Films of ZnO:Al were produced by weakly reactive dual-target magnetron sputtering. Optical band gaps, evaluated from spectrophotometric data, were widened in proportion to the Al doping. The widening could be quantitatively reconciled with an effective-mass model for n-doped semiconductors, provided the polar character of ZnO was accounted for.

Films of pure and Sn-doped semiconducting Inz03 were prepared by reactive e-beam evaporation. The spectral absorption coefficient was evaluated by spectrophotometry in the (2-6)-eV range. The extracted band gap increases with electron density (n,) approximately as n, for n, (10 ' cm This result is interpreted within an effective-mass model for n-doped semiconductors well above the Mott critical density. Because of the high degree of doping, the impurities are ionized and the associated electrons occupy the bottom of the conduction band in the form of an electron gas. The model accounts for a Burstein-Moss shift as well as electron-electron and electron-impurity scattering treated in the random-phase approximation. Experiments and theory were reconciled by assuming a parabolic valence band with an effective mass-0.6m. Earlier work on doped oxide semiconductors are assessed in the light of the present results.

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The spontaneous magnetization of a quantum point contact ͑QPC͒ formed between two large quantum dots by a lateral confinement of a high-mobility two-dimensional electron gas is studied for a realistic GaAs/Al x Ga 1Ϫx As heterostructure. The model of the device incorporates the contributions from a patterned gate, doping, surface states, and mirror charges. To explore the magnetic properties, the Kohn-Sham local spin-density formalism is used with exchange and correlation potentials that allows for local spin polarization. Exchange is the dominant mechanism behind local magnetization within the QPC, while the correlation part is less prominent. However, the correlation potential gives rise to an important correction in the QPC potential. Below the first conduction plateau we thus find a magnetized regime corresponding approximately to a single electron spin. Using an approximate separable saddle potential we compute the conductance and recover the so-called ϳ0.7 (2e 2 /h) conduction anomaly plus an additional anomaly at ϳ0.4 (2e 2 /h) below which the magnetization collapses.