Solar Energy Materials 13 (1986) 85-96 85 North-Holland, Amsterdam

I N D I U M - T I N - O X I D E S ( ITO) T H I C K F I L M S F O R S O L A R C E L L S

H.B. S A I M and D.S. C A M P B E L L

Department of Electrical and Electronic Engineering, University of Technology, Loughborough, Leicestershire, LEII 3TU, UK

J.A. A V A R I T S I O T I S

University of Athens, Department of Physics, Division of Electronics, Panepistimioupolis, Kitiria TYPA, Athens, Greece

Received 28 September 1985

Films of transparent semiconducting oxides (indium-tin-oxide) have been prepared by conven- tional thick-film printing methods. When fired at approximately 650°C in a furnace with a continuous air flow, a continuous thick film of the semiconducting oxides, about 3000 ,~ thick is formed. The sheet resistance of the as-deposited film is about 6 kfl/D which can be reduced to about 300 fl/D upon heat-treatment at 300°C in a vacuum. A slight increase of the sheet resistance to 1 kl]/[3 occurs on ageing at ambient atmosphere for a few days. Investigations were done to characterise the electrical, optical, structural and compositional properties of the films particularly with regard to the effects of vacuum treatment of the films. The feasibility of forming diodes with silicon wafers which can eventually lead to the fabrication of SIS solar cells has been examined and will be reported subsequently.

1. Introduction

Transpa ren t conduc t ing oxides have been p roduced by var ious me thods inc luding sput te r ing [1,2], chemical vapou r depos i t ion [3,4], spraying [5,6] and thermal evapora t ion [7,8]. I t is not surpr is ing therefore, there is a marked scat ter ing of results on the proper t ies of the fi lms so obta ined . The s t ructure of the coat ings are found to be polycrysta l l ine , and depend ing on the me thods and p repa ra t i on condi t ions , the size of the crystal l i tes varies f rom less than a few nanomete r s (poss ib ly amorphous ) to several hundred nanometers . This large var ia t ion in the size of the crystat l i tes will affect some of the electrical p roper t ies such as e lect ron mobi l i ty , which varies be tween a few c m / ( V s) to over 100 c m / ( V s) [9]. (Hal l mobi l i t ies in SnO 2 for example can be as high as 300 cm 2 V-1 s -1 [10].)

Qui te recent ly there has been an interest in the s tudy of t r ansparen t conduc t ing f i lms by screen-pr int ing, using convent iona l thick-f i lm techniques and equ ipment [11,12]. This p a p e r descr ibes some of the character is t ics of i n d i u m - t i n - o x i d e ( ITO) f i lms depos i ted f rom organometa l l i c pas te (resinate) of a homogeneous ly dissolved combina t i on of meta l l ic i nd ium and tin together with organic c ompone n t s and m anufac tu r ed by Electro-Science L a b o r a t o r y (ESL). Subs t ra tes of high grade a lumina (2" × 1") as used in the hybr id microe lec t ronic circuit work, have been used for the

0165-1633 /86 /$03 .50 © Elsevier Science Publ ishers B.V. ( N o r t h - H o l l a n d Physics Publ ishing Divis ion)

86 H.B. Saim et al. / ITO thick films

initial assessment studies. The films have also been deposited on quartz and also Corning 7059 glass to facilitate the optical studies of the films. The production of SIS devices has involved the use of etched, single crystal p-type silicon wafers. Using the method of Auger analysis, the films were found to contain an In : Sn : 02 ratio of 7 : 1 : 12 in terms of atomic abundance.

In general the properties of ITO films prepared by thick-film printing techniques depend on composition, thickness and firing conditions. Initial film composition is determined by the concentration of indium and tin organometallic compounds in the ink formulation. The following sections describe the process conditions required for deposition and the various methods of controlling and optimizing their properties.

2. Experimental details

2.1. Samples deposition

A semi-automatic DEK1200 printing machine has been used for the deposition process. It is essentially a stencil process in which the printing medium (the paste or ink) is forced through the open area of a mesh-reinforced stencil onto the surface of the substrate. A 325 mesh stainless steel screen with a 10-20 rtm thick emulsion backing is employed since a coarser screen will deposit an excess of material which can cause crazing and blistering of the film during firing.

The paste is applied to the upper surface of the screen and a flexible squeegee is traversed across the pattern area. The passage of the squeegee presses the screen into contact with the substrate surface and forces the ink through the open meshes of the screen. Behind the squeegee, the screen, due to tension, peels away from the substrate, leaving a printed pattern of ink on the substrate surface. A detailed description of the printing pro~:dures and processes can be found elsewhere [13,14].

