Highly conductive transparent F-doped tin oxide films were prepared by photo-CVD and thermal-CVD

ELSEVIER

Thin Solid Films 281-282 ( 1996) 228-231

Highly conductive transparent F-doped tin oxide films were prepared by photo-CVD and thermal-CVD Tadashi Ishida a.*, Osamu Tabata b, Jung il Park c, Sung Ho Shin c, Hiroyuki Magara d, Shigeharu Tamura Shoichi Mochizuki a, Toshiyuki Mihara a il,

• Osaka National Research lnstitute, Ikeda 563, Japan " Thill Film Lab., 5-2JO, 2·9·2, Fushi-odai, Ikfda 563, Japan C National Industrial Technology Institute, Kwacheon, Kyeon-ggi-do, Korea d lndustrial Technology Center of Fukui Prefecture, FI/kui 910, Japan

Abstract Although high-quality 8002 films have been prepared above 400°C, we prepared highly-conductive transparent F-doped 8n0 2 films below about 350°C and at high growth rates, using a new raw material system of.Sn{ CH 3) 4, O2 containing 5 mol.%0 3, and HF-acid. Ata substrate temperature of350 "C, the films, which had properties such assheet resistances of 1.6 and 4.5 IUD, resistivities of3.4 and 4.5 X 10- 4 n em, and transmittances includ.ng substrates of 70% and 80% at 550 nm, were prepared by thermal-CVD and photo-CVD (chemical vapour deposition), respectively. Several optical and electrical properties ofthe films prepared by both CVD methods were compared. Keywords: Chemical Vapour Deposition (CVD); Tinoxide

1. Introduction

Low pre8IUI'e HgLamp

High-quality 8n02 films have been prepared by chemical vapour deposition (CVD) methods [1-5]. Recently, photochemical vapour deposition (photo-CVD) methods [6] were reported to beeffectiveinincreasing thedeposition rates atlow temperatures. However, it isbelieved that high-quality Sn02films can bedeposited above 400°C. This paper reports on highly-conductive transparent Fdoped Sn02 films prepared at about 350°C by composing a highly-reactive raw material system and the various properties of the films prepared by therrnal-CVD and photo-CVD.

CootroDer

2. Experiment Fig. I. Schematic diagram of theexperimental apparatus.

Fig. 1 shows a schematic diagram of the experimental setup. A 200 W low-pressure mercury lamp was used as a light source. In order to prevent the suprasil window, which introduces the light onto the substrate, from being contaminated, He gas was strongly blown to the window out of the nozzles and a porous polymer film was also set under the nozzles. TMT (Sn(CH3)4) ~nd020rTMTandOzcontaining about 5 mol.% 0 3 were used as the source materials. HF

* Corresponding author. 0040-6090/96/$15.00 © 1996 Elsevier Science SA All rights reserved Pl/ S0040-6090{ 96) 08619·1

(hydrogen fluoridej-acid was used as theF-doping material. The TMT and HF-acid were put into containers maintained at 20 -c Firstly, the reaction chamber was evacuated to 1 X 10- 2 Torr, and a slide glass substrate was maintained ata constant temperature. Secondly, the Ar carrier gas of HF-acid was introduced into the chamber at 90 ml min -I flow rate when a F-doped film Sn02 (F) was prepared. Then O2 or Oz containing 5 moJ.% 3, and TMT vapour were also introduced

°

T.lshida eral. /Thin Solid Films 281-282 (1996) 228-23/

at 300 ml min " and 8 ml min-I, respectively. FinaIly, the total pressure was adjusted to 10 Torr by the main valve before the deposition of the film. The electrical resistivity of the film was measured by the van der Pauw method. The carrier concentration and the mobility were mr.ii::ur~~1 using the Hall-effect measurement. The optical transmittance was measured by a Shimadzu spectrophotometer UV-1600PC. The morphologies of the films were obtained using a JEOL JSM-5800 8EM.

229 Subetlate Tempt:tature(CC)

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The growth rates of the films are given in Fig. 1. In the figure, the term 8n02 is used to mean pure Sn02 films and SnOz(F) means F-dopcd Sn02 films. If the raw material system is 'tMT and 2, or TMT and O2 containing 3, the CVD ismimed 02CVD or03CVD, respectively. The growth rates of the films generated using 02CVD were much lower than those generated using 03CVD. Therefore, it is very difficux to prepare a really useful film using OzCVD. In the case ofSn02(F), the growth rate using photo-03CVD (refer to the large 0 symbols) was lower than that using thermal03CVD (refer to the large • symbols). This was also the case for 02CVD. Photo-etching is, therefore, considered to occur during thedeposition when HP·acid isadded tothe raw material system in the case of photo-CVD. In the case of 5n02(F) and photo-OzCVD (refer tothe small 0 symbols) , a film does not seem to grow in proportion to the deposition time because ofthe low growth rate ofthe 0zCVD and photoetching, although the growth rate at 350°C is marked. Even the sheet resistance of the film prepared for 2 h could not be measured. In the case oftheffilal-02CVD, only tile SnOz(F) film prepared at 400 °C for 2 h had a thickness (,f sufficient size (O.7l£m) to allow it to be electrically measured, The sheet resistance and the resistivity were 12 IUD aad 8 X10- 4 (J em, and the optical transmittance, including the substrate

