamber and the partial pressure of oxygen was set to 0.08 Pa. The filament current was then increased and the films were deposited at a deposition rate of 4-5 nm/min, which was much slower than normal deposition rates of 120 nmfmin [ 31. After completing the evaporation, films were cooled to room temperature at the same oxygen partial pressure. The design of the heater used to prepare these films was such that it produced a sinusoidal variation of temperature on the substrate surface. Although a molten zone was not established the difference in the local substrate temperatures forced the impu~ties to segregate into regions of higher temperature through Ail rights reserved.
123
Volume 18. number 3
MATERIALS LETTERS
a process similar to the zone-refining process [ 21. The result was alternating regions (width x20-50 nm) of heavily doped material and relatively impurity-free material. Tin concentrations in the films were measured by energy dispersive X-ray (EDX) analyses in a Cambridge Instruments S250 scanning electron microscope. Optical transmission data were obtained with a Perkin-Elmer spectrophotometer from samples of size 1 cmx 1 cm and a computer fitting programme [ 41 was used to obtain the values for the refractive index n at 2 = 600 nm and the average charge carrier concentration N. The thickness of the films was determined by an interferometric method using a Varian A-scope interferometer. The four-point probe van der Pauw method [ 51 was used to measure the resistivity p of the film. As N and p were determined independently the dc-conduction mobility pc was calculated using the relation [ 3 ] : &=l/Nep.
t-~orx~
December 1993
3. Results and discussion Figs. 1 and 2 show the room temperature optical transmission through the film-substrate composite for the specimens prepared with different dopant concentrations at substrate temperature of 250 and 300°C respectively. Table 1 gives the refractive indices calculated from the optical transmission spectra shown in figs. 1 and 2 at il=600 nm for all the specimens. Table 2 gives the measured dc-resistivity p, carrier density N and the conduction mobility Pi. Tables 1 and 2 show that the specimen with the highest value of N (prepared with 5.1 at% tin at 300°C) has the lowest refractive index, while the specimen with the lowest value of N (prepared with 6.6 atoh tin at 250°C) has the highest refractive index. This behaviour would be expected from dielectric theory because with increasing N the plasma frequency shifts towards the visible part of the solar spectrum causing the refractive index to decrease. Table 2 shows the measured dc-resistivity as a function of dopant concentration and substrate temperature. In each case, three specimens, prepared un-
nm
Tin concentration
= 4.2 at.% ___
Ttn concentration
= 5.1 at.%
Tin
1200 Wavelength
concentration
= 9.3
at.%
--. -
1700 (nm)
Fig. 1. Optical transmission through the film-substrate composites for the tin-doped indium oxide films prepared at a substrate temperature of 250°C (t represents the film thickness).
124
December 1993
MATERIALS LETTERS
Volume 18, number 3
:ll#a?:‘..
. .._.__
-
-._
1 =
-7
-
-
. . . .
._
----__ --.-.-_.-.___._.
