vacuum temperature-dependent ellipsometric studies on wo_3 thin films

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Vacuum temperature-dependent ellipsometricstudies on WO3 thin films

Zahid Hussain

Vacuum temperature-dependent ellipsometric studies on WO3 thin films are reported at a single wave-length, l 5 0.633 mm, and across a temperature range of 100 , T # 453 K. All the measurements weremade in an optical cryostat fixed in the sample compartment of the ellipsometer. Experimental resultsinvolving reduction and oxidation of WO3 are discussed in terms of electrochromic characteristics andstructural changes, which can be helpful for many and various technological applications. Temperature-dependent drifts in the real part of the refractive index n and extinction coefficient k have been explainedby use of a variety of chemical relations and have also been utilized to evaluate their temperaturecoefficients. Moreover, polaronic excitations between localized states around the Fermi level are putforward to explain the ellipsometric results at or above room temperature, and both polaronic andbipolaronic transitions are proposed for interpreting low-temperature ellipsometric measurements.© 1999 Optical Society of America

OCIS codes: 310.6860, 240.0310, 230.0250, 160.2100, 130.5990, 260.2130.

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1. Introduction

Tungsten trioxide WO3 is the highest oxide of tung-sten, where tungsten w is a 5d transition metalhaving an electronic configuration 5d46s2 and consid-erable ionic character too. The structure of WO3shows considerable deviation from that of the idealperovskite type, and the distortions in WO3 corre-spond to displacements of W atoms and also dependon mutual rotations of the oxygen octahedra.1–3 It isdifficult to determine whether WO3 is ferroelectric ~orntiferroelectric! at room temperature, simply be-ause of its high electric conductivity and higher val-es of static dielectric constant. Nevertheless WO3

behaves as a piezoelectric insulator at low tempera-tures and a semiconductor at high temperatures withsome expected changes in its space group.4

Microcrystalline WO3 thin film has a bandgap of3.3 eV,5–8 and this band is alterable when WO3 is

injected with hydrogen or metallic ions such as H1,Li1, or Na1. The electrochromic WO3 films afteroloration can be used for smart windows,9–11 micro-

Z. Hussain [email protected]! is with Department of Elec-trical and Electronic Engineering, Imperial College of Science,Technology and Medicine, Exhibition Road, London SW7 2BTUnited Kingdom.

Received 13 April 1999; revised manuscript received 30 July1999.

0003-6935y99y347112-16$15.00y0© 1999 Optical Society of America

7112 APPLIED OPTICS y Vol. 38, No. 34 y 1 December 1999

electronics, two-dimensional displays ~of revers-ible writing and erasing of optical information!,14,15

and for many other technological applications. WO3film is useful for chemical sensing16,17 and can also beused as a negative-type electron resist to heat.18,19

WO3 films have high contrast and high resolution,8,18

and their electrochromic effects rely on the interval-ence charge transfer ~or small-polaron! absorptionband in the near-IR visible region.

Therefore, considering the importance of the abovetechnical applications, extensive optical studies ofWO3 thin films are needed for their wide-rangingscientific exposition. In this paper, vacuum ellipso-metric temperature-dependent studies on WO3 thinfilms are carried out at a single wavelength ~l 50.633 mm! to determine their temperature-dependentlectrochromic characteristics and the structuralhanges involved occurring at different levels of re-uction and oxidation process in terms of ellipsomet-ic parameters: real part of refractive index n andxtinction coefficient k.

2. Ellipsometry

A. Principles

Fresnel’s equations giving the variation of the planeof vibration of the reflected light with the angle of

a

El

rT

tF

pfl

incidence can be derived @from Fig. 1 ~upper part!#nd expressed by the following complex equations20:

tan c9 5uE1suuE1pu

< 1, (1)

E2p

E2s5

uE2puuE2su

exp@i~dp 2 dp!#, (2)

where dp and ds are the phase angles of the two com-ponents of the reflected wave. Equation ~2! can berewritten in the following form:

r 5 tan c exp~iD!. (3)

quation ~3! is known as the basic ellipsometric re-ation. In Eq. ~3! r is the ratio of the components of

the complex amplitude of the reflected light, i desig-nates the imaginary unit ~21!1y2, and D gives theeflective phase difference between the components.he refractive index of the substrate ns can be com-

puted from r by using21,22

ns 5 nair tan fF1 24r sin2 f

~r 1 1!2 G1y2

. (4)

If the substrate is absorbing, its refractive index willbe complex. The exact Drude equations23–25 for the~complex! reflection coefficients for a film-coveredsubstrate are

Rp 5r12

p 1 r23p exp D

1 1 r12pr23

p exp D, (5)

Rs 5r12

s 1 r23s exp D

1 1 r12sr23

s exp D, (6)

Fig. 1. Schematic of PCW

where

D 5 24pi cos f9dl

. (7)

Equations ~5! and ~6! give the leading ellipsometricrelation as

r 5Rp

Rs 5 tan c exp~iD!. (8)

In Eqs. ~5! and ~6!, subscripts 1, 2, and 3 are used forhe medium, film, and substrate, respectively.resnel coefficients r12 and r23 refer to the ~complex!

reflection coefficients between air and film and be-tween film and substrate, respectively. In Eq. ~7! f9is the ~complex! angle of refraction ~not shown in Fig.1!, d is the film thickness, and l is the wavelengthbeing used. The polarizing angle f is designed to be;60°, which is a good compromise for this piece ofwork.

B. Design of Angle of Incidence

Let light fall on a plate of glass with its plane ofvibration making an azimuthal angle C 5 45° to the

lane of incidence as shown in Fig. 2~a!. After re-ection the relations23,24 of ~D, C! to the angle of

incidence f are given as

D 5 tan21 ~2B sin f tan f!

