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nonlinear optical characteristics
Accepted Manuscript Title: Sputtered CuO mono-phase thin films: structural, compositional and spectroscopic linear/nonlinear optical characteristics Authors: Asim Jilani, M. Sh. Abdel-Wahab, Mohd Hafiz Dzarfan Othman, Sajith VK, Ahmed Alsharie PII: DOI: Reference:
S0030-4026(17)30745-3 http://dx.doi.org/doi:10.1016/j.ijleo.2017.06.075 IJLEO 59338
To appear in: Received date: Revised date: Accepted date:
28-1-2017 17-6-2017 19-6-2017
Please cite this article as: Asim Jilani, M.Sh.Abdel-Wahab, Mohd Hafiz Dzarfan Othman, Sajith VK, Ahmed Alsharie, Sputtered CuO mono-phase thin films: structural, compositional and spectroscopic linear/nonlinear optical characteristics, Optik - International Journal for Light and Electron Opticshttp://dx.doi.org/10.1016/j.ijleo.2017.06.075 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sputtered CuO mono-phase thin films: structural, compositional and spectroscopic linear/nonlinear optical characteristics
Asim Jilani1,2,*, M.Sh. Abdel-wahab2, Mohd Hafiz Dzarfan Othman2,3, Sajith VK2, Ahmed Alsharie2 1
Centre of Nanotechnology, King Abdul-Aziz University, Jeddah 21589, Saudi Arabia
2
Advanced Membrane Technology Research Centre, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru,
Johor, Malaysia 3
Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor,
Malaysia
*Corresponding
author address: [email protected],
Abstract Mono phase CuO thin films through DC sputtering at various oxygen pressure (10, 20, 30, 40 sccm) was deposited on glass substrate. The structural analyses of the DC sputtered thin films were performed through X-ray diffraction (XRD) technique. The atomic force microscope (AFM) exposed the variation in surface roughness with the change in the deposited Oxygen pressure of CuO thin films. The band gap at various oxygen pressures was also estimated. The dielectric properties in term of real/imaginary dielectric constants and dielectric loss have also been investigated. The surface chemical composition of deposited CuO thin films has been examined through X-ray photoelectron spectroscopy (XPS). The Cu2p spectra indicated the presence of Cu2+covalent bond with d9 configuration at ground state. The O1s spectra proved the increment in the chemisorbed oxygen (Oi) with the enchantment of deposition oxygen pressure. The nonlinear optical constant such as nonlinear refractive index, linear optical susceptibility and third order nonlinear susceptibility were also calculated through very simple inexpensive
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method. The advantage of this work is to calculate the linear and nonlinear optical investigations through spectroscopic approach rather than expensive experimental Z- scan method. Keywords: Mono phase thin films; Linear &nonlinear; XPS Chemical state; structural 1. Introduction CuO thin films is an emerging material for various advance application such as solar cell, Li-ion batteries because of its innocuous nature and having low environmental impact [1]. The p-type nature of this material makes suitable for junction[2]. Moreover it is being used as catalysts[3, 4], photovoltaic applications[5]. The CuO can harvest more energy because of its indirect band gap nature that is suitable for solar cell application[6, 7]. Moreover, CuO also plays a vital role in water splinting by facilitating the electron-hole pair separation even under visible light as reported by various research groups[8-10] The CuO thin films can be grown by physical and chemical methods e.g. successive ionic layer adsorption and reaction SILAR[11], pulsed laser deposition [12] and atomic layer deposition[13]. Direct current sputtering technique has advantage over the other deposition method because of its fast process and easy to control deposition parameter to get uniform thickness of the thin films. Nonlinear optics has become the backbone of new era of photonic devices[14]. The nonlinearity in thin films technology has certain advantages over the single crystals photonics devices in terms of compatibility. The switching devices, organics semiconductors, frequency multiplayer, liquid crystal are the example of some examples of nonlinear optics. In this article, we studied the linear and nonlinear optical constants of mono-phase CuO thin films by the very simple spectroscopic approach rather than expensive Z-scan technique. Moreover, the information about nonlinearity of mono-phase CuO thin films through spectroscopic method had not been investigated till now. The structural properties of deposited thin films were measured through X-ray diffraction method. Topographic information was obtained via atomic force microscope. The x-rays photoelectron spectroscopy was employed to find the compositional and chemical state analysis.
