M. Potemski

Filling factor dependent Landau level broadening studied with inter- and intraband magneto-optics in GaInAs/AlInAs MDQWs

Abstract The density of electronic states of a two-dimensional electron gas in a perpendicular magnetic field is studied with both interband luminescence and cyclotron resonance experiments. Various oscillatory phenomena with a 1 B periodicity are observed, depending on the nature of scatterers which are outside the well or deliberately introduced in the middle of the two-dimensional layer.

Stark and Zeeman effects in excitons in GaAs/GaAlAs quantum wells

Abstract We have studied the effects of electric and magnetic fields in the excitonic spectrum of GaAs/GaAlAs quantum wells by means of low-temperature photoluminescence excitation spectroscopy. The electric field, perpendicular to the layers, couples different excited states of the heavy-hole exciton with the ground state of the light-hole exciton. As a result of this coupling, fine structure becomes visible in the spectra. A small magnetic field (∼0.5 Tesla) is applied to remove degeneracies of the excitons and to enhance the oscillator strength of excited exciton-states. These states are resolved with the use of circularly polarized light, which enables us to separate the Zeeman components of the excitons. We are able to assign all the peaks appearing in the complicated excitonic fine structure by comparison with calculations, which take into account valence-band mixing and electric and magnetic field effects.

Interband magneto-optics in GaAs/AlGaAs quantum wells in a parallel field

Abstract We present a theoretical and experimental study of interband magneto-optics in GaAs/AlGaAs quantum wells (QWs) in the presence of a magnetic field applied in the plane of the barriers. In this configuration both electrons and holes experience a diamagnetic shift and a spin splitting of the levels. The mixture of the heavy and light character for the hole subbands depends on the competition between the magnetic and the confinement potentials, i.e., on the ratio between the magnetic length and the well width. For thin wells confinement effects dominate over the effect of the magnetic field. In this case, the heavy-hole splitting is almost negligible. On the contrary, the light-hole splitting can be shown to depend only on the hole effective magnetic moment κ, the parameter introduced by Luttinger in analogy to the effective magnetic moment g ∗ of electrons. Therefore a measure of such splitting yields directly a determination of this parameter which is not easy to obtain from measurements of the bulk or of quantum wells in a perpendicular field where the observable quantities are mainly determined by the hole band parameters γ1, γ2, γ3. From a comparison between theory and experiments we obtain a value of κ = 0.7 for GaAs which is smaller than the value commonly assumed in the literature.

Magneto-optical properties of self-organized strained InGaAs quantum disks

Abstract We have measured the optical properties of self-organized strained InGaAs quantum disks in high magnetic fields. The disks are formed during spontaneous reorganization of a sequence of AlGaAs and strained InGaAs epitaxial films grown on GaAs (311)B substrates by metallorganic vapor-phase epitaxy. The magneto-luminescence properties of these InGaAs quantum disks have been studied for various disks sizes with lateral widths from 100 nm down to 20 nm. We have confirmed that when the lateral confinement exists the exciton binding energy increases even when the spatial extent of the effective potential is greater than the exciton in-plane Bohr radius. This enhancement starts being noticeable when the spatial extent of the potential is two, three times larger than the Bohr radius, where the center-of-mass quantization is negligibly small.

Temperature dependence of spin relaxation observed with high field magneto-optics in GaAs/GaAlAs quantum wells

Abstract We have investigated the temperature dependence of the spin-relaxation time of photocreated carriers in GaAs/GaAlAs quantum wells in high magnetic fields by luminescence experiments under selective optical excitation. An analysis of the ratio between the luminescence intensity of the two components of the heavy-hole exciton ground state, using rate equations, gives us a lower limit value of the spin-relaxation time. Exchange interaction between electrons and holes appears to be the dominant spin relaxation mechanism.

Magneto-optics of dense electron plasmas in modulation-doped GaInAs/AlInAs single quantum wells

Abstract Dense electron plasmas in modulation-doped GaInAs/AlInAs single quantum well structures have been studied with magnetoluminescence at 2 K. An electron-sheet-concentration ( n s )-dependent band-gap renormalization is measured in the magnetic field. The data agree well with those measured at zero magnetic field. A rigid effective-mass increase for electrons with different k -states in the conduction band is observed as the n s decreases, which is contrary to the results obtained for an electron-hole plasma in GaInAs/InP quantum well.

Determination of the Hole Effective Magnetic Moment in Quantum Wells in a Parallel Magnetic Field

We present a theoretical and experimental study of interband magnetooptics in GaAs-AlGaAs quantum wells in a magnetic field parallel to the layers. This study yields a precise and direct determination of the parameter κ, describing the hole effective magnetic moment. Experimentally, for thin wells, both transitions from heavy and light hole states experience a diamagnetic shift but only those deriving from light hole states are split by the field. The observed light hole splitting is shown to depend solely on the parameter κ as far as the well width is less than the magnetic length. From a comparison between theory and experiments, we obtain for GaAs, a value of κ = 0.7 which is substantially smaller than the value commonly assumed in the literature.

Magnetic Levels in Fibonacci Superlattices and Temperature Dependence of Spin Relaxation in Quantum Wells at High Magnetic Fields

New experimental results on magnetic levels in Fibonacci superlattices and on spin relaxation in quantum wells at high magnetic fields are presented. In the case of the Fibonacci superlattices a unique self-similarity in the shape of magneto-optical spectra is demonstrated. In the case of spin-relaxation in quantum wells, a strong temperature dependence of the rate of the relaxation of conduction band spin character of excitonic states which decreases with temperature as T−3/4 has been observed.

Spin Memory of Photocreated Carriers in Quantum Wells in High Magnetic Fields

Spin relaxation of photocreated carriers in GaAs/GaAlAs quantum wells in high magnetic fields is investigated by luminescence experiments under selective optical excitation. The complete quantization of the two-dimensional energy structure in high magnetic fields implies a simultaneous exchange of energy and momentum for spin relaxation and makes spin conserving relaxation processes, both in thermalization and recombination, much faster than spin-flip processes. This results in a new spectroscopic tool to identify magneto excitonic states with the same electronic spin orientation. We apply this technique to study the zero field splitting of the heavy hole exciton ground state often observed in high quality GaAs/GaAlAs quantum wells. We find that each peak of this doublet splits in a magnetic field into two components, and using the selective relaxation we can attribute the conduction band spin character to each of these. It turns out that the ordering of the electron spins of the Zeeman splitting of the upper component of the doublet is inverted with respect to that of the lower component. This observation strongly suggests a magnetic mechanism, a possibility would be exchange, as the origin of the zero field splitting.

Excitons in Low Dimensional Semiconductors

We present high resolution pseudo-absorption spectra of GaAs/GaAIAs quantum wells. Information on the energy spectrum of excitons is obtained from low temperature photoluminescence excitation spectroscopy. The application of an external electric field tunes the energy of the excitons, and fine structure is observed as a result of the interaction of high-angular momentum states and the ground state of the first light-hole exciton. Their oscillator strength is enhanced by means of an external magnetic field. The observed structures are assigned to excited states of excitons by comparison with calculations of excitonic mixing in the presence of external electric and magnetic fields.