Optical and Electrical Properties of Copper Oxide Thin Films

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Aug 31, 2015 - spray pyrolysis technique (SPT) [9], radio frequency magnetron sputtering [12], spin coating [13], dip coating [14], SILAR [15], thermal ...

Sensors & Transducers, Vol. 191, Issue 8, August 2015, pp. 21-27

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Optical and Electrical Properties of Copper Oxide Thin Films Synthesized by Spray Pyrolysis Technique 1 1

S. S. Roy, 2 A. H Bhuiyan, 2 J. Podder

Department of Physics, Dwarika Paul Mohila Degree College, Sreemongal-3210 Department of Physics, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh Tel.: 88-01711983489 E-mail: [email protected]

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Received: 14 July 2015 /Accepted: 17 August 2015 /Published: 31 August 2015 Abstract: Copper oxide (CuO) thin films have been synthesized on to glass substrates at different temperatures in the range 250-450 °C by spray pyrolysis technique from aqueous solution using cupric acetate Cu(CH3COO)2H2O as a precursor. The structure of the deposited CuO thin films characterized by X-ray diffraction, the surface morphology was observed by a scanning electron microscope, the presence of elements was detected by energy dispersive X-ray analysis, the optical transmission spectra was recorded by ultravioletvisible spectroscopy and electrical resistivity was studied by Van-der Pauw method. All the CuO thin films, irrespective of growth temperature, showed a monoclinic structure with the main CuO (111) orientation, and the crystallite size was about 8.4784 Å for the thin film synthesized at 350 °C. The optical transmission of the asdeposited film is found to decrease with the increase of substrate temperature, the optical band gap of the thin films varies from 1.90 to 1.60 eV and the room temperature electrical resistivity varies from 30 to18 Ohmcm for the films grown at different substrate temperatures. Copyright © 2015 IFSA Publishing, S. L. Keywords: Spray pyrolysis, CuO, Band gap, Substrate temperature.

1. Introduction Copper oxide (CuO) thin film has been reported to exhibit p-type conduction and shows a band gap of 1.64 and it is a monoclinic crystal structure with lattice parameters a = 4.6837 Å, b = 3.4226 Å, c = 5.1288 Å and β = 99.54o [1-3]. CuO, an important transition metal oxide semiconductor has been extensively studied for a number of applications like gas sensors [4-6], solar cells [7-8], lithium ion electrode [9], etc. Thermal preparation methods result in resistivities in the range 102-104 Ω-cm and electrodeposition produces films with resistivities in the range 104-106 Ω-cm. [10-11]. There are various established ways of fabricating CuO thin films like spray pyrolysis technique (SPT) [9], radio frequency magnetron sputtering [12], spin coating [13], dip

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coating [14], SILAR [15], thermal evaporation [16], ctc. SPT is simple, fast, inexpensive, vacuum less process and is suitable for mass production among all of these. So, the aim is to grow CuO thin film by SPT and to study the effect of the subtrate temperature (Ts) on the physical and chemical properties of CuO thin films.

2. Experimental Details CuO thin films have been synthesized by SPT using 0.10 mole of cupric acetate (Cu(CH3COO)2H2O) which was dissolved in deionized 90 ml water and 10 ml ethanol. The distance between substrate to spray nozzle was 25 cm and air pressure was 1 bar. The solution was sprayed onto the ultrasonically cleaned glass substrates heated at

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Sensors & Transducers, Vol. 191, Issue 8, August 2015, pp. 21-27 five different Ts, namely 250, 300, 350, 400 and 450°C. The Ts was recorded using a Cromel-alumal thermocouple. The flow rate of the solution during spraying was adjusted at about 1 ml/min and was kept constant throughout the experiment and the spray time was 5 min. The possible chemical reaction that takes place on the heated substrate to produce

CuO thin film when the droplets of the solution reached the heated substrate, chemical reaction of the copper acetate water solution takes place under stimulated temperature as shown below and provides the formation of CuO film. Thickness of the thin films was determined by Fizeau fringe interferometric method.

Cu ( CH 3 COO ) 2 . H 2 O  CH 3 CH 2 OH  H 2 O  Heat    CuO   2 CH 3 COOCH

for all the CuO thin films, obtained at different Ts, indicating the formation of monoclinic phase CuO in all the cases. Although (111) and (200) reflections are present, no other phases are present for Cu2O. The lattice constants of the CuO thin films are found to be: a = 4.6749 Å, b = 3.4536 Å and c = 5.1207 Å, and are in good agreement with the standard JCPDS data for monoclinic structured CuO. It is observed in the XRD patterns that the intensity of the peaks increases. For peak (111) the calculated values of the crystallite size for the CuO thin films are presented in Table 1.