2.2. Drying and firing of samples

After printing, the film is generally dried in an oven at about 125°C for 10-15 min. This drying stage tends to remove volatile organic solvents and make the prints adherent to the substrates. Drying also helps to ease the handling of the films. The films are then fired at 650°C, for times up to 1 h, in order to develop the ultimate characteristics of the films. A typical firing profile for ind ium-t in-oxide is shown in fig. 1 [15]. The process involves several major consecutive processes in the furnace. The first stage involves the removal of the organic vehicle which otherwise will cause excessive carbonization and hence influence the bonding mechanism of the films. Sintering of the metallic particles occur during the second stage when the substrates are kept at a temperature plateau at the maximum desirable temperature. The printed layer adheres to the substrates and the final characteristics of the ink are developed. The final stage requires the cooling of the printed layers to room temperature. It is not very critical although excessive cooling rates could quench defect structures into the film. The thickness of the films were measured by a

H.B. Saim et al. / ITO thick films 8 7

640 '

560

480

U 400

320

24C

160

8O

0 I 1 I I I • I

i0 20 30 40 50 60

Time tn m:nutes

Fig. 1. Firing profile for In203SnO 2 films on soda lime glass substrates [15].

talystep (Taylor Hobson Talystep 1) and also by ellipsometry. A thickness of - 0.3 rtm is normally obtained for the fired films.

3. Results and discussions

3.1. Sheet resistance measurements

Two methods of measuring sheet resistances were employed - the four-point probes and using thick film silver paste as electrical contacts.

Figs. 2 and 3 show the effects of varying the firing parameters. In fig. 2 the sheet resistance was measured for a batch of samples deposited under the same condition but fired under a different firing temperature. It shows that the lowest sheet resistance is obtained at a firing temperature between 600 and 700°C. A comparison of the graphs in fig. 2 shows that there exists an opt imum firing time for a particular temperature and that the resistances are appreciably lower when the samples are fired at 650°C.

The increase in sheet resistance when the firing temperature is raised might be due to grain growth and subsequent creation of voids in the film or because of a

88 H.B. Saim et al. / ITO thick films

12.0

ii.

ii. • = o

10.5

i0.0

~ 9.5

c

~ 9 .0 m

~ 8.5

= o

8 .0

30 35 40 45 50 55 60

Firing time (mlnutes)

Fig. 2. Changes of sheet resistance with firing time ( > 35 rain) at 600°C and 700°C.

decrease in oxygen deficiency in the film. This can also explain the increase in sheet resistance due to longer firing time. Fig. 3 shows the variation of sheet resistances with firing time at 650°C. It shows that the optimum firing time is about 35 min.

3.2. Effects of vacuum annealing

A study of the annealing behaviour of the film was performed by annealing in both high and low vacuum (10- 3 and 10- 5 Torr) and also in air. The substrates with adherent indium-tin-oxide films were placed in the vacuum installation. Both ends of the film were connected to silver electrodes which had also been screen-printed, through which a change of sheet resistance can be measured. The heating and cooling rates of the substrates for the measurements were about 15 and 8°C min-t respectively. Heating at the maximum temperature was carried out for approxi- mately 10 min. A heating coil was used to heat the films and the temperature was monitored with a thermocouple fixed to the substrate.

H.B. Saim et al. / ITO thick films 89

12.0

ii.0

I0.0

9.0

v 8.0

7.0

6 .0

5.0

3A

4.0

3.0 I

i0

I I I I I

2O 3O 40 50 60

Firing tlme (minutes)

Fig. 3. Variations of sheet resistances for three samples, 3A, 3B and 3C, with firing time (5 < t < 60). The firing temperature was 650°C.

In figs. 4 -6 are plotted the curves showing sheet resistance variation as a function of annealing temperature, for oxide films having a thickness of - 0.3 ~tm and an initial sheet resistance of - 6 kf~/D. At a voltage of 0-15 V, all the samples produce a linear I - V curve. In fig. 4 the films measured were separately heated in air, at a pressure of 10 -3 and at 10 -5 Torr (films A, B and C respectively). All the films show a decrease in R~ and thus in resistivity with increasing annealing temperature. The fall in R~ was slow at first but became more rapid at temperatures between 100-200°C and stabilize at around 200°C. This shows that there is no unique activation energy within the annealing temperature range. There is no further reduction in R~ observed for annealing temperatures above 300°C.

