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Study of annealed indium tin oxide films prepared by rf reactive magnetron sputtering
Vacuum/volume 46lnumber 7lpages 673 to 68011995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved
Pergamon
0042-207x/95
$9.50+.00
0042-207x(94)00159-2
Study of annealed indium tin oxide films prepared by rf reactive magnetron sputtering Li-jian Meng*, Portugal received
A Maqarico and R Martins,
CEMOP/UNINOVA,
Quinta de Torre, 2825 Monte
da Caparica,
14 October 1994
Tin doped indium oxide (ITO) films were deposited on glass substrates by rf reactive magnetron sputtering using a metallic alloy target (In-Sn, 90-10). The post-deposition annealing has been done for IT0 films in air and the effect of annealing temperature on the electrical, optical and structural properties of IT0 films was studied. It has been found that the increase of the annealing temperature will improve the film electrical properties. The resistivity of as deposited film is about 1.3 x lo- ’ R *cm and decreases down to 6.9 x 10e3 R *cm as the annealing temperature is increased up to 500°C. In addition, the annealing will also increase the film surface roughness which can improve the efficiency of amorphous silicon solar cells by increasing the amount of light trapping.
1. Introduction Indium tin oxide (ITO) film has been widely used as a transparent conductor due to its high transparency to visible light and its low electrical resistivity. Applications which use IT0 as a transparent conductor include liquid crystal display, solar cells and various light sensitive solid state devices’m3. High quality IT0 films have been prepared by various deposition methods such as electron beam evaporation”‘, thermal evaporation’, CVD’, spray pyrolysis’, laser ablation’, dc sputthese deposition tering’@14 and rf sputtering’5~m’9. Among methods, reactive sputtering deposition is widely used for IT0 film deposition. Desirable features of reactive sputtering employing alloy targets for practical use are : a high deposition rate ; accurate control of the film thickness and easy fabrication of the alloy target. In general, a post-deposition annealing in air or in an oxygen free environment for reactively sputtered IT0 coatings from metal alloy target is needed in order to improve the film electrical properties’“.“.2g”. In this paper, IT0 films have been deposited on glass substrates by rf reactive magnetron sputtering method and the effect of the post-deposition annealing temperature (2OO”C-500°C) on the film structural, optical and electrical properties has been studied.
2. Experimental details IT0 films were deposited on glass substrates at ambient temperature by rf reactive magnetron sputtering technique. The schematic of deposition system used in this work is shown in Figure
* On leave from Changchun Institute of Physics, Chinese Academy of Sciences, Changchun 130021, P.R. China.
1. The system is equipped with both rf and dc sputtering sources. in this work, only the rf source is used. The chamber is pumped with a turbomolecular pump (TPU510) backed by a rotary pump (EDWARDS E2M40). The pressure of the chamber is monitored by a Pirani-Penning gauge combination and measured by Balzers TPG 300 total pressure gauge controller. The target is In-Sn (90 : 10) alloy of 99.99% purity (125 x 500 mm* ALCATEL CIT). The distance between target and substrate is about 40 mm. The cathode is cooled by running water. A RFX 13.56 MHz, 2500 W generator and an ATX tuner (Advanced Energy) are used as rf power supply. The vacuum chamber is evacuated down to pressure 3 x lo-’ mbar prior to deposition. Then the oxygen reactive gas is introduced into the chamber and the required pressure (9 x 10e4 mbar) is set. Argon gas is then introduced thereafter till the preset pressure is reached (4 x 10m3 mbar). After that, the oxygen reactive gas is shut off and the alloy target is presputtered (600 W rf power) in an argon atmosphere for about 10 min with a shuttle covering the substrate in order to remove the surface oxide layer formed during exposure to air. Then the oxygen reactive gas is introduced again and the shutter is removed and film deposition process starts. Both argon inert gas flow and oxygen reactive gas flow are controlled by a mass flow controller (Datametrics, controller 1605). The rf forward and reflected powers during the film deposition are 1200 W and 12.3 W respectively. The deposition time is about 25 min. The deposition conditions are listed in Table 1. After the deposition, the films are annealed in a tube furnace for about 30 min in air. The annealing temperatures are 200°C 300°C 400°C and 500°C respectively. The film thickness is measured by a SLOAN DEKTAK IID (the film thickness is about 610 nm). The film resistivity is measured by Veeco FPP-5000 four-point probe. The film by Shimadzu double-beam transmittance is measured 673
L-J Meng et al: Annealed
indium tin oxide
films
Table 2. X-ray diffraction
Gas mixture cylinder
conditions
X-ray tube Wavelength (a 1, ~2) lrradiated length Receiving slit Range (20) Step width, time
Cu KU, normal focus, 40 kV, 20 mA 1.54060A, 1.544388, 12 mm 0.2mm 10.00~ ~70.000” 0.020”, 1.25 s
eV) operating at 5 kV and 20 mA. Photoelectrons are detected with a hemispherical analyzer with an input lens positioned normal to the surface. Spectrometer pass energy is 20 eV. The atomic concentration of the various elements are computed from measured peak areas (C Is, 0 Is, Sn 3d5/2 and In 3d5/2) using VGS 5250 software with the following sensitivity factors: C, 1 ; 0, 2.85; Sn, 14.63 and In, 13.23. Linear background subtraction and Gaussian peak deconvolutions are performed for all peaks using VGS 5250 software. All peak positions are calibrated by taking C 1s peak (286.4 eV) as a reference because many authors have suggested that the C 1s peak produced by adsorbed, adventitious carbon could be assigned a fixed value and all other binding energies could be referenced to it24.
In:% (9O:lO) (12.5 mm x 500
3. Results and discussions
Figure 1. The magnetron
planar
sputtering
system
used in the exper-
iments.
