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In 0.21 Ga 0.79 As multiple quantum wells MQW, with different well thickness L, are grown on [001] and [113] A GaAs growth directions by molecular beam epitaxy MBE. An asymmetric photoluminescence PL line shape denoted LE A and LE B in the lower energies side has been observed in both structures. These emissions of deep localized states can be related to the energy potential modulation associated to Indium cluster formation. Temperature dependence of photoluminescence properties has been reported. Localized state ensemble LSE model has investigated atypical behaviors of PL peak energies and the full width at half maximum FWHM of both emissions. These abnormal behaviors are explained by carriers captured by localized recombination centers. Competition processes between localized and delocalized excitons have been occurred to interpret the PL properties. The degree of localization induced by quantum-dot-like states and critical temperatures between different temperatures regions increase as far as away [001] growth direction.

Photoluminescence study PL has been performed for the In 0.21 Ga 0.79 As multiple quantum wells MQW grown by molecular beam epitaxy MBE on (001) and (113) A GaAs substrates. The electronic structure was obtained by solving the Schrodinger equation, including piezoelectric field and strain effect on the conduction and valence bands of the unequal QWs. We critically review the explanation of S-shape in temperature dependence of PL peak energy for polar Middle In 0.21 Ga 0.79 As QW at intermediate temperatures. This abnormal behavior is merely linked to the impact of carrier localization and polarization-induced electric fields in optical properties. A significant blue shift of 18 meV for polar and a negligible shift for non-polar In 0.21 Ga 0.79 As/GaAs Middle QW has been observed. In order to follow the evolution of the PL peak energies for each QW in both samples versus temperature, three theoretical models (Varshni, Vina and Passler) have been reported. A comparison between theoretical and experimental data demonstrates that the Passler model is the most accurate fit despite none of the classical models can replicate the excitonic PL energy evolution at cryogenic temperature for Middle QW in the structure grown on (113).

In this paper we explore the structure parameters, hydrostatic pressure and temperature effects on Nonlinear optical rectification (NOR) in an asymmetric vertically coupled lens-shaped InAs/GaAs quantum dots. During epitaxial growth, lens-shaped quantum dots (QDs) are formed on the wetting layer (WL). Many theoretical works have neglected WL and its effect on nonlinear optical properties of QD-based systems for sake of simplicity. However, in this work the WL has been shown to be so influential in the intersubband energy and nonlinear optical rectification magnitude. Also, a detailed and comprehensive study of the nonlinear optical rectification is theoretical investigated within the framework of the compact density-matrix approach and finite difference method (FDM). It's found that nonlinear optical rectification coefficient is strongly affected not only by the WL, but also by the pressure, temperature and the coupled width between the QDs. Obtained results revealed that a red or a blue shift cane be observed. This behavior in the NOR gives a new degree of freedom in regions of interest for device applications.

In this paper, we report the theoretical calculation of the linear, third-order nonlinear and total optical absorption coefficient in a lens shaped InAs/GaAs quantum dot. The simultaneous effects of QD radius, hydrostatic pressure, temperature and applied electric field on the total optical absorption coefficient in lens-shaped InAs quantum dots are investigated under the compact density matrix formalism and the effective mass approximation. From our calculation, it is found that the sub-band energies and total optical absorption coefficient are quite sensitive to the quantum dot radius, applied hydrostatic pressure, temperature and electric field. The results show that the total optical absorption peaks position and in magnitude change as quantum dot radius, hydrostatic pressure, temperature and electric field value varies.

We report a theoretical study of the wetting layer thickness, In x Ga (1−x) As quantum-well thickness and Indium composition effects on the physical properties of InAs quantum dots (QDs) and quantum dots-in-well (QD-in-WELL) grown on GaAs high index substrates. Finite element method is used to calculate the strain, piezoelectric field distributions and the electronic structure. Coulomb interaction has been taken as a perturbation in the interband transition energy. The ground–state transition energy is influenced by the wetting layer thickness (WL) and the substrate orientation; however, it is not affected by the InGaAs quantum-well thickness. We have found that the tensile strain at the interface is the main factor responsible for the difficulty of self-assembled InAs QDs formation on GaAs(111) substrate. On the other hand, the stability of the relaxed strain into QD–IN–WELL depended on the Indium composition and the quantum-well thickness as well as the orientation substrate. The appropriate Indium composition in the InGaAs quantum-well is found to be 0.3 for the QD–IN–WELL grown on GaAs(111) and 0.2 for the QD–IN–WELL grown on GaAs(119). This work can be helpful to controlling the wavelength of QD–IN–WELL grown on high index substrates by changing the In composition or the quantum well-thickness.