Fig. 2. Optical transmission through the film-substrate composites for the tindoped indium oxide films prepared at a substrate temperature of 300°C (t represents the film thickness). Table 1 Computed values of the refractive index n for all the samples prepared with different dopant concentration Cs, and at two different substrate temperatures (250 and 300°C) c (&) 4.2 5.1 6.6 9.3
Substrate temperature ( “C) 250
300
1.73 1.86 1.86 1.82
1.74 1.60 1.83 1.80
der identical conditions, were measured. The lowest and the highest values of resistivity obtained are reported. The specimen with 5.1 at% tin prepared at 300°C shows the lowest value of 4.4. x 10m7 ii2m for the resistivity which is about a factor of 4 lower than previously reported values [ 11. For both substrate temperatures, the specimen with 6.6 at% tin shows the highest value of resistivity. An intergranular amorphous phase has been reported [ 6,7 ] to appear at this value of tin concentration, so that the presence of an inter~anular non-c~stalline phase might
be responsible for the highest value of the resistivity. Table 2 also shows the conduction mobilities as a function of dopant concentration. The dc-conduction mobilities for specimens prepared at a substrate temperature of 300°C are considerably higher than those for specimens prepared at 250°C. The better crystallinity at higher substrate temperatures might be the reason for higher mobilities. This could also indicate that the process discussed in the beginning of this Letter works better at higher substrate temperatures. The conduction mobility for the specimen prepared with 5.1 at% tin at a substrate temperature of 300°C is much higher than previously reported values for tin-doped indium oxide films [ 3 1. In fact, it is comparable to the mobility values reported [ 81 for flux-grown single crystals of tin-doped indium oxide. The solid line in fig. 3 shows the theoretical lower limit for the resistivity expected from ionized impurity scattering. Calculations are based on the work of Dingle [ 91 and Moore [ lo] for a degenerate free electron gas, according to which the resistivity is given as 12.5
Volume 18, number 3
MATERXALSLETTERS
December 1993
Table 2 Resistivity p, carrier density N and conduction mobility & for specimens prepared with different dopant concentration Cs, and at two different substrate temperatures (250 and 300°C)
c
Substrate temperature 250°C
Substrate temperature 300°C
(%J)
4.2 5.1 6.6 9.3
p(nm)
N (ma3)
k (ms V-l s-l)
P (am)
N (m-‘)
k (m2V-r s-‘)
(1.7-2.9)~10-~ (6.8-9.0) x 1O-6 9.3x 1O-6-3.Ox 1O-5 (4.7-6.1) x 1O-6
9.8 x 5.7x 5.6x 7.0x
0.0038 0.0016 0.0012 0.0019
(1.2-2.7)x1O-6 (4.4-6.9) x lo-? (3.4-6.0) x 1O-6 (1.9-4.2)x10-6
95x1026 13.8x10Z6 6.5x IO= 7.6x 10z6
0.0055 0.0103 0.0028 0.0043
1026 1026 lO= lo=
Carrier concentration (mv3) Fig. 3. Experimental resistivity data compared with the theoretical lower limit which can be obtained for a certain value of carrier concentration.
theoretical calculations assume that the impurities are homogeneously distributed in the specimen. Although they increase the carrier density, they simultaneously decrease the carrier mobility because of ionized impurity scattering. As discussed earlier, the specimens for present work were prepared in such a way that the impurities are forced to segregate to certain regions so that there are two “phases” of the material; the “dopant rich phase”, i.e. the regions where most of the electrons are generated and the “purer indium oxide phase”, i.e. the regions where mobility will be higher. If that is the case, the mobility will certainly be higher than that expected from a homogeneous distribution of dopant atoms. If the dopant in the segregated phase has adopted the host structure, the free carrier density will remain unaffected, resulting in a lower value of resistivity.
4. Conclusiims where Ni is the density of impurities, Z the charge on impurities, N the free electron density and er is the low frequency relative permittivity. The function f(kr) is given as
f(h=)=ln(l+B2)-
p2 1+p2
1
where b= 2kFjkTF, kTFis the Thomas-Fermi screening wavevector. The scattered points in fig_ 3 are the results of the present investigation. Apart form one specimen (the best,p=4.4~ lo-‘52 m), all the specimens show resistivities which lie above the theoretical limit. The 126
In summary, tin-doped indium oxide thin films prepared by combining e-beam reactive evaporation and a zone-confining process, have resistivity better by a factor of about 4 than previously reported values and have mobilities comparable to that for the flux-grown single crystals of tin-doped indium oxide. The improved properties of the present ftims are attributed to the new process used to prepare these ftims. The better c~st~nity in the films is a consequence of exceptionally slow deposition rates and impurity segregation achieved by control of substrate temperature.
