Microstructure of annealed low-temperature-grown GaAs layers

The crystal structure and orientation of As precipitates in annealed low-temperature GaAs (LT-GaAs) layers have been investigated by transmission electron microscopy. Three types of As precipitates were identified in layers grown by molecular beam epitaxy at substrate temperatures from 180° to 210° C. In the monocrystalline LT-GaAs layers small pseudocubic As precipitates (2–3 nm diameter) coherent with the GaAs lattice were observed. These precipitates lose their coherency when a certain critical size is exceeded. Precipitates of similar sizes are occasionally found for which a TEM lattice image cannot be obtained. These precipitates are believed to be amorphous. Larger As precipitates with a hexagonal structure (>4 nm diameter) were also found in the layers. These hexagonal As precipitates were observed to be largest near structural defects. The effect of these precipitates on the structure and on the electronic properties of the host GaAs is discussed.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Preparation and Raman Spectrum of Rutile Single Crystals Using Floating Zone Method

With anatase-type titanium dioxide as the raw materials, the futile type titanium dioxide single crystal is prepared using the floating zone method. The results of XRD measurement show that the grown crystal is highly crystalline with a futile structure, which has orientation to the c-axis. The four Raman vibration characteristic peaks （143, 240, 450 and 610 cm^-1） at room temperature show that the crystalline structure of the single crystal is a typical futile phase, meanwhile a new Raman peak at around 690 cm^-1 is found. The results of the Raman measurement at various temperatures for the single crystal show that the Raman frequency shifts are different.     

X-ray diffraction and ab initio determinations of the structure of Rb4CdCl6

The structure of Rb₄CdCl₆ has been examined both at the experimental and at the ab initio levels. Experimentally, new X-ray diffraction measurements have been performed on a single crystal grown at room temperature from aqueous solution. The compound is confirmed to crystallize with a rhombohedral unit cell (space group ) but with a distorsion from cubic symmetry noticeably smaller than in a previous study [Kristallografiya, 37 (1992) 815]. From the ab initio point of view, three distinct sets of first-principles density functional theory calculations (Perdew-Wang generalized-gradient-approximation (PWGGA), Perdew, Burke and Ernzerhof parametrization and Becke-Lyp exchange-correlation functions) have been carried out. These ab initio results corresponding to the ideal crystal (i.e. infinite, periodic in a definite phase and defect-free) are in correct agreement with our own experimental findings, especially for the distorsion. Nevertheless, due to the relative flatness of the ideal crystal potential energy surface with respect to distorsion around the minimum, it is quite plausible the two presently available X-ray diffraction analyses, made on two different real (imperfect) samples, are both correct even though they come out onto significantly different mean distorsion parameters. The shape, orientation and relative sizes of the thermal vibration ellipsoids are discussed in relation with relevant aspects of the calculated potential energy surface. The PWGGA wavefunction is used to plot valence electronic charge density maps. The comparison of the charge distribution in Rb₄CdCl₆ with that in Rb₂CdCl₄ is discussed in terms of compound structures.     

Structural and Magnetic Properties of Codoped ZnO based Diluted Magnetic Semiconductors

Zn1-xCoxO （x = 0.01, 0.02, 0.05, 0.10 and 0.20） diluted magnetic semiconductors are prepared by the sol-gel method. The structural and magnetic properties of the samples are studied using x-ray diffraction （XRD）, extended x-ray absorption fine structure （EXAFS） and superconducting quantum interference device （SQUID）. The XRD patterns does not show any signal of precipitates that are different from wurtzite type ZnO when Co content is lower than x = 0.10. An EXAFS technique for the Co K-edge has been employed to probe the local structures around Co atoms doped in ZnO powders by fluorescence mode. The simulation results for the first shell EXAFS signals indicate that Zn sites can be substituted by Co atoms when Co content is lower than x = 0.05. The SQUID results show that the samples （x 〈 0.05） exhibit clear hysteresis loops at 300K, and magnetization versus temperature from 5 K to 350K at H = 100 Oe for the sample x = 0.02 shows that the samples have ferromagnetism above room temperature. A double-exchange mechanism is proposed to explain the ferromagnetic properties of the samples.     