In this work, we have investigated the optical study of In0.21Ga0.79As multiple quantum wells (MQW) with different well thickness (LW = 30, 60 and 90 A), grown by molecular beam epitaxy MBE on (0 0 1) and (1 1 3) A GaAs substrates. A blue-shift of energy transition is observed for the MQW grown on (1 1 3) A. This result could be explained by the presence of significant piezoelectric field inside the structure. Yet, the exciton dynamics properties have been measured by picosecond time-resolved photoluminescence (TRPL) measurements. We have found that the piezoelectric effect in MQW grown on high index plane induces slower exciton relaxation compared with those grown on (0 0 1). The PL lifetime versus temperature was performed to study the carrier transfer between the quantum wells of both samples.

Simulation of quantum dots (QD) is usually performed on the basis of abrupt changes between neighboring materials. In practice, it is not possible to construct such QD because in a real structure the interface between two adjacent materials is not a step. In the work discussed in this paper, vertically coupled InAs/GaAs quantum dots (VCQD) with a non-abrupt change between two neighboring materials are considered. A potential function in the form of a Gaussian distribution was used to show this effect. We also focused on studying the effect of structure, applied electric (F) and magnetic (B) fields, pressure (P), and temperature (T) on second-harmonic generation (SHG). The analytical expression for SHG was investigated theoretically by use of the density matrix approach, the effective mass, and the finite-difference method (FDM). It was found that the major resonant peak value of SHG is a non-monotonic function of the barrier width (L B). Moreover, the major resonant peak of SHG is blue-shifted (red-shifted) and its magnitude increases (decreases) monotonically with increasing temperature (pressure). The results obtained also show that the magnitude and position of the resonant peaks of SHG are affected by changes in external conditions, for example applied electric and magnetic fields, structural dimensions of the coupled QD system, and relaxation time (T 0). Calculations also show that SHG in a VCQD structure with a non-abrupt potential change can be controlled and optimized by appropriate choice of structural dimensions and the external conditions mentioned above.

We investigate with photoluminescence (PL) measurements the optoelectronic properties of self-organized InAs quantum dots (QDs) grown on nominal InP(0 0 1) substrate. InAs/InP(0 0 1) QDs are grown by Molecular Beam Epitaxy (MBE) method with optimized conditions in Stranski-Krastanov regime. A lateral coupling behavior was shown by photoluminescence spectroscopy. This phenomena is considered as a degradation source of the optoelectronic properties of InAs/InP(0 0 1) QDs used in lasers applications. In order to overcome this disadvantage behavior, we have studied the optical properties of InAs quantum islands (QIs) grown on vicinal InP(0 0 1) with 2° off miscut angle toward the [1 1 0] direction. From Polarized Photoluminescence (PPL) measurements, we have deduced that InAs quantum nanostructures have quantum dashes (QDas) form elongated in [1] , [2] , [3] , [4] , [5] , [6] , [7] , [8] , [9] , [10] direction. From excitation density PL measurements, we have evidenced that the different observed PL peaks are attributed to the emission of InAs QDas of different size. The lateral coupling behavior is completely eliminated in the case of this sample. The temperature-dependent PL measurements show a good thermal stability and an emission wavelength at room temperature around 1.55 μm of the vicinal sample. All these properties prove that this sample possess favorable characteristics for microlasers based devices functioning at room temperature and for optical telecommunication with long range weapon. The broad emission range observed at 300 K of the vicinal sample gives the possibility to use it as an active zone in solar cells and in infrared photodectectors of high optical gain and excellent sensitivity on a wide energy range.

Effects of growth conditions on the formation of InAs quantum dots (QDs) grown on GaAs (1 1 5)A substrate were investigated by using the reflection high-energy electron diffraction (RHEED) and photoluminescence spectroscopy (PL). An anomalous evolution of wetting layer was observed when increasing the As/In flux ratio. This is attributed to a change in the surface reconstruction. PL measurements show that QDs emission was strongly affected by the InAs deposited amount. No obvious signature of PL emission QDs appears for sample with 2.2 ML InAs coverage. Furthermore, carrier tunneling from the dots to the non-radiative centers via the inclination continuum band is found to be the dominant mechanism for the InAs amount deposition up to 4.2 MLs.

