Edge emission of AlxGa1−xN

The photoluminesence in the edge emission region of Al_xGa_1−xN> (0 ≦ x ≦ 0.24) has been studied. The compositional dependence of the band gap was studied and the bowing parameter was determined: b = 0.98 eV. With increasing x the edge emission peak broadens, explanation of the mechanism of which has been attempted.     

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.     

An X-ray and TEM study of inhomogeneous ordering in AlxGa1−xN layers grown by MOCVD

Ordering along the c-axis has been observed both by high resolution X-ray diffraction and transmission electron microscopy in Al_xGa_1−xN grown by low pressure metal-organic vapor phase epitaxy on sapphire (0001). Beside the disordered alloy, AlGaN₂ (1:1), and Al_0.25Ga_0.75N (3:1), there is an additional type with a quite large unit cell. In the latter, the c parameter is six times larger than that of the disordered alloy.     

Intersubband absorption in strained AlxGa1−xN/GaN quantum wells with InyGa1−yN nanogroove layers

The intersubband absorption in a four-energy-level system consisting of a strained Al_xGa_1?xN/GaN quantum well with an In_yGa_1?yN nanogroove layer is calculated by considering the strain modification on the material parameters and polarization effect. It is found that the In_yGa_1?yN nanogroove layer in the middle of quantum well can enhance the confinement of electrons and their energy levels which consequently affect the intersubband absorption. With increasing the In composition and the groove thickness or applying a moderate compressive stress, an inflexion of energy levels appears when the lowest energy potentials of the left well and the groove are equivalent. The intersubband absorption spectrum exhibits multiple peaks contributed by different transitions. The position and height of absorption peaks are sensitive to the structural parameters (i.e., In composition and nanogroove thickness) and the strain induced by the groove layer.     

Analysis of critical melt supercooling for heteroepitaxy of AlxGa1−xSb by GaSb

Thermodynamic computations of the critical supercooling of a melt are performed for the case of heteroepitaxy of a solid Al_xGa_1–xSb solution on a GaSb substrate for which there should be no substrate etching. Three kinds of supercoolings are examined, where _cr ⁽¹⁾ is the supercooling for which the change in the system Gibbs energy should equal zero because of dissolution, _cr ⁽¹⁾ is the supercooling for which the diminution in the system Gibbs energy due to substrate dissolution equals the energy being liberated during crystallization of the Al_xGa_1–xSb solid solutions layer. Finally, the influence of the specific free interphasal energy of the substrate-melt interface on the result of computing the critical supercooling (the supercooling _cr ⁽¹⁾ is considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 47–52, February, 1988.In conclusion, the authors are grateful to senior scientific worker A. P. Vyatkin and scientific worker L. E. Épiktetova for interest in the research and for useful discussion.     

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.     

The refractive index dispersion of Hg1−xCdxTe by infrared spectroscopic ellipsometry

The refractive indices of Hg_1−xCd_xTe (x=0.276, 0.309, and 0.378) bulk samples in the region below, in, and above the fundamental band gap have been measured by infrared spectroscopic ellipsometry at room temperature. A refractive index peak, in which the corresponding energy equals approximately the band gap energy, is observed for each refractive index spectrum with different compositions. Above the band gap, the refractive index drops quickly near the gap, then decreases slowly as photon energy increases. The refractive index n above the band gap is found to follow the Sellmeier dispersion relationship n²(λ)=A+B/λ²+C/λ⁴+D/λ⁶ as a function of the wavelength of light λ.     

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...     

Ternary mixed crystal effects on interface optical phonon and electron-phonon coupling in zinc-blende GaN/AlxGa1−xN spherical quantum dots

The theoretical investigations of the interface optical phonons, electron–phonon couplings and its ternary mixed effects in zinc-blende spherical quantum dots are obtained by using the dielectric continuum model and modified random-element isodisplacement model. The features of dispersion curves, electron–phonon coupling strengths, and its ternary mixed effects for interface optical phonons in a single zinc-blende GaN/Al_xGa_1−xN spherical quantum dot are calculated and discussed in detail. The numerical results show that there are three branches of interface optical phonons. One branch exists in low frequency region; another two branches exist in high frequency region. The interface optical phonons with small quantum number l have more important contributions to the electron–phonon interactions. It is also found that ternary mixed effects have important influences on the interface optical phonon properties in a single zinc-blende GaN/Al_xGa_1−xN quantum dot. With the increase of Al component, the interface optical phonon frequencies appear linear changes, and the electron–phonon coupling strengths appear non-linear changes in high frequency region. But in low frequency region, the frequencies appear non-linear changes, and the electron–phonon coupling strengths appear linear changes.     

