Fabrication of n-CdO NPs/PS Heterojunction by laser ablation in ethanol Sevan Hussein Ali Erbil Technology Institute / Erbil Polytechnic University
[email protected]
Nawroz Ismael Hassan Hawler Health Technical Collage/ Erbil Polytechnic University
[email protected]
ABSTRACT CdO/ PSi heterojunction have been fabricated. CdO NPS has been prepared by laser ablation in ethanol at 500 pulses and 400 mJ as laser energy . The optical , structural and morphological properties of the CdO thin film have been studied. The Photoelectrochemical etching process has been used as a technique to control the porosity of silicon wafer by controlling the etching time for (20 min). The minimum value of filling factor was (0.30) for the junction prepared. The results of efficiency showed that these junctions had two depletion regions, the first one between CdO film and PSi layer, the second between the latter and the crystalline silicon. The minimum value of efficiency was (1.61). Keywords: Nanoparticles , laser ablation , XRD , AFM ,CdO
INTRODUCTION: Metal oxides can be used as transparent conducting oxides (TCO’s) possessing a conducting property (Stiebig H. et al., 1997) Most of the studied transparent conducting metal oxides are anion deficient (Oxygen deficient) and hence are always n-type conductors [(Andreas , 2012), Yousif et al. , 2012)and ( Khudheir et al. 2012)]. The transparent conducting metal oxides are also referred as oxide semiconductors. Role of metal oxide thin films are very important in the field of science and technology. These films can exhibit various characteristics of metals, semiconductors and insulators with improved electrical and optical properties. So they can be used as electrodes in optoelectronic devices(Radi et al., 2006), display devices (Rao et al., 2007) and photovoltaic cells ( Yu Yang et al., 20120. The n-type CdO thin films exhibit rock salt 1
structure (FCC) with band gap of 2.2eV. It also has a good optical conductivity and transmission in the visible region (Raid and Omer, 2007). CdO films can be synthesized by various methods such as spray pyrolysis(Ortega et al., 1999), sputtering (Dong, 2004), sol-gel spin coating (Subramanyam et al., 1998) activated reactive evaporation (Carballeda-Galicia et al., 2000) metal Organic Chemical Vapor deposition (Ye et al., 2006), pulsed laser deposition (Guo-hua et al., 2007) The main goal of this work is to seek a simple, non-vacuum and economic deposition technique for efficient transparent films. In this study CdO films were synthesized using laser ablation in ethanol. Optical studies such as transmission, bandgap, refractive index, absorption coefficient, extinction coefficient, dielectric constant (imaginary and real) and Urbach energy were obtained.
EXPERIMENTAL WORK The laser used for the ablation was Nd:YAG operating at 10 Hz repetition rate, with 7 ns pulse width and wavelength of 1064 nm. The laser pulses were focused by a 20 cm positive lens onto a cleaned 2 mm thick CdO bulk sample (99.99% purity provided by porch company) immersed in methanol at safe laser energy (400 mJ) with an ablation time of 20 min. The energy of laser pulse was measured using calibrated Joule meter after taking into account the effect of ethanol transmittance. The CdO target was placed in the bottom of a quartz vessel filled with 10 ml of ethanol, see Figure 1. X-ray diffractometer (XRD 6000, Shimadzu, X-ray, diffractometer) with Cukα radiation at a wavelength of ( = 0.154056 nm) was utilized to investigate the structural properties of CdO NPs deposited on a glass substrate. The optical absorption of colloidal CdO, NPs was measured using a spectrophotometer (Cary, 100 cans plus, UV-Vis-NIR, Split Beam Optics, Dual detectors) in the range of (200–900) nm. The morphology of the CdO NPs was investigated by using AFM (AA 3000 Scanning Probe Microscope). The thickness of the films was measured using ellipsometer (Angstrom sun Technologies Ins). All above measurements were carried out at room temperature at 200 nm thickness .
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RESULTS AND DISSCUSION
Figure 1 shows XRD patterns of CdO thin films deposited on FTO substrate . Xray diffraction patterns of CdO thin films revealed polycrystalline nature with a cubic structure. The observed diffraction patterns were indexed with standard values (JCPDS card no. 78-0653). X-ray diffraction patterns also show that various diffraction peaks at 2ɵ values 26.42˚,33.62˚, 37.67˚, 55.52˚ and 69.44˚, were identified to originate from (200) (110) (220) (222) and (111) planes, respectively, which corresponds to CdO face centered cubic structure. The large peak at 37.67˚ indicates that CdO thin film is preferentially oriented along the (200) crystallographic plane. The structural properties were calculated using expressions given by Abd (Ahmed et al., 2014), crystallite size (D) was in the order of 20 nm to 76 nm.The microstrain () , number of crystallites (N) and dislocation density (σ) of the CdO thin film is listed in Table 1.
400
Intensity (a.u.)
