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Transparent conducting oxides of metals and alloys made by reactive magnetron sputtering from elemental targets
Vacuum/volume 36/numbers Printed in Great Britain
1-3/pages
0042-207X/86$3.00
95 to 98/l 986
Transparent conducting alloys made by reactive from elemental targets R Lewin,
R P Howson,
Technology,
Loughborough,
+ .OO
Pergamon Press Ltd
C A Bishop
and M I Ridge,
Leics LE7 1 3TU
oxides of metals and magnetron sputtering Department
of Physics, Loughborough
University
of
UK
Oxide films were made in a continuous process onto plastic sheet wound from reel to reel over a drum exposed to the sputtering material and residual active gas. Single and double coaxial magnetron sources were used to create conducting elemental oxides of indium, cadmium, tin and zinc and alloy oxides of a combination of these materials. Optimum stoichiometric composition of oxygen was determined for each material using continuous monitoring of electrical conductivity and visual transparency. The composition of the films was determined with’ XPS, AES and electron microprobe analysis. The films were assessed for electrical conductivity, visual transparency and the Hall effect parameters of carrier density and mobility. lndium and cadmium oxides proved the most conducting and stable with resistivities of 4. lo- %rn being obtainable with good visual transparency. The addition of a small quantity of tin to cadmium oxide had the effect of considerably changing the blue transmittance, removing the yellow appearance from the film. For the less conducting oxides of tin and zinc film resistivities of approximately 2.7 O- 4Qm were obtained.
1. Introduction We have previously reported the production of conducting oxides based on indium, indium-tin and cadmium-tin which had high electrical conductivities whilst being transparent in the visible region of the spectrum’. These films were deposited onto polyester sheet (ICI-Melinex) transferred from roll to roll over a drum. They were made by carefully controlled reactive planar magnetron sputtering, in some cases with simultaneous rf plasma bombardment 2. The successful manufacture of films having high electrical conductivities onto low temperature substrates had been obtained by following conventional wisdom in the choice of materials and most of our attention had been directed in creating techniques and conditions suitable for them. What had emerged was that very careful control of sputtering rates and gas flows had to be maintained to enable the very small ‘windows’ of conditions for satisfactory coatings to be created. Of great importance in this was the stability of the supplies and of the process and of the ability to ‘tune’ conditions by continuous monitoring of the electrical resistance and optical transparency of the film very shortly after preparation. This was done with probes within the vacuum chamber, which as well as allowing the achievement of optimum conditions in a short time, gave a large experimental productivity for the observation of other changes in conditions. The development of these techniques now allowed the investigation of oxide materials that might be expected to show some electrical conductivity whilst remaining transparent. These investigations are reported here for a range of materials prepared by the reactive sputtering of metal targets of a planar magnetron cathode
in an an atmosphere of argon and oxygen. The materials were chosen to be in elemental and alloy form and optimum alloying effects were investigated with a dual source unit.
2. Experimental The apparatus used in the current investigation has been described previously2. The planar magnetrons were constructed with pole pieces brought into the target area, this had been found to be satisfactory in previous experiments because of the nature reactive sputtering; sputtering takes place in a narrow region of the target, the ‘race-track’, in other regions deposition of the oxide takes place and so there is no contamination by the exposed iron material. A dual source unit consisted of two concentric and electrically isolated targets separated by a high magnetic permeability material. The magnetrons have also been previously reported 3. Indium, tin, zinc, bismuth and lead oxide films have been prepared from elemental targets. IT0 from an alloy target (10% Sn) and CT0 from an alloy target (33% Sn). The dual source magnetron has been used to investigate the addition of tin to indium oxide, tin to cadmium oxide, indium to zinc oxide and antimony to tin oxide. The substrate material was 50 pm thick polyethylene terephthalate (PET-ICI-Melinex). In performing the measurements that were made on the film we had to be mindful of the nature of the substrate. The techniques that we evolved include a refractive index and thickness measurement from investigation with an ellipsometer. In this case great care had to be taken with the birefringence exhibited by the sheet and by its very. thin form, 95
R Lewin et al: Transparent
conducting
oxides
of metals
and alloys
requiring elimination of reflection from the uncoated surface by roughening and blackening. Conventional electrical measurements could be made using a four-point probe with ‘soft’ contacts and by the Hall effect on samples cut from the roll. These samples had to be handled carefully because with some samples cracking of the films occured with flexing preventing continuous electrical continuity. Optical properties were also measured4. Dissolving off the PET substrate with ortho-chlorophenol made possible examination of the deposited thin films (typically 100 nm thick) with transmission electron microscopy’. Consequently it has been possible to estimate the crystal size, determine the crystal structure and lattice constant from the electron diffraction pattern. I
3. Results
Cd/Sri
The electrical characteristics of the optimum oxide films that were created are shown in Table 1. The precision of sputtering power and gas admission rates needed to produce these films is illustrated in Figure 1 for cadmium 4% tin metal composition. Other effects in sputtering alloy targets were noticed in that the energetic conditions needed for conducting films led to a
140 130
-
Ix)-
-/I
110-
-
100-
-9
10
-
-8
?E
a0 -
-7
8 -
70
-
-6
*
60
-
-5
r F
50
-
-4
4
90
”
).