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2. Experimental details 2.1 CuO Thin films deposition To deposit the high quality mono phase CuO thin film we have used the direct current (DC) sputtering of “Syskey Technologies, Taiwan”. Prior to thin films deposition, we have minimized the probability of contamination on substrate through acetone cleaning. The target for Cu (3 × 0.6 inch) metal was used of high purity (99.99%) to deposit the mono phase CuO thin films. The Table.1 concisely describes the experimental condition that were kept constant except the operating oxygen pressure (10, 20, 30, 40 sccm).to get the thin film on glass substrate. 2.2 Characterizations The structure investigations were performed through x-ray diffractometer (XRD) of Ultima-IV Rigaku Japan. The optical observations were done through UV-visible spectrometry (Perkin Elmer Lambda 750, USA). All the UV measurements were studied at room temperature. The surface topography of the mono phase CuO thin films were examined by atomic force microscope (Omicron- VTA-AFM) and field emission scanning electron microscope (JOEL JSM-7600F). Surface composition and chemical state analysis were performed through X-ray photoelectron spectroscopy (XPSPHI 5000 Versa Probe II USA). The thickness of the deposited thin films at various oxygen pressure was measured by the DektakXT( Bruker Germany ) surface profiler. We have noticed the change in the thickness of the CuO thin films with the oxygen pressure and it was 130nm, 112.5nm, 78.4nm and 50.9nm at 10, 20 30 and 40 sccm respectively. 3. Results and discussion 3.1 Structural Analysis The structural analysis of the sputtered CuO thin films has been investigated through XRD technique. The Fig.1 shows XRD pattern of CuO thin films at different Oxygen flow rate. Crystallographic information indicates the formation of the single tenorite CuO mono-phase, no indication of any other impurities (JCPDS # 00-005-0661).The main diffraction was observed along (0 0 2) following by (1 1 1) plane. The existence of other diffraction planes were also observed (110), (112), (11-3), (022), (31-1), (004). The maximum intense diffraction peak was at oxygen flow rate of 20sccm while further increase in the oxygen flow rate results the decrease in Page 3 of 29
the intensity of the peaks which in turns the decrease in the crystallinity of the CuO thin films. In our experiments, it has been noticed that the oxygen flow rate during the deposition of CuO thin films has a key impact on the crystallinity of the CuO thin films. This fact is evidently proved from the XRD pattern (Fig. 1) at oxygen flow rate 20sccm. The slight shift in the peak position towards lower diffraction angle was observed with the increase in the oxygen flow rate from 10 to 30 sccm while at 40 sccm of oxygen the peaks shifted to higher diffraction angle. This shifting behavior of the diffraction planes related to the micro strains presences in the sputtered thin films and the variation in the stoichiometry of the sputtered CuO thin films[15-17]. Debye Scherrer's formula has been employed to calculate the grain size of the sputtered CuO thin films[18]:
D
0.9 cos
(1)
The dislocation density of sputtered CuO thin films was determined by the following relation[19]:
1 D2
(2)
The lattice strain of the thin films was calculated through following equation [20]
cos 4
,
(3)
The lattice parameter and unit cell volume of monoclinic structure (JCPDS # 00-005-0661) sputtered CuO thin films is calculated by the following equation respectively [21]. (4) (5) The lattice structure parameter and cell volume of the sputtered CuO thin films at different Oxygen flow rate are listed in Table. 2 while grain size, dislocation density, lattice strain are shown in Table. 3. 3.2 Surface topography Page 4 of 29
The surface topographic of thin films at different oxygen pressure via atomic force microscope is shown in Fig. 2. The scan size was 1µ x 1µ was kept constant for all samples. It has been observed that the change in the oxygen pressure affected the surface morphology of the deposited films. The mean grain size and roughness were decreased with the increase in the oxygen pressure except at 20 sccm. These AFM finding is also consistent with the XRD findings. The change in the mean grain size and roughness may explain as “whenever the gain size of the films decreases then grain boundaries tends to increases which in results the decrease in the surface roughness . The route mean square (RMS) of the AFM analysis can be concise as: 23.71 nm, 23.97 nm, 21.97 nm and 19.82 nm for each film deposited at 10, 20, 30 and 40 sccm oxygen. The complete AFM results are documented in the Table. 4. The mean grain size shows the decreasing trend with the increase in oxygen pressure except 20 sccm. This could be also see from Fig. 2 (20 sccm) a prominent grain and valley. 3.3 Optical investigations The transmittance spectra of sputtered CuO thin films at different oxygen flow rate are shown in Fig.3. The optical transmittance of the CuO thin films was found dependent on the oxygen flow rate. The transmittance was found increased with the increase of oxygen flow rate. In other words, with the increase in the oxygen flow, the thicknesses of the films decreased which allows absorbing less photon in thin films. So, when the absorption of the photons is less than the transmittance of the film will rise. Shariffudin et.al [22] also reported the change in the transmittance of the thin films with thickness. It could be noted, the presences of small oscillation in the transmittance spectra which results due to the difference of refractive index between the sputtered CuO thin films and substrate [23]. The band gap of sputtered CuO thin films has been estimated through the following relation[24]. (6) In the above equation, h is the photon energy and α is the absorption coefficient while A is a constant. The Fig 4 (a) shows the direct band gap while Fig4 (b) shows the indirect band gap of sputtered CuO thin films at various oxygen pressure. The direct band was found 1.95 eV to 2.17 while the indirect band gap was in the range of 1.25 eV to 1.54 eV depending upon the oxygen pressure. Further, the change in band gap also depends on the surface defects as reported by Radhakrishnan et al[25] which can be evidently observed in our AFM findings. The range of our Page 5 of 29
reported band gap also agrees with the previous literature[25] [26-28]. Moreover, the quantum confinement effect is the basic reason for the variation in the band gap of deposited CuO thin films. The surface to volume ratio has the tendency to increase with the reduction of particle size which leads to change in the band gap as reported by Lee et al[29]. The refractive index of the subjected material is the response for its suitability to the various optical applications such as solar cell. The refractive index of the deposited CuO thin films has been studied by the following relation [30].
n
( I R) 4R k2 2 (1 R) (1 R)
(7)
The refractive index of deposited thin films at different oxygen pressure is shown in Fig. 5. The refractive index of the thin films at 10 sscm was found maximum and the continuous reduction was observed with the increase in oxygen pressure. So the oxygen pressure has a key impact our deposited thin films in terms of optical and structural point of view. 3.4 Dielectric investigations The dielectric constant and dielectric loss are important for the material being under investigation for optical and dielectric purpose. Moreover, it gives the information about the response of the subjected material to the polarizability. We have employed the following relation for the real and imaginary dielectric constant of the deposited CuO thin films at different oxygen pressure [31]. (8) (9) The Fig 6(a) shows the real parts of the dielectric constant while Fig 6(b) imaginary part of dielectric constants versus photon energy of the CuO thin films at different oxygen pressure. It has been observed that the dielectric constants of the thin films increase with the decrease in the oxygen pressure. The inverse relation was noticed between the oxygen pressure and the dielectric constants. The dielectric losses of the thin films are also very important as it reflects the anhromonic lattice force and the losses of the periodicity defects in the thin films. These dielectric losses could be due the change in the grain boundaries and the change in the dislocation density[32]. The dialectic losses of the deposited CuO thin films have been calculated by the by the following relation [33].
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(10) The Fig.7 represents the tangent losses of the deposited thin films as a function of the photon energy. The oxygen pressure has effect the tangent loss of the CuO thin films. From the figure it is clear that tangent loss decrease with the increase in the oxygen pressure. Moreover the tangent loss descends rapidly up to photon energy of 1.25 hv afterwards it decrease slowly and then about 2.00 hv it again fall briskly. 3.5 Nonlinear investigations The electron polarization of the CuO thin films can be ascribed in term of the light interaction with the atom of the deposited thin films. So, when the light interacts with the particular material then the electric field produces due to the polarization of the material. This induced electric field is the main cause for the nonlinear electron polarizability PNL and may represents by the following relation [34, 35] p (1) E PNL
(11)
in the above equation the PNL is may written as [34, 35] PNL ( 2) E 2 (3) E 3
(12)
In the above two equations (1), (2) and (3) are 1st order linear optical susceptibility 2nd order nonlinear optical susceptibility and 3rd nonlinear optical susceptibility respectively. The linear optical susceptibility in term of refractive index may written as [36].
