M.D. Sturge

Electronic states in GaAs-AlAs short-period superlattices: energy levels and symmetry

We have made a comprehensive photoluminescence (PL) study of the low lying conduction band states of (GaAs)m/(AlAs)n type-II short-period superlattices (SL) with m, n ⩽ 4, in order to determine their energy and symmetry. The symmetry is found from the shift and splitting of the levels under uniaxial stress, and from the no-phonon oscillator strengths determined by time-resolved PL. Our samples are found to be true superlattices obeying the optical selection rules predicted by the space group symmetry, which determines whether a transition is indirect or pseudo-direct. However, selection rules depending on parity with respect to reflection in the growth plane are not obeyed. The results are compared with theoretical calculations from the literature. When strain due to lattice mismatch is taken into account, the ordering of the levels is found to agree with the most recent calculations, except for the case m = n = 1. In this SL the lowest conduction band state is found to derive from the bulk X valley, rather than from the L valley as predicted. We confirm that this discrepancy can be resolved if there is an ordered interchange of a certain fraction of the Ga and Al atoms, and our data support the theoretical prediction that such an ordered intermixed SL may in fact be more stable than either the perfect SL or the random alloy.

Resonant Raman scattering and exciton localization in GaP:N and GaAsxP1−x:N

Abstract We present a study of the luminescence and resonant Raman scattering (RRS) by optical phonons which are due to excitons bound to isoelectronic nitrogen impurities in GaP and in GaAsxP1-x. The exciton bands are inhomogeneously broadened. In GaP the broadening results from nitrogen centers which are perturbed by other distant impurities (V-band). In GaAsxP1-x, the N-bound exciton band reflects the random potential fluctuations in the alloy. Under resonant excitation at liquid He temperatures, strong Raman lines are observed involving the LOΓ, TOΓ phonons and a nitrogen-induced forbidden scattering by the zone-edge LOx phonon. The LOx RRS spectrum is shown to be determined mainly by the density of terminal states, namely, those N-bound excitons which do not tunnel to other sites within the A-B thermalization time. In GaP, this RRS shows a resonance which is strong in the V-band but is essentially missing at the peak of the A-line. This is interpreted as an indication of a fast dephasing rate within the A-line due to resonant transfer, as compared to slow tunneling between the perturbed centers.

Hidden symmetry and the magnetically induced “Mott transition” in quantum wells containing an electron gas

Abstract The magnetoluminescence spectra of symmetric quantum wells containing an electron gas show an abrupt transition from Landau level behavior (i.e. a linear shift of energy with field) to quadratic (exciton-like) behavior as the magnetic field is increased. This so-called “Mott transition” always occurs at the field at which the electron filling factor ν e is 2, i.e. when the lowest Landau level is just filled. We show that the transition is a natural consequence of a hidden symmetry which has been shown to hold in two-dimensional systems at high fields. The mechanism driving it has nothing to do with that driving the true Mott transition, and it would be better named a “symmetry-driven transition”.

Transient photoluminescence of GaAs/AlGaAs quantum wires

Abstract We have studied the temperature dependence of exciton decay and rise times in a strain confined quantum wire and its host quantum well. The decay time in the wire increases as T 1.7 rather than the predicted T 0.5 . At high temperature, the radiative decay time in the wire is longer than in the well.

Symmetry of the lowest conduction band states in very short-period GaAsAlAs superlattices

Abstract The symmetry of the lowest conduction band minimum (CBM) in type-II (GaAs) n (AlAs) n short-period superlattices (SL) was investigated by photoluminescence (PL) decay, PL excitation (PLE) and PL under uniaxial compressive stress. The results show that for n ⩾ 4, the CBM is in the center of the SL Brillouin zone (BZ) and the wave function is mainly derived from the bulk X z state. For n ⩽ 3, the CBM is away from the center of the SL BZ, and derives mainly from the bulk X x , y states. Interface disorder plays an essential role in both the transition probabilities and the energy levels.

Diffusion of free trions in mixed-type GaAsAlAs quantum wells

Abstract We show that an emission line which depends on the simultaneous presence of excitons and 10 cm −2 electrons in a 20 nm GaAs quantum well is due to free negatively charged excitons or ‘trions’. Using time- and spatially-resolved photoluminescence, we find that trions and excitons are localized at 5 K, and with increasing bath temperature both species become mobile. The diffusivity of trions and excitons increases linearly with bath temperature, and the trion mobility is found to be about one-half the exciton mobility.