Comparing between XRD specfra of WO 3 films and WO 3 powder, we can not deduce any difference m crystalline structure. And one possible reason is Uie existence of nano phases.. It me[r]
Trang 1VNĨU J o u n ia l o f S c ic n c c , M aứ icm atics - P h ysics 25 (2 0 0 9 ) 4 7 -5 5
Survey o f WO3 thin film structure built on ITO /glass
substrates b y the Raman and X R D spectroscopies
Le Van N g o c ’’*, Tran Cao V inh’, Le Quang T oai’, N guyen Due Thinh'
Huynh Thanh Dat^, Tran T u a n \ Duong A i Phuong'
‘ University o f Science, Vietnam N ational University - Ho Chi Minh city, 227 Nguyen Van Cu, Vietnam
^ Vietnam N ational University - Ho Chi Minh city Link Trung, Thu Due, Vietnam
Received 17 January 2009; received in revised form 12 March 2009
Abstract Tungsten oxide film was deposited on ITO-coated glass by using RF magnefron sputtering mcUiod from WO3 ceramic target Thin film preparation - process took place in Ar + O2 plasma The dependence o f tungstai oxide film stìTỉCture on experiment conditions was
investigated by X-ray dinraction (XRD) Raman spectroscopy In this paper, we considered ù at the ứiickness o f ITO layers about 150 nm to 350 nm clearly effects on ứie Raman and XRD
spectrograms o f WO3 films
Keywords: W 03 sưucturc, W 03 /ITO/glass, Raman spccưoscopy.
1 In trod u ction
W O 3 thin film s have been studied for a long rime due to their unique properties and potential applications A nd the m ost prom ising application o f W O3 thin film s is electrochrom ic devices based
on electrochrom ism , in w hich optical properties W O 3 alter reversft)ly under electrical bias applied [ 1- 3] Moreover, recently W O3 thin film s have been studied to fabricate toxic gas sensors, such as N 0 « H2S , NH3, CO and som e popular others like H2, O2, O3, CI2, SO2, CH4 [4-7] Both electrochrom ism and gaseous sensitization are based on the reversible diffusion o f particles along the vacant tunnels o f
W O 3 perovskite structure Thus, having large and oriented vacant tunnels w ill be a great advantage Furthermore, many m ethods are used to prepare W O3 thin film s, such as sputtering [8- 1 1 ], sol - gel [12], spray pyrolysis [13-14], anodizing technique [15], thermal evaporation [15-21] And different prqjaration m ethods have respective advantages in film quality and application.
Besides optical and electrical properties, W O 3 crystalline structure has been studied by utilừ ing
X R D and Raman Spectroscopy X R D surveys focus on 20° - 25° range o f diffraction angles and Raman sp ecữ oscop y surveys focus on 2 0 0 c m ' - 1000 cm ' range o f w ave number.
In this paper, WO3layers w ere dq)osited on ITO/glass subsữates The thickness o f ITO layers is measured approxim ately 150, 20 0 , 25 0 , 300, 350 nm, respectively From X R D specứogram s, we considered that the thickness o f ITO coaters clearly effect on W O 3 crystalline structxire In order to
Corresponding auứior T el: 0908283530
E-mail: lvngoc@phys.hcmuns.edu.vn
4 7
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understand what occurred inside and whether nano particle phases exist, w e used their Raman specừoscopy And 60 0 cm ' - 1000 cm ' range was analyzed into different basic vibration With samples with ITO layer about 300nm thickness and more, in Raman spectrum there is an odd peak at
680 cm ', w hich could b e related to nano phases However, the absence o f 950 c m ' refuses that assumption The origin o f this peak w ill be focused on in this paper.
2 E xp erim en tal
In this research, ITO and W O3 layers were prq)ared by magnetron sputtering in (O 2 + Ar) plasma
O xygen and argon gases with high purity (99.999% ) were used in dqjosition processes O iư sputtering chamber was evacuated down to 10'^ to ư by using turbo purtp before inư oducing gases ITO layers were dqjosited on glass substrates by DC magnefron sputtering with their thickness about 150, 200,
250, 300, 350 nm respectively Then W O3 layers were dqposited on ITO/glass by RF magnetron sputtering The pow er is 100 w and the dqjosition time is about 30 minutes.
