Abstract
Resistivity, ρ(T), and magnetoresistance (MR) are investigated in the Cu2ZnSnxGe1−xS4 single crystals, obtained by the chemical vapor transport method, between x = 0–0.70, in the temperature range of T ~ 50–300 K in pulsed magnetic field of B up to 20 T. The Mott variable-range hopping (VRH) conductivity is observed within broad temperature intervals, lying inside that of T ~ 80–180 K for different x. The nearest-neighbor hopping conductivity and the charge transfer, connected to activation of holes into the delocalized states of the acceptor band, are identified above and below the Mott VRH conduction domain, respectively. The microscopic electronic parameters, including width of the acceptor band, the localization radius and the density of the localized states at the Fermi level, as well as the acceptor concentration and the critical concentration of the metal-insulator transition, are obtained with the analysis of the ρ(T) and MR data. All the parameters above exhibit extremums near x = 0.13, which are attributable mainly to the transition from the stannite crystal structure at x = 0 to the kesterite-like structure near x = 0.13. The detailed analysis of the activation energy in the low-temperature interval permitted estimations of contributions from different crystal phases of the border compounds into the alloy structure at different compositions.
Highlights
In the last years, utilization of the Cu-based group I2-II-IV-VI4 chalcogenide semiconductors became one of the leading streams in the development of the low-cost thin film solar cells
Such behavior is attributable to a different proximity of samples to the metal-insulator transition (MIT) at different x, which will be verified later
Such behavior is attributable to a possible existence of the SN phase at x = 0, which is expectable in the pure CZGS compound, as well as the KS phase
Summary
Utilization of the Cu-based group I2-II-IV-VI4 chalcogenide semiconductors became one of the leading streams in the development of the low-cost thin film solar cells. The Cu2ZnSnxGe1−xS4 (CZTGS) solid solutions exhibit an increase of the band gap, Eg, with increasing Ge concentration, reaching the values of Eg up to ~2.3 eV for the pure Cu2ZnGeS4 (CZGS) compound[9,10]. The latter was found to be crystalized in the kesterite (KS)[26] and wurtzstannite (WS)[27] types of structure, and the electronic properties were studied for each type of CZGS24,25 Both CZTS and CZGS compounds exhibit similar activated character of the temperature dependence of resistivity, including the Mott variable range hopping (VRH) conduction within a wide temperature range[22,23,24,25]. The purpose is to establish the conductivity mechanisms in various www.nature.com/scientificreports/
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