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at 550 nm. was 80%. These prcperties are not superior to those offilms generated using 03CVD (Table 1).Inthe case ofSnOz, high-quality films were not prepared [7].Theproperties of the films are not described here. When considering the grow rates of the films, 03CVD is much more useful than 02CVD. Inthe case ofphoto-03CVD,

Table 1 Electric resistivity, sheet resistance. band gap. transmittance (including thesubstrate) at SSO nm and film thickness forSnOz(F) films prepared by thermalCVD & photo·03CVD at various substrate temperatures Substrate temperature

Thickness (/Lm)

Resistivity Cacm)

(0C)

Sheet resistance

Bandgap (eV)

Transmittance at 550nm

(%)

(nlD)

SnOz(F) films generated using photo-CVD 0.2 250 300 0.4 0.4 325 350 1.0

5.2x 10- 3 I.1X 10- 3 9.5x 10- 4 4.5xI0- 4

260 36 24 4.5

3.95 ± 0.04 4.00±O.04 3.92 ± 0.04 4.00±0.04

84 83 86 80

SnOz(F) films generated using thermal-CVD 0.5 250 300 0.8 340 1.7 2.2 350 360 2.0

1.3 3.9x 10- 3 5.6X iQ-4 3.4X 10-. 4 3.9x 10'-4

26000 49.5 3.3 1.6 1.8

3.7310.04 3.83±0.04 3.93±0.04 3.94±0.04 3.92±0.04

86 86 81 70 74

T. Ishida etal./TilinSolid Films 281-282 (1996) 228-231

230

a comparatively high-quality 8n02(F) film was prepared at 250 C'C (Table 1) and 8n02 films were prepared at growth rates of 2-3 nm min -I even at low temperatures of 100150DC (Fig. 2). TMT issaid tobedecomposed above about 400DC. Inthe case of 03CVD, the film prepared at 400DC was abnormally non-uniform, Some parts ofthe substrate glass were not covered by the film. The reason seems to be that chemical reactions occur in the space above 400 DC and form particles just above a substrate. Therefore, we did not prc::}are fi1m ~ above 400DC. However it is very important tolower the substrate

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temperature because itsaves a lot ofenergy, does notdamage the substrate, and allows us to use other kinds of substrate. The optical transmittance for the 8n02(F) film alone (500 nm thickness) deposited ona slide glass by thermal·03CVD at 250 DC was measured. Fig. 3 shows thesquare absorption coefficients rr as a function of photon energy gained by the transmittance. Extrapolating to the abscissa, we gained a bandgap of 3.73 eV [4]. Fig. 4 shows the electric resistivity, p, carrier concentration, n, and Hall mobility, Jot, oftheSn02( F) film asafunction of thesubstrate temperature. Table 1 summarizes the thickness, resistivity, p, and sheet resistance, bandgap and transmittance at 550 nm of the Sn02(F) film prepared by photo-03CVD and thermal03CVD for 30 min. Fig. 5 and Fig. 6 show the surface morphologies (SEM photograph taken using aJEOL, J8M 5800) oftheSn02(F) films prepared by thermal-03CVD and .photo-03CVD, According to Fig. 2 and Table I, the films seem tohave been photo-etched during the photo-CVD. For this reason, the shapes of the grain crystals seem to be round.

T. Ishida etal. /ThinSolid Films 28J-282 (J996) 228-231

231

4. Conclusions

References

We prepared highly conductive transparent F-doped 8n02 films, for example, with sheet resistances and transmittances (including the substrates at 550 nm) of 1.6 .aID and 70%, 4.!i 0 and 80% etc., using thermal-03CVD and photoO~CVD at about 350°C and at high growth rates of 33-73 nm min - I, using a new raw material system of TMT, O2 containing 5 mol.% 0 3 and HF-acid. We prepared 8n02 films using photo·03CVD at low temperatures of 1OG-150 °C and at growth rates of 2-3 nm min-· I .

[ I) T.P. Chow, M. Ghezzo and B.O. Baliga, J. Electrochem. Soc; 129 (1982) 1040. [2} A.K. Saxena, R. Thangaraj, S.P. Singh and D.P. Agnihotri, Thin Solid Films. IJI (1985) 121. [3] T.Maruyama and K. Tabata, J. Appl. Phys., 68 (1990) 4Z~2. [4} O.Sanon, R.Rup and A. Mansingh, Thin Solid Films, 190 (1990) 287. [5] K.H. Kim and co. Park,!. Electrochem: Soc; /38 (1991) 2408. [6J T.Maruyama and T. Morishita, Thin Solid Films, 25/ (1994) 19. [7] J.I. Park, T. Ishida, S. Kimura, S. Tamura, T. Mihara and O. Tabata, Surface Finishing, 45 (1994) 547, inJapanese.

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