Fig. 5 shows the effects of cooling the three samples above (A, B C) to room temperature. The curves show that all the samples, except the one in air (curve A) show no further increase in R s. Fig. 6 shows the irreversibility of the annealing process in a vacuum of 10-3 Torr. When the sample has been annealed in vacuum to about 180°C and cooled to ambient, the sheet resistance is much lower than the

90 H.B. Saim et al. / ITO thick filrns

g u

o3

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0 i i i I

30 70 ii0

A

\ I

\

L I I I I I I I I L

150 190 230 270 310

Annealing Temperature T {°C) A

Fig. 4. Variations in sheet resistance of ITO films with temperature during heating process. (Film A, annealed at normal pressure, film B, annealed at 10 3 Torr and film C annealed at 10 5 Torr.)

initial value and subsequently is reduced further on heating to 300°C. The direction of the sheet resistance changes as indicated by the arrows on the curves.

3.3. Ageing of ITO films

Ageing studies of the ITO films give a more or less similar pattern. The samples were left under laboratory conditions and the change of resistances recorded at intervals for about six days. Fig. 7 illustrates the general change of sheet resistance for the three films from three different firings that have been annealed in vacuum on ageing at room temperature. The different rates of increase in resistance might be due to the differences in the quenched, oxygen deficient defect structures. Besides, differences in film thickness, porosity and other processes which might also play a role. However, despite the differences in kinetics of the ageing reactions, the film eventually stabilize after about three days. This might be due to the indium-tin-oxide films reaching an equilibrium oxygen content as a result of room temperature ageing processes.

H.B. Saim et al. / 1TO thick films 91

8 . 0

7 . 0

6 . 0

"~ 5 . 0

r.

4 . 0

3 . 0 4~

2.0

1 . 0 B

0 . 0 I 1 I l I I I I I I I I I "1

30 70 ii0 150 190 230 270 310

Annealing Temperature T A (°C)

Fig. 5. Variations in sheet resistance of ITO films with temperature during cooling process. (Film A, in air, film B in vacuum of 10 -3 Torr and f i lm C in 10 -5 Torr.)

3.4. Structural studies

Structural studies using the scanning electron microscope shows that the films are polycrystalline with crystal sizes of 100-200 ,~. There is no obvious change in crystal sizes due to annealing process. Thus grain growth of the film may not be the main contribution to the increase in conductivity. Electron diffraction Studies also show no obvious change due to the annealing process. However the diffraction patterns resembles more closely that of In203 (bcc) with a lattice constant of - 1 0 . 0 (compared to - 10.12 .~ for powder In203).

3.5. Optical studies

Transmission speetra of ITO films in a wavelength region from near ultraviolet to near infrared have been examined at room temperature. All the samples show high transmittivity in the visible region of the spectrum.

Fig. 8 shows the transmission spectra for three samples having different values of resistivities. For curve A, the film was unfired and the resistivity was > 1.0 fl cm. For curve B, the film was fired but unannealed and the resistivity was about

92 H.B. Sairn et al. / ITO thick filrns

7.0

:4

uo

6.0

4.0

3.0

2.0

Stage l(h)

1.0 Stage 2(h)

I Stage l(c)

Stage 2(c) i i I I i i

30 70 ii0 150 190 230 270 310

Annealing Temperature T A (°C)

Fig. 6. Effects of two consecutive stages of heating and cooling on the sheet resistance of ITO film in a vacuum of 10 -3 Torr ((h): heating, (c): cooling).

2 × 10- ] f~ cm. The film for curve C was annealed in a vacuum of 10 3 Torr, having a resistivity of about 3 × 10 2 f~ cm.

The data of fig. 8 show that the absorption edge of the annealed ITO film (lower resistivity) shifts to higher energies compared to the one which is unannealed or unfired. These shifts can be described in terms of an increase in mobility and carrier concentrat ion which lowers the Fermi level and reduces the magni tude of the Burstein effect [16]. Thus the absorption edge moves from about 3.7 eV for the unfired film to about 4.3 eV for the annealed film.

Annealing also caused the development of a free-electron absorpt ion edge at about 1.7 ~m (film C). This result can reasonably be understood if one considers that the " reduc ing procedure" (vacuum treatment) brings about an increase in the amount of defects and that defects form shallow donor levels in the i nd ium- t i n -ox ide film [17].

3. 6. Compositional studies

Auger electron spectroscopy studies on the film were carried out which allows for accurate composit ional analysis. In-depth profiling of the elements under investiga-

H.B. Saim et al. / ITO thick films 93

1 .5

1 . 4

"~" 1 .3

~ 1 .2

el .,-t

-~ 1 .0

0 . 9

0 . 8

0.7 I I I I I I l I I I I I r I I

0 20 40 60 80 i00 120 140 160

Ageing time t G (hours)

Fig. 7. Ageing of ]TO films under laboratory conditions. All the three films have been vacuum annealed before left at ambient.

