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International audience; The exciton blocking layer (EBL) as an interfacial layer is extremely critical in determining the organic photo-voltaic cell (OPV) performances. Here, we studied inverted planar heterojunction solar cells PHJ-OPVs with the following configuration ITO/EBL/C60/CuPc/MoO3/Al. Upon the EBL functionality which can act as an exciton blocking layer and allows the electron collection at the cathode, we proposed the insertion of hybrid EBL con -sisted of KI/Alq(3) thin layer. The Alq(3) is known as an EBL due to its broad bandgap, whereas we found that when a thin layer of 1 nm of KI is introduced in ITO/Alq(3) interface, the KI decomposed during the thermal deposition, and only potassium interacts and diffuses in the Alq(3) layer, which effectively enhances the electrons collection at the ITO/C-60 interfaces leading to the improvement of open-circuit voltage (Voc), and device power conversion efficiency by 36% than the device using Alq(3) alone as EBL.

The impact ionization in semiconductor materials is a process that produces multiple charge carrier pairs from a single excitation. This mechanism constitutes a possible road to increase the efficiency of the p-n and p-i-n solar cells junctions. Our study considers the structure of InN/InGaN quantum dot solar cell in the calculation. In this work, we study the effect of indium concentration and temperature on the coefficient θ of the material type parameter of the impact ionization process for a p(InGaN)-n(InGaN) and p(InGaN)-i(QDs-InN)-n(InGaN) solar cell. Next, we investigate the effect of perturbation such as temperature and indium composition on conventional solar cell’s (p(InGaN)-n(InGaN)) and solar cells of the third generation with quantum dot intermediate band IBSC (p(InGaN-i(QD-InN)-n(InGaN)) by analyzing their behaviour in terms of efficiency of energy conversion at the presence of the impact ionization process. Our numerical results show that the efficiency is strongly influenced by all of these parameters. It is also demonstrated that θ decreased with the increase of indium concentration and temperature which contributes to an overall improvement of the conversion efficiency.

In this work, a numerical study using radial basis functions (RBF) is performed on a Open image in new window-i-Open image in new window junction GaAs solar cell. So, we solve the differential equations satisfied by the density of excess photogenerated minority carriers in the front and rear regions of this junction. We observe the effect of back surface recombination velocity on the minority carrier distribution and the internal quantum efficiency (IQE) in the p type and n type regions. Next, we study the effect of insertion into the i region multiple of InGaAs/GaAsP quantum wells (QWs) with ultra-thin GaAs spacers inserted between the QW and the barriers. Precisely, we focus attention on the effect of In content and the number of QWs on IQE.

The behavior of InN/InxGa1-xN spherical quantum dots solar cell is investigated, considering the internal electric field induced by the polarization of the junction. In order to determine the position of the intermediate band (IB), we present an efficient numerical technique based on difference finite method to solve the 3D time-independent Schrodinger's equation in spherical coordinates. The resultant n × n Hamiltonian matrix when considering n discrete points in spatial direction is diagonalized in order to calculate energy levels. Thus, the interband and intersubband transitions are determined, taking into consideration the effect of the internal electric field, size dots, interdot distances, and indium content on the energy levels, optical transition, photo-generated current density, open-circuit voltage and power conversion efficiency of the QD-IBSCs.

In this work we describe the physical properties of the organometallic thin films. We investigate the optical properties of the Bis-(8-hydroxyquinoline) magnesium (Mgq 2 ) thin film grown by Physical Vapor Deposition on glass substrate. Spectral properties of these films were examined using classical and time-dependent photoluminescence investigations. We found that the Mgq 2 thin films have a strong luminescence intensity while measuring in room temperature, however, this intensity decrease in the temperature range from 33 K to 300 K.

