Materials Science and Engineering, 1322 (1994) 233-240 233

Spray pyrolysis deposition for indium oxide doped with different impurities

C. H. L e e a n d C. S. H u a n g

Graduate Institute of Materials Science and Engineering, Cheng Kung University, Tainan 70101 (Taiwan)

(Received March 10, 1993; in revised form May 17, 1993)

Abstract

Spray pyrolysis including the doping process of impurities of different valence for preparation of transparent conductive indium oxide thin films is investigated. In this work, the oxidized Si wafers and pyrex glass slides are used as substrates. Low concentration InCl3-methanol solution is the main source chemical with ZnC]2, SbCl 3 and TeCl 4 as additives for dopants. The deposition process and film properties are characterized as functions of various process parameters includ- ing the substrate temperature, the flow rate and the concentrations of reactants. In general, the deposition rates are optimized to reach 40 to 50 nm/min at substrate temperatures between 300 °C and 325 °C. All the films are smooth and transparent in the visible and IR range of radiation with transparency greater than 90%. The refractive index is between 1.8 and 2.1. The resistivity of the In203 films without doping is in the range of 2 x 10 -3 to 8 x 10 -3 fl-cm. It increases with Zn doping by an order of magnitude while decreases with Sb and Te as dopants. In this work, the lowest resistivity from Te doping reaches to 5 x 10 -4 fl-cm.

I. Introduction

In203 and ln203:Sn(ITO ) have been subjected to many investigations, for various applications, since they were found to be electrically conductive and opti- cally transparent in the late 1960s [1]. Indium oxide is physically stable and chemically inert, as a transparent conductor it is similar in nature to S n O 2 and is superior for applications in many aspects [2-10]. Consequently, it has replaced the SnO2-based materials in the tradi- tional market of transparent conductors. In particular, the transparent conductive material of more expensive and durable goods are made of In203.

For example, indium oxide has been used as an anti- reflection coating for energy efficient windows [3] and transparent electrodes in solar cells [4,5], liquid crystal displays and other optical devices [6,7].In recent years, In2Oa:Sn (ITO) has also been used as an anode in the photochemical cells of space power systems [8]. In addition, for ohmic contact electrodes of InP devices [9], ITO provides the most compatible physical and electronic interface properties. Thus, there is renewed interest in process development of In203 thin films for electronic applications.

1n20 3 crystallizes into a cubic bixtype structure which exhibits a direct band gap between 3.55 and 3.75 eV [10]. It has a melting point temperature of 1910°C. However, it sublimes at 850°C. In:O 3 has

very interesting optical properties. It absorbs IR light waves of 3. > 900 nm, yet it is highly transparent to the visible waves of ;t = 400 to 700 nm.

Because In203 is mostly used in the form of thin conductive films ( - 2 0 0 nm), it is generally charac- terized in terms of thin film properties which are some- what different from those of the bulk of this material. In general most of the I n 2 0 3 thin films are polycrystalline, with grain sizes varying from 10 to 100 nm depending on the method of preparation and the subsequent pro- cessing steps. In203 thin films are generally n-type, with electron concentration ranging from 1017 to 102° cm- 3 and mobility between 30 and 60 cm V-t s-1 resulting in a resistivity between 1 and 10-3 f~-cm depending on the conditions of preparation. However, films can be doped with different impurities to result in a resistivity of 2× 10 -4 f~-cm, though 1n20 3 films are invariably n-type irrespective of the dopants.

Thin films of indium oxides can be prepared by different methods including evaporation [11-14], sputtering [15,16], chemical vapor deposition [17-19] and spray pyrolysis [20-22]. Since spray pyrolysis is a process which can be easily applied to dope impurities of high concentration, and is the most popular process for preparation of transparent conducting thin films on glass substrates, we chose this technique to study the doping effect of different impurities on In20 3. The details of our study are reported in this paper.

0921-5107/94/$7.00 © 1994 - Elsevier Sequoia. All rights reserved

234 ('.tt. Lee. (.S. t humg /' Spray i~yrolysis deposition

2. Experimental details

The experimental work of spray pyrolysis for growth of In203 thin films was carried out in a home- made reaction chamber, schematically illustrated in Fig. 1. The deposition system consists of three sections of an apparatus which includes: (a) the reactants and carrier gas manifold connected to the spray nozzle at the entrance of the reaction chamber, (b) the pumping and exhausting gas scrubbing systems and (c) the reac- tion chamber in which there is a resistance heated pedestal for the substrate to sit on for film deposition. The pedestal can revolve with variable speeds.