4O30
-
20
-
-3
ki k
-2
s
I
i7c IO’0 ‘10 0’12 0’14 16 16 lo ’ 0
0
O2 Partial
pressure
0
Table 1. The electrical
96
characteristics
of reactively
I 08
09
ratio
Figure 2. The electrical resistivity of oxide films, made by sputtering an alloy target of cadmium/tin in different partial pressures of oxygen, shown as a function of the resulting metal composition of the film.
composition of tin to cadmium in a film being quite different from that of the target and of that ratio specified by Cd,SnO,, taken as necessary for good film8, see Figure 2. Optical properties given by the most conducting films made in this way are shown in Figure 3. Investigation of alloy ratios revealed that the effect of adding tin to an indium film prepared at an optimum partial pressure of oxygen gave worse characteristics (Figure 4) but adding small quantitites of tin to cadmium gave better visible transparency (Figure 5) and the loss of mobility was compensated for by the increase in carrier density (Figure 6). To date TEM measurements have been made on indium oxide, CT0 and the doped cadmium oxide (&25% Sn) films. Indium oxide and the doped cadmium oxide films show similar crystal sizes (20-50 nm). The doped cadmium oxides show no lines additional to those associated with the Cd0 structure (Figure 7). CT0 films prepared from the alloy target however show an essentially amorphous structure7. Also the resulting diffraction pattern shows a large degree of line broadening, consistent with the small crystal size, unfortunately to the extent where no other information could be deduced (Figure 8). 4. Discussion
(Pa)
Figure 1. The effect of changes in the oxygen partial pressure, during reactive sputtering, on the electrical properties of CdO(il%Sn) films. The argon partial pressure was 0.4 Pa and the total magnetron current was 1.2 A.
IO IT0 CT0 ZnO ZnO(S%In) Cd0 Cd0(4%Sn) SnO, SnO,(lO%Sb)
I 07
06
sputtered
The
range
of metals
that
were
chosen
to be investigated
stable
sputtering
targets
when
operated
oxide films
Sheet resistance
Thickness
Resistivity
@im)
(nm)
(x lO_Qm)
24.3 15.6 15.5 9,700 2,780 36 20.3 443 17,200
172 270 260 133 81 110 101 250 218
4.2 4.2 4.0 1,300 225 3.95 2.1 111 3.750
here
in a partial pressure of oxygen, but the conditions necessary for the production of electrically conducting oxides often depended strongly on
provided
Mobility (cmZV-‘SK’)
Carrier
density
Refractive
(x 10Z0cm~3)
41 17 34 _
3.6 8.7 4.6
46 28.3 13 _
3.4 11 0.43
1.94 1.9x 2.0 2.16 1.90 2.29 2.01 2.08 1.95
index
R Lewin et a/: Transparent
conducting
oxides of metals and alloys
I-
3-
,31
5
--_
I
IO
I
Wavelength
(pm) O-
Figure 3. The selective
optical
properties
obtained
with variot
1s II
O_
%
Sn
Figure 6. The influence of tin on the electrical properties films.
%/amp
of cadmium
oxide
(In=0.5A)
Figure 4. The effect on the electrical caused by the addition of tin.
properties
of indium
oxide films Figure 7. Transmission electron micrograph (x lOO,ooO) of cadmium oxide film. Insert, the electron diffraction pattern of the same sample.