Table 1. Deposition
conditions
Base pressure Oxygen partial pressure Total pressure Forward power Reflected power Target-substrate distance Sputtering time
3 x 10m5 mbar 9 x 10 -4 mbar 4 x IO-’ mbar 1200 w 12.3 W 40 mm 25 min
3.1. Structural properties. Figure 2 shows the X-ray diffraction patterns of the IT0 films as deposited and annealed at different temperatures. The diffraction patterns contain only In,O, peaks”. All the films show the (400) plane texturing. In general, IT0 films prepared by evaporation show the (222) plane texturing4,26 *’ and prepared by sputtering show the (400) plane texturing’4.23. However, some different findings have also been reported. Davis got amorphous IT0 films by dc reactive sputtering” and Chiou et al got random orientation IT0 films by rf sputtering”. These different results indicate that the IT0 film structure orientation does not only depend on the deposition method but also on the deposition parameters. From Figure 2 it can be seen that the (002) peak intensity
T -I 500
spectrophotometer UV-3101PC. The film diffuse transmittance and reflectance are measured by using the integrating sphere attachment which has been installed in the sample compartment of the UV-3 1OlPC. The film X-ray diffractions (XRDs) have been made using a Philips geiger counter PW1710 computercontrolled diffractometer. Cu Kcc radiation from an X-ray tube with normal focus was used. The diffraction conditions are listed in Table 2 The surface topology and cross-sectional structure of the films are observed by means of scanning electron microscopy (SEM). In order to prevent charge build-up, a thin carbon film is coated on the sample surface before SEM is carried out. X-ray photoelectron spectroscopy (XPS) analysis is performed with a VG Escalab 200A electron analyzer operating under computer control using VGS 5250 Datasystem software. The samples are placed in the vacuum system arid pump down to the lo-” mbar range before the measurement. Photoelectrons are excited over a nominal 10 x 10 mm2 area with Al Kcc radiation (1486.6 674
7 9 5 Q) z
I4001
Oc
--I 400 “c
12221
I4401 A
_
-
,I.
A
-
-
1.
A
-
-
A
,L,_
I
-L : 300 “c
20
&
30
40
50
60
70
Diftraction Angle ( 2 theta) Figure 2. X-ray diffraction patterns annealed at different temperatures.
of IT0
films as deposited
and
L-J Meng et al: Annealed
indium tin oxide films 1
0.15 T
ratio of 2 : 1. By fitting the measured curve using the method mentioned as above, the width B, was found to be 0.057’. The corrected peak width can be calculated by the formula B* = Bf B i **, and the crystalline dimension D, along a line normal to the (400) reflecting plane, was then estimated according to
i
0.1
S
a
8
0.5 7 i; N c!
8 c? T I - 0.05 t
-
i(222) I l(440) 0
0 0
100
200
300
400
500
Annealing Temperature (% )
Figure 3. The variations of peak intensity ratios of I (222)/I (400) and I (222)/I (440) as a function of the annealing temperature.
increases as the annealing temperature is increased although all the films have (400) preferred orientation. Figure 3 shows the variations of the 1(222)/1(400) and 1(222)/1(440) with annealing temperature. It is clear that the intensity ratio increases as the annealing temperature is increased. The (400) diffraction peak width was used to estimate the crystalline dimension along the a axis. The Cu Ka radiation contains two lines of wavelength 1.54060 and 1.54439 8, with an intensity ratio of 2 : l**. Indium oxide with a spacing of 2.529 A along the (400) direction will give two (400) diffraction peaks separated by A(20) = 0.090” with an intensity ratio of 2 : 1. These two diffraction peaks were broadened and only one single peak could be observed in the spectra because the crystallites were small. In order to find the true peak width B, corresponding to a monochromatic X-ray, the (400) peaks were fitted by two Gaussian curves with a height ratio of 2 : 1, width ratio of 1 : 1 and separated by 0.090”. The instrumental broadening was obtained by measuring the diffraction peak width of monocrystal silicon (111) plane, which has 20 around 28.5” and is close to the indium oxide (400) diffraction angle 28 (about 35.5”). There were two well-separated silicon (111) diffraction peaks with an intensity
l&J
where I is the X-ray (K,,) wavelength and 0 is the Bragg diffraction angle*‘. The results are shown in Figure 4 (a). It can be seen that the grain size along (400) direction increases as the annealing temperature is increased up to 300°C and no clear variation after the annealing temperature is over 300°C. The (400) crystal plane distance was calculated from the diffraction angle which has been obtained by fitting the diffraction peak. The results are shown in Figure 4 (b). The line in the figure is the standard value d,, (2.529 A). It shows that the d values of all the films are larger than L&.However, as the annealing temperature is increased, the difference between the standard value and the calculated value becomes small. That means that there is compressive stress in all the films and the stress value decreases as the annealing temperature is increased. Figure 5 shows the scanning electron microscopy (SEM) photographs of IT0 films as deposited and annealed at different temperatures. It is clear that all films have columnar structures. The average crystallite size along the sample surface is about 100 nm and no clear variation for the films annealed at different temperatures. This crystallite size is smaller than that along the direction normal to the sample surface estimated by XRD method. From Figure 5 it can also be seen that the voids between the crystallite grain become small as the annealed temperature is increased. That means the annealing improves the film density. In addition, as the annealing temperature is increased, the film surface becomes rough which can improve the efficiency of amorphous silicon solar cells by increasing the amount of light trapping. 3.2. Optical and electrical properties. Figure 6 shows the transmittance of the films as deposited and annealed at different temperatures from UV to near IR region. In order to see the variation clearly in the visible region, the transmittance spectra of the films at the wavelength between 300 nm and 900 nm are shown in Figure 7. As shown in Figure 6, the transmittance decreases as 2.54 -
T I
. (b) .*
(a)
. l
l
l
s
0
.
300
400
2 2.53 -0
0
140
100 Annealing
Figure 4. (a) Grain
’
,I,,/,,,,:,,“:““:“‘.( 0
size and (b) d-values
200
300
Temperature
400
500
( OC)
of IT0 films vs annealing
2,522 0
loo Annealng
temperature.
200
Temperature
500
(‘Cl
The line in (b) gives the d-value of In,O, powder. 675
L-J Meng et al: Annealed
indium tin oxide films
Figure 5. SEM micrographs of IT0 films as deposited deposited, annealed at 300°C and 5OO”C, respectively.
