Effects of working pressure on physical properties of tungsten ... - AVS

Effects of working pressure on physical properties of tungsten-oxide thin films sputtered from oxide target I. Riecha兲 and M. Acosta Laboratorio de Ciencia de Materiales, Facultad de Ingeniería, Universidad Autónoma de Yucatán, A. P 150. Cordemex, Mérida, Yucatán 97130, Mexico

J. L. Peña and P. Bartolo-Pérez Departamento de Física Aplicada, CINVESTAV-IPN Unidad Mérida, A.P. 73 Cordemex, Mérida, Yucatán 97130, Mexico

共Received 2 February 2009; accepted 25 January 2010; published 4 March 2010兲 Tungsten-oxide films were deposited on glass substrates from a metal-oxide target by nonreactive radio-frequency sputtering. The authors have studied the effect that changing Ar gas pressure has on the electrical, optical, and chemical composition in the thin films. Resistivity of WO3 changed ten orders of magnitude with working gas pressure values from 20 to 80 mTorr. Thin films deposited at 20 mTorr of Ar sputtering pressure showed lower resistivity and optical transmittance. X-ray photoelectron spectroscopy 共XPS兲 measurements revealed similar chemical composition for all samples irrespective of Ar pressure used. However, XPS analyses of the evolution of W 4f and O 1s peaks indicated a mixture of oxides dependent on the Ar pressure used during deposition. © 2010 American Vacuum Society. 关DOI: 10.1116/1.3333423兴

I. INTRODUCTION Semiconducting metal oxide sensors have been tested for monitoring pollutant components of the atmosphere such as O3, NOx, H2S, CO, and NH3. The sensing mechanism of metal-oxide based gas sensors lies in a resistance change resulting from gas-phase species adsorption and catalytic reactions at the material surface. It is well known that tungsten-oxide resistivity is determined by stoichiometric defects such as oxygen vacancies present in the material.1 These oxygen vacancies associated with substoichiometric compounds WO3−x, where x is the oxygen composition parameter 共0 ⬍ x ⬍ 1兲, determine the gas-sensing properties of the films. WO3 thin films have been prepared by several methods: thermal evaporation,2 radio-frequency sputtering,3,4 screen printing,5 pulsed electrodeposition,6 sol gel,7 and spray pyrolysis,8 among others. Sputtering technique is widely used because its composition reproducibility and thin-film formation on a large-area substrate. Often, thin films are obtained by sputtering a metallic target by means of plasma using argon and oxygen as carrier and reactive gases, respectively. Physical properties of tungsten-oxide films deposited using Aru O2 reactive atmosphere depend on processing parameters, such as oxygen flow ratio. Several authors pointed out the oxygen-partial pressure influence on physical parameters, such as conductivity, surface morphology, chemical composition, and optical properties.9–13 In general, reactive sputtering of a metallic target produces oxide films with good stoichiometry and crystallinity depending on deposition parameters. However, oxide thin films obtained by nonreactive sputtering of a metal-oxide target normally are oxya兲

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gen deficient. For gas-sensor applications, this characteristic may be useful, considering that oxygen vacancies play an important role in the conductivity changes under the presence of gas. There are several studies on physical properties of tungsten-oxide thin films obtained from an oxide target, where the sputtering is not reactive and plasma is formed only by Ar carrier gas. This study analyzes the relationship between physical properties of WO3 thin films prepared by rf sputtering from an oxide target and Ar atmosphere in the deposition chamber. X-ray photoelectron spectroscopy 共XPS兲 analysis has been combined with electrical and optical measurements to correlate changes in oxidation states with electrical and optical behavior. II. EXPERIMENT Tungsten-oxide thin films were deposited on Corning glass 2947 substrates by rf magnetron sputtering. The films were prepared at room temperature, using WO3 共99.9%兲 target and Ar 共99.999%兲 as a sputtering gas without external oxygen gas injection. Thin films were deposited varying the Ar sputtering pressure from 20 to 80 mTorr in increments of 20 mTorr. We used an on-axis configuration for the sputtering process, and the distance between the target and substrate was held at 5.0 cm. The vacuum chamber was evacuated to a pressure lower than 2 ⫻ 10−5 Torr before the deposition, and the target was presputtered for 10 min to remove any surface contamination. The thin films were amorphous, as confirmed by x-ray diffraction measurements, with thicknesses of about 0.5 ␮m and average deposition rate of 50 nm/min. More information about deposition mechanisms are presented in Ref. 14. The XRD spectra were obtained using a Siemens D500 diffractometer with Cu K␣ radiation. Transmittance measurements were done in the spectral range from 300 to 1100 nm using an Agilent 8453 spectro-

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©2010 American Vacuum Society

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Riech et al.: Effects of working pressure on physical properties

