Search

An analytical approach to the calculation of the spontaneous emission of a quantum dot or other “active area” embedded into a spherical Bragg microcavity has been developed. Spontaneous emission spectra have been calculated.

Microcavity structures containing InGaAs–GaAs quantum dots (QDs) emitting at 1.3 μm at 300 K have been studied. The angle-resolved photoluminescence measurements revealed a large spectral splitting of TE and TM components, which amounts up to 12 nm for an angle of emission of 30°. This effect is tentatively explained by diffraction of the electro-magnetic field at a single anisotropically shaped QD and the collective effect of the anisotropy of the QD array.

The effect of interface potential fluctuations of a Si/SiGe multiple quantum well structure upon the low temperature exciton luminescence is studied. A possible exciton localization at such potential fluctuations, with a lateral period of 9–12 nm, is observed as a blue shift with increasing excitation power in the low temperature photoluminescence. Moreover, annealing the sample at temperatures well below the growth temperature leads to the formation of striations with a period of several micrometers along the 011 direction. To clarify whether or not a metallic decoration of misfit dislocations is needed to observe D1-line photoluminescence (PL) emission, annealing was performed in two different ways. Only in the case of annealing in direct contact with a metal, D1-line emission was observed in photoluminescence experiments.

The photoluminescence of InP quantum wires embedded in the dielectric template has been studied by site-selective pump-probe spectroscopy. The enhancement of the emission in the spectral window around the pump frequency and the long-lasting memory of pumping have been observed and explained on the basis of changes in energy states of template defects under intensive pumping. The effect of wire–template interface upon the appearance of the photoluminescence spectrum is discussed.

We have fabricated a polymeric photonic crystal by impregnating an opal with a Coumarin 6 dye-containing polymer which was subsequently polymerised inside the opal to form a dye-polymer–opal composite. A strong spectral redistribution of the luminescence properties of the composite has been found by angle resolved photoluminescence measurements. The emission spectrum shows a marked distortion compared with a dye-polymer reference sample, which changes with the rotation of the crystal due to the angular dispersion of the stop band. At the maximum of the dye-emission band a strong directionality of the photoluminescence recorded from the back surface was found.

A new class of copolymers potentially suitable for an imprint process is investigated. They are composed of methyl-methacrylate and a methacrylic acid, where the acid component increases the glass temperature and thus the thermal stability. The results obtained are compared to previous experiments with a methyl/butyl acrylate and the suitability for an imprint process is discussed. A modified processing sequence is proposed, where pressure is applied during the heating phase, resulting in superior imprint quality of, for example, periodic patterns.

Porous host materials of opal and asbestos have been infilled with InP by atmospheric pressure MOCVD. These were characterised by a variety of techniques including PL at 4 and 300 K. The PL gave clear evidence of quantum confinement with the InP bound exciton blue shifted by at least 0.2 eV. OpalInPgave the additional characteristic of controlling the stop-band of the host. These all represent a new method of producing easily contactable quantum device structures.

Similar effects are responsible for self-organization of periodically corrugated surface structures and ordered dot arrays on crystal surfaces. Strain relaxation on facet edges may result in the appearance of periodically corrugated surfaces for lattice-matched growth. Strain relaxation on facet edges and island interaction via the strained substrate act as driving forces for the formation of ordered arrays of uniform, strained lattice-mismatched islands on a crystal surface. A pseudoperiodic square lattice is manifested for the InAs-GaAs(100) system. Less ordered dots are formed on the GaAs(100) surface with a 4 monolayer GaSb deposition. New experimental methods are applied for the characterization of faceted nanoscale structures. For GaAs-AlAs multilayer structures grown on (311)A substrates, interface corrugation results in optical anisotropy of the same sign as expected from the low symmetry growth direction, making the main origin of the anisotropy unclear. Our quantitative optical reflectance and reflectance anisotropy studies show that the interface corrugation plays an important role for thin (less than 4 nm) GaAs layers. Mesa arrays from samples with InAs quantum dots grown on (100) surface are fabricated. The photoluminescence intensity is found to depend only weakly on the mesa size (1000 nm to 250 nm). The estimated electron-hole pair capture time into the InAs dot at room temperature is less than 1 ps. We also found a weak dependence of the threshold current density on the deep mesa stripe width (down to 3 μm) in the case of room temperature operated quantum dot injection lasers.

Resonant luminescence studies of InAs quantum dots (QDs) embedded in a GaAs matrix grown by molecular beam epitaxy are presented, showing marked differences for modulation-doped and undoped QDs and indicating that the doping leads to different exciton formation and carrier relaxation mechanisms. The LO-phonon assisted relaxatioin of excitons between sub-levels is identified for the modulation-doped QDs.

We have proposed and realized a new type of optoelectronic structures we refer to as excitonic waveguides which allow an ultimate shift towards the violet spectral range (460 nm at 300 K for the ZnMgSSe/GaAs materials system). The excitonic waveguiding effect is realized at the low energy side of the strong exciton absorption peak due to localized states, caused by ultrathin narrow-gap insertions. To realize the zero-phonon lasing mechanism resonant to the spectral region of resonantly enhanced refractive index one needs to break the k-selection rule preventing direct radiate exciton recombination having large in-plane k-vector dominating at high excitation densities and observation temperatures. This can be done by localizing excitons at nanoscale CdSe-rich islands formed by ultrathin CdSe insertions.

It was found that RIE using CH4/H2 is a high resolution dry etching technique which is suitable for a variety of II-VI semiconductors. The optimum conditions for etching of ZnTe are given. The quality of surfaces etched under these conditions has been examined using XPS and Raman spectroscopy, both techniques indicating little overt damage.

Atmospheric-pressure MOVPE has been used to prepare unintentionally doped Al0.6Ga0.4As/GaAs heterostructures which support a high-quality two-dimensional hole gas (2-DHG). p-type doping of the Al0.6Ga0.4As layer was achieved with the residual carbon background characteristic of the trimethylaluminium precursor. A hole concentration of 1.55 (±0.15) × 1012cm−2 has been measured by both the quantum Hall effect and Van der Pauw measurement with a mobility up to 6000cm2 V−1s−1 at 2K. Parallel conduction has been eliminated from the structure by incorporating an n-type Al0.45Ga0.55As buffer rather than a similar layer unintentionally doped p-type.

The fabrication and room temperature operation of 50 nm quantum dot light emitting diodes based on Si-Si0.7Ge0.3 superlattices is described. The diodes emit light efficiently at 1.3 µm at room temperature with a threshold injection current of ~0.1 pA/dot.