The surface morphology of the films was examined by a HITACHI S-3400N model scanning electron microscope (SEM), the elemental analysis was performed by an electron dispersive spectrometer attached to the SEM, X-ray diffraction (XRD) patterns were recorded by a Philips PW3040 X’Pert PRO X-ray diffractometer. The optical transmission spectra for as-deposited thin films were obtained in the ultraviolet UV-visible (UV-visible) and near infrared regions (300-1100 nm) using a UV-VIS spectrophotometer (Model: 1201V, Shimazdu). The observed transmittance data were recorded using an identical uncoated glass substrate as reference. The electrical property of the samples was determined in air by Van-der Pauw method.

Table 1. Crystallite size for the CuO thin films at various Ts. 250 °C Ts Crystallite 8.4784 size in Å

3. Results and Discussion 3.1. X-ray Diffraction Analysis

10

35

40

45

2

50

60

9.6223

40

(0 2 0)

(1 1 0)

(0 2 0)

20 10 0

0

30

30

35

40

45

50

55

35

60

2

Fig. 1. XRD patterns of CuO thin film synthesized at various Ts.

22

0

450 C

(2 0 0)

50

30

20

55

9.6545

(0 0 2)

350 C

10

30

9.5743

(-2 0 2)

(-2 0 2) 0

30

0

9.0257

60

Intensity

40

(2 0 0 )

(0 0 2)

Intensity

50

(1 1 0)

20

(0 2 0)

(1 1 0)

30

(-2 0 2)

40

(2 0 0 )

(0 0 2)

Intensity

60

50

450 °C

70

70

0

250 C

400 °C

80

70 60

350 °C

(1 1 1)

80

300 °C

It is seen in the Table 1 the crystallite size increases with Ts up about 400 oC and then to decrease. For CuO there are many dangling bonds related to the copper and/or oxygen defects at the grain boundaries. As a result, these defects are favorable to the merging process to form larger CuO grains while increasing Ts. It implies that the crystallinity of the CuO thin films is improved at higher Ts. This may be due to gaining enough energy by the crystallites to orient in proper equilibrium sites at high Ts, resulting in the improvement of crystallinity and degree of orientation of the CuO thin films [16-18].

(1 1 1)

(1 1 1)

XRD patterns for CuO thin films synthesized at different Ts viz, 250, 350, and 450°C are shown in Fig. 1. The diffraction peaks observed at 2θ values of 32.2, 35.5, 38.3, 39.1, 48.85 and 52.7 correspond to the diffraction lines produced by (110), (002), (111), (200), (-202), and (020) planes of the end-centered monoclinic structured CuO (JCPDS card No. 89-5895). Crystallite size of the prepared CuO thin film was determined from the strongest peak of (111) for every XRD pattern using Scherrer formula. The (111) surface of CuO thin film is energetically the most stable and the predominant crystal face found in polycrystalline samples. It is observed from Fig. 1 that the diffraction peak positions are identical 80

 2 H 2O 

3

40

45

2

50

55

60

Sensors & Transducers, Vol. 191, Issue 8, August 2015, pp. 21-27

3.2. Elemental and Surface Morphological Analysis Fig. 2 shows EDX spectra of CuO thin film prepared at Ts = 350°C. The EDX analysis revealed the presence of copper (Cu) and oxygen (O) and other elements from the glass substrates which confirm that the CuO thin films prepared through the chemical oxidation route are free from impurities. The atomic ratio between Cu and O was found to be 2:1. This implies that the prepared samples are made up of Cu and O. A careful analysis of XRD and EDX results reveal that the EDX signals coming from the glass substrate are solely responsible for the excess Si atomic percentage observed through the EDX analysis. Hence, it is confirmed that phase pure CuO is only formed through this facile solution phase chemical oxidation route by SPT.

highest transmittance of about 80 % for the thin films grown at Ts = 350 °C. The increase in transmittance may be due to the transition of the CuO films from amorphous to polycrystalline structure.

10 μm

(a)

10 μm Fig. 2. EDX spectra of CuO thin films at Ts 350 ºC.

SEM images were recorded to examine the surface morphology of the as deposited CuO thin films and the images are shown in Fig. 3 (a, b, c). The as-deposited films have islands of different sizes and shapes, and their distribution on the surface is not homogeneous. These could be the result of the chemical reaction during the deposition. SEM micrographs reveal the formation of particles with different shapes and sizes, it seems appropriate to consider that the particles which appear in SEM images are, in fact, grain agglomerates.

