Structure characterization of MHEMT heterostructure elements with In<Subscript>0.4</Subscript>Ga<Subscript>0.6</Subscript>As quantum well grown by molecular beam epitaxy on GaAs substrate using reciprocal space mapping

The crystallographic parameters of elements of a metamorphic high-electron-mobility transistor (MHEMT) heterostructure with In_0.4Ga_0.6As quantum well are determined using reciprocal space mapping. The heterostructure has been grown by molecular-beam epitaxy (MBE) on the vicinal surface of a GaAs substrate with a deviation angle of 2° from the (001) plane. The structure consists of a metamorphic step-graded buffer (composed of six layers, including an inverse step), a high-temperature buffer of constant composition, and active high-electron-mobility transistor (HEMT) layers. The InAs content in the metamorphic buffer layers varies from 0.1 to 0.48. Reciprocal space mapping has been performed for the 004 and 224 reflections (the latter in glancing exit geometry). Based on map processing, the lateral and vertical lattice parameters of In_xGa_1–xAs ternary solid solutions of variable composition have been determined. The degree of layer lattice relaxation and the compressive stress are found within the linear elasticity theory. The high-temperature buffer layer of constant composition (on which active MHEMT layers are directly formed) is shown to have the highest (close to 100%) degree of relaxation in comparison with all other heterostructure layers and a minimum compressive stress.     

Characterizing Plasma-exposed In0.52Al0.48As Surface Using Photoreflectance-, Raman-, and Photoluminescence Spectra Method

We have performed Photoreflectance (PR), Raman Scattering (RS), and Photoluminescence (PL) experiments to characterize the In_0.52Al_0.48As surface exposed to plasma by a gas mixture of CH₄/H₂/Ar, PR spectra indicate that RIE (plasma) causes defects such as nonradiative recombination centers, scattering centers, and defects leading to the decrease of signal intensity, broaden line width and red shift of the transitions by increasing the rf power. In the Raman scattering study, RIE causes defects against InAs-like and AlAs-like LO modes vibration. As the rf power increased, the maximum of two LO modes shifts towards lower frequency and the line shape becomes increasingly asymmetric. Also, the intensity degrades gradually by disorder and point defects with increasing rf power. The PL transition energies show a red-shift with increasing the rf power. In addition, the spectral feature broadens, and the intensity decreases with rf power higher than 200 w. The consistence of the PL, PR, and RS results indicate that these three methods can be used as sensitive probes to evaluate the near surface damage of the epilayer.     

Electrophysical characteristics and structural parameters of metamorphic HEMT nanoheterostructures In0.7Al0.3As/In0.7Ga0.3As/In0.7Al0.3As containing superlattices with different numbers of periods in the metamorphic buffer

The results of studying the electrophysical characteristics and structural parameters of metamorphic In_0.7Al_0.3As/In_0.7Ga_0.3As/In_0.7Al_0.3As HEMT nanoheterostructures epitaxially grown on GaAs (100) substrates have been presented. A linear metamorphic buffer with inserted unbalanced superlattices characterized by different numbers of periods is used. Transmission electron microscopy has shown that an increase in the number of superlattice periods from 5 to 30 promotes the improvement of the crystal structure. In this case, the electrophysical parameters of metamorphic HEMT nanoheterostructures are also significantly improved.     

Epitaxial low-temperature growth of In<Subscript>0.5</Subscript>Ga<Subscript>0.5</Subscript>As films on GaAs(100) and GaAs(111)<Emphasis Type="Italic">A</Emphasis> substrates using a metamorphic buffer

A complex investigation of epitaxial In_0.5Ga_0.5As films grown on GaAs substrates with crystallographic orientations of (100) and (111)A in the standard high- and low-temperature modes has been performed. The parameters of the GaAs substrate and In_0.5Ga_0.5As film were matched using the technology of step-graded metamorphic buffer. The electrical and structural characteristics of the grown samples have been studied by the van der Pauw method, atomic force microscopy, scanning electron microscopy, and transmission/ scanning electron microscopy. The surface morphology is found to correlate with the sample growth temperature and doping with silicon. It is revealed that doping of low-temperature In_0.5Ga_0.5As layers with silicon significantly reduces both the surface roughness and highly improves the structural quality. Pores 50–100 nm in size are found in the low-temperature samples.     

