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.     

Influence of arsenic flow on the crystal structure of epitaxial GaAs grown at low temperatures on GaAs (100) and (111)<Emphasis Type="Italic">A</Emphasis> substrates

The influence of arsenic flow in a growth chamber on the crystal structure of GaAs grown by molecular-beam epitaxy at a temperature of 240°C on GaAs (100) and (111)A substrates has been investigated. The flow ratio γ of arsenic As4 and gallium was varied in the range from 16 to 50. GaAs films were either undoped, or homogeneously doped with silicon, or contained three equidistantly spaced silicon δ-layers. The structural quality of the annealed samples has been investigated by transmission electron microscopy. It is established for the first time that silicon δ-layers in “low-temperature” GaAs serve as formation centers of arsenic precipitates. Their average size, concentration, and spatial distribution are estimated. The dependence of the film structural quality on γ is analyzed. Regions 100–150 nm in size have been revealed in some samples and identified (by X-ray microanalysis) as pores. It is found that, in the entire range of γ under consideration, GaAs films on (111)A substrates have a poorer structural quality and become polycrystalline beginning with a thickness of 150–200 nm.     

Metamorphic growth of tensile strained GaInP on GaAs substrate

Optical and structural properties of tensile strained graded Ga_xIn_1−xP buffers grown on GaAs substrate have been studied by photoluminescence, X-ray diffraction, atomic force microscopy, and scanning electron microscopy measurements. The Ga composition in the graded buffer layers was varied from x=0.51 (lattice matched to GaAs) to x=0.66 (1% lattice mismatch to GaAs). The optimal growth temperature for the graded buffer layer was found to be about 80–100 °C lower than that for the lattice matched GaInP growth. The photoluminescence intensity and surface smoothness of the Ga_0.66In_0.34P layer grown on top of the graded buffer were strongly enhanced by temperature optimization. The relaxation of tensile GaInP was found to be highly anisotropic. A 1.5 μm thick graded buffer led to a 92% average relaxation and a room temperature photoluminescence peak wavelength of 596 nm.     

X-ray analysis of multilayer In<Subscript>0.52</Subscript>Al<Subscript>0.48</Subscript>As/In<Subscript>0.53</Subscript>Ga<Subscript>0.47</Subscript>As/In<Subscript>0.52</Subscript>Al<Subscript>0.48</Subscript>As HEMT heterostructures with InAs nanoinsert in quantum well

In_0.52Al_0.48As/In_0.53Ga_0.47As/In_0.52Al_0.48As HEMT heterostructures on InP substrates with elastically strained InAs insert in combined quantum well (QW) have been investigated using a combination of X-ray methods: double-crystal X-ray diffraction, X-ray reflectivity, and reciprocal space mapping. This approach has provided detailed complementary information about the layered and real crystal structures of the samples. The data obtained have made it possible to perform structural analysis of the multilayer systems and compare their characteristics with specified technological parameters, due to which the HEMT growth technology can be corrected and improved.     

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.     

Low-Temperature epitaxial growth of InGaAs films on InP(100) and InP(411)<Emphasis Type="Italic">A</Emphasis> substrates

The structural and electrical characteristics of In_0.53Ga_0.47As epitaxial films, grown in the low-temperature mode on InP substrates with (100) and (411)A crystallographic orientations at flow ratios of As4 molecules and In and Ga atoms of γ = 29 and 90, have been comprehensively studied. The use of InP(411)A substrates is shown to increase the probability of forming two-dimensional defects (twins, stacking faults, dislocations, and grain boundaries), thus reducing the mobility of free electrons, and As_Ga point defects, which act as donors and increase the free-electron concentration. An increase in γ from 29 to 90 leads to transformation of single-crystal InGaAs films grown on (100) and (411)A substrates into polycrystalline ones.     

Gallium vacancy ordering in Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin layers on Si(100), Si(111), and Si(123) substrates

Thin Ga₂Se₃ layers deposited on silicon substrates with the (100), (111), and (123) orientations are studied by transmission electron microscopy and X-ray microanalysis. Some features and regularities of the stoichiometric gallium vacancy ordering at different substrate orientations are discussed. The Ga₃Se₄(100)с(2 × 2) and Ga₂Se₃(111)(√3 × √3)-R30° ordered structures are formed on the Si(100) and Si(111) surfaces, respectively.     

