207 research outputs found
Conditions for resistivity from electron-electron scattering
Many complex oxides (including titanates, nickelates and cuprates) show a
regime in which resistivity follows a power law in temperature (). By analogy to a similar phenomenon observed in some metals at low
temperature, this has often been attributed to electron-electron (Baber)
scattering. We show that Baber scattering results in a power law only
under several crucial assumptions which may not hold for complex oxides. We
illustrate this with sodium metal () and
strontium titanate (). We conclude that an
observation of is not sufficient evidence for
electron-electron scattering.Comment: 5 pages, 4 figure
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
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