TABLE I. Room-temperature resistivity and optical-band gap values as a function of Ar pressure. Ar pressure 共mTorr兲

Electrical resistivity 共⍀ cm兲

Optical band gap 共eV兲

20 40 60 80

1.52⫻ 10−2 8.86⫻ 107 3.94⫻ 108 ¯

2.38 3.06 3.10 3.14

330 100

Transmittance (%)

330

80 mTorr

75

60 mTorr 50

25

40 mTorr

20 mTorr

0

photometer. Thin-film structures were examined by XPS using a Perkin-Elmer PHI 560/ESCA-SAM system, equipped with a double-pass cylindrical mirror analyzer, with base pressure of 10−9 Torr. The samples were excited with 1486.6 eV Al K␣ x rays. All XPS spectra were obtained after 5 min of Ar sputtering performed with 4 keV ions and 0.36 ␮A / cm2 current beams, yielding a sputtering rate of about 3 nm/min. The low current density in the ion beam and the short cleaning time reduce possible modifications in the stoichiometry of the sample surface. XPS spectra were obtained under multiplex repetitive-scan mode, setting a energy window for each XPS peak. No signal smoothing was attempted and a scanning step of 1 and 0.2 eV/step with an interval of 50 ms was used for survey and multiplex modes, respectively. The spectrometer was calibrated using the Cu 2p3/2 共932.4 eV兲 and Cu 3p3/2 共74.9 eV兲 lines. Bindingenergy calibration was based on C 1s at 284.6 eV. III. RESULTS AND DISCUSSION Table I lists typical values of resistivity at room temperature as a function of total Ar pressure. As seen, the electrical resistivity of thin films strongly depends on sputtering pressure. Resistivity values increase by ten orders of magnitude as PAr goes from 20 to 60 mTorr. The oxide layer deposited at 20 mTorr, exhibits a quasimetal behavior with a very low resistivity. For samples deposited at 40 and 60 mTorr, the resistivity values are greater than 107 ⍀ cm. Electrical resistivity of sample grown at 80 mTorr exceeds the detection limit of the measurement method. Maximum uncertainties in the resistivity data are estimated to be 10%. In general, perfectly stoichiometric tungsten trioxide is characterized by a very high intrinsic resistivity, and deviation from it allows conductivity enhancement. There are other mechanisms that can influence the electrical behavior of WO3 thin films, as grain size15,16 or morphology. In our case, all investigated films are confirmed to be in amorphous state, and atomic force microscopy micrograph indicates that the morphology of all our as-grown samples was characterized by fine uniformly sized grains and smooth surfaces with an average roughness of 1 nm. According to this analysis we suggest that the main phenomenon responsible for the conductivity changes in our samples is the oxygen vacancy formation with decreasing Ar pressure due to differences in the transport of sputtered species in the background gas.14 The abrupt increase in resistivity is similar to that reported by others authors using reactive sputtering.4,10 YamaJ. Vac. Sci. Technol. A, Vol. 28, No. 2, Mar/Apr 2010

400

600

800

1000

Wavelength (nm)

FIG. 1. Spectral transmittance for tungsten-oxide films deposited at different Ar pressures.

moto et al.10 used a metallic W target and varied the Ar/ O2 ratio at a total pressure of 5 mTorr. The changes in the resistivity of their films are considered to be due to the target’s surface changes from metallic to oxide, obtaining WO3 films for an oxygen flow rate around 20%. Our results demonstrate that it is possible to obtain a similar behavior in the resistivity just by varying the Ar pressure, without adding O2 as a reactive gas. Transmittance spectra for WO3 films were recorded in the 300–1100 nm wavelength range. Figure 1 shows typical spectral transmittance measured on the tungsten-oxide samples deposited at different Ar pressures. The oscillatory structures of the spectra result from the thin film interference effects. Films grown with PAr = 20 mTorr had transmittance values lower than 6% with a dark blue color. Films prepared with 40 mTorr had transmittance around 60% with a lighter blue color. All the films grown with PAr ⬎ 40 mTorr had high transmittance values 共70%兲 and were transparent. The blue color is correlated with WOx 共2 ⬍ x ⬍ 3兲 and transparent samples with stoichiometric WO3.10,11 The optical band gap was determined from the analysis of the spectral dependence of the absorption near the fundamental absorption edge. The absorption coefficient 共␣兲 near the band edge is given by ␣h␯ ⬃ 共h␯ − Eg兲2, where Eg is indirectband gap value. We observed that the working pressure in the sputtering chamber has a significant effect on the optical band gap of the films 共see Table I兲. Thin films deposited at 20 mTorr show the lower-band gap value, due to larger amounts of oxygen vacancies generated in the film at low Ar pressure. The formation of vacancies generate defect levels in the gap, and when its concentration increases, the orbitals overlap and lead to the formation of a “defect band” which explains the decrease in the band gap. Amorphous thin films deposited at 40 mTorr were subjected to post heat treatment at 300, 400, and 500 ° C in air. Figure 2 illustrates the absorptioncoefficient dependence on the energy for as-grown and thermally annealed samples. It can be seen that for temperatures lower than 500 ° C there are small changes in the absorption edge; but at 500 ° C, it shifts abruptly to lower energies. This behavior has been explained as structural modifications and


TABLE II. XPS analysis data for films deposited at different Ar pressures.