Structural and electrical properties of InAlAs/InGaAs/InAlAs HEMT heterostructures on InP substrates with InAs inserts in quantum well

A complex study of the influence of nanoscale InAs inserts with thicknesses from 1.7 to 3.0 nm introduced into In_0.53Ga_0.47As quantum wells (QWs) on the structural and electrical properties of In_0.52Al_0.48As/In^0.53Ga_0.47As/In_0.52Al_0.48As heterostructures with one-sided δ-Si-doping has been performed. The structural quality of a combined QW was investigated by transmission electron microscopy. A correlation between the electron mobility in QW with the thickness of InAs insert and the technology of its fabrication is established. Specific features of the InP(substrate)/InAlAs(buffer) interface are investigated by transmission electron microscopy and photoluminescence spectroscopy. A relationship between the energy positions of the peak in the photoluminescence spectra in the range of photon energies 1.24 eV < ?ω < 1.38 eV, which is due to the electronic transitions at the InP/InAlAs interface, and the structural features revealed in the interface region is established. It is found that an additional QW is unintentionally formed at the InP/InAlAs interface; the parameters of this QW depend on the heterostructure growth technology.     

Effect of ultrasound on the growth striation and electrical properties of Ga<Subscript>0.03</Subscript>In<Subscript>0.97</Subscript>Sb single crystals

The growth striation of impurity segregation and electrical properties of Ga_0.03In_0.97Sb single crystals grown by the Czochralski method in an ultrasonic field have been investigated. It is established that ultrasonic irradiation of the melt during growth significantly decreases the growth striation (in particular, it eliminates striations spaced at a distance of more than 14 μm). The Ga_0.03In_0.97Sb single crystals grown in an ultrasonic field had a higher charge-carrier mobility and thermoelectric power in comparison with the single crystals grown without ultrasound.     

Interface reconstruction in the Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript>/GaAs(100) and In<Subscript>2</Subscript>Se<Subscript>3</Subscript>/InAs(100) nanoheterostructures

The surface of GaAs(100) and InAs(100) substrates thermally treated in selenium vapor has been investigated by transmission electron microscopy (TEM) and reflection high-energy electron diffraction. Transmission electron microscopy and high-energy electron diffraction data on these heterostructures confirms the epitaxial pseudomorphic growth of the gallium selenide Ga₂Se₃(100) and indium selenide In₂Se₃(100) phases with ordered stoichiometric cation vacancies. A model of the atomic structure of the Ga₂Se₃(100) and In₂Se₃(100) surfaces is proposed, and the 2 × 2 reconstruction of the GaAs(100) and InAs(100) surfaces after treatment in selenium vapor is discussed within this model.     

Crystal structure of Ga<Subscript>0.5</Subscript>In<Subscript>1.5</Subscript>Se<Subscript>3</Subscript> solid solution

A solid solution of the GaIn₃Se₆ (2Ga_0.5In_1.5Se₃) composition with a hexagonal lattice (a = 7.051(3) Å, c = 19.148(2) Å, sp. gr. P6₁, z = 6, V = 824.4332(4) ų, ρ = 5.379(2) g/cm³) has been synthesized as a result of alloying Ga, In, and Se elements with a metal ratio of 1: 3. It was established that six out of nine In atoms in the lattice are located in a trigonal bipyramid, while the other three In atoms and three Ga atoms have a tetrahedral coordination.     

Magnetic susceptibility of quasicrystalline Al<Subscript>65</Subscript>Cu<Subscript>22</Subscript>Fe<Subscript>13</Subscript> powders

Magnetic properties of quasicrystalline Al₆₅Cu₂₂Fe₁₃ powders synthesized by the solid-phase diffusion method via thermal treatment in vacuum or a hydrogen atmosphere have been studied. The powders synthesized in vacuum are found to contain a ferromagnetic fraction. The formation of iron oxide Fe₃O₄ is shown to be the most probable cause of the existence of this fraction. It is possible to avoid the formation of the ferromagnetic fraction in the powders synthesized or annealed in hydrogen. Removal of the ferromagnetic fraction from the powder by repeated magnetic separation made it possible to obtain a quasicrystalline fraction, for which the behavior of the magnetic susceptibility can be explained by the formation of nanoclusters. Such behavior is a general and integral property of quasicrystalline powders synthesized in both vacuum and hydrogen.     

