ELSEVIER Thill Solid Films 281·-2B2 (1996) 235-238

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Characterization of niobium oxide electrochromic thin films prepared byreactive d.c. magnetron sputtering

Kazuki Yoshimura a,*, Takeshi Mild 11, Saburo Iwama b, Sakae Tanemura a• Multifunctional Material Science Department, National lndustno! Research Institute o/Nag(lya, Nagoya 462, Japan

b Department ofElectronics Engineering, Daido Instill.te of1eclmology, Nagoyd 457, Japan

Abstract

Niobium oxide electrochromic thin films were prepared by reactive d.c. magnetron sputtering, and their physical properties, such as surfacestructure, composition, optical transmittance, reflectance and absorption, were studied. The surface morphology ofthe heated sample is verydifferent from that ofthe unheated sample. Auger electron spectroscopy (AES) measurements show that the composition ofthe amorphousfilm isclose to Nb20S as well as the crystallized sample. The crystallized and amorphous samples show very different absorption behavior.The band gap is estimated to be3.41 eV for the crystallized sample and 3.45 eV for the amorphous sample.

Keywords.' Amorphous; Crystallized; Niobium oxide; Surface morphology

1. Introduction

In previous papers [1,2], we have reported that niobiumoxide films prepared by reactive d.c, sputtering within a cer­tain oxygen flow rate range show reasonably good electro­chromism. In particular, well-crystallized films consisting ofpolycrystalline Nb20 5 exhibit a much wider optical modu­lation range and better durability than amorphous films. Thistendency is very different from thatof W03 films. Crystal­lized W03 films show poorer electrochromic behavior thanamorphous films [3]. This implies that the electrochromicmechanism of niobium oxide is different from thatofW03•

Several studies onclectrochromic niobium oxide films pre­pared by thermal oxidation [4,5], anodization [6,7], chem­ical vapor deposition (CVD) [8], sputtering [9] and thesol-gel process [10] have been reported. However, theirphysical properties have not been investigated in detail. Inthis paper, to clarify thedifference between the electrochro­mism of crystallized and amorphous films, the surfacemorphology, composition and optical properties wereinvestigated.

2. Experimental procedures

Niobium oxide films were prepared by reactive d.c. mag­netron sputtering with a metallic niobium target. Thedetails

• Corresponding author.

0040-6090/96/$15.00@ 1996 Elsevier Science SA All rights reservedPI/S0040-6090( 96) 08640-3

of the preparation conditions have been described elsewhere[1,2]. The flow rates ofargon and oxygen gaswere 100 seemand 10seem respectively. The total pressure was 2 Pa andthe induced power was 80W.

Thesubstrates were 11 mm X 11 mm X 1 mm glass plateswith a conductive coating of indium-tin oxide (ITO). Thecoating was 300 nm thick with a sheet resistivity of 10ilJO.

The surface morphology was observed by atomic forcemicroscopy (AFM) using a Topometrix TMX·2000.

The composition of theas-deposited films was studied byinsitu Auger electron spectroscopy (AES). Deposited filmswere transferred from the; sputteringchamber tothe analyzingchamber under ultrahigh vacuum conditions. AES measure­ments wereperformed using adouble-passCMA with coaxialelectron gun (PHI 15-255G) in a vacuum of5 X 10-7 Pa,

Directional hemispherical spectral transmittance with anincident angle of 12" in thewavelength range 300-2500 omwas measured using a Hitachi U3400 spectrometer.

3. Results and discussion

Two kinds of sample were prepared by sputtering. Onewas sputtered on a heated substrate at a temperature of500 "C, The other was deposited on an unheated substrate.The thickness ofboth samples was about 500nm, determinedbya surface profilometer (RTIl Talysurf).

The surface morphologies of the as-sputtered niobiumoxide films were studied using AFM. AFM images of the

236 K. Yoshimura etal. /tu« Solid Films 28/-282 (1996) 235-238

heated and unheated samples are shown inFig. 1. The surfacestructures ofthe two samples arevery different. The unheatedsample exhibits a compact granular structure that is oftenobserved for sputtered films. The average grain size is 150­300 nm in diameter. This is much larger than that of a sput­tered nickel oxide film on an unheated substrate [11]. Thegrain size of theheated sample is much smaller, 50-100 nmin diameter. Some grains are fused. Thedifference betweenthe diffusion properties of Li ions in crystallized and amor­phous niobium oxide films [2] may becaused by the differentsurface morphologies.