676
and annealed
at different
temperatures.
The RT, 300 and 500 in the pictures
mean that as
L-J Meng et al: Annealed indium tin oxide films Glass Substrate
T
‘m
t
!
0
:_2
, , ,I 600
I
lcoo
1400
,)/ 1600
I
: 2200
, , 2600
Wavelength (nm)
Figure 6. Specular transmittance spectra of IT0 films as deposited and annealed at different temperatures.
the annealing temperature is increased at wavelengths above 1000 nm. This is because of the free-carrier absorption which increases as the carrier concentration increases. The majority carriers in IT0 are electrons. That means the electron concentration in the film increases as the annealing temperature is increased. It is well known for the IT0 films that the electrons are liberated from the substitutional entered Sn atoms in the cation sublattice and from the doubly charged oxygen vacancies. Tin can exist as either SnO (valence 2) or SnO, (valence 4). Since indium has valence 3 in In,O, the presence of SnO, would result in n doping of the lattice because the dopant would add electrons to the conduction band. In contrast, the presence of SnO would lower the electron density in the conduction band. Some authors believe that, at low substrate temperature, tin is present in the IT0 films as SnO resulting in low carrier densities and annealing the films will transfer SnO into Sn02 and results in the formation
loo1
0.
300
500
700
900
Wavelength (nm) Figure 7. Specular transmittance in the visible region of IT0 films as deposited
and annealed
at different
temperatures.
of an n-type semiconductor with high carrier density and low resistance29.30. In order to know if there is also this kind of transition in our films, we have performed the X-ray photoelectron spectroscopy (XPS) measurement for all the films as shown in Figure 8. XPS makes it possible to measure the binding energy with a high energy resolution which makes it possible to detect the different chemical states of an element. From literature it can be found that the binding energies of tin 3d5/2 for SnO and SnO, are 486.8 and 486.6 eV respectively”. In order to get accurate binding energy of tin in our films, we have performed linear background subtraction and Gaussian peak deconvolutions for Sn 3d5/2 photoelectron peaks using VGS 5250 software. The results show that the Sn 3d5/2 peaks for all samples can be deconvoluted by one peak which is located at 486.6 eV and no clear variation of the peak shape is observed. This suggests tin is present in our films as Sn02. Therefore, the increase of the carrier concentration, as the annealing temperature is increased, is not from the transition of SnO to SnO,. We believe that the increase of the carrier concentration is due to the decrease of the donor sites trapped at the dislocations or point defect aggregate?. As the annealing temperature is increased, the film crystallinity is improved. The improvement of the crystallinity results in a decrease in concentration of donors trapped at crystalline defects and hence an increase of carrier concentration. From Figure 3 it can be seen that the (222) diffraction peak intensity increases as the annealing temperature is increased. This variation may also contribute to the increase of the carrier concentration. However, it is also known that annealing IT0 film at temperature of more than 300°C in air will cause the thermal oxidation of the oxygen vacancies and thus the extinction of associated carriers3’. This effect results in the similar free-carrier absorptions for the films annealed at 300°C and 400°C. The atomic concentration of the various elements in the films have been computed from measured peak areas with the relative sensitivity factors given in the introduction. The results show that the ratios of O/In and Sri/In for different films are about 1.2 and 0.09, respectively. It is clear that the ratio of Sri/In in the films is slightly lower than that in the target. However. it should be noted that the ratios in the sample surface may have some differences with that in deeper layers because of the strong carbon contamination on the surface. From Figure 7 it can be seen that the fundamental absorption edge of IT0 film shifts to shorter wavelengths as the annealing temperature is increased. This shift resulted from an increase of the carrier concentration and is known as the Burstein-Moss shift, where in the heavily doped n-type semiconductor, the Fermi level is inside the conduction band and the states near the bottom of the conduction band are filled. Therefore, the absorption edge shifts to higher energy. Absorption coefficients of the films at different wavelengths have been calculated from transmission and reflection data. The absorption coefficient for the direct allowed transition can be written as t( = (hv - Eg)li2, where hv is the photon energy and E,! is the transition energy gap34. Figure 9 shows the photon energy dependence of c? for IT0 films as deposited and annealed at 3OO’C temperature. Extrapolations of the straight regions of the plots to tl = 0 give EC The values of E, are given in Table 3. As it can be seen, the E, increases as the annealing temperature is increased up to 300°C. After that the Eg has no clear variation even if the annealing temperature is increased further. This variation can be attributed to the variation of the carrier concentration as shown in Figure 6. As the carrier concentration is 677
L-J Meng etal:Annealed
indium
tin oxide
films
RT
. 482
5
spectra of
IT0
515
528.5
Binding Energy ( eV )
Binding Energy ( eV ) Figure 8. X-ray photoelectron
4 99
films as deposited
542
Binding Energy ( eV )
and annealed at different temperatures. 5
.*-\
-..--.-
Noannealing
__-_
*@yc
-_-__3000c
-
$3
.‘\
3.8 hu
4
4.2
-I 400
‘.
”
I
”
”
500
I
increased, the optical band gap shifts to the high energy direction. However, as shown in Figures 6 and 7, although the carrier concentration increases further as annealing temperature is increased from 400°C to 5OO”C, the optical band gap E, does not increase. These results can be explained as the effects of electronelectron and electron-ion scattering. It is well known that the variation of the optical band gap Eg can be explained as the net result of two competing mechanisms: a widening due to the Burstein-Moss effect as has been stated as above, and a narrowing due to electron-electron and electron-ion scattering. Figures 10 and 11 show the diffuse transmittance and reflectance of the films as deposited and annealed at different temperatures. As shown in these figures, both the diffuse transmittance and reflectance increase as the annealing temperature is increased. From Figure 5 it can be seen that the film
---._._.___.-._.___~ ---._
”
600 Wavelength
(eV)
Figure 9. Square of the absorption coefficient (II) plotted as a function of photon energy (hv) for two IT0 films.
-..
,--. . ..*’
‘._.___.--.._
0
3.6
Glass
‘T
2
%.