Nanolayer of the A2IIIB3VI (111) phase with ordered cation vacancies on GaAs(111) and InAs(111)

The surface of GaAs(111) and InAs(111) substrates has been investigated by transmission and scanning electron microscopy after thermal treatment in selenium vapor. A pseudomorphic growth of single-crystal phases of indium selenide In₂Se₃(111) and gallium selenide Ga₂Se₃(111) is found; these compounds are crystallized into a sphalerite lattice with ordered stoichiometric cation vacancies. A model of an atomic surface is proposed for the In₂Se₃(111) and Ga₂Se₃(111) structures. The reconstruction of the (√3 × √3)-R30° surface of GaAs(111) and InAs(111) after treatment in Se vapor is considered within this model.     

Growth and structure of La<Subscript>3</Subscript>Zr<Subscript>0.5</Subscript>Ga<Subscript>5</Subscript>Si<Subscript>0.5</Subscript>O<Subscript>14</Subscript> crystals

The complete X-ray structure determination of Czochralski grown La₃Zr_0.5Ga₅Si_0.5O₁₄ single crystals with the Ca₃Ga₂Ge₄O₁₄ structure is performed (sp. gr. P321, a = 8.226(1) Å, c = 5.1374(6) Å, Z = 1, Mo K_α1 radiation, 1920 crystallographically independent reflections, R = 0.0166, R_w = 0.0192). The absolute structure is determined. It is shown that possible transition of some of La atoms (～1.2%) from the 3e to 6g position may give rise to the formation of structural defects.     

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.     

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.     

Structural and electrical properties of metamorphic nanoheterostructures with a high InAs content (37–100%) grown on GaAs and InP substrates

A complex study of the structural and electrical properties of nanoheterostructures containing a metamorphic barrier with a high InAs content (37–100%) in the active region have been performed by the Van der Pauw and X-ray diffraction methods. All peaks observed in the rocking curves for the samples studied (throughout the entire structure) have been revealed and identified. It is shown that, having properly chosen the design of the metamorphic buffer and the compositional gradient in it, one can obtain mobilities and concentrations of the 2D electron gas in the In _x Ga_1−x As quantum well in the heterostructures formed on GaAs substrates that are comparable with the corresponding values for the nanoheterostructures grown on InP substrates. It is established that the mobility and concentration of 2D electron gas depend both on the metamorphic barrier design and on the structural quality of heterostructure as a whole.     

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 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.     

Study of the relationship between the crystal structure of nanolayers and electrical properties in Al x Ga1? x As/In y Ga1? y As pseudobinary heterostructures by double-crystal X-ray diffraction

The structural parameters of individual layers of samples of a Al _x Ga_1−x As/In _y Ga_1−y As/GaAs pseudomorphic heterostructure have been determined by double-crystal X-ray diffraction. A relationship of the technological parameters of fabrication of heterostructures with their structural and electrical properties is established. The increase in the mobility of the 2D electron gas in the samples under study, caused by the increase in the growth temperature of the Al _x Ga_1−x As spacer layer and the decrease in the time of silicon δ doping from the two sides of the quantum well, correlates well with the degree of the sample structural quality. Original Russian Text ? R.M. Imamov, I.A. Subbotin, G.B. Galiev, 2008, published in Kristallografiya, 2008, Vol. 53, No. 2, pp. 210–213.     

Accurate crystal structure refinement of La<Subscript>3</Subscript>Ta<Subscript>0.25</Subscript>Ga<Subscript>5.25</Subscript>Si<Subscript>0.5</Subscript>O<Subscript>14</Subscript>

An accurate X-ray diffraction study of an La₃Ta_0.25Ga_5.25Si_0.5O₁₄ single crystal has been performed using two data sets obtained independently for the same sample in different orientations on a diffractometer with a 2D CCD detector. This structure was refined with an averaged set of these data (a = 8.1936(15) Å c = 5.1114(6) Å, sp. gr. P321, Z = 1, R/wR = 0.75/0.71%, 4030 independent reflections). This analysis was aimed at determining the character of the occupancies of the cation position in the structure. The octahedra at the origin of coordinates turned out to be statistically occupied by gallium and tantalum ions of similar sizes, whereas the tetrahedra on the threefold symmetry axes are occupied by gallium and silicon whose ionic radii differ significantly. The latter circumstance caused the splitting of oxygen positions and made it possible to reliably establish the structural position of statistically located [SiO₄] and [GaO₄] tetrahedra of different sizes.     

