SEMICONDUCTOR MATERIALS

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

Sonet Kumar Saha1, M. Azizar Rahman1, M. R. H. Sarkar2, M. Shahjahan2 and M. K. R. Khan2

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 Corresponding author: M. Azizar Rahman, E-mail: azizar@phy.buet.ac.bd

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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



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Fig. 1.  XRD patterns of Zn$_{1-x}$Co$_{x}$O thin films.

Fig. 2.  SEM micrograph of Zn$_{1-x}$Co$_{x}$O thin films with (a) Co $=$ 0 %, (b) Co $=$ 2 % and (c) Co $=$ 4 %.

Fig. 3.  (Color online) Transmittance spectra of Zn$_{1-x}$Co$_{x}$O thin films as a function of photon energy.

Fig. 4.  (Color online) Determination of direct and indirect band gaps from the plots of (a) $\left( {\alpha h\nu} \right)^{1/2}$ versus photon energy $(h\nu)$ and (b) $\left( {\alpha h\nu} \right)^2$ versus photon energy $(h\nu)$, respectively, for Zn$_{1-x}$Co$_{x}$O thin films.

Fig. 5.  Variation in conductivity with temperature of Zn$_{1-x}$Co$_{x}$O thin films.

Fig. 6.  (a) Arrhenius plot of DC conductivity. This plot shows separately for the determination of activation energy in two different temperature regions of (b) 363-403 K and (c) 408-473 K.

Fig. 7.  Temperature dependence of (a) thermo-emf and (b) thermoelectric power of Zn$_{1-x}$Co$_{x}$O thin films.

Table 1.   Data for lattice parameters, packing fraction and grain size.

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Table 2.   Data for conductivity ($\sigma )$, activation energy ($E$) and band gap energy $(E_{\rm g})$ of nanostructured Zn$_{1-x}$Co$_{x}$O thin films.

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    Received: 20 August 2014 Revised: Online: Published: 01 March 2015

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      Sonet Kumar Saha, M. Azizar 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]. Journal of Semiconductors, 2015, 36(3): 033004. doi: 10.1088/1674-4926/36/3/033004 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.Export: BibTex EndNote
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      Sonet Kumar Saha, M. Azizar 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]. Journal of Semiconductors, 2015, 36(3): 033004. doi: 10.1088/1674-4926/36/3/033004

      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.
      Export: BibTex EndNote

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

      doi: 10.1088/1674-4926/36/3/033004
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      • Corresponding author: E-mail: azizar@phy.buet.ac.bd
      • Received Date: 2014-08-20
      • Accepted Date: 2014-10-21
      • Published Date: 2015-01-25

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