Charge Transport in the Impurity Band of Dilute Magnetic Semiconductors: Experiment and Theory

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By incorporating magnetic atoms into conventionally doped semiconductors one enters an interesting field between magnetism and semiconductor physics. On the one hand, in dilute magnetic semiconductors (DMS) such as Zn1-xMnxSe:Cl, spin-effects on the electronic structure can be tuned continuously up to orders of magnitude by varying x. On the other hand, the evolution of the Cl-donor energy levels into a so called impurity band (IB) by varying the donor concentration ND provides an excellent test-ground for verifying fundamental physical concepts of charge transport in solid media. DMS show a very different transport behavior compared to non-magnetic systems. This holds for both regimes on either side of the metal-insulator transition (MIT) – hopping conductivity at low donor concentrations and metallic conductivity at high doping levels. These differences must arise from the interplay of enhanced spin effects in the scattering processes, in the shape of the donor wave functions, and in the spin-dependent changes of the density of states of impurity and conduction bands. However, conventional models cannot account for the observed behavior. Most theories of charge transport are applicable only to non-magnetic systems assuming that the impurity levels are spin-degenerate and independent of external magnetic field. We will perform low-temperature magneto-transport and magneto-optical spectroscopic measurements on different series of n-type Zn1-xMnxSe, which is an ideal model system as concentrations x and ND can be varied independently. The experimental study will be combined with the development of an adequate transport theory to model the experimental results and, thus, to clarify the fundamental problems of charge transport and MIT in DMS.