Mechanisms of charge transfer and electronic properties of Cu2ZnGeS4 from investigations of the high-field magnetotransport

Abstract

Recent development of the thin film solar cells, based on quaternary compounds, has been focused on the Ge contain compounds and their solid solutions. However, for effective utilization of Cu2ZnGeS4, deeper investigations of its transport properties are required. In the present manuscript, we investigate resistivity, ρ (T), magnetoresistance and Hall effect in p-type Cu2ZnGeS4 single crystals in pulsed magnetic fields up to 20 T. The dependence of ρ (T) in zero magnetic field is described by the Mott type of the variable-range hopping (VRH) charge transfer mechanism within a broad temperature interval of ~100–200 K. Magnetoresistance contains the positive and negative components, which are interpreted by the common reasons of doped semiconductors. On the other hand, a joint analysis of the resistivity and magnetoresistance data has yielded series of important electronic parameters and permitted specification of the Cu2ZnGeS4 conductivity mechanisms outside the temperature intervals of the Mott VRH conduction. The Hall coefficient is negative, exhibiting an exponential dependence on temperature, which is quite close to that of ρ(T). This is typical of the Hall effect in the domain of the VRH charge transfer.

Magnetotransport and conductivity mechanisms in Cu2ZnSnxGe1−xS4 single crystals

“…As has been demonstrated recently 24,32 , the most probable mechanism of nMR in CZGS is connected with quantum interference effects in the Mott VRH conduction interval 46–50 . Therefore, we can utilize this mechanism for interpretation of nMR in our CZTGS alloys, too, especially taking into account the close similarity of the MR behavior in both materials, mentioned in Section “Experimental results”.…”

“…Generally, such features of nMR are quite uncommon for conventional doped semiconductors. However, they have been already observed in the pure CZGS 24 and, partially, in the Cu 2 ZnSn x Ge 1−x Se 4 alloys 32 .

Figure 2The dependences of Δρ/ρ on B for x  = 0 ( a ), 0.13 ( b ), 0.51 ( c ) and x  = 0.70 ( d ).

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“…On the other hand, investigations of the electronic transport in CZTGS solid solutions are lacking up to date. The only available data have been obtained in the pure CZTS 20–23 and CZGS 24,25 border compounds. 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 CZGS 24,25 .…”

“…The only available data have been obtained in the pure CZTS 20–23 and CZGS 24,25 border compounds. 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 CZGS 24,25 . 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 22–25 .…”

“…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 22–25 . However, the behavior of magnetoresistance (MR) in these compounds is different, being completely positive (pMR) in CZTS 22 and possessing a negative contribution (nMR) in CZGS 24 . Therefore, similar activated conductivity with the Mott VRH conduction within a broad temperature range is expectable for the CZTGS solid solutions, too.…”