We report on the design of highly reflecting two-dimensional photonic crystal (2D-PC) mirrors, which can be integrated in a short InGaAs QWs laser source, operating around 970 nm. These mirrors consist of a triangular lattice of air holes into the GaAs slab, whose geometrical parameters have been calculated by the effective index method and 2D plane wave expansion method. A 2D finite-difference time-domain (2D-FDTD) model has also been used in order to evaluate the transmission and reflection characteristics of 2D-PC front and back mirrors of the in-plane laser. Finally, a detailed description of the nanotechnology processes necessary for the fabrication of ultra-compact in-plane laser is shown.

We present the fabrication of high refractive index contrast two-dimensional photonic crystal (2D-PC) slab waveguides. 2D photonic crystal structures, consisting of a periodic array of holes in a GaAs matrix, are obtained by nanopatterning of epitaxially grown GaAs/AlGaAs heterostructures. Two different PC structures will be described in detail: (i) an asymmetric 2D-PC slab waveguide obtained by selective oxidation which transforms the Al-rich AlGaAs cladding layer underneath the PCs into aluminium oxide and (ii) a symmetric 2D-PC slab waveguide obtained by removing the oxide (free standing structure).

We report on Coulomb correlation effects in the luminescence of strain-induced quantum dots. In single dots, under low power excitation, we observe the rising of sharp lines associated to the formation of excitonic molecules. In the grand-ensemble, in magnetic fields up to 45 T, we observe Darwin-Fock stales of the dots to merge into a unique Landau level, with a considerable reduction in the total diamagnetic shift due to the enhanced electron-hole correlation caused by the increased degeneracy of the slate.

We present an experimental and theoretical investigation of the longitudinal quantum confined Stark effect in strain-induced parabolic InGaAs quantum dots. The electric field is applied in the plane perpendicular to the growth axis (quantum well plane), through a sub-micron gap opened in a metallic contact, which contains just a few dots. The changes in the microphotoluminescence (µ-PL) spectra were measured versus the bias, at low temperature. Due to the device geometry, the electric field profile in the gate is not constant resulting in an asymmetric Stark effect as a function of the applied field polarity. Calculations of the field profile in the conatct gap and of the distorsion of the parabolic-like potential as a function of the applied field have been made in order to correlate the observed changes in the electro-optical properties of the dots with the carrier spill over and wavefunction modifications under electric field.

We investigate the effects of few-particle population of a single strain-induced quantum dot by optical excitation. The low-power photoluminescence spectra consist of sharp lines with energy separation of a few meV, associated to the formation of excitonic molecules in the single dot. With increasing photoexcitation intensity, the population of higher states is observed; however, we also observe a clear intensity dependence of the transition energies, inconsistent with a simple filling of exciton levels. Based on a theoretical model for interacting electron-hole pairs in the dot, we obtain good agreement with experiment and show that exciton-exciton interactions control the spectral changes as the number of pairs is increased.

We have investigated the photocurrent of GaN and AlGaN epilayers grown on SiC substrates. The peculiar temperature and intensity dependence of the exciton photocurrent reveal the existence of two different carrier localization mechanisms, one due to shallow point defects, which is thermally activated above 150 K and the other due to extended defects, which causes the sublinear intensity dependence of the photocurrent in the whole temperature range comprised between 10 and 300 K. Comparison with temperature dependent photoluminescence experiments suggests that Ga-vacancies can be responsible for the observed localization at point defects, whereas the high-density of native extended defects (as inferred by Rutherford back-scattering experiments) is responsible for the intensity dependence of the photocurrent. However they are localized close to the interface and do not propagate across the bulk of the layer resulting in a good quality of the sample.

We have studied the recombination mechanism of InGaAs/GaAs V-shaped quantum wire lasers under electrical injection and in high magnetic field, from well below to above lasing threshold. The emission originates from free-carrier recombination independent of temperature and injection density. No excitonic contribution is found, indicating that excitons are weakly bound in these wires due to the internal piezoelectric field causing a strong Stark effect. A quantitative analysis of the piezoelectric field is performed by measuring the screening induced blue-shift of the electroluminescence at different densities and comparing it with the piezoelectric potential calculated from the quantum wire cross-sections observed by transmission electron microscopy. (C) 1999 Elsevier Science Ltd. All rights reserved.

We have fabricated InGaAs/GaAs V-shaped quantum wire lasers operating with threshold in the 100 A cm−2 range at low temperature. The investigation of the electroluminescence and lasing under electrical injection and in high magnetic field elucidates the fundamental physical properties of the device. In contrast to recent claims, the emission is found to originate from free-carrier recombination independent of temperature and injection density. No excitonic lasing is found, indicating that excitons are weakly bound in these wires due to the internal piezoelectric field causing strong Stark effect.