In this work, oxidized (100)Si with 100 nm SiO 2 was used as a substrate. The spray solution was prepared by dissolving different amounts (e.g. 3, 4, 5 g) of InC13 in the solvent containing 85% CH3OH, 10% H20 and 5% HC1. To study the doping characteristics of differ- ent impurities, we added the proper amount of the impurity chloride into the standard solution. The impurities for doping study included Zn, Sb and Te. The deposition parameters include the substrate tem- perature, varying from 200 to 375°C, the reaction pressure fixed at 50 Torr, the distance of the nozzle from the substrate, kept at 25 cm, and the carrier gas flow rate, fixed at 3 1 min- J, with the flow rate of the solution varying from 0.3 to 1.1 cc rain- ~.

The properties of the thin films in this work were characterized by various techniques. Namely, the thick- ness and refractive index were measured by an a-step meter and an ellipsometer respectively. For composi- tion, microstructure, resistivity and transmittance, the films were analyzed by means of Auger electroscopy, X-ray diffraction, four point probe measurement and UV spectroscopy respectively.

3. Results and discussion

In order to establish a reference point, the first series of experiments was carried as process parameter investigation, without the introduction of dopants. For instance, by using the 4 and 5 g solutions for growth at 325 °C with a fixed flow rate of 0.3 cc min i, the thick- ness variation with deposition time is determined, as shown in Fig. 2. This is linearly proportional to the deposition time, implying that the growth condition can be held stable and the experimental results are repro- ducible. By using the 4 g solution with a flow rate of 0.3 and 0.5 cc min-~, the growth rates as a function of growth temperature were determined, as shown in Fig. 3. One notices that, for both flow rates, the growth rates have a similar trend of variation with substrate temperature,In general, in the lower temperature range (-< 300 °C) the growth rates increase with growth tem- perature because of the increase in reaction rates of hydrolysis of the InC13 solute molecules. They reach a maximum at about 300 °C and then decrease as the temperature further increases, because of depletion of reactants owing to volume reaction. For growths at 300 °C the effect on growth rates of the flow rates of three solutions of different concentration is plotted in Fig. 4. The data indicate that the growth rates increase with the flow rates because of greater amounts of reactant participating in the deposition process. While for higher flow rates the rate of increase is slower, showing the phenomenon of reactant saturation effect. For the same reason, the growth rates of the three solutions of different concentration are not exactly in proportion with the concentrations.

so[u~ion ~j~

w N2

o sprayer

pyrex

/subs±r~±e

E VACUUM

Fig. 1. Schematic illustration of apparatus for spray pyrolysis.

I0000

.< : v 8000

G') I.,J 6000 z ,,/ o -r- 4000 y...

2000

O

cl

5 10 15 e0 2 5 30

TIME (mln)

Fig. 2. Film thickness vs. deposition time, T = 325 °C flow rate 0.3ccmin-l,m C=4g/100ml, o C=5g/100ml.

C.H. Lee, C.S. Huang

800

700

~ 600

"<I: 500

Ld I.-- <~ 400 re

-r" 300 I.--

r'~ 200 r'v" L:.3

l o g

l , ,I, I l , I

: '50 275 . 300 325 350

TEMPERATURE ('C)

I

375

Fig. 3. Growth rate vs. substrate temperature: • C = 4 g/100 ml, F=0.3 cc min-I; • C=4 g/100 ml, F=0.5 cc min-I; • C=5 g/100 ml, F= 0.3 cc rain- i.

Spray pyrolysis deposition 235

800

700

~" 600

v 50O

I,I I.-- <~ 4O0 n /

"1- 300 F--

r"l :=00 P/ L.3

IO0

f •

1 I I I I 1 I P I ' !

0.~ 02 0.3 0,¢ 0.5 o.6 0.7 0.~ 0.9 t.o t.:

F'LOW RATE (cc/mln)

Fig. 4. Growth rate vs. solution flow rate, T= 300 °C, • C = 5 g/100ml,• C=4g/100ml,• C=3g/100ml.