(1) n 2 1/ 4
(13)
and the 3rd nonlinear optical susceptibility in form of 1st order linear optical susceptibility[37]
(3) A 1
4
(14)
So by the above two equations [38].
( 3)
A
4
4
n
2 0
1
4
(15)
Where A is a constant and its value is 1.7x10-10. The nonlinear refractive index in term of 3rd nonlinear optical susceptibility is written as under[36].
n2
12 (3) no
(16)
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The calculated nonlinear refractive index for deposited CuO thin films at different oxygen pressure is shown in Fig. 8. The nonlinear refractive index at 10 sccm was observed 2.0x10-12. Furthermore it was changed with the increase in the oxygen pressure. The1st order linear optical susceptibility and linear optical susceptibility and 3rd nonlinear optical susceptibility of the deposited CuO thin films are shown in Fig. 9 (a) and Fig. 9 (b) respectively .The value of 3rdnonlinearoptical susceptibility at 10 sccm is 2 x10-13esu while it increasing with the increase in oxygen pressure and it was maximum at 30 sccm i.e 3x10-12esu.The Chen et al[39] reported the 3rd nonlinear optical susceptibility of laser deposited CuO thin films through Z-scan technique but our mono phase CuO thin films deposited via DC sputtering have relatively high 3rd nonlinear optical susceptibility (calculated through spectroscopic method). 3.6 XPS Investigation We have employed the XPS to find out the surface composition and chemical state of the deposited CuO thin films at different oxygen pressure. The Fig. 10 express the survey scan overlay spectra of CuO thin films at different oxygen pressure. Table 5 shows the elemental compositions of the detected element through survey scan. The change in elemental composition was noticed with the variation in the deposition oxygen pressure furthermore the variation in Cu concentration was also observed. In addition to Copper and oxygen there was also the carbon on the surface of the thin films. The finding of carbon is being considered as a contamination on the surface during the transfer of thin films from sputtering unit to the XPS analysis chamber [40]. We have also analyzed the chemical state of Cu and O for better understanding about the nature of CuO thin films. The deconvolution Cu 2p spectra at different oxygen pressure of CuO deposited thin films is shown on Fig. 11.In all the spectrums the main binding energy peak was associated with the small peaks which is called shakeup satellite peak. It is also noticeable that all the detected spectrums have nearly the same position, which also proved the mono phase nature of CuO thin films. Furthermore, the presence of shakeup satellite peak is the also the main characteristics of CuO phase and belong to d9 configuration at ground state[41]. The main peak at 933.0 ±0.02 eV represents the Cu2+covalent bond of the CuO deposited thin films while the shakeup satellite peaks at 941.16 ± 0.14 eV and 943.5±0.13 attributed to the various types of Cu-O binding species, It is also notable that with the increases in deposition oxygen pressure, slightly border
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shakeup satellite was observed which also the confirms the increase in Cu-O oxygen species as detected during the survey scan[42]. The O1s spectrum of CuO thin films at various oxygen pressures is shown in Fig. 12. The peak position was observed about 529.5±0.1 and 531.2±0.1. The peak at lower binding energy attributed to the Cu-O in the CuO while higher binding energy position is the surface chemisorbed oxygen (Oi) of the deposited thin films[43]. The chemical state analysis of the O1s spectra of CuO thin films also revealed the change in the chemisorbed oxygen with the change in the oxygen pressure. The chemisorbed oxygen was noticed about 43.20 % at oxygen 10sccm while with the increase in oxygen pressure to 40 sccmit was 44.89 %. The information of the Oi of deposited CuO thin films could quit useful for the different advance application such as photodegradation process. The literature shows that Oi plays an important role for enhanced photodegradation process of thin films technology [44]. Conclusion The highly smooth mono-phase CuO thin films have been deposited through sputtering method. The effect of oxygen deposition pressure has been evaluated on structural, optical, dielectric and surface composition of the thin films. The X-ray diffraction and X-ray photoelectron spectroscopic observation confirmed the nature of CuO thin films as mono-phase. The grain size of the deposited thin films decreased from 9.4 nm to 6.3 nm with the increase of oxygen pressure from 10 sccm to 40 sccm.
The variation of 1.00 A0was observed with the alteration in the
deposition conditions. The dislocation density was 1.937E-02 to 4.145E-02 while the lattice strain lays from 4.507E-03 to 6.464E-03. The optical band gap was noticed 1.86-2.1 eV. The XPS chemical state showed the Cu2+covalent bond in d9 configuration at ground state in all deposited thin films. The O1s spectra reveled the Cu-O bonding and the chemisorbed oxygen (Oi). Furthermore, O1s also showed the increment of Oi with the increase in oxygen deposition pressure. The nonlinear refractive index was 3x10-12 to 4x10-11. The first order nonlinear susceptibility was 1.8 and third order nonlinear optical susceptibility was found maximum at 30 sccm about 3x10-12. In short, the linear and nonlinear optical calculation through cheapest method give the idea of suitability of CuO thin films for different advance optical purpose such as solar cell. The XPS chemical state information O1s would be useful for the advance environmental issues such as enhanced photo-degradation process.
