Observation of the anisotropic hopping conductivity of p-CdSb in a magnetic field

Abstract

The resistivity of the anisotropic semiconductor p-CdSb is investigated in a wide temperature range of T = 1.9-300 K in pulsed magnetic fields up to B = 25 T. Two not intentionally doped single-crystalline samples oriented along the crystallographic axes [100] (no. 1) and [010] (no. 2) are used for measurements of the resistivity, ρ, in transversal magnetic field configuration. Below T ≈ 8 K the resistivity follows the laws ln ρ ∼ C B 2 for B < B c and ln ρ ∼ S B 7/12 for B > B c , where the coefficients C and S do not depend on T , and B c ≈ 5-7.5 T. These are characteristics of nearest-neighbour hopping conductivity. The coefficients C and S depend on the direction of B, and their ratios agree completely with the values calculated with the components of the hole effective mass. The acceptor concentrations, N 1 ≈ 4.3 × 10 16 cm −3 and N 2 ≈ 7.6 × 10 16 cm −3 for no. 1 and no. 2, respectively, are relatively close to the critical value of the metal-insulator transition (MIT), N c ≈ 1.2×10 17 cm −3 . This leads to enhancement of the mean localization radii, a * 1 ≈ 78 Å in no. 1 and a * 2 ≈ 126 Å in no. 2, with respect to the value of a * 0 ≈ 50 Å far from the MIT determined by an asymptote of the wavefunction at a large distance from the impurity centre.

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

“…As follows from the percolation model of the hopping charge transfer over the nearest sites (or NNH conductivity) [10], the positive MR is connected to shrinkage of the impurity wave functions in the direction perpendicular to B and is given in strong magnetic field by the expression [6,10] ln…”

“…The anisotropic quantum transport revealed by Shubnikov-de Haas effect [4] and negative magnetoresistance [5] are observed on the metallic side of the MIT, whereas anisotropic hopping conductivity is realized on the insulating side of the MIT [6].…”

“…On the other hand, the onset of the temperature dependence of η j in # 3 observed below 3 K (Fig. 4) cannot be attributed to transition to the VRH conductivity (as e. g. in undoped p-CdSb [6]), because for any VRH mechanism positive MR is predicted to strengthen when T is decreased [10,12], that is the slopes in the bottom part of Fig. 4 are expected to increase with T, but not vise versa.…”

Electronic structure and chemical bonding of the electron-poor II-V semiconductors ZnSb and ZnAs

“…This characteristic results in a high thermoelectric power and -in conjunction with the orthorhombic symmetry -strongly anisotropic transport properties . 8,9 CdSb is an interesting material for thermoelectric sensor applications. [10][11][12] With respect to the well understood tetrahedrally bonded III-V and II-VI systems, II-V semiconductors pose some peculiarity into their bonding properties.…”

Mechanisms of hopping conductivity of p‐CdSb:Ni in magnetic field

“…However, the dependence of ln ρ (B) on temperature is different for each hopping regime. The law ln[ρ(B)/ρ(0)] j = C j B 2 is expected for the NNH conductivity, where C j = t e 2 a p j 2 /(ħ 2 N A ) does not depend on T, t = 0.036, e is the elementary charge, a is the mean localization radius, p j = [m j 2 /(m k m l )] 1/6 is the anisotropy coefficient (m j , m k and m l are the components of the hole effective mass), with j, k, l = 1, 2 , 3 (j ≠ k ≠ l) and j = 3, 1, 2 for # 1, # 2 and # 3, respectively, corresponding to the direction of the magnetic field along the [001], [100] and [010] axes, respectively, ħ is the Planck constant and N A is the acceptor concentration [3,4]. For the Mott-VRH conductivity in low fields one gets an expression similar to that of the NNH conductivity, but with C j replaced by A j (M)…”