Abstract
Polarity-controlled growth of ZnO by chemical bath deposition provides a method for controlling the crystal orientation of vertical nanorod arrays. The ability to define the morphology and structure of the nanorods is essential to maximizing the performance of optical and electrical devices such as piezoelectric nanogenerators; however, well-defined Schottky contacts to the polar facets of the structures have yet to be explored. In this work, we demonstrate a process to fabricate metal-semiconductor-metal device structures from vertical arrays with Au contacts on the uppermost polar facets of the nanorods and show that the O-polar nanorods (∼0.44 eV) have a greater effective barrier height than the Zn-polar nanorods (∼0.37 eV). Oxygen plasma treatment is shown by cathodoluminescence spectroscopy to affect midgap defects associated with radiative emissions, which improves the Schottky contacts from weakly rectifying to strongly rectifying. Interestingly, the plasma treatment is shown to have a much greater effect in reducing the number of carriers in O-polar nanorods through quenching of the donor-type substitutional hydrogen on oxygen sites (HO) when compared to the zinc-vacancy-related hydrogen defect complexes (VZn-nH) in Zn-polar nanorods that evolve to lower-coordinated complexes. The effect on HO in the O-polar nanorods coincides with a large reduction in the visible-range defects, producing a lower conductivity and creating the larger effective barrier heights. This combination can allow radiative losses and charge leakage to be controlled, enhancing devices such as dynamic photodetectors, strain sensors, and light-emitting diodes while showing that the O-polar nanorods can outperform Zn-polar nanorods in such applications.
Highlights
ZnO belongs to the 6mm point group with a wurtzite structure that leads to opposing polar facets that are predominantly Znor O-terminated.[1,2] On bulk ZnO crystals, the physical and chemical properties of these facets have been well-studied often to determine the surface band bending, electronic properties, and the resultant effect on electrical contacts to the polar surfaces.[3−6] Here, we extend this investigation to ZnO nanostructures to study the differences between electrical contacts fabricated on the uppermost but opposing polar facets of polarity-controlled vertical ZnO nanorods.[7]
After several measurements, it was found that the I−V characteristics were the same when using only one probe on the Au top contact (Figure 1c) and the sample stage as the low-potential probe, which was electrically connected to the large-area Au contact at the base of the nanorods, forming a MSM structure on each vertical nanorod
These initial measurements immediately displayed some difference between the two wire types with the Zn-polar nanorods showing greater electrical conductivity and weaker rectification
Summary
ZnO belongs to the 6mm point group with a wurtzite structure that leads to opposing polar facets that are predominantly Znor O-terminated.[1,2] On bulk ZnO crystals, the physical and chemical properties of these facets have been well-studied often to determine the surface band bending, electronic properties, and the resultant effect on electrical contacts to the polar surfaces.[3−6] Here, we extend this investigation to ZnO nanostructures to study the differences between electrical contacts fabricated on the uppermost but opposing polar facets of polarity-controlled vertical ZnO nanorods.[7]. Control of the crystal polarity provides an opportunity to determine the structure, electrical properties, and the all-important metal−semiconductor contacts that must be fabricated on the array tips to integrate them into nanotechnological devices. This is critical in the applications of piezoelectricity including nanogenerators and pressure/strain sensors,[8,9] which benefit from the buildup of an electric field induced by piezoelectric polarization at a potential barrier.[10,11] In many of these devices, the formation of ZnO nanorods is achieved by chemical bath deposition on polycrystalline seed layers, which typically results in the creation of arrays with an uncontrolled/mixed polarity. Extensive work on bulk ZnO crystals shows some variability in the behavior of Schottky contacts on the polar facets, while theoretical studies show a large difference in barrier height when comparing the O-polar and Zn-polar surfaces.[14,15] This suggests that there is an opportunity for selecting the contact and nanorod properties that have yet to be exploited on nanostructured crystals, and the issue remains largely unexplored in the case of ZnO nanorods despite its primary importance
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