Localized states in InxGa1−xN epitaxial film

This study investigated localized states from In_0.36Ga_0.64N epitaxial film grown on a Si (111) substrate by performing macro-photoluminescence, micro-photoluminescence and time-resolved micro-photoluminescence experiments. Experimental data revealed two localized states — single and extended. The single localized state is a single-quantum-dot-like deep confined energy state, which is responsible for the bright line emissions. The extended localized state is a shallow confined energy state, which is related to a broad background emission. This work suggests that the origin of single and extended localized states is the indium-rich In_xGa_1?xN cluster and the spatial indium concentration fluctuation, respectively.     

Exciton binding energies in GaN/AlxGa1 − xN pseudomorphic quantum wells

Interband transitions of pseudomorphic GaN/Al_xGa_1 − xN quantum wells are analysed theoretically with respect to the piezoelectric field utilizing a 6  ×  6 Rashba–Sheka–Pikus (RSP) Hamiltonian. Band structure modifications due to the built-in Stark effect explain a shift of the emission peak in GaN/Al_0.15Ga_0.85N of up to 400 meV. Quantum well exciton binding energies are calculated by the variational method and are discussed in terms of spatial separation of electrons and holes by the built-in electric field, as well as the interaction between valence subbands.     

Specifics of Heat Transfer in AlxGa1 – xN/GaN Heterostructures on Sapphire

Physics of the Solid State - The thermal conductivity of AlxGa1 – xN/GaN heterostructures (0.05 ≤ x ≤ 1) fabricated on sapphire by molecular beam epitaxy is...     

Optimization of optical gain in AlxGa1−xN/GaN/AlxGa1−xN strained quantum well laser

In this paper the optical gain in wurtzite AlGaN/GaN quantum well is studied. The effects of temperature, carrier concentration, quantum well width, and barrier width are analyzed theoretically taken into account the strong built-in electric field effect due to the piezoelectric and spontaneous polarization in the nitride materials. The numerical results clearly show that the increasing of carrier concentration, and decreasing of temperature and well widths, the optical gain increases.     

Raman scattering study of IrxRu1−xO2 mixed metal oxide nanowires (0 ≤ x ≤ 1)

Recent interests in mixed metal oxide nanostructured materials especially Ir_xRu_1−xO₂ compounds have been mainly driven by the technological application as electrocatalyst and electrode materials. We present room temperature Raman scattering results of single crystalline Ir_xRu_1−xO₂ (0 ≤ x ≤ 1) nanowires grown by atmospheric pressure chemical vapor deposition. We observed that the E_g, the A_1g, and the B_2g phonon modes of a single Ir_xRu_1−xO₂ nanowire are blue-shifted linearly with respect to the Ir contents from which we could get stoichiometry information. We also observed that the asymmetric lineshape and the broadening of the full width at half maximum of the E_g mode that involves the out-of-plane oxygen vibration. The unusual asymmetric broadening of the E_g phonon can be explained by the activation of the non-zone-center phonons due to substitutional disorder present in the system. We also found that there is a mixed mode of the A_1g and the B_2g phonons due to the substitutional disorder, in the range of 630–750 cm⁻¹.     

Determination of the critical indium composition corresponding to the metal–insulator transition in InxGa1−xN (0.06 ⩽ x ⩽ 0.135) layers

The low-temperature conductivity of In_xGa_1−xN alloys (0.06 ⩽ x ⩽ 0.135) is analyzed as a function of indium composition (x). Although our In_xGa_1−xN alloys were on the metallic side of the metal–insulator transition, neither the Kubo-Greenwood nor Born approach were able to describe the transport properties of the In_xGa_1−xN alloys. In addition, all of the In_xGa_1−xN alloys took place below the Ioeffe–Regel regime with their low conductivities. The observed behavior is discussed in the framework of the scaling theory. With decreasing indium composition, a decrease in thermal activation energy is observed. For the metal–insulator transition, the critical indium composition is obtained as x_c = 0.0543 for In_xGa_1−xN alloys.     

