We have studied the effect of the Al interdiffusion on the electronic states of AlGaSb-GaSb quantum wells. Measuring the interdiffusion lengths by means of highly depth-resolved secondary ion mass spectrometry we find that the effect of Al diffusion results in the transformation of the ternary-binary system in a ternary-ternary structure. The modified energy levels, calculated on the basis of the measured chemical profile using a Posch-Teller potential, are consistent with the blue shifts of the PL peaks measured in samples grown with increasing growth temperature. (C) 1998 Elsevier Science B.V. All rights reserved.

High quality quantum dots have been fabricated by using self-organized InP islands as stressors. The tensile strain due the islands creates local potential minima in an InGaAs/GaAs quantum well under the islands, and confines both electrons and holes into these minima. The ground state emission from the dots is redshifted by up to 105 meV from the quantum well emission due to this lateral confinement potential, and clearly resolved emission peaks are observed from the excited states. From the time-resolved photoluminescence measurements an interlevel relaxation time of 0.6 ns between the first excited state and the ground state and a radiative lifetime of 0.9 ns for the quantum dot ground state are obtained. Photoluminescence up-conversion measurements show subpicosecond onset of the dot luminescence at high excitation densities, suggesting that Coulomb scattering is responsible for the fast capture process. A large Zeeman splitting of the higher angular momentum states is observed in a magnetic field perpendicular to the sample surface.

Amorphous SiC/SiO2 distributed Bragg reflectors (DBR) deposited by ion beam sputtering at room temperature is reported on in this letter. The DBR consists of only 2.5 pairs and exhibits high peak reflectivity (84%) around ∼1.7 μm with a full width at half maximum of about 1000 nm. The measured reflectivity spectrum is well reproduced by the equivalent layer theory by using the measured refractive indices of SiC and SiO2 and including absorption losses.

We report the observation of a size-induced phase transition (from Zincblende to Wurtzite) occurring in CdSSe nanocrystallites (quantum dots) of radius R > 4 nm. The preferential Zincblende phase seems to be a peculiar property of the ultra small dots. The observed results may be interpreted qualitatively as follows. When the surface energy of the nanocrystallites with the zincblende structure is less than that in the case of wurtzite structure crystallites, then with increasing quantum dot size the relative contribution of the surface energy to the total cluster energy decreases and a following subsequent transition from Zincblende structure to the bulk Wurtzite structural modification occurs.

The occurrence of correlation enhancement at the Fermi edge is demonstrated in the luminescence spectra of modulation-doped Zn1-xCdxSe/ZnSe quantum wells. The Fermi-edge singularity (FES) is observed at low temperature and is rapidly quenched with increasing temperature. A line-shape analysis reveals that recombination involves momentum-nonconserving transitions between electrons and localized holes. The temperature increase causes thermal activation of the trapped carriers and the consequent recovery of the momentum-conserving recombination. Many-body analysis of the FES line-shape indicates a characteristic localization length of about 14 nm for the holes, which we associate with compositional fluctuations in the ternary alloy quantum wells.

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.

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.

An unprecedentedly well resolved Zeeman effect has been observed when confined carriers moving along a closed mesoscopic path experience an external magnetic field orthogonal to the orbit plane. Large Zeeman splitting of excited higher angular momentum states is observed in the magnetoluminescence spectrum of quantum dots induced by self-organized InP islands on InGaAs/GaAs. The measured effect is quantitatively reproduced by calculations including the vertical quantum well confinement and strain induced, nearly parabolic, lateral confinement, together with the magnetic interaction.

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.

The exciton-to-free-carrier transition in GaAs and In_xGa_{1-x}As V-shaped quantum wires is revealed by means of temperature-dependent magnetoluminescence experiments. The experimental results are in excellent agreement with the diamagnetic shift obtained from a solution of the full two-dimensional Schrodinger equation for electrons and holes including magnetic-field and excitonic effects. In the GaAs wires, the exciton-to-free-carrier transition is found to occur at temperature consistent with the exciton binding energies. In the In_xGa_{1-x}As wires the diamagnetic shift of the luminescence is found to be free-carrier-like, independent of temperature, due to the weakening of the exciton binding energy induced by the internal piezoelectric field.

The exciton stability in Zn1−xCdxSeZnSe multiple quantum wells has been investigated by absorption and magneto-luminescence spectroscopies in undoped and modulation-doped samples. The well width and the composition dependence of the exciton binding energy has been determined experimentally and compared with the value obtained with theoretical calculations. The absorption saturation has been investigated in modulation-doped samples as a function of the sheet carrier density. The excition stability has been correlated to the actual lasing mechanism in samples with different structure and composition. Freecarrier recombination dominates the stimulated emission of Zn1−xCdxSe quantum wells with relatively low exciton binding energy, whereas a dominant excitonic character is found in the lasing of deep and narrow quantum wells. Stimulated emission experiments in high magnetic fields performed on various samples confirm this attribution.

We report a comprehensive study of excitonic nonlinearities in Zn1-xCdxSe/ZnSe multiple quantum wells under optical and electrical excitation. The stability of the heavy-hole exciton is studied as a function of Cd concentration by using pump and probe transmission spectroscopy. The heavy-hole exciton absorption peak is investigated as a function of the intensity of the pump beam until the exciton bleaching is reached. The experimental results are fitted with a theoretical model which includes band-gap renormalization, phase space filling and density dependent lifetime broadening. The correlation of these results with stimulated emission measurements performed also in magnetic field up to 8 T, allowed us to elucidate the role of excitons and free carriers in the lasing process in Zn1-xCdxSe/ZnSe multiple quantum wells. Electric field induced effects are also studied in order to elucidate the role of quantum confined Stark effect on the optical modulation. Further insight in the nonlinear optical properties of excitons has been obtained by photoluminescence measurements under external bias.

Systematic temperature-dependent studies of optical absorption and photoluminescence in ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Cd}}_{\mathit{x}}$Se/ZnSe strained-layer multiple-quantum-well samples grown by molecular-beam epitaxy were used to evaluate the well-width dependence and the composition dependence of the major excitonic properties. Exciton binding energies calculated by means of a variational method were found in good agreement with the experimental values obtained from the analysis of the absorption line shapes. The well-width dependence of the excitonic eigenstates were well reproduced by envelope-function calculations that included the effect of pseudomorphic strain.

The localization of the wave function on the scale length of a single monolayer has been studied by magnetophotoluminescence in GaSb/AlGaSb quantum wells. The studied range of well width includes the direct-indirect transition involvingL-point conduction states and Γ-point valence states induced by quantum size effects. Separate carrier localization dominates at higher field values (B>2T), whereas the excitonic effects are important only in the low field range. Variational calculations of the excitonic transverse extension provide a quantitative description of the experimental data. The dependence of the effective reduced mass on the well width has been obtained experimentally by magnetoluminescence that is highly sensitive to the modifications of the wave function, even on the scale of a single monolayer.

Quantum wire heterostructures, such as V- and T-shaped wires, are very promising candidates for low-threshold lasing. A crucial issue is the excitonic vs. free-carrier nature of the radiative recombination. Here, we report on magnetophotoluminescence studies of GaAs and InGaAs V-shaped wires that allow to discriminate different regimes of radiative recombination.