We have performed theoretical calculation of the nonlinear optical rectification in a lens shape InAs/GaAs quantum dot (0D). The combined effects of hydrostatic pressure and temperature on the nonlinear optical rectification in lens-shaped InAs QDs are studied under the compact density matrix formalism and the effective mass approximation. From our calculation, it is found that the subband energies and optical rectification susceptibility are quite sensitive to the applied hydrostatic pressure and temperature. The results show that the resonant peak of the optical rectification can be red-shifted or blue-shifted and their intensity also varied by external probes such as hydrostatic pressure and temperature. In addition, the oscillator strength is strongly affected by these parameters.

We have investigated the optical properties of InAs/GaAs (1 1 3)A quantum dots grown by molecular beam epitaxy (MBE) with different growth rates by photoluminescence spectroscopy (PL) as a function of the excitation density and the sample temperature (10–300 K). Reflection high-energy electron diffraction (RHEED) is used to investigate the formation process of InAs quantum dots (QDs). A redshift of the InAs QDs PL band emission was observed when the growth rate was increased. This result was explained by the increase of the InAs quantum dot size with increasing growth rate. A significant redshift was observed when the arsenic flux was decreased. The evolution of the PL peak energy with increasing temperature has showed an S-shaped form due to the localization effects and is attributed to the efficient relaxation process of carriers in different InAs quantum dots and to the exciton transfer localized at the wetting layer.

We have investigated the optical properties of InAs/GaAs (113)A quantum dots grown by molecular beam epitaxy (MBE) capped by (In,Ga)As. Reflection high-energy electron diffraction (RHEED) is used to investigate the formation process of InAs quantum dots (QDs). A broadening of the PL emission due to size distribution of the dots, when InAs dots are capped by GaAs, was observed. A separation between large and small quantum dots, when they are encapsulated by InGaAs, was shown due to hydrostatic and biaxial strain action on large and small dots grown under specifically growth conditions. The PL polarization measurements have shown that the small dots require an elongated form, but the large dots present a quasi-isotropic behavior.

We have investigated the temperature dependence of photoluminescence (PL) peak position of InAs self-assembled quantum dots (QDs) grown on GaAs(11N)A (N = 3, 5) substrates. The interband transition energy is calculated by the resolution of the 3D Schrodinger equation for a parallelepipedic InAs QD, with a width of about 8 nm and a height around 3 nm. Experimentally, it was found that the PL spectra quenches at about 160 K. In addition, the full width at half maximum (FWHM) has an abnormal evolution with varying temperature. The latter effect maybe due to the carrier repopulation between QDs. The disorientation of the GaAs substrate and the low width of terraces which was presented in the high index surfaces have an important contribution in the PL spectra. Despite the non-realist chosen shape of QD and the simplest adopted model, theoretical and experimental results revealed a clear agreement.

Self-assembled InAs/GaAs (001) quantum dots (QDs) were grown by molecular beam epitaxy using ultra low-growth rate. A typical dot diameter of around 28 ± 2 nm and a typical height of 5 ± 1 nm are observed based on atomic force microscopy image. The photoluminescence (PL) spectra, their power and temperature dependences have been studied for ground (GS) and three excited states (1–3ES) in InAs QDs. By changing the excitation power density, we can significantly influence the distribution of excitons within the QD ensemble. The PL peak energy positions of GS and ES emissions bands depend on an excitation light power. With increasing excitation power, the GS emission energy was red-shifted, while the 1–3ES emission energies were blue-shifted. It is found that the full width at half maximum of the PL spectra has unusual relationship with increasing temperature from 9 to 300 K. The temperature dependence of QD PL spectra shown the existence of two stages of PL thermal quenching and two distinct activation energies corresponding to the temperature ranges I (9–100 K) and II (100–300 K).

Resonant quasi-level lifetime of the lowest quasi-bound state localized within the quantum well region of a InGaAs/GaAs double barrier resonant tunneling structure have been analytically calculated for three different GaAs substrate orientations such as (0 0 1), (3 1 1)A and (1 1 1)A. The calculation is based on the solution of the time-independent Schrodinger equation and takes into account the effective mass changes between the well and barrier materials. A significant dependence of the ground state energy and its associated lifetime on the effective masses of the well and barrier layers, indium concentration in the well In x Ga 1− x As material, which is associated to the barrier height, and GaAs substrate orientation has been observed. We believe that the structure grown on GaAs (1 1 1)A substrate is more useful for developing new resonant tunneling two-dimensional (2D) devices. Finally, the coupling effect between transverse and longitudinal wave vector has also been investigated. With increasing the transverse wave vector the lowest energy and its associated lifetime decrease and this for the different GaAs substrate orientations.