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List of figures
Figure 1: XRD spectra of CuO thin films at various oxygen pressures Page 14 of 29
Figure 2: AFM surface analysis of CuO thin films at various oxygen pressures
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Transmittace (%)
100 80 60 40
10 sccm 20 sccm 30 sccm 40 sccm
20 0
500
1000 1500 2000 Wavelenghth (nm)
Figure 3: Transmittance spectra of deposited CuO thin films
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2500
Figure 4: Band gap analysis of deposited CuO thin films (a) direct band gap (b) indirect band gap
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Refractive index (n)
5
10 sccm 20 sccm 30 sccm 40 sccm
4 3 2 1
500
1000 1500 2000 Wavelength (nm)
Figure 5: Refractive index of CuO thin films at various oxygen pressures
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2500
Figure 6: Dielectric constant of CuO thin films (a) Real dielectric constants (b) imaginary dielectric constants
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Dielectric loss (Tan
0.30
10 sccm 20 sccm 30 sccm 40 sccm
0.25 0.20 0.15 0.10 0.5
1.0 1.5 2.0 Photon energy (hv)
Figure 7: Dielectric loss of CuO thin films at various oxygen pressures
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2.5
2.0
10 sccm 20 sccm 30 sccm 40 sccm
n2 x10-11
1.5 1.0 0.5 0.0 -0.5
1000
1500 2000 Wavelength (nm)
Figure 8: Nonlinear refractive index of CuO thin films
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2500
Figure 9: Optical susceptibility of CuO thin films (a) liner susceptibility (b) 3 rd order nonlinear susceptibility
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Intensity (c/s)
Cu Cu Cu Cu Cu Cu
O Cu Cu
O
C 40 sccm 30 sccm 20 sccm 10 sccm
0
500 1000 Biniding energy (eV)
Figure 10: XPS survey scan of CuO thin films at various oxygen pressure
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Figure 11: Cu2p spectra of CuO thin films at various oxygen pressures
Page 24 of 29
Figure 12: O1s spectra of CuO thin films at various oxygen pressures
Page 25 of 29
List of Tables:
Table: 1. Experimental conditions for CuO thin films
No Deposition parameters
Value
1
Base pressure
9×10-6 Torr
2
Operating pressure
5×10-3 Torr
3
Deposition time
900 sec
4
Substrate temperature
25 (RT)°C
5
DC power for Cu target
200 W
6
Argon flow rate
20 SCCM
7
Oxygen flow rate
10,20,30 and 40 SCCM
8
Target – substrate distance 14 cm
9
Substrate rotation speed
15 rpm
Page 26 of 29
Table: 2. Cell volume of CuO thin films at various oxygen pressures
Sample details a(Ao ) b(Ao ) c(Ao ) β(o )
Cell volume(Ao )
10 sccm
4.60
3.51
5.11
98.3
81.7
20 sccm
4.58
3.52
5.12
97.83 82.0
30 sccm
4.61
3.51
5.10
98.21 81.9
40 sccm
4.70
3.45
5.16
99.26 82.7
Page 27 of 29
Table: 3.Structural parameter of CuO thin films at various oxygen pressures
Sample Details 10 sccm
size,
Dislocation
Lattice
(nm)
density
strain
35.448
15.39195
4.221E-03
2.252E-03
38.268
11.05032
8.189E-03
3.137E-03
51.8818
4.789922
4.359E-02
7.237E-03
61.714
10.21
9.593E-03
3.395E-03
64.7783
6.001875
2.776E-02
5.775E-03
66.9778
6.609262
2.289E-02
5.245E-03
2o
mean value 20 sccm
9.008888
1.937E-02
4.507E-03
32.1294 12.32083
6.587E-03
2.813E-03
35.4239
14.28422
4.901E-03
2.427E-03
38.1864
10.42456
9.202E-03
3.325E-03
51.7984
4.370616
5.235E-02
7.931E-03
61.6626
7.977701
1.571E-02
4.345E-03
64.451
4.78065
4.375E-02
7.251E-03
66.8631
6.085301
2.700E-02
5.696E-03
74.5916
14.97806
4.457E-03
2.314E-03
mean value 30 sccm
Grain
9.402743
2.050E-02
4.513E-03
32.1247
11.26362
7.882E-03
3.077E-03
35.4029
11.76718
7.222E-03
2.946E-03
38.2087
8.522751
1.377E-02
4.067E-03
52.0745
3.439275
8.454E-02
1.008E-02
67.3118
2.124483
2.216E-01
1.632E-02
mean value
7.423461
6.699E-02
7.297E-03
40 sccm 35.2664
9.105542
1.206E-02
3.807E-03
38.1467
6.671582
2.247E-02
5.196E-03
67.063
3.336458
8.983E-02
1.039E-02
mean value
6.371194
4.145E-02
Page 28 of 29
6.464E-03
Table: 4. AFM Surface analysis of CuO thin films at various oxygen pressures
Sample details Mean grain size (nm) Mean surface roughness (nm) 10Sccm
23.71
1.18
20Sccm
23.97
1.12
30Sccm
21.97
0.77
40Sccm
19.82
0.61
Table: 5. XPS elemental composition of CuO thin films at various oxygen pressures.
