Epitaxial growth of silicon carbide layers by sublimation “Sandwich method” (II) structural defects and growth mechanism

Structural defects of α-SiC epitaxial layers grown by sublimation “sandwich-method” in vacuum at the temperatures ranging from 1600 to 2100 °C have been investigated by X-ray topography and optical microscopy methods. It was shown, that perfect SiC layers with the homogeneous polytype structure and small dislocation density (≦ 10² cm⁻²) may be obtained on the substrates with any crystallographic orientation at the conditions close to quasi-equilibrium one. The presence of impurities and silicon deficiency in the vapour phase, lead usually to the deterioration of morphological and structural perfection of SiC layers. There are the following structural defects: uncoherent polytype inclusions (mainly β-SiC), pores, dislocations, specific stacking faults. Morphological peculiarities of the SiC epitaxial layers and possible growth mechanisms are discussed.     

Magnetic phase transitions in LaMnO3+λ

Magnetic properties of nonstoichinometric lanthanum manganite LaMnO_3+λ as a function of the Mn⁴⁺ concentration (from 0 to 27%) and temperature (from 77 to 300 K) are investigated. The Mn⁴⁺ ions concentration depends on the degree of oxidation λ. It is shown that at 0–10% Mn⁴⁺ there exists an antiferromagnetic ordering and at 10–14% — mixed ferro-antiferromagnetic ordering, while at Mn⁴⁺ concentration exceeding 14% the ferromagnetic ordering takes place. The concentrational antiferromagnetic-ferromagnetic transition at 14% Mn⁴⁺ is due to the crystal structural O' — O orthorhombic transition. In ferromagnetic O-orthorhombic lanthanum manganite the phase ferromagnetic-paramagnetic transition occurs via intermediate magnetic state where in the paramagnetic matrix there are noninteracting ferromagnetic impurity clusters. The presence of such magnetic state gives rise to anomalous behaviour of magnetic and galvanomagnetic properties of LaMnO_3+λ near Curie temperature.     

Neutron diffraction studies of the orthorhombic lattice distortions and magnetic properties of lanthanum manganite obtained under different technological regimes

Neutron diffraction studies have been carried out on two series of lanthanum manganite compositions obtained under different technological regimes. Based on the plot of angle dependence β° characterizing the degree of monoclinic distortions on “tolerance factor” t Mn³⁺ concentration is determined in the compositions obtained by us. With partial oxygen pressure increasing in annealing atmosphere the crystalline transition (orthorhomb-cube) temperature lowers from 583 ± 10°K to 508 ± 10°K. The correlation between magnetic properties of the compositions obtained and the degree of orthorhombic distortions γ′ = \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm b}\sqrt {{\rm 2/c}} $\end{document}\end{document} is established. Neutron diffraction studies of magnetic properties of LaMnO₃ compositions show that with decrease of orthorhombic distortion degree the decrease of antiferromagnetic component and lowering of the Néel point occur, while the ferromagnetic contribution increases and the Curie point rises.     

Precision measurements of plastic deformation of β-CuZn at helium temperatures

By attaching condenser plates directly to the tensile specimen, creep rates of β-CuZn single crystals have been measured by the capacitance change in the strain-rate range of 10⁻⁷ to 10⁻⁵ s⁻¹ and at temperatures between 0.7 and 17 K using a ³He cryostat. Slope of the temperature variation of the stress to give a constant strain-rate decreases below 5 K and seems to level off at 0 K. The strain-rate sensitivity becomes almost constant below 2 K. The conventional activation analysis showed that an ARRHENIUS type strain-rate equation breaks down drastically below 5 K. For an effective temperature T^* which gives 2 K at 0 K and asympototically coincides with the testing temperature at 5 K, the experimental results can be fitted to the ARRHENIUS strain-rate equation with an activation enthalpy ΔH(τ) for the PEIERLS mechanism. The above result is interpreted in terms of the quantum-mechanical oscillation of the dislocations.     

