Hopping conduction in n- type ZnO: a critical revisit

To clarify conduction mechanisms, simultaneous fits have been performed to the experimental data of the temperature dependence of the electrical conductivity σ(T), the Hall coefficient RH(T) and the Hall mobility μH(T) on 16 samples of doped and undoped non-degenerate n-type ZnO single crystal which were grown by different methods in six studies. It is shown that the experimental data of σ(T), RH(T) and μH(T) can be simultaneously fitted by including both nearest-neighbor hopping and Efros-Shklovskii type variable-range hopping in addition to free-electron conduction for all of the samples investigated without assuming Mott-type variable-range hopping or hopping in the top Hubbard band, i.e. so-called ϵ2 conduction, whereas both the latter hopping mechanisms were assumed in some of the original articles. It is also shown that neutral impurity (NI) scattering has little effect and that it is needless to assume the considerable temperature dependence of the Hall factor for NI scattering which was assumed in one of the original articles for explaining the temperature dependence of the Hall mobility. By plotting the estimated values of the ionisation energy ED of the shallow donor level against the cube root of the compensating acceptor concentration NA, the relation of ED = 49 meV – αA NA1/3 with αA = 5.8 × 10−5 cm meV is obtained for the residual donor level in undoped samples. In addition, the piezoelectric coupling coefficient parallel to the c-axis has been determined to be 0.26.