Investigation on quality of cubic GaN/GaAs(100) by double-crystal X-ray diffraction

Cubic GaN was grown on GaAs (100) by low pressure metal organic chemical vapor deposition (MOCVD). X-ray diffraction, scanning electron microscope (SEM) and photoluminescence (PL) spectra were performed to characterize the quality of the GaN film. The PL spectra of cubic thin films being thicker than 1.5 μm were reported. Triple-crystal diffraction to analyze orientation distributions and strain of the thin films was also demonstrated.     

α-Sn thin film grown on GaAs substrate by MBE and investigation of its multiquantum well structure

α-Sn thin films have been grown on GaAs (001) single crystal substrates by molecular beam epitaxy (MBE). The α-Sn growth process has been characterizedin situ by reflection high energy electron diffraction (RHEED), and the transmission electron microscope (TEM) was used to analyze the interface structures. The measurement results indicate that our metastable a-Sn films have both higher temperature stability which increases by 30°C (from 70 to 100°C) and thickness stability which increases by 200 nm (from 500 to 700 nm) in comparison with previous reports. Other improvements in electrical properties have also been observed. In addition, a new model of multiquantum well structure has been suggested. Project supported by the National Natural Science Foundation of China (Grant No. 69586001).     

High-T c superconducting thin film/GaAs MESFET hybrid microwave oscillator

A high-T _c superconducting (HTSC) thin film/GaAs MESFET hybrid microwave oscillator operated at 10.6 GHz has been designed, fabricated and characterized. Microstrip line structures were used throughout the circuit with superconducting thin film YBa₂Cu₃O_7-δ(YBCO) as the conductor material. The YBCO thin films were deposited on 15 mm×10 mm×0.5 mm LaAlO₃ substrates. The oscillator was common-source, series feedback type using a GaAs-MESFET (NE72084) as the active device and a superconducting microstrip resonator as the frequency stabilizing element. By improving the unloaded quality factorQ ₀ of the superconducting microstrip resonator and adjusting the coupling coefficient between the resonator and the gate of the MESFET, the phase noise of the oscillator was decreased. At 77 K, the phase noise of the oscillator at 10 kHz offset from carrier was −87 dBc/Hz. Project supported by the National Center for Research and Development on Superconductivity of China.     

Study of ion implanted Al0.25Ga0.75As/GaAs by Raman spectroscopy

This paper reported the effect of the weak damage in the Al_0.25Ga_0.75As/GaAs epitaxial layer induced by 0.8 MeV Si ions implantation with ion dose from 1 × 10¹⁴ to 5 × 10¹⁵ cm^-2. The Raman spectra measured on these samples showed that there were two kinds of phonon modes existing in the epitaxial Al_0.25Ga_0.75As films. The strains induced in the implanted layer and the corresponding lattice parameters were also evaluated as a function of the implanted dose. In addition, the Rutherford backscattering spectrometry/channeling (RBS/C) was also measured on these samples. These two measurement techniques all confirmed that the implantation only induced weak damage in the material.     

Study of ion implanted Al0.25Ga0.75As/GaAs by Raman spectroscopy

This paper reported the effect of the weak damage in the Al_0.25Ga_0.75As/GaAs epitaxial layer induced by 0.8 MeV Si ions implantation with ion dose from 1 × 10¹⁴ to 5 × 10¹⁵ cm^-2. The Raman spectra measured on these samples showed that there were two kinds of phonon modes existing in the epitaxial Al_0.25Ga_0.75As films. The strains induced in the implanted layer and the corresponding lattice parameters were also evaluated as a function of the implanted dose. In addition, the Rutherford backscattering spectrometry/channeling (RBS/C) was also measured on these samples. These two measurement techniques all confirmed that the implantation only induced weak damage in the material.     