On the Structural and Electronic Characteristics of Titanium Sulphide (Tis1.7) Crystals

TiS_1.7 crystals have been grown by vapour transport technique employing a two-zone furnace with the temperatures of reaction and growth zone maintained at 1073 K and 973 K, respectively. We have measured the variation of electrical conductivity (s̀) with temperature (T) of TiS_1.7 single crystals. It has been found that the conductivity increases at temperatures T > 433 K, which provides convincing evidence that the TiS_1.7 crystal is a semiconductor. Another electronic characteristic of TiS_1.7 crystals observed in the present investigation is the occurrence of voltage controlled negative resistance (VCNR) at a field of 32.1 V cm^-1 to 35.7 V cm^-1. All the polytypes of TiS_1.7 were found to exhibit VCNR nearly at the same field which indicates that the VCNR is polytype independent property. The occurrence of VCNR has been explained on the intervally transfer of electrons in the conduction band.     

Experimental determination of the solid–liquid equilibrium,metastable zone,and nucleation parameters of the flunixin meglumine–ethanol system

Measurements of the metastable zone and solubility for flunixin meglumine–ethanol system were obtained. The solubility was measured within the temperature range from 288.15 to 328.15 K. The mole fraction solubility was correlated satisfactorily with the temperature by the equation: x_eq=2.35×10^?12e^0.07121T. The value of enthalpy of dissolution, enthalpy of fusion and enthalpy of mixing were determined to be 49.04, 64.03 and ?14.99 kJ mol^?1 respectively. The metastable zone width of flunixin meglumine was measured by an electric conductivity method. A comparison of the nucleation temperatures from electric conductivity measurement and from focused beam reflectance measurement (FBRM) shows that both detection techniques give almost the same results for flunixin meglumine. The nucleation parameters of flunixin meglumine in ethanol were determined from the metastable zone data. Over the equilibrium temperature range from 312.28 to 325.55 K, the nucleation rate constant was varied from 0.00001 to 0.00120 #/m² min, whereas the nucleation order was varied from 2.23022 to 3.39299. The obtained high values of nucleation order indicated a high rate of nucleation.     

Three-layer monoclinic 3M 2 polytype of muscovite revealed from oblique-texture electron diffraction patterns

The inhomogeneous (complex) polytype 3M ₂ of finely dispersed muscovite with the structure formed by layers with different parity of azimuthal orientations (561) or relative rotations $[11\bar 2]$ was identified for the first time using the oblique-texture electron diffraction patterns. The specimen had no rigorous order in layer alternation, which is reflected in the distortion of the reflection positions and intensities. It is shown that the most probable defects reduce to the replacement of the layer orientations characteristic of the 3M ₂ polytype by layer orientation characteristic of the 2M ₂ polytype. This fact indicates that, under certain conditions, the prismatic coordination of interlayer cations in micas is preferable.     

Cation clustering and formation of free oxide ions in sodium and potassium lanthanum silicate glasses: nuclear magnetic resonance and Raman spectroscopic findings

Alkali silicate glasses containing lanthanum oxide are useful model systems for understanding the structural role of rare earth cations in optical and other types of materials. We report ²⁹Si and Raman spectra of sodium and potassium silicate glasses, both with added La₂O₃ and with La₂O₃ substituted for Na₂O or K₂O on an equal-oxygen basis. In the former series, silicate speciation changes show the formation of more non-bridging oxygens (NBO) as more of the network-modifying La₂O₃ is added. In the latter series, however, in which the nominal ratio of NBO to Si is constant, silicate speciation changes indicate that the actual ratio decreases significantly as La is substituted for 3 Na or K. The simplest explanation of this finding is that up to several percent of the oxygen in the La-rich glasses is not bonded to any Si, but instead forms `free oxide' ions that are part of La-rich domains. Although the size of these domains remains unconstrained, the lack of evidence for phase separation and continuity of trends in structure with composition suggests that the metastable liquid structure at the glass transition contains substantial intermediate-range heterogeneity.     

