Investigation of the effects of coating numbers of thin films and metal contact type on physical properties of undoped ZnO, Fe-doped ZnO, and Fe–B co-doped ZnO thin films
Authors: Üzar Neslihan , Abdulaziz Ubade 
Published: June 2024

This study was designed for three purposes. The first objective was to examine the effects of iron (Fe) and boron (B) elements on the physical properties (structural, electrical, optical, and optoelectronic) of zinc oxide (ZnO) material. For this reason, pristine ZnO, 6% Fe-doped ZnO (Zn0.94Fe0.06O), and 6% Fe-4% B co-doped ZnO (Zn0.90Fe0.06B0.04O) thin films with different thicknesses (4, 6, 8, and 10 layers of coatings for each sample type) were produced using sol–gel dip coating and spraying method on glass and silicon (Si) substrates. In the second stage, we examined the effects of film thickness on optical, electrical, and optoelectronic properties for these three sample types. In the final stage, the MIS (metal/interlayer/semiconductor) structures were created using the three groups of samples produced as interlayers. Gold (Au) was initially applied as the metal contacts in these MIS structures. We investigated optoelectronic and electrical properties such as ideality factor, barrier height, and series resistance for all samples with Au contacts. Afterward, aluminum (Al) contacts were coated on the sample that yielded the best results with Au contacts, and the same properties were re-examined, thereby determining the effects of the contact material, especially on optoelectronic properties. All samples were produced as pure and wurtzite ZnO polycrystalline with preferred orientation along the (002) plane. Although Hall measurement results indicated that all sample groups were n-type semiconductors, the carrier density decreased from − 7.5 × 1013 for pristine ZnO to − 8.7 × 1011 with Fe–B co-doping. The irregular nanodots-shaped surface morphology of ZnO transformed into a homogeneous and smooth one by incorporating boron into the structure. In all sample groups except the 6% Fe-doped ZnO thin films, the band gaps of the thin films decreased as the film thickness increased. For pure ZnO and Fe-B co-doped ZnO sample groups, the band gap energy decreased from 3.245 to 3.215 eV, and from 3.540 to 3.180 eV, respectively, depending on the thicknesses of films. On the other hand, the band gap energy of only Fe–doped ZnO samples increased from 3.34 eV to 3.46 eV. It was observed that as the thicknesses of films increased, the ideality factor of Au/ZnO/p-Si, Au/Zn0.94Fe0.06O/p-Si, and Au/Zn0.90Fe0.06B0.04O/p-Si diodes increased, and the barrier heights of them decreased in the three sample groups. However, when we look at the average value of the electrical properties including all layers, we can say that the best results were obtained for the Fe–B co-doped sample group. Specifically, Fe–B co-doped ZnO sample with 6 layers of coating exhibited an ideality factor of 3.25, a barrier height of approximately 0.51 eV, and a serial resistance of 8.42 kΩ. The best performance as solar cell and photodiode was again obtained for this sample. While the solar cell efficiency of this sample (6 layers of coated Zn0.90Fe0.06B0.04O) was 0.04% with Au contacts, it increased to 0.08% with Al contacts.

In summary, it was observed that the electrical, optical, structural, and optoelectronic (as solar cell and photodiode) properties of ZnO material were improved very well made with Al contact and 6 layers of coated Fe and B co-doping. Therefore, Zn0.90Fe0.06B0.04O sample may be promising material for optoelectronic devices.