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The temperature dependence of electrical properties of (Au/Pd)/Al0.22 Ga0.78N/ GaN Schottky barrier diodes (SBDs) have been deeply analyzed by means of forward Igs (Vgs) measurements over the temperature range of 60-320 K. First results indicate that the barrier height (ϕb), ideality factor (n) and series resistance (Rs) have demonstrated a strongly temperature dependence which suggesting the presence of barrier inhomogeneities at the interface. The effect of spatial inhomogeneity causes fluctuations in the barrier height around an average value. The series resistances obtained from Cheung's function are increasing with increasing temperature. Werner's model consisted in the Schottky contact modeling through inhomogeneities of the barrier which induces the presence of three different average barrier heights each associated with a standard deviation σ. Such abnormal behavior is indeed due to the lateral inhomogeneity of the Schottky barrier, assuming three Gaussian distributions of the barrier height (BH) in three different temperature ranges of 220-320K, 120-200K, and 60-100K, respectively. The (Au/Pd)/ GaN Schottky barrier diode have been shown a Gaussian distribution giving mean s (ϕb) of 0.83, 0.43 and 0.20 eV and standard deviation σ of 139, 74.18 and 35.15 mV for the three temperature regions, respectively. A modified Richardson plot ln (I0/T2) − q2σ2/2k2T2 vs. 1/kT have given ϕb and A* as 0.95 eV and 10.56 A/cm2K2, 0.45 eV and 24.45A/cm2 K2, 0.20 eV and 28.18 A/cm2 K2, respectively. Moreover, the determination of the energy characteristic relating to the transmission probability by tunneling effect E00 made it possible to highlight two conduction modes: conduction by tunnel effect (FE) and a thermionic conduction assisted by field effect (TFE).

The structural, elastic, electronic and magnetic properties of cubic double perovskite Pb2FeTaO6 have been investigated using self-consistent ab-initio calculation through full-potential linearized augmented plane wave (FP-LAPW) method within the frame work of the spin-polarized density functional theory (DFT), considering generalized gradient approximation (GGA) described by Perdew–Burke–Ernzerhof (PBE), GGA+U and mBJ-GGA. Pb2FeTaO6 is found as ductile material from the elastic calculations. The obtained magnetic results show that Pb2FeTaO6 is stable in the ferromagnetic state; the electronic properties show a half-metallic behavior using GGA and GGA+U and a semiconducting one with TB-mBJ, magnetic moments of each atom are also discussed in this study. These results confirm the use of this compound in spintronic devices.

A cryogenic investigation of the Kink effect with drain-source bias sweeping process during output characteristics is suggested. An exhaustive study of the field effect dependence on the emission rate from hole traps in AlGaN/GaN HEMT transistors has been realized by means of current DLTS spectroscopy (I-DLTS). We have found that the Kink effect was induced by impact ionization of electron trapped in acceptor-like deep levels with activation energies at about 0.85 eV overhead the valence band of the GaN buffer layer. Using I-DLTS method, three holes traps, labeled A, H1, and H5, have been distinguished. The H1 deep level might correspond to the carbon substituting the N site (CN) which is supposed to be the main cause of the Kink effect. The major H5 trap seems to be gallium vacancy complex (VGa–ON). For the hole trap H1, the phonon-assisted tunneling emission is the dominant mechanism for holes to escape from the trapping centers while for the H5 trap their field dependence shows a classical pure tunneling effect.

In this work, interband and intraband optical transitions from direct bandgap strained GeSn/Ge quantum dots are numerically tuned by evaluating the confined energies for heavy holes and electrons in Г-and L-valley. The practically exploitable emission wavelength ranges for efficient use in light emission and sensing should fulfill specific criteria imposing the electrons confined states in Г-valley to be sufficiently below those in L-valley. This study shows that GeSn quantum dots offer promising opportunity towards high efficient group IV based infrared optical devices operating in the mid-IR and far-IR wavelength regions.