Preparation of Y-Ba-Cu-O film on metal substrate with buffer layers deposited by ion beam assisted deposition

Ion beam assisted pulsed laser deposition of biaxially aligned yttria-stabilized zirconia (YSZ) was used to produce a buffer layer for YBCO film on polycrystalline metallic substrate. The YSZ layers were biaxially aligned with (001) axis normal to the substrate. The minimum FWHM of (111) phi-scan of the YSZ was 19°, and the minimum FWHM of the rocking curve of YSZ was 4.5°. Highly c-axis oriented biaxially aligned YBCO thin films were epitaxially grown by laser ablation on these layers, with J_C(77K, 0T) = 2.1 × 10⁵ A/cm², . Project supported by the National Center for R&D on Superconductivity of China.     

Properties of GaAs single crystals grown by molecular beam epitaxy at low temperatures

Properties of GaAs single crystals grown at low temperatures by molecular beam epitaxy (LTMBE GaAs) have been studied. The results show that excessive arsenic atoms of about 10²⁰ cm⁻³ exist in LTMBE GaAs in the form of arsenic interstitial couples, and cause the dilation in lattice parameter of LTMBE GaAs. The arsenic interstitial couples will be decomposed, and the excessive arsenic atoms will precipitate during the annealing above 300°C. Arsenic precipitates accumulate in the junctions of epilayers with the increase in the temperature of annealing. The depletion regions caused by arsenic precipitates overlap each other in LTMBE GaAs, taking on the character of high resistivity, and the effects of backgating or sidegating are effectively restrained.     

Growth morphology and structural characteristic of C70 single crystals

Large size C₇₀ single crystals with the dimension of more than 5 mm are grown from the vapor phase by controlling nucleation. X-ray diffraction and electron diffraction confirm that in the C₇₀ single crystal a phase of the hexagonal close-packed (hcp) structure coexists with a minor face-center-cubic (fcc) phase at room temperature. The morphologies and their formation mechanism of the C₇₀ single crystals are investigated by means of scanning electron micrascopy and optical microscopy. The influence of growth conditions on the morphologies of C₇₀ single crystals is discussed. Project supported by the National Natural Science Foundation of China (Grant No. 59772026).     

Analysis of arbitrarily shaped planar cracks in two-dimensional hexagonal quasicrystals with thermal effects. Part II: Numerical solutions

The extended displacement discontinuity (EDD) boundary element method is developed to analyze an arbitrarily shaped planar crack in two-dimensional (2D) hexagonal quasicrystals (QCs) with thermal effects. The EDDs include the phonon and phason displacement discontinuities and the temperature discontinuity on the crack face. Green's functions for uniformly distributed EDDs over triangular and rectangular elements for 2D hexagonal QCs are derived. Employing the proposed EDD boundary element method, a rectangular crack is analyzed to verify the Green's functions by discretizing the crack with rectangular and triangular elements. Furthermore, the elliptical crack problem for 2D hexagonal QCs is investigated. Normal, tangential, and thermal loads are applied on the crack face, and the numerical results are presented graphically.     

Formation of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloy under high pressure

The formation of nanocrystalline Fe_73.5 Cu₁Nb₃Si_13.5 B₉ alloy by annealing an amorphous Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy at a temperature of 823 K under pressures in the range of 1–5 GPa is investigated by using X-ray diffraction, electron diffraction, and transmission electron microscopy. The high pressure experiments are carried out in belt-type pressure apparatus. Experimental results show that the initial crystalline phase in these annealed alloys is a-Fe solid solution (named a-Fe phase below), and high pressure has a great influence on the crystallization process of the a-Fe phase. The grain size of the a-Fe phase decreases with the increase of pressure (P). The volume fraction of the a-Fe phase increases with increasing the pressure as the pressure is below 2 GPa, and then decreases (Pδ2 GPa). The mechanism for the effects of the high pressure on the crystallization process of amorphous Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy is discussed Project supported by the National Natural Science Foundation of China (Grant No. 19674070) and the Natural Science Foundation of Hebei Province.     

