Phase evolution in zirconia thin films prepared by pulsed laser deposition

Applied Surface Science 258 (2012) 5157–5165

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Phase evolution in zirconia thin films prepared by pulsed laser deposition Maneesha Mishra a , P. Kuppusami a,∗ , Akash Singh a , S. Ramya b , V. Sivasubramanian c , E. Mohandas a a

Physical Metallurgy Group, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India Corrosion Science and Technology Group, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India c Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India b

a r t i c l e

i n f o

Article history: Received 30 September 2011 Received in revised form 4 January 2012 Accepted 29 January 2012 Available online 6 February 2012 Keywords: Zirconia Pulsed laser deposition Phase transformation Crystallite size Optical properties

a b s t r a c t Zirconia thin films were deposited on silicon (1 0 0) and quartz substrates using pulsed laser deposition. Phase formation in zirconia films was monitored as a function of substrate temperature (473–973 K) and oxygen partial pressure (0.001–1 Pa). Volume fraction of tetragonal zirconia is determined from X-ray diffraction and Raman analysis. Tetragonal volume fraction of zirconia films varies from 10 to 76% for different substrate temperature and oxygen partial pressure. Zirconia films show a good transparency in the visible region, except for the films deposited at 473 K and at 0.002 Pa. The band gap values and refractive index of the films are discussed in relation with the microstructure and phase composition of the zirconia films as well as with 8 mol% yttria stabilised zirconia films. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Zirconium oxide (ZrO2 ) is a transition metal oxide which offers several interesting technological applications due to its properties like high melting point, high refractive index, good thermal, chemical and mechanical stabilities, wide band gap, high dielectric constant and electrical resistivity. It finds applications in optical coatings, protective coatings and insulating layers [1–3], catalyst for alcohol synthesis [4], in fuel cells [5] as well as in thermal barrier coatings [6]. Three different phases of zirconium oxide are reported in literature, namely, monoclinic zirconia (m-zirconia) in the temperature <1443 K, tetragonal (t-zirconia) in the range 1443–2643 K and cubic phase (c-zirconia) in the range 2643–2903 K [7]. Among the three phases, tetragonal and cubic phases are formed at very high temperatures. But these high temperature phases can be retained at room temperature by doping with some suitable oxides such as aluminium oxide [8], yttrium oxide [9], magnesium oxide [10], and calcium oxide [9,11]. However, it is interesting to note that un-doped zirconium oxide can also be retained in tetragonal phase at room temperature in thin film forms [12,13], as a result of small crystallite size [12–16] and compressive stress [17]. Garvie [18] prepared tetragonal zirconia in powder form at room temperature and correlated the stability of the tetragonal phase

∗ Corresponding author at: Physical Metallurgy Group, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India. Tel.: +91 44 27480306; fax: +91 44 27480121. E-mail address: [email protected] (P. Kuppusami). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.160

with mean crystallite size and surface energy. The critical size for tetragonal to monoclinic transition has been reported to be 30 nm [13,18]. The critical size for the stabilisation of zirconia in tetragonal phase has been found to vary according to the preparation method. The reported value of critical crystallite size for the stabilisation of high temperature zirconium oxide phase is 50–55 nm for the films deposited by metal oxide chemical vapour deposition (MOCVD) [12], 18 nm for films deposited by thermal spray method [15]. Influence of crystallite size is mainly based on the increase in surface energy of zirconia by decreasing the crystallite size [18,19]. To stabilise the tetragonal phase at lower temperature, the following condition should be satisfied [18,19]: (Gt − Gm ) + (St t − Sm m ) ≤ 0

(1)

where Gt , Gm are the molar free energy,  t and  m are the molar surface energy and St , Sm are the surface area of t-zirconia and m-zirconia phases, respectively. When crystallite size decreases, molar surface area increases and beyond certain value of molar surface area, Gt becomes less than Gm and the whole term in the above equation becomes negative, resulting in the stabilisation of tetragonal phase [18]. Compressive stress in the film can also stabilise the tetragonal phase at lower temperature when the molar volume of t-zirconia is lower than that of the m-zirconia [17,20]. Various methods such as MOCVD [12,21], chemical vapour deposition (CVD) [22], ion assisted deposition (IAD) [23], ultrasonic nebulisation and pyrolysis technique [2], pulsed laser deposition (PLD) [24–26], sol–gel method [27], sputtering technique [13,28], thermal oxidation [29], plasma and ion beam deposition [30] and electron beam deposition [31] have been used to prepare zirconia

M. Mishra et al. / Applied Surface Science 258 (2012) 5157–5165

70

60

m (140)

50

m (-401)

m (211) m (-202)

m (120) m (-112) m (201)

