J. Semicond. > Volume 36 > Issue 3 > Article Number: 033004

Effect of Co doping on structural, optical, electrical and thermal properties of nanostructured ZnO thin films

Sonet Kumar Saha 1, , M. Azizar Rahman 1, , M. R. H. Sarkar 2, , M. Shahjahan 2, and M. K. R. Khan 2,

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Abstract: Nanocrystalline Zn1-xCoxO (where x varies from 0 to 0.04 in steps of 0.01) thin films were deposited onto glass substrate by the spray pyrolysis technique at a substrate temperature of 350 ℃. The X-ray diffraction patterns confirm the formation of hexagonal wurtzite structure. The crystal grain size of these films was found to be in the range of 11—36 nm. The scanning electron micrographs show a highly crystalline nanostructure with different morphologies including rope-like morphology for undoped ZnO and nanowalls and semispherical morphology for Co-doped ZnO. The transmittance increases with increasing Co doping. The optical absorption edge is observed in the transmittance spectra from 530 to 692 nm, which is due to the Co2+ absorption bands corresponding to intraionic d—d* shifts. The direct and indirect optical band gap energies decrease from 3.05 to 2.75 eV and 3.18 to 3.00 eV, respectively for 4 mol% Co doping. The electrical conductivity increases with increasing both the Co doping and temperature, indicating the semiconducting nature of these films. The temperature dependence thermal electromotive force measurement indicates that both undoped and Co-doped ZnO thin films show p-type semiconducting behavior near room temperature. This behavior dies out beyond 313 K and they become n-type semiconductors.

Key words: ZnO:Co thin filmsstructural propertiesoptical propertieselectrical propertiesthermal properties

Abstract: Nanocrystalline Zn1-xCoxO (where x varies from 0 to 0.04 in steps of 0.01) thin films were deposited onto glass substrate by the spray pyrolysis technique at a substrate temperature of 350 ℃. The X-ray diffraction patterns confirm the formation of hexagonal wurtzite structure. The crystal grain size of these films was found to be in the range of 11—36 nm. The scanning electron micrographs show a highly crystalline nanostructure with different morphologies including rope-like morphology for undoped ZnO and nanowalls and semispherical morphology for Co-doped ZnO. The transmittance increases with increasing Co doping. The optical absorption edge is observed in the transmittance spectra from 530 to 692 nm, which is due to the Co2+ absorption bands corresponding to intraionic d—d* shifts. The direct and indirect optical band gap energies decrease from 3.05 to 2.75 eV and 3.18 to 3.00 eV, respectively for 4 mol% Co doping. The electrical conductivity increases with increasing both the Co doping and temperature, indicating the semiconducting nature of these films. The temperature dependence thermal electromotive force measurement indicates that both undoped and Co-doped ZnO thin films show p-type semiconducting behavior near room temperature. This behavior dies out beyond 313 K and they become n-type semiconductors.

Key words: ZnO:Co thin filmsstructural propertiesoptical propertieselectrical propertiesthermal properties



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[1]

Han B Q, Lavernia E J, Mohamed F A. Mechanical properties of nanostructured materials[J]. Rev Adv Mater Sci, 2005, 9: 1.

[2]

Mandal S K, Nath T K. Microstructural, magnetic and optical properties of ZnO:Mn (0.01 ≤ x ≤ 0.25) epitaxial diluted magnetic semiconducting films[J]. Thin Solid Films, 2006, 515: 2535.

[3]

Deepa M, Bahadur N, Srivastava A K. Optical properties and mechanical characteristics of transparent nanostructured Zn1-xMnxO thin films[J]. J Phys Chem Solids, 2009, 70: 291.

[4]

Soki T, Hatanaka Y, Look D C. ZnO diode fabricated by excimer-laser doping[J]. Appl Phys Lett, 2000, 76: 3257.

[5]

Liu Y, Gorla C R, Liang S. Ultraviolet detectors based on epitaxial ZnO films grown by MOCVD[J]. J Electron Mater, 2000, 29(1): 69.

[6]

Shinde V R, Gujar T P, Lokhande C D. Nanobeads of crystalline ZnO synthesis from pyrolytic decomposition[J]. J Cryst Growth, 2006, 296: 6.

[7]

Ohno H. Making nonmagnetic semiconductors ferromagnetic[J]. Science, 1998, 281: 951.

[8]

Saeki H, Tabata H, Kawai T. Magnetic and electric properties of vanadium doped ZnO films[J]. Solid-State Commun, 2001, 120: 439.