Evolution with temperature of a single sol-gel TIO2 layer on sapphire (α-Al2O3) (0 0 0 1)

TiO₂ sol-gel layer has been deposited on single crystal sapphire (0 0 0 1) substrate. Evolution of the layer microstructure with the thermal treatment in the range 100-1100 °C has been studied using X-ray diffraction and X-ray reflectometry. TiO₂ layer density first increases with temperature up to 800 °C and then decreases with the appearance of a high roughness finally leading to anatase islands formation. The single crystal nature of the substrate seems to contribute to hinder the transformation of the anatase phase into the rutile phase and to induce a preferred orientation of the TiO₂ islands.     

Growth and characterization of rocksalt MnS/GaAs epilayers by hot-wall epitaxy

Epitaxial growth characteristics of α-MnS on GaAs(1 0 0) substrates have been investigated by X-ray diffraction and double crystal rocking curve measurements. Growth of stoichiometric α-MnS films has been performed by hot-wall epitaxy using Mn and ZnS as a source of sulfur. The films on GaAs(1 0 0) at low substrate temperature exhibit multiphase crystal structures of zincblende and rocksalt, and the main structure is changed to rocksalt with increasing substrate temperature. Photoluminescence spectrum of the α-MnS epilayer at 5 K exhibits broad emission bands, which are attributed to Mn²⁺ ions. The band gap energy of the α-MnS epilayer at room temperature was also estimated to be about 3.3 eV by reflection.     

α- and β-Mn precipitates in the spinodal Fe30Ni20Mn25Al25 alloy

Precipitates in the spinodal alloy Fe₃₀Ni₂₀Mn₂₅Al₂₅, produced by drop-casting followed by hot extrusion, have been investigated using transmission electron microscopy. The as-extruded microstructure consisted of alternating, coherent body-centred cubic and B2 (ordered body-centred cubic) rods aligned along ?100? directions with a wavelength of ～60?nm. Upon annealing at 550°C, precipitates were formed having two distinct morphologies: relatively spherical precipitates with the α-Mn crystal structure and more elongated precipitates with the β-Mn crystal structure. Both precipitates existed simultaneously after annealing at this temperature for up to at least 99?h. The compositions of the precipitates and the orientation relationships between the precipitates and the spinodal matrix were determined.     

Characterization of the charge ordering state by maximum entropy method

In this work we introduce the use of a combined Rietveld refinement and Maximum Entropy Method (MEM), as a helpful new tool to gain insight into the charge ordering pattern present in CO-transition metal compounds. In particular we show the study made on two CO oxyborates A₂OBO₃ (A = Mn and Fe): (a) the Mn-oxyborate, that exhibits a strong charge lattice coupling, and whose charge ordering pattern had been previously satisfactory established from BVS analysis of single crystal X-ray diffraction data and that we have used as a test to confirm the validity of the new method; (b) the weakly charge-lattice coupled Fe-oxyborate, a compound not suitable for the BVS approach and for which nevertheless this method gives evidence for a new charge ordering pattern.     

TEM characterization of yttrium silicide layers synthesized by ion implantation

The structure of Si implanted with high doses of yttrium has been investigated by varying implantation doses and energies. As implantation doses increase into the low 10¹⁷ cm^–2 range, silicide precipitates form. The precipitates are thin and long and lie parallel to {111} planes in the Si matrix. As dopant concentrations increase, the precipitates themselves become more equiaxed, aspect ratios decrease, and precipitates densities increase until the precipitates coalesce to form a continuous buried layer of yttrium silicide within the Si matrix. The layer thickness is relatively uneven. As implant doses increase to 4×10¹⁷ cm^–2, the layer thicknesses become more uniform although there are still defects present. As the implant doses increase further, the precipitate bands on either side of the continuous layer decrease due to gettering of yttrium to the layer. As the energy of the implant is increased, the appearance of the sample is similar to that of the lower energy implants except that the layer is buried deeper in the Si matrix. Observations of the silicide are consistent with its having the AlB₂ structure with ordered vacancies on the Si sublattice.Address from July 1, 1992: Arizona State University, Tempe, AZ     

The growth of crystalline vapor deposited carbon-60 thin films

Thin films (25-2500 Å) of C₆₀ molecules have been deposited on both (001) NaCl and mica substrates at varying temperatures by resistive evaporation. Both electron diffraction and high resolution microscopy have been used to assess the degree of crystallinity, the orientational ordering and the nature of the defects present in these face-centered-cubic films. For NaCl, optimum conditions yielded polycrystalline films with a tendency towards a 110 orientation, while for mica, extended single crystal films have been fabricated which exhibit a 111 direction normal to the film surface.     