40

m (231)

30

m (220) m (022) t (112) m (-122) m (003) m (310) m (-113) t (103) m (-203) m (311) m (-312) m (113) m (230) m (-132)

20

m (200) m (020) t (002) m (002)

t (011)

Intensity (arb.units)

m (111)

m (-111)

thin films. Also, the structural [2,26,32–34], optical [2,26,34–40], electrical [2,36,41–45], wear [28], luminescence [46], bioactivity and cytocompatibility [35] properties of the thin films of zirconium oxide were reported. In the present work, zirconia thin films were prepared on (1 0 0) oriented silicon (Si) and quartz substrates by the PLD technique at different substrate temperature and oxygen partial pressure. Substrate temperature was varied from 473 to 973 K and the oxygen partial pressure from 0.001 to 1 Pa. The effect of substrate temperature and oxygen partial pressure on the formation of mzirconia and t-zirconia phases was also investigated by Raman spectroscopy. Several experiments as a function of temperature and oxygen partial pressure over a wide range were carried out to establish phase mapping of zirconia in the films prepared by PLD technique. Variation in the optical properties is also studied at two different oxygen partial pressures and substrate temperature of 773 K and is discussed in relation with the microstructure of the zirconia films.

m (110) m (011)

5158

80

2θ (degrees) 2. Experimental procedure

Fig. 1. XRD pattern of a zirconia target used in the PLD experiments.

A zirconia target (25 mm diameter and 3 mm thickness) was prepared by using commercially available zirconia powder (99.99% purity) at a pressure of 10 MPa using a uniaxial press and was sintered at 1673 K for 6 h. Zirconia thin films were ablated using this target by the PLD technique at different substrate temperature and oxygen partial pressure. Excimer laser (Lambda Physik, Compex205) with laser energy of 200 mJ/pulse and repetition rate of 10 Hz was used as an energy source. Substrate temperature was varied from 473 to 973 K and oxygen partial pressure was varied from 0.002 to 2 Pa. Both Si (1 0 0) and quartz substrates were ultrasonically cleaned with soap solution, water and methyl alcohol and then mounted onto the substrate holder using silver paste. The chamber was evacuated to a base pressure of 0.002 Pa using a turbo molecular pump and a rotary pump. The target was rotated with a speed of 10 rpm to avoid grooving of the target. A limited number of thin films of 8 mol% yttria stabilised zirconia (YSZ) were prepared from a sintered YSZ target on Si(1 0 0) and quartz substrates under similar deposition conditions for a comparative study. The thickness of the films was measured using a Dektak profilometer (DEKTAK 6M-stylus profiler). Phase identification and phase evolution of the deposited films were studied in an INEL XRG-3000 X-ray diffractometer (XRD) attached with a curved position sensitive detector using CuK␣1 (0.15406 nm) radiation. The crystallite size (D) was calculated for (2 0 0) and (0 0 2) reflections of m-zirconia and tzirconia, respectively, using the following Scherrer formula with an accuracy of ±3 nm: D=

0.9 ˇ cos 

(2)

where  is the wavelength of X-rays used,  is the angle of diffraction and ˇ is the full width at half maximum (FWHM) after √ subtracting instrumental broadening and ˇ = (B2 − b2 ), where B is the FWHM of the peak and b is the instrumental broadening obtained from the standard Si powder received from National Physical Laboratory, New Delhi. From the XRD analysis of the films, the crystallite size, volume fraction and strain of both monoclinic and tetragonal phases were calculated. Volume fraction of tetragonal phase was qualitatively calculated using the prominent peak intensities of monoclinic (Im(2 0 0) ) and tetragonal (It(0 0 2) ) phases by using Eq. (2). Vt =

It(0 0 2) Im(2 0 0) + It(0 0 2)

(3)

The preferred orientation is, however, not taken into consideration in the above equation. The strain in the films was calculated using the following equation for the monoclinic and tetragonal phases: ε=

ˇ 4 tan 

(4)

