Hopping conductivity of Ni-doped p-CdSb

Abstract

Mechanisms of the resistivity, ρ, of single crystal samples oriented along the [100] (No 1), [010] (No 2) and [001] (No 3) axes of anisotropic semiconductor p-CdSb doped with 2 at.% of Ni are investigated. In zero magnetic field the Mott type variable-range hopping (VRH) conductivity is observed in No 2 and the Shklovskii–Efros type in No 1 and No 3 at T≤2.5 K. The magnetoresistance (MR) of the samples obeys the law lnρ∼B2 up to B∼6 T. However, the temperature dependence of MR gives evidence for the Mott-VRH conductivity in No 1 at T≤4.2 K and the nearest-neighbor hopping conductivity in No 2 between T = 3 and 4.2 K and in No 3 between 1.5 and 4.2 K. From the experimental data the values of the localization radius and dielectric permittivity and details of their critical behavior near the metal–insulator transition, as well as the widths and the values of the density of the localized states, the acceptor energies, their concentrations and the anisotropy coefficients, are obtained.

Hopping conductivity of Ni-doped p-CdSb in strong magnetic fields

“…In addition, MR decreases with increasing T up to ~ 30 K and then starts to increase, in agreement with the behavior of ρ (T) at B = 0 in Fig. 1 [7].…”

“…As observed by X-ray diffraction, the ingots of volume ~ 1 cm 3 were of single phase As can be seen from Fig. 1 the values of ρ (T) at B = 0 decrease smoothly below 300 K until a minimum is attained at ~ 30 K followed by strong increase of ρ with further decreasing of T [7].…”

“…given by these equations, is always much weaker than the exponential magnetic-field dependence of MR in the region of hopping conductivity (see below), which takes place in p-CdSb:Ni below T ~ 4.2 K [7]. This explains the difference in the behavior of MR in Fig.…”

“…Namely the variable-range hopping (VRH) conductivity at B = 0 transforms into the nearest-neighbor hopping (NNH) conductivity in nonzero field. This transition is accompanied with expansion of the temperature interval of the hopping conductivity already in weak magnetic fields, in line with presence of magnetic disorder or strongly inhomogeneous magnetization due to Ni-rich nanoclusters [7], which is damped when B is increased.…”

“…1 exhibit two intervals of activated behavior characterized by different slopes between ~ 5 − 20 K and below ~ 5 K, corresponding to the conductivity determined mainly by activation of holes from shallow acceptor states to the valence band and to the hopping conductivity over the states of the impurity band, respectively [7]. In semiconductors transversal MR, connected to non-generate carriers in conduction or valence band (VB), depends on B as follows:…”