Reflexion high-energy electron diffraction (RHEED) and energetic balance between surface and elastic strain energies, based on continuum elastic theory, were used to investigate the dependences of the formation process and the strain on the growth temperature of InAs quantum dots (QDs). This latest was grown on (1 1 3)A GaAs substrates by using molecular beam epitaxy (MBE). The thickness of the InAs wetting layer, which preceded the formation of the InAs QDs, was significantly affected by the substrate temperatures. The magnitude of the strain of the InAs QDs, which formed at a low substrate temperature, increases when the temperature drops.

We present calculations of the two-dimensional electron density in a Si δ-doped AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (P-HEMT), at low temperature, for three different growth directions (001) and (111)A/B. The calculations are made using a self-consistent resolution of Schrodinger and Poisson equations. The presence of a strong built-in piezoelectric field in (111)A/B growth directions causes changes of the confining potential shape and the carrier distribution in the InGaAs channel. We discuss the influence of GaAs substrate orientation on the conduction-band structure and thereafter on the two-dimensional electron gas (2DEG) concentration in the channel. Our results show that the calculated 2DEG concentration in the normal P-HEMT structure grown on a (111)A GaAs substrate is significantly higher than those grown on (001) and (111)B GaAs substrates. We also note an increase of the average separation between the ionized donors and the carriers. On the other hand, the GaAs (111)B substrate orientation appears as inadequate for the type of structure (normal P-HEMT) on account of the charge-transfer reduction in the channel compared with the (001) orientation. In contrast, we demonstrate that the calculated 2DEG Si δ-doped GaAs/InGaAs/AlGaAs pseudomorphic inverted high electron mobility transistor (PI-HEMT) grown on aGaAs (111)B substrate is appreciably higher than that grown on (001) and afterwards an enhancement of the spatial separation between confined electrons in the channel and ionized dopants occurs. These effects might result in considerably improved devices of great interest regarding high electron mobility.

Monitoring the intensity of the reflected spot in a reflection high-energy electron diffraction (RHEED) image is the most important method used to control the growth of semiconductors in MBE. The accurate control of both layer thickness and alloy composition is particularly important for the growth of high quality heterostructures. Indeed, under such conditions, extremely uniform and high quality epitaxial devices become possible. RHEED intensity oscillation can be used as accurate, quick and direct measure of the growth rates and alloy compositions as well. Although analog signal could be obtained by using an optical fiber associated to a photo-detector PM tube and a plotter, this method is troublesome and limited. In some application, the availability of the intensity signal as a digital voltage is useful to realize further advanced analysis, and achieve feedback between growth dynamic and the external parameters, such as the cells temperatures and the synchronization of the shutters. In this paper we describe the advantages gained with the upgrade of our analog system into a digital package using CCD camera, frame grabber and a home made software. Its main purpose is to track RHEED intensity changes and measures the rate of oscillation. A state-of-the-art RHEED digital image analysis system gives us the necessary tools to gain insight into the thin film growth process and optimize material quality.

Electronic properties of the Si δ-doped AlGaAs/GaAs heterostructure such as the electron density and electron mobility have been studied when the Al concentration in the vicinity to the silicon (x Al (Si)) is varied. Shubnikov-de Haas and quantum Hall effect measurements at 4.2 K have been carried out to investigate the existence of a quasi-two-dimensional electron gas (2DEG) in such structures. We have studied the effects of the Al concentration x Al (Si) on the electronic properties. Results show an improvement of the electron transport properties when x Al (Si) decreases. But as far as the electron mobility is concerned, a slight difference between the theoretical and experimental values was observed. The value of interface charge density (N BI ) is necessary to explain the experimental mobility at low temperature. For N BI 6 × 10 9 cm -2 , we have obtained a good agreement between the theoretical and experimental electron mobility.

