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E lectronic and E lectrical Engineering. Uniuer.iity College London, Torrington Place, London W C IE 7JE, U K

Abstract

W e report the growth o f single- and multilayer films o f titanium dioxide (TiO^) on Si(lOO) substrates at low temperature by a new photo-induced so l-g el process. Polymeric T iÛ2 sols prepared by the hydrolysis and condensation o f titanium isopropoxide were spin-coated on the silicon substrate and then irradiated by an Xe^ excimer vacuum ultraviolet (V U V ) lamp operating at a wavelength o f 172 nm. Films with thicknesses between 10 and 200 nm were achieved readily by this technique. The effects o f spin speed, irradiation time and substrate temperature on the films formed have been studied. The chemical bonding changes in the thin films were analysed by Fourier transform infrared spectroscopy (F T IR ), while thickness and refractive indices were determined by ellipsom etry. F T IR confirmed the removal o f H2O and O H groups after V U V irradiation for 10 min at 300 'C. The refractive index reached a value o f 2.4, which compares favourably with the value o f 2.58 recorded for the bulk material, while optical transm ittance values in the visible region o f the spectrum between 85 and 95% were obtained on quartz substrates. © 2000 Elsevier Science S.A. A ll rights reserved.

K eyw ords: E llip s o m e tiy ; E x cim er lam p; F T I R ; P h o to -in d u c e d p ro ce ssin g ; S o l- g e l; T ita n iu m d io x id e th in film s

1. Introduction

TIOt is currently attracting much interest as a nanocrystalline material for a range o f potential applica­ tions, such as in dye-sensitised solar cells [1,2], as an anti-reflective coating for solar cells [3,4], as an insulator for memory devices [5], as a catalyst [6], and as a humidity and fibre-optic chemical sensor [7].

TiO i has been deposited by many techniques includ­ ing pyrolysis [8], electron-beam evaporation o f TiOz [9], pulsed laser deposition [10], m etal-organic chemical vapour deposition (M O C V D ) [11,12], molecular-beam epitaxy [13], atomic-layer deposition (A L D ) [14,15], hydrothennal techniques [16,17], plasma-enhanced chemical vapour deposition (PECVD ) [18], reactive sputtering [19,20] and sol-gel processes [21-23],

S ol-gel processing via the polymerisation o f m etal- organic com pounds involves ceramic synthesis by the hydrolysis o f metal alkoxides, resulting in metal oxides or hydrated oxides [21 ], A gel with a continuous network is thus fonned. Thin, transparent, multi-component oxide layers can be readily deposited on various sub-

* C o r r e s p o n d in g a u th o r .

E -m ail address: n k a liw o h ^ rlee.u c l.a c .u k ( N . K a liw o h )

strates by this process. A firing step is required to displace organic (in this case propyl) com pounds.and the hydroxyl groups. The m ethod has been widely used to fabricate high-quality ceramics and glasses [24]. Advantages o f this technique as compared with other processes include low cost, relative simplicity, and easier control o f the com position o f the deposited films [7,24], In this work, TiOz produced from titanium isoprop­ oxide [T i(O C3Hz)4] by an excimer-lamp based photo­

induced sol-gel process has been studied. The interest in ultraviolet (U V ) processing and studies o f the dynamics o f photochemical reactions have burgeoned in recent years [25], Photochemical processing by exci­ mer lamps has an added advantage o f a low thermal budget due to the ability o f processing at low temperatures.

2. Experimental details

The excimer lamp system used comprises four lamps, employs the dielectric barrier discharge principle and is described m ore completely elsewhere in the literature [26], Polymeric sols were prepared by the hydrolysis

0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. Pit: 8 0 2 5 7 - 8 9 7 2 ( 9 9 ) 0 0 6 1 7 - 9

N. K aliw oh et al. / S u r fa c e a n d C oalini;s Technoloi’y 125 ( 20 0 0 ) 424 -4 2 7 4 2 5

and condensation o f T i(0 0 3 1 4 7 ) 4 in C^H^OH in the

presence o f HCl as shown in Fig. 1. The resulting transparent mixture was sealed in a container and stirred continuously at room temperature. The solution was spin-coated on to silicon wafers at various speeds, first after 2 h, then at 24 h intervals, and was analysed with an u ltraviolet-visible (U V -V IS ) spectrophotom eter (Lam bda 14), where the polym erisation process was m onitored by observing changes in the absorbance spectra. Thickness and refractive index were measured by ellipsom etry (R u d olp h A uto El II) and Fourier transform infrared spectroscopy (F T IR ) was used to m onitor the absorption o f OH and C H3 groups to

provide inform ation on the presence and removal o f H2O and propyl radicals. Initially, substrates were irradi­

ated with 172 nm radiation from an Xe^ excimer lamp for different times and at temperatures o f 80, 1 2 0 and

N o i r r a d i a t i o n O H 5 min 10 min I 172 4000 3400 2800 2200 1600 1000 W a v e n u m b e r (cm'^> 400 F ig . 2. F T I R sp e c tr a o f film s ir r a d ia ted a t a te m p e r a tu r e o f 8 0 ° C fo r e x p o s u r e t im e s o f 0 , 5, 10 a n d 2 0 m in u s in g 172 n m e x c im e r la m p r a d ia tio n . Transparent so! Irradiation Spin - coating Ti02 Films irrad iatio n s c m u h 01 > 5% 120“C 3400 2800 2200 1600 1000 400 4000 F ig . 1. F l o w c h a r t fo r th e p r e p a r a tio n o f T i 0 2 film s b y p h o t o - in d u c e d s o l - g e l p r o c e s s in g . W a v e n u m b e r (cm'^) F ig . 3. F T I R sp e c tr a o f film s ir r a d ia te d a t d iffe r e n t te m p e r a tu r e s fo r a fix e d e x p o s u r e tim e o f 10 m in , u s in g th e 172 n m V U V ir r a d ia tio n .