Analysis of arbitrarily shaped planar cracks in two-dimensional hexagonal quasicrystals with thermal effects. Part I: Theoretical solutions

An extended displacement discontinuity (EDD) boundary integral equation method is proposed for analysis of arbitrarily shaped planar cracks in two-dimensional (2D) hexagonal quasicrystals (QCs) with thermal effects. The EDDs include the phonon and phason displacement discontinuities and the temperature discontinuity on the crack surface. Green's functions for unit point EDDs in an infinite three-dimensional medium of 2D hexagonal QC are derived using the Hankel transform method. Based on the Green's functions and the superposition theorem, the EDD boundary integral equations for an arbitrarily shaped planar crack in an infinite 2D hexagonal QC body are established. Using the EDD boundary integral equation method, the asymptotic behavior along the crack front is studied and the classical singular index of 1/2 is obtained at the crack edge. The extended stress intensity factors are expressed in terms of the EDDs across crack surfaces. Finally, the energy release rate is obtained using the definitions of the stress intensity factors.     

Microtwins and twin inclusions in the 3C-SiC epilayers grown on Si(001) by APCVD

Microtwins in the 3C-SiC films grown on Si(001) by APCVD were analyzed in detail using an X-ray four-circle diffractometer. The Φ scan shows that 3C-SiC films can grow on Si substrates epitaxially and the epitaxial relationship is revealed as (001)3C-SiC//(001)Si，［111］3C-SiC//［111］Si. Other diffractions emerged in the pole figures of the (111) 3C-SiC. We performed the (1010)h-SiC and the reciprocal space mapping of the (002) plane of twins for the first time, finding that the diffractions at χ=15.8° result from not hexagonal SiC but microtwins of 3C-SiC, and twin inclusions are estimated to be 1%.     

Diamond nucleation on surface of C60 thin layers

Diamond nucleation on the surface of C₆₀ thin layers and intermediate layer of Si substrates are studied by scanning electron microscopy (SEM). The cross-section SEM images of diamond films show that diamond grains really nucleate on the surface of C₆₀ thin layers. The SEM images of diamond nucleating sites show the nucleating aggregation of diamond on C₆₀ surfaces. The preferential oriented diamond films are observed. The plasma pretreatment of C₆₀ sublimating layers is a key factor for diamond nucleation.     

Multiplicity factor and diffraction geometry factor for single crystal X-ray diffraction analysis and measurement of phase content in cubic GaN/GaAs(001) epilayers

On the basis of integrated intensity of rocking curves, the multiplicity factor and the diffraction geometry factor for single crystal X-ray diffraction (XRD) analysis were proposed and a general formula for calculating the content of mixed phases was obtained. With a multifunction four-circle X-ray double-crystal diffractometer, pole figures of cubic (002), 1₁₁₁ and hexagonal 1010 and reciprocal space mapping were measured to investigate the distributive character of mixed phases and to obtain their multiplicity factors and diffraction geometry factors. The contents of cubic twins and hexagonal inclusions were calculated by the integrated intensities of rocking curves of cubic (002), cubic twin 111, hexagonal 1010 and 1011.     

Exact asymptotics for Boundary crossings of the brownian bridge with trend with application to the Kolmogorov test

We consider a boundary crossing probability of a Brownian bridgeB ₀ and a piecewise linear boundary functionu(t)−γh(t). The main result of this paper is an asymptotic expansion for γ→∞ of the boundary crossing probability thatB ₀ (t) is larger than the piecewise linear boundary functionu(t)−γh(t) for somet. Such probabilities occur for instance in the context of change point problems when the Kolmogorov test is used. Examples are discussed showing that the approximation is rather accurate even for small positive γ values. Supported by the Swiss National Science Foundation Grant 20-55586.98.     

On pairs of unbounded self-adjoint operators satisfying an algebraic relation

Unbounded pairs of self-adjoint operatorsA andB satisfying the algebraic relationF ₁(A)B=BF ₂(A) are studied. For these relations, various definitions of integrable pairs of operators are presented and the class of tame relations is indicated; for the tame relations, the irreducible pairs are described and a structure theorem is presented.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 45, No. 9, pp. 1253–1258, September, 1993.     