4 . M i c r o s t r u c t u r e

Since the electrical and optical properties are microstructurally dependent, the films were subjected to morphological and microstructural investigations as discussed in this section. The techniques used for the study include X-ray diffraction, scanning electron micrographic and transmission electron microscopic analysis. Figures 5(a), (b) and (c) show the SEM images of three films prepared at 250, 300 and 350°C respectively. The images clearly indicate that all the films are polycrystalline and consist of fine grains. The grains of the 250 °C film are much finer than those

Fig. 5. SEM images of the films grown at (a) 250 m *C, (b) 300 °C and (c) 350 *C.

grown at higher temperatures, because the reaction species are immobile for grain growth at low tempera- tures. The 350 °C growth consists of the most coarse grains, which means that grain growth during film deposition is significant. Figure 6 shows a set of X-ray diffraction data for the corresponding samples. The data indicate clearly that the high temperature growths do have better crystallinity. The results of the X-ray diffraction pattern are also verified by TEM analysis. Figures 7(a), (b) and (c) show a series of TEM images and diffraction patterns of the same set of samples. The TEM results indicate that, for growth at 250 *C, no clear contrast of crystalline structure shows up in the micrograph and the diffraction pattern. While for growths at 300 and 350 °C, the contrast of the poly- crystalline grains of good crystallinity is very clear.

236 CH. Lee, C.S. Huang /

r~

7- r , A

3 5 0 1 C

25o'c ' " i ! I i -

t0 b 35 3o 4, b 50 Spectral angle

Fig. 6. X-ray diffraction data of the films grown at different temperatures.

Spray pyrolysis deposition

However, the 350 °C film has larger grain sizes than the 300 °C film.

5. Optical properties

The refractive indices, as determined by means of ellipsometry, are plotted against growth temperature in Fig. 8. The indices increase with growth temperature because growths at higher temperatures result in films of greater density and better crystaltinity. The data also indicate that the films grown with lower flow rates have a greater index, this is because they have a lower growth rate resulting in a denser film.

The transmittance data of two films of thickness about 300 nm, grown at 250 and 350 °C, are shown in Fig. 9 as a function of the wavelength in the range 300 to 900 nm. The average transmittance of the 350 °C film is about 93%, which is significantly higher than that of the 250 °C film. This means that the 350 °C film has larger grain sizes and better crystallinity, resulting in better optical properties. The fluctuations in the UV range of transmittance are due to the reflective interfer- ence from the surface of the film and the film substrate interface.

In Fig. 10, the transparency data of the films grown at 350°C under different flow rate conditions are plotted against the flow rates. The data show a gradual decrease with increase in flow rates. This means that growths with higher flow rates have greater growth rates, resulting in higher defect densities and smaller grain sizes. These changes constitute the main cause of light-wave scattering and the decrease in transmittance, in a similar manner to that of the growth at 250 °C as shown in Fig. 9.

2 . 2

Fig. 7. TEM images and diffraction patterns of the films grown at (a) 250 °C, (b) 300 °C and (c) 350 °C.

X bJ ~:~

Z

bJ > p- (-I

rv b- i i rv

?.0

1.8

1.6 I I I I I I 0 ~50 2 7 5 300 3 2 ~ 350 3 7 5

TEMPERATURE ('C)

Fig. 8. Film refractive index vs. growth temperature, flow rate 0.5 ccmin -1 (A)and 0.3 ccmin -l (m).

C.H. Lee, C.S. Huang /

100

8O

v

:7 60 <C h - l ' -

~-- 40

Z <E EE ~'-- 20

I

200

25Q'C

~ 350 C

I I I I I I I

3oo 4oo 5oo 6oo 70o aoo 9oo

WAVELENGTH (nm)

Fig. 9. Film transmittance vs. light wavelength.

Spray pyrolysis deposition

t¢

237

E U I E

o

)- . I - - M

> i - - , i

I - -

i - - i

Q~

io"

z6"

16 i 1 • I 1 I -1 250 275 300 325 350 375

TEMPERATURE ( 'C)

Fig. 11. Film resistivity vs. growth temperature, C = 4 g/100 ml, flow rate 0.3 cc min-1 (m) and 0.5 Cc min-1 (A).