Ternary effects on the optical interface phonons in onion-like GaN/AlxGa1 − xN quantum dots

The interface optical phonons and its ternary effects in onion-like quantum dots are studied by using dielectric continuum model and the modified random-element isodisplacement model. The dispersion relations, the electron–phonon interactions and ternary effects on the interface optical phonons are calculated in the GaN/Al_xGa_1 − xN onion-like quantum dots. The results show that aluminium concentration has important influence on the interface optical phonons and electron–phonon interactions in GaN/Al_xGa_1 − xN onion-like quantum dots. The frequencies of interface optical phonons and electron–phonon coupling strengths change linearly with increase of aluminium concentration in high frequency range, and do not change linearly with increasing aluminium concentration in low frequency range.     

Synthesis and electrical properties of nanocrystalline Ca1 − xEuxMnO3 ± δ (0.1 ≤ x ≤ 0.4) powders prepared at low temperature using citrate gel method

Nanopowders of Ca_1 − xEu_xMnO₃ (0.1 ≤ x ≤ 0.4) manganites were synthesized as a single phase using the auto gel-combustion method. The citrate method shows to be simple and appropriate to obtain single phases avoiding segregation or contamination. The Ca_1 − xEu_xMnO₃ system has been synthesized at 800 °C during 18 h, against the conventional method of mixing oxides used to obtain these materials at higher temperatures of synthesis. The formation reaction was monitored by X-ray diffraction (XRD) analysis and an infrared absorption technique (FTIR). The polycrystalline powders are characterised by nanometric particle size, ∼ 48 nm as determined from X-ray line broadening analysis using the Scherrer equation. Morphological analysis of the powders, using the scanning electron microscope (SEM), revealed that all phases are homogeneous and the europium-substituted samples exhibit a significant decrease in the grain size when compared with the undoped samples. The structure refinement by using the Rietveld method indicates that the partial calcium substitution by europium (for x ≥ 0.3) modifies the orthorhombic structure of the CaMnO₃ perovskite towards a monoclinic phase. All manganites show two active IR vibrational modes around 400 and 600 cm^− 1. The high temperature dependence of electrical resistivity (between 25 and 600 °C) allows us to conclude that all the samples exhibit a semiconductor behaviour and the europium causes a decrease in the electrical resistivity by more than one order of magnitude. The results can be well attributed to the Mn⁴⁺/Mn³⁺ ratio.     

Nonlinear properties in InxGa1 − xAs/GaAs multiple quantum well laser structures

In this paper we investigated nonlinear properties and lasing in In_xGa_1 − xAs/GaAs multiple quantum wells grown by metal-organic chemical vapour deposition. A systematic study, performed by high excitation photoluminescence measurements as a function of excitation intensity, allowed us to identify the minimum well width for observing stimulated emission from well states. We also determined the threshold for stimulated emission for well and barrier lasing. Radiative recombination energies are identified by using theoretical data obtained in the effective mass approximation, including boundary conditions and strain.     

Elastic Constants and Related Mechanical Properties of YxIn1 – xN Ternary System

Physics of the Solid State - Using a pseudo-potential approach within the virtual crystal approximation, the elastic properties of YxIn1 – xN semiconductor ternary alloys in...     

Electro-absorption modulator using a type-II quantum well in the InxGa1 − xAs/InAlAs/InP system

In this paper photoluminescence measurements at low temperature under different excitation powers were carried out on an InGaAs tensile strained (x =  0.3) quantum well with InGaAs barriers lattice matched (LM) to InP. Evidence of a type-II recombination was found between carriers confined in the tensile layer and in the LM layer. This study allows us to deduce an accurate determination of the conduction band offset in the In_0.3Ga_0.7As/In_0.53Ga_0.47As/InP system. Moreover, we include the previous type-II structure between InAlAs barriers in order to confine both electrons and holes. This structure has potential applications in electro-optical modulators. We simulate its optical modulation by solving the Schrödinger equation using the envelope function approximation and calculating the absorption spectrum taking into account excitonic effects.     

