Optical spectra in the region of exciton resonances in quantum wells and quantum dots of In0.3Ga0.7As/GaAs heterostructures

The transparency, reflection and luminescence spectra of In_0.3Ga_0.7As structures with 8 nm thickness and quantum wells limited by the barrier layer GaAs of a 9 nm (upper layer) and 100 nm (bottom layer) thickness had been studied in the region of photon energy 0.5–1.6 eV. Lines associated with the transitions hh,lh1-e1(1s,2s,3s), hh2,lh2-e2(1s,2s,3s), hh1,lh1-e2(1s) and hh3,lh3-e3(1s) had been revealed in reflection spectra. The shapes of the reflection and transparency lines had been calculated using a single oscillator model of dispersion relations and the Kramers–Kronig integrals. The binding energy of hh,lh1-e1 excitons, the effective mass m_hh^∗ and m_lh^∗ and the damping factor for the optical transitions to QW and QD had been determined. The lifetime of charge carriers on quantum dots varies in the range of 0.04–0.1 ps, while the radiative lifetime of excitons in quantum wells in the considered structure is around 2 ps.     

Direct observation of dark exciton states in single carbon nanotubes

We have studied magneto-photoluminescence (PL) spectra of a single carbon nanotube at low temperatures. A single PL peak arising from optically allowed (bright) exciton state was observed under the zero-magnetic field, and an additional PL peak from optically forbidden (dark) exciton state was enhanced with increasing the magnetic field. Excitons populate in the lower dark state at low temperatures, and the optically forbidden transition is observed due to the Aharonov-Bohm effect.     

Kinetic effects in recombination of optical excitations in disordered quantum heterostructures: Theory and experiment

An overview of recent experimental and theoretical results on stationary and time-dependent photoluminescence spectra in disordered semiconductor heterostructures is presented. In particular, temperature-dependent peak position and linewidth of the luminescence spectra, as well as the luminescence intensity are considered along with the time evolution of the luminescence intensity after pulsed excitation. Emphasis is given on the comparison between experimental and theoretical results aiming at a characterization of disorder in the underlying structures.     

The effect of the excitation and of the temperature on the photoluminescence circular polarization of AlInAs/AlGaAs quantum dots

In this paper, we present a study of photoluminescence (PL) from AlInAs/AlGaAs quantum dots (QDs) structures grown by molecular beam epitaxy. Specifically, we describe the effects of the temperature and of the excitation density on the photoluminescence circular polarization. We have found that the circular polarization degree depends on temperature. On the other hand, the study of the excitation density dependent circular polarization PL degree shows that the last increases in the case of the sample of weak dot density. However, in the case of large dot density, it is almost constant in the excitation density range from 0.116 W cm⁻² to 9 W cm⁻².     

Inter-Layer Energy Transfer through Wetting-Layer States in Bi-layer InGaAs/GaAs Quantum-Dot Structures with Thick Barriers

The inter-layer energy transfer in a bi-layer InGaAs/GaAs quantum dot structure with a thick GaAs barrier is studied using temperature-dependent photoluminescence. The abnormal enhancement of the photoluminescence of the QDs in the layer with a larger amount of coverage considering the resonant Forster energy transfer between the at 110K is observed, which can be explained by wetting layer states at elevated temperatures.     

Temperature Dependence of Photoluminescence from Single and Ensemble InAs/GaAs Quantum Dots

We investigate the temperature dependence of photoluminescence from single and ensemble InAs/GaAs quantum dots systematically. As temperature increases, the exciton emission peak for single quantum dot shows broadening and redshift. For ensemble quantum dots, however, the exciton emission peak shows narrowing and fast redshift. We use a simple steady-state rate equation model to simulate the experimental data of photoluminescence spectra. It is confirmed that carrier-phonon scattering gives the broadening of the exciton emission peak in single quantum dots while the effects of carrier thermal escape and retrapping play an important role in the narrowing and fast redshift of the exciton emission peak in ensemble quantum dots.     