3.4
/
700
678
RT 3.72 1.3x lo-’ 2.1 X 10’
200°C 3.78 5.1 X 10 z 8.4 x 10’
““I
800
(nm)
Figure 10. Diffuse transmittance
spectra of IT0 films as deposited and annealed at different temperatures.
surface becomes more rough as the annealing temperature is increased. The rough surface results in the increases of the diffuse transmittance and reflectance. From Figure 7 it can be seen that the film transmittance in the visible region decreases as the annealing temperature is increased. Considering the light scattering by the rough surface (diffuse transmittance and reflectance) it can be concluded that the decrease of the transmittance results from the light scattering. Comparing Figure 10 with Figure 11 it can be found that, for the same film, the diffuse transmittance is higher than diffuse reflectance. Therefore, the light scattering processes in the films are mainly Rayleigh type scatteringx5. Figure 12 shows the variation of the film electrical resistivity with the annealing temperature. The film sheet resistance and electrical resistivity have been given in Table 3. As shown in
Table 3. Optical and electrical properties of IT0 films
Annealing temperature Optical band gap (eV) Resistivity (Q* cm) Sheet resistance (Q)
4.&C
so&
-
i 0
3.2
.
-. ‘a._
: t -
. . -
300°C 3.82 1.9x10 2 3.1 X 102
400°C 3.82 9.8 x 10m3 1.7 x lo2
500°C 3.82 6.9x IO-’ 1.2x IO2
L-J Meng
Annealed
eta/:
indium
tin oxide
films
-------
Noannealing
1
400
----
200%
-.-_-
3oo*c
600
500
800
700
(nm)
Wavelength
Figure 11. Diffuse reflectance spectra of IT0 films as deposited and annealed at different temperatures.
about 30 min. The annealing temperature was varied from 200°C to 500°C with an interval of 100°C. All the films show a preferred orientation along (400) crystal plane. But the (222) diffraction peak intensity increases as the annealing temperature is increased. The grain size along (400) direction increases as the annealing temperature is increased. All the films have the compressive stress and the stress value decreases as the annealing temperature is increased. The film surface becomes more rough as the annealing temperature is increased. Tin is present in all films as Sn02, no transition from SnO to SnO, has been observed in our films. As the annealing temperature is increased, the electron, which is liberated from the donor sites trapped at the dislocations or point defect, aggregates due to annealing, concentration increases and results in the decrease of the near IR transmittance and increase of the optical band gap. Both the film diffuse transmittance and reflectance increase because of the increase of the surface roughness as the annealing temperature is increased. As the annealing temperature is increased, the film electrical resistivity decreases. The resistivity of as deposited film is about 1.3 x IO-’ R*cm and decreases down to 6.9 x 10m3 0*cm as the annealing temperature is increased up to 5OO’C. Acknowledgements
1000 7 t
We would like to thank A Azevedo of the Geology Department of the University of Minho for X-ray diffraction measurements, Carlos Sk from CEMUP (University of Porto) for XPS measurements and August0 Luis of the Ceramic Department of the University of Aveiro for SEM measurements. One of the authors (LJM) is thankful to the CEMOP/UNINOVA for providing the scholarship. References
. .
1
I 0
I
100
200
Annealing Figure 12. Variation
300
Temperature
400
500
(% )
in resistivity of IT0 films with annealing tempera-
ture.
Figure 12, the film resistivity decreases gradually as the annealing temperature is increased. Both the increase of the carrier density and the carrier mobility will improve the film electrical resistivity. From Figure 6 it has been seen that the carrier concentration increases as the annealing temperature is increased and will improve the film electrical resistivity. In addition, the results of the XRD and SEM have shown that the film structural property has been improved by increasing annealing temperature. It will improve the carrier mobility and result in the decrease of the film resistivity. 4. Conclusions IT0 films were deposited on the glass substrates by rf reactive magnetron sputtering technique using metallic In-Sn (90-10) target. After the deposition, the films were annealed in air for
’ K L Chopra, S Major and D K Pandya, Thin Solid Films, 102,l (1983). ‘Z Ovadyahu and H Wiesmann, J Appl Phys, 52,5865 (1981). ‘M J Tsai, A L Fahrenbruch and R H Bube. J Appl Phys, 51, 2696 (1980). 4P Manivannan and A Subrahmanyam, J Phys, D: Appl Phys, 26, 15 IO (1993). ‘1 Hamberg and C G Granqvist, J Appl Phys, 60, RI23 (1986). ‘P Nath, R F Bunshah, B M Basal and 0 M Staffsud, Thin Solid Films, 72,463 (1980). ‘T Maruyama and K Fukui, J Appl Phys, 70,3848 (1991). ’ H Haitjema and J J Ph Elich, Thin Solid Films, 205,93 (1991). ‘)J P Zheng and H S Kwok, Appl Phys Left, 63, I (1993). ‘“T Karasawa and Y Miyata, Thin Solid Films, 223, 135 (1993). ” L Davis, Thin Solid Films, 236, I (1993). I2S Ishibashi, Y Higuchi, Y Ota and K Nakumura. J Vuc, Sci Technol A, 8, 1399 (1990). “M Higuchi, S Uekusa, R Nakano and K Yokogawa, J Appl Phys, 74, 6710 (1993). I4Y Shigesato, S Takaki and T Haranoh, J Appl Phys, 71,3356 (19920. “B S Chiou and S T Hsieh, Thin Solid Films, 229, 146 (1993). 16S Ray R Banerjee, N Basu, A K Batabyal and A K Barua. J Appl Phys, 54: 3497 (1983). “W G Haines and R H Bube, J Appl Phys, 349,304 (1978). Ix S B Lee, J C Pincenti, A Cocco and D L Naylor, J Vat Sci Technol A, 11,2742 (1993). “‘S Kulkarni and M Bayard, J Vat Sci Technc)l A. 9, 1193 (1991). ‘“A Kawada, Thin Solid Films, 191,297 (1990). ” L Gupta, A Mansingh and P K Srivastava. Thin Solid Films, 176, 33 (1989). ‘*S Chaudhuri, J Bhattacharyya and A K Pal, Thin Solid Films. 148,279 (1987). 23Y Shigesato
and D C Paine, Thin Solid Films, 238,44 ( 1994). 24D Briggs and M P Seah (Eds), Practical Su@ce Analysis by Auger and XPS, John Wiley & Sons (1987). 679
L-J Meng et a/: Annealed
indium tin oxide films
“Powder Diffraction File, Joint Committee on Powder Diffraction Standards, 1967 (ASTM, Philadelphia, PA, 1967) Card 6-0416. Z6Y Shigesato and D C Paine, J Appl Phys, 73, 3805 (1993). *‘T Oyama, N Hashimoto, J Shimadzu, Y Akao, H Kojima, H Aikama and K Suzuki, J Vat Sci Technol A, 10, 1682 (1992). 28B D Cullity, Elements of X-ray Diffraction 2nd ed. Addison-Wesley, Reading, MA (1978). 29G Frank and H Kostlin, Appl Phys, 27A, 197 (1982). “C H L Weijtens, J Electrochem Sot, 138, 3432 (1991).