Sample detail
Surface elements (At.wt%) Copper
Oxygen
Carbon
10 sccm
32.2
63.4
4.4
20 sccm
30.8
64.3
4.9
30 sccm
28.0
67.2
4.8
40 sccm
27.9
68.0
4.1
Page 29 of 29
S0030-4026(17)30745-3 http://dx.doi.org/doi:10.1016/j.ijleo.2017.06.075 IJLEO 59338
To appear in: Received date: Revised date: Accepted date:
28-1-2017 17-6-2017 19-6-2017
Please cite this article as: Asim Jilani, M.Sh.Abdel-Wahab, Mohd Hafiz Dzarfan Othman, Sajith VK, Ahmed Alsharie, Sputtered CuO mono-phase thin films: structural, compositional and spectroscopic linear/nonlinear optical characteristics, Optik - International Journal for Light and Electron Opticshttp://dx.doi.org/10.1016/j.ijleo.2017.06.075 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sputtered CuO mono-phase thin films: structural, compositional and spectroscopic linear/nonlinear optical characteristics
Asim Jilani1,2,*, M.Sh. Abdel-wahab2, Mohd Hafiz Dzarfan Othman2,3, Sajith VK2, Ahmed Alsharie2 1
Centre of Nanotechnology, King Abdul-Aziz University, Jeddah 21589, Saudi Arabia
2
Advanced Membrane Technology Research Centre, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru,
Johor, Malaysia 3
Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor,
Malaysia
*Corresponding
author address: [email protected],
Abstract Mono phase CuO thin films through DC sputtering at various oxygen pressure (10, 20, 30, 40 sccm) was deposited on glass substrate. The structural analyses of the DC sputtered thin films were performed through X-ray diffraction (XRD) technique. The atomic force microscope (AFM) exposed the variation in surface roughness with the change in the deposited Oxygen pressure of CuO thin films. The band gap at various oxygen pressures was also estimated. The dielectric properties in term of real/imaginary dielectric constants and dielectric loss have also been investigated. The surface chemical composition of deposited CuO thin films has been examined through X-ray photoelectron spectroscopy (XPS). The Cu2p spectra indicated the presence of Cu2+covalent bond with d9 configuration at ground state. The O1s spectra proved the increment in the chemisorbed oxygen (Oi) with the enchantment of deposition oxygen pressure. The nonlinear optical constant such as nonlinear refractive index, linear optical susceptibility and third order nonlinear susceptibility were also calculated through very simple inexpensive
Page 1 of 29
method. The advantage of this work is to calculate the linear and nonlinear optical investigations through spectroscopic approach rather than expensive experimental Z- scan method. Keywords: Mono phase thin films; Linear &nonlinear; XPS Chemical state; structural 1. Introduction CuO thin films is an emerging material for various advance application such as solar cell, Li-ion batteries because of its innocuous nature and having low environmental impact [1]. The p-type nature of this material makes suitable for junction[2]. Moreover it is being used as catalysts[3, 4], photovoltaic applications[5]. The CuO can harvest more energy because of its indirect band gap nature that is suitable for solar cell application[6, 7]. Moreover, CuO also plays a vital role in water splinting by facilitating the electron-hole pair separation even under visible light as reported by various research groups[8-10] The CuO thin films can be grown by physical and chemical methods e.g. successive ionic layer adsorption and reaction SILAR[11], pulsed laser deposition [12] and atomic layer deposition[13]. Direct current sputtering technique has advantage over the other deposition method because of its fast process and easy to control deposition parameter to get uniform thickness of the thin films. Nonlinear optics has become the backbone of new era of photonic devices[14]. The nonlinearity in thin films technology has certain advantages over the single crystals photonics devices in terms of compatibility. The switching devices, organics semiconductors, frequency multiplayer, liquid crystal are the example of some examples of nonlinear optics. In this article, we studied the linear and nonlinear optical constants of mono-phase CuO thin films by the very simple spectroscopic approach rather than expensive Z-scan technique. Moreover, the information about nonlinearity of mono-phase CuO thin films through spectroscopic method had not been investigated till now. The structural properties of deposited thin films were measured through X-ray diffraction method. Topographic information was obtained via atomic force microscope. The x-rays photoelectron spectroscopy was employed to find the compositional and chemical state analysis.
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2. Experimental details 2.1 CuO Thin films deposition To deposit the high quality mono phase CuO thin film we have used the direct current (DC) sputtering of “Syskey Technologies, Taiwan”. Prior to thin films deposition, we have minimized the probability of contamination on substrate through acetone cleaning. The target for Cu (3 × 0.6 inch) metal was used of high purity (99.99%) to deposit the mono phase CuO thin films. The Table.1 concisely describes the experimental condition that were kept constant except the operating oxygen pressure (10, 20, 30, 40 sccm).to get the thin film on glass substrate. 2.2 Characterizations The structure investigations were performed through x-ray diffractometer (XRD) of Ultima-IV Rigaku Japan. The optical observations were done through UV-visible spectrometry (Perkin Elmer Lambda 750, USA). All the UV measurements were studied at room temperature. The surface topography of the mono phase CuO thin films were examined by atomic force microscope (Omicron- VTA-AFM) and field emission scanning electron microscope (JOEL JSM-7600F). Surface composition and chemical state analysis were performed through X-ray photoelectron spectroscopy (XPSPHI 5000 Versa Probe II USA). The thickness of the deposited thin films at various oxygen pressure was measured by the DektakXT( Bruker Germany ) surface profiler. We have noticed the change in the thickness of the CuO thin films with the oxygen pressure and it was 130nm, 112.5nm, 78.4nm and 50.9nm at 10, 20 30 and 40 sccm respectively. 3. Results and discussion 3.1 Structural Analysis The structural analysis of the sputtered CuO thin films has been investigated through XRD technique. The Fig.1 shows XRD pattern of CuO thin films at different Oxygen flow rate. Crystallographic information indicates the formation of the single tenorite CuO mono-phase, no indication of any other impurities (JCPDS # 00-005-0661).The main diffraction was observed along (0 0 2) following by (1 1 1) plane. The existence of other diffraction planes were also observed (110), (112), (11-3), (022), (31-1), (004). The maximum intense diffraction peak was at oxygen flow rate of 20sccm while further increase in the oxygen flow rate results the decrease in Page 3 of 29