We report a theoretical investigation of the photovoltaic conversion efficiency of solar cells based on the introduction of I n x G a 1 − x N / I n N quantum dot supracrystals arrayed in the i-region of a p − i − n photodiode. The position and width of intermediate bands induced by the discrete quantized energy levels of electrons and holes originating from QDs are determined by using the Kronig-Penney model. Thus, interband and intersubband transitions are determined for different dot sizes and inter-dot distances. Taking into account the hole level and its impact on the band offset usually neglected in the same studies, all characteristic parameters of the cell such as open circuit voltage, short circuit current density and photoelectric conversion efficiency are determined as a function of the I n -concentration, mean size and inter-dot spacing of QDs. The results show that the performances of this new generation of solar cell increase considerably and can be adjusted by controlling the size, inter-dot spacing and I n -concentration.

In this paper, we have investigated the structural and optoelectronic properties of A 2 S n B r 6 (A = K, Cs, and Rb) using the density functional theory with generalized gradient approximation (GGA) for potential exchange-correlation. The modified Becke-Johnson (mBJ) potential approximation is also used for calculating the optoelectronic properties of the material. The results show that the band structure of the perovskites A 2 S n B r 6 has a semiconductor behavior with direct bandgap at Γ-Γ direction and that they have strong absorptions, and low reflections. As a result, these compounds are light harvester competent for photovoltaic applications. The spectroscopic limited maximum efficiency (SLME) was used to calculate the efficiency η of the A 2 S n B r 6 double perovskite solar flat cells.

The main problem of the photovoltaic conversion device is that low energy photons cannot excite charge carrier to the conduction band, so that the concept of the quantum dot solar cell is proposed in the intrinsic region of the p − i − n structure. In this work, numerical simulation has been proposed using the Matlab software. We simulate and measure the effects of quantum dots size and temperature on efficiency of GaAs p − i − n structure solar cells incorporating quantum dots (QDs) InAs under illumination. The results show that the efficiency of p − i − n solar cell depend strongly on size of QDs and temperature.

Capacitance-voltage (C-V) have been mainly performed on single diode, have not yet been fully implemented for measuring multijunction (MJ) solar cells because a signal composed by different contributions of each subcell which are not easy to determine. We demonstrate how, using C-V technique, the parameters of each sub-cell through DualJunction (DJ) solar cells can be determined. In this work, we propose a new approach using simple model and C-V method to characterize and evaluate the properties of the subcells in a monolithic DJ stack by extracting the important values of physical parameters, specifically the doping level (N) and the built-in voltage Vbi. To reach this goal, first we have measured the C-V characteristic of global dual-Junction GaAs/Ge solar cells. After, we have used Matlab software assuccess fully sound way to reach the behaviour of every single subcell.

We describe two methods of extracting physical parameters of a non ideal p-n diodes. These include the ideality factor, saturation current and series resistance. The proposed techniques that treat extraction parameters, using the current-voltage characteristic (I,V) forward biased. The first method is to learn three different points of the curve, with the coordinates of these points, one can generate a system of nonlinear equations expressing the electrical parameters. The resolution of these non-linear equations was performed by the numerical method of Newton-Raphson. The second is based on the least squares method. Both methods are tested using programs developed in Matlab code based on the experimental characteristic (I-V) of two different diodes in silicon. Key words: Diode characteristic, electrical parameters, w-function Lambert, equations of non-linear system, method of least squares.

International audience; Recently, the new tunable optoelectronic devices associated to the inclusion of the single dopant are in continuous emergence. Combined to other effects such as magnetic field, geometrical confinement and dielectric discontinuity, it can constitute an approach to adjusting new transitions. In this paper, we present a theoretical investigation of magnetic field, donor position and quantum confinement effects on the ground state binding energy of single dopant confined in ZnS/CdSe core/shell quantum dot. Within the framework of the effective mass approximation, the Schrödinger equation was numerically been solved by using the Ritz variational method under the finite potential barrier. The results show that the binding energy is very affected by the core/shell sizes and by the external magnetic field. It has been shown that the single dopant energy transitions can be controlled by tuning the dopant position and/or the field strength.