100

B 80 e ~ •

60

f. 40

h 6i00 i i , i 16'00 i f i , 200 400 800 I000 1200 1400 1800 2000 2200 2400 26100

Wavelength, 1 (nm)

Fig. 8. Op t i c a l t r an smi s s ion of ] T O fi lms o n q u a r t z as a func t ion o f f r equency for fi lm A: un f i r ed ; B:

f i red b u t u n a n n e a l e d ; a n d C: f i red a n d a n n e a l e d a t 3 0 0 ° C (at 10 - 3 Torr) . These fi lms have been

depos i t ed o n q u a r t z subs t ra tes .

94 H.B. Saim et al. / ITO thick films

Table 1 Auger analysis: in-depth profiling of sample 1 (6 kf~/D) unannealed

Depth Elements detected (at%) Ratio

(nm) In Sn O C S CI Na I n / O S n / O

0 26.5 6.8 55.9 4.0 1.4 4.3 1.2 0.47 0.12 15 35.3 4.8 59.9 0 0 0 0 0.59 0.08 45 35.4 5.0 59.2 0 0 0.4 0 0.60 0.08 75 35.9 5.3 58.4 0 0 0.4 0 0.61 0.09 90 36.2 5.2 58.2 0 0 0.1 0 0.62 0.09

105 36.2 4.8 56.7 0 0 1.2 0 0.64 0.08 120 35.7 5.0 55.1 0 0 1.4 0 0.65 0.09

tion were obtained. Tables 1 and 2 give the results of the in-depth profiling of the two ITO films and also compare the effects of annealing in vacuum.

The presence of a relatively high amount of carbon on the surface of film 2 might be due to the sample being placed in the vacuum installation during annealing - which obviously contains some organic vapours. This is also true for the presence of sulphur atoms. Chlorine atoms were present in both the samples.

The effect of annealing in vacuum has little effect on the I n / O ratio but slightly increases the ratio of S n / O of the film, which may be used as an indication of the degree of increase in oxygen vacancies of the film during the annealing (the sheet resistance was lowered to 400 ~2/[2 on annealing).

3. 7. Hall effect m e a s u r e m e n t s

Hall effect measurements on the films allow the determination of carrier con- centrations and the mobility of the carriers as well as the sign of the carriers. For this experiment, the sample was deposited in such a way that the aspect ratio of the length and breadth of the film was 4 : 1 to meet practical requirements. The Hall voltage was measured across the breadth of the sample. Contacts were made at four different points of the film.

Table 2 Auger analysis: in-depth profiling of sample 2 (400 fl/t~) after annealing in vacuum at 300°C

Depth Elements detected (atomic %) Ratio

(nm) In Sn O C S CI Na I n / O S n / O

0 15.9 5.1 37.4 33.0 2.3 2.7 3.5 0.43 0.14 15 36.1 7.2 55.7 0 0.7 0.3 0 0.65 0.13 45 35.9 7.0 56.2 0 0.6 0.3 0 0.64 0.13 75 35.7 6.7 56.6 0 0.6 0.5 0 0.63 0.12 90 35.5 7.0 56.3 0 0.6 0.6 0 0.63 0.12

105 34.0 6.9 54.5 0 0.8 0.7 0 0.62 0.13 120 20.5 3.8 52.2 0 0.4 0.6 0 0.39 0.07

H.B. Saim et al. / ITO thick films 95

Table 3 Hall effect results for sample 3

Samples Hall parameters

R H (cm 3 C - I ) n (cm -3) 0 (f~ t c m - 1 ) #H ( cm2 V I s - l )

Before annealing 10.00× 10 -2 6.24×1019 6.10 6.10×10 - I After annealing 8.78 × 10- 2 7.79 x 1019 9.52 × 101 8.01 After annealing and ageing 9.22 × 10 2 6.77 × 1019 6.67 x 101 6.15

The Hall coefficient R H is obtained from the relation

R . = V H d / I B , (1)

where R H is given in cm3/C if I (along the x-direction) is measured in A, V H (along the y-direction) in V, thickness d in cm, and the negative field B (along the z-direction) in T.

The carrier concentration, n (or p) can then be calculated from the formula

n = 1 / R . q , (2)

where q is the charge on an electron. The conductivity a can be obtained by measuring across the current I and voltage

V configuration and the mobility/x H can then be calculated from the relation.

t~.= R . o . (3)

Table 3 summarizes the electrical characteristics of the film before and after annealing and also after ageing for a few days.

