Broadening of the optical absorption spectra in ZnO nanowires induced by mixed-phase Mg x Zn1− x O shells

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.