100

80

i.i (D Z 6Q <E

I-. i - - i "~" 4O oo Z < ¢-/ I-- 2o

I I 1 I I , 1

o.~ o.a o.~ 0,7 o.9 ~.~

FLEI~/ RATE (c ¢ Imln)

Fig. 10. Film transmittance vs. solution flow rate, T--350"C, C-- 5 g/100 ml.

TABLE 1. Absorption edges and energy gaps

Film Deposition Absorption Eg (direct) Eg (indirect) temperature edge (eV) (eV) (°c) (eV)

A 250 3.54 3.79 3.20 B 350 3.55 3.78 3.31

From the transmittance spectra in the UV region, the absorption coefficients were calculated and the absorption edges determined. The results are listed in Table 1, where the higher values of the 350 °C film doped with Te are due to the Burnstein shift, and the

lower values of the 250 °C film are probably due to the band edge diffusion, caused by the morphologic nature of the defect structure accompanied by the high carrier concentration.

6. Resist ivi ty

The resistivities of the films, as given by the product of the sheet resistance and the thickness, are plotted against the substrate temperature in Fig. 11. In the tem- perature range studied, the resistivity shows an increase with growth temperature, reaching a constant value of about 5 x 10 -3 fk-cm at 350 °C. This is prob- ably due to the change in stoichiometric condition of the films, which is growth temperature dependent as determined by AES analysis. As shown in Figs. 12(a) and (b), the AES spectra of the growths at 250 and 300°C are very different, resulting in the stoichio- metric ratios of In/O = 1.57 and 1.28 respectively. The chlorine contents also decrease with the increase in growth temperature, from 0.05 to 0.01. Both the increase in oxygen and decrease in chlorine ions reduce the concentration of the conductive carder because of the increase in substrate temperature from 250 to 3000C. As a consequence, the resistivity increases with the growth temperature. However, the film morphology and microstructure also change with the growth temperature, as discussed previously, having little effect in this case of a polycrystalline semi- conductor of high carrier concentration.

The effect of the solution flow rate on resistivity is shown by the data in Fig. 13. The resisitivity shows a gradual decrease with increase in flow rate for growths at 300 °C with the 4 g solution. This is apparently due

238 C.H. Lee, ('.S. ttuang / ~Spraypyrolysisdq~osition

Cl ~ 5

0

393

(a) In

i J

0

(b) In

Fig. 12. Auger spectra of the films grown at (a) 250 °C and (b) 300 °C.

to the fact that the higher flow rates result in greater growth rates, and a higher concentration of oxygen vacancy and chlorine ion content. The effects of heat treatment and annealing, ambient on the resistivity for growths at different growth temperatures, are shown in Fig. 14. The data indicate that the resistivity decreases after annealing in nitrogen because of the lack of oxygen to generate lattice vacancies, i.e. charge carriers, while it increases after annealing in oxygen

u [

E (-

0 "xJ

>- H-

>

Ld C~

i0

16'

zo" l i I l ..... I 0.2 0.4 0.6 0.8 LO 1.2 ~'

FLOW RATE (cc/mL)

Fig. 13. Film resistivity vs. solution flow rate, T = 300 °C, C = 4 g/100 ml.

/'%

E LJ I

c- O

~J

>- I-.-

(,,9

IM c v

io" =

1 0 t,., I l I I I 250 27~ 300 325 350 375

TEMPERATURE (°C)

Fig. 14. Film resistivity vs. growth temperature prior and post annealing in 02 and N: • as-grown; m annealed at 400 °C O2; zx annealed at 400 °C N z.

due to vacancy reduction. However, the effect of the growth temperature is minor, because it is lower than the annealing temperature. For annealing in oxygen the resistivity appears to reach the same value regardless of the growth temperature. This means that annealing in oxygen results in an equilibrium number of oxygen vacancies,depending on the annealing temperature.