the intensity of the peaks which in turns the decrease in the crystallinity of the CuO thin films. In our experiments, it has been noticed that the oxygen flow rate during the deposition of CuO thin films has a key impact on the crystallinity of the CuO thin films. This fact is evidently proved from the XRD pattern (Fig. 1) at oxygen flow rate 20sccm. The slight shift in the peak position towards lower diffraction angle was observed with the increase in the oxygen flow rate from 10 to 30 sccm while at 40 sccm of oxygen the peaks shifted to higher diffraction angle. This shifting behavior of the diffraction planes related to the micro strains presences in the sputtered thin films and the variation in the stoichiometry of the sputtered CuO thin films[15-17]. Debye Scherrer's formula has been employed to calculate the grain size of the sputtered CuO thin films[18]:
D
0.9 cos
(1)
The dislocation density of sputtered CuO thin films was determined by the following relation[19]:
1 D2
(2)
The lattice strain of the thin films was calculated through following equation [20]
cos 4
,
(3)
The lattice parameter and unit cell volume of monoclinic structure (JCPDS # 00-005-0661) sputtered CuO thin films is calculated by the following equation respectively [21]. (4) (5) The lattice structure parameter and cell volume of the sputtered CuO thin films at different Oxygen flow rate are listed in Table. 2 while grain size, dislocation density, lattice strain are shown in Table. 3. 3.2 Surface topography Page 4 of 29
The surface topographic of thin films at different oxygen pressure via atomic force microscope is shown in Fig. 2. The scan size was 1µ x 1µ was kept constant for all samples. It has been observed that the change in the oxygen pressure affected the surface morphology of the deposited films. The mean grain size and roughness were decreased with the increase in the oxygen pressure except at 20 sccm. These AFM finding is also consistent with the XRD findings. The change in the mean grain size and roughness may explain as “whenever the gain size of the films decreases then grain boundaries tends to increases which in results the decrease in the surface roughness . The route mean square (RMS) of the AFM analysis can be concise as: 23.71 nm, 23.97 nm, 21.97 nm and 19.82 nm for each film deposited at 10, 20, 30 and 40 sccm oxygen. The complete AFM results are documented in the Table. 4. The mean grain size shows the decreasing trend with the increase in oxygen pressure except 20 sccm. This could be also see from Fig. 2 (20 sccm) a prominent grain and valley. 3.3 Optical investigations The transmittance spectra of sputtered CuO thin films at different oxygen flow rate are shown in Fig.3. The optical transmittance of the CuO thin films was found dependent on the oxygen flow rate. The transmittance was found increased with the increase of oxygen flow rate. In other words, with the increase in the oxygen flow, the thicknesses of the films decreased which allows absorbing less photon in thin films. So, when the absorption of the photons is less than the transmittance of the film will rise. Shariffudin et.al [22] also reported the change in the transmittance of the thin films with thickness. It could be noted, the presences of small oscillation in the transmittance spectra which results due to the difference of refractive index between the sputtered CuO thin films and substrate [23]. The band gap of sputtered CuO thin films has been estimated through the following relation[24]. (6) In the above equation, h is the photon energy and α is the absorption coefficient while A is a constant. The Fig 4 (a) shows the direct band gap while Fig4 (b) shows the indirect band gap of sputtered CuO thin films at various oxygen pressure. The direct band was found 1.95 eV to 2.17 while the indirect band gap was in the range of 1.25 eV to 1.54 eV depending upon the oxygen pressure. Further, the change in band gap also depends on the surface defects as reported by Radhakrishnan et al[25] which can be evidently observed in our AFM findings. The range of our Page 5 of 29
reported band gap also agrees with the previous literature[25] [26-28]. Moreover, the quantum confinement effect is the basic reason for the variation in the band gap of deposited CuO thin films. The surface to volume ratio has the tendency to increase with the reduction of particle size which leads to change in the band gap as reported by Lee et al[29]. The refractive index of the subjected material is the response for its suitability to the various optical applications such as solar cell. The refractive index of the deposited CuO thin films has been studied by the following relation [30].
n
( I R) 4R k2 2 (1 R) (1 R)
(7)
The refractive index of deposited thin films at different oxygen pressure is shown in Fig. 5. The refractive index of the thin films at 10 sscm was found maximum and the continuous reduction was observed with the increase in oxygen pressure. So the oxygen pressure has a key impact our deposited thin films in terms of optical and structural point of view. 3.4 Dielectric investigations The dielectric constant and dielectric loss are important for the material being under investigation for optical and dielectric purpose. Moreover, it gives the information about the response of the subjected material to the polarizability. We have employed the following relation for the real and imaginary dielectric constant of the deposited CuO thin films at different oxygen pressure [31]. (8) (9) The Fig 6(a) shows the real parts of the dielectric constant while Fig 6(b) imaginary part of dielectric constants versus photon energy of the CuO thin films at different oxygen pressure. It has been observed that the dielectric constants of the thin films increase with the decrease in the oxygen pressure. The inverse relation was noticed between the oxygen pressure and the dielectric constants. The dielectric losses of the thin films are also very important as it reflects the anhromonic lattice force and the losses of the periodicity defects in the thin films. These dielectric losses could be due the change in the grain boundaries and the change in the dislocation density[32]. The dialectic losses of the deposited CuO thin films have been calculated by the by the following relation [33].
Page 6 of 29
(10) The Fig.7 represents the tangent losses of the deposited thin films as a function of the photon energy. The oxygen pressure has effect the tangent loss of the CuO thin films. From the figure it is clear that tangent loss decrease with the increase in the oxygen pressure. Moreover the tangent loss descends rapidly up to photon energy of 1.25 hv afterwards it decrease slowly and then about 2.00 hv it again fall briskly. 3.5 Nonlinear investigations The electron polarization of the CuO thin films can be ascribed in term of the light interaction with the atom of the deposited thin films. So, when the light interacts with the particular material then the electric field produces due to the polarization of the material. This induced electric field is the main cause for the nonlinear electron polarizability PNL and may represents by the following relation [34, 35] p (1) E PNL
(11)
in the above equation the PNL is may written as [34, 35] PNL ( 2) E 2 (3) E 3
(12)
In the above two equations (1), (2) and (3) are 1st order linear optical susceptibility 2nd order nonlinear optical susceptibility and 3rd nonlinear optical susceptibility respectively. The linear optical susceptibility in term of refractive index may written as [36].
(1) n 2 1/ 4
(13)
and the 3rd nonlinear optical susceptibility in form of 1st order linear optical susceptibility[37]
(3) A 1
4
(14)
So by the above two equations [38].