After dqjosition, W O 3/IT 0 /g la ss system s were annealed in the atmosphere at 400°c tenperature during four hours The crystalline structure o f W O3 thin film s was investigated by X R D patterns using
Cu Kj, radiation at 1.5406 Â w avelength and Raman spectroscopy using He - N e excitation (632.8 nm) In order to analyze broad peaks, included many basic vibration m odes o f Raman spectrum, we used Origin 7.5 program with Gaussian function This information giv es us exact evaluation o f the existence o f different phases in our films.
3 R esults and d iscu ssion
3.1 The effect o f the th ic h ie ss o f ITO layer on X R D spectrum
A ll W O3 thin film s, created in these experimental conditions, were transparent in visible region In
o f 20 diffraction angle due to the existence o f three highest peaks Figure 1 show s X R D pattern o f
W O3 powder sa n p le with tlưee peaks with strongest magnitudes (001), (020), (200) They correspond with 20 = 23.29°, 23.77°, 2 4 5 1 “ with lattice plane distances d = 0 382 nm, 0.374 nm, 0.363 n n \ respectively Therefore, our W O3 powder sa n p le has a m onoclinic structure (m -W Os) with a =
=90°, 3 = 90.15°, a = 0.7285 nm, b = 0.7517 nm, c = 0.3835 nm [22] and tw o highest peaks, located at (001) and (200) (international JCPDC database, JCPDC 5 - 363).
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9 0 0 0
2 5 0 0
2 0 0 0
I '
•i
0 0 0
5 0 0
0
Fig, 1 XRD spccưuin o f ni-WOs powder.
w 0 ^(001)
W 0 ^ ( 2 0 0 )
ỵ' 7 / 7 / W O3/ I T O 3 5 0 n m
W O3/ I T O 3 0 0 n m 7 ^ y - yAN O3/ I T O 2 5 0 n m
" W O j / l T O 2 0 0 n m O ^ / I T O 1 5 0 n m
O j p o w d e r
2 t h e t a ( d e g )
Fig 2 XRD spcclra o f WO3 films on layers ITO wiUi difTcrcnt ưũcknesscs.
Figure 2 is X R D spectra o f film s on layers ITO with different thicknesses However their peaks distribute in such a sm all range o f the angle 20, that we couldn’t confirm whether our film s have a
m on oclin ic structure (with parameters a = Y = 90°, p = 90, 15“, and a = 0,7285 nm, b = 0,7517 nm, c =
0 ,3 8 3 5 nm) or a orthorhombic one (0-W O 3 with a = p = Y = 90“, a = 0,7341 nm, b = 0,7570 nm, c =
0 ,3 8 7 7 nm) bccause the values aren’t clearly distinctive.
A nalyzin g figure 2, w e recognized that ITO layer with thickness about 150 nm, X R D shows a sharp peak (2 0 0 ), a cco n p a n ied by a weaker one (001) B etw een these peaks was a even weaker peak (0 2 0 ), like a shoulder o f (200) font W ith an increase in the thickness from 150 nm to 350 nm, X RD spectra expose a gradual decrease o f the magnitude o f peak (200) and a raise o f peak (001) Moreover, peak (020) is sh ow n obviou sly in the case o f 250 nm W hen the ITO layer have a thickness about 300
nm, a growth o f WO3 sh ow a great anom aly in orientation due to the appearance o f peaks in larger
diffraction angles W ith 3 5 0 nm thickness ITO player, W O3 film preferentially grows along dừection (001) with larger lattice plane distance (0.4001 nm) This value is nearly equal to (001) lattice plane
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distance o f tefragonal sừucture (t-W Oj) Therefore; w e assum ed that the crystalline structure o f this
W O3 film is tetragonal (w ith a = 0 = ^ = 90", a - b = 0.525 nm, c = 0 3 9 2 nm) W ith this structure, vacant tunnels along c ~ axis are naưow er than these ones o f m -W 03 and 0-W O 3 W O 3 film preferentially grow along direction (001), (200), however, both o f these tw o p ossib le grow th directnons lead us to the conclusion that vacant tunnels grew perpendicular to film surface A nd w ith preferentially grow ing along (200) direction o f 0-W O3, vacant tunnels achieved the largest s iz e o f
WO3 “ perovskite
In this research, all the peaks o f W O3 film in X R D spectra are shifted to sm aller diffraction angles than these ones o f pow der sa n p le This result show s that the lattice plane distance increases due to a con p ressed sfress, because W O 3 has a coefficient thermal expansion sinaller than glass does and because o f the heteroqjitaxial growth o f film s W O 3 in w hich the parameters o f plane ITO (440) are slightly larger than the ones o f W O3 planes The relation betw een shifts o f X R D peaks and total íĩlm stress is given by equation:
^ _ E M 2 9 )
Where ơf is total film stress, 0 is Bragg diffraction angle, E is Y oung m odulus, V is Poisson
coefficient A(20) w ill get a minus value if the total film stress is com pressed stress [23], Thus due to this thermal stress, in order to survey WO3, we have combined the results from both XRD and Raman specfrum investigarions B esid e that, from XRD patter, the gram size o f W O 3 film w ere determ ined by Scheưer equation and all o f them valued in 30 nm to 35 nm.