Eu 3 + doped phosphors with charge compensation are potential candidates of red emitting phosphors for lamp applications. Charge compensation improves the luminescence performance of the material. The charge compensation can most probably be achieved by three possible mechanisms: (a) two Zn 2 + ions are replaced by one Eu 3 + ions and one monovalent cation, 2 Zn 2 + → Eu 3 + + Li + , where Li + is acting as a charge compensator; (b) the charge compensation is provided by a zinc vacancy ( V Z n ) defects, 3 Zn 2 + → 2 Eu 3 + + V Z n , the subscript Zn denotes an ion in a normal zinc site in the lattice; (c) two Zn 2 + ions are replaced by one Eu 3 + ions with the presence of interstitial oxygen ( O i ), 2 Zn 2 + → 2 Eu 3 + + O i . Electronic structures of the crystals corresponding to the three models are evaluated by the first-principles quantum mechanical calculations based on the density functional theory. It is found that the charge compensator defects make Eu 3 + doping in ZnO energetically more favorable. They break the local symmetry around the Eu 3 + ion and lead to deep states below the empty upper band, the conduction band that could facilitate intra-4f shell transitions, which can obviously improve the emission intensity of Eu 3 + -doped ZnO. Therefore, the effect of these defects on the host crystals electronic band states relative to the Eu 3 + states is reported, since both electron transfer and electronically energy transfer processes enhance the performance of optoelectronic devices based on this material. These theoretical insights are helpful for designing rare-earth doped oxide materials with high photoluminescence (PL) performance.

The impact of vertical electrical field on the electron related linear and 3rd order nonlinear optical properties are evaluated numerically for pyramidal GeSn quantum dots with different sizes. The electric field induced electron confining potential profile&rsquo, s modification is found to alter the transition energies and the transition dipole moment, particularly for larger dot sizes. These variations strongly influence the intersubband photoabsorption coefficients and changes in the refractive index with an increasing tendency of the 3rd order nonlinear component with increasing both quantum dot (QD) size and applied electric field. The results show that intersubband optical properties of GeSn quantum dots can be successively tuned by external polarization.

The effects of self-organized GeSn/Ge quantum dot’s size and shape on the direct band gap interband emission energy, oscillator strength and radiative lifetime are evaluated. The electron’s and the hole’s confined energies and their corresponding wave function are driven by solving the 3D Schrodinger equation in Cartesian coordinates using the finite elements method. Dome and pyramidal shaped quantum dots are considered within practically exploitable size and aspect ratio that allows direct band gap transition energies. It is found that bigger quantum dots with higher aspect ratio are likely to have higher oscillator strength and longer radiative lifetime. For smaller aspect ratio, the emission energy, oscillator strength and radiative lifetime are found to be very sensitive to the quantum dot’s size and shape. These results demonstrate that this quantum dot QD system could be very interesting for CMOS compatible light emitters and detectors operating in the mid-IR range. Keywords: GeSn, Quantum dot, Direct band gap, Interband transition, Oscillator strength, Radiative lifetime

Intersubband optical transitions, refractive index changes, and absorption coefficients are numerically driven for direct bandgap strained GeSn/Ge quantum dots. The linear, third-order nonlinear and total, absorption coefficients and refractive index changes are evaluated over useful dot sizes&rsquo, range ensuring p-like &Gamma, electron energy state to be lower than s-like L-electron energy state. The results show strong dependence of the total absorption coefficient and refractive index changes on the quantum dot sizes. The third order nonlinear contribution is found to be sensitive to the incident light intensity affecting both total absorption coefficient and refractive index changes, especially for larger dot sizes.

Strain-engineered self-assembled GeSn/GeSiSn quantum dots in Ge matrix have been numerically investigated aiming to study their potentiality towards direct bandgap emission in the mid-IR range. The use of GeSiSn alloy as surrounding media for GeSn quantum dots (QD) allows adjusting the strain around the QD through the variation of Si and/or Sn composition. Accordingly, the lattice mismatch between the GeSn quantum dots and the GeSiSn surrounding layer has been tuned between − 2.3 and − 4.5% through the variation of the Sn barrier composition for different dome-shaped QD sizes. The obtained results show that the emission wavelength, fulfilling the specific QD directness criteria, can be successively tuned over a broad mid-IR range from 3 up to7 μm opening new perspectives for group IV laser sources fully integrated in Si photonic systems for sensing applications.