Structural relation and epitaxial properties of hexagonal InN and oxidized cubic In2O3

The epitaxial properties and structural relation between hexagonal InN and cubic In₂O₃ phases were studied by synchrotron X-ray scattering and X-ray photoelectron spectroscopy. The cubic bixbyite In₂O₃ phase on the sapphire(0001) substrate was formed after an annealing time of 10 min at 10⁻⁵ Torr after the hexagonal InN film was grown at 550 °C, above the dissociation temperature of InN, by RF-magnetron sputtering. The crystal orientation was cubic In₂O₃(222), parallel to Al₂O₃(0001) and parallel to hexagonal InN(0002) before the oxidation process. The cubic In₂O₃ phase was believed to be formed layer by layer by the oxidation of the hexagonal InN phase.     

X-ray pole-figure study of the epitaxial growth of C60 thin films on mica (001)

The growth of epitaxial C₆₀ thin films on mica(001) by thermal evaporation has been studied in detail by X-ray pole-figure measurements. The influence of the deposition rate, the substrate temperature and the film thickness on the in-plane epitaxial arrangements and the formation of twins has been investigated. It has been demonstrated that the C₆₀ growth is determined by two independent and equivalent C₆₀-crystal grain alignments (type-A and type-B). The nearly six-fold symmetry of the mica(001)-substrate surface offers the three-fold fcc-(111)-oriented C₆₀-crystal grains two equivalent crystal alignments. A high deposition rate of 0.5 Å/s is responsible for the formation of twins at a substrate temperature of 150°C, which diminishes by a higher substrate temperature of 200°C. By a decrease of the deposition rate down to 0.08 Å/s the twins vanish at a film thickness of 200 nm and at the substrate temperature of 150°C. Under the same sublimation conditions, in addition to the type-A and type-B crystal orientations, the growth of the thin C₆₀ films starts with a slight fibre texture which does not appear at a larger film thickness.     

Mechanical properties of sol-gel derived lead zirconate titanate thin films by nanoindentation

Lead zirconate titanate (PZT) thin films are deposited on platinized silicon substrate by sol-gel process. The crystal structure and surface morphology of PZT thin films are characterized by X-ray diffraction and atomic force microscopy. Depth-sensing nanoindentation system is used to measure mechanical characteristics of PZT thin films. X-ray diffraction analyses confirm the single-phase perovskite structures of all PZT thin films. Nanoindentation measurements reveal that the indentation modulus and hardness of PZT thin films are related with the grain size and crystalline orientation. The increases of the indentation modulus and hardness with grain size are observed, indicating the reverse Hall-Petch effect. Furthermore, the indentation modulus of (1 1 1)-oriented PZT thin film is higher than those of (1 0 0)- and random-oriented films. The consistency between experimental data and numerical results of the effective indentation moduli for fiber-textured PZT thin films using Voigt-Reuss-Hill model is obtained.     

Early precipitation stages of aluminum alloys—The role of quenched-in vacancies

We present preliminary results on the structure of nano-sized particles (clusters of solute atoms or Guinier-Preston-Bagaryatsky (GPB) zones) in aluminum alloys. Hindering the motion of dislocations, these GPB zones (precipitates) are responsible for the strength of AlCuMg-alloys - used e.g. as AA2024 - for the outer shell plates in aircraft industry. We will discuss the role of quenched-in vacancies for the formation and growth of the precipitates. Using positron annihilation (Doppler broadening together with the recently developed tool of high-momentum analysis (HMA)) and X-ray absorption spectroscopy measurements, we are able to characterize the local atomic environment in the vicinity of vacancies and selected elements (Cu) forming the precipitates. The interpretation will be based on a comparison to numerical calculations of positron lifetimes and momentum distributions as well as simulated X-ray absorption spectra. Often reliable numerical calculations of experimental quantities are possible only if relaxed atomic positions - calculated employing an ab-initio code like SIESTA - are used as input.     

