ELSEVIER Applied Surface Science 92 (1996) 163-166

sur face sc ience

Characteristics of spray pyrolytic ZnO thin films

C.H. Lee *, L.Y. Lin Graduate School of Materials Science and Engineering, Cheng gung University, Tainan, Taiwan, ROC

Received 12 December 1994; accepted for publication 4 March 1995

Abstract

This paper discusses the characteristics of spray pyrolytic ZnO thin films, which include the morphology, microstructure, resistivity and transmittance. The oscillation nature of resistivity versus growth temperature is analyzed in terms of phase change in microstructure. The effect of AI and S doping is also investigated.

1. Introduction

ZnO has been of interest to many investigators for a long time, because it is one of the few metal oxides which can be made simultaneously optically trans- parent and electrically conductive, and are known as transparent conductors. Zinc oxides are widely used as electrode materials for applications to solar cells, photochemical batteries, display pannels and many other opto-electronic devices [1]. ZnO thin film is also an excellent piezo-electfic material which has been extensively used in acoustic body and acoustic surface wave devices [2]. In addition, ZnO is an excellent material for varistor and various sensor applications [3,4]. However, the recent renewed in- terest in studying ZnO came from its applications as a transparent electrode in photo-electrochemical cells, ot-Si:H solar cells and flat-pannel display devices, because it showed better stability against photo-cor- rosion and glow discharge plasma damage [5].

Zinc oxide can be prepared by various means which include chemical vapor deposition [6], sputter- ing [7], evaporation [8] and spray pyrolysis [9]. For

interests of application to substrates of large contact area, the samples of this work are prepared by spray pyrolysis in the reaction chamber of an experimental apparatus under a reduced pressure of about 50 Torr using a source of aqueous solution of Zn(C2H30) 2 dissolved in CH3OH with variable solute concentra- tions from 0.1 to 0.5 mol% [10]. The experimental substrates include thin quartz plate, (100) Si and oxidized Si wafers. The deposition temperature ranges from 200 to 350°C. For A1 and S doping study, A1CI 3 and thiourea, CS(NH2) 2 are used as additional solutes to the solution. The properties of the thin films are measured by means of various standard commercial instruments and are character- ized as functions of process parameters. Because of space limitation, only the morphology, microstruc- ture, resistivity and transmittance data are discussed in this paper.

2. Experimental results and discussion

2.1. Morphology and raicrostructure

* Corresponding author. Our concerns about the ZnO films as electronic

material prepared by spray pyrolysis are the surface

0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00223-5

164 C.H. Lee, L.Y. Lin /Applied Surface Science 92 (1996) 163-166

0.4

0.2

0.0

.0.2

-0.4

1

1

ttowrate 0 . 4 ¢ e J , , , ; " ] ........................ i ................................................................... -] ] ....................... i ......................................................................... [!

0 500 I0O0 1500 I.TM

Fig. 1. a-step measurement of smoothness of ZnO films deposited with a solution of O.15M.

smoothness and microstructure, although all the films as grown look mirror smooth. In this work, the surface smoothness of a typical sample which is grown at 300°C from the 0.15M solution is analyzed by means of scanning with an (x-step meter probe. The results are shown in Fig. 1, where the smooth- ness is close to the sensitivity limit of the technique, i.e. 10 nm. This should meet the requirement for most of the modem applications.

Fig. 2 shows three scanning electron micrographs of the films grown at 275, 300 and 350°C from the 0.15M solution to show the morphology. Though the films are all visually alike, i.e. mirror smooth, the nature of their morphology is quite different. Namely,

( 002 ) C - O . 15M

3zs'c ~ . L k

3o0"c - - - ~-~ ,L _

~ ,,. ~ _

S i O z / S ~ - -

20 24 2S 32 36 40 44 48 52 5(~ 60 2 e

Fig. 3. X-ray diffraction data of ZnO thin films grown at different temperatures.

the geometry of the aggregates grown at lower tem- peratures ( < 300°C) appears to be arc shaped, whereas those grown at higher temperatures (> 300°C) become circular grains. This indicates that there is a phase change occurring near 300°C. The observation agrees with what is detected from the data of X-ray analysis, as shown by the diffrac- tion spectra in Fig. 3. In the series of the X-ray spectra of the films grown at different temperatures,

Fig. 2. SEM micrographs of ZnO thin films grown at different temperatures: (a) 275°C, (b) 300°C, (c) 350°C.

C.H. Lee, L.Y. Lin /Applied Surface Science 92 (1996) 163-166 165

growths below 300°C are amorphous, while those grown at temperatures above 300°C show the crys- talline phase with the diffraction peak of (002). The intensity of the peak increases with the growth tem- perature indicating enhancement of crystallinity of the film at higher temperatures.