Metastable phase formation in rapidly quenched Zr80Pt20 alloys: Linkage to oxygen

Melt spun Zr₈₀Pt₂₀ ribbons having oxygen contents ranging from <200 to ≈5000 mass ppm are shown to form various metastable phases during quenching. In the presence of sufficient oxygen, a big cube Zr₆Pt₃O phase forms, but this structure is de-stabilized towards an icosahedral quasicrystalline structure at lower oxygen levels. Further reduction of oxygen promotes increasingly stable ZrPt bonds, and a metastable β-Zr(Pt) structure is formed.     

Crystallization of amorphous Ni55Pd35P10 alloys

The crystallization of Ni₅₅Pd₃₅P₁₀ alloys was examined by isochronal resistivity and hardness measurements, along with observations of the phase transformation in the heating stage of a transmission electron microscope (TEM). The resistivity temperature coefficient between 26 and 250°C is low (9.38 × 10^?5/°C); a few metastable phases form on crystallization. The hardness-temperature curve shows the hardness to increase as the metastable phases appear.     

Non-stoichiometry of apatites at high temperatures

Schottky defects in apatites are formed by a decomposition reaction at temperatures in excess of 1000°K. In this investigation thermogravimetrie experiments on fluorapatite single crystals and powders have been carried out in the temperature range 1000–1600°K. Relative weightlosses of 1.2 × 10⁻³ per hour corresponding to Ca/P decreases of 0.0002 per hour have been observed at 1675°K. In the temperature range studied the number of vacancy triplets formed per second is in the order of 10¹⁸. The activation energy (1.15 eV) involved indicates that dislocations might play an important role in the decomposition process.     

Studies on syntactically coalesced Silicon Carbide polytype structures

The structure of two new silicon carbide polytypes, found in syntactic coalescence with a basic structure 6 H, have been worked out. The structure of polytype 21 H is [(33)₂63] and that of polytype 93R is [(33)₄34]₃. The polytype 93R belongs to the structure series [(33)_n34]₃. One of the (00.1) growth face reveals the growth spiral. The splitting of diffraction spots indicates the existence of a high period polytype in the crystal. This high period polytype have been identified as 558H or 1674R. The growth mechanism have been discussed in the light of above observations.     

Crystallization kinetics and phase transformations of zinc- and cadmium diphosphide films

The kinetics of isothermal crystallization of amorphous ZnP₂ films in the temperature range from 730 to 810 K and amorphous CdP₂ films between 630–770 K has been investigated. The kinetic curves of isothermal transformation of amorphous films into crystalline ones have been obtained. For ZnP₂ the transformation occurs as follows: amorphous films → α-ZnP₂ (tetragonal modification) → β-ZnP₂ (monoclinic modification). For CdP₂ – amorphous films → γ-CdP₂ (cubic modification) → β-CdP₂ (tetragonal modification). From the experimentally determined kinetic curves the stability diagrams for ZnP₂ and CdP₂ films have been plotted. The most stable modification in the investigated temperature range are β-ZnP₂ and β-CdP₂ other ones are metastable. The values of effective energy of activation of transformation have been determined.     