3.2 M icro - Raman Studies
Due to structiưal m odifications o f W O3 film s, deposited on o f ITO layers with Sifferent thicknesses, w e investigated their Raman spectra to find out more helpful information W e divided ITO layers into two groups, basing on their thicknesses: 150 - 250 nm group and 3 0 0 - 3 5 0 mil giou^j.
3.2.1 Raman spectrum o f WO} thin f i l m on r r o layers with thickness, altering from 150 to 250 run
Figure 3 show s X R D an d R am an sp ecừ a o f WO3 p o w d er s a n p le a n d WO3 film s, d ep o sited on ITO
layers/150 nm, 200 nm and 2 5 0 nm thickness Generally, these film s have ratios I<2oo/I<ooi) in X R D pattern quite greater than this one o f powder sa n p le Raman spectra o f all three s a n p le s show sharp peaks, sited at 26 5 6 - 2 6 9 7 c m ', 703.8 - 709.9 c m ' and 7 9 9 9 9 - 803.5 c m ‘ A ll these peaks are characterized for crystalline phase o f W O 3 The Raman peak at 2 5 6 6 cm ' indicates the deformation vibration of o - w - o bond and 600 - 900 cm ' region relates to stretching vibrations of w - o bonds [24] The Raman peak at 9 5 0 cm ‘, atứibuted to w = o boundary bonds does not exist S o w e assumed that the surface and volum e rate is negligftilc.
In the other hand, three sharp peaks, characterizing WO3 crystalline phase, located at 263 cm ',
709 cm ', 802 cm ' do not indicate any difference in structure betw een WO3 film s and WO3 pow der
sa n p le and betw een one film to another These numbers show no difference in structure between WO3
powder and W O3 film s H ow ever, w e could not elim inate a probability o f orthorhombic phase in these films because the parameters o f orthorhombic and m onoclinic primary cells are nearly the same.
Trang 53.2.2 Raman spectrum o f wo 3 thin films on ITO layers with thickness, altering from 300 to 350 nm
L V N g o e et al / VNU Jou rn al o f Science, M ath em atics - P h ysics 25 (2009) 47 -5 5 51
( 2 0 0 )
w o / IT o I S O n n i
( 0 0 1 )
2 T h * u ( d » s )
Fig 3 XRD patterns and Raman specưum
o f WO3 thin films on ITO layers with
difTcrent thichnesses
a) 150 nm ITO; b) 200 nm ITO; c) 250 nm ITO; d) WO3 powder.
Figure 4 shows the Raman and XRD spectrum of W O 3 films, deposited on 300 nm and 350 nm ITO layers and W O 3 powder sanple From XRD specừa of WO3/ ITO 300 nm film intensity of peak (0 0 1) exceeds intensity of peak (2 0 0) And W O3 film, deposited on 350 nm ITO layer preferenilly grew in (0 0 1) dừection.
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2 Theta (deg)
c)
Fig 4 Raman spectrum and XRD patterns o f WO3 on ITO layers witli
dino-ent thicknesses a) 300 nra ITO; b) 350 nm ITO;
c) WO3 powder
Raman shift (em ')
Comparing between XRD specfra of WO3 films and WO3 powder, we can not deduce any difference
m crystalline structure However, m the Raman spectrum of WO3 /ITO 300 ran (fig 4a) besides tw o
characteristic peaks of crystalline structure, which are located at 267.66 cm ' of the deformation mode and 802.5 cm' of covalent bonds w - o, there is only one clear modification, conparing to Raman specfra of film on thinner rro layers The only difference is a conpositíon of two peaks, one at 680 cm ' and the other at 709 cm' And one possible reason is Uie existence of nano phases [25]
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Nevertheless, like Raman spectra of WO3/ITO 150 nm, WO3/ITO - 200 nm and WOj/ITO - 250
nm, 950 cm ' peak, which coưesponđs to w = o boundary bonds, does not appear due to the annealing proces;s It means that the ratio of surface to volume is negligiTile And some calculations, basmg on formula Scheưer also shows that the size of grains in all WO3 films is about 30 - 35 nm Moreover the annealing process and the breakage o f the double bonds w = o also result in the limit o f the shift to the 68 0 cm ' Thus, m the films WO3/ITO 300 nm nanocrystal phase does not produce any strong peak about 680 cm ' Therefore, we believe that the appearance of peak closed to 680 cm ' have another origin, such as phase tetragonal (t-W Os) or phase orthorhombic (0-W O 3).