According toThornton's structural model [12], sputteredfilm structures are d~ssified using a parameter TITffit whereT is the substrate temperature and Tm is the melting point ofthe designated material. Themelting point of NbzOs is 1793K.This means that TITm= 0.18 forthe unheated sample amiTITm=0.43 for the heated sample. The Tlrm value of theunheated sample belongs to zone 1 (TITm=O.l-Q.3) in thismodel. Thestructure observed has the typical shape of zone1materials. On the other hand, theTITmvalue ofthe unheatedsample belongs to zone 2 (TITm=0..3-Q.5). TIM structureobserved also exhibits the characteristic features oi''l'::!.~(..vialsin this zone. These results indicate that Thornton's model isapplicable to the structure ofsputtered niobium oxide films.

X-Ray diffraction (XRD) measurements show that thesample with a substrate temperature of 500°C is well crys­tallized [1:1. Theobserved peak positions coincide with thoseofNbzOs (JCPDS file 28-317).Thus th, heated sample con­sists of Nb20 S' The XRD pattern of the unheated sample.l~:,..~WS no structure in the deposited film, i.e. this film is",1·~;'n'Phous. ByXRD measurements, it isimpossible todeter­T,i;,!(C whether unheated films arecomposedofNb20sorotherr .obium oxides.

Therefore wedetermined the composition ofas-depositedfilms by in situ AES measurements. The acceleration voltageof the primary beam was 3 keV and the sample current wasabout 100 nA. N(E) curves were recorded using a pulsecounting method. The composition ratio of Nb to°wascalculated from the intensity ratio of the peak height of theNbpeak (E= 164 eV) to that of theopeak (E=503 eV) inthe Auger electron spectra.

Theresulting composition ratio ofNb to0 is 1:2.7 for thecrystallized sample and 1:2.4 for the amorphous sample.These results indicate that the amorphous film also consistsof NbzOs. The crystallized sample is slightly oxygen rich.This may be related to the fact that the grain size of thecrystallized sample is sma!ler than that of the amorphoussample asobserved in the AFM images.

Thecoloring and bleaching of the prepared samples wasperformed by cyclic voltammetry using 1 M LiCI04+propylene carbonate as electrolyte. When the sample wasremoved from the voltammetric cell, the colored sample wasquickly bleached by exposure to theatmosphere. Hence thecolored sample was rinsed with ethanol and dried inanargonatmosphere.

Fig. 2 shows the change in transmittance, reflectance andabsorption in the bleached state and colored state of crystal­lized and amorphous niobium oxide films by ex situ opticalmeasurements. TIle integrated solar transmittance T( sol) andsolar reflectance R(81"1) [13] were calculated from the trans­mittance and reflectance spectra in Fig. 2 respectively. Theresults are summarized in Table 1. As yet, ex situ opticalproperties over a wide wavelength range forthe colored sam­plehave not been reported.

Both crystallized andamorphous samples were quite trans­parent. Thesolar transmittance was 74.9% forthe crystallizedsample and 68.7% for the amorphous sarnple. Because thesolar transmittance of precoated ITO is 64.7%, thetransmit­tance of the deposited niobium oxide isquite high.

The transmittance change between the bleached state andcolored state was much larger than the reflectance change.This means thattheelectrochromism of niobium oxide is ofabsorption type as observed for most other electrochromicmaterials.

Theabsorption change of the crystallized sample is quitecharacteristic. Within the wavelength range of500-2000 nm,the absorption isflat. 'The coloredstate ofW03 has anabsorp­tion maximum around 800 nm, This implies thatthe absorp­tion mechanism of niobium oxide is different from that ofW03•

The absorption change of theamorphous sample isdiffer-ent from that of the crystallized sample. Theabsorption dif-

2!Dlnm

l000nm

OIU' I

Onm , lDJrvn

(a) heated2D1lrn

(b) unheatedFig. 1. AFM images ofas-oeposited samples: (a) heated sample; (b) unheated sample.

K. Yoshimura etaI./Thin Solid Films 28/-282 (/996)235-238 237

100 100

g 80 i 80

j 60 j 60

J40 i 40

20 ~ 20

0 0

500 1000 1500 2000 2500 500 1000 1500 2000 2500Wllv.lenalh rnm) W~vlll.nglh (nm)

100 no

~ 80 t 80 bleached.....

j 60 j GO

colored 40== iGIa: a: 20

0

2500 500 1000 1500 2000 2500Wavelonglh (nm)

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1 80 g 80.....~ 60 ~ 60

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J !-.: 20 20

0 0

500 100,,) 1500 2liOO 2500 500 1000 1500 2000 2500Wavelenglh (nm) Wavelengtn (nm)

(a) crystallized (b) amorphousFig.2.Exsitutransmittance, reflectance andabsorption in thebleached state and colore'! stateofcrystallized and amorphous niobium oxide films.