660
” C D Wagner, W M Riggs, L E Davis, J F Moulder and G E Muilenberg (Eds), Handbook of X-ra_v Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, (1979). ” Y Shigesato, S Takaki and T Haranoh, Appl Surf Sci, 48/49,269 (199 I). “M Mizuhashi, Thin Solid Films, 70,91 (1980). 14D Curie, Luminescence of Inorganic Solids, Ed. B Di Bartolo, Plenum Press, New York, (1978). 35I Fanderlik, Optical Properties of Glass, Elsevier, Amsterdam (1983).
Pergamon
0042-207x/95
$9.50+.00
0042-207x(94)00159-2
Study of annealed indium tin oxide films prepared by rf reactive magnetron sputtering Li-jian Meng*, Portugal received
A Maqarico and R Martins,
CEMOP/UNINOVA,
Quinta de Torre, 2825 Monte
da Caparica,
14 October 1994
Tin doped indium oxide (ITO) films were deposited on glass substrates by rf reactive magnetron sputtering using a metallic alloy target (In-Sn, 90-10). The post-deposition annealing has been done for IT0 films in air and the effect of annealing temperature on the electrical, optical and structural properties of IT0 films was studied. It has been found that the increase of the annealing temperature will improve the film electrical properties. The resistivity of as deposited film is about 1.3 x lo- ’ R *cm and decreases down to 6.9 x 10e3 R *cm as the annealing temperature is increased up to 500°C. In addition, the annealing will also increase the film surface roughness which can improve the efficiency of amorphous silicon solar cells by increasing the amount of light trapping.
1. Introduction Indium tin oxide (ITO) film has been widely used as a transparent conductor due to its high transparency to visible light and its low electrical resistivity. Applications which use IT0 as a transparent conductor include liquid crystal display, solar cells and various light sensitive solid state devices’m3. High quality IT0 films have been prepared by various deposition methods such as electron beam evaporation”‘, thermal evaporation’, CVD’, spray pyrolysis’, laser ablation’, dc sputthese deposition tering’@14 and rf sputtering’5~m’9. Among methods, reactive sputtering deposition is widely used for IT0 film deposition. Desirable features of reactive sputtering employing alloy targets for practical use are : a high deposition rate ; accurate control of the film thickness and easy fabrication of the alloy target. In general, a post-deposition annealing in air or in an oxygen free environment for reactively sputtered IT0 coatings from metal alloy target is needed in order to improve the film electrical properties’“.“.2g”. In this paper, IT0 films have been deposited on glass substrates by rf reactive magnetron sputtering method and the effect of the post-deposition annealing temperature (2OO”C-500°C) on the film structural, optical and electrical properties has been studied.
2. Experimental details IT0 films were deposited on glass substrates at ambient temperature by rf reactive magnetron sputtering technique. The schematic of deposition system used in this work is shown in Figure
* On leave from Changchun Institute of Physics, Chinese Academy of Sciences, Changchun 130021, P.R. China.
1. The system is equipped with both rf and dc sputtering sources. in this work, only the rf source is used. The chamber is pumped with a turbomolecular pump (TPU510) backed by a rotary pump (EDWARDS E2M40). The pressure of the chamber is monitored by a Pirani-Penning gauge combination and measured by Balzers TPG 300 total pressure gauge controller. The target is In-Sn (90 : 10) alloy of 99.99% purity (125 x 500 mm* ALCATEL CIT). The distance between target and substrate is about 40 mm. The cathode is cooled by running water. A RFX 13.56 MHz, 2500 W generator and an ATX tuner (Advanced Energy) are used as rf power supply. The vacuum chamber is evacuated down to pressure 3 x lo-’ mbar prior to deposition. Then the oxygen reactive gas is introduced into the chamber and the required pressure (9 x 10e4 mbar) is set. Argon gas is then introduced thereafter till the preset pressure is reached (4 x 10m3 mbar). After that, the oxygen reactive gas is shut off and the alloy target is presputtered (600 W rf power) in an argon atmosphere for about 10 min with a shuttle covering the substrate in order to remove the surface oxide layer formed during exposure to air. Then the oxygen reactive gas is introduced again and the shutter is removed and film deposition process starts. Both argon inert gas flow and oxygen reactive gas flow are controlled by a mass flow controller (Datametrics, controller 1605). The rf forward and reflected powers during the film deposition are 1200 W and 12.3 W respectively. The deposition time is about 25 min. The deposition conditions are listed in Table 1. After the deposition, the films are annealed in a tube furnace for about 30 min in air. The annealing temperatures are 200°C 300°C 400°C and 500°C respectively. The film thickness is measured by a SLOAN DEKTAK IID (the film thickness is about 610 nm). The film resistivity is measured by Veeco FPP-5000 four-point probe. The film by Shimadzu double-beam transmittance is measured 673
L-J Meng et al: Annealed
indium tin oxide
films
Table 2. X-ray diffraction
Gas mixture cylinder
conditions
X-ray tube Wavelength (a 1, ~2) lrradiated length Receiving slit Range (20) Step width, time
Cu KU, normal focus, 40 kV, 20 mA 1.54060A, 1.544388, 12 mm 0.2mm 10.00~ ~70.000” 0.020”, 1.25 s
eV) operating at 5 kV and 20 mA. Photoelectrons are detected with a hemispherical analyzer with an input lens positioned normal to the surface. Spectrometer pass energy is 20 eV. The atomic concentration of the various elements are computed from measured peak areas (C Is, 0 Is, Sn 3d5/2 and In 3d5/2) using VGS 5250 software with the following sensitivity factors: C, 1 ; 0, 2.85; Sn, 14.63 and In, 13.23. Linear background subtraction and Gaussian peak deconvolutions are performed for all peaks using VGS 5250 software. All peak positions are calibrated by taking C 1s peak (286.4 eV) as a reference because many authors have suggested that the C 1s peak produced by adsorbed, adventitious carbon could be assigned a fixed value and all other binding energies could be referenced to it24.
