High-temperature Mott transition in wide-band-gap semiconductor quantum wells

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Using a density-functional approach, we investigate the reactivity of supported metal nanoclusters. We focus on the sequential adsorption of N-2 molecules on a Fe nanocluster supported by an MgO substrate. For an increasing number of N atoms preadsorbed on the nanocluster, we found that the binding energy of the N-2 molecule increases, and can become higher than that of its dissociation products, in marked contrast with the behavior at the respective metallic surface. We identify the electrostatic interacti

We present a theoretical model of the time-resolved excitonic photoluminescence in quantum wires. We study the role of disorder on the temperature dependence of the photoluminescence lifetime. We find that at low temperature the exciton population is not thermalized in disordered quantum wires and that the lifetime is mainly determined by the localization length of the exciton. However for large localization lengths or high enough temperatures the exciton population thermalizes within its lifetime and the t

Microcavity exciton polaritons, the fundamental optical excitations of semiconductor microcavities with quantum wells inside, have been proposed as promising candidates for observing stimulated scattering, condensation and other phenomena related to the bosonic nature of excitons. Having a light mass, quantum degeneracy of polaritons can be reached at low densities and high temperatures. But the radiative time of polaritons is very short (in the picosecond range) and usually prevents an efficient thermaliza

Using reflectivity and picosecond time-resolved photoluminescence, we have studied the intrinsic optical properties of negatively charged excitons in modulation doped CdTe quantum wells. In emission, we observe simultaneously a low energy exponential tail in the charged exciton spectral line and a linear increase of its radiative lifetime with temperature. In absorption, we find a consistent decrease of the charged exciton oscillator strength with temperature. For low electron concentrations these observati

We have studied the effect of disorder on the radiative properties of semiconductor quantum wires by time-resolved photoluminescence spectroscopy. The dependence of the radiative lifetimes is measured over a temperature range extending from 8 to 150 K. At low temperatures we find that the measured dependence does not conform to the theoretical prediction for recombination of free excitons in a quasi-one-dimensional system due to the major influence of disorder. An extension of the theory is developed in ord

Microcavity light emitting diodes (MCLEDs) present several interesting features compared to conventional LEDs such as narrow linewidth, improved directionality and high efficiency. We report here on MCLEDs with a top emitting geometry. The MCLED layers were grown using molecular beam epitaxy on GaAs substrates. They consist of a 3-period Be-doped distributed Bragg reflector (DBR) centered at 950 nm wavelength, a cavity containing three InGaAs quantum wells and a 15-periods Si-doped DBR. Different values for

Semiconductor quantum wires (QWRs) are promising structures for optoelectronics applications, since they can provide quantum confinement for charge carriers in two dimensions. The advantage that they offer over conventional quantum wells (QWs) is due to the sharper density of states characteristic of these structures, yielding narrower spectral lines and higher optical gain. However, to exhibit clear confinement characteristics, QWRs must meet stringent requirements in terms of size, uniformity and interfac

We present second-harmonic generation experiments in p-doped asymmetrically stepped quantum wells using the emission of a free-electron laser in the wavelength interval between 13 and 18 mu m. The wells are designed such that the three fewest valence-subbands are about equally spaced. We measured the angular and frequency dependence of the second-harmonic signal excited by intense laser irradiation chosen to be nearly resonant with the transitions between the three lowest valence subbands. We see evidence o

The coexistence of localized and free holes in modulation doped InGaAs/InP quantum wells, which show Fermi edge singularities in the cw photoluminescence and excitation spectra, has been determined by picosecond time-resolved luminescence spectroscopy. A long-lived peak at the tow-energy side of the emission spectrum is observed at 5 K and weak excitation. The peak, which vanishes on either increasing the temperature at similar to 30 K or increasing the excitation density, is attributed to recombination of

Using femtosecond resonant luminescence, we have measured the intersubband scattering rate of electrons in wide GaAs quantum wells at very low excitation densities. Even when the spacing be tween the first two electron subbands is smaller than the LO phonon energy, we observe that intersubband scattering is a subpicosecond process, much faster than previously measured or anticipated. Our experimental results are in perfect agreement with Monte Carlo calculations, which show that carrier-carrier interaction

The temperature and excitation intensity dependencies of the photoluminescence (PL) spectra of GaAs/Al0.3Ga0.7As superlattices (SL's) having randomly distributed well widths are studied. Our results indicate that the electronic properties of disordered SL's are similar to those of other disordered semiconductors: at low energies the electronic states are essentially localized, while at higher energies they are extended. Further, the PL spectra feature a disorder-induced fine structure, and they shift to the

This thesis deals with the monolithic integration of long wavelength (1.3-1.55 µm) p-i-n photodiodes with HEMT field effect transistors for the realization of wide bandwidth OEIC receivers for lightwave communication systems. Part of this work was carried out in collaboration with the Laboratory for Electromagnetic Fields and Microwave Electronics (IFH) of the ETH-Zürich. The p-i-n photodiode structure consisting of a n+-InP lower contact layer, a 1 µm undoped InGaAs absorption layer and a 100 nm p+-InGaAs

High-temperature Mott transition in wide-band-gap semiconductor quantum wells

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High efficiency top-emitting microcavity light-emitting diodes

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Efficient intersubband scattering via carrier-carrier interaction

Temperature and excitation intensity dependencies of the photoluminescence spectra of GaAs/(AlGa)As disordered superlattices

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