First-principles Study Of Magnetic Properties Of Wide Band Gap Semiconductors Doped By Non-Magnetic Ions

Wide band-gap semiconductors (Eg> 2.3eV), including diamond, GaN, titanium dioxide etc., are known as the third-generation semiconductor materials. As compared with the first and second generation semiconductors, the wide band-gap semiconductors possess many superior properties, such as high breakdown electric field strength, high saturated electronic drift velocity, high thermal conductivity, small dielectric constant, strong anti-radiation ability and outstanding chemical stability, which would made them excellent candidates for the production of anti-radiation, high-frequency, high power and high-density integration of optoelectronic devices, and have a wide range of applications in the national defense industry, biological medicine, electronic circuits, energy storage and environmental protection.It is found that there is ferromagnectism in the wide band-gap semiconductors such as TiO2, ZnO doped with Mn,Fe,Co etc., and Mn-doped general semiconductors such as GaAs,InAs,Ge,Si also show ferromagnectism. Such magnetic semiconductors called Diluted magnetic semiconductors (DMS). DMS holds both semiconductor and magnetism properties concurrently. Namely, it is the best candidate of spintronics as both function of electron charge and spin is available in materials.Most of the previous experimental and theoretical studies mainly focus on magnetism of 3d transition metal doping in semiconductor. Recently years, it is found that many non-magnetic ion doped semiconductors show room temperature ferromagneticsm, which can't understand by the common magnetic theory. Based on the density functional theory, we have studied the origin of weak ferromagnetism in C-doped GaN, the effects of C on the enhanced ferromagnetism in C/Mn codpoed GaN, the mechanism of ferromagnetism occurs in Gd/Si codpoed GaN and the origin of the ferromagnetism in Al-doped 4H-SiC, respectively. The dissertation is arranged as follows:In chapter 1, we introduce the background and significance of this dissertation. The recent experimental and theoretcial progress for GaN and SiC are presented, respectively.In chapter 2, we briefly introduce the basic concept of DFT and review its recent progress. Finding good approximation for exchange-correlation functional is one of the main targets in DFT research. With the development of functional, DFT leads to more and more accurate results from the initial LDA, GGA to hybridization functional. Besides the improvement of exchange-correlation, extension of DFT to the time dependent region and combination of DFT with dynamic mean field theory (DMFT) are also active topics recently. All these progress leads DFT application to a broad range of problems.In chapter 3, based on DFT calculations, we calculate the C/Mn codoping in GaN bulk, and study the mechanism of C enhancing the ferromagnetism of Mn-doped GaN. The results sustain to the experimental studies.In chapter 4, we analyze the weak ferromagnetism in C-doped GaN crystal. Our results indicate that non-metal atom also can arouse magnetism as C-doped ZnO.In chapter 5, we study the enhanced ferromagnetism in Gd/Si codpoed GaN crystal. Like Al atom in 4H-SiC, Si atom in Gd:GaN brings no spin polarization. The Si atoms can make Gd easier doping into GaN and work as donor who brings electrons to make system ferromagnetic.In chapter 6, the geometric and electronic properties of intrinsic defects and Al doping defects in 4H-SiC are investigated in detail. Our results indicate that the Al impurity prefers Si's sites in the lattice, and brings no spin polarization. It is the substitutional Al for Si atom introduces hole and mediates spin-polarizations resulting from silicon vacancy to form ferromagnetism in 4H-SiC.In chapter 7, we summarize the work in this dissertation and preview the further studies.