First-Principles Studies Of Doped Semiconductors

Spintronics, as potential materials for second-generation electronics, focused on the trans-mission of charge and spin of electrons. It has more extensive application prospects than mi-croelectronics as the first-generation electronics that only studies charges of electrons. In recent years, there has been the increasing interest in the spintronic materials, especially the half-metallic ferromagnets which has hold a great promise for spintronic applications. The aim of the present work is to investigate the half-metallicity, electronic and magnetic properties of possible ternary alloy films of alkaline-earth or rare-earth doped tetrahedrally coordinated semi-conductor SiC and GaN by using density functional theory (DFT) base on first-principles, both biaxial strain and pressure condition are considered. DFT is a very useful theoretical method to study this kind of materials.At first we gived a short introduction to the development and the basic concept of DFT. Only simple H atom can be calculated by quantum mechanics at the very beginning of its foundation. Several years later, E. A. Hylleraas and D. R. Hartree calculated the ground state of Helium atom by variational and self-consistent field method, respectively; V. Fork improved the self-consistent field method forming the well-know Hartree-Fork method. This method is the prototype of many modern methods for electronic structure calculation, The DFT was founded in1960’s, it is widely used in materials simulations and computational chemistry because of its high precision and moderate computational consumption.In the third chapter, the electronic, magnetic and structural properties of rare earth Yb doped on SiC are investigated theoretically based on DFT. Both zincblence (ZB) and rocksalt (RS) structure of SiC has been calculated. We find Si3YbC4with ZB exhibits a complete half-metallic characteristic with a wide gap. However, the half-metallicity is destroyed with RS. The sensitivity of magnetic moments of Si3YbC4as a function of pressure is also discussed, there are two magnetic phase transition points with pressure increased. The exchange interaction between local Yb-4f electrons and conduction electrons play an important role in their heavy fermion characters. The exchange splitting of the conduction band is confirmed to be much larger than that of the valence band in Si3YbC4, which makes the holes-mediated ferromagnetism in this material.In the fourth chapter, the electronic and magnetic properties of zinc blende (Si,AE)C and (Ga,AE)N (AE=Mg, Ca, Sr and Ba) superlattice are investigated using first-principles calcula-tions. We focus on molecular-beam epitaxy method by delta doping structures with the alkaline earth substituting Si or Ga. Four potential materials are found to be half-metallic ferromagnets according to our calculations. The tendency to exhibit half-metallicity on the isotropic or biax-ial strain in the superlattices are investigated. Furthermore, Phase transitions from AFM to FM states are also considered. Magnetic property of GaN substrates with Ca or Sr was not much af-fected by compression and elongation, which means that they also have robust half-metallicity property. This chapter provides a general picture of the magnetic and half-metallic properties of alkaline earth-alloyed SiC and GaN thin films, which is helpful in exploring half-metallic ferromagnets in future.In the final chapter, we made a conclusion for this thesis, and made a plan for the following work.