We report here on the preparation, X-ray diffraction with Rietveld refinement, AC susceptibility measurements and effect of heat treatments in (Y1-xSmx)SrBaCu3O6+z. Each sample was subject to two types of heat treatment: oxygen annealing [O] and argon annealing followed by oxygen annealing [AO]. For each x, the [AO] treatment increases the orthorhombicity epsiv = (b -a) / (b + a) (for 0 les x les 1), T c (for x ges 0.4) and the distance d [Cu(1) - (Sr / Ba)] (decrease T c ) for x 0.5 . When x increase from 0 to 1, epsiv decreases with T c[O]. However, T c[AO] decreases with epsiv until x = 0.2 and after it increases by 6 K to 85 K for x = 1 [AO]. A remarkable correlations were observed between Tc(x) and the volume of the unit cell V (x) ; and between deltaT c(x) = T c[AO] - T c[O] and deltaepsiv(x). A combination of several factors such as decrease in d [Cu(1) - (Sr / Ba)]; increase in cationic and chain oxygen ordering; the number p sh(x) of holes by Cu(2) - O2 superconducting plans and in-phase purity for the [AO] samples may account for the observed data.

Electron or proton irradiations introduce recombination centers, which tend to affect solar cell parameters by reducing the minority-carrier lifetime (MCLT). Because this MCLT plays a fundamental role in the performance degradation of solar cells, in this work we present a new approach that allows us to get accurate values of MCLT. The relationship between MCLT in p -region and n -region both before and after irradiation has been determined by the new method. The validity and accuracy of this approach are justified by the fact that the degradation parameters that fit the experimental data are the same for both short-circuit current and the open-circuit voltages. This method is applied to the p + / n -InGaP solar cell under 1 MeV electron irradiation.

Ferroelectric liquid crystal (FLC) materials exhibiting the cholesteric (N∗), smectic A (SmA) and chiral smectic C (SmC∗) phases have been studied by electro-optical and dielectric investigations. In the dielectric measurements, we have studied the soft mode in the SmC∗ and SmA phases. From experimental data, we have evaluated the soft mode rotational viscosity and the electroclinic coefficient in the SmA phase. All results are discussed and compared with previous studies performed on other homologues of the same series. The main result is that the relaxation process, observed in N∗ phase for the C8, C10 and C11 materials and identified as a soft-mode-like mechanism, disappears for C12.

Structural, electro-optical, and dielectric investigations of a ferroelectric liquid crystal (FLC) exhibiting the chiral smectic C phase (SmC∗) are reported. This study has been performed on the pure FLC showing the SmC∗–SmA–N∗ phase sequence and having high spontaneous polarization and a large relaxation frequency. We have determined the Goldstone rotational viscosity and the twist elastic constant in the SmC∗ phase from the experimental data of helical pitch, tilt angle, polarization, dielectric strength, and relaxation frequency of the Goldstone-mode relaxation. An Arrhenius-type behavior of the Goldstone rotational viscosity was obtained, and the corresponding activation energies were evaluated.

We report here on the preparation, AC magnetic susceptibility and resistivity measurements of LnSrBaCu3O6+z (Ln = rare earth = Eu, Sm, Nd). Each sample was subject to two types of heat treatment: oxygen annealing [O] and argon annealing followed by oxygen annealing [AO]. In the normal state, the heat treatment [AO] reduced considerably the linear resistivity parameters indicating a diminution of the interaction of carrier charges with phonons. For each ionic radius r(Ln+3), Tc increases remarkably by 10 K, 6 K and 6 K to Tc[AO] = 78 K, 85 K and 87 K, respectively for Ln = Nd, Sm and Eu. Tc(chi') and Tc(p = 0) were in good agreement. There was a remarkable improvement in the shielding effect in the case of the samples [AO] at all T < Tc and for any applied field. An enhancement of the irreversibility line was observed due to the argon treatment. A remarkable correlation was observed between Tc(r) and the number psh(r) of holes by Cu(2) - O2 superconducting planes. A combination of several factors such as the ionic size of the rare earth, increase in cationic and chain oxygen ordering; psh and in-phase purity for the [AO] samples may account for the observed data.