120 100

as grown 300° C 400° C 500° C

80

4

-1

D (10 cm )

331

Atomic concentration 共%兲

PAr 共mTorr兲

60

20 40 60 80

40 20 0 1.5

2.0

2.5

3.0

69.4 69.1 68.3 69.3

30.6 30.9 31.7 30.7

3.5

hQ (eV)

FIG. 2. Spectral variation in absorption coefficient for tungsten-oxide samples deposited at 40 mTorr before 共solid curve兲 and after annealing in air for 1 h at 300 ° C 共dashed line兲, 400 ° C 共dotted line兲, and 500 ° C 共dotteddashed line兲.

arrangements such as transformation from amorphous WOx into a different compound, such as monoclinic WO3 共Ref. 9兲 or full WO3 crystallization.17,18 The XRD data showed that irrespective of the Ar pressure, all films as deposited were amorphous. On the other hand all films crystallized upon heating at 300, 400, and 500 ° C in air, see Fig. 3. The peaks that can be observed in this figure for films annealed at 300 ° C, can be attributed to a hexagonal h-WO3 phase.19 For films annealed at 400 ° C there is a combination of hexagonal h-WO3 phase and monoclinic ␣-WO3 phase.19,20 For films annealed at 500 ° C only the peaks of the monoclinic ␣-WO3 phase are observed. In Fig. 2 can be seen that the absorption edge shifts to lower energies as the WO3 films crystallize. It is important to note that whatever the value of PAr, the annealed films always presented the same behavior. Considering the previous results we observed that using nonreactive sputtering, electrical and optical properties of thin films are very different depending on Ar pressure. To monitor the WOx stoichiometry as a function of this growth parameter, and correlate with resistivity and optical data, XPS analyses were systematically carried out on each sample. Compositions of thin films calculated from curve-

Intensity (arbitrary units)

W 共S = 2.8兲

O共S = 0.67兲

area analyses of XPS results for different Ar pressure are shown in Table II. Maximum uncertainties in the results are estimated to be 5%. No appreciable changes in the film compositions varying sputtering pressure were observed, however it should be mention that XPS is a surface-sensitive technique, so the values reported in Table II are mainly related with the surface region. Other point that we should take into account is the detection limit of XPS technique. This technique has sensitivity capable of detecting surface atomic concentration changes of 1%. It means, that is possible to detect variations of 1013 atoms/ cm2. According to resistivity data, the difference in carrier concentration between samples deposited at 20 and 40 mTorr is around 109 cm−3, that represent surface concentration of 106 atoms/ cm2. As we explain above, the most common defect in tungsten oxide is oxygen vacancies, which can be neutral 共V0兲, singly charged 共V+兲, or doubly charged 共V2+兲 with respect to the undisturbed lattice. The most energetically favorable to form is a doubly charged vacancy and two W5+ ions.11 In any case, one or two electrons can be transferred to conduction band. In that way, a carrier concentration change of 108 cm−3 corresponds to about 108 oxygen vacancies in volume. This value represents a change of around 106 oxygen atoms/ cm2 which is very small to be detected by the XPS. Figure 4 shows the evolution of the W 4f peak for thin

WO

W

3

0

500 C 0

400 C 0

N(E) (a. u.)

331

(d)

300 C

(c)

WO3 monoclinic

(b)

WO3 hexagonal

(a)

20

25

30

35

40

45

2T (degrees) FIG. 3. X-ray diffraction patterns of the WO3 films annealed at 300, 400, and 500 ° C, JVST A - Vacuum, Surfaces, and Films

45

40

35

30

Binding Energy (eV)

FIG. 4. W 4f photoemission spectra for samples deposited at 共a兲 20 mTorr, 共b兲 40 mTorr, 共c兲 60 mTorr, and 共d兲 80 mTorr of Ar pressure.

332


N(E) (a. u.)