We have calculated the low-temperature electron density in a δ-doped AlGaAs/InGaAs/GaAs heterostructure in the presence of a piezoelectric field. Growth of a strained InGaAs layer on (N11) GaAs substrates causes a piezoelectric field to be built into the quantum well of a pseudomorphic high electron mobility transistor (HEMT). The presence of this field modifies the electronic properties of strained-layer heterostructures. A self-consistent analysis is made of these δ-doping systems to solve simultaneously the Schrodinger and Poisson equations taking into account exchange-correlation and strain effects. Thus, we have found the confining potential, the electron density, the subband energies, the eigenenvelope wavefunctions, and the Fermi-energy level in the quantum well. We have studied the effects of the InGaAs channel width and the indium composition on the electron density. Our results show a larger increase of the electron density when the calculated value is for a (111) GaAs substrate rather than for equivalent (001) and (311) substrates. We have also investigated the effects of the environmental Al concentration seen by the Si δ-doped on the electron density.

We have grown single, 10 and 20 InAs/GaAs quantum dots (QDs) multilayers by molecular beam epitaxy in Stranski-Krastanov growth mode with and without growth interruption. Multilayer structures of InAs QDs have been studied by photoluminescence (PL) and atomic force microscopy (AFM) techniques. Between 1 and 10 layers of QDs, 10 K PL shows a shift energy, and a PL linewidth reduction. Moreover, AFM image of the 10 layers sample shows that the InAs QDs size remains constant and almost uniform when the growth is without interruption. These effects are attributed to electronic coupling between QDs in the the columns. However, we show the possibility of extending the spectral range of luminescence due to InAs QDs up to 1.3 μm. Realisation of such a wavelength emission is related to formation of lateral associations or coupling of QDs (LAQDs or LCQDs) during InAs deposition when growth interruption (20 s) is used after each InAs QDs layer deposition. The growth interruption applied after the deposition of the InAs layer allows the formation of well-developed InAs dots (large dot size).

Coherent InAs islands separated by GaAs spacer (d) layers are shown to exhibit self-organized growth along the vertical direction. A vertically stacked layer structure is useful for controlling the size distribution of quantum dots. The thickness of the GaAs spacer has been varied to study its influence on the structural and optical properties. The structural and optical properties of multilayer InAs/GaAs quantum dots (QDs) have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The PL full width at half maximum (FWHM), reflecting the size distribution of the QDs, was found to reach a minimum for an inter-dots GaAs spacer layer thickness of 30 monolayers (ML). For the optimized structure, the TEM image shows that multilayer QDs align vertically in stacks with no observation of apparent structural defects. Furthermore, AFM images showed an improvement of the size uniformity of the QDs in the last layer of QDs with respect to the first one. The effect of growth interruption on the optical properties of the optimized sample (E30) was investigated by PL. The observed red shift is attributed to the evolution of the InAs islands during the growth interruption. We show the possibility of increasing the size of the QDs approaching the strategically important 1.3 μm wavelength range (at room temperature) with growth interruption after InAs QD deposition.

A self-consistent theoretical analysis, using both Schrodinger and Poisson equations, is made to investigate and propose novel Hall devices based on AlGaAs/GaAs heterostructures. The novel heterostructures are designed and optimized with respect to the measurements of low magnetic field du to their high sensitivity. In this study, we attempt to show that the electron mobility of the studied heterostructure may be enhanced without loss in interface electron concentration by both increasing the spacer thickness and by inserting a δ-doping in a narrow quantum well within the AlGaAs barrier where the Al concentration in the well is less than in the barrier. Therefore, we predict a significant enhancement of device sensitivity to low magnetic field without compromise in noise performance.

A self-consistent theoretical analysis, using both Schrodinger and Poisson equations, is made to investigate and propose novel Hall devices based on AlGaAs/GaAs heterostructures. The novel heterostructures are designed and optimized with respect to the measurements of low magnetic field du to their high sensitivity. In this study we attempt to show that the electron mobility of the studied heterostructure may be enhanced without loss in interface electron concentration by both increasing the spacer thickness and by inserting a δ-doping in a narrow quantum well within the AlGaAs barrier where the Al concentration in the well is less than in the barrier. Therefore, we predict a significant enhancement of device sensitivity to low magnetic field without compromise in noise performance. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