A new higher‐order accurate numerical method for solving heat conduction in a double‐layered film with the neumann boundary condition

Heat conduction in multilayered films with the Neumann (or insulated) boundary condition is often encountered in engineering applications, such as laser process in a gold thin‐layer padding on a chromium thin‐layer for micromachining and patterning. Predicting the temperature distribution in a multilayered thin film is essential for precision of laser process. This article presents an accurate finite difference (FD) scheme for solving heat conduction in a double‐layered thin film with the Neumann boundary condition. In particular, the heat conduction equation is discretized using a fourth‐order accurate compact FD method in space coupled with the Crank–Nicolson method in time, where the Neumann boundary condition and the interfacial condition are approximated using a third‐order accurate compact FD method. The overall scheme is proved to be convergent and hence unconditionally stable. Furthermore, the overall scheme can be written into a tridiagonal linear system so that the Thomas algorithm can be easily used. Numerical errors and convergence rates of the solution are tested by an example. Numerical results coincide with the theoretical analysis. © 2014 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 30: 1291–1314, 2014     

Characterization of ZnO: Al films deposited by r.f. magnetron-sputtering at low temperature

Transparent conducting ZnO: Al thin films with good adhesion and low resistivity have been prepared on organic substrates and Corning 7059 glass substrates by r.f. magnetron-sputtering technique at low substrate temperature (25–210°C). Structural and photoelectric properties of the deposited films are investigated. The deposited films are polycrystalline with hexagonal structure and a preferred orientation with the c-axis perpendicular to the substrate. Only the (002) peak is observed. High quality films with resistivity as low as 1.0 x 10^-3Ω·cm and 8.4 x 10^-4Ω·cm, the average transmittance over 74% and 85% in the wavelength range of the visible spectrum have been obtained on different substrates.     

Collinear periodic cracks and/or rigid line inclusions ofantiplane sliding mode in one-dimensional hexagonal quasicrystal

For a one-dimensional (1D) hexagonal quasicrystal (QC), there is the periodic (x₁,x₂)-plane of atomic structures with the quasiperiodic direction x₃-axis along which there exists a phason displacement. The macroscopically collinear periodic cracks and/or rigid line inclusions are placed on the periodic (x₁,x₂)-plane for finding out the influence of phason displacement on the related physical quantities. These two models are reduced to the Riemann–Hilbert problem of periodic analytic functions to obtain the closed-form solutions for the antiplane sliding mode. It is found that the phonon and phason stress intensity factors of cracks as well as the phonon and phason stress field intensity factors of rigid line inclusions are not related to the coupling of phonon and phason fields. These mean that there is not the influence of phason displacement on both the phonon stress intensity factor (usual stress intensity factor) of cracks and the phonon stress field intensity factor of rigid line inclusions. However, the energy release rates of periodic cracks and/or rigid line inclusions are obtained and affected not only by the periodicity of cracks and/or rigid line inclusions but also by the phason displacement.     

Dynamic simulation on the preparation process of thin films by pulsed laser

An ablation model of targets irradiated by pulsed laser is established. By using the simple energy balance conditions, the relationship between ablation surface location and time is derived. By an adiabatic approximation, the continuous-temperature condition, energy conservation and all boundary conditions can be established. By applying the analytical method and integral-approximation method, the solid and liquid phase temperature distributions are obtained and found to be a function of time and location. The interface of solid and liquid phase is also derived. The results are compared with the other published data. In addition, the dynamics process of pulsed laser deposition of KTN (Kta_0.65Nb_0.35O₃) thin film is simulated in detail by using fluid dynamics theory. By combining the expression of the target ablation ratio and the dynamic equation and by using the experimental data, the effects of laser action parameters on the thickness distribution of thin film and on the thin film component characteristics are discussed. The results are in good agreement with the experimental data.