Volume 18, number 3
MATERIALS LETTERS
Acknowledgement
I would like to thank the Ahmadiyya Muslim AsUK for generous financial support. Many useful discussions with Dr. P.H. Gaskell and Professor L.M. Brown are gratefully acknowledged. sociation
References [ 1] L.A. Ryabova, V.S. Salun and LA. Serbinov, Thin Solid
December 1993
[3] I. Hamberg and C.G. Granqvist, J. Appl. Phys. 60 ( 1986) R123. [4] I.A. Rauf, Ph.D. Thesis, University of Cambridge, Cambridge, UK ( 199 1). [ 51L.J. van der Pauw, Philips Res. Rept. 13 (1958) 1. [ 6 ] LA. Rauf, in Proceedings of the 12th International Congress for Electron Microscopy, 12-18 August 1990, Seattle, Washington, USA, Vol. 4 (San Francisco Press, San Francisco, 1990) p. 716. [ 71 I.A. Rauf, M.G. Walls and P.H. Gaskell, Trans. Roy. Microsc. Sot. 1 ( 1990) 165. [8] Y. Kanai, Japan. J. Appl. Phys. 23 (1984) L12. [9] R.B. Dingle, Phil. Mag. 46 (1955) 831. [lo] E.J. Moore, Phys. Rev. 160 (1967) 618.
Films 92 (1982) 327. [2] I.A. Rauf, J. Mat. Sci. Letters, in press.
127
18 (1993) 123-127
Low resistivity and high mobility tin-doped indium oxide films I.A. Rauf l MP Group, CavendishLaboratory,MadingleyRoad, Cambridge, UK Received 24 April 1993; in tinal form 19 September t993; accepted 22 September 1993
Tin-doped indium oxide Sims have been prepared by e-beam reactive evaporation using a done-con~ning process. The lowest resistivity of 4.4x IO-’ Q m is lower by a factor of about 4 than previously reported values. Despite such a low resistivity the optical transparency is h&b (above 70% for film-substrate composite) and the mobility is 0.0103 m* V-’ s-’ which is comparable to the mobility observed for flux-grown single crystals of indium oxide.
1. Introduction
There is high interest in the optical and electrical properties of undoped and tin-doped indium oxide films because they are wide band gap (3.5-4 eV) materials exhibiting high visible transmission, fairly good electrical conductivity and high infrared reflectivity. However, transparency in the visible region is strongly affected by the electrical properties of the films. Good electrical properties can be achieved but often at the expense of transmission. The required quality of the films increases with increasing sophistication of application. Achievement of the lowest possible resistivity is of practical significance in that it provides some freedom in selecting the film thickness to achieve high optical transmission while still maintaining iow sheet resistance. Much effort has been expended to obtain low resistivities. The lowest resistivity reported ( 1] in the literature is of the order of 1.6~ lop6 R m. This Letter reports the preparation of polycrystalline tindoped indium oxide films with very low resistivity and high mobility.
r Present address: Department of Materials and MetaUtugicai Engineering, Nicol Hall, Queen’s University, Kingston, Ontario, K7N 3N6, Canada. 0167-577x/93/$ 06.00 0 1993 Elsevier Science Publishers B.V.
2, Experimental Thin films of tin-doped indium oxide were prepared by reactive e-beam evaporation as follows: tin oxide and indium oxide powders were mixed using a mortar and pestle in different proportions (ranging from 4.9 to 6.8 at% Sn). These mixtures were pressed to form pellets, which were sintered in air at 675 *C for 24 h and the sintered pellets were then loaded into a crucible for subsequent evaporation. Fused quartz slides, cleaned with acetone in an ultrasonic bath for I.5 min, were placed at a distance of 30 cm above the crucible and then the chamber was evacuated to its base pressure of less than 5 x 10m4 Pa, The slides were then heated to various deposition temperatures by means of a substrate heater (described in detail in ref. [ 21). Oxygen was then admitted to the chamber and the partial pressure of oxygen was set to 0.08 Pa. The filament current was then increased and the films were deposited at a deposition rate of 4-5 nm/min, which was much slower than normal deposition rates of 120 nmfmin [ 31. After completing the evaporation, films were cooled to room temperature at the same oxygen partial pressure. The design of the heater used to prepare these films was such that it produced a sinusoidal variation of temperature on the substrate surface. Although a molten zone was not established the difference in the local substrate temperatures forced the impu~ties to segregate into regions of higher temperature through Ail rights reserved.