( 3)
A
4
4
n
2 0
1
4
(15)
Where A is a constant and its value is 1.7x10-10. The nonlinear refractive index in term of 3rd nonlinear optical susceptibility is written as under[36].
n2
12 (3) no
(16)
Page 7 of 29
The calculated nonlinear refractive index for deposited CuO thin films at different oxygen pressure is shown in Fig. 8. The nonlinear refractive index at 10 sccm was observed 2.0x10-12. Furthermore it was changed with the increase in the oxygen pressure. The1st order linear optical susceptibility and linear optical susceptibility and 3rd nonlinear optical susceptibility of the deposited CuO thin films are shown in Fig. 9 (a) and Fig. 9 (b) respectively .The value of 3rdnonlinearoptical susceptibility at 10 sccm is 2 x10-13esu while it increasing with the increase in oxygen pressure and it was maximum at 30 sccm i.e 3x10-12esu.The Chen et al[39] reported the 3rd nonlinear optical susceptibility of laser deposited CuO thin films through Z-scan technique but our mono phase CuO thin films deposited via DC sputtering have relatively high 3rd nonlinear optical susceptibility (calculated through spectroscopic method). 3.6 XPS Investigation We have employed the XPS to find out the surface composition and chemical state of the deposited CuO thin films at different oxygen pressure. The Fig. 10 express the survey scan overlay spectra of CuO thin films at different oxygen pressure. Table 5 shows the elemental compositions of the detected element through survey scan. The change in elemental composition was noticed with the variation in the deposition oxygen pressure furthermore the variation in Cu concentration was also observed. In addition to Copper and oxygen there was also the carbon on the surface of the thin films. The finding of carbon is being considered as a contamination on the surface during the transfer of thin films from sputtering unit to the XPS analysis chamber [40]. We have also analyzed the chemical state of Cu and O for better understanding about the nature of CuO thin films. The deconvolution Cu 2p spectra at different oxygen pressure of CuO deposited thin films is shown on Fig. 11.In all the spectrums the main binding energy peak was associated with the small peaks which is called shakeup satellite peak. It is also noticeable that all the detected spectrums have nearly the same position, which also proved the mono phase nature of CuO thin films. Furthermore, the presence of shakeup satellite peak is the also the main characteristics of CuO phase and belong to d9 configuration at ground state[41]. The main peak at 933.0 ±0.02 eV represents the Cu2+covalent bond of the CuO deposited thin films while the shakeup satellite peaks at 941.16 ± 0.14 eV and 943.5±0.13 attributed to the various types of Cu-O binding species, It is also notable that with the increases in deposition oxygen pressure, slightly border
Page 8 of 29
shakeup satellite was observed which also the confirms the increase in Cu-O oxygen species as detected during the survey scan[42]. The O1s spectrum of CuO thin films at various oxygen pressures is shown in Fig. 12. The peak position was observed about 529.5±0.1 and 531.2±0.1. The peak at lower binding energy attributed to the Cu-O in the CuO while higher binding energy position is the surface chemisorbed oxygen (Oi) of the deposited thin films[43]. The chemical state analysis of the O1s spectra of CuO thin films also revealed the change in the chemisorbed oxygen with the change in the oxygen pressure. The chemisorbed oxygen was noticed about 43.20 % at oxygen 10sccm while with the increase in oxygen pressure to 40 sccmit was 44.89 %. The information of the Oi of deposited CuO thin films could quit useful for the different advance application such as photodegradation process. The literature shows that Oi plays an important role for enhanced photodegradation process of thin films technology [44]. Conclusion The highly smooth mono-phase CuO thin films have been deposited through sputtering method. The effect of oxygen deposition pressure has been evaluated on structural, optical, dielectric and surface composition of the thin films. The X-ray diffraction and X-ray photoelectron spectroscopic observation confirmed the nature of CuO thin films as mono-phase. The grain size of the deposited thin films decreased from 9.4 nm to 6.3 nm with the increase of oxygen pressure from 10 sccm to 40 sccm.
The variation of 1.00 A0was observed with the alteration in the
deposition conditions. The dislocation density was 1.937E-02 to 4.145E-02 while the lattice strain lays from 4.507E-03 to 6.464E-03. The optical band gap was noticed 1.86-2.1 eV. The XPS chemical state showed the Cu2+covalent bond in d9 configuration at ground state in all deposited thin films. The O1s spectra reveled the Cu-O bonding and the chemisorbed oxygen (Oi). Furthermore, O1s also showed the increment of Oi with the increase in oxygen deposition pressure. The nonlinear refractive index was 3x10-12 to 4x10-11. The first order nonlinear susceptibility was 1.8 and third order nonlinear optical susceptibility was found maximum at 30 sccm about 3x10-12. In short, the linear and nonlinear optical calculation through cheapest method give the idea of suitability of CuO thin films for different advance optical purpose such as solar cell. The XPS chemical state information O1s would be useful for the advance environmental issues such as enhanced photo-degradation process.
Page 9 of 29
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[22] C. Huvier, E. Conforto, H. Alami, D. Delafosse, X. Feaugas, IOP Conf. Series: Materials Science and Engineering, in, IOP publishing, 2009. [23] S. Tan, B. Chen, X. Sun, W. Fan, H.S. Kwok, X. Zhang, S. Chua, Blueshift of optical band gap in ZnO thin films grown by metal-organic chemical-vapor deposition, Journal of Applied Physics, 98 (2005) 13505-13505. [24] A.A. Al-Ghamdi, M. Khedr, M.S. Ansari, P. Hasan, M.S. Abdel-wahab, A. Farghali, RF sputtered CuO thin films: Structural, optical and photo-catalytic behavior, Physica E: Lowdimensional Systems and Nanostructures, 81 (2016) 83-90. [25] A.A. Radhakrishnan, B.B. Beena, Structural and optical absorption analysis of CuO nanoparticles, Indian Journal of Advances in Chemica l Science, 2 (2014) 158-161. [26] N. Mukherjee, B. Show, S.K. Maji, U. Madhu, S.K. Bhar, B.C. Mitra, G.G. Khan, A. Mondal, CuO nano-whiskers: electrodeposition, Raman analysis, photoluminescence study and photocatalytic activity, Materials Letters, 65 (2011) 3248-3250. [27] Y. Wang, P. Miska, D. Pilloud, D. Horwat, F. Mücklich, J.-F. Pierson, Transmittance enhancement and optical band gap widening of Cu2O thin films after air annealing, Journal of Applied Physics, 115 (2014) 073505. [28] S. Masudy-Panah, K. Radhakrishnan, H.R. Tan, R. Yi, T.I. Wong, G.K. Dalapati, Titanium doped cupric oxide for photovoltaic application, Solar Energy Materials and Solar Cells, 140 (2015) 266-274. [29] K.D. Lee, W.C. Jung, J.H. Kim, Thermal degradation of black chrome coatings, Solar energy materials and solar cells, 63 (2000) 125-137. [30] M. Dongol, A. El-Denglawey, M.A. El Sadek, I. Yahia, Thermal annealing effect on the structural and the optical properties of Nano CdTe films, Optik-International Journal for Light and Electron Optics, 126 (2015) 1352-1357. [31] M.A. Omar, Elementary solid state physics, Pearson Education India, 1975. [32] H.S. Nalwa, Handbook of low and high dielectric constant materials and their applications, two-volume set, Academic Press, 1999. [33] U. Zhokhavets, R. Goldhahn, G. Gobsch, W. Schliefke, Dielectric function and onedimensional description of the absorption of poly (3-octylthiophene), Synthetic metals, 138 (2003) 491-495.