Wavelength
Figure 5. The optical transmittance small percentages of tin. A = CdO,
(nm)
of films of cadmium oxide containing B = CdO(Z%Sn), C = Cd0(4%Sn).
Figure 8. Transmission electron film. Insert, electron diffraction
micrograph ( x 100,000) of CT0(42%Sn) pattern of the same sample.
R Lewin
et a/: Transparent
conducting
oxides
of metals
and alloys
the previous history of the target surface and without a continuous monitoring technique the deposition of conducting films would be very difficult. This is particularly true for the creation of both tin and zinc oxide films. In general, the addition of a dopant to an elemental oxide resulted in worse electrical characteristics but this could be compensated in some cases by lower optical absorptance (Cd-Sn), better handleability (In-Sn) and a relaxation in the conditions required for the preparation of satisfactory films (In-Sn).
Indium oxide was almost as good in electrical properties and was optically neutral, apart from effects introduced by interference. The addition of small amounts of tin to indium oxide resulted in deteriorating performance. Indium is expensive and cadmium toxic, other possibilities that offer solutions where such high conductivities are not required were found to be oxides of tin and zinc.
References 5. Conclusion A high performance, transparent, electrically conducting oxide should have a high energy gap, to give good visible transmission, and have a high conductance without the free carrier optical absorption affecting the appearance. The material with the best electrical characteristics proved to be cadmium oxide, it had however a yellow appearance due to absorption in the blue region of the spectrum. This could be reduced, if not eliminated, by the addition of small amounts of tin.
98
’ R P Howson, crnd IPAT’83,
M
I Ridge, C A Bishop and K Suzuki, Proc~ lSlAT’X3
p 933, (1983).
* M I Ridge, M Stenlake.
R P Howson
and C A Bishop.
Thin Solid Film.\.
80. 31 (1981).
’ C A Bishop, M I Ridge and R P Howson. Proc ISfAT’X3 md IPAT’X?. p 945, (1983). 4 C A Bishop and R P Howson, Proc IPAT’85, 377 (1985). ’ J E Morris, C A Bishop, M I Ridge and R P Howson. Thin Solid Film\. 62, 19 (1979). ’ G Haake, Pm SIC pho-opt Enqrs. 324, 10 (19X2). ’ A J Nozik, Phys Ret), B6, 453 (1972).
1-3/pages
0042-207X/86$3.00
95 to 98/l 986
Transparent conducting alloys made by reactive from elemental targets R Lewin,
R P Howson,
Technology,
Loughborough,
+ .OO
Pergamon Press Ltd
C A Bishop
and M I Ridge,
Leics LE7 1 3TU
oxides of metals and magnetron sputtering Department
of Physics, Loughborough
University
of
UK
Oxide films were made in a continuous process onto plastic sheet wound from reel to reel over a drum exposed to the sputtering material and residual active gas. Single and double coaxial magnetron sources were used to create conducting elemental oxides of indium, cadmium, tin and zinc and alloy oxides of a combination of these materials. Optimum stoichiometric composition of oxygen was determined for each material using continuous monitoring of electrical conductivity and visual transparency. The composition of the films was determined with’ XPS, AES and electron microprobe analysis. The films were assessed for electrical conductivity, visual transparency and the Hall effect parameters of carrier density and mobility. lndium and cadmium oxides proved the most conducting and stable with resistivities of 4. lo- %rn being obtainable with good visual transparency. The addition of a small quantity of tin to cadmium oxide had the effect of considerably changing the blue transmittance, removing the yellow appearance from the film. For the less conducting oxides of tin and zinc film resistivities of approximately 2.7 O- 4Qm were obtained.
1. Introduction We have previously reported the production of conducting oxides based on indium, indium-tin and cadmium-tin which had high electrical conductivities whilst being transparent in the visible region of the spectrum’. These films were deposited onto polyester sheet (ICI-Melinex) transferred from roll to roll over a drum. They were made by carefully controlled reactive planar magnetron sputtering, in some cases with simultaneous rf plasma bombardment 2. The successful manufacture of films having high electrical conductivities onto low temperature substrates had been obtained by following conventional wisdom in the choice of materials and most of our attention had been directed in creating techniques and conditions suitable for them. What had emerged was that very careful control of sputtering rates and gas flows had to be maintained to enable the very small ‘windows’ of conditions for satisfactory coatings to be created. Of great importance in this was the stability of the supplies and of the process and of the ability to ‘tune’ conditions by continuous monitoring of the electrical resistance and optical transparency of the film very shortly after preparation. This was done with probes within the vacuum chamber, which as well as allowing the achievement of optimum conditions in a short time, gave a large experimental productivity for the observation of other changes in conditions. The development of these techniques now allowed the investigation of oxide materials that might be expected to show some electrical conductivity whilst remaining transparent. These investigations are reported here for a range of materials prepared by the reactive sputtering of metal targets of a planar magnetron cathode
in an an atmosphere of argon and oxygen. The materials were chosen to be in elemental and alloy form and optimum alloying effects were investigated with a dual source unit.
