A newer technique to modify the surface of a-Fe2O3 with Zn dots was attempted by the authors' group in 2007 . These samples exhibited much better PEC responses, compared to doping or layering of the same metal. We modified the surface of 1.5 at% Zn-doped iron oxide (prepared by spray pyrolysis) by depositing thermally evaporated Zn through a mesh of pore diameter ^0.7 mm, using a vacuum-coating unit. The heights of the Zn dots were varied in the range 100 to 260 A by varying the processing time of thermal evaporation. The schematic diagram explaining the method of preparation of sample is shown in Figure 13.14.
All dotted samples exhibited n-type behavior. The resistivity in Zn-doped samples was found to increase by one order of magnitude , whereas the presence of Zn-dotted islands on the surface of Zn-doped a-Fe2O3 thin films gave a decrease in resistivity, which could be due to a difference in the work functions of the two materials. A decrease in the resistivity from 2.6 x 107 to 5.7 x 106 O cm was observed on increasing the height of the Zn island to 230 Aon the a-Fe2O3 surface. However, a slight increase in resistivity up to 9.2 x 106 O cm was observed after loading with 260 A thick Zn-dotted islands. This increase in resistivity at a height of 260 A was due to interdiffusion of the Zn islands in the iron oxide near the interface, which resulted in the loss of the charge carriers.
Doping of 1.5 at% Zn in a-Fe2O3 thin films prepared by the spray-pyrolysis method enhanced the photocurrent density from 0.061 to 0.321 mA cm-2 at 0.7 V versus SCE. Deposition of Zn dots on 1.5 at% Zn-doped a-Fe2O3 thin films with increasing height of dots resulted in continuous enhancement in the photocurrent, as shown in Figure 13.15. Structures with —230 A thick Zn islands exhibited the maximum photocurrent density of — 1.282 mA cm 2, at an applied potential of 0.7 V versus SCE. The increase in photocurrent density in this study was due to conversion of the Zn dots on the surface of a-Fe2O3 into wide-bandgap ZnO, at the time of annealing, which acted as an efficient catalyst for the swift migration of the photogenerated charge carriers. However, a decrease in photocurrent density with a-Fe2O3 deposited with thicker dots (260 A) was correlated with an increase in resistivity and a decrease in the a-Fe2O3 surface area, due to interdiffusion of Zn. A minor change in donor density and flatband potential was observed on increasing the height of the dots. Observed values of photocurrent density on the surface of a-Fe2O3 and onset potential, along with resistivity, for various modified/unmodified thin films of a-Fe2O3 are given in Table 13.2.
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