200 and 300°C. Subsequent irradiation was carried out at 300°C for 10 min. The optical properties o f the films prepared on quartz and irradiated by the 172 nm radia­ tion were investigated using a U V -V IS spectrophotom e­ ter. T iÛ2 multilayers were subsequently prepared on

Si ( 100) in a step-wise process, with thickness and refrac­ tive index determined for every step.

3. Results and discussion

The synthesis o f titanate polym ers by the so l-g el process involves the hydrolysis and condensation o f the starting alkoxide, in this case titanium isopropoxide, in

4.16 A’. Kdlm oli et ul. Surface am! Coatiugs Technology 125 ( 2000) 424 -4 2 7 th e I'ollovving r c a c l i o n s : H y d r o l y s i s T i ( O C , , H ^ ) j + H / ) ^ T i ( ( ) C + C o n d e n s a t i o n T i ( ( ) C , H - ) _ , - f T i ( O C . , H ~ ) , O H ^ T i / ) ( O C j H , ) , + C 3 H , O H W a t e r c o n d e n s a t i o n T i ( ( X - , H - ) , O H + T i ( ( ) C , H - ) , O H ^ T U O l O C ^ H . j e + H , 0 . (1) (2) (3)

Fig. 2 slio w s the F T IR spectra o f film s irradiated at a tem perature ot'XO C. C h an ges tak in g place w ithin the gel under the in llu en ce o f irradiation can be observed by c o n sid erin g the sp ectral range .'^100-3400 c m “ ^ w here the O H stretch in g v ib ra tio n s o f w ater and O H grou p s app ear, at 550 cm ' w h ere Ti O v ib ration s occu r and I 30 0 -1 500 cm ‘ (d u e to C H 0 A s the irradiation tim e is increased from 0 to 20 m in. the in ten sity o f these b a n d s decreases. T h e spectra in the 3 1 0 0 -3 4 0 0 cm ‘ range sh o w the d eh y d ra tio n and d eh y d ro x y la tio n o f the polym er. T h e increase in tem perature betw een 0 and 80 C results in lo ss o f w eight th rou gh the ev a p o ra tio n o f adsorb ed w ater and the loss o f hyd roxyl groups. T he ap p earan ce and grow th o f new a b so rp tio n features b elow 730 cm ' in d icate the presence o f Ti — O b o n d s [27]. N o significan t ch a n g es were observed up on raising the tem p erature to 200 C. At 3 0 0 "C, the O H peak

I n , I IS,I I ' in Si) I I ( 'I s IN II in b e i o f la v e r s

I i;j. 4, D ependence o f tlie ttiickness ( d ) an d refraetive inciex (/;) o f

T'KN after irrad iatio n at 172 nm for 10 m in on spin deposition speed an d n um ber o f lavers.

disap peared, and the Ti O peak shif'ted to sh orter w aven u m b ers due to d en sifica tio n o f the films. F ig. 3 sh o w s the F T IR spectra o f these film s irradiated at different tem peratures for a fixed exp osu re tim e o f 10 m in. T he spectra sh ow that c o n v e rsio n o f the gel in to a co m p lex T iO , film takes place after irradiation for 10 m in at 3(KFC, w ith the rem oval o f O H and C H3

grou p s being achieved.

T iO i m ultilayers were d ep o sited at 2000 and 3000 rev m in " ' from the sam e so lu tio n w ith a 24 h tim e difference. T h ese co a tin g sp eed s w ere ch o sen a s h o m o ­ g en e o u s film s are form ed at these sp in -on speeds. R efractive index va lu es ranging from 2 to 2 .4 w ere

m easured for m ultilayers co a ted at 2000 and

3000 rev m in " ‘, after each irradiation step, as seen in Fig. 4. T h ese valu es com p are w ell w ith th o se for bulk m aterial in rutile single crystals, w h ich is reported to be 2.58 at 633 nm [28].

T he m ultilayers d ep osited at the sam e speed exh ib ited a higher refractive index than the m on olayers. R a o and M oh an [9] also observed an increase in refractive ind ex with an increase in thickness o f their T iO i films. T h ick n esses o f up to 200 nm w ere achieved for our m ultilayers, in d icatin g that a predeterm ined th ick n ess o f film can therefore be achieved readily by e m p lo y in g this d e p o sitio n tech n iq u e to produ ce layers in stack form . T h e thin film d ep osited on qu artz crystals sh ow ed g o o d o p tical properties in the 2 5 0 -1 1 0 0 nm w av elen g th

II so c 41) II I 1011 (Oil W a v e l e n g t h ( n m )

Fig. 5. Spectral tran sm ittan ce o f l iO , films d eposited on q u a rtz after irrad iatio n at 172 nm for 10 min.