2.2. Resistivity

The resistivity is determined from sheet resistance measurements by means of a set of four point probes. As a function of growth temperature the resistivities of the films grown from 0.15M solution with a flow rate of 0.2 cm3/min are plotted in Fig. 4. In general, the data exhibit the relationship of reproducible os- cillation. Namely, in the lower temperature range, the resistivity decreases which is due to microstruc- tural and compositional change of the grains because of the increase of growth temperature. This leads to the reduction of grain boundary effect and a change in conduction mechanism. However, the resistivity increases with the growth temperature to a maximum value near 300°C because of carrier reduction as the deposition reaction becomes more complete and the crystallinity further improves. On the other hand, for growths above 300°C, the increase of oxygen vacan- cies is likely a source of an increase in carrier concentration which results in a decrease of resistiv- ity, though better crystallinity may also enhance the carrier mobility for increase of conductivity. In an attempt to correlate the microstructure and the com- plex behavior of the electronic property of the films,

10

, 1

O ~ 1 0 -t

"~ I0 "

' \

I I ~ I I 10 "225 250 275 300 325 350 375

T e m p e r a t u r e (*C)

Fig. 4. A n a l y s i s o f s inuso ida l d e p e n d e n c e o f res i s t iv i ty on depos i - t ion tempera ture .

I0

, 1 E

. ¢ o

10 -i

.a m

,~ tO "

o: . /

• ~ , , o / \ / " T e m B . . 3 0 0 , C \ /

"~, . .9" C = O . 1 5 N

" ~ ' g n O : & l F - 0 . 2 c . c . / a i n .

-a 0 I I t t ~ T I t I llO 1'1 10 1 2 3 4 5 6 7 8 9 12 Concentration( % )

Fig. 5. D e p e n d e n c e o f res i s t iv i ty on dop ing concen t ra t ion o f AI

and S.

the microscopic observations of the morphology of the films grown at 275 and 350°C are sketched along the the resistivity data in Fig. 4, which indicate there is a phase change in the vincinity of 300°C, consis- tent with the X-ray diffraction data.

As a transparent conductor, it is hoped to reduce and to be able to control the resistivity of the ZnO films by doping impurities during deposition. In this work, the effect of S and AI doping is investigated. The experimental data of resistivity of the doped samples are plotted against the impurity source con- centration in atomic fraction in Fig. 5. For low concentration of AI doping, the resistivity decreases with the increase of A1 concentration which reaches to a minimum value of about 2 × 10 -3 1"~. cm for the concentration ratio of A1/Zn = 0.03. This is known to occur from the fact that AI atom replaces Zn in the lattice as a donor to give an extra electron for conduction. Thus, the resistivity decreases with the increase of A1 concentration. However, for A1/Zn ratio greater than 3% there are AI atoms in the ZnO lattice to exhibit the AI203 behavior which has a much wider band gap, 5.6 eV, and causes a reduc- tion of the intrinsic carrier concentration resulting in an increase of resistivity with the increase of AI concentration.

Regarding the S-doping experiments, because S is an iso-electronic impurity in ZnO, it forms ZnS with Zn atoms in ZnO lattice. Since ZnS has a wider band gap, 3.7 eV, it is understandable that when S is doped in high concentrations, the resistivity (Fig. 5) of the ZnO:S films would increase with S concentra-

166 C.H. Lee, L.Y. Lin / Applied Surface Science 92 (1996) 163-166

too

9c

8G

~" 7c ~ 6c ~ 5c ~ 4c e, .a 3c

2o 10

. . . . . . : S , S / Z n - O . 0 3

- - : & l , & l / ; a - O . 0 3 3

T e | p . = ] O 0 " C

C - 0 . 1 S N

r - 0 . 2 c.c . /a£n.

I I I I I 400 500 600 700 800 900

Waveleng th( r tm)

Fig. 6. Comparison of the transmittance of ZnO films doped with AI and S.

tion. However, the sudden change in resistivity in the concentration range of 4% and beyond may be due to lattice defects formed during the doping process.

using the solutions made of Z n ( C 2 H 3 0 ) 2 and CH3OH, are mirror smooth and have a preferred orientation of (002) of which the X-ray diffraction intensity is enhanced by the increase of growth temperature. The resistivity of the films varies from 10 -2 to 1 l~- cm and shows a reproducible oscilla- tion behavior with growth temperature in the range between 250 and 350°C. The doping experiments show that the resistivity increases with the concentra- tion of S to a maximum of 3 1). cm and decreases with the concentration of A1 to a minimum of 2 × 10-3 l-l. cm. The transparency of the film of about 3000 ,~ is better than 90% in the wavelength range of 380 to 850 nm. The band gap energies of the films grown at 300°C are determined from the absorption data which have values of 3.203, 3.136 and 3.092 eV for the intrinsic, the S- and the Al-doped samples respectively.

3. Transmittance and band gap

The transmittance of the two films doped with S and AI are shown in Fig. 6 with an undoped sample for comparison. These films are grown at 300°C from the 0.15M solution with a flow rate of 0.2 cm3/min and have a thickness of about 3000 ,~. It is apparent that the ZnO films doped with S and A1 in a concentration ratio of about 3% do not alter the transparency significantly, though minor decreases of transparency in the long wave-length range and greater increases in the UV range are seen for the Al-doped sample. The band gaps of these films are determined by means of a graphical method from the optical absorption coefficients derived from the transmittance data. For the film grown at 300°C, the band gap energies are 3.203, 3.16 and 3.092 eV for the intrinsic, the S- and the Al-doped samples respec- tively, which are self-consistent with the process condition.

4. Conclusion

ZnO thin films deposited on Si by means of spray pyrolysis, at temperatures between 200 and 400°C

Acknowledgement

This work is sponsored by the National Science Council of ROC Contract #NSC81-0417-E-006-05.

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