Structural,optical and electrical properties of undoped and Si-doped AlxGa1−xN thin films on Si (1 1 1) substrate grown by PA-MBE

We have studied the structural properties of undoped and Si-doped Al_xGa_1?xN/GaN/AlN on Si (1 1 1) substrate prepared by plasma-assisted molecular beam epitaxy (PA-MBE) using high-resolution X-ray diffraction (HR-XRD) and atomic force microscopy (AFM). In comparison with undoped AlGaN, the roughness and dislocation density on the surface of the AlGaN layer decrease with Si doping. Full width half maximum (FWHM) of the undoped and Si-doped samples were equal to 0.69° and 0.52°, respectively. This indicates that the Si doping improves the crystalline quality of the Al_xGa_1?xN layer compared with the undoped one. Raman scattering measurement reveals that the optical phonon modes of A₁(LO) and E₂(H) of the AlGaN show a one-mode and two-modes behavior, respectively. The Fourier-transform infrared reflectance (FTIR) investigation confirms the one-mode (two-mode) behavior of the LO (TO) phonon in our samples. This is in good agreement with Raman measurement. Finally, the barrier height (Φ_B) of undoped and Si-doped Al_xGa_1?xN samples was found to be 0.86 and 0.74 eV, respectively.     

NMR and electric impedance study of lithium mobility in fast ion conductors LiTi2 − xZrx(PO4)3 (0 ≤ x ≤ 2)

Materials of the LiTi_2 − xZr_x(PO₄)₃ series (0 ≤ x ≤ 2) were prepared and characterized by powder X-ray (XRD) and neutron diffraction (ND), ⁷Li and ³¹P Nuclear Magnetic Resonance (NMR) and Electric Impedance techniques. In samples with x < 1.8, XRD patterns were indexed with the rhombohedral R3̅c space group, but in samples with x ≥ 1.8, XRD patterns display the presence of rhombohedral and triclinic phases. The Rietveld analysis of the LiTi_1.4Zr_0.6(PO₄)₃ neutron diffraction (ND) pattern provided structural information about intermediate compositions. For low Zr contents, compositions deduced from ³¹P MAS-NMR spectra are similar to nominal ones, indicating that Zr⁴⁺ and Ti⁴⁺ cations are randomly distributed in the NASICON structure. At increasing Zr contents, differences between nominal and deduced compositions become significant, indicating some Zr segregation in the triclinic phase. The substitution of Ti⁴⁺ by Zr⁴⁺ stabilizes the rhombohedral phase; however, electrical performances are not improved in expanded networks of Zr-rich samples. Below 300 K, activation energy of all samples is near 0.36 eV; however, above 300 K, activation energy is near 0.23 eV in Ti-rich samples and close to 0.36 eV in Zr-rich samples. The analysis of electrical data suggests that the amount of charge carriers and entropic terms are higher in Zr-rich samples; however, the increment of both parameters does not compensate lower activation energy terms of these samples. As a consequence of different contributions, the bulk conductivity of Zr-rich samples, measured at room temperature, is one order of magnitude lower than that measured in Ti-rich samples.     

n doping effect modeling in 1.3 μm GaN0.58yAs1−1.58yBiy/GaAs quantum wells

The effect of n doping on the band structure of lattice-matched GaNAsBi/GaAs quantum wells was investigated using a self-consistent calculation combined with the 16-band anti-crossing model. Bi/N incorporation and doping effects can offer a huge potential to engineer the electronic band structure of such materials suitable for the design of photodetectors and emitters operating at 1.3 µm. The increase of the doping density induces a blue-shift of the fundamental transition energy in the doping range between 6×10¹⁷ and 5×10¹⁸ cm⁻³. The absorption spectra dependence on the well width are discussed. To maintain the fundamental transition fixed at the wavelength 1.3 µm, we have adjusted the Bi composition for the well width range between 4.5 and 10 nm with respect of the confinement conditions.     