(b)

10 μm

3.3. Optical Properties 3.3.1. Transmittance and Optical Band Gap The optical property of as-deposited CuO thin films deposited at various Ts were investigated by means of the transmission spectra recorded shown in Fig. 4 at various Ts. It is seen that the transmittance is high in the visible and near infrared regions and minimum at wavelength ~ 300 nm. An average 60 to 80 % transmittance is observed in the wavelength range of 800-1100 nm and below 800 nm transmittance decreases gradually. The transmittance increases from 5 to 10 % with Ts, and shows the

(c) Fig. 3. SEM image of CuO thin films at Ts (a) 250 ºC, (b) 350 ºC and (c) 400 ºC.

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Sensors & Transducers, Vol. 191, Issue 8, August 2015, pp. 21-27 for sample obtained at 350 °C. It can be seen that a band gap tuning of 0.30 eV occurs when the Ts is changed by about 100 °C. The value of the α and Eg decrease as the Ts increases gradually up to 350 °C where it starts to increase with further increasing of Ts. It may be due to the removal of defects and disorderness in the as-deposited film by increasing Ts.

100 T s= T s= T s= T s= T s=

Transmittance, T (%)

80

250 300 350 400 450

°C °C °C °C °C

60

40

20 0.6

600

800

1000

1200

Wavelength (nm)

Fig. 4. Optical transmittance vs. wavelength of CuO thin films at various Ts.

5.0x10

3

4.5x10

3

4.0x10

3

3

3.0x10

3

2.5x10

3

2.0x10

3

1.5x10

3

1.0x10

3

5.0x10

2

(h)

2

-1

(m eV)

2

3.5x10

Ts= 250 °C Ts= 300 °C Ts= 350 °C Ts= 400 °C Ts= 450° C

0.0

0.3 T s= T s= T s= T s= T s=

0.2 0.1

250 300 350 400 450

°C °C °C °C °C

0.0

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

h  (eV)

Fig. 6. Variation of extinction coefficient with hυ at various Ts. 2.0

3.0

1.9

2.9

Eg n

1.8

2.8

1.7

2.7

1.6

2.6

1.5

250° C

300° C

350° C

400° C

450° C

2.5

o

Substrare temperature, Ts C

Fig. 7. Variation of band gap and refractive index with Ts.

3.3.2. Refractive Index and Extinction Coefficient 1.5

2.0

2.5

3.0

3.5

4.0

4.5

h (eV)

Fig. 5. Variation of (αhυ) with (hυ) for CuO thin films at various Ts.

The α was found in the order of 106 m-1 which may be suitable for a transparent conducting film. The Eg of the CuO thin films is plotted in Fig. 6. It is seen in Fig. 7 that for the CuO thin films deposited at Ts = 250 °C the optical band gap is found to be 1.90 eV and then a minimum value 1.60 eV is found

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0.4

The variations of refractive index, n for CuO thin films increases with Ts, as seen in Fig. 7. The n of CuO thin film have been obtained 2.72 at Ts of 250 °C and it become lowest 2.52 at Ts 350 °C, This value is very close to the reported values 2.65 of CuO thin film [21] and it is lower than that of bulk CuO and this low value of refractive index may probably due to the smaller density of the films. The variation of extinction coefficient, k with hυ is shown in Fig. 6. It is observed that the k increases with the increase of Ts. The rise and fall in k is directly related to the absorption of light. The k about 0.1 in the range of wavelength 800-1100 nm

Refractive index, n

A relatively high transmittance value for the thin film deposited at 350 °C may be attributed to less scattering due to the decrease in the degree of irregularity in the grain size distribution [19]. The transmittance values are decreased for the next higher Ts. This suggests that the decrease in the transmittance of CuO thin films with increasing in Ts may lead to increase in the degenerate (metallic) nature of the films, which results in light absorption. The optical band gap for the direct band gap semiconductors is determined using the Tauc model and parabolic bands [20], (αhν)2 = A(hν-Eg), where A is a proportionality constant, hν is the incident photon energy, α is the absorption coefficient, and Eg is the optical band gap. Fig. 5 shows the absorption coefficient squared (αhν)2 as a function of, hν for the CuO thin films deposited at various Ts.