68.37.Lp, 78.55.Cr, 78.67.Hc, 78.67.Pt, 79.40.+z InAs/GaAs vertically stacked self-assembled quantum dot (QD) structures with different GaAs spacer layer thicknesses are grown by solid source molecular beam epitaxy (SSMBE) and investigated by trans-mission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. An increase in the polari-zation anisotropy is observed when the spacer layer thickness decreases. For a 10 monolayer (ML) thick inter-dots GaAs spacer, the TEM image shows an increase in the QD size when moving to the upper layer accompanied by the generation of dislocations. Consequently, the corresponding temperature-dependant PL properties are found to exhibit an unusual behaviour. The main PL peak is quenched at a temperature around 190 K giving rise to a broad background correlated with the formation of a miniband in the growth direction due to the strong interlayer coupling. For a thicker GaAs spacer layer (30 ML), multilayer QDs align vertically in stacks with no apparent structural defects. Over the whole temperature range, the exci-tonic band energies are governed by the Varshni empirical relation using InAs bulk parameters and the PL line width shows a slight monotonic increase. For a thinner GaAs interlayer, the thermal activation ener-gies of the carrier emission out of the quantum dots are found to be considerably small (about 25 meV) due to the existence of defects. By combining these structural and optical results, we can conclude that a thinner GaAs spacer has a poorer quality.

Low aluminum composition x, in a series of Al x Ga 1-x As samples, has been investigated by the multi-mode elastic peak electron spectroscopy (M-M EPES). This spectroscopy is a non-destructive method, and is very sensitive to the surface sample. Indeed, all electrons are elastically reflected from the first overlayers of the substrate (J. Surf. Anal. 5 (1) (1999) 90). Measurements of the percentage of the reflected current are depending on different parameters such as the atomic number of the substrate, the primary electron beam energy (Surf. Interface Anal. 30 (2000) 341). The EPES results must be associated to a Monte Carlo program for efficient sample interpretation. The goal of this paper is to report that the Monte Carlo model, based on a realistic description of the GaAs and AlAs materials and their surface regions allowed us to explain the EPES measurements in ternary Al x Ga 1-x As alloys when the aluminum composition is changed.

The electronic subband of δ-doped AlGaAs/GaAs heterostructure has been studied theoretically by the finite differential method. We use an efficient self-consistent analysis to solve simultaneously the Schrodinger and Poisson equations. The results show the possibility to increase the electron density in the channel by the introduction of the silicon δ doping in a quantum well, where the Al concentration is smaller than in the barrier. The effect of the quantum well width is studied on the electron density. To test the validity of our calculation, we have grown, by molecular beam epitaxy, a series of δ-doped AlGaAs/GaAs heterojunctions having various alloy compositions seen by the silicon. If we consider the spreading of the silicon in space during the δ-layer growth, we show that the theoretical model explains well the experimental Hall data for all aluminum concentrations.

With the help of photoluminescence Spectroscopy (PLS), we have investigated the optoelectronic properties of two different families of InAs quantum dashes (QDashes) grown on misoriented InP(001) substrate with 2[Formula: see text]off miscut angle toward the [110] direction (2[Formula: see text]F type). The lowest full width at half maximum (FWHM) of the PL spectrum measured at 12 K indicates the good self organization of InAs QDashes. The weak ratio of the integrated PL measured in 12–300 K temperature range denotes the good spatial confinement of the photogenerated carriers in InAs QDashes. The fast redshift of the PL peaks energy and the anomalous decrease of the FWHM with the increase of the temperature are attributed to an efficient thermal relaxation process of photogenerated carriers in the vicinal sample. This result is highlighted with the help of theoretical modeling of the PL peak energy as a function of the temperature, using three models (Varshni, “Vina, Logothetidis and Cardona” and Pässler). From experimental and theoretical results, we have evidenced the contribution of longitudinal acoustic-phonons (LA-phonons) in the PL of InAs/InP QDashes, via the deformation potential, especially in high temperatures range. We have attributed this behavior to the strained InAs/InP QDashes and/or to the topography of the vicinal InP(001) substrate which favors the presence of stepped phonons polarized along the steps. These vibrational modes can further interact with the excitons at high temperatures. The measured thermal activation energies of each family of InAs QDashes demonstrate that the InAs wetting layer act as a barrier for the thermoionic emission of photogenerated carriers. This result confirms the good spatial confinement of excitons in this sample.