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List of figures
Figure 1: XRD spectra of CuO thin films at various oxygen pressures Page 14 of 29
Figure 2: AFM surface analysis of CuO thin films at various oxygen pressures
Page 15 of 29
Transmittace (%)
100 80 60 40
10 sccm 20 sccm 30 sccm 40 sccm
20 0
500
1000 1500 2000 Wavelenghth (nm)
Figure 3: Transmittance spectra of deposited CuO thin films
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2500
Figure 4: Band gap analysis of deposited CuO thin films (a) direct band gap (b) indirect band gap
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Refractive index (n)
5
10 sccm 20 sccm 30 sccm 40 sccm
4 3 2 1
500
1000 1500 2000 Wavelength (nm)
Figure 5: Refractive index of CuO thin films at various oxygen pressures
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2500
Figure 6: Dielectric constant of CuO thin films (a) Real dielectric constants (b) imaginary dielectric constants
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Dielectric loss (Tan
0.30
10 sccm 20 sccm 30 sccm 40 sccm
0.25 0.20 0.15 0.10 0.5
1.0 1.5 2.0 Photon energy (hv)
Figure 7: Dielectric loss of CuO thin films at various oxygen pressures
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2.5
2.0
10 sccm 20 sccm 30 sccm 40 sccm
n2 x10-11
1.5 1.0 0.5 0.0 -0.5
1000
1500 2000 Wavelength (nm)
Figure 8: Nonlinear refractive index of CuO thin films
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2500
Figure 9: Optical susceptibility of CuO thin films (a) liner susceptibility (b) 3 rd order nonlinear susceptibility
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Intensity (c/s)
Cu Cu Cu Cu Cu Cu
O Cu Cu
O
C 40 sccm 30 sccm 20 sccm 10 sccm
0
500 1000 Biniding energy (eV)
Figure 10: XPS survey scan of CuO thin films at various oxygen pressure
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Figure 11: Cu2p spectra of CuO thin films at various oxygen pressures
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Figure 12: O1s spectra of CuO thin films at various oxygen pressures
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List of Tables:
Table: 1. Experimental conditions for CuO thin films
No Deposition parameters
Value
1
Base pressure
9×10-6 Torr
2
Operating pressure
5×10-3 Torr
3
Deposition time
900 sec
4
Substrate temperature
25 (RT)°C
5
DC power for Cu target
200 W
6
Argon flow rate
20 SCCM
7
Oxygen flow rate
10,20,30 and 40 SCCM
8
Target – substrate distance 14 cm
9
Substrate rotation speed
15 rpm
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Table: 2. Cell volume of CuO thin films at various oxygen pressures
Sample details a(Ao ) b(Ao ) c(Ao ) β(o )
Cell volume(Ao )
10 sccm
4.60
3.51
5.11
98.3
81.7
20 sccm
4.58
3.52
5.12
97.83 82.0
30 sccm
4.61
3.51
5.10
98.21 81.9
40 sccm
4.70
3.45
5.16
99.26 82.7
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Table: 3.Structural parameter of CuO thin films at various oxygen pressures
Sample Details 10 sccm
size,
Dislocation
Lattice
(nm)
density
strain
35.448
15.39195
4.221E-03
2.252E-03
38.268
11.05032
8.189E-03
3.137E-03
51.8818
4.789922
4.359E-02
7.237E-03
61.714
10.21
9.593E-03
3.395E-03
64.7783
6.001875
2.776E-02
5.775E-03
66.9778
6.609262
2.289E-02
5.245E-03
2o
mean value 20 sccm
9.008888
1.937E-02
4.507E-03
32.1294 12.32083
6.587E-03
2.813E-03
35.4239
14.28422
4.901E-03
2.427E-03
38.1864
10.42456
9.202E-03
3.325E-03
51.7984
4.370616
5.235E-02
7.931E-03
61.6626
7.977701
1.571E-02
4.345E-03
64.451
4.78065
4.375E-02
7.251E-03
66.8631
6.085301
2.700E-02
5.696E-03
74.5916
14.97806
4.457E-03
2.314E-03
mean value 30 sccm
Grain
9.402743
2.050E-02
4.513E-03
32.1247
11.26362
7.882E-03
3.077E-03
35.4029
11.76718
7.222E-03
2.946E-03
38.2087
8.522751
1.377E-02
4.067E-03
52.0745
3.439275
8.454E-02
1.008E-02
67.3118
2.124483
2.216E-01
1.632E-02
mean value
7.423461
6.699E-02
7.297E-03
40 sccm 35.2664
9.105542
1.206E-02
3.807E-03
38.1467
6.671582
2.247E-02
5.196E-03
67.063
3.336458
8.983E-02
1.039E-02
mean value
6.371194
4.145E-02
Page 28 of 29
6.464E-03
Table: 4. AFM Surface analysis of CuO thin films at various oxygen pressures
Sample details Mean grain size (nm) Mean surface roughness (nm) 10Sccm
23.71
1.18
20Sccm
23.97
1.12
30Sccm
21.97
0.77
40Sccm
19.82
0.61
Table: 5. XPS elemental composition of CuO thin films at various oxygen pressures.
Sample detail
Surface elements (At.wt%) Copper
Oxygen
Carbon
10 sccm
32.2
63.4
4.4
20 sccm
30.8
64.3
4.9
30 sccm
28.0
67.2
4.8
40 sccm
27.9
68.0
4.1
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