where ε is the strain in the film, ˇ is the FWHM of (2 0 0) and (0 0 2) reflections of monoclinic and tetragonal peak of zirconium oxide, respectively and  is the angle of diffraction. Raman spectroscopy was performed using a spectrometer (HR 800, Jobin Yvon) equipped with 1800 grooves/mm holographic grating. The samples were placed under an Olympus BXFM-ILHS optical microscope mounted at the entrance of the Raman spectrograph. Argon ion laser of wavelength 488 nm was used as an excitation source. The laser spot size of 3 mm diameter was focused tightly on the sample surface using a diffraction limited (numerical aperture = 0.25 at 10×) long distance objective. The laser power at the sample was ∼15 mW. The slit width of the monochromator was 400 ␮m. The back scattered Raman spectra were recorded using super cooled (∼110 ◦ C) charge-coupled device (CCD) detector. Three sets of zirconia films were prepared on quartz substrates for optical property measurements. Two sets were prepared for different substrate temperatures (473–973 K) at oxygen partial pressure of 1 Pa and 0.002 Pa. The third set of samples were prepared at 773 K and different oxygen partial pressures (2–0.002 Pa). Optical properties of the films, such as transmittance, absorbance, band gap and refractive index were measured using an ultraviolet–visible (UV–vis) spectrometer (Jasco V-650, spectrophotometer) in transmission mode and band gap values were determined from Tauc plot [47] with an accuracy of ±0.1 eV. 3. Results and discussion 3.1. X-ray analysis The XRD pattern of the sintered zirconia pellet indicates that the pellet contains a mixture of both tetragonal and monoclinic phases of zirconia and absence of any impurity phase in the target (Fig. 1). The XRD plot shows reflections at angles (2) 28.175◦ , 34.16◦ , 35.309◦ , 45.20◦ , 49.75◦ , 55.20◦ , 58.9◦ , 60.20◦ and 29.98◦ , 34.81◦ , 35.25◦ , and 50.37◦ corresponding to m-zirconia and tzirconia phases, respectively. The XRD patterns were indexed using the JCPDS File no. 37-1484 and 50-1089 for m-zirconia and tzirconia phases, respectively.

M. Mishra et al. / Applied Surface Science 258 (2012) 5157–5165

5159

973 K 873 K

20

30

40

50

60

(122) m (013) t (311) m

(112) t

(-202) m

(-111) m

(011) t (200) m (002) m (002) t

Intensity (arb. units)

(-322)m

(013)t (311)m

(122)m

(112)t

(211)m

(011)t (200)m (002)m

(-111)m

Intensity (arb. units)

(002)t

a

0.002 Pa

0.02 Pa

773 K 673 K

0.2 Pa

473 K

2 Pa

70

80

20

30

40

50

60

70

80

90

2 θ (degrees)

2 θ (degrees)

b

(013)t (311)m

(221)m

(-202)m

Intensity (arb. units)

(200)m (002)m (002)t

Fig. 3. XRD pattern of zirconia thin films deposited on Si (1 0 0) substrates at different oxygen partial pressures (2–0.002 Pa) and at substrate temperature of 973 K.

973 K 873 K 773 K 673 K 473 K

30

40

50 60 2 θ (degrees)

70

80

90

Fig. 2. XRD pattern of the zirconia thin films deposited on Si (1 0 0) substrates at different substrate temperatures (473–973 K) and at oxygen partial pressures of (a) 2 Pa and (b) 0.002 Pa.

Fig. 2(a) shows the XRD patterns of zirconia thin films that were deposited at different substrate temperatures (473–973 K) at oxygen partial pressure of 2 Pa. From the results, it is observed that all the films are polycrystalline in nature, having both monoclinic and tetragonal phases. XRD pattern of zirconia thin films that were deposited at the substrate temperature of 473 K mostly show reflections at 2 values of 35.2◦ , 55.2◦ and 60.2◦ corresponding to (2 0 0), (2 2 1) and (3¯ 2 1) planes of m-zirconia with small amount of t-zirconia phase identified using (0 0 2) reflection at 34.8◦ (2). With increase in the substrate temperature, the intensity of the peaks at 2 values of 34.81◦ and 58.9◦ , corresponding to (0 0 2) and (0 1 3) planes of tetragonal phase increases. At substrate temperature of 973 K, the peaks corresponding to the tetragonal phase have the highest intensity with a preferred orientation of (0 0 2) for the tetragonal phase. The study indicates that the tetragonal phase content increases with an increase in the substrate temperature. It is also observed that at 473 K, the film is less crystalline and the fractions of monoclinic and tetragonal phases are similar to that of the target material. As the substrate temperature increases to 773 K, the intensity of monoclinic peaks