Crystal growth, thermal and optical studies of nonlinear optical material: Glycine potassium sulphate

A new nonlinear optical material glycine potassium sulphate (GPS) has been synthesized and optical quality crystals were grown from aqueous solution. This material has positive temperature coefficient revealed from the solubility studies. The grown crystals were characterized by employing several techniques such as single crystal and powder X-ray diffraction, thermo gravimetric analysis, FTIR and UV-vis-NIR spectra. The etching studies have been done on the (1 0 0) plane of the grown crystal. Preliminary measurements to find second harmonic generation efficiency of GPS has been carried out.     

Curved crystal analysis using a triple-crystal X-ray spectrometer

A Bragg crystal spectrometer using cylindrically curved crystals is designed to focus a divergent X-ray beam. The spectral resolution and precision of focus is determined by the quality of the concavely curved crystal. The analysis of the quality of the crystal was done using a triplecrystal X-ray spectrometer, comparing parallel and antiparallel X-ray scans of the third curved crystal, determining flaws resulting from the bend. Topics discussed include the derivation of the crystal's quality, conditions of alignment for the Bragg reflection, and X-ray beam width contributions to the FWHM. Organic and inorganic crystals with varying values of Bragg angles are used.     

Structural and Optical Properties of Zinc Nitride Films Prepared by Pulsed Filtered Cathodic Vacuum Arc Deposition

Polycrystalline zinc nitride films are deposited on Coming 7059 glass substrates by pulsed filtered cathodic vacuum arc deposition （PFCVAD）. The crystallographic structure is studied by means of x-ray diffraction. These measurements show that all the films are crystallized in the cubic structure, in a preferred orientation along the （332） and （631） directions. Weak XRD signal shows small crystallites distributed in an amorphous tissue. A small improvement of crystallinity is observed with annealing. Optical parameters such as absorption, energy band gap, Urbach tail, extinction coefficients have been determined. The Urbach tail energy is decreased with annealing at 500℃ for one hour. Energy band gap values are found to be increased by annealing.     

Growth and design of novel nonlinear optical material (NLO) ⿿ Glycine barium nitrate potassium nitrate (GBNPN) crystal

A potentially useful semi organic nonlinear optical (NLO) material ? glycine with barium nitrate and potassium nitrate (GBNPN) has been synthesized by slow evaporation technique. Good transparent GBNPN crystals were obtained in a time span of 3 weeks. The grown crystals were characterized by single crystal/powder XRD, UV?vis?IR absorption, FTIR, thermal analysis and powder SHG measurements have been studied. The grown crystals were thermally stable up to 137.53 °C. The GBNPN crystal exhibits second harmonic generation efficiency of about 1.35 times than that of potassium di hydrogen phosphate (KDP). Mechanical properties such as micro hardness (H_v) and Mayer's index, n, have been carried out by indentation method. The refractive index (μ) has been measured by the Brewster's angle method.     

The magnetic properties of Be-codoped GaMnN grown via molecular beam epitaxy

GaMnN and Be-codoped GaMnN were grown via molecular beam epitaxy using a single GaN precursor and their structural and magnetic properties were examined. X-ray diffraction and superconducting quantum interference device (SQUID) measurements revealed that the grown layers are homogeneous without precipitates. The saturation magnetization of GaMnN has increased from ∼4 to ∼16 emu/cm³ via codoping of Be. The d–d exchange interaction between Mn atoms was discussed for the ferromagnetism of GaMnN.     

A Comparison of GaN Epilayers with Multiple Buffer Layers and with a Single Buffer Layer Grown on Si（111） Studied by HRXRD and RBS/Channeling

Two hexagonal GaN epilayers （samples A and B） with multiple buffer layers and single buffer layer are grown on Si （111） by metal-organic vapour phase epitaxy （MOVPE）. From the results of Rutherford backscattering （RBS）/channeling and high resolution x-ray diffraction （HRXRD）, we obtain the lattice constant （a and c） of two GaN epilayers （aA = 0.3190 nm, cA = 0.5184 nm and aB = 0.3192 nm, CB = 0.5179 nm）, the crystal quality of two GaN epilayers （ ХminA=4.87%, ХminB =7.35% along 〈1-↑213〉 axis） and the tetragonal distortion eT of the two samples along depth （sample A is nearly fully relaxed, sample B is not relaxed enough）. Comparing the results with the two samples, it is indicated that sample A with multiple buffer layers have better crystal quality than sample B with a single buffer layer, and it is a good way to grow GaN epilayer on Si （111） substrates using multiple buffer layers to improve crystal quality and to reduce lattice mismatch.