Structural Features of PLZT Films

Lead zirconate titanate (PZT) films doped with lanthanum, Pb(_1–х)La_х(Zr_0.48Ti_0.52) (х = 0, 0.02, 0.05, 0.08, or 0.01), have been investigated by electron microscopy and X-ray diffraction. Films were formed on Si–SiO₂–TiO₂–Pt substrates by chemical vapor deposition from a solution and annealed at temperatures T = 650 and 750°C. The main structural features of the films, differing them from undoped PZT films fabricated by the same method, have been established. It is found that doping with lanthanum delays the pyrochlore–perovskite transformation in the film bulk, i.e., in the regions distant from the film–substrate interface. The fraction of metastable pyrochlore phase increases with an increase in the La molar content in the films. The main reason for the delay is the deficit of lead in the intergranular perovskite space, especially in the upper part of the film. Annealing at T = 750°C reduces the content of pyrochlore phase but does not completely remove it, which was never observed for undoped PZT films. Doping with lanthanum leads to a change in the lattice period c and a tetragonal distortion of the perovskite lattice (c/a ratio). Hence, the [100] texture of the films obtained, in contrast to the typical [111] texture of PZT films, is due to the increase in the lattice mismatch between the film and platinum layer when lead atoms are replaced with lanthanum. Lattice distortions of “transrotational” character, whose value exceeds 160 deg/μm, are found to arise in growing crystals.     

Metastable states in high carbon C–(Fe,Ni, Co) films obtained by three-electrode ion-plasma sputtering

Abstract

The effect of ion-plasma deposition on the structure of high-carbon films (at. %) Fe–(20–84) % С, Co–(5–52) % С, Ni–(7–61) % С was investigated. The lattice periods and crystallite sizes of nonequilibrium phases in the as-deposited state and after heating are determined. The temperatures of the beginning and end of the decay of metastable phases during heating at a constant speed are established. The transition from an amorphous to an equilibrium crystalline state in Fe–C films passes through the stage of formation and subsequent decomposition of an intermediate, metastable hcp phase of variable composition. The electrical and hysteretic magnetic properties of the films were measured in the as-deposited state and after heat treatment. The compositions and conditions for producing films with low values of the temperature coefficient of electrical resistance and high coercive force are established. Thus, high-carbon films of Ni–61% C in the as-depoteted state and Fe–69% C films after heating to 900?K are characterized by small TCR values (± 10^?6 К^?1) over a wide temperature range.     

Zur Epitaxie von Galliumnitrid auf Korund im System GaCl/NH3/Ar

The vapour growth of single crystalline layers of GaN on {0001} oriented sapphire substrates in the temperature range of 500–1100°C is described for the system GaCl/NH₃/Ar. The RHEED micrographes of samples grown in the high temperature range of 1000–1050°C show KIKUCHI pattern, thus indicating a good crystalline perfection of the layers. The large free electron concentration of about 10²⁰ cm⁻³ decreases below 700°C rapidly with decreasing growth temperature. Simultaneously the crystalline perfection is remarcably lowered due to increasing misorientation of the subgrains, but no arcing of the RHEED spots due to polycrystalline behaviour is observed. The layers grown at 500°C are contaminated by chloride. Sometimes a dendritic overgrowth of the layers by the GaN sphalerite polytype takes place.     

Strukturelle Umklapp-Vorgänge in metastabilen β1-Kupfer-Zink-Mischkristallen bei tiefen und höheren Temperaturen (I). Gitterdynamische Skizze1)