Table 1Integrated solar transmittance TCsol) andsolar reflectance R(sol) in the bleached andcolored states for thecrystallized and amorphous samples----------_._--------------------------Sample TCsol) (%) (bleached) T(sol) (%) (colored) R(sol) (%) (bleached)

Crystallized sampleAmorphous sample

74.968.7

17.525.2

13.620,9

4.913.0

o+ __..,._..':A~~_,. "'"3.0 3.2 3.. 3.6 3.8 4.0

Photon Energy (eV)

Fig. 3.Relation between theabsorption coefficier.tandtheenergy ofincidentphotons forthecrystallized sample and amorphous sample.

ference between the bleached and colored states decreaseswith incr.easing wavelength. Such a feature is similar to thatof thenickel oxide system [11].Thus it ispossible that otherabsorption species areformed in the colored niobium oxide.

The optical band gaps of the Nb20S filnc, were estimatedfrom the optical absorption measurements for 100 nm thicksputtered films on glass substrates. Fig. 3 shows theabsorp­tion coefficients of thecrystallized and amorphous samplesas a function of thephoton energy. The extrapolation to theabscissa shows an optical band gap Eg of 3.41 eV for thecrystallized sample and 3.45 eV for the amorphous sample.The band gap of the amorphous sample is about 1% widerthan that of the crystallized sample.

Thereported values of the band gap exhibit a wide rangefor samples obtained by different preparation methods.Spectrophotometric measurements on anodic films yieldedEg =3.2eV [7]. Films prepared by the CVD method exhib-

30

25

r 20~

~b 15lCl!

l 10

5

238 K. Yoshimura etal./ Thin Solid Films 281-282 (1996) 235-238

ited E~=3.5 eV [8]. Maruyama and Arai [9] estimated thatthe band gap of r.f sputtered films was as wide as 3.8 eV.Belozclov et al, [7] suggested that lattice disorder leads toband gap widening. According to this explanation, our sput­tered samples are less disordered than r.f sputtered films.This may he one reason why oursamples show much betterelectrochromism than r.f stuttered films.

4. Conrlusions

The surface morphology ofthe crystallized and amorphoussamples isvery different. Thornton's structural model seemsto explain the difference well. In situ AES measurementsshow that sputtered amorphous films consistofNU20S aswellas the crystallized films, The absorption behavior of Nb20S

isvery different from that ofW03• Inaddition, the absorptionbehavior ofthe crystallized sample isvery different from thatof the amorphous sample. The band gap is estimated to be3.41 eV for the crystallized sample and 3.45 eVfor the amor­phous sample.

Acknowledgements

This work was financially supported by grants from the"NewSunshine Project" of the Agency ofIndustrial Scienceand Technology, MIT!.

References

[1] K Yoshimura, T.Miki, S.lwamaand S.Tanernura, Ipn. J.Appl.P1Iys.,34 (1995) LI293.

[21 ~. Yoshimura, T. Miki, M. Tazawa, P. Jin and S. Tanemura, Sol.Energy, tobepublished.

[3} M. Kitao.~, Yamada, T. Yoshioka, H. Akram and K. Urabe, Sol.Energy Matt I:' Sol. Ceffs, 25 (1992) 241.

[4] B.Reichman and A.Y. Bard, J. Electrochem: Soc., 127(1980) 241.[5] R. Cabanel, J. Chaussy, J. Mazuer, G,Delabouglise, J.C. Joubert, G.

Barra! and C.Montella, J. Electrochem. Soc.• 137(1990) 1444.[6] C.K. Dyer and J.S.L, Leach. 1. Electrochem. Soc.• J?5 (1978) 23.[7] V.V. Belozerov, Y.I. Malyuk and L.I. Skatkov, u: re« Fiz., 59

(1989) 172.[8] M.T. Duffy, C.C. Wang, A. Waxman end K.H. Zaininger, J.

Electrochem. Soc; /3/ (1969) 234.[9] T. Maruyama and S.Ami. Appl. Phys. Leu; 63 (1993) 869.

[10] C.O. Avellaneda, M.A. Macedo, A.O. Plorer.lno and M.A. Aegerter.Proc. Optical Materials Technology for Energy EJticieney andSolarEnergy Conversion XIII: Chromogenics for Smart Window. 1728(1994) 40.

[ In K. Yoshimura, T.Miki and S.Tanemura,Jpn. J. Appl. Phys.•34(1995)2440.

[12] I.A.Thontton,J. Yac.Scl. Technol.i Il (1974) <'66.[] 3] lS.E.M. Svensson and e.G. Granqvist, Appl. Ph)!s. Lett; 49 (1986)

1566.