~A2 1 B2 2 sin2 f tan2 f!, (9)

C 5 tan21

3 SA2 1 B2 2 2A sin f tan f 1 sin2 f tan2 f

A2 1 B2 1 2A sin f tan f 1 sin2 f tan2 fD1y2

.

(10)

for manual ellipsometer.
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1 December 1999 y Vol. 38, No. 34 y APPLIED OPTICS 7113

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In the case of a dielectric, one has k 5 0; so

A 5 ~n2 2 sin2 w!1y2, (11)

B 5 0. (12)

quations ~9! and ~10! can be reduced to

D 5 arctan 0

r

D 5 0, p, (13)

C 5 tan21@~n2 2 sin2 f!1y2 2 sin f tan f#1y2. (14)

With an increase in the angle of incidence, reflectedlight remains plane polarized, but the plane of vibra-tion shifts farther away from the plane of incidence.When f 5 f# ~Brewster’s angle!, and in the case of aglass plate, f# is ;57.6° as shown in Fig. 2~b!; then E2p> 0, but the amplitude E2s ~or Rs! continues to in-crease, whereas the reflected light phase difference~E2p 2 E2s) remains constant from beginning to end.In the experimental situation the value of C is foundto be sensitive to any oxide film ~on the glass plate!within 2–3 deg of Brewster’s angle, while D reachespeak sensitivity even for a fraction of ;0.50° offBrewster’s angle. So when f ; 60° and n 5 1.51 areinserted, Eqs. ~13! and ~14! give D 5 0 ~or p! and C 5.77°, which are the estimated substrate ~7059 glass!haracteristics.

Fig. 2. Azimuths and amplitudes of plane-polarized light re-flected from a glass: ~a! at any angle of incidence; ~b! near Brew-ter’s angle of incidence.


3. Experiments

A. Alignment Procedure

The whole apparatus was dismantled and alignedwith the same standard as described in Ref. 26. Inthe first stage, the polarizer prism was aligned withrespect to the principal plane of polarization of thelaser output. The beam aperture was then reducedin size to ;1 mm in diameter, and this yielded thehighest extinction ratio, which in the best case was;3.0 3 1027. The change in the intensity of lightwas found to be less than 0.2%.

In the second stage, the quarter-wave plate wasremoved, polarizer P and analyzer A were placed inthe straight-through position with a photomultipliertube. The P scale was set to read zero, and the Ascale was adjusted near 90° until minimum trans-mission was achieved. The reading on the A scalewas noted to be ~P 1 90°! 1 j°, and, when its positionin its holder was adjusted, j° was reduced to zero. Inthis position two Glan Thomson prisms were in theplane of incidence and the normal to the plane ofincidence.

In the third stage, by using a stainless-steel mirroras the reflecting surface and by keeping the angle ofincidence very close to 60°, ellipsometric readingswere obtained again by fixing A and adjusting P forthe minimum transmission. At this stage of devel-opment, the principal axes of the polarizer and theanalyzer were found to be parallel and perpendicularto the plane of incidence, respectively. A was thenmoved by 0.2°, again P was adjusted, and readingswere noted once again. The experiment was contin-ued in the direction in which the meter readings gavethe lower values. A series of readings was taken inthis manner by changing A in steps of 0.2° and ad-justing P for minimum transmission each time. A

Fig. 3. Calibration curve of analyzer ~A! versus polarizer ~P! forthe manual ellipsometer.

Cwfaammimw

tsa

Table 1. Vacuum Ellipsometric Alignment Data with Reference to the

plot of analyzer ~A°! versus polarizer ~P°! scale read-ings as shown in Fig. 3 gave the values of ~P!° and~A!°. Quantity j9 ~different from j°! was the amountby which both scales were displaced from the trueazimuths. The A scale was then set according to thevalue as shown in Fig. 3, but for the polarizer j9 wasreduced to zero by turning its prism by a smallamount in its holder and setting its scale to a mini-mum transmission.

In the fourth stage, the reflecting surface was re-moved, and the polarizer and analyzer were put backto the straight-through position. The P scale was

Stainless-Steel Mirror

P ~deg! Q ~deg! Amin ~deg!

0.00 0.00 90.190.00 90.00 90.000.00 180.00 90.230.00 270.00 90.02

90.00 0.00 359.9490.00 90.00 0.0090.00 180.00 359.9590.00 270.00 0.01

180.00 0.00 90.18180.00 90.00 89.99180.00 180.00 90.22180.00 270.00 90.03

270.00 0.00 359.94270.00 90.00 0.01270.00 180.00 359.95270.00 270.00 0.01

Total mean error 5 10.06

Note: f 5 60.34°, l 5 632.8 nm.

Table 2. Vacuum Ellipsometric Alignment Data when Platinum ThickFilm on a Microscopic Glass Slide is Used

PolarizerSettingsP ~deg!

Quarter-WaveSettingsQ ~deg!

Average of Two Measurementsof Minimum Analyzer Settings

Amin ~deg!

0.00 0.00 90.000.00 90.00 89.950.00 180.00 90.160.00 370.00 89.98

90.00 0.00 0.0090.00 90.00 0.0290.00 180.00 359.9790.00 270.00 0.01

180.00 0.00 90.00180.00 90.00 89.96180.00 180.00 90.16180.00 270.00 89.99

270.00 0.00 359.94270.00 90.00 0.01270.00 90.00 359.95270.00 270.00 0.05

Total mean error 5 10.01

set at zero, and the A scale ~90° 1 j9! was adjusted toa minimum value. The analyzer prism was turnedin its holder until its scale read 90°. Finally ellipso-metric alignment was testified by putting all the el-lipsometric components in order and using platinumthick film ~on 7059 glass! as the testing material.All the alignment results are listed in Tables 1 and 2.