O 1s

(d)

(c)

(b) (a) 535

530

525

Binding Energy (eV) FIG. 5. O 1s photoemission spectra for samples deposited at 共a兲 20 mTorr, 共b兲 40 mTorr, 共c兲 60 mTorr, and 共d兲 80 mTorr of Ar pressure.

films deposited at different pressures. In this figure are indicated the positions of 4f 5/2 and 4f 7/2 doublet of WO3 and tungsten metal. In general, W 4f peaks are very broad with highly asymmetric shapes, suggesting that tungsten atoms are in a range of different coordination environments. This behavior corresponds to amorphous films where the structure is disordered and additionally, mixed W different oxidation states are present. XPS spectrum of sample deposited at 20 mTorr shows a shoulder in the lower binding-energy region, suggesting a more metallic behavior than other samples, which exhibit a nearly linear shape in the same spectra energy range. As Ar pressure increases, the position of main peak slightly shifts to higher energy region which is attributed to the presence of intermediate WOx oxides with higher order of oxidation. In all cases we observe curves with left shoulders that correspond to WO3 region. Additionally, these results can be correlated with the oxygen XPS peak. It has been reported that, the O 1s peak for WO3 is formed by two components. The binding energy of the principal component is 530.5 eV, assigned to the oxygen atoms that form W v O chemical bonds in the oxide.21 The second component is about 531.7 eV and has been attributed to oxygen atoms in oxide stoichiometry in form hydroxyl groups and a mixture of substoichiometic oxides.22 Figure 5 shows the XPS spectra of the O 1s peak for samples prepared in this work. In our case, the peak maximum was shifting gradually from 530.5 eV for the blue metallic sample to 531 eV, corresponding to the more resistive ones. The positions of the O 1s peaks did not reach 531.7 eV, suggesting that we do not have components that correspond to OH groups. This is explained by the fact that all samples were surface cleaned by Ar+ sputtering before XPS measurements. In order to know the position of O 1s peak in the stoichiometric oxide we used WO3 in the form of power pressed into J. Vac. Sci. Technol. A, Vol. 28, No. 2, Mar/Apr 2010

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pellets as a reference. The O 1s peak in stoichiometric WO3 was found at 531.0 eV which is the same binding energy for thin film deposited at PAr = 80 mTorr. Thus from XPS analysis we can conclude that deposited films contain a mixture of oxides including WO3 and other phases with tungsten in a lower oxidation states. Film chemical composition did not change significantly compared with resistivity and transmittance measurements. That can be explained because to calculate concentrations we take the area under the XPS spectrum, which are similar for all films. However, the individual peaks of W 4f and O 1s showed variations, according to the electrical and optical data interpretation. On the other hand, stoichiometric changes in the bulk, which are correlated with electrical and optical behavior cannot be detected by XPS technique. On the basis of the above-mentioned results, we suggest that at 20 mTorr Ar pressure, thin films are oxygen deficient because in the sputtering process, tungsten 共ten times heavier than oxygen兲 is able to travel through the background gas more easily. Meanwhile, oxygen suffers a stronger scattering and consequently films are oxygen deficient.14 Consequently, thin film deposited at PAr = 20 mTorr shows low resistivity and transmittance. XPS analyses of the W 4f peak for this sample exhibit a shoulder in the low-energy regions that correspond to W. Further increase in the Ar pressure leads to the thermalization process of sputtered species in the background gas, and the transport is by diffusion. In this diffusive regime, both species are affected equally by scattering and therefore reach the substrate roughly in the same proportion as they are sputtered from the target. Then, the ratio between tungsten and oxygen arriving to the substrate is no longer dependent on the argon pressure.14 Thin films deposited at PAr ⱖ 40 mTorr show an abrupt increase in electrical resistivity compared with those deposited at 20 mTorr, which indicates the decrease in oxygen vacancies. Samples became transparent similar to near-stoichiometric tungsten-oxide thin films and XPS analyses of the W 4f peak show a mixture of oxides, but without a shoulder corresponding to W. It is important to notice that changes in the resistivity values and in the shape of W 4f peak for samples deposited at 40, 60, and 80 mTorr are small. This means that as Ar pressure increases, the formation process tends to be independent of this parameter. IV. CONCLUSIONS This work has shown that the use of different Ar pressure in a nonreactive sputtering of WO3 target enables us to prepare tungsten-oxide thin films with a wide range of resistivity and transmittance values. As deposited films are amorphous but after annealing in air at temperatures of 300 ° C or higher, all films crystallize and the defect states annealed out. The decrease in Ar working pressure resulted in oxygendeficient samples. Thin-film electrical resistivity increases with the Ar sputtering pressure. Tungsten-oxide films deposited at 20 mTorr were blue and showed the lowest optical transmittance and resistivity. The optical-band gap values ranged from 2.38 to 3.14 eV. The XPS analysis did not show

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significant quantitative changes in the surface atomic composition due to the technique detection limits. However, the electrical and optical dependence were found to be related to the formation of different mixtures of nonstoichiometric WOx oxides as revealed by XPS peak shapes. Further investigations on these films should focus on sensor performance and relationship with deposition conditions during the sputtering process from an oxide target. ACKNOWLEDGMENT This work was partially supported by Universidad Autonoma de Yucatan under PRIORI Project No. FING-06-007. 1

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