[9]

Farley N R S, Staddon C R, Zhao L. Sol—gel formation of ordered nanostructured doped ZnO films[J]. J Mater Chem, 2004, 14(7): 1087.

[10]

Liu E, Xiao P, Chen J S. Ni doped ZnO thin films for diluted magnetic semiconductor materials[J]. Curr Appl Phys, 2008, 8: 408.

[11]

Kim K J, Park Y R. Spectroscopic ellipsometry study of optical transitions in Zn1-xCoxO alloys[J]. Appl Phys Lett, 2002, 81: 1420.

[12]

Rode K, Anane A, Mattana R. Magnetic semiconductors based on cobalt substituted ZnO[J]. J Appl Phys, 2003, 93: 7676.

[13]

Choi C H, Kim S H. Fabrication of TiO2 nanotube film by well-aligned ZnO nanorod array film and sol—gel process[J]. Thin Solid Films, 2007, 515: 2864.

[14]

Ma J, Hao W, Luo R. Effect of crystallization quality on ferromagnetism in Zn1-xCox O nanopowders[J]. Mater Lett, 2008, 62: 403.

[15]

Ismail B, Abaab M, Rezig B. Structural and electrical properties of ZnO films prepared by screen printing technique[J]. Thin Solid Films, 2001, 383: 92.

[16]

Supriya V, Sugiyama T S, Reddy K T R K. Structural and optical properties of cobalt doped sprayed ZnO films[J]. Opt Adv Mat, 2010, 4: 2064.

[17]

Kamruzzaman M, Khan M K R, Rahman M M. Structural and dielectric properties of Zn(1-x-y)CdxLiyO solid solution[J]. J Bangladesh Academy Sci, 2008, 32: 183.

[18]

Cullity B D. Elements of X-ray diffraction[J]. Addison-Wesley Publications Company Inc, Reading, Massachusetts, 1956.

[19]

Khan M K R, Rahman M A, Shahjahan M. Effect of Al-doping on optical and electrical properties of spray pyrolytic nano-crystalline CdO thin films[J]. Curr App Phys, 2010, 10: 790.

[20]

Dinia A, Schmerber G, Meny C. Room-temperature ferromagnetism in Zn1-xCoxO magnetic semiconductors prepared by sputtering[J]. J Appl Phys, 2005, 97: 123908.

[21]

Hodgson J N. Optical absorption and dispersion in solids[J]. London: Chapman & Hall, 1970.

[22]

Liu X C, Shi E W, Chen Z Z. Structural, optical and magnetic properties of Co-doped ZnO films[J]. J Cryst Growth, 2006, 296: 135.

[23]

Fouchet A, Prellier W, Mechin L. Growth and characterizations of ZnO and Co-doped ZnO films for their use in spintronic[J]. Superlattice and Microstructures, 2007, 42: 185.

[24]

Yoo Y Z, Fukumura T, Jin Z. ZnO—CoO solid solution thin films[J]. J Appl Phys, 2001, 90: 4246.

[25]

Sato T, Suzuki H, Kido O. Production of transition metaldoped ZnO nanoparticles by using RF plasma field[J]. J Cryst Growth, 2005, 275: 983.

[26]

Kumar R, Khare N. Temperature dependence of conduction mechanism of ZnO and Co-doped ZnO thin films[J]. Thin Solid Films, 2008, 516: 1302.

[27]

Mott N F, Davis E A. Electronic processes in non-crystalline materials. 2nd ed[J]. Oxford: Clarendon Press, 1979.

[28]

Rahman M M, Khan M K R, Islam M R. Effect of Al doping on structural, electrical, optical and photoluminescence properties of nano-structural ZnO thin films[J]. J Mater Sci Tech, 2012, 28: 329.

[29]

Salunkhe R R, Dhawale D S, Gujar T P. Structural, electrical and optical studies of SILAR deposited cadmium oxide thin films: annealing effect[J]. Mater Res Bull, 2009, 44: 364.

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S K Saha, M. A. Rahman, M. R. H. Sarkar, M. Shahjahan, M. K. R. Khan. Effect of Co doping on structural, optical, electrical and thermal properties of nanostructured ZnO thin films[J]. J. Semicond., 2015, 36(3): 033004. doi: 10.1088/1674-4926/36/3/033004.

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Manuscript received: 20 August 2014 Manuscript revised: Online: Published: 01 March 2015

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