B. Substrate Cleaning Procedure

The 7059 glass slides were washed with microclean-ing fluids ~purchased from International Products

orporation! and rinsed in tap water and distilledater. After being soaked in freshly distilled water

or several hours, the substrates were flushed withcetone and immersed in freshly distilled isopropyllcohol. The substrates were then subjected to 3in of ultrasonic cleaning, which removed the re-aining dust particles. Finally the substrates were

mmersed in the vapor of boiling isopropyl alcohol forore than 15 min, and at this stage the cleaning runas complete.

C. Preparation of WO3 Thin Films

The source material was prepared by taking tungstenpowder ~Koch Light Ltd.! and heating it in a platinumcrucible in air as high as 973 K for 24 h and then for15 h at 1573 K. This caused extensive sintering andgrain growth. The sintered material was brokeninto small lumps and sieved, resulting in a pale greenfine powder.

In the next stage, WO3 powder was put in a ~resis-ivity heated! silica tube that was 18 cm off the sub-trate. The final base pressure was 3 3 1026 Torr,nd it was reduced to 4 3 1027 Torr with the liquid

nitrogen trap filled. The WO3 was outgassed at1073 K for 30 min in a vacuum chamber. In the nextstep, the temperature was further increased, andwhen the deposition rate was 1.5 nmys, a shutter wasopened and WO3 was deposited onto a 7059 glassslide. The rates of evaporation and film thicknessboth were controlled by a calibrated quartz crystalmonitor. Details of the apparatus are given else-where.6,27

The thermally evaporated WO3 thin films werefound to be composed of microcrystalline fine-grainedparticles of grain size in the range of 20–80 Å, ascharacterized in a 1-MeV TEM.6,28,29 Some otherresearchers more or less came to the same conclu-sion.30,31 The film thicknesses were between 0.15and 0.50 mm, and each film thickness was measuredfour times with a Talysurf profilometer to get a highprecision of 620 Å. Owing to a slow rate of evapo-ration, porosity was reduced and the film density wasmeasured to be 6.0 6 0.3 gycc; this value is alsoconfirmed by many other researchers.28,29 The bulkdensity of WO3 is 7.3 gycc; and features such as thepolar nature and the lattice deformation, as observedin bulk tungsten oxide, do not seem to be preserved inthe films prepared by thermal evaporation. Stoichi-ometry of the fresh evaporated films as measured byproton backscattering32 was of the value of ~OyW! ;2.7, and from Auger electron spectroscopyyelectron


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tttt

tntaoppwwdmwi

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7

spectroscopy for chemical analysis,33–35 this ratio~OyW! was found to be ;2.72. Nevertheless all thereported films are designated WO3 throughout the

aper.The reduction of WO3 thin films was done by in-

erting hydrogen plasma into WO3, which was main-tained at a temperature around 373 K and at apressure of approximately 6 3 1021 Torr. Briefly,the molecular state of hydrogen was broken intoatomic hydrogen with the help of a Microtron200MKP microwave power generator unit. The re-sulting concentration of the hydrogen content in afilm was proportional to exposure time. The revers-ibility of the hydrogen-induced coloration was alsotested by exposing the colored WO3 films to the oxy-gen plasma.

D. Apparatus and Operation of Ellipsometer

A picture of the apparatus built up in the laboratoryis shown in Fig. 4, and its schematic arrangement ofpolarizer, compensator ~quarter-wave plate!, silicaoptical window ~before reflection!, sample, silica op-tical window ~after reflection!, and analyzer ~PC-WSW9A! is also shown in the lower part of Fig. 1. Inhis manual ellipsometer the incident side included ae–Ne laser ~L! with 1-mW output, a first quarter-

Fig. 4. Manual ellipsometer constructed and composed of an alumThe incident section is composed of L, a He–Ne laser; Q1, mica

homson polarizer prism mounted on a Bellingham and StanlayB and S head ~scale to 0.01°!, C, sample vacuum cell setup on specsection is composed of A, a Glan–Thomson analyzer prism mountedconnected to Keithly to monitor output signal.


wave plate ~Q1 set at 645°!, a Glan Thomson polar-zer ~P!, and a second quarter-wave plate ~Q set at45°!. The resulting elliptically polarized light then

ell normally ~within 2–3°! on a fused silica opticalindow ~W!, and finally it was incident on a 7059lass plate ~having its back surface roughened! at anngle of ;60°. The substrate ~7059 glass! was fixed

~in a horizontal or vertical position! inside a ~cryostat-ype! vacuum cell ~C! that was made firm on a spec-rometer table and was also connected to aemperature controller through an alumel–chromelhermocouple.

The reflected side included another fused-silica op-ical window ~W9! through which laser light passedormally ~within 2–3°!, and then it was transmittedhrough another Glan Thomson prism acting as annalyzer ~A!. After being transmitted through anptical filter, it finally passed through a photomulti-lier tube ~PM! connected to Keithly to read the out-ut signal. To minimize eye strain, all the scalesere illuminated. Polarizer and analyzer azimuthsere read on graduated circles equipped with twoiametrically opposite verniers. Note that all azi-uthal angles were measured positive counterclock-ise from the plane of incidence when one is looking

nto the light beam. Moreover P, A, and C ~or Q!

m alloy base plate to which ellipsometric components were fixed.er-wave plate producing circularly polarized light; P, the Glan–~scale to 0.01°!; Q, second mica quarter-wave plate mounted in ater table and connected to temperature controller. The reflectedB and S head ~scale to 0.01°!; PM, detector ~photomultiplier tube!

iniuquartheadtrome

in a

l

awM

s

Table 3. Ellipsometer Readings

i

Table 4. Vacuum Ellipsometric Measurements on a 7059 Glass Slide

m

scales were adjusted to read angles with respect tothe plane of incidence.