The environmental problems have been arisen and the importance of electronic, optical and thermoelectric applications is being well re-recognized. Orthorhombic structure of perovskite materials ARhO3 (A = Bi, Lu) have been gaining interest recently due to their unique and tunable electronic and crystal structure. First principle calculations reveal a semiconductor behavior for both materials. Band structure and density of states using TB-mBJ potential, show that our compounds having an indirect band gap for BiRhO3 and direct one for LuRhO3. Moreover, the optical spectra are analyzed by dielectric constants, refractive index and absorption of light. BoltzTraP code is executed to investigate the thermoelectric characteristic through Boltzmann transport theory. The maximum figure of merit ZT calculated at 200 K for n-type and p-type BiRhO3 are ~0.95 and ~0.016, respectively, and at 400K for n-type and p-type LuRhO3 are ~1.09 and ~0.29, respectively. Results suggest that both crystals can be promising candidates for thermoelectric devices.

First principles calculations were performed to investigate the structural, electronic, elastic, optical and thermoelectric properties of Ruddlesden-Popper (RP) Ba2MO4 (M ​= ​Sn, Pb) compounds in the tetragonal structure using the full potential linearized augmented plane wave method. The main results are found as follows: (1) the two compounds are semiconductors having indirect band gaps, our results are in agreement with the available experimental and theoretical data. (2) The elastic constants Cij indicate that the tetragonal phase is mechanically stable. (3) The real and imaginary parts of dielectric function for the tetragonal phases for both compounds demonstrate an optical band gap similar to the electronic one and a substantial absorption in the ultraviolet region of the spectrum in elevated energy level. (4) Thermoelectric properties are reported as a function of temperature ensuring a beneficial trend of these materials in energy storage applications.

A cryogenic investigation of the Cgs (Vgs) and the transfer Ids (Vgs) characteristics is suggested to monitor the evolution of the pinch-off voltage Vpinch-off of the AlGaN/GaN HEMTs transistors. An excessive gate leakage current of 117 mA has been identified from Igs (Vgs) measurements from 60 to 320 K. We have found that the negative shift of the Vpinch-off voltage was induced by an electron trap with activation energies at about 0.63 eV overhead the conduction band. The E2 deep level might be correspond to the nitrogen anti-sites, possibly located in the buffer layer near to the interface AlGaN/GaN which is usually procreated from NH3-MOCVD growth technique.

In this report, the gate leakage mechanism for AlGaN/GaN high electron mobility transistors (HEMTs) is consistently analyzed by means of temperature-dependent reverse gate current–voltage characteristics in the temperature range of 60–320 K. The gate leakage current of the AlGaN/GaN HEMT is studied by adjusting the experimental data using Poole–Frenkel (PF), and Fowler–Nordheim (FN) tunneling. In the temperature range of 200–325 K, the reverse bias leakage current is well dominated by a Frenkel-Poole emission model. The scenario describing the PF mechanism assumes that a defect level at φt = 0.45eV could be considered as an intermediate level through which carriers can transit to reach another level attributed to dislocations closer to conduction band. On the other hand, the FN mechanism seems to operate over the entire temperature range of 60–320 K and starts to dominate the gate leakage current due to triangular barrier formation.

The electronic structures of the non-doped and Li+, Eu3+ co-doped Ca2SnO4 materials were calculated employing the density functional theory with the WIEN2k package. The later is based on the full potential linearized augmented plane wave method (FP-LAPW), an approach which is among the most precise and reliable ways to calculate the electronic structure of solids. The generalized gradient approximation (GGA) method was applied to the non-doped material. In order to describe better the strongly correlated 4f electrons of the Eu3+ the GGA+U method was used. The GGA+U method requires as the input the Coulomb repulsion strength (Hubbard parameter U) and the exchange parameter J (magnetic coupling constant), which are related to the Slater integrals. In this study, the effects of Eu3+ and Li+ co-doping on the lattice parameters, and also on the electronic structures are presented and analyzed in details.

The defect calculations are firstly performed by replacing a single cation (namely Zn and Sn) with a single Mn atom in the pure chalcopyrite ZnSnAs2 supercell, and their corresponding formation energies show that the substitution of a Sn atom (rather than Zn) by Mn is strongly favored. Thereafter, a comparison of total energy differences between ferromagnetic (FM) and antiferromagnetic (AFM) are given. Surprisingly, the exchange interaction between a Mn pairs is found to oscillate with the distance between them. Consequently, the AFM alignment is energetically favored in Mn-doped ZnSnAs2 compounds, except for low impurity concentration associated with lower distances between neighboring Mn impurities, in this case the stabilization of FM increases. Moreover, the ferromagnetic alignment in the Mn-doped ZnSnAs2 systems behaves half-metallic; the valence band for majority spin orientation is partially filled while there is a gap in the density of states for the minority spin orientation. This semiconducting gap of ~1 eV opened up in the minority channel and is due to the large bonding-antibonding splitting from the p-d hybridization. Our findings suggest that the Mn-doped ZnSnAs2 chalcopyrites could be a different class of ferromagnetic semiconductors.