Effective line strengths of trans-nitrous acid near 1275 cm−1 and cis-nitrous acid at 1660 cm−1

We determined the effective line strengths of the trans conformer of nitrous acid (HONO) near 1275 cm^?1 (R-branch of ν₃ mode, NOH bend) and of the cis conformer at 1660 cm^?1 (R-branch of ν₂ mode, NO stretch), both at a spectral resolution of 0.001 cm^?1 by tunable infrared laser differential absorption spectroscopy (TILDAS) utilizing continuous-wave quantum cascade (cw-QC) lasers. Absorbance of one conformer was measured while simultaneously quantifying the mixing ratio of total HONO by catalytic conversion to nitric oxide (NO) followed by calibrated absorption spectroscopy. Line strengths obtained here are consistent with previously reported band strengths for the trans conformer but are lower by a factor of approximately 2.4 for the cis conformer.     

H adsorption at Ag/Si interfaces in epitaxially grown Ag(1 1 1) films on Si(1 1 1)7 × 7 substrates

The interaction of atomic H with Ag(1 1 1)/Si(1 1 1)7 × 7 surfaces was studied by thermal desorption (TD) spectroscopy and scanning tunneling microscopy (STM) at room temperature. TD spectroscopy revealed an intense peak from mono H–Si bonds, even though the Si surface was covered by the Ag atoms. This peak was not observed from Ag-coated SiO₂/Si substrates. STM observation showed no clear change of the Ag surface morphology resulting from H exposure. All these results indicate that the atomic H adsorbs at neither the Ag surfaces nor Ag bulk sites, but at the Ag/Si interface by diffusing through the Ag film.     

Modification of B-site doping of perovskite LaxSr1 − xFe1 − y−zCoyCrzO3 − δ oxide by Mg2+ ion

Co-doping B-site of perovskite oxide La_xSr_1 ? xCo_yFe_1 ? yO_3 ? δ (LSCFO) with Cr⁶⁺ and Mg²⁺ ions has been attempted in this research for revamping chemical stability and oxygen ionic conductivity of this mixed conducting oxide. It is known that partial substitution for B-site cations of LSCFO by Cr gives rise to a significant improvement on chemical and thermal stability of the perovskite oxide. On the basis of this doped structure, introduction of an immaterial dose of Mg²⁺ ion into its B-site results in a microstructure consisting of smaller grains with higher density than its precursor. Furthermore, the resulting perovskite oxide La_0.19Sr_0.8Fe_0.69Co_0.1Cr_0.2 Mg_0.01O_3 ? δ (LSFCCMO) displays higher O^2? conductivity than the solely Cr-doped LSCFO besides the improved chemical stability against reduction in 5% CH₄/He stream at 850 °C. A detailed examination of the oxidation states of B-site transition metal ions by XPS has also been conducted as a part of structural characterizations of LSFCCMO. The assessment of relative O^2? conductivity shows that the grain boundary area plays a more important role than the bulk phase in facilitating ion transport, but with comparable boundary areas the higher densification level is favorable.     

Thermoelectric performance of quaternary Mg2(1+x)Si0.2Ge0.1Sn0.7 (0.06 ≤ x ≤ 0.12) solid solutions with band convergence

The study of Mg₂Si based thermoelectric materials has received widespread attention. In this research, quaternary Mg_2(1+x)(Si_0.2Ge_0.1Sn_0.7)_0.99Sb_0.01 (0.06≤x ≤ 0.12) solid solutions with an optimized Sb doping were prepared by B₂O₃ flux method combined with spark plasma sintering (SPS) technique. The Seebeck coefficient, electrical conductivity and thermal conductivity were measured as a function of Mg excess between 300 K and 780 K. The electron concentration, electrical conductivity and lattice thermal conductivity increase while the Seebeck coefficient decreases with increasing magnesium excess content. The electron effective mass enhancement for x ≥ 0.08 suggests the conduction band convergence of Mg₂Si_0.2Ge_0.1Sn_0.7. Mg_2.16(Si_0.2Ge_0.1Sn_0.7)_0.99Sb_0.01 with a maximum dimensionless figure of merit of 0.94 at 780 K stand out as one of the best materials for intermediate temperature applications, providing a good nontoxic alternative to PbTe.