Phase transitions in compounds with the Ca<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> structure

Possible structural changes described by the group-subgroup relationships in the Ca₃Ga₂Ge₄O₁₄-type structure (sp. gr. P321) are considered. The most probable phase transitions seem to be those accompanied by lowering of the symmetry to the maximal non-isomorphic subgroups P3 and C2. It is shown that only destructive phase transitions accompanied by symmetry rise up to the minimal non-isomorphic supergroups for the given structure type can take place. The change of the trigonal symmetry to monoclinic is revealed in La₃SbZn₃Ge₂O₁₄, whose crystal structure is refined as a derivative structure of the Ca₃Ga₂Ge₄O₁₄ structure type within the sp. gr. A2 (C2). At ～250°C, La₃SbZn₃Ge₂O₁₄ undergoes a reversible phase transition accompanied by symmetry rise, A2 ? P321. Similar phase transitions, P321 ? A2, are also observed in La₃Nb_0.5Ga_5.5O₁₄ and La₃Ta_0.5Ga_5.5O₁₄ under the hydrostatic pressures 12.4(3) and 11.7(3) GPa, respectively. The mechanisms of compression and phase transition are based on the anisotropic compressibility of a layer structure. With the attainment of the critical stress level in the structure, the elevated compressibility in the (ab) plane gives rise to a phase transition accompanied by the loss of the threefold axis. Attempts to reveal low-temperature phase transitions in a number of representatives of the langasite family have failed.     

Proton-Conducting Composites Based on the Cs<Subscript>4</Subscript>(HSO<Subscript>4</Subscript>)<Subscript>3</Subscript>(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>) Compound

Proton-conducting composites xCs₄(HSO₄)₃(H₂PO₄) + (1–x)AlPO₄ in the composition range x = 0.9–0.5 have been obtained. Their transport properties are studied by impedance spectroscopy. The dependences of the phase composition of the materials on the component ratio are investigated by X-ray diffraction analysis. The spatial phase distribution in the materials is analyzed using scanning electron microscopy.     

Ga and In incorporation rates in Ga1−xInxAs growth by chemical beam epitaxy

GaAs, InAs and Ga_1?xIn_xAs layers were grown by chemical beam epitaxy (CBE) using triethylgallium, trimethylindium and tertiarybutylarsine as precursors for Ga, In and As, respectively. The growth rate during the homoepitaxial growth of GaAs and InAs, deduced from the frequency of reflection high-energy electron diffraction intensity oscillations, was used to calibrate the incorporation rates for the III elements. The In content of the Ga_1?xIn_xAs layers was measured by Rutherford backscattering spectrometry and compared with the value predicted from the above calibration data; while the measured In mole fraction is close to the predicted value for the samples grown for low In to Ga flux ratios (x<0.2), the In incorporation is enhanced for larger values of this ratio. The results obtained on layers grown at different substrate temperatures show that In mole fraction is almost constant at growth temperatures in the range 400–500 °C, but a strong dependence on the substrate temperature has been found outside this range. The above results, not observed for samples grown by solid source molecular beam epitaxy, indicate that some interaction between Ga and In precursors at the sample surface could take place during the growth by CBE.     

Zum Wachstum von AIIIBV-Halbleitern aus nichtstöchiometrischen Schmelzen (II) Dotierung von GaAs und Ga1−xAlxAs mit Zink

Doping of GaAs and Ga_1−xAl_xAs with Zn in the LPE process was studied by a radioanalytical method. It is found that Zn diffuses from the Ga-(Al)-As-Zn-solution into the n-GaAs substrate before epitaxial growth starts. This “pre-diffusion” and the following diffusion of Zn out of the epitaxial layer into the substrate results in three regions with distinct Zn-graduation in the vicinity of the pn-junction. There are no striking differences for GaAs/GaAs and Ga_1−xAl_xAs/GaAs structures. The Zn concentration in GaAs epitaxial layers decreases exponentially from the substrate up to the layer surface. From this profile the temperature dependence of the Zn segregation coefficient is calculated. At 900°C a value k_eff = 5,2 · 10⁻² is found. The doping profile in Ga_1−xAl/As layers is more complex. It is influenced by the changings of temperature during the growth of the layer and by the nonuniform Aldistribution over the layer thickness.