corresponding to the reflections (2 0 0), (0 0 2) and (2 2 1) increases, whereas above 873 K, the intensity of the monoclinic peaks decreases and intensity of tetragonal peaks corresponding to the reflections of (0 0 2) and (0 1 3) increases. The initial increase in the intensity of the monoclinic phase up to the temperature of 873 K is attributed to an improved crystallinity with substrate temperature, while the decrease in the intensity of the monoclinic phase at >873 K indicates the partial transformation of the film to tetragonal phase. The transformation from m-zirconia to t-zirconia occurs as per the phase diagram for bulk materials. However, the transformation temperature is low because of the non-equilibrium conditions provided by the laser induced plasma during film deposition. Fig. 2(b) shows the XRD pattern of the zirconia thin films deposited at different substrate temperatures (473–973 K) and oxygen partial pressure of 0.002 Pa. From the XRD pattern, it is clear that all the films are polycrystalline in nature and they consist of both m-zirconia and t-zirconia phases. The XRD pattern shows peaks corresponding to 2 values of 33.7◦ , 35.5◦ , 45.1◦ , 55.2◦ and 61.8◦ corresponding to (2 0 0), (0 0 2), (2¯ 0 2), (2 2 1) and (3 1 1) reflections of m-zirconia and 34.4◦ and 58.9◦ corresponding to (0 0 2) and (0 1 3) reflections of t-zirconia. Intensity of the peaks corresponding to t-zirconia increases with increase in the substrate temperature, which indicates that formation of tetragonal phase is favoured at higher substrate temperature [48]. Also, the increase in the intensity of the peaks of t-zirconia with increasing substrate temperature indicates better crystallinity at higher temperature. Fig. 3 shows the XRD pattern of zirconia thin films deposited at different oxygen partial pressures (2–0.002 Pa) and at substrate temperature of 973 K. At all oxygen partial pressures, the film has preferred orientation along (0 0 2) plane of tetragonal phase. XRD pattern shows 2 values of 28.1◦ , 33.5◦ , 35.6◦ , 45.0◦ , 55.4◦ and 61.7◦ corresponding to (1¯ 1 1), (2 0 0), (0 0 2), (2¯ 0 2), (1 2 2) and (3 1 1) reflections of m-zirconia and 29.9◦ , 34.8◦ , 49.7◦ , 59.1◦ corresponding to (0 1 1), (0 0 2), (1 1 2) and (0 1 3) reflections of t-zirconia. XRD results clearly indicate the polycrystalline nature of zirconia films for all oxygen partial pressure. There is a decrease in the intensities of the peaks of m-zirconia with increase in the oxygen partial pressure. At oxygen partial pressure of 2 Pa, almost all the peaks corresponding to m-zirconia has disappeared except the (1 2 2) and (0 0 2) reflection. This indicates that higher oxygen partial pressure is favoured for the formation of t-zirconia.

M. Mishra et al. / Applied Surface Science 258 (2012) 5157–5165

Table 1 Crystallite size of monoclinic zirconia deposited on Si(1 0 0) at different substrate temperatures (473–973 K) and oxygen partial pressures of (2–0.002 Pa). Temperature (K)

473 673 773 873 973

Oxygen partial pressure (Pa) 2

0.2

0.02

0.002

6 16 19 24 17

9 17 20 23 18

14 18 19 23 17

22 30 22 24 19

a % Volume fraction

5160

Table 2 Crystallite size of tetragonal zirconia deposited on Si(1 0 0) at different substrate temperatures (473–973 K) and oxygen partial pressure (2–0.002 Pa).

473 673 773 873 973

Oxygen partial pressure (Pa)

473 673 773 873 973

74 72 70 68

400

2

0.2

0.02

0.002

12 18 24 23 35

7 15 21 21 26

17 15 18 23 28

9 19 16 25 27

Table 3 Volume fraction of tetragonal phase of zirconia at different substrate temperatures (473–973 K) and oxygen partial pressures (2–0.002 Pa). Temperature (K)

Tetragonal Monoclinic

66

Oxygen partial pressure (Pa) 2

0.2

0.02

0.002

26 26 24 66 76

22 26 23 40 70

23 13 10 64 68

19 10 36 45 72

Crystallite size calculated for the (2 0 0) and (0 0 2) peaks of m and t-zirconia, respectively, are listed in Tables 1 and 2, respectively. For monoclinic phase, the crystallite size calculated for (2 0 0) peak of m-zirconia varies from 6 nm to 24 nm at 2 Pa, where it varies from 19 to 30 nm at 0.002 Pa in the temperature range 473–873 K. Crystallite size of the monoclinic peak decreases to ∼17–19 nm for the substrate temperature > 873 K. Whereas the crystallite size calculated for the (0 0 2) plane of t-zirconia increases from 12 to 35 nm and 9 to 27 nm, with increase in the substrate temperature from 473 to 973 K, for oxygen partial pressure of 2 and 0.002 Pa, respectively. As the substrate temperature increases, the mobility of adatoms increases at the substrate surface and form the lowest energy structure due to the increase in the mobility, which results in the increase in the crystallite size in accordance with the trend reported in the literature [49]. Volume fraction of the phases has been calculated using Eq. (2) from the intensity of (2 0 0) and (0 0 2) reflections corresponding to m-zirconia and t-zirconia phases, respectively and the values are listed in Table 3. The value of tetragonal volume fraction is found to increase from 10 to ∼76% with increasing temperatures and oxygen partial pressures. The monoclinic volume fraction increases up to 773 K for all oxygen partial pressures except for 0.001 Pa and then it decreases. In the case of oxygen partial pressure of 0.001 Pa, the volume fraction increases only up to 673 K and then decreases indicating an increase in the tetragonal phase content with increase in the substrate temperature. The influence of oxygen partial pressure on the crystallite size and volume fraction of the phases could not be clearly brought out due to the changes that occur simultaneously in the stoichiometry, crystallite size and phase transformation of the films. In summary, results of the XRD analysis show that higher substrate temperature and oxygen partial pressure favour the