The dependence of the structural and optical properties on the Te mole fraction in ZnS1−xTex/GaAs heterostructures

Lattice-mismatched ZnS_1−xTe_x epilayers with various Te mole fractions on GaAs (100) substrates were grown by double well temperature gradient vapor deposition. X-ray diffraction patterns showed that the grown ZnS_1−xTe_x layers were epitaxial films. The photoluminescence spectra showed that the peak position of the acceptor-bound exciton (A⁰, X) varied dramatically with changing the Te mole fraction and that the behavior of the (A⁰, X) peak position of the ZnS_1−xTe_x epilayers with a small amount of the Te mole fraction was attributed to a bowing effect. The reflectivity and ellipsometry spectra showed that the absorption energy peak was significantly affected due to the Stoke's effect. These results provide important information on the structural and optical properties of ZnS_1−xTe_x/GaAs heterostructures for improving optoelectronic device efficiencies operating in the spectral range between near ultraviolet and visible regions.     

Barrier-disorder induced exciton relaxation via LO-phonons in GaAs/Al x Ga1−x As multiple quantum wells

Hot exciton relaxation is observed in GaAs/Al _x Ga_1–x As multiple quantum wells. The photolumnescence excitation spectra of the localized exciton emission at low temperatures and excitation densities are composed of narrow equidistant peaks exactly separated by the GaAs LO-phonon energy (36 meV). The relaxation mechanism via LO-phonons is found to be important for localized excitons in multiple quantum wells with GaAs layer thicknesses of about 50 Å, where pronounced alloy fluctuations in the barriers provide a strong additional lateral potential which suppresses the dissociation of hot excitons.     

Electromagnetically induced transparency and slow light with n-doped GaAs

The suitability of bound exciton system in semiconductors is studied for use in nonlinear optical schemes based on EIT, such as “slow” or “stored” photons. We match the desired properties of such a system exhibiting EIT with the known physical realities of a semiconductor system, and suggest, in particular, two suitable schemes using donor impurities in GaAs. In addition to generic properties, we also focus on the influence of many neighboring levels and continuum levels, and on the effect of strong hole-mixing.     

Optimization of Metamorphic InGaAs Quantum Wells on GaAs Grown by Molecular Beam Epitaxy

We investigate the molecular beam epitaxy growth of metamorphic InxGal-xAs materials （x up to 0.5） on GaAs substrates systematically. Optimization of structure design and growth parameters is aimed at obtaining smooth surface and high optical qualdty. The optimized structures have an average surface roughness of 0.9-1.8 nm. It is also proven by PL measurements that the optical properties of high indium content （55%） InGaAs quantum wells are improved apparently by defect reduction technique and by introducing Sb as a surfactant. These provide us new ways for growing device quality metamorphic structures on GaAs substrates with long-wavelength emissions.     

Exact polarizability of low-dimensional excitons

Wannier excitons polarized by a static electric field and confined to low-dimensional structures are studied. Effective non-integer dimensions are included by considering hydrogenic states in α-dimensional space, with α as a parameter. Exact expressions for lowest order energy and wave function corrections are obtained as a function of α. For the 1s state, we demonstrate that the exciton polarizability decreases with reduced dimensionality and eventually vanishes as (α−1)⁴ as the 1D limit is approached. The low-dimensional 2s and 2p polarizabilities are also decreased but remain finite in the 1D limit.     

Recrystallization behavior of high-fluence N-implanted GaAs studied by Raman spectroscopy

Raman spectroscopy was used to study the evolution of host lattice recrystallization in high-fluence N⁺-implanted GaAs. A high-fluence of N⁺ ions (>10¹⁵ cm⁻²) was introduced into semi-insulating GaAs by the combinatorial implantation method. Subsequent thermal annealing at 800 °C was carried out to re-grow the implantation-induced amorphous layers. The dependence of Raman parameters on N contents was systematically observed for each recrystallized cell. The volume of the newly formed crystallites with original orientation decreases with increasing fluences, whereas that of crystallites of other orientations increases after high-fluence implantation and annealing. The correlation length L, representing the size of crystalline regions with preserved translational symmetry, was determined by fitting the LO phonon signal with spatial correlation model. For 10¹⁶ cm⁻² implantation, the recrystallized layer consists of nano-meter-sized crystallites (∼30 nm). The dimension of the recrystallized crystallites decreases with increasing N⁺ fluences, in good agreement with the model.     