In order to understand more about the mentioned peak, we used program Origin 7.5 to analyze the doublet 701.7 cm ' of thin film WO3/ITO 300nm and investigated XRD and Raman specứa of WOs/riO 350 nm Analyzing the specữa of WO3/ITO 300 lun, we received one peak which is not sharp and corresponds to the vfljration mode of WO3-H2O The two other sit at 703 cm ' and 678 cm ' And the best fit was get by considering the peak 802 cm ' as a combinarion of two sqjarated ones at
803 cm ' and 797 cm ' as shown in figure 5 Moreover, in some previous researches of other authors, there were only two peaks in the range from 600 to 900 cm ', which are attributed to covalent vft)ration
of chemical bonds w - o in the octahedral WOe around w centers of lattice Therefore, the existence
of both four peaks 803 cm '; 797 cm '; 703 cm ' and 678 cm ' proves that in our thin films, there are two tj-pes of crystalline structures: m-WOs phase with 803 cm ’ and 703cm ' and t-WOs phase or 0-
WO3 phase with the two characteristic peaks at 797 cm ' and 678 cm '.
Ram an shift (cm ’^) Fig 5 Raman spccưa of WO3 thin film/ITO 300 nm/glass subsừate.
However, from the experiments of E.Cazzanelli [26], the phase transirion from orthorhoiribic pha.'e to tetragonal phase is acconpanied by the disappearance of peak at 700 cm ', only peak at 800
cm remains So, in W O 3/ITO 350 nm the t-WOs phase could not exist, the two possftle ones are
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W O 3 and 0- W O 3 Therefore, w e b elieved that a pair o f peaks at 803 c m 708 c m ' correspond to nio-
W O 3 phases and the another pair goes for the 0-W O 3 phases In that case, the vibrational frequencies
o f m-WOa are quite higher than these o f 0- W O 3 A nd the reason is that parameters o f crystalline structure o f m-WOa are sm aller than that o f 0-W O 3 or ứie force constant in W -O bonds in the former is larger than in the later phase.
Raman shift (cm'^)
Fig 6 Raman spccưa o f WO3 tliin film/ITO 350 nin/glass subsưatc.
X R D spectrum o f W Oj/ITO 35 0 nm sam ple show s that W O3 film grow s alon g (001) direction preferentially, (0 20) and (200) peaks are weakened down to the font o f (0 0 1 ) peak In these films W O 3 particles are w ell crystallized For a Raman specừ um o f that film , peaks, characterizing crystalline phases, such as 7 9 9.4 cm ', 688.3 cm ' and 25 3 2 6 cm ' have appeared and the last tw o o f them are shifted to sm aller w ave numbers, co n p arin g to specfra 3a, 3b and 3c Furthermore, peak 688,3 cm ' in the spectrum o f W O 3/ITO 3 50 nm is a com bination o f peaks at 68 0 cm ' o f 0-W O 3 and at 700 cm ' o f
m -W O s, where the former is m uch sữonger than the later as show n in fígiư e 6.
4 C o n clu sio n
In this research, w e have prqjared W O 3 film s on ITO/ glass substrates by magnetron sputtering and annealing in the atmosphere W ith thin ITO coalers, 150 nm, W O 3 film grew along (200) plane preferentially A n increase in the thickness o f ITO layer, W O3 film orientatìon changcs from (200) to (001) gradually.
Trang 9In our experim ents, film s have many different crystalline structures with m -W Oj and o-W Os
to be the majority With layer ITO with thickness 250 nm or thinner, the majority is m-WOs with Raman peaks at 263 cm'*; 709 cm ’; 802 cm * H ow ever w hen the thickness is 300 nm or higher, the peaks^ characterizing to orthorhoiTibic phase at 797 cm*; 678 cm* em erge clearly Films with thickness 3 0 0 nm could b e regarded as a transitional phase with both m -W 03 and 0-W O 3 phases.
L V N goe et aỉ / VNƯ Jou rnal o f Science, M athem atics - P h ysics 25 (2009) 4 7 -5 5 5 5
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