Review of defect physics and doping control in wide-band-gap semiconductors

Wide-band-gap semiconductors are important materials for current and the post-Moore era technologies. Both basic research and technical applications of the wide-band-gap semiconductors have advanced rapidly in recent years. The wideband-gap semiconductors have already been widely applied in high temperature, radiation resistant, high frequency, high power, and high-density integrated electronics. However, the wide-band-gap semiconductors are known for their hard-todope properties. Thus, effective doping and defect control is the key to the applications of wide-band-gap semiconductors. In this paper, we review some recent work on doping properties of wide-band-gap carbide, oxide, and nitride semiconductors, which including: (1) The electrical and kinetic properties of the intrinsic defects in 4H-SiC and the underlying physical mechanism of the experimentally observed hydrogen passivation in 4H-SiC. The carbon vacancy VC is determined to be the main defect that affects the electrical properties of 4H-SiC. The configurations of the defect complex V-C+nH (n=1-4) in 4H-SiC are identified, and VC+4H is found to be able to effectively passivate all the deep defect levels, which successfully explains the origin of the experimental phenomenon of hydrogen passivation. (2) The physical properties of transition metal doping in In2O3 and the design principles of transition metal doping in TCOs. The dualdoping behavior of Mo in In2O3 depending on the doping site is discovered, and it is expected that the transition metals Zr, Hf, and Ta can be the promising n-type dopants in In2O3. (3) Strategies to achieve p-type doping in Ga2O3 by raising its valence band maximum through alloying with Bi and by properly choosing doping impurities (e.g., CuGa). Also the general approach to effectively doping wide-band-gap semiconductors with strongly correlated band edges is brought up. (4) The doping behaviors of Be and Mg in GaN, and clarifying the origin of why the acceptor level of Be is deeper than that of Mg in GaN. It is demonstrated that both the bigger size-mismatch induced structure relaxation and the covalency of the bonding between the Be and N atoms have contributed to the deeper BeGa acceptor. This has improved the understanding of nitrides doping with IIA elements, and provided theoretical guidance for more effective p-type doping in wide-band-gap nitrides. (5) A quantum mechanical modulation doping technique to achieve highly efficient p-type doping in AlGaN. A nonequilibrium doping method is proposed and implemented to modulate the valence band of AlGaN materials, the activation energy of Mg acceptor in AlGaN is reduced, and the p-type doping efficiency is greatly enhanced. (6) Studying the universal behavior and basic rules for the defect doping properties as a function of the applied strain, and designing how to achieve high efficient p-type doping in GaN through strain. It is proposed that the positive strain applied to GaN materials could reduce the ionization energy of MgGa to achieve higher performance of p-type doping. These pieces of work not only deepen our understanding on the electronic structure and doping properties in wide-band-gap semiconductors, but also play an important role in guiding and promoting the device designs and practical applications of wide-band-gap semiconductors.