When MgZnO serves as the shell to passivate a ZnO nanowire, the proportion of Mg is usually small to avoid the unexpected spectrum from the wurtzite to rock salt transformation. Using the effective mass approximation, we investigate the impact of a mixed-phase MgxZn1−xO shell on the optical absorption spectra in ZnO nanowires. The results show that the dual absorption peaks from the coexisting two sets of band offset tend to appear as an intrinsic line broadening. This is because the spacing of dual peaks is small and even less than the full width at half maximum to be distinguished. The dual peaks get closer by increasing x or core size since the energy levels become less insensitive to the difference of the potentials. Enhanced confinement of an electron from higher x and smaller core size induces not only a blueshift and a slower saturation but also a sharper peak. The above two aspects make the dual peaks appear always as a broadening in inter-band V1–C1 transitions, while only appear as a broadening in inter-subband C1–C2 transitions when the core radius gets larger than the critical value for a certain x. The broadening from the mixed-phase MgZnO-coated ZnO nanowire could be restricted by increasing the core size or the proportion of Mg in the shell.
MgxZn1−xO shells are commonly used as a passivation barrier for improving electron mobility in ZnO nanowires by preventing electrons from charged surfaces. However, a high Mg mole fraction x instead makes lower electron mobility, which is usually attributed to the appearance of mixed-phase MgxZn1−xO as x increases. This work aims to find the optimal x for optical phonon limited electron mobility by considering the phase transformation in the MgZnO shell from wurtzite to rock salt, leading to a mixed-phase range of x. Our calculations show that the electron mobility μT can be effectively enhanced by keeping x below 0.057 when confined (CO1) optical phonons are only permitted for small wave vectors, and there is no interface (IF) optical phonon. Once x gets over 0.057, the propagating optical phonons are transformed into IF ones while CO1 phonons become permitted for all wave vectors resulting in a largely strengthened scattering effect and thus a drastic drop in the total electron mobility μT from 1215 to 310 cm2/V s. From then, μT begins to fall slowly as x increases even when the rock salt component in the shell appears to take the place of the wurtzite part, while the scattering from CO1 optical phonons remains primary. Furthermore, the enlarging core radius can weaken the electron–CO1 phonon interaction to enhance mobility.
Within the framework of dielectric continuum and Loudon’s uniaxial crystal models, the transfer matrix method (TMM) is developed to investigate interface optical phonons (IOPs) in cylindrical wurtzite core-multishell nanowires (CMSNWs) consisting of ternary mixed crystals (TMCs). The IOPs in GaN/InxGa1-xN/InyGa1-yN and GaN/InxGa1-xN/InyGa1-yN/InzGa1-zN CMSNWs are calculated as examples. The results show that there may be several types of IOPs existing in certain frequency regions in CMSNWs for a given component due to the phonon dispersion anisotropy in wurtzite nitrides. The IOPs are classified by possible combinations of the interfaces in CMSNWs. Furthermore, the dispersion relations and electro-static potentials of each kind of IOPs are discussed in detail. The dispersion relations of IOPs in CMSNWs is found to be the combination of that in each nearest two layer CSNW. It can explain the fact that the total branch number of IOPs obey the 2n rule. It is also found that the peak positions of electro-static potentials are decided by the layer component order from the inner layer to outside in CMSNWs. The results indicate that TMM for IOPs is available and can be commodiously extended to other cylindrical wurtzite III-nitride CMSNWs. Based on this method, one can further discuss the IOPs related photoelectric properties in nitride CMSNWs consisting of TMCs.
We report here the experimental observations that the tip topography of ZnO nanoprisms sensitively depends on the percentage of oxygen in the flux of the carrying gas in vapor growth. At a relatively high oxygen concentration, a number of thin filaments can be nucleated atop nano-prisms, forming a unique fish-spear-like multi-tip morphology. The length and density of the "spear tines" depend on the flux of the carrying gas. The field emission properties of the nanorod array with different tip morphology are investigated. The structures with longer and denser spear tines possess lower turn-on electric field and higher electric current density. The cathodoluminescence properties of the ZnO nano-prisms have also been studied. The luminescence related to defects in multi-tip nano-prisms possesses the strongest intensity, and the nanorod without any tine structure possesses the lowest defect luminescence intensity. The intrinsic luminescence of ZnO around 385 nm, however, has the opposite tendency. We suggest that our observation is inspiring in optimizing the emission properties of the nanowire devices.
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