500

600

700

800

900

1000

Temperature (K)

b

Tetragonal

76

% Volume fraction

Temperature (K)

76

74

72

70

68 0.0

0.5

1.0

1.5

2.0

Oxygen partial pressure (Pa) Fig. 4. Phase evolution diagram of zirconia films at (a) different substrate temperature and oxygen partial pressure of 2 Pa and at (b) different oxygen partial pressure and at substrate temperature of 973 K.

tetragonal phase formation. Fig. 4(a) and (b) shows the phase evolution diagram of zirconia at different substrate temperatures (at oxygen partial pressure of 2 Pa) and at different oxygen partial pressures (at substrate temperature of 973 K), respectively. In both the figures, only the phase, which has more than 50% volume fraction (dominating phase) is represented. Fig. 4(a) shows that the m-zirconia is dominating up to the substrate temperature 773 K and beyond 773 K, t-zirconia is the dominating phase. Similarly for the films grown at 973 K and at different oxygen partial pressures (Fig. 4b), tetragonal phase is found to dominate. The results obtained from Raman analysis are in favour of the XRD results. The increase in the stress in the films is generally attributed to the peening effect of the ablated atoms at the substrate surface and is influenced by the substrate temperature and oxygen partial pressure. Strain values of the zirconia films as a function of the substrate temperature and oxygen partial pressure were calculated using Eq. (3) and are shown in Fig. 5. The strain value for tetragonal phase decreases with the increase in the substrate temperature as a consequence of stress relaxation due to the increased crystallite size. The change in the stress of the films as a function of oxygen partial pressure could be related to reduction in the kinetic energy of the ablated species with the increase in the oxygen partial pressure. It is expected that the ablated species lose their kinetic energy with the increase in the oxygen partial pressure due to the collisional process with surrounding oxygen molecules, leading to less strain at higher oxygen partial pressures. In contrast, Shen et al. [50] have

M. Mishra et al. / Applied Surface Science 258 (2012) 5157–5165

973 K 873 K 773 K 473 K

0.012

0.010 0.009 0.008 0.007 0.006

400

500

600

700

800

900

1000

110

120

130

140

-1

150

160

170

180

190

200

210

220

-1

Temperature (K)

reported the formation of t-zirconia by electron beam evaporation with decreasing oxygen partial pressure in the range 2 × 10−3 to 2 × 10−2 Pa at a temperature of 573 K which are at least two orders of magnitude lower than the pressures used in the present study. These contradicting results suggest that the phase formation in zirconia is more complex and depends on the method of deposition, process conditions, energetics of the depositing species, etc.

973 K 873 K 773 K

(148 cm )

b

-1

Fig. 5. Variation in strain for tetragonal zirconia deposited at different substrate temperatures and oxygen partial pressures.

Intensity (arb. units)

Wavenumber (cm )

-1

0.003

2t t0.2 t0.02 t0.002

(189 cm )

0.004

-1

0.005

(178 cm )

Strain

0.011

191 cm

0.013

-1

Intensity (arb. units)