Self-supported thick (200–500 μm), non-intentionally doped, epitaxial GaAs layers are good candidates for X-ray imaging for the following reasons. Their electronic properties are homogeneous over large areas, they can be grown at low cost, the technology to realize pixel detectors of various size is standard, the defect concentration is low and the fluorescence yield is small. Here, we characterize the defects present in the material and evaluate the mobility-lifetime product, using Deep Level Transient Spectroscopy combined with current–voltage and charge collection measurements.

The ab-plane reflectivity of YBa2Cu3O6+x thin films was measured in the 500–25 000 cm−1 range in samples with x = 1 (Tc = 90 K), x = 0.85 (Tc = 82 K), x = 0.6 (Tc = 55 K), x = 0.45 (Tc = 35 K) and x = 0.38 (Tc = 20 K) in normal state. The effective charge carrier number obtained from the spectral weight increases with increasing x. Variation is consistent with higher dc resistivity for x

The aim of this paper is to predict the degradation induced by proton and electron irradiations on the parameters (short-circuit current, open-circuit voltage and maximum power) of solar cells versus fluence, by a direct calculation now that the characteristics of the recombination centers induced by the irradiation have been determined. The calculation is performed for any energy of the irradiating particle and for any thicknesses and doping levels in the base and emitter. This approach allows also deducing the degradation of multijunction cells. The validity of this method is illustrated for the case of GaAs cells of different origins.

Proton and electron irradiations induce the degradation of space solar cells. The rate of the degradation depends on the current regime in the junction; it is four times lower when the cell is in the diffusion current regime than in the recombination regime. The analysis of the current regime shows that the diffusion regime is predominant for voltages larger than a critical value, which reduces with large values of the surface recombination velocity, small-base doping levels and increasing concentration of recombination centres. We examine the conditions for which we can keep as minimum the degradation rate under irradiation.

We first briefly review the performances for X-ray detection which are obtained using thin epitaxial GaAs layers. We then show that good detectors can be realized on thick and large area epitaxial GaAs layers which are now available, making them suitable for X-ray imaging. We finally discuss the main limitation imposed by the epitaxial nature of this new material and ways to overcome it.

We describe a way to obtain the degradation, induced by proton and electron irradiations, of solar cell parameters (short-circuit current, open-circuit voltage and maximum power) versus fluence, directly from the calculation of the characteristics of the cell and of the irradiation-induced recombination centres. The calculation can be performed for any energy of the irradiating particle and for any specific thicknesses and doping levels of the base and emitters. The validity of this approach is illustrated in the case of GaAs cells of different origins and extended to GaInP cells. It will allow us to deduce the degradation of multijunction cells.

We demonstrate how, using electroluminescence, the parameters characterizing the recombination center induced by irradiation in a solar cell can be measured. Because electroluminescence is able to provide information on an individual cell in a multijunction (MJ) cell device, independently of the others, we apply this technique to measure these parameters in InGaP/GaAs/Ge MJ cells. We then calculate the variations of the open-circuit voltage and short-circuit current of such cells versus fluence. The results are compared with experimental data obtained for 1 MeV electron irradiations.

International audience; We study the effect of an external electric field on an exciton bound to an ionized donor (D+, X) confined in a spherical quantum dot using a perturbative-variational method where the wave function and energy are developed in series of powers of the electric field strength. After testing this new approach in the determination of the band gap for some semiconductor materials, we generalize it to the case of (D+, X) in the presence of the electric field and for several materials ZnO, PbSe, and InAs, with significant values of the mass ratio. Three interesting results can be deduced: First, we show that the present method allows to determine the ground state energy in the presence of a weak electric field in a simple way (E = E-0 - alpha f(2)) using the energy without electric field E-0 and the polarizability a. The second point is that our theoretical predictions show that the polarizability of (D+, X) varies proportionally to R-3.5 and follows an ordering alpha(D0) < alpha(X) < alpha((D+, X)). The last point to highlight is that the Haynes rule remains valid even in the presence of a weak electric field.

We have studied the current–voltage and capacitance–voltage characteristics of p/i/n structures made on non-intentionally doped epitaxial GaAs layers grown by the chemical reaction method. Deep level transient spectroscopy demonstrates that these layers contain a low defect concentration. X-ray photoconductivity shows that the diffusion length is large. The homogeneity of the properties of these layers, which has been evaluated over large area (cm 2 ), is confirmed by photoluminescence mapping.