The ellipticity of the light due to reflection ~fromeither a bare or film-covered substrate! was compen-sated by rotating the polarizer and analyzer until theminimum signal was obtained through Keithly. Toensure accurate results, it was necessary to sweepthe polarizer ~or analyzer! across the observed mini-mum to one side and then to the other side of equalintensity and then take the average to cancel theeffects of eccentricity and to get the true minimum.This entire process was repeated for each measure-ment. The various readings were taken in four setscalled zones, two with the fast axis of the quarter-wave plate set at py4 ~called 2 and 4! and two set at2py4 ~called 1 and 3!.

4. Results and Error Analysis

The ellipsometric scale reading p is related to thepolarizer azimuth angle P; ap is related to the ana-yzer azimuth angle A ~in zones 1 and 4!, and as is

related to the A readings ~in zones 2 and 3!. Assum-ing a perfect quarter-wave plate, P and A are associ-ated with ellipsometric angles ~D and C! by

D 5 62P 6 py2, (15)

C 5 ap 5 as. (16)

The relations of P and A to p and ap ~or as! and to Dnd C in all four zones are in Table 3. Equation ~8!as solved with the help of an improved version of thecCrackin package36 by using Fortran 77, and the

temperature-dependent ~experimental! optical con-stants ~n and k values! were then calculated for theubstrate and for the reported WO3 thin films. Ta-

ble 4 shows the refractive index of 7059 glass ~with

ZoneaCompen-

sator P ~deg! A ~deg!C ~deg! orD ~deg!b

1 2py4 p ap C 5 ap 5 as

~in all zones!p 1 p ap

p p 1 ap D 5 2p 1 py2p 1 p p 1 ap

3 2py4 py2 1 p p 2 as D 5 2p 2 py23py2 1 p p 2 as

py2 1 p 2p 2 as

3py2 1 p 2p 2 as

2 1py4 py2 2 p as D 5 22p 1 py23py2 2 p as

py2 2 p p 1 as

3py2 2 p p 1 as

4 1py4 p 2 p p 2 ap D 5 22p 2 py22p 2 p p 2 ap

p 2 p 2p 2 ap

2p 2 p 2p 2 ap

ap, ap, as are analyzed into four zones in relevance to the polar-zer, P ~deg! and analyzer, A ~deg!, azimuths.

bConversion formulas for ellipsometric angles c° and D°.

the back surface roughened! with and without opticalwindows. This value of the refractive index was alsocompared with an automatic ellipsometer and Abberefractometer data as shown in Table 5. Similarly,optical data for freshly evaporated WO3 thin filmwere also compared with the autoellipsometer andare listed in Table 6. These results show that themanual ellipsometer was aligned with high precision.

Both the vacuum temperature-dependent and an-nealed data are, respectively, given in Tables 7 and 8,and their optical plots versus temperature are shownin Figs. 5 and 6, respectively. The optical data ofWO3 in the temperature range 100 , T , 400 K~along with time periods where necessary! are col-lected in Table 9 and plotted in Fig. 7. Reductionand oxidation effects on WO3 thin film are shown inTables 10 and 11 and are, respectively, plotted inFigs. 8 and 9. The data related to different rates ofevaporation and different substrate temperaturesare both, respectively, listed in Tables 12 and 13.Figure 10 manifests the effects of substrate temper-ature on the basic WO3 data.

The lack of parallelism of the prism faces due to

~Owing to Strain Effects because of Optical Windows! Kept UnderVacuum and in Open Aira

PressureOptical

Windows D ~deg! C ~deg! n k

2 3 1025 Torr With 1.77 5.51 1.5296 0.00041In air ~atm! With 1.32 5.55 1.5276 0.00031In air ~atm! Without 1.58 5.53 1.5284 0.00036

aAt room temperature in both cases.

Table 5. Vacuum Ellipsometric Data on a 7059 Glass Slide ~with BackSurface Roughened by Silicon Carbide!

Instrument Type D ~deg! C ~deg! ne

AbbeReadings

~deg! na

Manual 0.06 5.34 1.5296 — —ellipsometer

Automatic 1.96 5.04 1.5264 — —ellipsometer

Abbe — — — 24.743 1.5283

Note: Data ~on the same glass slide! from other optical instru-ents set at a wavelength of 632.8 nm ~and all maintained at room

temperature! are also included.

Table 6. Manual and Autoellipsometric ~Comparative! Data on WO3

Thin Films Deposited on 7059 Glass Slides

TypeThickness

~mm! D ~deg! C ~deg! n k

Manual 0.280 156.49 11.79 2.0410 0.00059Auto 0.286 130.08 10.16 2.0220 0.00058Manual 0.370 283.22 4.28 1.9905 0.01514Auto 0.364 82.68 6.84 1.9810 0.01612

Note: f ; 60.


a6

W

Table 7. Vacuum Ellipsometric Temperature-Dependent Data on WO

7

misalignment could cause a deviation of the beam,which results in a change in the angle of incidence,which is again a function of A and P ~the analyzer andthe polarizer azimuths!. Thus a change of 60.05° atan angle of incidence could correspond to an error of60.0012 in n and 60.00005 in k values of a 7059 glasssubstrate. The same kind of uncertainty in the an-gle measurements plus a typical error of 60.002 mmin the thickness measurements gave a composite er-ror of 60.0078 in n and of 60.0023 in k values of WO3thin films. Birefringence effects ~due to fused silicawindows! on 7059 glass produced an error of60.000035 in n and 60.00005 in k. The errors in nand k ~due to birefringence effects! were found to be60.0002 and 60.00029, respectively, for WO3 thinfilms.