0.014

147 cm-1

0.015

178 cm

a

0.016

5161

3.2. Raman analysis Raman analysis of zirconia films was carried out on two sets of samples at different substrate temperature and oxygen partial pressure of 2 Pa (Fig. 6(a)) and 0.002 Pa (Fig. 6(b)). Fig. 6(a) shows Raman bands at 147 for tetragonal and 180, 191 cm−1 for monoclinic phase, respectively in accordance with the literature [51–54]. Raman bands are mainly affected by the parameters such as crystallite size and stoichiometry. The intensity and FWHM of the Raman band affect crystallite size, while the peak position is known to affect the stress in the films [55]. The intensity of the Raman peak at 147 cm−1 , which belongs to the t-zirconia (Fig. 6(a)), indicates that the tetragonal phase formation is more favoured at higher substrate temperature and oxygen partial pressure. The position of the Raman peak at 147 cm−1 (corresponding to t-zirconia) shifts towards lower wave number side with the increase in the substrate temperature, indicating strain relaxation, which is also observed from the XRD results. Whereas the Raman peaks at 178 and 191 cm−1 (corresponding to m-zirconia) shift to higher wave number side with increase in the substrate temperature, indicating increase in strain in the monoclinic phase. Fig. 6(b) shows the Raman spectra of the zirconia films deposited at different substrate temperature and oxygen partial pressure of 0.002 Pa. No Raman peak corresponding to tetragonal phase (147 cm−1 ) is observed for substrate temperature of 773 and 873 K. Raman peak at 147 cm−1 (corresponding to t-zirconia) is present only in the zirconia film deposited at 973 K and 0.002 Pa of oxygen partial pressure. Tetragonal volume fractions in the films were calculated qualitatively from the area under the peaks of 147 and 180 cm−1 belonging to tetragonal and monoclinic phases, respectively using the following equation: At Vt = At + Am

110 120 130 140 150 160 170 180 190 200 210 220 -1

Wavenumber(cm ) Fig. 6. Raman spectra of zirconia films deposited on Si(1 0 0) at (a) substrate temperature of 473–973 K and oxygen partial pressure of 2 Pa and (b) substrate temperature in the range 773–873 K and 0.002 Pa.

where Vt is the volume fraction of t-zirconia, At is the area under the peak at 147 cm−1 and Am is the area under peak at 180 cm−1 . Volume fraction calculated from the Raman data shows that the highest volume fraction of t-zirconia (∼67%) is obtained for the films deposited at the substrate temperature of 973 K and oxygen partial pressure of 2 Pa. The trend in the volume fraction of the phases as a function of temperature and oxygen partial pressure obtained from Raman analysis is comparable with the XRD results. 3.3. Optical properties The absorbance of the films was measured in the wavelength range of 200–900 nm using the UV–vis spectrometer. Since we obtained similar structural and microstructural properties of zirconia films deposited on silicon and quartz substrates in agreement with our earlier work on yttria thin films [56], the optical properties obtained on quartz substrates alone are presented in the following section. The absorption coefficient (˛) was calculated using Eq. (6) [38,56,57]: ˛=

(5)

2.303 × Ab t

(6)

where Ab is the absorbance, t is the thickness of the film. The band

5162

a

2.2 473 K 673 K 773 K 873 K 973 K

2.0

Absorbance

1.8 1.6 1.4

100 80

% Transmittance

a

M. Mishra et al. / Applied Surface Science 258 (2012) 5157–5165

1.2 1.0 0.8 0.6

60 40 473 K 673 K 773 K 873 K 973 K

20

0.4 0

0.2 225

275

300

325

350

375

400

300

450

b

1.8 473 K 673 K 773 K 873 K 973 K

1.6 1.4 1.2

Absorbance

425

0.8 0.6 0.4

50 40 30

10

300

c 2 Pa 0.2 Pa 0.02 Pa 0.002 Pa

1.2 1.0

400

500

600

700

0.6 0.4 0.2

100

70 60 50 40 30

2 Pa 0.2 Pa 0.02 Pa 0.002 Pa

20 10

0.0 0 300

250 300 350 400 450 500 550 600 650 700 750 800 850

400

500

600

700

800

900

Wavelength (nm)

Wavelength (nm) Fig. 7. Absorbance versus wavelength plot of the zirconia films deposited on quartz substrates at (a) different substrate temperature and oxygen partial pressure of 2 Pa, (b) different substrate temperature and oxygen partial pressure of 0.002 Pa and (c) different oxygen partial pressure and substrate temperature of 773 K.

gap (Eg ) for the films was measured by Tauc plot using the following equation [56]: n

900

80

0.8

(˛h) = A(h − Eg )

800

Wavelength (nm) 90

% Transmittance

1.4

473 K 673 K 773 K 873 K 973 K

20

800

Wavelength (nm)

Absorbance

60

0

700

900

70

-10

600

800

100

-0.2 500

700

Wavelength (nm)

0.0 400

600

80

1.0

300

500

90

0.2

c

400

Wavelength (nm)

% Transmittance

b

250

(7)

where h is the Planck’s constant and  is the frequency of the incident radiation. Fig. 7(a) and (b) shows the absorbance versus wavelength plot of zirconia thin films deposited at different substrate temperatures and oxygen partial pressure of 2 Pa and 0.002 Pa, respectively. Similarly, Fig. 7(c) shows the absorbance