2. Experimental The apparatus used in the current investigation has been described previously2. The planar magnetrons were constructed with pole pieces brought into the target area, this had been found to be satisfactory in previous experiments because of the nature reactive sputtering; sputtering takes place in a narrow region of the target, the ‘race-track’, in other regions deposition of the oxide takes place and so there is no contamination by the exposed iron material. A dual source unit consisted of two concentric and electrically isolated targets separated by a high magnetic permeability material. The magnetrons have also been previously reported 3. Indium, tin, zinc, bismuth and lead oxide films have been prepared from elemental targets. IT0 from an alloy target (10% Sn) and CT0 from an alloy target (33% Sn). The dual source magnetron has been used to investigate the addition of tin to indium oxide, tin to cadmium oxide, indium to zinc oxide and antimony to tin oxide. The substrate material was 50 pm thick polyethylene terephthalate (PET-ICI-Melinex). In performing the measurements that were made on the film we had to be mindful of the nature of the substrate. The techniques that we evolved include a refractive index and thickness measurement from investigation with an ellipsometer. In this case great care had to be taken with the birefringence exhibited by the sheet and by its very. thin form, 95
R Lewin et al: Transparent
conducting
oxides
of metals
and alloys
requiring elimination of reflection from the uncoated surface by roughening and blackening. Conventional electrical measurements could be made using a four-point probe with ‘soft’ contacts and by the Hall effect on samples cut from the roll. These samples had to be handled carefully because with some samples cracking of the films occured with flexing preventing continuous electrical continuity. Optical properties were also measured4. Dissolving off the PET substrate with ortho-chlorophenol made possible examination of the deposited thin films (typically 100 nm thick) with transmission electron microscopy’. Consequently it has been possible to estimate the crystal size, determine the crystal structure and lattice constant from the electron diffraction pattern. I
3. Results
Cd/Sri
The electrical characteristics of the optimum oxide films that were created are shown in Table 1. The precision of sputtering power and gas admission rates needed to produce these films is illustrated in Figure 1 for cadmium 4% tin metal composition. Other effects in sputtering alloy targets were noticed in that the energetic conditions needed for conducting films led to a
140 130
-
Ix)-
-/I
110-
-
100-
-9
10
-
-8
?E
a0 -
-7
8 -
70
-
-6
*
60
-
-5
r F
50
-
-4
4
90
”
).
4O30
-
20
-
-3
ki k
-2
s
I
i7c IO’0 ‘10 0’12 0’14 16 16 lo ’ 0
0
O2 Partial
pressure
0
Table 1. The electrical
96
characteristics
of reactively
I 08
09
ratio
Figure 2. The electrical resistivity of oxide films, made by sputtering an alloy target of cadmium/tin in different partial pressures of oxygen, shown as a function of the resulting metal composition of the film.
composition of tin to cadmium in a film being quite different from that of the target and of that ratio specified by Cd,SnO,, taken as necessary for good film8, see Figure 2. Optical properties given by the most conducting films made in this way are shown in Figure 3. Investigation of alloy ratios revealed that the effect of adding tin to an indium film prepared at an optimum partial pressure of oxygen gave worse characteristics (Figure 4) but adding small quantitites of tin to cadmium gave better visible transparency (Figure 5) and the loss of mobility was compensated for by the increase in carrier density (Figure 6). To date TEM measurements have been made on indium oxide, CT0 and the doped cadmium oxide (&25% Sn) films. Indium oxide and the doped cadmium oxide films show similar crystal sizes (20-50 nm). The doped cadmium oxides show no lines additional to those associated with the Cd0 structure (Figure 7). CT0 films prepared from the alloy target however show an essentially amorphous structure7. Also the resulting diffraction pattern shows a large degree of line broadening, consistent with the small crystal size, unfortunately to the extent where no other information could be deduced (Figure 8). 4. Discussion
(Pa)
Figure 1. The effect of changes in the oxygen partial pressure, during reactive sputtering, on the electrical properties of CdO(il%Sn) films. The argon partial pressure was 0.4 Pa and the total magnetron current was 1.2 A.