Stimulated emission of atmospheric water vapour between 2 cm−1 and 30 cm−1 photoinduced by infrared radiation

Fourier spectra of humid air in a 0.8 m path when it was illuminated by a combined flux of millimetre waves, infrared and ultraviolet showed millimetre wave maser action pumped by the infrared component.     

Analysis of infrared thermoreflectance spectra of NbxTi1−xS2

Thermoreflectance spectra of Nb_xTi_1−xS₂ crystals in the infrared are reported and discussed. In particular the plasmon lineshape, an interband transition at ≅0.8 eV and the effect of cation substitution on infrared thermoreflectance spectra are examined.     

Synthesis and optical properties of La1−XCeXMnO3 studied by infrared reflectivity measurements

The bulk polycrystalline La_1−xCe_xMnO₃ (0.0 ≤ x ≤ 0.8) manganites were synthesized through citrate sol-gel auto-combustion method for a wide range of composition 0.0 ≤ x ≤ 0.8. The x-ray diffraction (XRD) analysis suggests the single phase rhombohedral (R-3c) symmetry for parent LaMnO₃ compound, however a secondary cubic (Fm-3 m) phase of cerium oxide (CeO₂) is appeared when Ce is incorporated into the LaMnO₃. A detailed XRD study of these materials were discussed, and specifically the infrared reflectivity measurements on La_1−xCe_xMnO₃ materials are discussed in this study for the first time. The phonon softening and hardening was observed for different modes but softening was found to be more pronounced which implied strong electron–phonon coupling. The most affected phonon mode was appeared for the doping range 0.2 ≤ x ≤ 0.4, in which the largest softening was observed. The Lyddane–Sachs–Teller relation is verified by experimental FTIR data.     

Photoluminescence of AlxGa1−xAs/GaAs quantum well heterostructures grown by molecular beam epitaxy

Low-temperature photoluminescence measurements on nominally undoped Al_xGa_1–xAs/GaAs quantum well heterostructures (QWHs) grown by molecular beam epitaxy (MBE) exemplified the exclusivelyintrinsic free-exciton nature of the luminescence under moderate excitation conditions. Neither any spectroscopic evidence for alloy clustering in the Al_xGa_1–xAs barriers nor any extrinsic luminescence due to recombination with residual acceptors has been detected in single and double QWHs when grown at 670 °C under optimized MBE growth conditions. Carrier confinement in Al_xGa_1–xAs/GaAs QWHs starts at a well width ofL _z30 nm when x0.25. The minor average well thickness fluctuation ofL _z=4×10^–2nm as determined from the excitonic halfwidth allowed the realization of well widths as low asL _z=1 nm and thus a shift of the free-exciton line as high as 2.01 eV which is close to the conduction band edge of the employed Al_0.43Ga_0.57As confinement layer. The measurements further revealed a strongly enhanced luminescence efficiency of the quantum wells as compared to bulk material which is caused by the modified exciton transition probabilities due to carrier localization.     

Electrical properties of infrared photovoltaic CdxHg1−xTe detectors

p-n junctions in epitaxial graded-gap Cd_xHg_1−xTe layers were produced by the Hg-diffusion method, with molar compositions x from 0.18 to 1. Current-voltage, differential resistivity and capacity characteristics were measured as function of bias in the temperature range 4.2–300 K. Carrier transport mechanisms are discussed, including both generation-recombination and tunnelling processes in the junction. The dependence of electric and photoelectric properties of p-n junctions on both the physical and technological parameters of epitaxial Cd_xHg₁−xTe layers are also analysed.     

Surface-barrier structures on AlxGa1−xSb. II. Band gap dependence of barrier height

The short-circuit photocurrent is used to find the forbidden gap width E_g and barrier height _B of Pd-n-Al_xGa_1–xSb (0.1 s x 0.7) surface-barrier structure fabricated electrochemically. The barrier height was also determined from the capacitive voltage cutoff and the current-voltage characteristics. For metal-n-Al_xGa_1–xSb contacts the linear approximation to _B(E_g) is given by _B = 0.64E_g + 0.16 eV. In some samples the photoresponse spectrum has a hump, indicating the effects of deep levels.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 26–31, June, 1979.     

The influence of growth patterns on the transmission properties of nonabrupt GaAs/AlxGa1−xAs heterojunctions

The influence of growth patterns in the transmission properties of nonabrupt GaAs/Al_xGa_1−xAs heterojunctions is investigated. Five interfacial growth patterns, representative of interfacial alloy variations generated by different growth techniques, are used. It is shown that carrier transmission depends on the type of the aluminum molar fraction variation through the interface. A study of the role of the interface width in carrier transmission is done for each compositional growth profile.     

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.