Theoretical studies of the electronic, magneto-optical and transport properties of diluted magnetic semiconductors

Spintronics has recently become one of the key research areas in the magnetic-recording and semiconductor industries. A key goal of spintronics is to utilize magnetic materials in electronic components and circuits. A hope is to use the spins of single electrons, rather than their charge, for storing, transmitting and processing quantum information. This has invoked a great deal of interest in spin effects and magnetism in semiconductors. In my work, the electronic and optical properties of diluted magnetic semiconductors (DMS), especially (In,Mn)As and its heterostructures, are theoretically studied and characterized. The electronic structures in ultrahigh magnetic fields are carefully studied using a modified eight-band Pidgeon-Brown model, and the magneto-optical phenomena are successfully modeled and calculated within the approximation of Fermi's golden rule. We have found the following important results: (i) Magnetic ions doped in DMS play a critical role in affecting the band structures and spin states. The sp-d interaction between the itinerant carriers and the Mn d electrons results in a shift of the cyclotron resonance peak and a phase transition of the III-V DMS from paramagnetic to ferromagnetic; (ii) g-factors of the electrons in DMS can be enhanced to above 100 by large spin splitting due to strong sp-d interaction. Also the effective masses of DMS systems strongly depend on interaction parameters; (iii) Two strong cyclotron resonance peaks present in p-doped DMS arise from the optical transitions of heavy-hole to heavy-hole and light-hole to light-hole Landau levels, in lower and higher magnetic fields, respectively; (iv) Electron-active cyclotron resonance takes place in p-doped DMS samples. This is unusual since a simple quasi-classical argument would suggest that one could not simultaneously increase angular momentum and energy for this type of polarized light in a hole system. This occurs because of the degeneracy in the valence bands; (v) Due to the magnetic circular dichroism, nonvanishing magneto-optical Kerr rotation up to several tenths of a degree occurs in DMS systems. The Kerr rotation in multilayer structures depends on quantum confinement and multi-reflections from the surfaces; (vi) Quantitative calculations show that in intrinsic bulk GaAs, the hole spin life time is around 110 femtoseconds, which is due to phonon scattering. However, in DMS, the p-d exchange interaction and the high density of impurities give rise to other spin flip scattering channels. The nonequilibrium spin life time is only a few femtoseconds. These research results should be helpful for gaining more understanding of the properties of DMS systems and should be useful in designing novel devices based on DMS.