Optical orientation in quantum wells

Snelling, Michael John (1991) Optical orientation in quantum wells. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

We have made a systematic investigation of spin relaxation and related phenomena in type l GaAs-Al_xGa_1-xAs quantum wells. The technique used was optical orientation, and measurement of the CW luminescence polarisation, in combination with time resolved measurements of luminescence decay times. This has been applied to quantum wells as a function of carrier type and concentration, magnetic field strength and direction, excitation and detection energies, and temperature. It is demonstrated that for type l quantum wells, unlike bulk zinc-blende semiconductors, in degenerate samples the minority photocreated carriers always have the longer spin relaxation time and govern the luminescence polarisation. The extended hole spin relaxation time over that obtained in bulk material is characteristic of the removal of degeneracy between the light and heavy hole band at itkɢ. Non-degenerate excitonic populations have been shown to have a far faster spin relaxation due to the coupling of the electron and hole spin relaxation. This forces the carrier with the faster relaxation time to govern the luminescence polarisation, which is found to be that of the heavy hole. Such behaviour is consistent with the anticrossing of the light and heavy hole bands away from the band centre. The electron, hole and excitonic effective Lande* g-factors and the electron-hole exchange splittings, have been measured as a function of quantum well thickness. The g-factors are seen to pass through zero while the behaviour of the electronic g-factor is modelled by itk . p theory. The exchange splitting between the optically active and inactive levels is consistent with theory, and falls rapidly with decreasing confinement. The finite exchange splitting of the optically active levels demonstrates the symmetry of the quantum well is less than D_2d, which is attributed to interface effects.

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