In:% (9O:lO) (12.5 mm x 500
3. Results and discussions
Figure 1. The magnetron
planar
sputtering
system
used in the exper-
iments.
Table 1. Deposition
conditions
Base pressure Oxygen partial pressure Total pressure Forward power Reflected power Target-substrate distance Sputtering time
3 x 10m5 mbar 9 x 10 -4 mbar 4 x IO-’ mbar 1200 w 12.3 W 40 mm 25 min
3.1. Structural properties. Figure 2 shows the X-ray diffraction patterns of the IT0 films as deposited and annealed at different temperatures. The diffraction patterns contain only In,O, peaks”. All the films show the (400) plane texturing. In general, IT0 films prepared by evaporation show the (222) plane texturing4,26 *’ and prepared by sputtering show the (400) plane texturing’4.23. However, some different findings have also been reported. Davis got amorphous IT0 films by dc reactive sputtering” and Chiou et al got random orientation IT0 films by rf sputtering”. These different results indicate that the IT0 film structure orientation does not only depend on the deposition method but also on the deposition parameters. From Figure 2 it can be seen that the (002) peak intensity
T -I 500
spectrophotometer UV-3101PC. The film diffuse transmittance and reflectance are measured by using the integrating sphere attachment which has been installed in the sample compartment of the UV-3 1OlPC. The film X-ray diffractions (XRDs) have been made using a Philips geiger counter PW1710 computercontrolled diffractometer. Cu Kcc radiation from an X-ray tube with normal focus was used. The diffraction conditions are listed in Table 2 The surface topology and cross-sectional structure of the films are observed by means of scanning electron microscopy (SEM). In order to prevent charge build-up, a thin carbon film is coated on the sample surface before SEM is carried out. X-ray photoelectron spectroscopy (XPS) analysis is performed with a VG Escalab 200A electron analyzer operating under computer control using VGS 5250 Datasystem software. The samples are placed in the vacuum system arid pump down to the lo-” mbar range before the measurement. Photoelectrons are excited over a nominal 10 x 10 mm2 area with Al Kcc radiation (1486.6 674
7 9 5 Q) z
I4001
Oc
--I 400 “c
12221
I4401 A
_
-
,I.
A
-
-
1.
A
-
-
A
,L,_
I
-L : 300 “c
20
&
30
40
50
60
70
Diftraction Angle ( 2 theta) Figure 2. X-ray diffraction patterns annealed at different temperatures.
of IT0
films as deposited
and
L-J Meng et al: Annealed
indium tin oxide films 1
0.15 T
ratio of 2 : 1. By fitting the measured curve using the method mentioned as above, the width B, was found to be 0.057’. The corrected peak width can be calculated by the formula B* = Bf B i **, and the crystalline dimension D, along a line normal to the (400) reflecting plane, was then estimated according to
i
0.1
S
a
8
0.5 7 i; N c!
8 c? T I - 0.05 t
-
i(222) I l(440) 0
0 0
100
200
300
400
500
Annealing Temperature (% )
Figure 3. The variations of peak intensity ratios of I (222)/I (400) and I (222)/I (440) as a function of the annealing temperature.
increases as the annealing temperature is increased although all the films have (400) preferred orientation. Figure 3 shows the variations of the 1(222)/1(400) and 1(222)/1(440) with annealing temperature. It is clear that the intensity ratio increases as the annealing temperature is increased. The (400) diffraction peak width was used to estimate the crystalline dimension along the a axis. The Cu Ka radiation contains two lines of wavelength 1.54060 and 1.54439 8, with an intensity ratio of 2 : l**. Indium oxide with a spacing of 2.529 A along the (400) direction will give two (400) diffraction peaks separated by A(20) = 0.090” with an intensity ratio of 2 : 1. These two diffraction peaks were broadened and only one single peak could be observed in the spectra because the crystallites were small. In order to find the true peak width B, corresponding to a monochromatic X-ray, the (400) peaks were fitted by two Gaussian curves with a height ratio of 2 : 1, width ratio of 1 : 1 and separated by 0.090”. The instrumental broadening was obtained by measuring the diffraction peak width of monocrystal silicon (111) plane, which has 20 around 28.5” and is close to the indium oxide (400) diffraction angle 28 (about 35.5”). There were two well-separated silicon (111) diffraction peaks with an intensity
l&J
where I is the X-ray (K,,) wavelength and 0 is the Bragg diffraction angle*‘. The results are shown in Figure 4 (a). It can be seen that the grain size along (400) direction increases as the annealing temperature is increased up to 300°C and no clear variation after the annealing temperature is over 300°C. The (400) crystal plane distance was calculated from the diffraction angle which has been obtained by fitting the diffraction peak. The results are shown in Figure 4 (b). The line in the figure is the standard value d,, (2.529 A). It shows that the d values of all the films are larger than L&.However, as the annealing temperature is increased, the difference between the standard value and the calculated value becomes small. That means that there is compressive stress in all the films and the stress value decreases as the annealing temperature is increased. Figure 5 shows the scanning electron microscopy (SEM) photographs of IT0 films as deposited and annealed at different temperatures. It is clear that all films have columnar structures. The average crystallite size along the sample surface is about 100 nm and no clear variation for the films annealed at different temperatures. This crystallite size is smaller than that along the direction normal to the sample surface estimated by XRD method. From Figure 5 it can also be seen that the voids between the crystallite grain become small as the annealed temperature is increased. That means the annealing improves the film density. In addition, as the annealing temperature is increased, the film surface becomes rough which can improve the efficiency of amorphous silicon solar cells by increasing the amount of light trapping. 3.2. Optical and electrical properties. Figure 6 shows the transmittance of the films as deposited and annealed at different temperatures from UV to near IR region. In order to see the variation clearly in the visible region, the transmittance spectra of the films at the wavelength between 300 nm and 900 nm are shown in Figure 7. As shown in Figure 6, the transmittance decreases as 2.54 -
T I
. (b) .*
(a)
. l
l
l
s
0
.