IO IT0 CT0 ZnO ZnO(S%In) Cd0 Cd0(4%Sn) SnO, SnO,(lO%Sb)
I 07
06
sputtered
The
range
of metals
that
were
chosen
to be investigated
stable
sputtering
targets
when
operated
oxide films
Sheet resistance
Thickness
Resistivity
@im)
(nm)
(x lO_Qm)
24.3 15.6 15.5 9,700 2,780 36 20.3 443 17,200
172 270 260 133 81 110 101 250 218
4.2 4.2 4.0 1,300 225 3.95 2.1 111 3.750
here
in a partial pressure of oxygen, but the conditions necessary for the production of electrically conducting oxides often depended strongly on
provided
Mobility (cmZV-‘SK’)
Carrier
density
Refractive
(x 10Z0cm~3)
41 17 34 _
3.6 8.7 4.6
46 28.3 13 _
3.4 11 0.43
1.94 1.9x 2.0 2.16 1.90 2.29 2.01 2.08 1.95
index
R Lewin et a/: Transparent
conducting
oxides of metals and alloys
I-
3-
,31
5
--_
I
IO
I
Wavelength
(pm) O-
Figure 3. The selective
optical
properties
obtained
with variot
1s II
O_
%
Sn
Figure 6. The influence of tin on the electrical properties films.
%/amp
of cadmium
oxide
(In=0.5A)
Figure 4. The effect on the electrical caused by the addition of tin.
properties
of indium
oxide films Figure 7. Transmission electron micrograph (x lOO,ooO) of cadmium oxide film. Insert, the electron diffraction pattern of the same sample.
Wavelength
Figure 5. The optical transmittance small percentages of tin. A = CdO,
(nm)
of films of cadmium oxide containing B = CdO(Z%Sn), C = Cd0(4%Sn).
Figure 8. Transmission electron film. Insert, electron diffraction
micrograph ( x 100,000) of CT0(42%Sn) pattern of the same sample.
R Lewin
et a/: Transparent
conducting
oxides
of metals
and alloys
the previous history of the target surface and without a continuous monitoring technique the deposition of conducting films would be very difficult. This is particularly true for the creation of both tin and zinc oxide films. In general, the addition of a dopant to an elemental oxide resulted in worse electrical characteristics but this could be compensated in some cases by lower optical absorptance (Cd-Sn), better handleability (In-Sn) and a relaxation in the conditions required for the preparation of satisfactory films (In-Sn).
Indium oxide was almost as good in electrical properties and was optically neutral, apart from effects introduced by interference. The addition of small amounts of tin to indium oxide resulted in deteriorating performance. Indium is expensive and cadmium toxic, other possibilities that offer solutions where such high conductivities are not required were found to be oxides of tin and zinc.
References 5. Conclusion A high performance, transparent, electrically conducting oxide should have a high energy gap, to give good visible transmission, and have a high conductance without the free carrier optical absorption affecting the appearance. The material with the best electrical characteristics proved to be cadmium oxide, it had however a yellow appearance due to absorption in the blue region of the spectrum. This could be reduced, if not eliminated, by the addition of small amounts of tin.
98
’ R P Howson, crnd IPAT’83,
M
I Ridge, C A Bishop and K Suzuki, Proc~ lSlAT’X3
p 933, (1983).
* M I Ridge, M Stenlake.
R P Howson
and C A Bishop.
Thin Solid Film.\.
80. 31 (1981).
’ C A Bishop, M I Ridge and R P Howson. Proc ISfAT’X3 md IPAT’X?. p 945, (1983). 4 C A Bishop and R P Howson, Proc IPAT’85, 377 (1985). ’ J E Morris, C A Bishop, M I Ridge and R P Howson. Thin Solid Film\. 62, 19 (1979). ’ G Haake, Pm SIC pho-opt Enqrs. 324, 10 (19X2). ’ A J Nozik, Phys Ret), B6, 453 (1972).