Search

A review of our recent experiments on the studies of spin-lattice relaxation in semimagnetic semiconductor quantum wells (QWs) is given. Three series of magnetic nanostructures are studied: undoped (Cd,Mn)Te/(Cd,Mg)Te QWs; n-doped (Cd,Mn)Te/(Cd,Mg)Te QWs with two-dimensional electron gas; p-doped (Cd,Mn)Te/(Cd,Mg,Zn)Te QWs with two-dimensional hole gas. In the experiments, we use a technique which combines an injection of nonequilibrium phonons and an optical detection of the induced changes via the exciton photoluminescence. We observe that doping induces a significant acceleration of the spin-lattice relaxation process, which is due to spin-flip transitions between Mn ions and free carriers. We demonstrate experimentally that it is possible to tune the value of spin-lattice relaxation time by changing the intensity of the laser excitation in the doped QWs.

The in-plane motion of excitons (X) and negatively charged excitons (X−) in a doped CdTe/CdMgTe quantum well has been measured at 4.2 K in magnetic fields up to 12 T, using spatially resolved photoluminescence (PL) spectroscopy. The populations of X and X− have been found to be in local dynamical equilibrium. The measured PL patterns in real space could therefore be described by considering free and mobile excitons in equilibrium with localised charged excitons.

Photoluminescence, photoluminescence excitation and reflectivity spectra of CdTe-based modulation-doped quantum well structures with 2DEG have been studied in magnetic fields. Resonant lines corresponding to combined optical transitions in which the photo-generation of excitons as well as trions is accompanied by the excitation of an additional electron from states below the Fermi level to states above the Fermi level were observed and analyzed.

We study the Landau level fan diagram for charged and uncharged states in n-type modulation doped quantum wells with photoluminescence (PL) and photoluminescence excitation (PLE) measurements. In magnetic fields, the adjacent peak distance is different for PL and PLE. The peak energy for PL oscillates periodically in inverse magnetic fields while the peak energy for PLE does not oscillate. Time resolved PL shows that the difference of adjacent peak distance in PL and PLE is due to the hole relaxation and breaking of the transition selection rule. Within the experimental resolution, we attribute the different oscillatory behavior of the PL and PLE peak energies to different self-energy corrections for charged and uncharged states.

We have performed magneto-optical studies on a series of thin CdTe/(CdMn)Te single quantum wells at low temperatures and in external magnetic fields up to 7.5 T. As a consequence of the high manganese content of the barrier layers ( x 20%) the antiferromagnetic exchange interaction between the manganese spins exhibit spin-glass behaviour. By comparison of the Zeeman splitting of barrier and quantum well excitons, we found a reduced antiferromagnetic coupling of Mn-spins near the heterointerface. We assume that the missing of nearest-neighbour manganese spins adjacent to the transition from magnetic to nonmagnetic materials results in a reduced probability of finding antiferromagnetically coupled Mn-Mn clusters. The mechanism of interface-related frustrated antiferromagnetism leads to the astonishing result of comparable or even larger Zeeman splittings of quantum well excitons compared with those observed in barrier layers. If we introduce a second interface for the magnetic layer as can be realized with ultrathin superlattices, missing nearest neighbours produce a more pronounced frustration of antiferromagnetic coupling due to the quasi two-dimensional character of the magnetic layer, which is correlated with a higher percolation threshold.

We have investigated the strong anisotropic polarization of the prominent Y line luminescence at 2.62 eV in thin ZnSe layers grown by molecular beam epitaxy on (0 0 1) -oriented GaAs substrates. For thin ZnSe films a pronounced anisotropic polarization collinearly to the [ 1 1 0] crystallographic direction with intensity ratios up to 1 : 8 for polarizer orientations parallel and perpendicular to [1 1 0] is found. In this samples the matrix luminescence shows no preferential orientation of the electric field vector in the plane perpendicular to the axis of growth. For ZnSe films with a layer thickness just below the critical thickness we observed a fine structure of up to nine subcomponents in the Y line spectrum. We interpret the Y line as a radiative recombination of an exciton bound to a 60° α-dislocation, which is dissociated into a 90° and 30° Shockley partial dislocation bounding a stacking fault ribbon. We relate the fine structure to a discrete set of the stacking fault distances.

Exciton luminescence of semimagnetic semiconductor quantum well structures based on Cd1−xMnxTe is used for the first time as spectrometer for acoustic phonons in the subterahertz region. The principle of such a phonon spectrometer is based on the changes of the giant Zeeman splitting of the exciton states in the presence of nonequilibrium phonons which by means of direct spin-phonon transitions raise the temperature of the spin system of Mn ions. The detection of the well known phonon spectrum of losses at a crystal/helium interface is demonstrated.

For the first time clear evidence of Shubnikov-De Haas oscillations in CdTe is reported. The investigated epitaxial films were grown by photoassisted molecular beam epitaxy using ZnCl 2 as source material for chlorine n-type doping. Degenerately doped samples with carrier concentrations of about 2 x 10 17 −10 18 cm −3 consistently show Shubnikov-De Haas oscillations at high magnetic fields and negative magnetoresistance at low magnetic fields. An estimate of the effective mass gives a value which agrees well with the one determined from optical measurements.