300
400
2 2.53 -0
0
140
100 Annealing
Figure 4. (a) Grain
’
,I,,/,,,,:,,“:““:“‘.( 0
size and (b) d-values
200
300
Temperature
400
500
( OC)
of IT0 films vs annealing
2,522 0
loo Annealng
temperature.
200
Temperature
500
(‘Cl
The line in (b) gives the d-value of In,O, powder. 675
L-J Meng et al: Annealed
indium tin oxide films
Figure 5. SEM micrographs of IT0 films as deposited deposited, annealed at 300°C and 5OO”C, respectively.
676
and annealed
at different
temperatures.
The RT, 300 and 500 in the pictures
mean that as
L-J Meng et al: Annealed indium tin oxide films Glass Substrate
T
‘m
t
!
0
:_2
, , ,I 600
I
lcoo
1400
,)/ 1600
I
: 2200
, , 2600
Wavelength (nm)
Figure 6. Specular transmittance spectra of IT0 films as deposited and annealed at different temperatures.
the annealing temperature is increased at wavelengths above 1000 nm. This is because of the free-carrier absorption which increases as the carrier concentration increases. The majority carriers in IT0 are electrons. That means the electron concentration in the film increases as the annealing temperature is increased. It is well known for the IT0 films that the electrons are liberated from the substitutional entered Sn atoms in the cation sublattice and from the doubly charged oxygen vacancies. Tin can exist as either SnO (valence 2) or SnO, (valence 4). Since indium has valence 3 in In,O, the presence of SnO, would result in n doping of the lattice because the dopant would add electrons to the conduction band. In contrast, the presence of SnO would lower the electron density in the conduction band. Some authors believe that, at low substrate temperature, tin is present in the IT0 films as SnO resulting in low carrier densities and annealing the films will transfer SnO into Sn02 and results in the formation
loo1
0.
300
500
700
900
Wavelength (nm) Figure 7. Specular transmittance in the visible region of IT0 films as deposited
and annealed
at different
temperatures.
of an n-type semiconductor with high carrier density and low resistance29.30. In order to know if there is also this kind of transition in our films, we have performed the X-ray photoelectron spectroscopy (XPS) measurement for all the films as shown in Figure 8. XPS makes it possible to measure the binding energy with a high energy resolution which makes it possible to detect the different chemical states of an element. From literature it can be found that the binding energies of tin 3d5/2 for SnO and SnO, are 486.8 and 486.6 eV respectively”. In order to get accurate binding energy of tin in our films, we have performed linear background subtraction and Gaussian peak deconvolutions for Sn 3d5/2 photoelectron peaks using VGS 5250 software. The results show that the Sn 3d5/2 peaks for all samples can be deconvoluted by one peak which is located at 486.6 eV and no clear variation of the peak shape is observed. This suggests tin is present in our films as Sn02. Therefore, the increase of the carrier concentration, as the annealing temperature is increased, is not from the transition of SnO to SnO,. We believe that the increase of the carrier concentration is due to the decrease of the donor sites trapped at the dislocations or point defect aggregate?. As the annealing temperature is increased, the film crystallinity is improved. The improvement of the crystallinity results in a decrease in concentration of donors trapped at crystalline defects and hence an increase of carrier concentration. From Figure 3 it can be seen that the (222) diffraction peak intensity increases as the annealing temperature is increased. This variation may also contribute to the increase of the carrier concentration. However, it is also known that annealing IT0 film at temperature of more than 300°C in air will cause the thermal oxidation of the oxygen vacancies and thus the extinction of associated carriers3’. This effect results in the similar free-carrier absorptions for the films annealed at 300°C and 400°C. The atomic concentration of the various elements in the films have been computed from measured peak areas with the relative sensitivity factors given in the introduction. The results show that the ratios of O/In and Sri/In for different films are about 1.2 and 0.09, respectively. It is clear that the ratio of Sri/In in the films is slightly lower than that in the target. However. it should be noted that the ratios in the sample surface may have some differences with that in deeper layers because of the strong carbon contamination on the surface. From Figure 7 it can be seen that the fundamental absorption edge of IT0 film shifts to shorter wavelengths as the annealing temperature is increased. This shift resulted from an increase of the carrier concentration and is known as the Burstein-Moss shift, where in the heavily doped n-type semiconductor, the Fermi level is inside the conduction band and the states near the bottom of the conduction band are filled. Therefore, the absorption edge shifts to higher energy. Absorption coefficients of the films at different wavelengths have been calculated from transmission and reflection data. The absorption coefficient for the direct allowed transition can be written as t( = (hv - Eg)li2, where hv is the photon energy and E,! is the transition energy gap34. Figure 9 shows the photon energy dependence of c? for IT0 films as deposited and annealed at 3OO’C temperature. Extrapolations of the straight regions of the plots to tl = 0 give EC The values of E, are given in Table 3. As it can be seen, the E, increases as the annealing temperature is increased up to 300°C. After that the Eg has no clear variation even if the annealing temperature is increased further. This variation can be attributed to the variation of the carrier concentration as shown in Figure 6. As the carrier concentration is 677
L-J Meng etal:Annealed
indium
tin oxide
films
RT
. 482
5
spectra of
IT0
515
528.5
Binding Energy ( eV )
Binding Energy ( eV ) Figure 8. X-ray photoelectron
4 99
films as deposited
542
Binding Energy ( eV )
and annealed at different temperatures. 5
.*-\
-..--.-
Noannealing
__-_
*@yc
-_-__3000c
-
$3
.‘\
3.8 hu
4
4.2
-I 400
‘.