Electrical and structural characteristics of metamorphic In0.38Al0.62As/In0.37Ga0.63As/In0.38Al0.62As HEMT nanoheterostructures

The influence of the metamorphic buffer design and epitaxial growth conditions on the electrical and structural characteristics of metamorphic In_0.38Al_0.62As/In_0.37Ga_0.63As/In_0.38Al_0.62As high electron mobility transistor (MHEMT) nanoheterostructures has been investigated. The samples were grown on GaAs(100) substrates by molecular beam epitaxy. The active regions of the nanoheterostructures are identical, while the metamorphic buffer In _x Al_{1 ? x} As is formed with a linear or stepwise (by Δ _x = 0.05) increase in the indium content over depth. It is found that MHEMT nanoheterostructures with a step metamorphic buffer have fewer defects and possess higher values of two-dimensional electron gas mobility at T = 77 K. The structures of the active region and metamorphic buffer have been thoroughly studied by transmission electron microscopy. It is shown that the relaxation of metamorphic buffer in the heterostructures under consideration is accompanied by the formation of structural defects of the following types: dislocations, microtwins, stacking faults, and wurtzite phase inclusions several nanometers in size.     

Crystal structure refinement of Sr<Subscript>3</Subscript>TaGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript>

An accurate X-ray diffraction study of Sr₃TaGa₃Si₂O₁₄ (STGS) crystal (a = 8.3023(10) Å, c = 5.0853(2) Å, sp. gr. P321, Z = 1, R/wR = 0.59/0.52%, 4004 independent reflections) is performed. The use of two independent data sets obtained on diffractometers with point and 2D detectors made it possible to determine the model structure characterized by the best reproducibility of parameters. The ordered distribution of atoms over crystallographic positions and the anharmonic character of displacements of all cations and one oxygen atom are established.     

Diffractometric study of structural transformations in Cu<Subscript>1.70</Subscript>Cd<Subscript>0.05</Subscript>S crystals

The influence of the partial replacement of copper atoms by cadmium atoms on the real structure, phase formation, and temperatures of the structural transformations in Cu_1.75S are studied by high-temperature X-ray diffractometry. It was shown that Cu_1.70Cd_0.05S crystals at room temperature, unlike Cu_1.75S and Cu_1.70In_0.05S crystals, form not only orthorhombic anilite and monoclinic djurleite phases but also the respective metastable high-temperature fcc₁ and fcc₂ phases.     

Structure of Nd<Subscript>5</Subscript>Mo<Subscript>3</Subscript>O<Subscript>16 + δ</Subscript> Single Crystals Doped with Tungsten

The structure of tungsten-doped Nd₅Mo₃O_{16 + δ} single crystals has been investigated by X-ray diffraction analysis, energy-dispersive X-ray spectroscopy (EDXS) microanalysis, and extended X-ray absorption fine-structure (EXAFS) spectroscopy. Tungsten atom, which partially replaces molybdenum atoms in the structure, is found to be located close (at a distance of 0.57 Å) to the Mo atom site. The interstitial oxygen atom О3 shifts from the crystallographic site with coordinates (0.5, 0.5, 0) to the site with coordinates (0.43, 0.48, 0.04) and complements the tungsten coordination to octahedral. The decrease in the unit-cell parameter and conductivity with an increase in the tungsten concentration in the sample is shown to be related to the fact that the interstitial oxygen atom О3 (responsible for conductivity) is in the coordination environment of tungsten atom.     

Microhardness and fracture toughness of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-and Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>-based nanocrystalline laser ceramics

The microhardness and fracture toughness of laser nanocrystalline ceramics based on the cubic oxides Y₂O₃ and Y₃Al₅O₁₂ are determined experimentally. It is shown by comparative measurements that the fracture toughness and microhardness of Y₂O₃ ceramics exceed the corresponding parameters of Y₂O₃ single crystals by factors of 2.5 and 1.3, respectively. The fine morphology of grains and grain boundaries in fractures is investigated. It is ascertained that changes in the mechanical properties of the nanocrystalline ceramics under study are related to both the sizes and structure of grains and the structure of grain boundaries. It is suggested that twinning processes determine the mechanisms of formation of nanocrystalline ceramics.     

Structural and electrophysical analysis of MHEMT In0.70Al0.30As/In0.75Ga0.25As nanoheterostructures with different strain distributions in metamorphic buffer

A complex structural and electrophysical analysis of MHEMT In_0.70Al_0.30As/In_0.75Ga_0.25As nanoheterostructures grown on (100)GaAs substrates using two radically new designs of metamorphic buffer (providing different internal-strain distributions) has been performed. The lattice parameters of the constant-composition layers entering the metamorphic buffer have been determined by X-ray diffraction using symmetric and asymmetric (400) and (422) reflections. It is shown that, having chosen a proper design of metamorphic buffer in nanoheterostructures on GaAs substrates, it is possible to obtain electron mobility and concentration comparable with those for nanoheterostructures on InP substrates. The compositions of smoothing layers, determined from the peaks on rocking curves, are found agree well with the process values.     