Fig. 8. Transmittance versus wavelength plot of the zirconia films deposited on quartz substrates at (a) different substrate temperature and oxygen partial pressure of 2 Pa, (b) different substrate temperature and oxygen partial pressure of 0.002 Pa and (c) different oxygen partial pressure and substrate temperature of 773 K.

versus wavelength plot of zirconia thin films deposited at different oxygen partial pressure (2–0.002 Pa) and substrate temperature of 773 K. All the films show two absorption edges at ∼225 nm and ∼250 nm, corresponding to two phases of zirconia. The result is in agreement with the observation by Balakrishnan et al. [38]. Fig. 8(a) and (b) shows the variation of % transmittance verses wavelength plot for the films deposited at different substrate temperatures and constant oxygen partial pressure of 2 and 0.002 Pa, respectively. Fig. 8(c) represents the % transmittance of zirconia films deposited

M. Mishra et al. / Applied Surface Science 258 (2012) 5157–5165 Table 4 Band gap values of zirconia thin films deposited on quartz substrates at different substrate temperatures (473–973 K) and oxygen partial pressure of 2 Pa.

5163

Table 5 Band gap values of zirconium oxide thin films deposited on quartz substrates at different substrate temperatures (473–973 K) and oxygen partial pressure of 0.002 Pa.

Temperature (K)

Band gap of tetragonal phase (eV) (±0.1 eV)

Band gap of monoclinic phase (eV) (±0.1 eV)

Refractive index ()

Temperature (K)

Band gap of tetragonal phase (eV) (±0.1 eV)

Band gap of monoclinic phase (eV) (±0.1 eV)

Refractive index ()

473 673 773 873 973

4.52 4.55 4.77 4.96 5.1

5.0 5.05 5.11 5.13 5.58

1.76 1.7 1.88 1.93 2.47

473 673 773 873 973

– 5.15 5.1 5.14 5.16

– 5.21 5.21 5.21 5.41

– 1.62 1.92 2.0 2.5

band bending effect in nanocrystallities [59]. We have also recently reported such effects in the yttria thin films prepared by PLD technique [56]. Similarly, the band gap energy calculated for the films deposited at different substrate temperature and oxygen partial pressure of 0.002 Pa, increases from 5.15 to 5.16 and from 5.21 to 5.43 corresponding to tetragonal and monoclinic phases, respectively [40,60]. For the films deposited at different oxygen partial pressure, the band gap of t and m-zirconia increases from 4.42 to 5.1 eV [32] and from 5.11 to 5.25 eV, respectively in accordance with the reported values [39,60]. The refractive index of the films was calculated using the following equation [38,61]:  = [N + (N 2 − n20 n21 )

1/2 1/2

]

(8)

(n20

+ n21 )/[2 + 2n0 n1 ((Tmax − Tmin )/Tmax × Tmin )], n0 is where N = the refractive index of air (1.0), n1 the refractive index of the quartz substrate (∼1.458), Tmax and Tmin are the maximum and minimum transmittance, respectively. Refractive index calculated for the zirconia films deposited at different substrate temperature and at the oxygen partial pressure of 2 and 0.002 Pa are listed in Tables 4 and 5, respectively. The results clearly show an increase in the refractive index with the increase in the substrate temperature and decrease with the oxygen partial pressure. The refractive index of the zirconia films deposited at different substrate temperatures and oxygen partial pressure of 2 Pa, increases from 1.76 (±0.1) to 2.47. Similarly, the refractive index for the films deposited at different substrate temperature and oxygen partial pressure of 0.002 Pa increases from 1.62 to 2.5 in accordance with the literature [39,50].

(111)

at different oxygen partial pressures and substrate temperature of 773 K. Transmittance of a film is highly dependent on factors like thickness, roughness, porosity and oxygen vacancies [56,58] in the films. In the present study, all the films show transmittance in the range 65–92% (measured at 550 nm) except the film deposited at 973 K (∼60%). The decrease in the transmittance at higher temperature may be related to the loss of light by scattering, as a consequence of the increase in surface roughness values due to the increase in the crystallite size. Fig. 8(b) shows the transmittance of the zirconia films deposited at different substrate temperature and at oxygen partial pressure of 0.002 Pa. The transmittance values vary from 60 to 85% as a function of substrate temperature. However, the films deposited at 473 K and at 0.002 Pa show the lowest value of the transmittance and it can be related to the loss of crystallinity in the film in accordance with the XRD results [58]. Fig. 8(c) shows the % transmittance of the zirconia films deposited at different oxygen partial pressure and substrate temperature of 773 K. All the films show good transmittance of 60–85%. The highest transmittance is obtained for the films deposited at higher oxygen partial pressure, and it decreases with the decreasing partial pressure of oxygen as a result of the lower content of oxygen in the films [38]. Band gaps were determined for the films calculated by Tauc plot [47]. A typical Tauc plot for the zirconia films deposited at the substrate temperature of 973 K and oxygen partial pressure of 2 Pa is shown in Fig. 9. All the zirconia films deposited at different substrate temperature and oxygen partial pressure show two band gaps belonging to tetragonal and monoclinic phases of zirconia (Table 4). Band gap values vary from 4.52 to 5.1 and 5.0 to 5.58 eV corresponding to tetragonal and monoclinic phases, respectively, for the films deposited at oxygen partial pressure of 2 Pa and at different substrate temperatures [32]. Band gap values are found to increase with increase in the substrate temperature due to the increase in crystallite size. This can be explained on the basis of