The present work is dedicated to simulate the band edges of AlInN/AlN/GaN high electron mobility transistors. In a second step, we have developed a model for the electron mobility by taking into account the predominant mechanisms. The confinement of subbands in the channel quantum well has also been accounted for in calculating the electron mobility. Obtained results have been used to calculate self-consistently the direct-current characteristics of AlInN/AlN/GaN HEMTs. As has been found, the drain current strongly depends on the electron band parameters. More especially, a drastic improvement in the electron transport is expected to be achieved by optimizing the deposited epilayers in terms of thicknesses and alloy composition. Based on an experimental support, the electronic model adopted is improved by including the effects of deep electron traps. As also shown, the AlInN/AlN/GaN HEMTs exhibit a self-heating. From the relevant direct current measurements, we have deduced the drain voltage-dependent temperature in the conductive channel.

This work reports on theoretical and experimental investigation of the impact of InAs quantum dots (QDs) position with respect to InGaAs strain reducing layer (SRL). The investigated samples are grown by molecular beam epitaxy and characterized by photoluminescence spectroscopy (PL). The QDs optical transition energies have been calculated by solving the three dimensional Schrödinger equation using the finite element methods and taking into account the strain induced by the lattice mismatch. We have considered a lens shaped InAs QDs in a pure GaAs matrix and either with InGaAs strain reducing cap layer or underlying layer. The correlation between numerical calculation and PL measurements allowed us to track the mean buried QDs size evolution with respect to the surrounding matrix composition. The simulations reveal that the buried QDs’ realistic size is less than that experimentally driven from atomic force microscopy observation. Furthermore, the average size is found to be slightly increased for InGaAs capped QDs and dramatically decreased for QDs with InGaAs under layer.

This paper investigates the impact of the deposition rate on the mean buried InAs/GaAs quantum dots’ (QDs) size by means of a coupled photoluminescence spectroscopy and numerical approach. The proposed method consists in tuning the theoretical transition energies by changing the QDs aspect ratio towards best fit of the photoluminescence emission energies arising from the state filling effect. The electron-hole confined states are obtained by solving the single particle one band effective mass Schrödinger equation in cylindrical coordinates for a lens shaped QD by finite element method taking into account the strain effects. The obtained evolution is in agreement with morphological data taken from similar uncapped QDs samples.

We report on a simple theoretical model allowing to investigate the rapid thermal annealing induced quantum dots intermixing and consequent inhomogeneous broadening. In this model, where the 3D Schrodinger equation has been solved, by the orthonormal wave function expansion method, for strained InAs QD, we assume a lens-shaped QD with a uniform indium composition and a constant aspect ratio during the intermixing process. The size and aspect ratio for as-grown InAs QD, have been estimated by matching the calculated interband optical transition energies to the experimental photoluminescence emission peaks from ground and excited states. The simulated results were correlated with photoluminescence data at various annealing temperatures. Keeping constant the QD aspect ratio, a good agreement has been found between experimental and calculated emission energies for different indium atomic diffusion lengths. Small QDs are found to be more sensitive to the intermixing than larger QDs. This study allows also to calculate the full width at half maximum (FWHM) and compare it with the experimental value. The theoretical calculations suggest that the origin of the inhomogeneous broadening is mainly related to the variation of the QDs size.

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.

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).

Deep level transient spectroscopy (DLTS) technique at low temperature has been performed to study the electrical properties of the In0.52Al0.48As epilayer lattice matched with InP(0 0 1). A new level, labelled N, has been detected at a temperature of about 36 K with an activation energy of about 20 meV. Photoluminescence (PL) studies together with numerical calculations based on the resolution of the Schrodinger equation have led us the proposition that the N signal is related to localized states due to compositional fluctuations in In0.52Al0.48As epilayer.