350
300 250 200 150 100 50 0 25
30
35
40
45 50 2 Theta (deg)
55
60
65
Figure 1: shows XRD patterns of the CdO thin films deposited on FTO substrate
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70
Table 1 :characterization of XRD patterns for CdO thin films deposited on FTO substrate σ x1014
x10-4
Nx 1018
lines.m-2
Lines-2.m-4
1/m2
8.95972
10.37171
80.9932
(110)
32.99681 9.184514
10.50102
84.06036
0.366
(220)
24.3946
14.204
208.031
69.44
0.126
(222)
76.44941 1.711007
4.532409
6.759032
33.62
0.4
(111)
20.64165
16.78645
343.3791
2 Theta
FWHM
hkl
D (nm)
(deg)
(deg)
plane
37.67
0.25
(200)
33.40818
26.42
0.246
55.25
16.8041
23.4699
Figure. 2 shows the 2D-3D AFM images of the synthesized CdO thin film deposited on an FTO substrate by drop casing method. The surface of the thin film has vertically closely packed ball –shaped, within the scanning area (2x2)µm.Using special software (4.62 imager), the estimated of the average grain was around 46 nm. Homogenous and a good roughness grain of CdO nanostructure were noticed.
Fig. 2: 2 and 3D AFM images of synthesized CdO nanoparticles
The transmittances of the individual components were measured with a spectrophotometer. UV-Vis-NIR spectrophotometer was used to measure the direct 4
transmittance. This instrument is a double-beam spectrophotometer where the detecting phototube compares the intensity of the sample beam with that of the reference beam, with the ratio of the two intensities recorded as per cent transmittance. Here, the cells containing the individual components were placed in the sample beam, and a quartz plate was placed in the reference beam. This latter plate cancelled out the effects of reflection and absorption by the quartz windows in the sample cells. Figure. 3 illustrates the UV-Vis spectra of ethanol solution containing CdO NPs prepared by laser ablation in liquid technique.The transmittance curve was subdivided into two regions at the visible and NIR regions . The transmittance curve increase sharply above 900 nm up to 1100nm but it seems to be nearly constant in the visible region. So the CdO NPS has an acceptable transmittance in the visible range, which can be used in solar cell and a smart window (Ahmed, 2015) .
Transmittance %
55
50 45 40 35 30 300
400
500
600
700
800
900
1000
1100
Wavelentgh (nm) Fig.3: UV-Vis transmittance spectra of nanoparticles colloid fabricated at wavelength CdO ablated with 400 mJ at 1064nm.
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Fig. 4 shows the variation of (αhυ)2 verses hυ for direct band gap which have been determined by the extrapolation of the linear portion versus the photon energy axis. It can be seen that the value of the energy gap was about 1.4 eV.
3.5E+10 3E+10
(h)² (eV/cm)²
2.5E+10
2E+10 1.5E+10 1E+10 5E+09 0 1
1.5
2
2.5
3
3.5
4
photon energy (eV) Fig.4: (αhυ)2versus photon energy plot for CdO NPS in ethanol.
In Fig. 5 the peak at 723 cm-1 and comparatively two small peaks confirm the presence of cadmium and oxygen in the sample. In the FTIR spectrum exhibit strong absorption in the range of 959 cm-1 to 986 cm-1 and the other couple of peaks in the range of 2905 and 2993 cm-1. The week absorption at 1641 was also found in the FTIR spectrum. The above cadmium reacting with oxygen in the air during the time of preparing CdO NPs. From the FTIR spectrum, it was concluded that the presence of cadmium and oxygen is in the range of 1641 to 1270 cm -1 which is in good agreement with Aldwayyan et al. (Aldwayyan et al. ,2013)
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120 100
T%
80 60 40 20 0 400
800
1200
1600
2000
2400
2800
3200
3600
4000
Wavenumber (1/cm
Fig. 5: FTIR for CdO Nanoparticles.
Figure 6 shows the I-V dark characteristics in forward and reverse direction of Al/CdO/ p-Si/Al. The forward current of photodetector was very small at voltages less than 1Volt . This current is known as recombination current which occurs at low voltages only (Hering, 2012). It is generated when each electron excited from valence band to conductive band. The second region at high voltage represented the diffusion or bending region ,which depends on a serries resistance .In this region; the bias voltage can deliver electrons with enough energy to penetrate the barrier between the two sides of the junction.
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Current (mA)
25 20 15 10 5 0 Voltage (volt) -5
-10
-8
-6
-4
-2
0
2
4
6
8
10
Figure 6 shows the I-V dark characteristics for CdO/Si heterojunction
Figure 7 shows that a linear relationship between 1/C2 and reverse bias voltage was obtained for the structure. This linear relationship represented the photodetector (CdO /Si) and (PSi/Si). The values of the built-in potential have been obtained and it had been found equal to 0.2volt .
-7
-6
-5
-4
-3
-2
-1
1/C2 nF-2
5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0
1
2
applied voltage (volt)
Fig. 7. 1/C2 versus reverse voltage of CdO/PSi Photodetectors.
Figure 8 displays the responsivity as a function of wavelength for CdO/p-Si photodetector , the maximum responsivity is located in the visible region and the 8
other at the NIR region, the spectral responsivity curve of CdO/p-Si consists of one peak of response; we think that the peak was located at 575 to 850 nm due to the absorption edge of CdO NPS and Si nanoparticles .
