Theory of Doping and Defects in III-V Nitrides

Doping problems in GaN and in AlGaN alloys are addressed on the basis of state-of-the-art first-principles calculations. For n-type doping we find that nitrogen vacancies are too high in energy to be incorporated during growth, but silicon and oxygen readily form donors. The properties of oxygen, including DX-center formation, support it as the main cause of unintentional n-type conductivity. For p-type doping we find that the solubility of Mg is the main factor limiting the hole concentration in GaN. We discuss the beneficial effects of hydrogen during acceptor doping. Compensation of acceptors by nitrogen vacancies may occur, becoming increasingly severe as x increases in Al_x Ga_(1-x)N alloys.Comment: 7 pages, 2 figures. Other related publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm

Phonon- and charged-impurity-assisted indirect free-carrier absorption in Ga2O3

Monoclinic β−Ga2O3 has a large band gap of 4.8 eV, and can therefore be used as a contact material that is transparent to visible and UV light. However, indirect free-carrier absorption processes, mediated by either phonons or charged impurities, will set a fundamental limit on transparency. We use first-principles calculations to accurately assess the absorption cross section and to elucidate the microscopic origins of these processes. Phonon-assisted absorption is dominated by the emission of phonons, and is therefore always possible. This indirect absorption is inversely proportional to the cube of the wavelength. The presence of charged impurities, whether intentional or unintentional, leads to additional absorption, but for realistic concentrations, phonon-assisted absorption remains the largest contribution. Direct free-carrier absorption also leads to below-gap absorption, with distinct peaks where optical transitions match energy differences to higher conduction bands. In contrast, indirect absorption uniformly reduces transparency for all sub-band-gap wavelengths