Considering the higher temperature range ~as high

3

Thin Film Deposited on a 7059 Glass Slide

Temperature~K! D ~deg!

C~deg! n k

First heating cycle295.0 281.17 4.79 2.06123 0.00869333.0 281.92 4.83 2.06182 0.00860373.0 282.93 4.80 2.06221 0.00889413.0 281.77 4.45 2.05986 0.00987453.0 278.30 4.46 2.05810 0.00918

First cooling cycle373.0 280.17 4.30 2.05830 0.01006297.0 282.29 4.45 2.06010 0.00996

Second heating cycle297.0 282.29 4.45 2.06010 0.00996453.0 278.93 4.69 2.05950 0.00860

Second cooling cycle295.0 281.20 4.57 2.06020 0.00928

Note: Thickness, 0.522 mm; f 5 60.32°.

Table 8. Vacuum-Annealed Ellipsometric Data on WO3 Thin FilmUpheld at Different Temperatures

Temperature~K! Time ~h! D ~deg! C ~deg! n k

First heating cycle295.0 — 2.56 5.48 2.0266 0.00020373.0 — 27.46 5.16 2.0323 0.00098373.0 14.0 257.07 4.31 2.0576 0.00655

First cooling cycle333.0 — 258.26 4.22 2.0578 0.00703303.0 48.0 258.71 4.15 2.0579 0.00733

Second heating cycle297.0 — 221.52 3.44 2.0386 0.00713413.0 — 268.54 4.41 2.0639 0.00772413.0 10.0 293.04 4.50 2.0769 0.01201

Second cooling cycle373.0 — 294.60 4.51 2.0776 0.01231333.0 — 296.32 4.53 2.0786 0.01270295.0 12.0 297.32 4.49 2.0789 0.01309

Third heating cycle295.0 — 296.41 4.49 2.0784 0.01289



s 453 K!, the total uncertainty was found to be0.0002 in n and 60.0001 in k values of the WO3

samples. Similarly, in the lower temperature range~as high as 100 K!, the maximum uncertainty wasfound to be 60.0015 in n and 60.0002 in k for the

O3 samples. Note that the analysis above will,however, be different for annealed films and also forthe films exposed to hydrogen or oxygen plasma.

5. Discussion

A. Structure and Phase Transitions

The structure of tungsten trioxide is a monoclinicReO3-type cubic lattice and is formed by corner-shared WO6 octahedra extending in three dimensions

Fig. 5. Plots of ~a! refractive index n and ~b! extinction coefficientk versus temperature of WO3 thin film under vacuum.

W

s

Table 9. Vacuum-Low-Temperature Ellipsometric Measurements on

with lattice parameters as determined by Salje andViswanathan4 as shown in Fig. 11. It has 8 W atomsand 24 oxygen atoms in its unit cell to maintain lat-tice stability.37 At higher than 1013 K, the WO3lattice is tetragonal37–39 and is changed into an or-thorhombic at 1013 K, and a orthorhombic–monoclinic transition occurs at 603 K.37–39 It ischanged to triclinic at 290 K, and a further triclinic–monoclinic II transition occurs at 233 K.37–39 Tet-ragonal, orthorhombic, and triclinic phases of WO3are shown in Figs. 12, 13, and 14, respectively.

Substoichiometric WO32x phases can be easily pro-duced in a reduced environment,40 and even intro-duction of a slightly small number of crystallographicshear planes can produce a large number of defective

Fig. 6. ~a! Plot of refractive index n versus temperature of WO3

thin film being annealed under vacuum for different periods oftime. ~b! Plot of extinction coefficient k versus temperature, of

O3 thin film under vacuum for different periods of time.

Fig. 7. Plot of refractive index n and extinction coefficient k ver-us temperature T of a WO3 thin film under vacuum across a

temperature range of 100–380 K. Solid lines refer to curve fittingfor the experimental n and k values.

WO3 Thin Film with Liquid Nitrogen Used as a Coolant and theTemperature Directed by a 407 Controller

Temperature~K! Time ~h! D ~deg! C ~deg! n k

First heating cycle295.0 — 93.18 8.78 2.0107 0.00771373.0 — 100.24 8.88 1.9931 0.00820

First cooling cycle333.0 — 98.70 9.09 1.9971 0.00996153.0 3.0 29.50 3.90 2.1123 0.00860105.0 1.0 21.83 5.41 2.1146 0.00980

Second heating cycle295.0 — 88.93 9.00 2.0200 0.00967

Second cooling cycle243.0 — 237.48 7.74 2.1282 0.00615101.0 — 245.45 7.29 2.1346 0.00390

Third heating cycle183.0 — 248.22 5.10 2.1314 0.00430298.0 — 227.72 2.44 2.1106 0.01485373.0 — 294.75 4.45 2.0773 0.01260373.0 6.0 291.23 3.81 2.0714 0.01377

Third cooling cycle297.0 4.0 294.94 3.85 2.0732 0.01446

Fourth heating cycle413.0 — 287.93 4.09 2.0718 0.01221413.0 4.0 283.51 4.05 2.0695 0.01143453.0 — 284.23 4.32 2.0714 0.01065453.0 2.0 251.92 3.33 2.0519 0.00948453.0 4.0 251.30 3.21 2.0514 0.00967

Fourth cooling cycle295.0 8.0 259.46 3.76 2.0568 0.00869

Note: Thickness 5 0.325 mm, f 5 60.34°.