Generally the changes in conductivities due to annealing are caused by changes of mobility (an increase of about 10 times). Changes in carrier concentrations are small but nevertheless significant. The increase in conductivity after annealing in a reduced pressure has also been observed in sputtered indium-t in-oxide films [18,19] and this was explained as due to an increase of degeneracy of the film with heat treatment. It has also been observed that vacuum annealing eliminates the effects of annealing in air. The nature of the annealing characteristics shows that the pressure of oxygen is the cause of the changes in electrical properties. The small changes in carrier concentrations might show that the effect of the diffusion of oxygen in a n d / o r out of the grains are small [20].

4. Conclusions

Transparent In203/SnO 2 semiconductive films may be formed on alumina, silicon or quartz substrates by the screen printing and firing of organometallic compositions. Sheet resistances of about 300 ~2/t2 are normally obtained by heat- treating of the samples in a reduced atmosphere. However the sheet resistances tend to increase slightly and stabilize at about 1 k~2/[3 after ageing at room temperature.

96 H.B. Saim et al. / ITO thick filrns

This increase is primari ly due to oxidation of the film. Moisture might also play a role in the ageing process.

The variations of the electrical and optical properties of I n 2 0 3 / S n O 2 films with heat- treatment, particularly in a vacuum is significant. It is thought that the origin of the conductivi ty in the as-deposited fims are due to the pressure of defects resulting

from the nonstoichiometr ic composi t ion of the material - i.e. oxygen vacancies a n d / o r interstitial tin atoms. The remarkable increase in conduct ivi ty when the samples are heated in a vacuum is a result of an increase in defects from shallow donor levels in the tin oxide. At ambient atmosphere, the film loses tin due to the gradual oxidation process which again lead to an increase in resistivity.

Annea l ing also causes an improvement in the carrier mobil i ty (electrons) by about 10 times and a small change in carrier concentrat ion. This result seems to agree with

the change in optical properties. Although the conductivit ies and mobilit ies of the films are not as high compared to those films prepared by thin film techniques, at tempts have been made to fabricate these films on single crystal p-type silicon, hence forming an SIS heterojunction. Typical value of the cells are Voc = 400 mV and J s c = 0.5 m A / c m 2 under a total i l luminat ion of 800 W / m 2.

References

[1] A.G. Sabnis and A.G. Moldvan, Appl. Phys. Lett. 33 (1978) 885. [2] R.P. Howson, J.A. Avaritsiotis, M.I. Ridge and C.A. Bishop, Thin Solid Films 63 (1979) 163. [3] K. Kane, H.P. Schweizer and W. Kern, Thin Solid Films 29 (1975) 155. [4] J.A. Aboaf, V.C. Marcotte and N.Y. Chen, J. Electrochom. Soc. 120 (1973) 701. [5] J.S. Mandes and T. Rodriquez, Thin Solid Films 69 (1980) 183. [6] J.C. Manifacier, J.P. Fillard and J.M. Bind, Thin Solid Films 77 (1981) 67. [7] M. Mizuhashi, Thin Solid Films 70 (1980) 91. [8] D. Laser, Thin Solid Films 90 (1982) 317. [9] F.T.J. Smith and S.L. Lyu, J. Electrochem. Soc. 128 (1981) 2388.

[10] E.E. Kohnke, J. Phys. Chem. Solids 23 (1962) 1557. [11] M.B. Field and L.R. Scudder, IEEE 12th IEEE Photovoltaic Specialists Conf. (1978) p. 303. [12] E.L. Ralph, Proc. llth Photovoltaic Specialists Conf. (1975) p. 315. [13] J. Savage, in: Handbook of Thin-Film Technology, eds. P.J. Holmes and R.G. Loasby (Electrochem-

ical publishing, 1970) pp. 14-48. [14] M.L. Topfer, Thick Film Microelectronics (Van Nostrand Reinhold, New York, 1971) ch. 1, pp.

1-22. [15] V. Shinta and J. Stein, 28th Electronics Components Conf., Anaheim, California (April 1978) p.

91-97. [16] E. Burstrein, Phys. Rev. 93 (1954) 632. [17] M. Nagasawa and S. Shionoya, Jap. Journal App. Phys. 10 (4) (19771) 472. [18] J.L. Vossen, RCA Review 32 (1971) 289. [19] J.A. Thornton and V.C. Hedgcoth, J. Vac. Sci. Tech. 18 (1976) 117. [20] E. Shanti, A. Bannerjee, V. Dutta and K. Chopra, Thin Solid Films 71 (1980) 237.