Schottky Contacts on Polarity-Controlled Vertical ZnO Nanorods

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.

Modulating the growth of chemically deposited ZnO nanowires and the formation of nitrogen- and hydrogen-related defects using pH adjustment

“…Additionally, a clear increase in the intensity of the red-orange emission band from the pristine series to the annealed series is seen in ZnO NWs grown with the pH 0 value of 7.00, which could again indicate the preferential formation of both V Zn -H and V Zn -N O -H defect complexes aer thermal annealing at 450 C. 35 The increase in the concentration of V Zn during the thermal annealing under an oxygen atmosphere is favorable for the preferential formation of low-coordinated V Zn -nH defect complexes. 70 Interestingly, a simultaneous strong increase in the intensity of the three defect emission bands is revealed in ZnO NWs grown with the pH 0 value of 10.55. The strong red-orange and yellow-green emission bands indicate the formation of the V Zn -H and V Zn -2H defect complexes in the bulk of ZnO NWs as well as of the V Zn , V Zn -H and V Zn -N O -H defects with unexpected charge states on their surfaces, which is very well correlated with the NBE widening towards the A X A , DAP, and FA emissions.…”

Engineering nitrogen- and hydrogen-related defects in ZnO nanowires using thermal annealing

“…At the annealing temperature of 500 °C, a general increase in the relative intensity of the visible emission bands correlates with the previously discussed broadening of the NBE emission, which appears to be due to the activation and generation processes of (V Zn -N O -H) defect complex. The thermal annealing at this temperature under oxygen atmosphere promotes the formation of N O -H bonds [76], and could be accompanied with the formation of (V Zn -nH) defect complex with a low coordination in the center and on the surfaces of ZnO NWs [37]. This can lead to the formation of (V Zn -N O -H) defect complex on the surfaces of ZnO NWs where holes are highly concentrated under electron beam excitation [14], leading to transition levels other than the expected (-1/-2) one.…”

“…The present approach offers a great opportunity to monitor the nature and concentration of hydrogen-related defects in ZnO NWs, but it has not been explored yet in detail. More recently, the alternative oxygen plasma treatment in ZnO nanorods has also been found to improve the Schottky contact with Au on their top c-plane faces through the strong effects on the nature and concentration of hydrogen-related defects [37]. Furthermore, the growth medium contains hydrogen, but also carbon and nitrogen species.…”

MoSe2 Nanosheets with Tuneable Optical Properties for Broadband Visible Light Photodetection

“…27 At the two metal− semiconductor junctions, due to slightly different junction areas, a barrier potential is developed. 28 The difference in barrier potential at the two junctions helps to transport the photogenerated carriers to opposite electrodes even without any external bias, giving rise to the zero bias photocurrent. Actually, we have used Pt-interdigited electrodes of 100 μm separation and a thin multilayer film (∼100 nm thick) of assynthesized MoSe 2 nanosheets deposited from solution on it.…”

“…This can be explained by the formation of a metal semiconductor Schottky barrier at the interface . At the two metal–semiconductor junctions, due to slightly different junction areas, a barrier potential is developed . The difference in barrier potential at the two junctions helps to transport the photogenerated carriers to opposite electrodes even without any external bias, giving rise to the zero bias photocurrent.…”