15

2

(α α hν )

15

1.5x10

15

1.0x10

14

5.0x10

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

hν (eV) Fig. 9. A typical Tauc plot for band gap calculation for zirconia film deposited on quartz substrate at 973 K and oxygen partial pressure of 2 Pa.

20

30

40

60

70

(331) (420)

50

(400)

(311)

Intensity (arb. units)

15

2.0x10

0.0 3.0

(200)

15

2.5x10

(220)

3.0x10

80

2θ (degrees) Fig. 10. XRD pattern of (8 mol%) YSZ thin films deposited on Si(1 0 0) at substrate temperature of 573–873 K and oxygen partial pressure of 0.2 Pa.

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M. Mishra et al. / Applied Surface Science 258 (2012) 5157–5165

The change in the refractive index as a function of the processing conditions could be related to the change in the microstructure of the films, especially, the crystallite size, porosity and packing density [58,62,63]. The lower value of the refractive index of the films deposited at higher oxygen partial pressure may be attributed to lower crystallite size and packing density of the films [38,50,64]. From the above results it can be concluded that the synthesis of a phase pure zirconia film from a target containing two phase structure by PLD poses constraint in the present range of substrate temperature and oxygen partial pressure. The variation in stress fields and the elastic properties of the particles could possibly prohibit the formation of a single phase film from a target containing a mixture of phases. The synthesis of a single phase becomes more tedious when we deal with varying particle sizes as a function of the process parameters. Therefore, efforts were taken to prepare phase pure cubic zirconia from a sintered target of 8 mol% yttria stabilised zirconia at oxygen partial pressure of 0.2 Pa and in the substrate temperature range 573–873 K. Fig. 10 shows the XRD pattern of the YSZ films deposited on Si (1 0 0) substrates at substrate temperature of 573–873 K and oxygen partial pressure of 0.2 Pa. XRD analysis shows peaks at 30.1◦ , 34.9◦ , 50.2◦ , 59.7◦ ,73.9◦ , 81.7◦ and 84.4◦ corresponding to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (4 0 0), (3 3 1) and (4 2 0) reflections of cubic zirconia (JCPDS file no: 49-1642). Transmittance of the films measured by UV–vis spectrometer shows a good transmittance of ∼90% at 500 nm. The detailed correlation between the microstructure and optical properties of the latter films will be published in the future.

4. Conclusions In summary, zirconia films were prepared by PLD at different substrate temperatures and oxygen partial pressures. XRD and Raman techniques were used to carry out the phase mapping of the zirconia in the temperature range 473–973 K and oxygen partial pressure in the range 2–0.002 Pa. (i) XRD analysis shows an increase in the crystallinity with increase in the substrate temperature. The effect of oxygen partial pressure on the crystallite size is not uniquely brought out as it influences the stoichiometry and transformation of the phases simultaneously at any substrate temperature. (ii) Highest volume fraction of tetragonal zirconia (∼76%) is obtained for the films deposited at a substrate temperature of 973 K and oxygen partial pressure of 2 Pa. The volume fraction obtained from Raman analysis also supports the results obtained from XRD. (iii) The strain value for tetragonal phase decreases with increase in the substrate temperature as a consequence of stress relaxation due to increased crystallite size. The strain in the films decreases with increasing oxygen partial pressure due to reduction in the kinetic energy of the ablated species. (iv) Except for the films deposited at 473 K and 0.002 Pa, all the films of zirconium oxide show good transmittance in UV–vis region. The band gap calculated for zirconia films shows an increasing trend with increase in the substrate temperature and decrease in the oxygen partial pressure. Refractive index also increases with increase in the substrate temperature and decrease in the oxygen partial pressure as a consequence of variation in the microstructure of the films.

Acknowledgements We thank Smt. Jyothi for X-ray analysis. We also thank Dr. M. Vijayalakshmi, Associate Director, PMG, Dr. T. Jayakumar, Director,

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