The quantity of the shear-modulus G′ = (c₁₁ – c₁₂)/2 is a measure for the probability of structural Umklapp-processes to occur in metastable β₁-Cu-Zn-solid solutions. Such processes take place, if G′ = G′(T, x) would fall below a critical limit, G′_crit, e.g. by lowering of temperature T or/and Zn-concentration x, because the Fermi-contribution to G′will sink in the 1^st and with him additional the Coulomb-contribution in te 2^nd case. Both ones are the authoritative stabilizing factors for β₁ and therefore specific fo its lattice-dynamical behaviour, especially in the longwavy range of thermoacoustic lattice-vibrations, by which the cooperative Umklapp-motion will escape. Umklapping can also be initiated by favourably oriented dislocations, is G′ would approach to G′_crit in their neighbourhood: G′_critG′(xM8), with xM8  Umklapp-concentration. This is possible by variation of x during isothermal reactions, in the course of whicht he β₁-matrix will heterogenize itself into Zn-poorer and -richer districts β and β (pre-diffusion by means of quenchy vacancies). Both kinds of districts, among which the latter ones will enhance their degree of stability, are joint together coherently. They build up a so called β /β -parquet. The parquet-bricks can reach a critical size, which is necessary that sufficient large atom-collectives can simultaneously be caused to an Umklapp-motion and to occupate new equilibrium positions by thermo-acoustic shear-waves (comparison with a sin-wave beeing changed to a zigzag-line). Only at higher temperatures the bricks come up to the critical size. Umklapping comes about only in the β -bricks, which turn by it to a transition lattice (β₂) with a structure similary to that of the lowtemperature-martensite β′′. After that β₂ changes to α-phase. The way β ⇒ α is marked by the following steps: prediffusion, Umklapping + dislocation, leading to β₂, and a further dislocation dissoziation, leading the atoms to the equilibrium positions in the α-lattice. The so stepped mechanism acts an nucleationmechanism of the α-phase. After the nucleation the α-nuclei grow at the cost of too much formed β (postdiffusion). By isothermal reactions at too low temperatures a mini-herterogenized state of the β₁-matrix will be caused comparable with coldhardening states of other alloys (Guinier-Preston-zones). A β₁-matrix in such a state is unable to isothermal Umklapping, so that α-crystals can be formed – provided that the mini-heterogenities are resolved by increasing of the reaction temperature.     

Incorporation of nitrogen into galliumarsenide grown by chloride VPE

The incorporation of nitrogen into GaAs in the halide-VPE system using NH₃ as the doping source is investigated. The concentration of incorporated N depends on the NH₃ flow rate and increases with decreasing deposition temperature. Concentrations up to 6 × 10¹⁷ cm⁻³ could be detected far above the calculated equilibrium value. By a comparison of SIMS-data with those obtained from IR it is found that most of the N-atoms are situated substitutionally on As lattice sites. Normal-pressure 2 K photoluminescence measurements show N-related luminescence peaks at 1.508 and 1.496 eV. Most probably this luminescence arises from complexes of N with undeliberately introduced impurities. For the 1.508 eV center a complex Si_Ga-N_As is discussed. — A new N-related metastable deep center with the energy levels E_c-0.33, E_c-0.41, and E_c-0.68 eV could be found by DLTS.     

Physical Properties of Zinc Oxide Films Prepared by dc Reactive Magnetron Sputtering at Different Sputtering Pressures

Thin films of zinc oxide were deposited by dc reactive magnetron sputtering onto glass substrates held at a temperature of 663 K and oxygen partial pressure of 1x10^‐3 mbar, and at different sputtering pressures in the range 3x10^‐2 ‐ 10x10^‐2 mbar. The effect of sputtering pressure on the structural, electrical and optical properties of the films were systematically studied. The films were polycrystalline in nature with preferred (002) orientation. The temperature dependence of Hall mobility indicated that the grain boundary scattering of the charge carriers are predominant in these films. The films formed at a sputtering pressure of 6x10^‐2 mbar showed a low electrical resistivity of 6.9x10^‐2 Ohm cm, optical transmittance of 83% with an optical band gap of 3.28 eV.     

Crystal structure of La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals doped with bismuth

Precision X-ray diffraction studies of La_{2 − x} Bi _x Mo₂O₉ (x = 0.04, 0.06, and 0.18) single crystals are performed. It is found that in the compounds doped with bismuth, analogously with the structure of the metastable β_ms phase of pure La₂Mo₂O₉ (LM), the La, Mo1, and O1 atoms deviate from the threefold axis on which they are located in the high-temperature β phase. It is shown that bismuth atoms substitute for part of lanthanum atoms and occupy a position at the threefold axis in the neighborhood of the split lanthanum position. The implantation of bismuth atoms in the LM structure results in the return of a part of the molybdenum atoms to the position at the threefold axis. The occupancy of this position is equal to the occupancy of the bismuth atomic position.