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S4.10 - Modifying Electrical and Optical Properties of Solution Derived ZnO Nanorods via Surface Doping 
April 8, 2015   11:45am - 12:00pm

ZnO nanostructures have attracted tremendous attention in the field of ultraviolet (UV) optoelectronic devices for their wide band gap and large exciton binding energy. Various methods have been applied to gain high quality ZnO nanorods (NRs) array. However, these methods usually require complex set-up and the product yield is low. Besides growth, intentionally incorporating dopant to manipulate the electrical performance of the UV devices based on ZnO NRs is of great significance. Nevertheless, the dopants introduce structural defects in the core of NRS, impairing the UV emission. To address above noted challenges, we propose a two-step methodology of growing Ga doped core-shell ZnO NRs with enhanced optical property. Firstly, undoped ZnO NRs array is grown on p-GaN wafer by low-cost hydrothermal method in our experiments. Successively, the NRs array is annealed and serves as template to be coated with gallium doped zinc oxide (GZO) thin film by pulsed laser deposition. The structure is designed to show two advantages. On the one hand, the Ga doped shell builds up high concentration electron layer to prevent the holes from diffusing to the surface and suppresses their recombination with deep energy levels. On the other hand, surface doping could enhance the n-type conductivity and circumvent the doping deficiency of hydrothermal method. Scanning Electron Microscopy shows that samples are well aligned and free of entanglement. High resolution transmission electron microscopy images clearly resolve the core-shell structure of the NRs and the thickness of the shell is measured. Temperature dependent photoluminescence are studied. In near band edge region, defect-related shoulder near 3.33 eV (10 K) is largely suppressed in the core-shell NRs. Two electron satellite of donor bound exciton peak emerge in the spectrum of the coated sample, from which the dopant ionization energy is calculated to be 56 meV, fairly close to the 54.5 meV ionization energy of Ga. As for deep level emission (DLE), an asymmetric broad peak dominates the visible region from 10K up to room temperature. The relative intensity of DLE (DLE/NBE) reduces by 57% after 5 nm GZO coating, indicating the improvement of the optical property. Furthermore, the broad DLE peak is decomposed to two components by Gaussian fitting. In contrast to the red-shift in the control sample, both the green component (c.a. 2.44 eV) and the orange-red peak (c.a. 2.12 eV) shift to higher energy due to Burstein Moss effect (BM) in the core-shell rod between 200 and 300 K, implying additional donors (e.g. Ga) are incorporated into the sample. X-ray photoelectron spectroscopy is conducted and further confirms the existence of Ga. A detailed study on the components of the element oxygen is implemented. The high energy component of O 1S reflects the amount of surface absorbed O. This component is largely reduced in the core-shell samples, indicating the decrease of surface depletion.

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