Lateral variation of quantum-well confinement energy in triangular-shaped dot-like structures grown by molecular beam epitaxy on patterned GaAs (311)A substrates

Received: 13 Oktober 1997/Accepted: 25 January 1998     

High quality GaAs quantum wires grown by flow rate modulation epitaxy

Accepted: 14 October 1997     

Bipolariton laser emission from a GaAs microcavity

Biexciton emission properties were studied in a single GaAs quantum well semiconductor planar microcavity by photoluminescence measurements at low temperatures. At high pump intensity a bipolariton emission appears close to the lower polariton mode. This new mode appears when we detune the cavity resonance out of the lower polariton branch, showing a laser-like behavior. Very small linewidths were measured, lying below 110 μeV and 150 μeV for polariton and bipolariton emission respectively. The input/output power (I/O) measurements show that the bipolariton emission has a weaker coupling efficiency compared to previous results for polariton emission. Varying the pump laser polarization, we were able to show the selection rules for the biexciton particle creation in the quantum well. Simultaneous photoluminescence and near-field measurements show that the polariton and bipolariton emission are spectrally and spatially separated.     

Interference effects on the photoluminescence spectrum of GaN/InxGa1−xN single quantum well structures

Interference effects in the spectra of the III-nitride epilayers can produce ambiguities and misinterpretations. This problem has been addressed in the literature (e.g. [L. Siozade, J. Leymarie, P. Disseix, A. Vasson, M. Mihailovic, N. Grandjean, M. Leroux, J. Massies, Solid State Commun. 115 (2000) 575]) and a few approaches are given to produce an interference function with which the interference effects can be removed. However, the calculated interference functions may provide over or underestimated corrections because of scattering in the values of the optical parameters of the materials used in the calculation. This paper first describes a model that is developed to derive an interference function. Then, it presents the fit parameters required in the calculation of the interference function that are found experimentally. The experiments compare the corrected spectrum with an interference free spectrum, which can be obtained if the geometry of the light collection is adjusted to the Brewster angle. In this way, one avoids large estimations errors, since the validation data are derived experimentally from the same material, and under the same excitation conditions. The model has been applied to correct the low-temperature, low excitation density photoluminescence measurements of a set of five GaN/InGaN single quantum well samples with different indium concentrations. The effectiveness of this proposed technique is illustrated by the so found results.     

Anisotropy of photoconductivity and nonlinear effects in GaSe monocrystals at high optical excitation

In this work, the photoelectric properties of gallium selenide (GaSe) monocrystals in the edge absorption region, with various configurations of current contacts, at low and high optical excitation levels are investigated. The photoconductivity spectrum behavior is determined by localized electronic and excitonic states along c-axis. It is shown that the localization of electronic and excitonic states in one-dimensional fluctuation potential along c-axis results to an anisotropy in photoconductivity spectrum at various current contacts configurations. At E⊥c the photoconductivity is observed in the hν < E_g and hν > E_g regions. In the case of hv < E_g, the maximum photoconductivity, in the impurity and exciton absorption region are observed at 1.975 eV and 2.102 eV, respectively. With rising of excitation energy level, suppression of photoconductivity in the exciton absorption region and increases in impurity absorption region is observed. At E||c contact configuration, the considerable photoconductivity is observed only in the impurity absorption region, which also increases with rising of excitation level. It is supposed that, suppression of photoconductivity in the exciton absorption region at high excitation levels is connected with exciton-exciton interaction, which results to a nonlinear light absorption. The results are compared with the absorption and photoluminescence measurements.     

Abnormal Energy Dependence of Photoluminescence Decay Time in InGaN Epilayer

We employ photoluminescence (PL) and time-resolved PL to study exciton localization effect in InGaN epilayers.By measuring the exciton decay time as a function of the monitored emission energy at different temperatures,we have found unusual behaviour of the energy dependence in the PL decay process. At low temperature, the measured PL decay time increases with the emission energy. It decreases with the emission energy at 200K, and remains nearly constant at the intermediate temperature of 12OK. We have studied the dot size effect on the radiative recombination time by calculating the temperature dependence of the exciton recombination lifetime in quantum dots, and have found that the observed behaviour can be well correlated to the exciton localization in quantum dots. This suggestion is further supported by steady state PL results.