High-field hopping magnetotransport in kesterites

摘要

Transport properties of the kesterite-like single crystals of Cu2ZnSnS4, Cu2ZnSnxGe1−xSe4and Cu2ZnGeS4are investigated in pulsed magnetic fields up toB = 20 T. The Mott variable-range hopping (VRH) conduction is established by investigations of the resistivity, ρ (T), in all the materials mentioned above within broad temperature intervals of ΔTv4 ∼ 50–150 K, 50–250 K and 100–200 K, respectively. In addition, the Shklovskii-Efros VRH conductivity belowTv2 ∼ 3–4 K, the nearest-neighbour hopping (NNH) charge transfer betweenT ∼ 250–320 K and the conductivity by activation of holes on the mobility threshold at temperatures outside ΔTv4, respectively, are observed in these materials. In Cu2ZnSnS4, magnetoresistance (MR) contains only a positive contribution, connected mainly to a shrinkage of impurity wave functions by the magnetic field. At the same time, a negative contribution to MR, attributable to interference effects in VRH, is observed in Cu2ZnSnxGe1−xSe4and, especially, in Cu2ZnGeS4. The joint analysis of the MR and ρ (T) data has yielded important electronic parameters of the materials. This includes widths of the acceptor bandWand of the Coulomb gap Δ, the NNH activation energyEn, the localization radiusa, the acceptor concentrationNAand the density of the localized states at the Fermi level,g(μ). A dramatic increase ofain Cu2ZnSnS4with decreasingTis observed, whereas in Cu2ZnSnxGe1−xSe4all the parameterW,En,g(μ),aandNAare non-monotonic functions ofx. Finally, in Cu2ZnGeS4the Hall coefficientRH(T) is negative (despite of thep-type conduction), exhibiting the dependence close to that of ρ (T) in the Mott VRH interval.