Effect Of Indium Doping On Structural, Optical And Electrical Properties Of Zinc Oxide Thin Films

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Date
2016-08
Authors
Md Aznan, Nornani
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Abstract
Transparent conducting oxide (TCO) such as metal-doped zinc oxide (ZnO) thin films have important applications in optoelectronic devices. Metal-doped ZnO thin films have an advantage as they can be fabricated in a relatively simple and economical process. However, their properties are sensitive to the amount of metal doping in the zinc oxide host material. In this work, the effect of indium (In) content as well as the annealing temperature on the structural, optical and electrical properties of ZnO were investigated. The Indium-doped ZnO (IZO) thin films were successfully deposited onto ultrasonically cleaned Si (100) and glass substrates by radio frequency (RF) magnetron sputtering from sintered ZnO/In2O3 target with different In content ranging from 1 to 7 wt. %. The IZO thin films were grown in argon environment at 150ºC with a bias power of 100W. Apart from zinc (Zn) and oxygen (O) elements, the energy dispersive x-ray (EDX) spectra of the IZO thin films also detected In element, indicating that In were successfully incorporated into the host material (ZnO). The scanning electron microscopy (SEM) shows that the IZO thin films have a continuous surface morphology without the presence of foreign particles. Un-doped ZnO films were also fabricated under the same sputtering conditions for comparison. The X-ray diffraction (XRD) analysis show that the thin films have preferential orientation along (002) plane. With increasing In content, the (002) peak position of the IZO thin films were shifted to lower angles when referenced to the un-doped ZnO sample. In addition, as the In content increases, the crystallite size becomes smaller while the lattice constant c elongates due to the incorporation of more In into ZnO lattice. The Raman spectra reveals the expected E2 (high) mode at 437 cm-1 corresponding to the ZnO hexagonal wurtzite structure. However, as the In content increases, the intensity of E2 (high) mode reduces indicating a slight deterioration of hexagonal wurtzite structure. The deterioration of the hexagonal wurtzite structure observed by the Raman analysis thus provides corroborating evidence of structural changes from the XRD data. The addition of In also introduces anomalous peak at 274 cm-1 related to intrinsic host lattice defect. Although previous studies relates the anomalous peak at 274 cm-1 to the nitrogen (N) induced local vibration mode, it was later shown that the ZnO thin film doped with other metal impurities, without the presence of N also produces the same anomalous mode. The optical transmittance reaches ~ 75 – 90% transparency in the visible region. The optical band gap (Tauc gap) widens from 3.29 eV for the un-doped ZnO to 3.48 eV with initial In doping followed by band gap narrowing with further addition of In content. The band gap widening can be explained by the Burstein Moss shift where the incorporation of sufficient metal dopant content shifts the absorption edge to higher energies, thus, widening the optical band gap. With higher In doping, the band gap narrows from 3.48 eV to 3.28 eV as a result of band tail states formation below the conduction band edge. The Zn2+ ions at Zn lattice sites are substituted by the In3+ ions, resulting in higher carrier concentration in the process. The carrier concentration of the IZO thin films increases from 1018 to 1020 cm-3 with increasing In content. In addition, indirect evidence of substitution of Zn by In is obtained from a nearly linear relationship between the In content and the changes of the lattice constant c of the IZO thin films which is in accordance to Vegard’s Law. The resistivity of the un-doped ZnO is 3.037 Ω cm while the lowest resistivity value of the IZO sample is 3.09 x 10-3 Ω cm. For IZO samples annealed in air within the range of 300 - 500ºC for 40 min, the optical transmittance is maintained at ~ 75 – 90% in the visible region while the optical band gap narrows from 3.42eV (un-annealed) to 3.21eV. Annealed at 500ºC caused the resistivity to increase to 1.972 x 102 Ω cm, making it less conductive than the un-annealed sample (1.285 x 10-2 Ω cm).
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Effect of indium doping on Structural, optical and electrical Properties , of zinc oxide thin films.
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