Temperature dependence of the effective band gap (BG) energy of strained InxGa1−xAs/GaAs single-quantum well and multi-quantum well structures grown by solid source MBE at varied substrate temperature is investigated by photoluminescence spectroscopy between 10K and room temperature. For low-temperature-grown heterostructure, the temperature-induced BG shrinkage exhibits a good correlation with that of unstrained material. However, no consensus is shown to occur for a relatively high-temperature-grown quantum wells (QWs). This discrepancy is interpreted in terms of indium segregation and reevaporation during epitaxy. The low-temperature range, where the well-known Varshni law fails to fit PL peak positions, is found to decrease with increasing QW width and is attributed to the interface-roughness-induced exciton localization. This study was propped by numerical solving of Schrodinger equation taking into account strain, indium segregation and desorption effects.

We present a study of the optical properties of InAs self-assembled nanostructures grown by molecular beam epitaxy on GaAs(11N)A substrates (N = 3–5). Photoluminescence (PL) measurements revealed good optical properties of InAs quantum dots (QDs) grown on GaAs(115)A compared to those grown on GaAs(113)A and (114)A orientations substrate. An additional peak localized at 1.39 eV has been shown on PL spectra of both GaAs(114)A and (113)A samples. This peak persists even at lower power density. Supporting on the polarized photoluminescence characterization, we have attributed this additional peak to the quantum strings (QSTs) emission. A theoretical study based on the resolution of the three dimensional Schrodinger equation, using the finite element method, including strain and piezoelectric-field effect was adopted to distinguish the observed photoluminescence emission peaks. The mechanism of QDs and QSTs formation on such a high index GaAs substrates was explained in terms of piezoelectric driven atoms and the equilibrium surfaces at edges.

Self-organized InAs/GaAs quantum dots (QDs) were grown by molecular beam epitaxy. The photoluminescence, its power, and temperature dependences have been studied for the ensembles of InAs QDs embedded in GaAs matrix to investigate the interband transition energies. Theoretical calculations of confined electron (heavy-hole) energy in the InAs/GaAs QDs have been performed by means of effective mass approximation, taking into account strain effects. The shape of the InAs QDs was modeled to be a convex-plane lens. The calculated interband transition energies were compared with the results of the photoluminescence spectra. The calculated interband transition energy from the ground electronic subband to the ground heavy-hole state was in reasonable agreement with the transition energy obtained by the photoluminescence measurement.

The pseudomorphic high electron mobility transistor (P-HEMT) structure materials Al0.33Ga0.7As/In0.1Ga0.9As/GaAs have been grown by molecular beam epitaxy (MBE) on (311)A and (111)A GaAs substrates. The epitaxy of strain heterostructure on high index GaAs substrate has led to new growth phenomena, material properties and device applications. The photoluminescence (PL) spectra of the structures have been measured at low temperature. The dominant emission in the PL spectra is due to the recombination from the first electron (e1) subband to the first heavy-hole (hh1) subband (E11: e1–hh1). This feature (E11) is a relatively broad peak and has a typical asymmetric line shape. The transformation of the PL spectra in the close vicinity of the Fermi edge (EF) under different excitation densities gives strong evidence for the Fermi Edge Singularity (FES) existence. The density of the quasi-two-dimensional electron gas (2DEG) determined by PL study (nsPL), is in sufficient agreement with the values found from Hall measurements nsHall at 77 K. The results prove an increase of the electron density in sample grown on GaAs (111)A and (311)A rather than in equivalent sample grown on (001) GaAs substrate. This effect is in good agreement with our theoretical prediction, which is based on a self-consistent solution of the coupled Schrodinger and Poisson equations.

Vertically stacked multilayers of self-organized InAs/GaAs quantum dots (QDs) structures with different GaAs intermediate layer thicknesses varying between 2.8 and 17 nm are grown by solid source molecular beam epitaxy (SSMBE) and investigated by photoluminescence spectroscopy (PL). For 17 nm thick GaAs spacer, the PL spectra show two well separated features attributed to the formation of two QDs family with a bimodal size distribution indicating no correlation between the dots in different layers. In the meanwhile, the structures having thinner spacer thickness demonstrate single PL peaks showing an enhancement of high energy side asymmetrical broadening when increasing the excitation power. The corresponding emission energies exhibit a red shift when the spacer layer thickness decreases and correlated with the enhancement of the vertical electronic coupling as well as the rise of the QD's size in the upper layers induced by the build up of the strain field along the columns. The spacer thickness of 8.5 nm is found to yield the best optical properties.