Although the defects introduced by irradiation in Si and GaAs have been extensively studied, the centres giving rise to non-radiative recombination have been neither characterized nor identified. We propose here a method allowing one to fully characterize recombination centres, i.e. to obtain their electron and hole capture cross sections as well as their concentration. It is based on a correlation between data extracted from the current–voltage characteristics of a junction in dark, deep level transient spectroscopy and the variations of the short-circuit current and open voltage of solar cells versus the logarithm of the irradiation fluence. The method is applied to characterize and tentatively identify the recombination centres introduced by electron irradiation in Si, GaAs and InGaP.

Using Deep Level Transient Spectroscopy (DLTS), we have investigated the properties of the DX center in GaAsP for the whole range of alloy composition x. We have determined the variation of the defect characteristics (thermal ionization energy E i , barrier for electron capture B, and energy level location E DX ) versus x. From the relationship that exists between E i , B and E DX , and between B and Δ, the energy difference between the L band and the bottom of the conduction band, we deduce that electron emission and capture occur from and to the DX center via the L band in the same fashion as in GaAlAs alloys. A good fit of the variation of E i and B versus x is obtained in a model where the DX level is a donor state associated with the L band which is 200 meV deep, like in GaAlAs, as a result of intervalley mixing.

There is a whole class of deep donor defects which exhibit a metastable character, i.e. transform into a metastable state after a specific photo-excitation. Typical examples are the EL2 defect in GaAs and the DX center in several ternary III-V compounds. We shall present other defects which behave similarly, demonstrating that this metastable character is not specific of a defect but is a common behaviour of a class of defects. We shall also demonstrate that for all these defects the thermal excitation of electrons takes place in the L conduction band and that the metastable character is a direct consequence of this. This leads to a new concept of a metastable state, a result of an electronic metastability due to a band structure effect and not to a strong electron-phonon interaction.

Monolithically stacked multijunction solar cells based on III–V semiconductors materials are the state-of-art of approach for high efficiency photovoltaic energy conversion, in particular for space applications. The individual subcells of the multi-junction structure are interconnected via tunnel diodes which must be optically transparent and connect the component cells with a minimum electrical resistance. The quality of these diodes determines the output performance of the solar cell. The purpose of this work is to contribute to the investigation of the tunnel electrical resistance of such a multi-junction cell through the analysis of the current-voltage (J-V) characteristics under illumination. Our approach is based on an equivalent circuit model of a diode for each subcell. We examine the effect of tunnel resistance on the performance of a multi-junction cell using minimization of the least squares technique.

The simple and efficient method for the extraction of all the parameters of a solar cell from a single current-voltage curve under one constant illumination level based on the Lambert W function. On calculating the lsqcurvefit function with constraints, between the experimental current-voltage characteristic and a theoretical arbitrary characteristic based on Lambert W-function. It is significant to understand the effect of the light intensity and temperature on output performance of the crystalline solar cells. The effect of light intensity and temperature on performance parameters of mc-si and pc-si solar cells is discusses. The experiments have been carried out under a solar simulator for various intensity levels in the range 1-2.5 sun and 25-60°C, respectively. The experiment was carried out employing solar cell simulator with varying cell temperature at constant light intensity. The results show that cell temperature has a significant effect on the photovoltaic parameters and it controls the quality and performance of the solar cell. The maximum power and efficiency are found to be decreased with cell temperature and the temperature coefficient of the efficiency and maximum output power is found to be negative.

The defect responsible for the minority‐carrier lifetime in p‐type Czochralski silicon introduced by electron irradiation has been detected and characterized by deep‐level transient spectroscopy and spin‐dependent recombination. From the isotropic g value (2.0055), the defect is tentatively identified as a Si dangling bond originating from a vacancy cluster. Its energetic location in the gap is at 630 meV below the conduction band. The electron and hole cross sections and their variation with temperature have been determined, and found to account for the minority‐carrier lifetime of the material.