Disorder and spectral broadening of vertically stacked InGaAs/GaAs V-grooved quantum wires have been investigated by means of microprobe luminescence. We show that the main spectral broadening mechanism originates from monolayer fluctuations at the bottom of the wire. A direct evidence of monolayer height islands of area 40x40 nm formed at the bottom of the grooves is provided by atomic force microscopy. Lateral and vertical wire-to-wire fluctuations are found to be negligible on the micron scale.

We have investigated the electro–optic properties of V-shaped InGaAs/GaAs quantum wires grown by MOCVD on patterned GaAs substrates. Two basic nanostructures have been fabricated: (i) single quantum wires and (ii) vertically stacked wires with different thickness of the barriers. The former exhibit a strong polarization anisotropy between the ground and the first excited states. The vertically stacked wires exhibit the formation of symmetric and antisymmetric states and a strong vertical coupling of the wavefunctions for narrow barrier widths. Based on these prototype structures we have fabricated an electro–optic modulator in wave-guide which exhibits a strong quantum confined Stark effect at room temperature, with bias as low as −2V. The multiple wire stack has been used for the fabrication of a p–i–n quantum wire light emitter with the unprecedentedly low current threshold of 0.3mA/cm2 at low temperature.

We have investigated the effect of a high magnetic field on the operation of an infrared electro-optic modulator constituted of In0.10Ga0.90As/GaAs quantum wells grown by metal-organic chemical vapor deposition. The magnetic field changes the working wavelength and the switching threshold of the device due to the diamagnetic blue-shift of the exciton ground-level energy and to the enhancement of the exciton oscillator strength, respectively. This is a consequence of the compression of the exciton wave function induced by the magnetic field.

We present the first observation of Quantum Confined Stark Effect in V-shaped InGaAs/GaAs quantum wires grown by MOCVD. P-i-n diodes with vertically staked quantum wires in the intrinsic region exhibit Stark threshold of the order of -2 V at room temperature and low temperature electroluminescence with a threshold current as low as 100 mu A under c.w. injection.

We have investigated the electro-optic properties of strained InGaAs/GaAs quantum wires of lateral width below 20 nm by means of electroluminescence and luminescence under electric bias. A novel room temperature electroluminescence is observed in MBE grown samples. The optical spectra reveal a strong red-shift and the suppression of the luminescence with increasing polarization either under reverse or direct bias. This anomalous Stark shift is due to the combination of the internal piezoelectric field and the external field.

The quantum-confined Stark effect (QCSE) in Zn1-xCdxSe/ZnSe multiple quantum wells embedded in the intrinsic region of p-i-n photodiodes has been investigated by means of photoluminescence spectroscopy under applied bias and photocurrent spectroscopy. Room-temperature excitonic bleaching is observed at reverse biases as low as 3 V. Both the redshift and the reduction of the oscillator strength of the excitonic emission have bean calculated by means of a variational model. We use the same model to clarify the main specific features of the QCSE in II-VI materials.

Strained multiple quantum wells of In(x)Gal(1-x)As/GaAs were grown by low pressure metalorganic chemical vapor deposition (LP-MOCVD) and characterized by secondary ion mass spectrometry, x-ray diffraction, and optical spectroscopy. The structural analysis demonstrates the excellent control of the interface morphology and composition achieved by MOCVD growth. Temperature dependent optical absorption, photoluminescence, and magnetotransmission were used to evaluate the well-width dependence of the major excitonic properties. The samples show sharp excitonic resonances with distinct excited states evolving into Landau-type excited states in high magnetic field. The well-width dependence of the excitonic eigenstates and of the exciton binding energy as well reproduced by envelope function and variational calculations, also in the presence of external electric field. Finally, nonlinear electro-optic modulation induced by the quantum confined Stark effect is demonstrated in a Schottky diode with extremely low switching threshold. (C) 1996 American Institute of Physics.

We report a clear evidence of bistability in the current–voltage characteristics of p-i-n heterostructures containing InGaAs V-shaped quantum wires. The observed phenomenon is explained in the framework of a single carrier transport model in which the quantum wires act like traps for the vertical current. The charge trapping phenomenon is indeed demonstrated by temperature-dependent photocurrent experiments.

We report a clear evidence of bistability in the current-voltage (I-V) characteristics of p-i-n heterostructures containing InGaAs V-shaped quantum wires. The observed phenomenon is explained in the framework of a single carrier transport model in which the quantum wires act like traps for the vertical current. The charge trapping phenomenon is indeed demonstrated by capacitance-voltage (C-V) and photocurrent experiments.

Photocurrent and absorption spectroscopy studies were performed in the 10<T<300K temperature range on p-i-n heterostructures grown by molecular beam epitaxy and incorporating Zn1-xCd(x)Se/ZnSe multiple quantum wells in the undoped region. An interpretation of photocurrent results is given.