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It is shown that absorption of circularly polarized infrared radiation achieved by inter-subband and intra-subband (Drude-like) transitions results in a monopolar spin orientation of free carriers. The monopolar spin polarization in zinc-blende-based quantum wells (QWs) is demonstrated by the observation of the spingalvanic and circular photogalvanic effects. It is shown that monopolar spin orientation in n-type QWs becomes possible if an admixture of valence band states to the conduction band wave function and the spin-orbit splitting of the valence band are taken into account.

Using the technique of photoluminescence imaging, self-organized patterns of high-electron density in homogeneous n-GaAs layers under homogeneous microwave irradiation are studied. The structures are shown to be analogous to current filaments in a static electric field. The symmetry of the microwave induced patterns is not constrained by the current feeding electrodes. It is, however, concluded that a feedback mechanism exists between the formation of high-conducting structures and the homogeneity of the incident microwave irradiation.

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Using a combination of the standard Hall technique and the photoluminescence imaging of galvanomagentic transport, free-electron density and mobility have been measured in the regime of filamentary current flow after the electric breakdown of n-GaAs at the temperature of liquid helium. The data show good agreement with those acquired by the geometrical magnetoresistance effect and by the optical Hall angle measurement. By comparing the mobilities obtained by independent techniques, arguments have been found indicating significant neutral impurity scattering in the post-breakdown regime. In the pre-breakdown regime variable range hopping has been concluded as the dominant transport mechanism.