We propose the use of electroluminescence to measure irradiation-induced degradation of multijunction photovoltaic cells in order to probe the degradation of each individual junction. We illustrate the technique by studying the decrease of the luminescence of GaInP, GaAs and Ge junctions of a triple-junction solar cell produced by 1 MeV electrons for fluences corresponding to space conditions.

Electron transport across rectangular barriers, made of 200 AA thick GaAlAs layers embedded in GaAs, and triangular barriers at the (n+)GaAs-(n)GaInP interface has been studied. Current versus voltage and temperature characteristics have been analysed in order to extract the different mechanisms that induce this transport, and to determine the temperature and electric field range in which they apply. At low temperature and high field the current is driven by the Fowler-Nordheim regime. At higher temperatures the current is dominated by a defect-induced mechanism. This mechanism consists of the thermal emission of electrons into the barrier conduction band from defects lying in the barrier that can be refilled by tunnelling. The defect involved appears to be the deep state associated with the donor impurity, i.e. the DX centre. This study demonstrates that the apparent band offset depends strongly on the experimental conditions under which it is measured.

The characteristic ionization energies, barriers associated with capture, energy levels, and introduction rates of the various electron and hole traps introduced by electron irradiation in n‐type GaP are determined using deep level transient spectroscopy. The same traps are created after 4 or 300 K irradiation. Their introduction rates correspond to those expected for primary displacements. From the similarity to the case of GaAs, we conclude that the corresponding defects are intrinsic defects (isolated vacancies and vacancy interstitial pairs) associated with the P (electron traps) and Ga (hole traps) sublattices.

A sensitive deep‐level transient spectrometer operating in the range 300–600 K has been used to detect the defect responsible for the minority‐carrier lifetime in p‐type Czochralski‐grown Si. The characteristics of this defect (energy level, barrier for hole capture, hole and electron capture rates, and concentration) have been determined. This level is, as expected, located near the middle of the forbidden gap. We verified that it is indeed responsible for the lifetime by a comparison between the calculated value and the result of direct measurements. If this defect is an impurity, it could be manganese.

We have analyzed electronic transport through a single, 200‐A‐thick, Ga0.74Al0.36As barrier embedded in GaAs. At low temperatures and high electric field, the Fowler–Nordheim regime is observed, indicating that the barrier acts as insulating layers. At higher temperatures the thermionic regime provides an apparent barrier height, decreasing with the field, which is equal to the expected band offset when extrapolated to zero field. However, for some samples, the current is dominated by the presence of electron traps located in the barrier. A careful analysis of the temperature and field behavior of this current allows to deduce that the mechanism involved is field‐enhanced emission from electron traps. The defects responsible are tentatively identified as DX centers, resulting from the contamination of the barrier by donor impurities.

We have characterized by capacitance‐voltage and deep level transient spectroscopy measurements the only defect detected in Si‐doped GaInP layers. This defect exhibits an ionization energy of 0.435 eV but is located only at ∼20 meV below the bottom of the conduction band. All its characteristics, i.e., energy level, apparent capture barrier, ionization energy, can be understood if the defect is a donor associated DX center. Its cross section for electron and hole capture have been measured. The effect of an electric field on the ionization energy confirms that the defect is indeed shallow and a donor.

Using deep‐level transient spectroscopy, we have characterized the energy levels, barriers for electron capture, and introduction rates of the defects introduced by electron irradiation in liquid‐phase epitaxy grown n‐type (Te)Ga1−xAlxAs layers of various alloy composition (x=0.25, 0.40, 0.60, and 0.80). We observed five defects which present various type behaviors: energy levels linked to the valence band or to the L conduction bands, constant barriers, or varying in a manner consistent with the band structure. These results are in agreement with the understanding obtained previously on electron‐induced defects in GaAs.

Deep level transient spectroscopy (DLTS) and capacitance‐voltage measurements at various temperatures have been used to characterize defects in Si‐doped (Ga1−xAlx)1−yInyAs materials for x=0.3 and different values of y (0, 0.005, and 0.07). We only detect DX centers, those associated with the doping impurity (Si), but also others associated with Te and, eventually, Sn not introduced intentionally. When the experimental conditions are chosen to obtain exponential transients, the shape of the DLTS spectrum and its variation with the filling pulse duration can be accounted for by this contamination; i.e., no sign of the so‐called alloying effect is detected.

