Optical phonon limited electron mobility in ZnO nanowires wrapped by MgZnO shells

Abstract

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.