”
I
”
”
500
I
increased, the optical band gap shifts to the high energy direction. However, as shown in Figures 6 and 7, although the carrier concentration increases further as annealing temperature is increased from 400°C to 5OO”C, the optical band gap E, does not increase. These results can be explained as the effects of electronelectron and electron-ion scattering. It is well known that the variation of the optical band gap Eg can be explained as the net result of two competing mechanisms: a widening due to the Burstein-Moss effect as has been stated as above, and a narrowing due to electron-electron and electron-ion scattering. Figures 10 and 11 show the diffuse transmittance and reflectance of the films as deposited and annealed at different temperatures. As shown in these figures, both the diffuse transmittance and reflectance increase as the annealing temperature is increased. From Figure 5 it can be seen that the film
---._._.___.-._.___~ ---._
”
600 Wavelength
(eV)
Figure 9. Square of the absorption coefficient (II) plotted as a function of photon energy (hv) for two IT0 films.
-..
,--. . ..*’
‘._.___.--.._
0
3.6
Glass
‘T
2
%.
3.4
/
700
678
RT 3.72 1.3x lo-’ 2.1 X 10’
200°C 3.78 5.1 X 10 z 8.4 x 10’
““I
800
(nm)
Figure 10. Diffuse transmittance
spectra of IT0 films as deposited and annealed at different temperatures.
surface becomes more rough as the annealing temperature is increased. The rough surface results in the increases of the diffuse transmittance and reflectance. From Figure 7 it can be seen that the film transmittance in the visible region decreases as the annealing temperature is increased. Considering the light scattering by the rough surface (diffuse transmittance and reflectance) it can be concluded that the decrease of the transmittance results from the light scattering. Comparing Figure 10 with Figure 11 it can be found that, for the same film, the diffuse transmittance is higher than diffuse reflectance. Therefore, the light scattering processes in the films are mainly Rayleigh type scatteringx5. Figure 12 shows the variation of the film electrical resistivity with the annealing temperature. The film sheet resistance and electrical resistivity have been given in Table 3. As shown in
Table 3. Optical and electrical properties of IT0 films
Annealing temperature Optical band gap (eV) Resistivity (Q* cm) Sheet resistance (Q)
4.&C
so&
-
i 0
3.2
.
-. ‘a._
: t -
. . -
300°C 3.82 1.9x10 2 3.1 X 102
400°C 3.82 9.8 x 10m3 1.7 x lo2
500°C 3.82 6.9x IO-’ 1.2x IO2
L-J Meng
Annealed
eta/:
indium
tin oxide
films
-------
Noannealing
1
400
----
200%
-.-_-
3oo*c
600
500
800
700
(nm)
Wavelength
Figure 11. Diffuse reflectance spectra of IT0 films as deposited and annealed at different temperatures.
about 30 min. The annealing temperature was varied from 200°C to 500°C with an interval of 100°C. All the films show a preferred orientation along (400) crystal plane. But the (222) diffraction peak intensity increases as the annealing temperature is increased. The grain size along (400) direction increases as the annealing temperature is increased. All the films have the compressive stress and the stress value decreases as the annealing temperature is increased. The film surface becomes more rough as the annealing temperature is increased. Tin is present in all films as Sn02, no transition from SnO to SnO, has been observed in our films. As the annealing temperature is increased, the electron, which is liberated from the donor sites trapped at the dislocations or point defect, aggregates due to annealing, concentration increases and results in the decrease of the near IR transmittance and increase of the optical band gap. Both the film diffuse transmittance and reflectance increase because of the increase of the surface roughness as the annealing temperature is increased. As the annealing temperature is increased, the film electrical resistivity decreases. The resistivity of as deposited film is about 1.3 x IO-’ R*cm and decreases down to 6.9 x 10m3 0*cm as the annealing temperature is increased up to 5OO’C. Acknowledgements
1000 7 t
We would like to thank A Azevedo of the Geology Department of the University of Minho for X-ray diffraction measurements, Carlos Sk from CEMUP (University of Porto) for XPS measurements and August0 Luis of the Ceramic Department of the University of Aveiro for SEM measurements. One of the authors (LJM) is thankful to the CEMOP/UNINOVA for providing the scholarship. References
. .
1
I 0
I
100
200
Annealing Figure 12. Variation
300
Temperature
400
500
(% )
in resistivity of IT0 films with annealing tempera-
ture.
Figure 12, the film resistivity decreases gradually as the annealing temperature is increased. Both the increase of the carrier density and the carrier mobility will improve the film electrical resistivity. From Figure 6 it has been seen that the carrier concentration increases as the annealing temperature is increased and will improve the film electrical resistivity. In addition, the results of the XRD and SEM have shown that the film structural property has been improved by increasing annealing temperature. It will improve the carrier mobility and result in the decrease of the film resistivity. 4. Conclusions IT0 films were deposited on the glass substrates by rf reactive magnetron sputtering technique using metallic In-Sn (90-10) target. After the deposition, the films were annealed in air for
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L-J Meng et a/: Annealed
indium tin oxide films
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” C D Wagner, W M Riggs, L E Davis, J F Moulder and G E Muilenberg (Eds), Handbook of X-ra_v Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, (1979). ” Y Shigesato, S Takaki and T Haranoh, Appl Surf Sci, 48/49,269 (199 I). “M Mizuhashi, Thin Solid Films, 70,91 (1980). 14D Curie, Luminescence of Inorganic Solids, Ed. B Di Bartolo, Plenum Press, New York, (1978). 35I Fanderlik, Optical Properties of Glass, Elsevier, Amsterdam (1983).