att

asWsr

s~r

s~r

Table 10. Ellipsometric Data Collected on WO Thin Film Owing to

7

phases. The phases occurring in the composition re-gion WO2–WO3 across the temperature range of 723–137 K ~together with different oxygen partialpressures! have been declared WO3, W20O58, W18O49,nd WO2, and these phases have been verified byransmission electron microscopy33 and x-ray diffrac-ion techniques.41 Moreover in these phases the av-

erage W–O distance increases monotonically with areduction in tungsten from W61 ~WO3! to W41 ~WO2!and also with decreasing distortion of the WO6 octa-hedra.42,43 Oxides W18O49 ~WO2.72! and W20O58~WO2.90! have peculiar structures with pentagonalnd rectangular columns sharing the corners, ashown in Figs. 15 and 16, respectively. OxideO2.72 is well ordered and can be considered as a

toichiometric phase with a very limited compositionange.43

Microcrystalline tungsten oxide films are built upfrom a disordered network of corner-sharing WO6octahedra, and these films consist of clusters havingat least two types of WO6 octahedra, one with a ter-

3

Hydrogen and Oxygen Plasma Treatments while Film Upheldat T 5 373 K

Type ofPlasma Time ~min! D ~deg! C ~deg! n k

No plasma — 156.61 11.73 2.0397 0.0006Hydrogen 5.0 117.58 8.14 1.8446 0.1982

3.0 112.71 9.08 1.8208 0.23483.0 104.13 9.35 1.7499 0.2559

35.0 89.07 12.74 1.5747 0.359915.0 89.73 12.54 1.5849 0.354545.0 89.87 12.32 1.5908 0.348040.0 90.05 11.97 1.5996 0.3375

Oxygen 90.0 97.77 9.74 1.6965 0.2722150.0 95.27 9.18 1.6826 0.2568270.0 97.70 8.72 1.7033 0.2428


Table 11. Ellipsometric Data Collected on WO3 Thin Film Owing toHydrogen and Oxygen Plasma Treatments while Film Upheld at

T 5 453 K

Type ofPlasma Time ~h! D ~deg! C ~deg! n k

No plasma — 158.52 11.86 2.0324 0.00108Hydrogen 0.92 96.83 10.61 1.6862 0.31270

0.83 94.68 10.55 1.6710 0.310500.83 93.11 10.56 1.6594 0.30990

Oxygen 0.75 97.44 10.02 1.6989 0.296540.50 97.96 9.98 1.7032 0.29546

14.00 99.50 8.38 1.7302 0.2531336.00 103.73 6.65 1.7618 0.209388.00 106.39 6.00 1.7740 0.193221.00 101.01 7.90 1.7278 0.217802.00 102.06 7.63 1.7342 0.209678.00 105.24 7.07 1.7505 0.19175

10.00 106.47 6.77 1.7549 0.18272



minal oxygen and one without. These represent ax-ially distorted and generally distorted octahedra,respectively. IR and Raman spectra44,45 show theexistance of the WO6 octahedra on the basis of theseevaporated WO3 thin films. These films have anopen pore network, and several defect bonds such asW™O™W or W¢O could exist in the WO3 films.48,49

Gabrusenoks et al.45 have shown that double bonds oftungsten and oxygen are always induced in the amor-phous WO3 films, and these W¢O bonds could act astrapping sites like W™OH bonds.45 Vacuum-

Fig. 8. Plot of refractive index n and extinction coefficient k ver-us time, of WO3 thin film being annealed at temperature, T 5373 6 1!K, under hydrogen and then in an oxygen plasma envi-onment.

Fig. 9. Plot of refractive index n and extinction coefficient k ver-us time, of WO3 thin film being annealed at temperature, T 5453 6 1!K, under hydrogen and then in an oxygen plasma envi-onment.

te

2t

0e

Table 12. Vacuum Ellipsometric Data on Evaporated WO Thin Films

evaporated WO3 films have microcrystallites of grainsize in the 20–80-Å range, and almost the same find-ing has been revealed by several other research-ers.28,29,50 Furthermore it has been observed thatwater is always essential for good electrochromic ef-fects.51,52 However, the relation between water con-ent and the local order is very complex forvaporated WO3 thin films.

B. Temperature-Dependent Drifts and Interpretations

The average value of n was computed to be around.04 at l 5 0.633 mm. In the literature the value ofhe real part of the refractive index n is between 1.8

Fig. 10. Plot of refractive index n and extinction coefficient k ofWO3 thin film versus substrate temperature T under vacuum.

3

Prepared at Different Substrate ~7059 Glass! Temperatures

SubstrateTemperature

~K!Thickness

~mm! D ~deg! C ~deg! n k

295.0 0.280 156.49 11.79 2.0410 0.0005353.0 0.520 283.35 4.80 2.0624 0.0089373.0 0.348 2100.10 5.92 2.1039 0.0133

Note: f 5 60.32°, rate of evaporation, 950 Åymin.

Table 13. Vacuum Ellipsometric Data on WO3 Thin Films when theRate of Evaporation is Varied while the Substrate ~7059 glass! Is at

Room Temperature

Thickness~mm!

Rate ofEvapo-ration

~Åymin!

Pressure31026

~Torr! D ~deg! C ~deg! n k

3220.0 900.0 1.8 88.05 7.56 2.0445 0.001473700.0 1200.0 2.2 282.57 6.04 1.9996 0.00096

Note: f 5 60.33°.

and 2.05 for microcrystalline ~or amorphous! tung-sten oxide thin films,5,8,53–55 and a narrow range, 2.4–2.6, exists for both polycrystalline56–59 and single-crystalline WO3.60,61 The spectral data of WO3 thinfilm were computed from the spectrophotometric da-ta,6,27 and the worked-out values of n and k at l 5.633 mm are found to be in good agreement with thellipsometric values, as shown in Fig. 17. Further-

Fig. 11. Model of a monoclinic ~ReO3-type! structure of WO3 atroom temperature: F, tungsten atoms; E, oxygen atoms.

