Appearance of amorphous phase in crystalline In-Y codoped ZnO thin films

Indium-Yttrium codoped ZnO (ZnO:In-Y) thin films were prepared by sol–gel spin-coating technique on glass substrates at room temperature and annealed at 500°C for 1 h. The indium was incorporated with a constant concentration of 2 at% and yttrium was introduced with different concentrations of 1, 2 and 3 at%. X-ray diffraction patterns indicate that ZnO:In-Y thin films are polycrystalline with a hexagonal wurtzite structure. The preferred orientation is along the (002) plane perpendicular to the surface of the glass substrate. No additional diffraction peaks corresponding to secondary phases confirmed that the In and Y ions were uniformly distributed inside the samples. Transmittance analysis showed high transparency in the visible region with some extension to the near-ultraviolet region. Each thin film is composed of two different phases implying two different optical bandgaps; the amorphous phase manifests evidently higher optical bandgap, higher Urbach energy and lower refractive index in comparison with the crystalline phase. The amorphous phase is created under the effect of the amorphous glass substrate having an ultrawide bandgap. Transparency improvement in the near ultraviolet region depends on the amorphous phase rate. The resistivity measurements revealed that the lowest electrical resistivity value was 4.49 x 10^-3Ω:cm obtained for ZnO:In (2 at%)-Y (1 at%) thin film. These ZnO:In-Y thin films can have a big interest in the solar cell industry; can be used to fabricate thin film transistors and ultraviolet light-emitting diodes. (author)