123
Volume 18. number 3
MATERIALS LETTERS
a process similar to the zone-refining process [ 21. The result was alternating regions (width x20-50 nm) of heavily doped material and relatively impurity-free material. Tin concentrations in the films were measured by energy dispersive X-ray (EDX) analyses in a Cambridge Instruments S250 scanning electron microscope. Optical transmission data were obtained with a Perkin-Elmer spectrophotometer from samples of size 1 cmx 1 cm and a computer fitting programme [ 41 was used to obtain the values for the refractive index n at 2 = 600 nm and the average charge carrier concentration N. The thickness of the films was determined by an interferometric method using a Varian A-scope interferometer. The four-point probe van der Pauw method [ 51 was used to measure the resistivity p of the film. As N and p were determined independently the dc-conduction mobility pc was calculated using the relation [ 3 ] : &=l/Nep.
t-~orx~
December 1993
3. Results and discussion Figs. 1 and 2 show the room temperature optical transmission through the film-substrate composite for the specimens prepared with different dopant concentrations at substrate temperature of 250 and 300°C respectively. Table 1 gives the refractive indices calculated from the optical transmission spectra shown in figs. 1 and 2 at il=600 nm for all the specimens. Table 2 gives the measured dc-resistivity p, carrier density N and the conduction mobility Pi. Tables 1 and 2 show that the specimen with the highest value of N (prepared with 5.1 at% tin at 300°C) has the lowest refractive index, while the specimen with the lowest value of N (prepared with 6.6 atoh tin at 250°C) has the highest refractive index. This behaviour would be expected from dielectric theory because with increasing N the plasma frequency shifts towards the visible part of the solar spectrum causing the refractive index to decrease. Table 2 shows the measured dc-resistivity as a function of dopant concentration and substrate temperature. In each case, three specimens, prepared un-
nm
Tin concentration
= 4.2 at.% ___
Ttn concentration
= 5.1 at.%
Tin
1200 Wavelength
concentration
= 9.3
at.%
--. -
1700 (nm)
Fig. 1. Optical transmission through the film-substrate composites for the tin-doped indium oxide films prepared at a substrate temperature of 250°C (t represents the film thickness).
124
December 1993
MATERIALS LETTERS
Volume 18, number 3
:ll#a?:‘..
. .._.__
-
-._
1 =
-7
-
-
. . . .
._
----__ --.-.-_.-.___._.
Fig. 2. Optical transmission through the film-substrate composites for the tindoped indium oxide films prepared at a substrate temperature of 300°C (t represents the film thickness). Table 1 Computed values of the refractive index n for all the samples prepared with different dopant concentration Cs, and at two different substrate temperatures (250 and 300°C) c (&) 4.2 5.1 6.6 9.3
Substrate temperature ( “C) 250
300
1.73 1.86 1.86 1.82
1.74 1.60 1.83 1.80
der identical conditions, were measured. The lowest and the highest values of resistivity obtained are reported. The specimen with 5.1 at% tin prepared at 300°C shows the lowest value of 4.4. x 10m7 ii2m for the resistivity which is about a factor of 4 lower than previously reported values [ 11. For both substrate temperatures, the specimen with 6.6 at% tin shows the highest value of resistivity. An intergranular amorphous phase has been reported [ 6,7 ] to appear at this value of tin concentration, so that the presence of an inter~anular non-c~stalline phase might
be responsible for the highest value of the resistivity. Table 2 also shows the conduction mobilities as a function of dopant concentration. The dc-conduction mobilities for specimens prepared at a substrate temperature of 300°C are considerably higher than those for specimens prepared at 250°C. The better crystallinity at higher substrate temperatures might be the reason for higher mobilities. This could also indicate that the process discussed in the beginning of this Letter works better at higher substrate temperatures. The conduction mobility for the specimen prepared with 5.1 at% tin at a substrate temperature of 300°C is much higher than previously reported values for tin-doped indium oxide films [ 3 1. In fact, it is comparable to the mobility values reported [ 81 for flux-grown single crystals of tin-doped indium oxide. The solid line in fig. 3 shows the theoretical lower limit for the resistivity expected from ionized impurity scattering. Calculations are based on the work of Dingle [ 91 and Moore [ lo] for a degenerate free electron gas, according to which the resistivity is given as 12.5
Volume 18, number 3
MATERXALSLETTERS
December 1993
Table 2 Resistivity p, carrier density N and conduction mobility & for specimens prepared with different dopant concentration Cs, and at two different substrate temperatures (250 and 300°C)
c
Substrate temperature 250°C
Substrate temperature 300°C
(%J)
4.2 5.1 6.6 9.3
p(nm)
N (ma3)
k (ms V-l s-l)
P (am)
N (m-‘)
k (m2V-r s-‘)
(1.7-2.9)~10-~ (6.8-9.0) x 1O-6 9.3x 1O-6-3.Ox 1O-5 (4.7-6.1) x 1O-6
9.8 x 5.7x 5.6x 7.0x
0.0038 0.0016 0.0012 0.0019
(1.2-2.7)x1O-6 (4.4-6.9) x lo-? (3.4-6.0) x 1O-6 (1.9-4.2)x10-6
95x1026 13.8x10Z6 6.5x IO= 7.6x 10z6
0.0055 0.0103 0.0028 0.0043
1026 1026 lO= lo=
Carrier concentration (mv3) Fig. 3. Experimental resistivity data compared with the theoretical lower limit which can be obtained for a certain value of carrier concentration.
