Effects of acceptor dopants on the enhanced piezoelectric potential of ZnO nanowires: limiting free charge-carrier density through neutralizing donors

Abstract

The piezoelectric potential of ZnO can be enhanced using acceptor dopants to neutralize the donor concentrations. In this study, unintentional n-type conductivity is assessed through modeling ZnO nanowires where the activation process of donors $(N_d^+)$ is given with a Fermi level $(E_F)$ close to the conduction band and followed by the introduction of an acceptor dopant $(N_a^-)$ in order to allow $E_F$ to be within the optimum range of $1\le E_F \le 3.2 \hbox { eV}$, which corresponds to the maximum piezoelectric potential calculated. The finite element method simulation reveals that the maximal range of ZnO piezoelectric potential can be obtained due to the intrinsic characteristics of the ZnO nanowire transformed using acceptor dopants, which implies that the limitations on the free-charge carriers (i.e. free-carrier depletion) could reduce the screening effects on the piezoelectric potential. Furthermore, the difference ${\vert }N_d^+ -N_a^- {\vert }$ is calculated to approach zero near the mid-gap and the energy band structure, which deviates from the normal flat line within the optimal range of $1\le E_F \le 3.2 \hbox { eV}$ under the external stress imposed.