Spring Conference of the E-MRS/IUMRS/ICEM, STRASBOURG, FRANCE, MAY 30-JUN 02, 2000; We have investigated the electron density of the delta -doped conventional AlGaAs/GaAs and pseudomorphic AlGaAs/InGaAs/GaAs heterojunctions. Shubnikov-de Hass (SdH) and quantum Hall effect measurements at 4.2 K have been carried out to investigate the existence of a quasi-two-dimensional gas (2DEG) in delta Si-doped AlGaAs/GaAs heterostructure. Cases have been also examined where the g-doping is placed in a narrow quantum well within the AlGaAs barrier with an Al concentration in the well (x(Al)(well)) less than in the barrier (x(Al)(Barr)). This is intended to increase the electron density. Secondary-ion mass spectroscopy (SIMS) measurements were also performed on one of these structures in order to demonstrate the spreading of the dopants. To improve the electron density, we have combined in only one structure the delta Si, in a narrow quantum well, and the InGaAs layer introduced at the heterointerface between the AlGaAs and the GaAs. A comparison between the theoretical and experimental results was performed. The variation trends of calculated electron density are in good agreement with the experimental ones. (C) 2001 Published by Elsevier Science B.V.

In this paper, we explore the effects of polarization, incidence angle, layer thickness, and permittivity profiles on reflection and transmission characteristics. Hill’s method used in this work is an efficient tool to obtain an analytical solution of the propagation equations and can be applied to all arbitrary profiles of permittivity and permeability. Using Hill’s method and an arbitrary profile of permittivity, numerical simulations for both polarizations transverse electric and transverse magnetic waves are performed to compute the reflection and transmission coefficients of spectrum analyzer, frequency selective surfaces (FSS), and electromagnetic bandgap (EBG) structures having optimized thickness.

Optical properties of InAs quantum dots (QDs) inserted in AlGaAs/GaAs modulation doped heterostructure are investigated. To study the effect of carrier transfer behavior on the luminescence of self-assembled quantum dots, a series of sample has been prepared using molecular beam epitaxy (Riber 32 system) in which we have varied the thickness separating the delta dopage and the InAs quantum dots layer. Photoluminescence spectra show the existence of two peaks that can be attributed to transition energies from the ground state (E1-HH1) and the first excited state (E2-HH2). Two antagonist effects have been observed, a blue shift of the emission energies result from electron transferred from the AlGaAs/GaAs heterojunction to the InAs quantum dots and a red shift caused by the quantum confined Stark effect due to the internal electric field existing In the AlGaAs/GaAs heterojunction.

In this paper, we have investigated the optical properties of InAs quantum dots (Qds) embedded near the channel of a delta-doped AlGaAs/GaAs high electron mobility transistor. In order to study the influence of the two-dimensional electron gas (2DEG) on the luminescence of QDs, we have prepared different structures in which we varied the thickness (d) separating the interface of AlGaAs/GaAs heterojunction from the InAs quantum dot layer. Various photoluminescence (PL) behaviors are observed when d decreases. PL spectra show the existence of two peaks which can be attributed to transition energies from the ground state (E1-HH1) and the first excited state (E2-HH2). A blueshift, a decrease in the PL intensity and an increase in the full width at half maximum of the PL peaks are observed, when the InAs QDs layer is closer to the 2DEG.

Laterally coupled quantum dot (QD) structures (artificial molecules) are investigated using one layer composed of InAs islands grown by molecular beam epitaxy. We have elaborated, at different InAs growth rates, a series of samples. Those elaborated at lower InAs growth rate present a weak and inhomogeneous QD density. The atomic force microscopy images show that the QDs are gathered by pair having a variable interdot separation. This is what enables us to study the lateral coupling. At 10 K, microphotoluminescence (micro-PL) measurements (realized on a design of 0.2 μm diameters) present either one, two, or four peaks. To understand the origin of these peaks, we have calculated the excitonic transitions of a system of two identical laterally coupled QDs according to the distance that separates them. The results of this model do not explain PL spectra. Then, we have considered a system of two QDs having slightly different sizes (asymmetric system). We have shown that when the interdot distance is ranging ...

We have investigated the consequences of a modified design of modulation doping pseudomorphic AlGaAs∕InGaAs∕GaAs and we propose a quantum well structure for a Hall device with the goal of improving its performances. From self-consistent calculations we find that the electron concentration ns in the interface region is increased. This implies that one can have a wider spacer layer and still have the same ns with the result that the mobility is improved. This result should be valuable for many types of devices. We specifically consider Hall sensors, where it is desirable to have a low electron concentration and a high mobility.