Fig. 12. Crystal structural model of ~a! tetragonal type 2 and ~b!tetragonal type 1, of WO3 with ReO3-type WO6 octahedra: F,tungsten atoms; E, oxygen atoms.


3s

1

L

d1

7

more from the literature perspective an opticalbandgap of as-evaporated thin-film WO3 lies in the.25–3.41-eV range,6,7,8,44,62 while polycrystalline andingle-crystalline WO3 both have an optical energy

gap in the 2.58–2.77-eV range.4,57,63–65

From Table 7 the thermal shift dnydT in the datavaries from 2.67 3 1026yK at 373 K to 29.61 3025yK at 453 K; similarly dkydK varies from 2.35 to

3.57 3 1024yK in the same temperature range.ow-temperature measurements in the range 100 ,

T # 373 K ~see Table 9! give temperature coefficientnydT from 21.12 3 104yK at 295 K to 23.18 3024yK at 101 K, while dkydT varies from 7.66 3

1024yK to 2.55 3 103yK in the same temperature

Fig. 13. ~a! Structural model of an orthorombic type of WO3: E,oxygen atoms; tungsten atoms are not shown. ~b! Numbersshown are W™O bond lengths in angstroms.

Fig. 14. Model of triclinic ~ReO3-type! structure of WO3: F,tungsten atoms; E, oxygen atoms.


conditions. Simple heating and cooling over a tem-perature range of 100–453 K are a good test for re-versibility ~see Figs. 5 and 7!. They are first-rateexamples of the first-order transition from the view-point of temperature hystersis. Previous studies ofwater absorption in WO3 films66 have demonstratedthat water can be incorporated in the film, causing aslight distortion of the WO3 lattice,49 which is good forelectrochromism. But thermal treatment of WO3 attemperatures higher than 490 K ultimately results infading out its electrochromic properties.55

Different minute discontinuities at low-temperature measurements ~see Fig. 7! cannot becorrelated with stable crystallographic transforma-tions or changes in the space group, as many otherresearchers observed in the case of single

Fig. 15. Structural model of WO2.72 viewed along the short axisand consisting of repeat units of the pentagonal column: F, tung-sten atoms; E, oxygen atoms.

Fig. 16. Structural model of tungsten oxide, WO2.90: F, tung-sten atoms; E, oxygen atoms.

Twap

Abwb

cd

vt

p

crystals.67–69 On the contrary, there may be somedrifts in the electronic band structure that may bedue to scattering of conduction electrons caused bygrain boundary or by phonon drag effects.

Reduction by hydrogen plasma, as shown in Figs. 8and 9, may cause hydrogen either to be adsorbed overthe surface, to enter the WO3 lattice-producing hy-drogen tungsten bronze, or to remove oxygen fromthe WO3 lattice-producing WO32x blue thin film.

wo kinds of electrocoloration processes are put for-ard, namely, the formation of interstitial hydrogennd hydroxyl-type hydrogen bronze and are both ex-ressed in the form of chemical relations:

WO3 1 xH1 1 xe23HxWO3, (17)

WO3 1 xH1 1 xe23WO32x~OH!x. (18)

ccording to atomic coordinate arrangements, it haseen suggested that there exists covalent O™H andeak hydrogen bonds, indicating that the hydrogenronze can be expressed by relation ~17!; this kind of

arrangement has been pointed out elsewhere.70,71

An hydroxyl type of hydrogen bronze as shown inrelation ~18! has been tested in UV photoemissionspectroscopy experiments by Maheshwar Sharon etal.72 In the reported temperature range, WO3 ishanged into hydrogen bronze but with a very minoregree of reduction. Complete reduction of WO3 to

metallic W occurs effectively at higher than 873 K~Ref. 73! according to the following chemical relation:

WO3 1 3H23W 1 3H2 O. (19)

The grain size increased as the reduction tempera-ture increased, resulting in a size of ;1000 Å whenapproaching 1273 K.73

Fig. 17. Plot of optical constants ~n, k! versus wavelength l ~de-termined by the spectrophotometric technique! of WO3 thin film of0.34-mm thickness. ~Spectrophotometric readings were taken atroom temperature in open air.!

Again annealing at a temperature of '453 K in anoxygen plasma environment did not bleach hydrogenbronzes more than 30% and 46% as shown in Figs. 8and 9, respectively. In this process, oxygen vacan-cies are always created in the tungsten oxide eitherdue to hydrogen plasma or during oxidative anneal-ing. As a result, an additive electrostatic force iscreated and the interatomic distance74 is changed,resulting in unsymmetrical coordination in the WO3matrix, and so the bleaching process becomes veryhard in the reported temperature range. Moreover,if an oxidation process is only a surface process, it canlead to stoichiometric oxide WO3 through

WO32x 1 xH2O3WO3 1 2xH1 1 2xe2. (20)

Note that in this case water molecules ~or the OHspecies! cannot penetrate the oxide easily.

Figure 6~a! and 6~b! indicate some grain growth ina simple annealing environment, but it is very diffi-cult to distinguish between crystallization effects andthe reduction of stoichiometry due to oxygen losses.However, in the beginning, as the annealing temper-ature increases, the k value of bare WO3 starts toincrease owing to water evolution. But at laterstages of annealing an increase and a decrease in kvalues more or less follow exponential dependence.In addition, the partial irreversible changes in the kalue can be attributed to a partial crystalline phaseransition in the microcrystalline WO3 film, and this

transition from the microcrystalline to crystallinestate appears to be controlled by the nucleation ofgrains at the film–substrate interface, which ulti-mately affects film conductivity. Annealing in airabove 623 K can produce crystallization,37,51,75,76 butWO3 thin film may still be nonstoichiometric.