0.5

Extinction coefficient

400

Band gap, Eg (eV)

0 200

Sensors & Transducers, Vol. 191, Issue 8, August 2015, pp. 21-27

3.4. Electrical Properties

5 .4 5 .2 5 .0 T s= T s= T s= T s= T s=

4 .8

-1

D.C. electrical resistivity measurements were made in air for as deposited CuO thin films from 300 to 470 K by Van-der Pauw (four probes) method. Room temperature electrical resistivity of CuO thin films with Ts is presented in Fig. 8.

present case the higher values of activation energy may suggest that the prepared sample is stoichiometric. For SEM and EDX observations, it is also found that CuO thin films are stoichiometric. The figure of merit is well-known as an index for evaluating the performance of transparent conducting films, and it is given by the equation F=(−ρlnT)−1 where ρ is the electrical resistivity and T is the average transmittance in the wavelength range of 800-1100 nm [24]. Fig. 11 shows the figure of merit values of CuO thin films deposited at various Ts.

ln ((Ohm.cm) )

(1-1.6 eV) which is lower than that of CuO thin films prepared by RF magnetron sputtering [21] and is very close to the reported value of CuO thin films prepared on to ITO glass substrates from an aqueous electrolytic bath containing CuSO4 and tartaric acid [22]. The k of CuO thin films increases rapidly for photon energies above 1.6 eV and tends to decrease above 2.3 eV.

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Resistivity in Ohm.cm

30 28

4 .6 4 .4

25 0 30 0 35 0 40 0 45 0

°C °C °C °C °C

4 .2 4 .0 3 .8 3 .6

26

3 .4 2 .0

24 22

2 .2

2 .4

2 .6

2 .8

-1

1 0 0 0 /T (K )

3 .0

3 .2

3 .4

Fig. 9. Variation of lnσ with respect to T-1 (K-1).

20 18

2 .4 250

300

350

400

450

2 .2

o

Fig. 8. Variation of resistivity at room temperature for various Ts.

ln  2k 1/ T

1 .8 1 .6 1 .4 1 .2 1 .0 0 .8 250

300

350

400

450

o

Ts ( C)

Fig. 10. Variation of activation energy versus Ts. 0.15 0.14

(1)

The temperature dependence of electrical conductivity (lnσ) is shown in Fig. 9 and variation of activation energy is shown in Fig. 10. The low value of activation energy may be associated with the localized levels hopping due to the excitation of carriers from donor band to the conduction band. A low activation energy of 0.14 eV was reported for sputtered CuO thin films [23]. This low value of activation energy was assumed due to the nonstoichiometry of the CuO thin film but in the

Figure of Merit (ohm.cm)

E  

2 .0

-1

The decrease in resistivity with Ts explained in term of stoichiometric changes induced by oxygen ion vacancies and neutral defects. The resistivity of the prepared CuO thin films decreases as Ts increases. It may be due to increase in the free path of carrier concentration. The formation of these defects depends on the sticking coefficient, nucleation rates and the migration of impinging copper and oxygen species on the substrate during deposition. The activation energy (ΔE) is calculated from the slope of a curve lnσ vs. (1/T). So the activation energy is given by

Activation Energy (eV)

Ts ( C)

0.13 0.12 0.11 0.10 0.09 250

300

350

400

450

o

T s ( C)

Fig. 11. Variation of Figure of merit versus Ts.

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Sensors & Transducers, Vol. 191, Issue 8, August 2015, pp. 21-27 The figure of merit for the CuO thin films deposited at 250, 300 and 350 °C were found to be 9.34×10−2, 10.62×10−2 and 14.24×10−2 Ω−1cm−1, respectively. The increase in the figure of merit of the CuO thin films is mainly due to the increase in the optical transmittance with increasing Ts. The experimental data suggest that a Ts of 350 °C is the best condition for depositing high-quality CuO films.

4. Conclusions XRD data confirmed that the CuO thin films were highly oriented along the CuO (111) plane. The average crystallite size and the average transmittance of the film deposited at 350 °C were about 9.5743 Å and 75 % in the wavelength range of 800-1100 nm respectively. The decrease in electrical resistivity at higher Ts can be explained by the increased carrier concentration improved stoichometry at higher Ts. The optical band gap varies from 1.90 to 1.65 eV. The lowest value of refractive index is 2.52 at Ts = 350°C and the extinction coefficient increases with the increase of Ts of the prepared CuO then films. The minimum resistivity is found to be 18 Ω-m for CuO thin film deposited at Ts = 350 °C. This reduction in resistivity with increase in Ts is due to the increase of carrier concentrations of Cu and lower scattering of excess conduction electrons. The highest figure of merit occurred for the film grown at 350 °C with an optical transmittance about 76 % in the wavelength range of 800-1100 nm. The results suggest that high-quality CuO thin film can be produced when deposited at a growth temperature of 350 °C. The obtained experimental results indicate the suitability of this material as transparent and conducting window materials in thin film solar cells and gas sensor devices.

Acknowledgement The authors are thankful to Dr. D. K. Saha, Chief Scientific Officer, Atomic Energy Center, Dhaka, Bangladesh for his kind help in taking X-ray diffractograms.

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