A series of unintentionally doped Ga1−xAlxAs epitaxial layers grown by molecular beam epitaxy, having various Al compositions (0.15, 0.18, and 0.29), have been Li diffused at 300 °C. Capacitance–voltage techniques show that the initial n‐type doping concentrations (of the order of 1016 cm−3) increase after diffusion by typically a factor 5 to 10 demonstrating that isolated Li interstitials do exist and behave as donors. However, secondary‐ion mass spectroscopy measurements show that the material contains 1019–1020 Li cm−3, which indicates that a large fraction of the Li impurities is not electrically active. Thus Li also produces defects as revealed by the fact that free electrons are frozen in the diffused layers below 77 K. Search for the existence of Li associated DX centers deep defects related to the donor impurities has been performed by deep level transient spectroscopy. Only the DX centers present before diffusion, i.e., associated with residual donor impurities, are detected in the layer of 0.29 ...

We have measured the cross sections associated with electron and hole capture on the DX center in liquid-phase-epitaxially grown, Te-doped ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As for various alloy compositions x. The results obtained demonstrate that the DX center in direct-band-gap material (i.e., up to x=0.4) is not responsible for the minority-carrier lifetime, which has been measured independently. The barriers associated with electron and hole captures have been determined from the variations of the corresponding cross sections versus temperature in the x=0.4 material. These barriers are not compatible with a capture process via multiphonon emission, which suggests that this defect is not associated with a large electron-phonon interaction.

The effect of the electric field on the emission rate of electrons from the DX centre has been studied in Te-doped, liquid phase epitaxy Ga1-xAlxAs materials of different alloy composition x. The results obtained-a decrease of the ionization energy or an increase in the emission rate-show that this emission is sensitive to the Poole-Frenkel effect. This clearly demonstrates first that the potential which binds the electron on the DX centre is not the short-range one expected if the DX centre is a deep defect, and second that the DX centre is a single donor state.

We report here carrier's magneto-transport properties and the band structure results for II-IV semiconductors. HgTe is a zero gap semiconductor when it is sandwiched between CdTe layers to yield to a small gap HgTe/CdTe superlattice which is the key of an infrared detector. Our sample, grown by MBE, had a period d (100 layers) of 18 nm (HgTe) / 4.4 nm (CdTe). Calculations of the spectra of energy E(kz) and E(kp), respectively, in the direction of growth and in the plane of the superlattice were performed in the envelope function formalism. The angular dependence of the transverse magnetoresistance follows the two-dimensional (2D) behavior with Shubnikov-de Haas oscillations. At low temperature, the sample exhibits p type conductivity with a hole mobility of 900 cm2/V.s. A reversal the sign of the weak-field Hall coefficient occurs at 25 K with an electron mobility of 3.104 cm2/Vs. In intrinsic regim, the measured Eg ≈ 38 meV agrees with calculated Eg(Γ,300 K) = 34 meV which coincide with the Fermi level energy. The formalism used here predicts that this narrow gap sample is semi metallic, quasi-two-dimensional and far-infrared detector.

We report an electron-paramagnetic-resonance (EPR) study of a group-VI donor in ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As (x=0.30). No EPR spectrum associated with the DX ground state could be detected. After photoexcitation with E\ensuremath{\ge}0.6 eV an EPR spectrum is observed, which is attributed to the ${\mathit{A}}_{1}$(1s) effective-mass state derived from the secondary ${\mathit{X}}_{1}$ conduction-band minimum. The occupation of this state results from hot-electron capture and is metastable below 40 K. The photoexcitation spectrum of this excited donor state is identical to the photoionization spectrum of the DX ground state previously determined by photocapacitance measurements. The asymmetrical line shape and the absence of the Te hyperfine doublets demonstrate modification of the isolated donor properties by donor-donor interaction.