Moreover an increase in substrate temperatureduring deposition can also result in larger values of nand k as shown in Fig. 10, indicating that microc-rytallites of WO3 start to grow bigger and becomemore closely packed. In this case the concentrationof the grain boundary increases, which causes a de-crease in the Rayleigh scattering coefficient as hasbeen observed by x-ray diffraction results.70 Notethat evaporation of WO3 onto a substrate held at atemperature below 600 K does not produce polycrys-talline WO3 thin film, and this fact has been observedby x-ray, electron diffraction, and Raman spectrumstudies.46,75,77 These aforementioned facts alto-gether are very critical in determining device sensi-tivity while controlling the film microstructure.

C. Explanation from the Polaronic Viewpoint

Freshly evaporated tungsten oxide thin films on un-heated substrates are deficient in oxygen, as esti-mated by ellipsometric studies. Moreover the slightbroadening of the electron spectroscopy for chemicalanalysis spectrum44,45,78 leads us to believe that as-evaporated films are not fully oxidized, i.e., the sur-faces contain small amounts of W51.45,78 Electron

aramagnetic resonance79 ~EPR! and x-ray photo-electron spectroscopy80,81 experiments prove the ex-


ip

tostocmla

vh

4ttwtt

tbtram

d

mou

7

istence of W51 states even in slightly oxygen-deficientthin film or lightly colored WO3 single crystal. Theinjection of guest ions ~H1, Li1, Na1, etc.! increasesthe number of W51 ions and also affects the way thelocalization process is involved in the films. For po-rous WO3 films, the electrons are less localized, thewave-function overlap on adjacent W51 and W61 sitess larger, and therefore the oscillator strength of theolaronic transition is enhanced.82 In other words,

the coloring efficiency is increased and more electronscan absorb light and transfer to a higher energy state.After thermal treatment, the film becomes more con-densed owing to the evolution of water and the elec-trons become more localized; therefore the oscillatorstrength of the polaronic transition decreases.

The reported thin films have a large bandgap com-pared with WO3 single crystal, and this may be due toa heavy topological disorder induced in the freshlyprepared films. If the disorder increases in the filmsbecause of either annealing or hydrogen ~or oxygen!plasma treatment, a dense population of polaronsmight build up, and in this case the optical transi-tions could be dominated by phonon-activated hop-ping processes between the in-gap impurity states.The optical phonons do make a major contribution tothe nature of the polaron state during annealing andcause change in the effective mass or in the couplingconstant, but it is very difficult to alter and measurethe coupling constant experimentally.

Polaronic effects at low temperatures give rise to acomplicated dielectric constant even in pureWO3.83–86 In a number of cases, pairing of W51

states and the formation of diamagnetic bipolaronshave been observed at low temperature.87 Polaronheory suggests that the two-dimensional characterf the polaronic or bipolaronic wave functions is re-tricted to only low values of mobility, and eventuallyhese low mobilities assist in stabilizing bipolaronicr polaronic states in the low-temperature range. Aonceivable model for bipolaronic transition is theutual hopping of the pair as shown in Fig. 18. A

ower bound for the binding energy of these bipol-rons is obtained from the stability condition84,87,88

Ebi , 2Eb, (21)

Fig. 18. Sketch of simple model of a bipolaron.


where Ebi is the bipolaron binding energy and Eb isthe single-polaron binding energy. Below room tem-perature, only polarons of small or intermediate sizeexist with bipolaronic ground states,80,87 but electronspin resonance ~ESR! spectra associated with W51 isobservable only below 60 K.87 Thin films of WO3after coloration were found to have an ESR signal at77 K.89 Also EPR spectra of W51 electrons can beobserved in WO3 single crystal after irradiating itwith visible-IR light at 20 K.85,90 Salje and Hopp-man83 obtained direct evidence for the hopping pro-cess of polarons in dielectric measurements and alsoin optical absorption, from which one can assign 0.18and 0.27 eV to the hopping energies of a single pol-aron and a bipolaron, respectively. Note that pol-arons and bipolarons in the metallic oxides areformed well above the superconductivity transitiontemperature Tc, simply owing to their strongelectron–phonon interaction,91,92 and in this low-temperature range the condition of the bipolaron bosecondensation is strongly determined by phonon dis-persion.93,94

There have not been quite so many types of theo-ries put forward for tungsten trioxide and other ox-ides, especially for their temperature-dependentdata, so it would be useful to develop a polaron modelto establish a correlation among the phonon disper-sion, carrier concentration, and the temperature ofthe concerned complex oxides. A challenging modelwith a reasonable computer simulation is needed fora good comparison between theory and experiment.

6. Summary

Ellipsometric temperature-dependent experimentswere done on WO3 thin films. All the components ofthe manual ellipometer were aligned with high pre-cision, and subsequently the manual ellipsometricresults compared well with the autoellipsometer.The value of n for WO3 film was found to be ;2.04 ata wavelength, l 5 0.633 mm, that, in the literature, isery close to the others. The results of vacuum-eating and cooling experiments on different WO3

samples across the temperature range of 100 , T #53 K are examples of first-order transitions. Buthe different discontinuities noted during the low-emperature measurements cannot be correlatedith stable crystallographic transformations; rather

hese results simply reflect some reversible struc-ural distortions.

A polaron model is suggested for room- or higher-emperature data, and both types of polaronic andipolaronic transitions are proposed for low-emperature measurements. But note that only aigorous theoretical treatment along with a reason-ble computer simulation could clarify the experi-ental results.Experiments of reduction and oxidation were also

one on WO3 thin films, and the related changes in nand k have been explained with a view to structural

odifications and electrochromic properties. More-ver the annealed data have also been elucidated bysing suitable chemical relations.

1

1

1

1

1

Physics of Thin Films, G. Haas, ed. ~Academic, New York,

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vacuum temperature-dependent ellipsometric studies on wo_3 thin films

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