The electrical and structural properties of InyGa1 ? yAs/InxAl1 ? xAs/InP quantum wells with different InAs content

In _x Al_{1 − x} As/In _y Ga_{1 − y} As/In _x Al_{1 − x} As/InP HEMT structures has been investigated with a change in the InAs molar fraction both in the quantum well and the buffer layer. The electrical parameters of the samples are measured at different temperatures. The structural parameters of the layers and the characteristics of the interfaces between them are determined by double-crystal X-ray diffraction. An increase in the Hall mobility and electron concentration, as well as in the structural quality of the samples, is observed alongside an increase in the InAs molar fraction in the quantum well. It is established that high electron mobility is retained at small (to 5%) mismatches between the buffer layer and substrate.     

Crystal structure of (Al,V)<Subscript>4</Subscript>(P<Subscript>4</Subscript>O<Subscript>12</Subscript>)<Subscript>3</Subscript>, archetype of double cubic ring tetraphosphate

The crystal structure of the (Al,V)₄(P₄O₁₂)₃ solid solution, obtained in the single-crystal form by hydrothermal synthesis in the Al(OH)₃-VO₂-NaCl-H₃PO₄-H₂O system, has been solved by X-ray diffraction analysis (Xcalibur-S-CCD diffractometer, R = 0.0257): a = 13.7477(2) Å, sp. gr. I \(\bar 4\)3d, Z = 4, and ρ_calcd = 2.736 g/cm³. It is shown that the crystal structure of the parent cubic Al₄(P₄O₁₂)₃ modification can formally be considered an archetype for the formation of double isosymmetric tetraphosphates on its basis.     

Investigation of the formation of quasicrystalline Al<Subscript>70</Subscript>-Pd<Subscript>20</Subscript>-Re<Subscript>10</Subscript> phase in situ during annealing

The change in the phase composition of thin-film layered AlPdRe nanostructures during annealing, which led to the formation of a quasicrystalline layer, has been studied in situ. It is shown that the Al₃Pd phase is formed at a temperature above 260°C, which transforms into the AlPd phase at 580°C, and the icosahedral quasicrystalline Al-Pd-Re phase is formed at 680°C.     

Synthesis and structure of new framework phosphates Li<Subscript>1/4</Subscript><Subscript>M</Subscript><Subscript>7/4</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>(<Emphasis Type="Italic">M</Emphasis> = Nb,Ta)

New lithium-niobium and lithium-tantalum phosphates Li_1/4 M _7/4(PO₄)₃(M = Nb, Ta) are synthesized by the solid-phase method. The compounds prepared are characterized using electron microprobe analysis, X-ray powder diffraction, and IR spectroscopy. The crystal structure of the Li_1/4Ta_7/4(PO₄)₃ phosphate is determined from the X-ray powder diffraction data (the Rietveld method) and belongs to the framework type. The framework of the structure consists of TaO₆ and LiO₆ vertex-shared octahedra and PO₄ tetrahedra. The isostructural phosphates Li_1/4 M _7/4(PO₄)₃ crystallize in the trigonal crystal system (space group R \(\bar 3\) c, Z = 6) and belong to the NaZr₂(PO₄)₃ structure type.     

Investigation of the structural conditionality for changes in physical properties of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals

With the aim of elucidating the nature of anomalies in the physical properties of K₃H(SO₄)₂ crystals that arise as the temperature grows, the dielectric and optical properties of the crystals are studied, an X-ray diffraction analysis of single-crystal and polycrystalline specimens are performed, and the morphology and chemical composition are studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. As a result of the studies performed, a phase transition from the phase with the monoclinic symmetry (space group C2/c) to the phase with the trigonal symmetry (space group R $ \bar 3 $ \bar 3 m) is found in a number of K₃H(SO₄)₂ specimens at a temperature of ≈457 K, the responsibility of the dynamically disordered hydrogen-bond system for the rise of high proton conductivity in the high-temperature phases of the crystals of this family is confirmed, and data on the solid-phase reactions proceeding at high temperatures are obtained.