theoretical calculations assume that the impurities are homogeneously distributed in the specimen. Although they increase the carrier density, they simultaneously decrease the carrier mobility because of ionized impurity scattering. As discussed earlier, the specimens for present work were prepared in such a way that the impurities are forced to segregate to certain regions so that there are two “phases” of the material; the “dopant rich phase”, i.e. the regions where most of the electrons are generated and the “purer indium oxide phase”, i.e. the regions where mobility will be higher. If that is the case, the mobility will certainly be higher than that expected from a homogeneous distribution of dopant atoms. If the dopant in the segregated phase has adopted the host structure, the free carrier density will remain unaffected, resulting in a lower value of resistivity.
4. Conclusiims where Ni is the density of impurities, Z the charge on impurities, N the free electron density and er is the low frequency relative permittivity. The function f(kr) is given as
f(h=)=ln(l+B2)-
p2 1+p2
1
where b= 2kFjkTF, kTFis the Thomas-Fermi screening wavevector. The scattered points in fig_ 3 are the results of the present investigation. Apart form one specimen (the best,p=4.4~ lo-‘52 m), all the specimens show resistivities which lie above the theoretical limit. The 126
In summary, tin-doped indium oxide thin films prepared by combining e-beam reactive evaporation and a zone-confining process, have resistivity better by a factor of about 4 than previously reported values and have mobilities comparable to that for the flux-grown single crystals of tin-doped indium oxide. The improved properties of the present ftims are attributed to the new process used to prepare these ftims. The better c~st~nity in the films is a consequence of exceptionally slow deposition rates and impurity segregation achieved by control of substrate temperature.
Volume 18, number 3
MATERIALS LETTERS
Acknowledgement
I would like to thank the Ahmadiyya Muslim AsUK for generous financial support. Many useful discussions with Dr. P.H. Gaskell and Professor L.M. Brown are gratefully acknowledged. sociation
References [ 1] L.A. Ryabova, V.S. Salun and LA. Serbinov, Thin Solid
December 1993
[3] I. Hamberg and C.G. Granqvist, J. Appl. Phys. 60 ( 1986) R123. [4] I.A. Rauf, Ph.D. Thesis, University of Cambridge, Cambridge, UK ( 199 1). [ 51L.J. van der Pauw, Philips Res. Rept. 13 (1958) 1. [ 6 ] LA. Rauf, in Proceedings of the 12th International Congress for Electron Microscopy, 12-18 August 1990, Seattle, Washington, USA, Vol. 4 (San Francisco Press, San Francisco, 1990) p. 716. [ 71 I.A. Rauf, M.G. Walls and P.H. Gaskell, Trans. Roy. Microsc. Sot. 1 ( 1990) 165. [8] Y. Kanai, Japan. J. Appl. Phys. 23 (1984) L12. [9] R.B. Dingle, Phil. Mag. 46 (1955) 831. [lo] E.J. Moore, Phys. Rev. 160 (1967) 618.
Films 92 (1982) 327. [2] I.A. Rauf, J. Mat. Sci. Letters, in press.
127