7. Doping effect

In order to increase conductivity and gain control of the resistivity, we studied doping of In203 thin films

C.H. Lee, C.S. Huang / Spray pyrolysis deposition 239

with different impurities during deposition. In this work Zn, Sb and Te were used as dopants. ZnCI2, SbCI 3 and T e C I 4 w e r e used as the source chemicals for the dopants, which were added to InC13 to make up the 5 g/100 cc solutions. The thin films were deposited at 350 °C, while the ratios of Zn/In, Sb/In and Te/In in the solutions were variable and the flow rate of 0.3 cc rain -1 was fixed. The resistivity, as determined by means of the four point probe measurement, is plotted against the atomic ratio of the impurities to indium as shown in Fig. 15. The results show that the resistivity increases with Zn concentration, reaching a saturation value at the ratio of Zn/In = 5. The data means that Zn atoms enter the In203 lattice to occupy the In sites as acceptors, which compensate the donors in the material to cause an increase in resistivity of In203. While doping with Sb and Te, the resistivities decrease with increase in impurity concentration, to a minimum value at the Sb/In and Te/In ratio of 0.3, the resistivity then increases with impurity concentration.With regard to Sb and Te, both the impurities are donors which serve to increase the electron concentration and decrease the resistivity. However, when the concentra- tions become too high, the mobility of the electrons decreases and the impurities may precipitate, resulting in defect structures which would trap electrons to cause an increase in resistivity as shown in Fig. 15. Te has one more valence electron than Sb, which appears to be more effective as a donor.

The effect of substrate temperature on resistivity was studied for the doping experiments. The results are shown in Fig. 16. For a ratio of Zn/In = 0.04 the resis- tivity remains at about 8 x 10 -2 C2-cm for all the

growth temperatures, while for Sb and Te the resistivit- ies decrease slightly with increase in substrate tempera- ture, to about 10 -3 ~t-cm and 6 x 1 0 -4 g'k-cm respectively at the growth temperature of 325 °C. The undoped sample has resistivity values from 2 x 10-3 to 7 x 10-3 ffbcm, depending on the growth temperature, with the maximum occurring at 325 °C, consistent with the data in Fig. 14.

8. Conclusions

Spray pyrolysis, including the doping process of impurities of different valence for preparation of trans- parent conductive indium oxide thin films, has been investigated. In this work, low concentration InC12-methanol solution was used as the main source chemical, with ZnC12, SbCI 3 and TeCI 4 as additives for dopants. On an oxidized Si wafer, the deposition rates were optimized at 40-50 nm min -~, at substrate temperatures between 300 and 325 °C. The film com- position, in terms of In/O, decreased with increase in substrate temperature as did the chlorine content. Annealing at 400 °C caused a reduction in In/O ratio. However, the optimum value of In/O was about 0.8.

All the films were smooth and transparent in the vis- ible and IR range of radiation, with transparency grea- ter than 90%. However, they proved highly absorptive in the UV region and did not appear to change with dopants in the concentration range investigated. How- ever, films from high flow rates of reactant solution consisted of whitish powdery species which deterio- rated the film transparency. The refractive index was

a- u I E c- to" o

>- F- > I---- t0"

10 "~

o'3

(/9 C,J rv

:o I 1 I I I I I I I I I t @ 3 4 5 I0

Sb/In

CONCENTRATION (%) zn / r~ ~ o n ~ c ~ - ~ o Te/In

Fig. 15. Film resistivity vs. dopant concentrat ion, T = 3 5 0 ° C , F = 0 . 3 cc min -~, C = 5 g /100 ml. • In203Zn, • In203Sb, • In203Te.

E U I E

..C 0

>-

I.--4 > 1.--

rv"

10

I¢

16'

• • 5 l

113" I I I I 1 1 250 ;a75 300 3~5 350 375

TEMPERATURE ('C)

Fig. 16. Film resistivity vs. growth tempera ture for different doping conditions: F = 0.3 cc m i n - ~, C = 5 g /100 ml, t~ undoped, • Z n / I n = 0.04, • Sb/In = 0.03, • Te/Tn = 0.03.

240 C.H. Lee, CS. ttuang / Spray l~yrolysis depositioll

between 1.8 and 2.1. It was found to increase with increasing substrate temperature and to decrease with increasing solution flow rate.

The resistivity of the InO films without doping was in the range (2 -8 )x 10 -30 -cm. It increased with Zn doping, by one order of magnitude, and decreased with Sb and Te doping. In this work the lowest resistivity obtained from Sb doping, with Sb/In=0.025, was about 10 -2 f2-cm, while from Te doping at Te/ ln=0.03 it reached 5 x 10 -4 fk-cm. Study of the dop- ing process indicated that the resistivity was predominately controlled by the impurity concetration and not by the substrate temperature. However, the resistivity of the films doped by Zn decreased upon annealing at 400 °C while it increased for those doped by Sb and Te.

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