0.9 0.8
R (A/W)
0.7 0.6 0.5 0.4 0.3
0.2 0.1 0 400
500
600
700
800
900
Wavelength(nm) Fig. 8: Spectral Responsivity plots for CdO/p-Si as a function of wavelength.
CONCLUSIONS
The synthesized CdO NPs were in nanosized of 46 nm prepared in ethanol by laser ablation in liquid method and the optical properties revealed that the direct band gap of CdO NP indicated to the effect of quantum size. X-ray diffraction (XRD) measurement disclosed that the CdO NPs are polycrystalline and have FCC crystal structure and no other phases were noticed. Deposition of CdO NPs on silicon (Si) gave suspensions photodetector characteristics enhanced the properties porous photodetectors. The spectral responsivity of Al/CdO /Si/Al photodetector was around 0.7 A/W at 790 nm
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REFERENCES [1] Stiebig H., Siebke F., Beyer W., Beneking C., Rech B., Wagner H., (1997) “Interfaces in a-Si:H solar cell structures” Solar Energy Materials and Solar Cells.48 (1-4): 351-363. [2] Andreas Stadler, (2012) “Transparent Conducting Oxides—An Up-To-Date Overview”, Materials 5(4) : 661-683. [3] Yousif, A A; Habubi, N F; Haidar, A A, (2012) “Nanostructure Zinc Oxide with Cobalt Dopant by PLD for Gas Sensor Applications”, Journal of Nano- and Electronic Physics 4 (2): 02007-1 (6pp). [4] Khudheir A. M., Sami S. Ch., Hazim L. M., Nadir F. H., ( 2012) “Influence of Copper Doping on the Structural and Optical Properties of Sprayed SnO2 Thin Films ”, Journal of Electron Devices (14): 1170-1177. [5] Radi P. A., Brito-Madurro A. G., Madurro J. M.
and DantasBraz N. O., (2006)
“Characterization and properties of CdO nanocrystals incorporated in olyacrylamide”, J. Phys. 36(2a): 412-414. [6] Rao C.N.R., Kulkarni G.U., & Thomas P.J., (2007) “Nanocrystals: Synthesis, Properties and Applications” Springer, 9.
[7] Yu Yang, Qinglan Huang, Andrew W. Metz, Shu Jin, Jun Ni, Lian Wang and Tobin J. Marks(2012), “Highly transparent and conductive CdO thin films as anodes for organic light-emitting diodes, Film micro structure and morphology effects on performance”, J Soc Inf Display 13(5): 383–387. [8] Raid A. I. and Omar A. A., (2007) “A new route for fabricating CdO/c-Si heterojunction solar cells”, Sol Energ Mat Sol C 91:903–907. [9] Ortega M., Santana G., Morales and Acevedo A., (1999)” Optoelectronic Properties of CdOSi Hterojunctions”, Superficies y Vacio, 9: 294-295. [10] Dong Ju Seo , (2004) “Structural and Optical Properties of CdO Films Deposited by Spray Pyrolysis” , J Korean Phys Soc 45(3-4): 1575-1579. [11] Subramanyam T.K , Uthanna S. and Srinivasulu B.N., (1998)” Preparation and characterization of CdO films deposited by dc magnetron reactive sputtering”, Mater Lett 35:214–220.
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[12] Carballeda-Galicia D.M, Castanedo-Pérez R., Jiménez-Sandoval O., Jiménez-Sandoval S., Torres- Delgado G., and Zúñiga-Romero C.I., (2000) “High transmittance CdO thin films obtained by the sol-gel method” , Thin Solid Films, 371(1-2): 105–108. [13] Ye X.R., Daraio C., Wang C., and Talbot J.B., (2006)” Room temperature solvent-free synthesis of monodisperse magnetite nanocrystals”, J. Nanoscience and Nanotechnology, 6(3): 852-856(5). [14] Guo-hua Z., Ming-fang L., and Ming-Li L., (2007)” Differential pulse voltammetric determination of dopamine with the coexistence of ascorbic acid on boron-doped diamond surface” CEJC, 5(4): 1114-1123. [15] Ahmed N. Abd, Raid A. I., and Nadir F. H., (2014) " Preparation of colloidal cadmium selenide nanoparticles by pulsed laser ablation in methanol and toluene", journal of Materials Science: Materials in Electronics, 25(7) : 3190-3194. [16] Ahmed N. Abd, (2015) " Improved photoresponse of porous silicon photodetectors by embedding CdS nanoparticles", World Scientific News (WSN),19: 32-49. [17] Aldwayyan A.S., Al-Jekhedab F.M., Al-Noaimi M., (2013)”Synthesis and Characterization of CdO Nanoparticles Starting from Organometalic Dmphen-CdI
complex”, Int. J.
Electrochem. Sci. 8:10506 - 10514. [18] Hering G., (2012)“Die Produktion von Solarzellen ist 2011 auf 37 Gigawatt gestiegen – und die Dominanz Chinas wächst. In”, Photon, Das Jahr des Drachen, Photon, 4: 42-63.
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