Leistungen
The properties of excitonic transitions in near surface III-V semiconductor quantum wells (QW), wires and dots have been investigated experimentally and theoretically in a large number of bi- and multilateral collaborations within the project. Near surface QWs, QW wires and clots were mainly fabricated at Wuerzburg University. By high resolution electron beam lithography and subsequent wet etching high quantum efficiency InGaAs/InP and InGaAs/GaAs QW wires and dots with as small a lateral size as 10 nm have been realised and transferred to the partners for the optical characterisation.
The joint work of the consortium shows that the optical properties of these systems cannot be described on the basis of a pure semiconductor picture. Such an approximation becomes too crude when the low dimensional quantum structure is located close to the surface. In different experiments and model calculations the partners of the consortium have studied surface effects on the electronic properties in semiconductor heterostructures:
i) A strong dielectric enhancement of the exciton binding energy in near surface QWs has been proven by photoluminescence excitation experiments and has been investigated in detail with the use of a magnetic field to control the exciton radius (ISSP, Chernogolovka, Wuerzburg University). The results agree well with theoretical calculations (GPI, Moscow University).
ii) Light polarisation effects originated from changes in the spatial distribution of the electro-magnetic field of the light wave at the semiconductor QW wire - vacuum interfaces, have been demonstrated for QW wire structures by the observation of strong light polarisation in difrerent optical measurements (luminescence, absorption, Raman scattering (Wuerzburg University with participation of ISSP colleagues and Madrid University)). The effect is in good agreement with calculations (GPI Moscow, ENS, Paris).
iii) A strong influence of the surface on dynamics of carrier-capture processes in QWs and QW wires and dots has been observed in time resolved optical experiments (Wuerzburg University, Rom University, ENS, Paris). In particular, in near-surface InGaAs/GaAs QWs the excitonic life time is strongly influenced by a fast trapping mechanism of carriers into surface states. This process can be controlled by surface passivation.
iv) A near-field scanning optical microscope has been used to investigate synchronously the photoluminescence with a topographic profile of near surface GaAs/AlGaAs QW wires and to study carrier diffusion in the QW layer to the side boundaries of the QW wire (Wuerzburg, Moscow and Basel groups).
The second part of the project activity was devoted to non-linear optical phenomena in low dimensional semiconductor structures. Here the participants investigated:
i) The problem of relaxation of hot carriers in QWs, interacting with strong resonance electro-magnetic field was solved, accounting for electron states and energy spectrum renormalisation by the field and many body effects. Obtained formulae describe also the energy exchange of the driving field and carriers, i.e. field absorption or amplification by the dense extremely nonequilibrium electron-hole system.
ii) The susceptibility of chaotic Hamiltonian system is studied and the dispersion relation for the susceptibility is established. The equations that connect the susceptibility and the power spectrum of coordinates and density of states are derived.
During these two years the colleges from ISSP (Chernogolovka) and CPI and MSU (Moscow) visited Wuerzburg University for joint experiments on magnetooptics of quantum wires and dots and for theoretical discussions. In March 1996, a project workshop was held at Wuerzburg where the participants of the project have discussed experimental and theoretical results of studies within the project and of some related problems.
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