In this work, we have theoretically investigated the intermixing effect in highly strained In$_{0.3}$Ga$_{0.7}$As/GaAs quantum well (QW) taking into consideration the composition profile change resulting from in-situ indium surface segregation. To study the impact of the segregation effects on the postgrowth intermixing, one dimensional steady state Schrodinger equation and Fick's second law of diffusion have been numerically solved by using the finite difference methods. The impact of the In/Ga interdiffusion on the QW emission energy is considered for different In segregation coefficients. Our results show that the intermixed QW emission energy is strongly dependent on the segregation effects. The interdiffusion enhanced energy shift is found to be considerably reduced for higher segregation coefficients. This work adds a considerable insight into the understanding and modelling of the effects of interdiffusion in semiconductor nanostructures., 6 pages, 4 figures

Capacitance– and reverse current–voltage (C–V) measurements have been performed on a Schottky barrier structure incorporating InAs quantum dots (QDs) embedded in n-type InAlAs matrix. The electron confined energy level positions in the QDs have been deduced from a capacitance–voltage analysis. Three electron levels have been found at about 240 meV, 180 meV and 70 meV via the InAlAs conduction band edge, and were attributed to the s ground state, the first p excited state and the d state, respectively. The variation of the leakage current density as a function of the depletion layer width has revealed an additional reverse current in the plane containing the dots. It was attributed to the discharging of the InAs QDs through the InAlAs barrier. The effective generation lifetime depth profile was derived from the C–V and the reverse current–voltage measurements. The effective generation lifetime was found to be controlled by the confined states in the QDs. Electrical field effect studies lead us to suggest a mechanism of electrons injection and emission into/from the confined states in the QDs.

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.

International audience; The electronic properties of InAs quantum dots (QDs) grown on InAlAs/InP(001) were studied by using capacitance-voltage (C-V) analysis and photoluminescence (PL) measurements. The level positions of electrons and holes could be studied separately by using n- and p-type InAlAs matrices, respectively. The holes are found to be more confined than electrons in these kinds of dots.

Single InAs/GaAs quantum dots are studied using micro-photoluminescence spectroscopy. Single dot spectroscopy shows an antibinding biexciton. Some of these quantum dots are laterally coupled due to an anisotropic dot repartition. At low excitation power density, four excitons emissions are observed and are interpreted as the four possible exciton recombinations of two coupled quantum dots. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Capacitance–voltage, C(V) studies have been carried out on Schottky barrier structure containing a sheet of self-organized InAs quantum dots (QDs) grown on InAlAs lattice matched to InP in order to deduce the electrical properties of the QDs. Three electron levels have been detected in n-type material, and were attributed to the s ground, the p excited, and the d excited states. Some parameters of the structure, such as the position of the InAs QD plane, the electron concentration in the QDs and an approximate QD height were deduced from the C(V) profile analysis. These results are in good agreement with the transmission electron microscopy (TEM) study realized on the structure.

Optical and morphological properties of solid source molecular beam epitaxy (MBE) grown InAs/GaAs quantum dots are investigated by photoluminescence spectroscopy (PL) and atomic force microscopy (AFM) as a function of the growth rate. The 10 K relatively high excitation density PL spectra of the investigated samples reveal the existence of multipeaks characterizing fundamental states and corresponding excited states. The emission energies are red-shifted by approximately 70 meV, when the InAs deposition rate is reduced from 0.069 ML/s to 0.013 ML/s. By further reducing the growth rate down to 0.003 ML/s an unexpected blue shift of the emission energy occurs. AFM observations, carried out on similar uncapped samples, show a monotonic decrease of the QDs density with decreasing QDs material's deposition rate. For a growth rate of 0.003 ML/s, islands density of 4.9 × 10 9 cm −2 has been achieved with a room temperature PL full width at half maximum (FWHM) of 22 meV. For the later growth rate, the blue shift of the emission energy has been correlated with desorption of In atoms during InAs deposition.

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.

Strain and crystalline quality of tensely strained silicon on insulator with thickness ranging from 8 to 100 nm have been evaluated by low temperature photoluminescence (PL). The strain conservation in the strained Si layers was checked by Raman spectroscopy. The PL clearly shows the emission related to the strained silicon optical band gap even for strained layers as much as seven times thicker than critical thickness (hc∼15 nm). For very thin layers (9 nm), a 21 meV blueshift is observed in the PL spectra, which corresponds to a 17 meV calculated one coming from quantum confinement in the sSi layer.

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 ...